I. Flashcards
Volume of the bony orbit
slightly less than 30 cm3
Orbital entrance height and width
h: 35 mm
w: 45 mm
Where is the maximum width of the orbit
approximately 1 cm behind the anterior orbital margin
Depth of the orbit in adults
40 to 45 mm from the orbital entrance to the orbital apex
Why is important the Whitnall tubercle?
It is a small elevation of the orbital margin of the zygomatic bone, lies approximately 11 mm below the frontozygomatic suture and it is the site of attachment for the following structures:
- ligament of the lateral rectus muscle
- suspensory ligament of the eyeball (Lockwood suspensory ligament)
- lateral palpebral ligament
- aponeurosis of the levator palpebrae superioris muscle
- Whitnall ligament
From where to where does the optic foramen lead?
from the middle cranial fossa to the apex of the orbit
The superior orbital fissure transmits the fo llowing structures ( !above the common tendinous ring
of the rectus muscles ):
- lacrimal nerve of CN V
- frontal nerve of CN V
- CN IV (trochlear nerve)
- superior ophthalmic vein
What does the optic foramen conduct
- optic nerve
- ophthalmic artery
- sympathetic fibers from the carotid plexus
Innervation of the conjunctiva
Ophthalmic division of CN V
Blood supply of the conjunctiva + where is the vascular watershed between the anterior and posterior territories
- Anterior ciliary arteries supply blood to the bulbar conjunctiva.
- The tarsal conjunctiva is supplied by branches of the marginal arcades of the lids.
- The proximal arcade, running along the upper border of the lid, sends branches proximally to supply the fornical and then the bulbar conjunctiva as the posterior conjunctival arteries.
- The Iimbal blood supply derives from the ciliary arteries through the anterior conjunctival arteries.
vascular watershed between the anterior and posterior territories lies approximately 3 or 4 mOl from the limbus
Precorneal tear film layers + expression
- superficial oily layer produced predominantly by the meibomian glands
- middle aqueous layer produced by the main and accessory lacrimal glands
- deep mucin layer derived from the conjunctival goblet cells.
The surface cells of the cornea and conjunctiva also express a mucinous glycocalyx.
Corneal epithelium basal cells width and density
12 µm, 6000 cells/mm2
Wing cells cornea - where? shape?
Overlying the basal cell layer in the corneal epithelium, 2 or 3 layers, polygonal
Microplicae and microvilli make the apical surfaces of the wing cells highly irregular; however, the
precorneal tear film renders the surfaces optically smooth
Superficial corneal epithelial cells thickness
30 µm
What nonepothelial cells can appear within the corneal epithelium?
- Wandering histiocytes
- macrophages
- lymphocytes
- pigmented melanocytes
are usually components of the peripheral cornea. - Antigen-presenting Langerhans cells are found peripherally and move centrally with age or in response to keratitis.
Bowman layer thickness
8-12 µm
The stroma’s collagen types
I, III, V, VI
What type of collagen forms the anchoring fibril ofthe epithelium?
VII
How many keratocytes does the cornea have? %?
~ 2.4 million, ~ 5% of the stromal volume
Keratocyte density in the cornea
anteriorly 1058 cells/mm2
posteriorly 771 cells/mm2
Macroperiodicy of the fibrils in the corneal stroma
640 Å
Collagen fibril orientation in the cornea
- obliquely oriented lamellae in the anterior third of the stroma (with some interlacing)
- parallel lamellae in the posterior two-thirds
The corneal collagen fibrils extend across the entire diameter of the cornea, finally winding circumferentially around the limbus.
What is the corneal stroma composed of?
- collagen-producing keratocytes
- ground substance
- collagen lamellae
Where are the goblet cells in the conjuctiva?
The goblet cells (unicellular mucous glands) are concentrated in the inferior and medial portion of the conjunctiva, especially in the region of the caruncle and plica semilunaris. They are sparsely distributed throughout the remainder of the conjunctiva and are absent in the limbal region.
Ground substance of the cornea?
- consists of proteoglycans that run along and between the collagen fibrils.
- Their glycosaminoglycan components (eg, keratan sulfate) are highly charged and account for the swelling property of the stroma.
Where are the keratocytes in the cornea?
In the stroma, they lie between the corneal lamellae
Corneal keratocytes - what do they synthetise?
- they synthesize both collagen and proteoglycans.
Corneal keratocytes - cell structures?
- highly active cells rich in mitochondria, rough endoplasmic reticula, and Golgi apparatuses
- have attachment structures, communicate by gap junctions, and have unusual fenestrations in their plasma membranes
Corneal keratocytes - role in light transmission?
- Their flat profile and even distribution in the coronal plane ensure a minimum disturbance of light transmission
How many types of corneal keratocytes are there?
Studies with vital dyes suggest that there may be at least 3 different types
Descemet membrane thickness
- at birth 3-4 µm
- in adulthood 10-12 µm
Thickness increases with age
What is the Descemet membrane composed of?
- anterior banded zone that develops in utero
- and a posterior nonbanded zone that is laid down by the corneal endothelium throughout life
( These zones provide a historical record of the synthetic function of the endothelium. )
What type of collagen is in the Descemet membrane?
It is rich in type IV collagen
like other basal laminae
What are Hassall-Henle warts?
- Peripheral excrescences of the Descemet membrane
- they are common, especially among elderly people
How are the central excrescences of the cornea called?
corneal guttae
appear with increasing age?!
What is the corneal endothelium composed of? Developmental origin?
a single layer of mostly hexagonal cells derived from the neural crest
Corneal endothelium developmental origin?
neuroectodermal
Number and density of corneal endothelial cells?
- ~ 500,000 cells
- ~ 3000/mm2
How can the size, shape, and morphology of the endothelial cells be observed?
by specular microscopy at the slit lamp
Orientation of the corneal endothelial cells
The apical surfaces of these cells face the anterior chamber; their basal surfaces abut the Descemet membrane
Corneal endothelial cells - biology
- Typically, young endothelial cells have large nuclei and abundant mitochondria.
- The active transport of ions by these cells leads to the transfer of water from the corneal stroma and the maintenance of stromal deturgescence and transparency
- Mitosis of the endothelium is rare in humans, and the overall number of endothelial cells decreases with age
How do the adjacent corneal endothelial cells interdigitate?
- in a complex way and form a variety of adherent junctions, but desmosomes are never observed between normal cells
- In cross section, pinocytotic vesicles and a terminal web (a meshwork of fine fibrils that increases the density of the cytoplasm) can be observed toward the apical surface of the cells.
- Junctional complexes are present at the overlapping apicolateral boundaries of contiguous cells.
- They form a significant but lesser barrier to ion and water flow than do the tight junctions of the epithelium.
What does corneal endothelial dysfunction and loss cause?
It may cause endothelial decompensation, stromal edema, and vision failure
In humans, endothelial mitosis is limited, and destruction of cells causes cell density to decrease and residual cells to spread and enlarge.
Causes of corneal endothelial dysfunction and loss
- surgical injury,
- inflammation,
- inherited disease (eg, Fuchs endothelial dystrophy
How are the superficial corneal epithelial cells attached to eachother?
occlusion of the zonular fibers (sometimes referred to as zonules).
These zonular fibers confer the properties of a semipermeable membrane to the epithelium
How are the deeper corneal epithelial cells attached to eachother?
desmosomes
How do the corneal keratocytes communicate?
by gap juctions
How much of the globe does the sclera cover?
posterior 4/5
Where do the tendons of the rectus muscles insert?
superficial scleral collagen
Sclera thickness
- thinnest (0.3 mm) just behind the insertions of the rectus muscles
- thickest (1.0 mm) at the posterior pole around the optic nerve head.
- 0.4-0.5 mm thick at the equator
- 0.6 mm thick anterior to the muscle insertion
Where can scleral rupture occur following a blunt trauma?
- in a circumferential arc parallel to the corneal limbus opposite the site of impact
- at the insertion of the rectus muscles
- at the equator of the globe
The most common site is the superonasal quadrant near the limbus
Scleral vessels
the sclera is essentially avascular except for the superficial vessels of the episclera and the intrascleral vascular plexus located just posterior to the limbus.
What are the emissaria (sclera)?
Numerous channels that penetrate the sclera, allowing for the passage of arteries, veins, and nerves.
Extraocular extension of malignant melanoma of the choroid often occurs by way of the emissaria.
What are the Axenfeld loops? DD?
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops posterior to the nasal and temporal limbus.
They are sometimes pigmented and, consequently, have been mistaken for uveal tissue or malignant melanoma.
They are typically found a constant distance from the corneoscleral junction, usually within 3-4 (?) mm of the limbus.
DD: Nevi Malignant Melanoma Cysts Scleral Foreign Bodies
What does the episclera consist of?
A dense vascular connective tissue that merges deeply with the superficial sclera and superficially with the Tenon capsule and the conjunctiva.
What does the scleral stroma consist of?
- bundles of collagen
- fibroblasts
- a moderate amount of ground substance
Diameter of the scleral collagen fibers
- outer scleral collagen fibers 1600 Å
- inner collagen fibers 1000 Å
What do the bundles of scleral collagen fibers contain?
- electron-dense bodies
- fibroblasts
- melanocytes
Why is the sclera not transparent like the cornea?
- greater variation in fibril separation and diameter
- the greater degree of fibril interweaving in the sclera
In addition, the lack of vascular elements, such as the scleral emissaria, contributes to corneal clarity.
Collagen fibrils of the sclera - orientation, buildup
- vary in size and shape and taper at their ends, indicating that they are not continuous fibers as in the cornea
- The inner layer of the sclera (lamina fusca) blends imperceptibly with the suprachoroidal and supraciliary lamellae of the uveal tract. The collagen fibers in this portion of the sclera branch and intermingle with the outer ciliary body and choroid.
Limbal structures
l. conjunctiva and limbal palisades
2. Tenon capsule
3. episclera
4. corneoscleral stroma
5. aqueous outflow apparatus
What is the Schwalbe line?
termination of the Descemet membrane
Zones of the surgical limbus
2 equal zones:
(1) an anterior bluish-gray zone overlying the clear cornea and extending from the Bowman layer to the Schwalbe line and
(2) a posterior white zone overlying the trabecular meshwork and extending from the Schwalbe line to the scleral spur, or iris root.
What is the limbus histologically?
- The transition from opaque sclera to clear cornea occurs gradually over 1.0-1.5 mm and is difficult to define histologically.
- The corneoscleral junction begins centrally in a plane connecting the end of the Bowman layer and the Schwalbe line
- Internally, its posterior limit is the anterior tip of the scleral spur
- Pathologists consider the posterior limit of the limbus to be formed by another plane perpendicular to the surface of the eye, approximately 1.5 mm posterior to the termination of the Bowman layer in the horizontal meridian and 2.0 mm posterior in the vertical meridian, where there is greater scleral overlap
supraorbital foramen (or notch) - location, transmission
- located at the medial third of the superior margin of the orbit.
- transmits blood vessels (supraorbital artery /ICA -> opthalmic artery -> supraorbital artery/ and vein) and the supraorbital nerve (CN V -> ophthalmic nerve -> frontal nerve -> supraorbital nerve)
anterior ethmoidal foramen - location, transmission
- located at the frontoethmoidal suture
- transmits the anterior ethmoidal vessels and nerve.
posterior ethmoidal foramen - location, transmission
- lies at the junction of the roof and the medial wall of the orbit
- transmits the posterior ethmoidal vessels and nerve through the frontal bone
zygomatic foramen - location, transmission
- lies in the lateral aspect of the zygomatic bone
- contains the zygomaticofacial and zygomaticotemporal branches of the zygomatic nerve and the zygomatic artery.
nasolacrimal duct location
travels inferiorly from the lacrimal fossa into the inferior meatus of the nose.
infraorbital canal - location, transmission
- continues anteriorly from the infraorbital groove and exits 4 mm below the inferior orbital margin.
- From here it transmits the infraorbital nerve, which is a branch of V 2 (the maxillary division of CN V), (and artery: maxillary artery -> infraorbital artery, but often arises in conjunction with the posterior superior alveolar artery.)
superior orbital fissure - location, size
located between the greater and lesser wings of the sphenoid bone and lies lateral to and partly above and below the optic foramen. It is approximately 22 mm long and is spanned by the common tendinous ring
of the rectus muscles (annulus of Zinn).
What is the annulus of Zinn?
consists of superior and inferior orbital tendons and is the origin of the 4 rectus muscles
- The upper tendon gives rise to the entire superior rectus muscle, as well as portions of the lateral and medial rectus muscles
- The inferior tendon gives rise to the entire inferior rectus muscle and portions of the medial and lateral rectus muscles
superior orbital fissure - transmission
1. Above the ring (annulus of Zinn): • lacrimal nerve of CN V 1 • frontal nerve of CN V 1 • CN IV (trochlear nerve) • superior ophthalmic vein
- Within the ring or between the 2 heads of the rectus muscle are the following:
• superior and inferior divisions of CN III (the oculomotor nerve)
• nasociliary branch of CN V 1
• sympathetic roots of the ciliary ganglion
• CN VI (the abducens nerve)
The course of the inferior ophthalmic vein is variable, and it can travel within or below the ring as it exits the orbit.
:) The nerves passing through the fissure can be remembered with the mnemonic, “Live Frankly To See Absolutely No Insult” - for Lacrimal, Frontal, Trochlear, Superior Division of Oculomotor, Abducens, Nasociliary and Inferior Division of Oculomotor nerve
Inferior orbital fissure - location
lies just below the superior fissure between the lateral wall and the floor of the orbit, providing access to the pterygopalatine and inferotemporal fos-
sae. Therefore, it is close to the foramen rotundum and the pterygoid canal.
Inferior orbital fissure - transmission
transmits the infraorbital and zygomatic branches of CN V 2, an orbital nerve from the pterygopalatine ganglion, and the inferior ophthalmic vein. The inferior ophthalmic vein connects with the pterygoid plexus before draining into the cavernous sinus.
Where is located the superior orbital fissure?
-between the greater and lesser wings of the sphenoid bone and lies lateral to and partly above and below the optic foramen
The superior orbital fissure transmits the fo llowing structures:
- lacrimal nerve of CN V
- frontal nerve of CN V
- CN IV (trochlear nerve)
- superior ophthalmic vein
Where is located the inferior orbital fissure?
It lies just below the superior fissure between the lateral wall and the floor of the orbit, providing access to the pterygopalatine and inferotemporal fossae.
What angle does the lateral wall of the orbit form with the medial plane in adults?
approximately 45°
Which fossae do the lateral walls of the orbit border?
middle cranial, temporal, and pterygopalatine fossa
Where is the ciliary ganglion?
approximately 1 em in front of the annulus of Zinn, on the lateral side of the ophthalmic artery between the optic nerve and the lateral rectus muscle
What roots does the ciliary ganglion receive?
- long sensory root
- short motor root
- sympathetic root
Long sensory root of the ciliary ganglion
arises from the nasociliary branch of CN V1. It is 10-12 mm long and contains sensory fibers from the cornea, the iris, and the ciliary body.
Short motor root of the ciliary ganglion
arises from the inferior division of CN III, which also supplies the inferior oblique muscle. The fibers of the motor root synapse in the ganglion, and the postganglionic fibers carry parasympathetic axons to supply the iris sphincter
Sympathetic root of the ciliary ganglion
comes from the plexus around the internal carotid artery. It enters the orbit through the superior orbital fissure within the tendinous ring, passes through the ciliary ganglion without sy.napse, and innervates ocular blood vessels and the dilator muscles of the pupil.
Which fibers synapse in the ciliary ganglion?
Only the parasympathetic fibers
Which fibers form the short ciliary nerves?
- the nonsynapsing sympathetic fibers
- the sensory fibers
- the myelinated, fast -conducting postganglionic parasympathetic fibers
Sympathetic fibers of the ciliary ganglion
postganglionic from the superior cervical ganglion and pass through it without synapse
Sensory fibers of the ciliary ganglion
Sensory fibers from cell bodies in the trigeminal ganglion carry sensation from the eye, orbit, and face.
How many short ciliary nerve arise from the ciliary ganglion?
Two groups of short ciliary nerves, totaling 6-10
Where do the short ciliary nerves travel?
both sides of the optic nerve and, together with the long ciliary nerves, pierce the sclera around the optic nerve. They pass anteriorly between the choroid and the sclera into the ciliary muscle, where they form a plexus that supplies the cornea, the ciliary body, and the iris.
Insertion of the rectus muscles
anteriorly on the globe. Starting at the medial rectus muscle and then proceeding to the inferior rectus, lateral rectus, and superior rectus muscles, the muscle insertions lie progressively farther from the limbus
What is the spiral of Tillaux?
An imaginary curve drawn through the insertions of the rectus muscles
Insertion of the superion oblique muscle
after passing through the trochlea in the superior nasal orbital rim, inserts onto the sclera superiorly, under the insertion of the superior rectus
Insertion of the inferior oblique muscle
inserts onto the sclera in the posterior inferior temporal quadrant
Origin of the medial rectus muscle
Annulus of Zinn
Insertion of the nedial rectus muscle
medially, in horizontal meridian, 5.5 mm from limbus
Blood supply of the medial rectus muscle
inferior muscular branch of ophthalmic artery
Size of medial rectus muscle
- 8 mm long
tendon: 3.7 mm long - 3 mm wide
Origin of the inferior rectus muscle
Annulus of Zinn at orbital apex
Insertion the inferior rectus muscle
Inferiorly, in vertical meridian, 6.5 mm from limbus
Blood supply the inferior rectus muscle
Inferior muscular branch of ophthalmic artery and infraorbial artery
Size of the inferior rectus muscle
40 mm long
tendon: 5.5 mm long
9. 8 mm wide
Origin of the lateral rectus muscle
Annulus of Zinn spanning the superior orbital fissure
Insertion of the lateral rectus muscle
Laterally, in horizontal meridian, 6.9 mm from limbus
Blood supply of the lateral rectus muscle
a single vessel derived from the lacrimal artery
Size of the lateral rectus muscle
- 6 mm long
tendon: 8 mm long - 2 mm wide
Origin of the superior rectus muscle
Annulus of Zinn at orbital apex
Insertion of the superior rectus muscle
superiorly, in vertical meridian, 7.7 mm from limbus
Blood supply of the superior rectus muscle
superior muscular branch of ophthalmic artery
Size of the superior rectus muscle
- 8 mm long
tendon: 5.8 mm long - 6 mm wide
Origin of the superior oblique muscle
the periosteum of the body of the sphenoid bone, above and medial to optic foramen, between annulus of Zinn and periorbita
Insertion of the superior oblique muscle
to trochlea through pulley, at orbital rim, then hooking back under superior rectus, inserting posterior to center of rotation
Blood supply of the superior oblique muscle
Superior muscular branch of ophthalmic artery
Size of the superior oblique muscle
40 mm long
tendon: 20 mm long
10,8 mm wide
Origin of the inferior oblique muscle
anteriorly, from a shallow depression in the orbital plate of the maxillary bone, at the anteromedial corner of the orbital floor, near the lacrimal fossa / near orbital rim (maxilla)/
Insertion of the inferior oblique muscle
posterior inferior temporal quadrant at level of macula, posterior to center of rotation
Blood supply of the inferior oblique muscle
Inferior branch of ophthalmic artery and infraorbital artery
Size of the inferior oblique muscle
37 mm long
no tendon
9.6 mm wide at insertion
Origin of the levator palpebrae superioris muscle
lesser wing of the sphenoid bone, at the apex of the orbit, just superior to the annulus af Zinn
In which direction does the tinferior oblique muscle extend?
From its origin, the inferior oblique muscle then extends posteriorly, laterally, and superiorly to insert into the globe
Lateral rectus muscle - innervation
CN VI
Superior oblique muscle - innervation
CN IV
Which extraocular mucles does CN III innervate
- the upper division innervates the levator palpebrae superioris and superior rectus muscles
- the lower division innervates the medial rectus, inferior rectus, and inferior oblique muscles.
The ratio of nerve fibers to muscle fibers in the extraocular muscles compared with the ratio of nerve axons to muscle fibers in skeletal muscle
very high 1:3-1 :5
(skeletal: 1:50-1:125).
Extraocular muscles - types of fibers
mixture of slow, tonic-type and fast, twitch-type fibers
Why is the high ratio of nerve fibers to muscle fibers in extraocular muscles important?
This high ratio enables precise control of ocular movements
Tonic-type muscle fibers
- unique to extraocular muscles.
- Smaller than twitch-type fibers, they contract slowly and smoothly and tend to be located more superficially in the muscle, nearer the orbital wall.
- they are innervated by multiple grapelike nerve endings (en grappe) and are useful for smooth pursuit movements
Twitch-type muscle fibers
- more similar to skeletal muscle fibers.
- Larger than tonic-type fibers
- located deeper in the muscle,
- they contract rapidly
- have platelike nerve endings (en plaque).
- they aid in rapid saccadic movements of the eye.
Size of the adult palpebral fissure
27-30 mm long and 8-11 mm wide
How far can the upper eyelid be raised?
15 mm by the m. levator pp sup alone and an additonal 2 mm if the frontalis muscle of the brow is used
Innervation of the m. levator palpebrae superioris
CN III
distinct segments of the eyelid from the dermal surface inward.
- skin
- eyelid margin
- subcutaneous connective tissue
- orbicularis oculi muscle
- orbital septum
- levator palpebrae superioris muscle
- Muller muscle
- tarsus
- conjunctiva
Where are the firmest attachments of the aponeurosis of the m. levator palpebrae superioris?
on the anterior aspect of the tarsus, approximately 3 mm superior to the eyelid margin.
What does the intermarginal sulcus of the eyelid correspond to?
orbicularis oculi muscle, the muscle of Riolan, and to the avascular plane of the eyelid
3 parts of the orbicularis oculi muscle?
orbital
preseptal
pretarsal
How are the muscle fibers of the orbocularis oculi muscle connected?
by myomyous junctions
Innervation of the orbicularis oculi muscle
CN VII (and end plates are arranged in clusters over the entire length of the muscle)
Insertion of the orbital part of the orbicularis oculi muscle
it inserts in a complex way into the medial canthal tendon and into other portions of the orbital rim and the corrugator supercilii muscle
Function of the orbital part of the orbicularis oculi muscle
it acts as a sphincter and functions solely as a voluntary muscle.
Function of the palpebral part of the orbicularis oculi muscle
it functions both voluntarily and involuntarily in spontaneous and reflex blinking.
Where do the preseptal and the pretarsal portions of the orbicularis oculi muscle unite?
along the superior palpebral furrow
Where does the pretarsal orbicularis oculi muscle adhere?
it adheres firmly to the tarsus; a portion of it attaches to the anterior lacrimal crest and the posterior lacrimal crest (sometimes called the Horner muscle)
Role of the pretarsal orbicularis oculi muscle
it plays a role in tear drainage
What is the muscle of Riolan?
Orbicularis fibers extend to the eyelid margin, where there is the small bundle of striated muscle fibers called the muscle of Riolan
What may the disinsertion of the lower eyelid retractors cause?
laxity of the lower eyelid, followed by spastic entropion
Parts of the lacrimal gland
orbital lobe, palpebral lobe
What separates the orbital and palpebral lobes of the lacrimal gland?
the lateral horn of the levator palpebrae superioris muscle
Function of the lacrimal gland
secretion: exocrine
content: aqueous
Location of the accessory lacrimal glands
- plica, caruncle
- Krause: eyelid
- Wolfring: eyelid
Function of the accessory lacrimal glands
secretion: exocrine
content aqueous
Location of the meibomian glands
tarsus
Function of the meibomian glands
secretion: modified holocrine sebaceous glands
content: oily
Location of Zeiss glands
- follicles of cilia
- eyelid, caruncle
Function of Zeiss glands
secretion: holocrine
content: oily
Location of Moll glands
eyelid
Function of Moll glands
secretion: eccrine
content: sweat
Location of goblet cells
conjunctiva, plica, caruncle
Function of goblet cells
secretion: holocrine
content: mucus
Orbital septum histology
connective tissue
Location of the orbital septum
- encircles the orbit as an extension of the periosteum of the roof and the floor of the orbit.
- It also attaches to the anterior surface of the aponeurosis of the levator palpebrae superioris muscle
Role of the orbital septum
it provides a barrier to anterior or posterior extravasation of blood or the spread of inflammation
Origin of the levator palpebrae superioris muscle
lesser wing of the sphenoid bone
What is the Whitnall ligament?
- superior transverse ligament
- it is formed by a condensation of tissue surrounding the superior rectus and levator muscles
Length of the levator palpebrae superioris muscle
Together, the levator muscle and tendon are 50-55 mm long. The muscle, which elevates the upper eyelid, is 40 mm long
Innervation of the levator palpebrae superioris muscle
superior division of CN III
Insertion of the levator palpebrae superioris muscle
- Near the Whitnall ligament, the levator muscle changes direction from horizontal to more vertical, and it divides anteriorly into the aponeurosis and posteriorly into the superior tarsal (Müller) muscle.
- The aponeurosis inserts into the anterior surface of the tarsus and passes by the medial and lateral horns into the canthal tendons
- The fibrous elements of the aponeurosis pass through the orbicularis oculi muscle and insert subcutaneously to produce the superior eyelid fold
- The aponeurosis also inserts into the trochlea of the superior oblique muscle and into the fibrous tissue bridging the supraorbital notch
- Aponeurotic attachments also exist with the conjunctiva of the upper fornix and the orbital septum
What is the Müller muscle?
a smooth (nonstriated), sympathetically innervated muscle that originates from the undersurface of the levator palpebrae superioris muscle in the upper eyelid.
Where is a smooth muscle similar to the Müller muscle?
A similar smooth muscle (but musch weaker than the Müller muscle) arises from the capsulopalpebral head of the inferior rectus in the lower eyelid
Where does the Müller muscle attach?
the upper border of the upper tarsus and to the conjunctiva of the upper fornix
Where does the capsulopelpebral muscle attach?
the lower border of the lower tarsus
Histology of the tarsal plates
connective tissue
How are the tarsal plates attached to the orbital margin?
- by the medial and lateral palpebral ligaments
Size of the tarsal plates
the upper and lower tarsal plates are similar in length (29 mm) and in thickness (1 mm), the upper tarsus is almost 3 times as wide vertically ( 11 mm) as the lower tarsus ( 4 mm)
How can the distribution and the number of meibomian glands within the eyelid be observed?
Infrared transilluination of the eyelid
How many meibomian glands are there?
A single row of 30-40 meibomian orifices is present in the upper eyelid, but there are only 20-30 orifices in the lower lid
Caruncle - histology
As a piece of modified skin, it contains sebaceous glands and fine, colorless hairs. The surface is covered by nonkeratinized, stratified squamous epithelium.
Plica semilunaris - histology
- highly vascular
- fold of conjunctiva
- histologically resembles the bulbar conjunctiva
- rich in goblet celles
- contains fat and some nonstraited muscle
- vestigial structure analogous to the nictitating membrane, or third eyelid, of dogs and other animals.
Where do the lacrimal ducts pass afterthe main orbital gland?
- > palpebral gland
The ducts con-
tinue downward, and about 12 of them empty into the conjunctival fornix approximately 5 mm above the superior margin of the upper tarsus.
Where is the biopsy of the lacrimal gland usually performed? Why?
Because the lacrimal excretory ducts pass through the palpebral portion of the gland, it is usually performed on the main part to avoid sacrificing the ducts.
Cell types of the lacrimal glands?
- acinar cells, which line the lumen of the gland
- myoepithelial cells, which surround the parenchyma and are covered by a base-
ment membrane
Blood supply of the lacrimal gland
lacrimal artery, a branch of the ophthalmic artery
Innervation of the lacrimal gland
- secretomotor cholinergic acts through inositol 1,4,5-triphosphate-activated protein kinase C),
-vasoactive intestinal polypeptide (VIP)-ergic, - sympathetic nerve fibers
(Cyclic adenosine monophosphate is the second messenger for VIP-ergic and B-adrenergic stimulation of the gland)(the gland also has alpha1 adrenergic receptors) - sensory innervation
via the lacrimal nerve (CN V1)
Where are the accessory lacrimal glands of Krause and Wolfring?
Proximal lid borders and the fornices
Histology of the accessory lacrimal glands of Krause and Wolfring
identical to the main lacrimal gland
Innervation of the accessory lacrimal glands of Krause and Wolfring
identical to the main lacrimal gland
The accessory lacrimal glands of Krause and Wolfring account for ?? % of the total lacrimal secretory mass
10%
The lacrimal drainage system includes
- upper and lower puncta,
- lacrimal canaliculi,
- lacrimal sac,
- nasolacrimal duct
Diameter of the lacrimal punctum
~0,3 mm
Distance of the lacrimal puncta from the medial canthus
The inferior punctum
is approximately 6.5 mm from the medial canthus; the superior punctum is 6.0 mm from it.
In ?% of people do the lacrimal canaliculi form a common canaliculus?
90%
Histology of the lacrimal puncta and canaliculi
- lined with stratified squamous nonkera-
tinized epithelium that merges with the epithelium of the eyelid margins. - Near the lacrimal sac, the epithelium changes into 2 layers:
1. a superficial columnar layer
2. a deep, flattened cell layer - goblet cells and occasionally cilia are present
- in the canaliculi, the substancia propria consists of collagenous connective tissue and elastic fibers
- The wall of the lacrimal sac resembles adenoid tissue and has a rich venous plexus and many elastic fibers.
Geographic zones of the conjunctiva
palpebral, forniceal, bulbar
Blood supply of the bulbar conjunctiva
anterior ciliary arteries
blood supply of the tarsal conjunctiva
- branches of the marginal arcades of the lids
- The proximal arcade, running along the upper border of the lid, sends branches proximally to supply the forniceal conjunctiva and then the bulbar conjunctiva as the posterior conjunctival arteries
Limbal blood supply
derives from the ciliary arteries through the anterior conjunctival arteries.
Histology of the conjunctiva
- nonkeratinizing squamous epithelium with numerous goblet cells
- thin, richly vascularized substantia propria containing lymphatic vessels, plasma cells, macrophages, and mast cells
- a lymphoid layer extends from the bulbar conjunctiva to the subtarsal folds of the lids. In places, specialized aggregations of conjunctiva-associated lymphoid tissue (CALT) correspond to mucosaassociated lymphoid tissue (MALT) elsewhere and comprise collections ofT and B lymphocytes underlying a modified epithelium. These regions are concerned with antigen processing.
Thickness of the conjunctival epithelium
2 to 5 cells
Shape of the cells of the conjunctival epithelium
The basal cells are cuboidal and evolve into flattened polyhedral cells as they reach the surface
Where is the intermuscular septum?
3 mm posterior to the limbus
What is the Tenon capsule?
- fascia bulbi
- an envelope of elastic connective tissue that fuses posteriorly with the optic nerve sheath and anteriorly with the intermuscular septum
- the cavity within which the globe moves
What is the Tenon capsule composed of?
compactly arranged collagen fibers and a few fibroblasts.
Where is the Tenon capsule the thickest?
equator of the globe
Where do the extraocular muscles penetrate the Tenon capsule ?
10mm posterior to their insertions
(The connective tissues form sleeves around the penetrating extraocular muscles, creating pulleys suspended from the periorbita. These pulleys stabilize the position of the muscles relative to the orbit during eye movements. The pulleys are connected to one another and to the Tenon fascia by connective tissue bands containing collagen, elastin, and smooth muscle)
Age-related connective tissue degeneration can lead to…
acquired strabismus
What is the suspesory ligament of Lockwood?
- fusion of the sheath of the inferior rectus muscle, the inferior tarsal muscle, and the check ligaments of the medial and lateral rectus muscles
- provides support for the globe and the anteroinferior orbit
An operation on the inferior rectus muscle may be associated with …?
palpebral fissure changes
(Note: the fusion of the sheath of the inferior rectus muscles, the Lockwood ligament, and the inferior tarsal muscle is an important consideration in surgery)
How many short ciliary arteries and nerves enter the globe and where?
- Approximately 20 short posterior ciliary arteries and 10 short posterior ciliary nerves
- in a ring around the optic nerve
How many long ciliary arteries and nerves enter the globe and where?
Usually, 2 long ciliary arteries and nerves enter the sclera on either side of the optic nerve close to the horizontal meridian
Origin of the posterior ciliary vessels
opththalmic artery
What do the posterior ciliary vessels supply
- the whole uveal tract
- the cilioretinal arteries
- the sclera
- the margin of
the cornea - the adjacent conjunctiva.
Example and consequence of the occlusion of the posterior ciliary vessels
- pl. giant cell arteritis
- may cause aion
Origin of the anterior ciliary vessels
ophthalmic artery
A single anterior ciliary vessel enters the lateral rectus muscle from the lacrimal artery.
What do the anterior ciliary vessels supply
usually supply
(in pairs) the superior, medial, and inferior rectus muscles. A single anterior ciliary vessel enters the lateral rectus muscle from the lacrimal artery.
How do the anterior and posterior ciliary vessels anastomose with the long posterior ciliary vessels?
via anastomoses that perforate the sclera anterior to the rectus muscle insertions.
Which vessel forms th intramuscular circle of the iris?
posterior ciliary vessel
Where is the intramuscular circle of the iris?
within the apex of the ciliary muscle,
what does the intramuscular circle of the iris supply?
- branches frm it supply the major arterial circle
- ciliary muscle
- iris
arrangement of the iris vessels
radial
Where do the vortex veins drain lood from?
choroid, ciliary body, and iris
How many vortex veins are there per eye?
Each eye contains 4-7 (or more) veins. One or more veins are usually located
in each quadrant
Where do the vortex veins exit the eye?
14-25 mm from the limbus between the rectus muscles.
Where are the ampullae of the vortex veins?
8-9 mm from the ora serrata and are visible by indirect
ophthalmoscopy. A circle connecting these ampullae corresponds roughly to the equator and divides the central or posterior fundus from the peripheral portion.
Shape and dimensions of th globe
- oblate spheroid shape
- The anteroposterior diameter of the adult eye is approximately 23-25 mm.
- The average transverse
diameter of the adult eye is 24 mm. - The radius of curvature of the cornea (8 mm) is
smaller than that of the sclera ( 12 mm),
How deep is the anterior chamber?
~ 3 mm(,15)
Volume of the anterior chamber in the emmetropic eye
200ul
Volume of the posterior chamber
60ul
Volume of the vitreus cavity
5-6 mL
Total volume of the average adult eye
6,5-7,5 mL
Meridians of the adult cornea
about 12 mm in the horizontal meridian and about 11 mm in the vertical
Thickness of the cornea
1 mm thick at its periphery and 0.5 mm thick centrally.
Why is the precorneal tear film vital
In addition to lubricating the surface of the cornea and conjunctiva, tears produce a smooth optical surface; allow for the diffusion of oxygen and other nutrients; and contain immunoglobulins, lysozyme, and lactoferrin.
Refractive power of the air-tear film interface at the surface of the cornea
+43 D
Shape and size of the central third of the cornea
- nearly spherical
- ~ 4 mm
Flattening of the cornea
The cornea becomes flatter in the periphery, but the rate of flatten-
ing is not symmetric. Flattening is more extensive nasally and superiorly than temporally and inferiorly. This topography is important in contact lens fitting.
Where is the anterior surface of the cornea derived from?
surface ectoderm
Histology of the corneal epitheliumand how is tha basal columnar layer attached to the basal lamina
nonkeratinized, stratified squamous epithelium whose basal columnar layer is attached to a basal lamina by hemidesmosomes
(The occasional recurrence of corneal erosion following a traumatic corneal abrasion may be due to improper formation ofhemidesmo-
somes after an epithelial abrasion)
How are the deeper endothelial cells of the cornea attached to eachother?
by desmosomes
What does the Bowman membrane consist of?
randomly dispersed collagen fibrils.
It is a modified region of the anterior stroma
Is the Bowman layer restored after injury?
NO. It is replaced by scar tissue
Is the Descemet membrane restored after injury?
Yes
The stroma constitutes approximately ??% of the total corneal thickness in humans
90%
What does the anterior chamber angle consist of?
- Schwalbe line
- Schlemm canal and trabecular meshwork
- scleral spur
- anterior border of the ciliary body (where its longitudinal fibers insert into the scleral spur)
- iris
Depth of the anterior chamber in a normal adult emmetropic eye
~ 3 mm at its center and reaches its narrowest point sightlycentral to the angle recess
Tha aqueous humor is produced by…
Ciliary epitheium in the posterior chamber
The uveoscleral pathway accounts for ? % of the aqueous outflow
Up to 50% in young people
What does the internal scleral sulcus accomodate?
- Schlemm canal externally
- trabecular meshwork internally
Anterior and posterior margns of the internal scleral sulcus
The Schwalbe line forms the anterior margin of the sulcus; the scleral spur is its posterior landmark
Structures of the scleral spur
- receives the insertion of the longitudinal ciliary muscle, and contraction opens up the trabecular spaces
- Contractile cells are found within the scleral spur
- as are structures resembling mechanoreceptors, which receive a sensory innervation.
Where are the myofibroblast-like cells of the scleral spur? How cab they be stimulated?
Myofibroblast-like scleral spur cells with contractile properties are disposed circumferentially within the scleral spur. They are connected by elastic tissue to the trabecular meshwork
- in experiments, stimulation with vasoactive intestinal polypeptide (VIP) or calcitonin gene-related peptide (CGRP) causes an increase in outflow facility
Innervation of the individual scleral spur cells
unmyelinated axons, the terminals of which contact the cell membranes of the spur cells without an intervening basal lamina
The nerve fibers in the scleral spur region are immunoreactive of ?? and therefore they are mediated by ??? pathways
- neuropeptide Y, substance P, CGRP, VIP, and nitrous oxide
- mediated by sympathetic, sensory, and pterygopalatine nerve pathways
- there are no cholinergic fibers in this region
Myelinated nerve fibers of the angle region
Myelinated nerve fibers extending forward from the ciliary region to the inner aspect of the scleral spur yield branches to the meshwork and to dub-shaped endings in the scleral spur. These endings have the morphologic features of mechanoreceptors found elsewhere in the body, such as in the carotid artery. The endings are incompletely covered by a Schwann cell sheath and make contact with extracellular matrix materials such as elastin.
Functions of the club-shaped myelinated nerve fiber endings of the scleral spur
Various functions have been proposed for these endings, including ( 1) proprioception to the ciliary muscle, which inserts into the scleral spur, signaling contraction of the scleral spur cells, and (2) baroreception in response to changes in intraocular pressure.
What is the trabecular meshwork?
circular spongework of connective tissue lined by trabeculocytes.
Properties of trabeculocytes
- contractile properties, which may influence outflow resistance.
- phagocytic properties
Shape of the trabecular meshwork? wheres is the apex and the base?
The meshwork is roughly triangular in cross section; the apex is at the Schwalbe line, and the base is formed by the scleral spur and the ciliary body. Some trabecular tissue passes posterior to the spur
Layers of the travbecular meshwork
- uveal portion
- corneoscleral meshwork
- juxtacanalicular tissue, which is directly adjacent to the Schlemm canal
The uveal portion and the corneoscleral meshwork can be divided by an imaginary line drawn from the Schwalbe line to the scleral spur. The uveal meshwork lies internal and the corneoscleral meshwork external to this line.
Histology and biology of the uveal trabecular meshwork
- composed of cordlike trabeculae
- has fewer elastic fibers than does the corneoscleral meshwork.
- The trabeculocytes usually contain pigment granules,
- the trabecular apertures are less circular and larger than those of the corneoscleral meshwork.
Histology and biology of the corneoscleral meshwork
- consists of a series of thin, flat, perforated connective tissue sheets arranged in a laminar pattern.
- Each trabecular beam is covered by a monolayer of thin trabecular cells exhibiting multiple pinocytotic vesicles.
- The basal lamina of these cells forms the outer cortex of the trabecular beam; the inner core is composed of collagen and elastic fibers
Aging changes to the trabecular meshwork
- increased piigmentation
- decreased number of trabecular cells
- thickening of the basement membrane beneath trabecular cells
These changes can cause resistance to aqueous outflow or possibly glaucoma
Where is the pericanalicular connective tissue?
Pericanalicular connective tissue invests the entire extent of the Schlemm canal
What is the endothelial meshwork?
- lies on the trabecular aspect af the pericanalicular connective tissue, between the outermost layers of the corneoscleral meshwork and the endothelial lining of the Schlemm canal
- multilayered collection of cells forming a loose network
- Between these cells are spaces up to 10 um wide through which aqueous humor can percolate to reach the endothelial lining of the Schlemm canal
Which region of the drainage system contributes the most to outflow resistance?
Pericanalicular connective tissue / endothelial meshwork
This region of the drainage system contributes the most to outflow resistance, partly because the pathway is narrow and tortuous and partly because of the resistance offered by extracellular proteoglycans and glycoproteins
Histology of the Schlemm canal
- circular tube that closely resembles a lymphatic vessel
- formed by a continuous monolayer of nonfenestrared endothelium and a thin connective tissue wall
- the basement membrane of the endothelium is poorly defined
how are the lateral walls of the endothelial cells of the Schlemm Canal joined?
tight junctions
Vesicles and vacuoles of the endothelial cells of the Schlemm canal
- micropinocytic vesicles are present at hte apical and basal surfaces of the cells
- larger vesicles (giant vacuoles) have been observed along the internal canal wall. These vacuoles are lined by a single membrane, and their size and number are increased by a rise in the intraocular pressure. They are thought to contribute to the pressure-dependent outflow of aqueous humor
Endothelial lining of the trabecular wall of the Schlemm canal
- highly irregular (cells show luminal bulges corresponding to cell nuclei and macrovascular configirations. The latter represent cellular invaginations from the basal aspect that eventually open on the apical aspect of the cell to form transcellular channels, through which aqueous humor flows down a pressure gradient. )
- the endothelial lining of a diverticulum is continuous with that if the canal. Such blind, tortuous diverticula course for a variable distance into the trabecular meshwork but remain separated from the open spaces of the meshwork by their continuous endothelial lining
The endothelial wall of the trabecular wall is supported by … ?
interrupted, irregular basement membrane and a zone of pericanalicular connective tissue of variable thickness. The cellular element predominates in this zone, and the fibrous elements, especially elastic fibers, are irregularly arranged in a netlike fashion. Here, the open spaces are narrower than those of the trabecular meshwork.
Corneoscleral trabeculR sheets and corneoscleral wall of the Schlemm canal - histology
- The corneoscleral trabecular sheets show frequent branching, and the endothelial covering may be shared between adjacent sheets
- The corneascleral wall of the Schlemm canal is more compact than the trabecular wall; a lamellar arrangement of collagen and elastic tissue predominates
How many collector channels arise from the Schlemm canal?
Approximately 25-30
Where do the collector channels drain from the Schlemm canal?
into the deep and midscleral venous plexuses
How many of the collector channels from the Schlemm canal drain directly into the episleral venous plexus as aqueous veins?
up to 8
visible in the conjunctiva by biomicroscopy
Changes due to aging in the trabecular meshwork
- twofold to threefold thickening of trabecular sheets (the cortex thickens and the core thins)
- Endothelial cellularity is lost
- connective tissue (eg, in the endothelial meshwork) increases
- debris accumulates in the meshwork
- glycosaminoglycans accumulate in the extracellular space.
- Such changes are exaggerated in chronic open-angle glaucoma.
What are Sondermann channels?
small, tortuous, blind diverticula that extend from the canal to tha trabecular meshwork
Parts of the uveal tract
- iris
- ciliary body
- choroid
the uveal tract is firmly attached to the sclera at tha ollowing sites:
- scleral spur
- exit points of the vortex veins
- optic nerve
These attachments account for the characteristic anterior balloons formed in choroidal detachment
What are Fuchs crypts?
- tthey are located on the iris on either side of the collaterette in the pupillary and ciliary portions and peripherally near the iris root
Vessels of the iris
major arterial circle in the ciliary body -> Radial branches of the arteries and veins extend toward the pupillary region. The arteries form the incomplete minor arterial circle, from which branches extend toward the pupil, creating capillary arcades
In humans, the anterior border layer is normally avascular
What is the stroma of the iris composed of?
- pigmented cells (melanocytes)
- nonpigmented cells
- collagen fibrils
- matrix containing hyaluronic acid
- Myelinated and nonmyelinated nerve fibers serve sensory, vasomotor, and muscular functions throughout the stroma.
where does the aqueous humor fow thorough the iris?
The aqueous humor flows freely through the loose stroma along the anterior border of the iris, which contains multiple crypts and crevices that vary in size, shape, and depth. This surface is covered by an interrupted layer of connective tissue cells that merges with the ciliary body
Difference between stromas in case of different iris colors
The overall structure of the iris stroma is similar in irides of all colors. Differences in color are related to the amount of pigmentation in the anterior border layer and the deep stroma. The stroma of blue irides is lightly pigmented, and brown irides have a densely pigmented stroma that absorbs light
What is pharmacokietics?
studies how the chemical substances cycle through the biological system
What is pharmacodynamics?
examines the biological and chemical effects of the chemical on the biological system
What is pharmacotherapeutics?
the study of how to achieve desirable effects or avoid or minimize adverse effects or toxicity of the drug
2 main branches of pharmacology?
pharmacokinetics and pharmacodynamics
What determines bioavailability?
pharmacikinetics and dose together
What is bioavailability?
concentration of the active drug at the therapeutic site
How is the pharmacodynamic action of a drug often described?
using the receptor for that drug; for example, a drug may be categorized as an a-adrenergic agonist or b-adrenergic antagonist.
in early neonatal life, the drug-metabolizing activities of the cytochrome P450-dependent, mixed-function oxidases and the conjugating enzymes are approximately ???% of adult values
50-70%
At what age does glucoronide formation reach adult levels?
3-4 years of life
When dies GFR reach the adult value?
6-12 months of life
Typess of local toxicity of topical drugs
- type I IgE mediated hypersenstivity reaction
- delayed reaction to either the medication or preservatives
Common preservatives in ophthalmic preparations
- quaternary cationic surfactants (pl. belzalkonium chloride or benzododecinium bromide)
- mercurials (pl. thiomersal, chlorobutanol, parahydroxybenzoates)
- aromatic alcohols
Ocular effects of preservatives in ophthalmic solutions
- can be toxic to the ocular surface following topical administration
- can enhance corneal permeability of various drugs
Methods to reduce the toxic effect on the ocular surface of newer preservatives
- preservative dissipates upon exposure to light or to the ions in the tear film (pl. stablized oxychloro complex - breaks down to sodium chloride and water, and sodium perborate - breaks down to hydrogen peroide before becoming hydrogen and oxygen)
- These “disappearing preservatives” theoretically should have no toxicity to the corneal surface - ionic buffer containing borate, sorbitol, propylene glycol, and zinc that breaks down into innate elements upon encountering the cations in the tear film of the eye
- polyquaternium -1 is a cationic polymer of quaternary ammonium structures that lacks a hydrophobic region. Although it is a detergent, human corneal epithelial cells tend to repel the compound
How much does human renal function decrease with age?
50%
What forms the bulk of the iris stroma
blood vessels
what is the thickest portion of the iris?
The collaterette - in this region, anastomoses occur between the arterial and venous arcades to form the minor vascular circle of the iris, which is often incomplete
Where is the major arterial circle of the iris located?
- apex of the ciliary body, not the iris
Histology o the iris capillaries
Their endothelium is nonfenestrated and is surrounded by a basement membrane, associated pericytes, and a zone of collagenous filaments. The intima has no internal elastic lamina
The posterior pigmented layer of the iris is continuous with…?
the nonpigmented epithelium of the ciliary body and thence with the neurosensory portion of the retina
what do the basal and apical surfaces of the posterior pigmented layer of the iris border?
The basal surface of the pigmented layer borders the posterior chamber. The apical surface faces the stroma and adheres to the anterior pigmented layer, which gives rise to the dilator muscle
Developmental origin of the iris?
neural ectoderm (not neural crest)
What is ectropion uveae?
pigmented layer extends farther onto the anterior surface of the iris, a condition called ectropion. The term ectropion uveae, which refers to an outfolding over the pupil of the iris pigment epithelium (IPE), is a misnomer because the IPE derived from neural ectoderm (not neural crest) and therefore is not considered part of the uvea.
Wher is the dilator of the iris derived form embriologically?
outer layer of the optic cup, which is neuroectoderm
Where is the dilator of the iris?
Parallel and anterior to the posterior pigmented epithelium
Histology of the dilator of the iris
- the smooth muscle cells cntain fine myofilaments and melanosomes
- the myofibrils are confined mainly to the basal portion of the cellsand extend anteriorly to the iris stroma
- the melanosomes and the nucleus are in the apical region of each myoepithelial cell
Innervation of the dilator of the iris
- dual sympathetic and parasympathetic innervation
- the dilator muscle contracts in response to sympathetic L1-adrenergig stimulation
- cholinergic parasympathetic stimulation may have an inhibitory role
Sympathetic chain that innervates the dilator of the iris
- The first-order neuron of the sympathetic chain begins in the ipsilateral posterolateral hypothalamus and passes through the brainstem to synapse in the intermediolateral gray matter of the spinal cord, chiefly at thoracic level 1.
- The second- order preganglionic neuron exits the spinal cord, passes over the pulmonary apex and through the stellate ganglion without synapsing, and synapses in the superior cervical ganglion
- The third-order postganglionic neuron originates here, joins the internal carotid plexus, enters the cavernous sinus, and travels with the ophthalmic division of cranial nerve (CN) V to the orbit and then to the dilator muscle.
What does the interruption of the sympathetic nerve supply of the dilator of the iris result in?
Horner syndrome, with miosis, in addition to ptosis and anhydrosis.
developmental origin of the sphincter of the iris
neuroectoderm
What is the sphincter of the iris composed of?
a circular band of smooth uscle fibers
Where is the sphincter of the iris located?
near the pupillary marginin the deep stroma, anterior to the pigment epithelium of hte iris
Innervation of the sphincter of the iris
Although dual innervation has been demonstrated morphologically, the sphincter muscle receives its primary innervation from parasympathetic nerve fibers that originate in the CN III nucleus, and it responds pharmacologically to muscarinic stimulation. The reciprocal sympathetic innervation to the sphincter appears to serve an inhibitory role, helping relax the sphincter in darkness.
Where do the nerve fibers serving the iris sphincter travel?
The fibers subserving the sphincter muscle leave the Edinger-Westphal subnucleus and follow the inferior division of CN III after it bifurcates in the cavernous sinus. The fibers continue in the branch supplying the inferior oblique muscle, exit, and synapse with postganglionic fibers in the ciliary ganglion. The postganglionic fibers travel with the short ciliary nerves to the iris sphincter.
They are unusual in that they are myelinated, presumably reflecting a need for fast conduction.
Shape of the ciliary body in cross section
triangular
direction of the apex of the ciliary body
towards the ora serrata
Where is the ciliary body atteched to the sclera?
- only at its base, via its longitudinal muscle fibers, where they insert into the scleral spur
What gives rise to the iris?
base of the ciliary body
What are the principial functions of the ciliary body?
- aqueous humor formation
- lens accomodation
It also plays a role in the trabecular and uveoscleral outflow of aqueous humor.
Width of the ciliary body
6-7 mm
Parts of the ciliary body
- pars plana
- pars plicata
How much of an average eyedrop is retained in the eye?
When a 50-IlL drop is delivered from the usual commercial dispenser, the volume of the tear lake rises from 7 uL to only 10 uL in the blinking eye of an upright patient. Thus, at most, 20% of the administered drug is retained (10 uL/50 uL)
- A rapid turnover of fluid also occurs in the tear lake-16% per minute in the undisturbed eye-with even faster turnover if the drop elicits reflex tearing. Consequently, for slowly absorbed drugs, at most only 50% of the drug that was initially retained in the tear reservoir (50% of the 20% of the delivered medication, or 10%) remains 4 minutes after instillation (0.844 = 0.50), and only 17% remains after 10 minutes, or 3.4% of the original dose.
What is the residence time of a medication?
The amount of time that a drug remains in the tear reservoir and tear film
What is the residence time affected by?
- durg formation
- timing of subsequent medication
- tear production
- and drainage
Pars plana of the ciliary body - description size
relatively avascular, smooth, pigmented zone; it is 4 mm wide and extends from the ora serrata to the ciliary processes
safest posterior surgical approach to the vitreous cavity
through the pars plana, located 3-4 mm from the corneal limbus
pars plicata of the ciliary body
The pars plicata is richly vascularized and consists of approximately 70 radial folds, or ciliary processes. The zonular fibers of the lens attach primarily in the valleys of the ciliary processes but also along the pars plana
blood supply and drainage of each ciliary process
The capillary plexus of each ciliary process is supplied by arterioles as they pass anteriorly and posteriorly from the major arterial circle; each plexus is drained by 1 or 2 large venules located at the crest of each process
What does sphincter tone within the arteriolar smooth muscle of the ciliary processes affect?
- capillary hydrostatic pressure gradient
- whether blood flows into the capillary plexus or directly to the draining choroidal vein, bypassing the plexus completely
What factors affect the rate of aqueous humor formation?
Neuronal innervation of the vascular smooth muscle and humoral vasoactive substances may be important in determining regional blood flow, capillary surface area available for exchange of fluid, and hydrostatic capillary pressure.
Ciliary epithelium - location of the layers
- The inner, nonpigmented epithelium is located between the aqueous humor of the posterior chamber and the outer, pigmented epithelium
How are the apices of the nonpigmented and pigmented cell layers of the ciliary epithelium fused?
by a complex system of junctions and cellular interdigitations
How are the epithelial cells of the nonpigmented ciliary epithelium attached to eachother?
Along the lateral intercellular spaces, near the apical border of the nonpigmented epithelium, are tight junctions (zonulae occludentes) that maintain the blood-aqueous barrier.
Basal laminae of the nonpigmented and pigmented epithelium of the ciliary body
- The basal surface of the nonpigmented epithelium, which borders the posterior chamber, is covered by the basal lamina, which is multilaminar in the valleys of the processes.
- The basal lamina of the pigmented epithelium, which faces the iris stroma, is thick and more homogeneous than that of the nonpigmented epithelium.
Shape of the cells of the pigmented epithelium of the ciliary body
cuboidal. They are characterized by multiple basal infoldings
(The pigmented epithelium is relatively uniform throughout the ciliary body)
Cell structures of the cells of the pigmented epithelium of the ciliary body
- large nucleus
- mitochondria
- extensive endoplasmatic reticulum
- many melanosomes
Shape and strusctures of the cells of the the nonpigmented epithelium of the ciliary body
- cuboidal in the pars plana but columnar in the pars plicata.
- multiple basal infoldings
- abundant mitochondria
- large nuclei
- The endoplasmic reticulum and Golgi complexes in these cells are important for aqueous humor formation.
- Sometimes melanosomes are present, especially anteriorly, near the iris.
What does the uveal portion of the ciliary body consist of?
- comparatively large fenesrated capillaries
- collagen fibrils
- fibroblasts
Main arterial supply of the ciliary body - what arterial plexus do they form?
comes from the anterior and long posterior ciliary arteries, which join together to form a multilayered arterial plexus consisting of a superficial episcleral plexus; a deeper intramuscular plexus; and an incomplete major arterial circle often mistakenly attributed to the iris but actually located posterior to the anterior chamber angle recess, in the ciliary body
Venous drainage of the ciliary body
- The major veins drain posteriorly through the vortex system
- although some drainage also occurs through the intrascleral venous plexus and the episcleral veins into the limbal region
How does the ciliary muscle change with age?
- the amount of connective tissue between the muscle bundles increases
- there is a loss of elastic recoil after contraction.
Where are the layers of the ciliary muscle located?
- the amount of connective tissue between the muscle bundles increases, and there is a loss of elastic recoil after contraction.
- The radial muscle fibers arise in the midportion of the ciliary body
- the circular fibers are located in the innermost portion
How do the ciliary muscles behave?
- Clinically, the 3 groups of muscle fibers function as a unit.
- The ciliary muscles behave like other smooth, nonstriated muscle fibers
Cell structures of the ciliary muscle fibers
Ultrastructural studies reveal that they contain multiple myofibrils with characteristic electrondense attachment bodies, mitochondria, glycogen particles, and a prominent nucleus.
-
histology of the ciliary muscle
- The smooth muscle cells are surrounded by a basal lamina separated from the cell membrane by a 300-A space
- Bundles of fibers are surrounded by a thin fibroblastic sheath rather than by collagen
Ciliary muscle - type of collagen
VI
it forms a sheath around the anterior elastic tendons
Where do the anterior elastic tendons of the ciliary body insert?
into the scleral spur and around the tips of the oblique and circular muscle fibers as they insert into the trabecular meshwork
What types of nerve fibers does the ciliary muscle have?
both myelinated and nonmyelinated
What % of the short ciliary nerves are directed to the ciliary muscle and the iris?
Approximately 97% of these ciliary fibers are directed to the ciliary muscle, and about 3% are directed to the iris sphincter
What is the role of the sympathetic fibers of the ciliary muscle?
may play a role in relaxing the muscle
Effect of cholinergic drugs on the ciliary muscle
contraction
Because some of the muscle fibers form tendinous attachments to the scleral spur, their contraction increases aqueous flow by opening up the spaces of the trabecular meshwork.
What does the choroid nourish?
outer portion of the retina
Thickness of the choroid
It averages 0.25 mm
Vessel layers of the choroid
- the choriocapillaris, the innermost layer
- a middle layer of small vessels
- an outer layer of large vessels
Where does the perfusion of the choroid come from?
!!! both from the long and short posterior ciliary arteries and from the perforating anterior ciliary arteries
Oxygen content of choroidal vessels
Blood flow through the choroid is high compared with that of other tissues. As a result, the oxygen content of choroidal venous blood is only 2%-3% lower than that of arterial blood.
Where does the Bruch membrane lie?
It extends from the margin of the optic disc to the ora serrata
Causes of defects in the Bruch membrane
- defects develop spontaneously in myopia or pseudoxanthoma elasticum
- trauma
- inflammation
Is Bruch membrane permeable to fluorescein?
Yes
What are the layers of the Bruch membrane?
1) basal lamina of RPE, 2) inner collagenous zone, 3) porous band of elastic fibers, 4) outer collagenous zone, 5) basal lamina of the choriocapillaris
What is Bruch membrane?
A PAS-positive layer resuting from the fusion of the basal laminae of the RPE and choriocapillaris
In to what does blood from the choroid drain?
vortex veins
What is the main arterial supply of the ciliary body?
the anterior ciliary arteries and long posterior ciliary arteries
What is the lining of the ciliary body?
2 layers of epithelial cells (non-pigmented and pigmented epithelial layers)
Which is more vascular, the pars plana or the pars plicata of the ciliary body?
the pars plicata is richly vascularized
Diameter of the capillaries of the choriocapillaris
40-60 um
Histology of the choriocapillaris
- it is a continuous layer oflarge capillaries
- The vessel walls are extremely thin and contain multiple fenestrations, especially on the surface facing the retina
- Pericytes are located along the outer wall.
Are the middle and outer choroidal vessels fenestrated?
no
Histology of the large choroidal vessels
The large vessels, typical of small arteries elsewhere, possess an internal elastic lamina and smooth muscle cells in the media. As a result, small molecules such as fluorescein, which diffuse across the endothelium of the choriocapillaris, do not leak through medium and large choroidal vessels.
Are the medium and large choroidal vessels permeable to fluorescein?
no, but the choriocapillaris is
Cells of the choroidal stroma
- abundant melanocytes
- occasional macrophages
- lymphocytes
- mast cells
- plasma cells
Intercellular space of the choroidal stroma
- collagen fibers
- nerve fibers
Why is the degree of pigmentation important for those performing photocoagulation?
it influences the absorption of laser energy
Focusing power of the lens of the average adult eye
20.00 D
Equatorial diameter of the lens
6.5 mm at birth; it increases in the first 2-3 decades of life and remains approximately 9-10 mm in diameter in late life
Anteroposterior width of the lens
approximately 3 mm at birth and increases after the second decade of life to approximately 6 mm by age 80 years
How does the growth of the lens through life affect the refractive power?
growth is accompanied by a shortening of the anterior radius of curvature of the lens, which would increase its optical power if not for a compensatory change in the refractive gradient across the lens substance
The increased thickness during accomodation is due to???
entirely due to a change in nuclear shape
Accomodative power of the lens
At age 8 years, the power is 14.00 D. By age 28 years, the accommodative power decreases to approximately 9.00 D, and it decreases further to 1.00 D by age 64 years
Causes of loss of the accomodative power
- increased size of the lens
- altered mechanical relationships
- increased stiffness of the lens nucleus secondary to changes in the crystalline proteins of the fiber cytoplasm
- Other factors, such as alterations in the geometry of zonular attachments with age and changes in lens capsule elasticity, may also play a role
Nourishment of the lens
After regression of the hyaloid vasculature during embryogenesis, the lens depends solely on the aqueous and vitreous for its nourishment
What is the lens capsule?
A basal lamina put down by the lens epithelial cells, rich in type IV collagen and other matrix proteins.
Does the thickness of the anterior capsule change during life?
Yes, its synthesis proceeds througout life so its thickness increases, while that of posterior capsule does not
Thickness of the lens capsule in adults
Values of 15.5 f.!m for the thickness of the anterior capsule and 2.8 1-lm for the posterior capsule have been cited for the adult lens.
Morphology of the lens capsule
it consists of fine filaments arranged in lamellae, parallel to the surface. The anterior lens capsule contains a fibrogranular material, identified as laminin, which is absent from the posterior capsule at the ultrastructural level
Where does the lens epithelium lie?
beneath the anterior and equatorial capsule, but it is absent under the posterior capsule.
Arrangement of the lens epithelium cells
- The basal aspects of the cells abut the lens capsule without specialized attachment sites.
- The apices of the cells face the interior of the lens, and the lateral borders interdigitate, with practically no intercellular space
Cell structures of the lens epithelium cells
Each cell contains a prominent nucleus but relatively few cytoplasmic organelles.
Regional differences of the lens epithelium
- The central zone represents a stable population of cells whose numbers slowly decline with age
- An intermediate zone of smaller cells shows occasional mitoses
- Peripherally, there are meridional rows of cuboidal preequatorial cells thal form the germinative zone of the lens
What forms the germinative zone of the lens?
peripherally, meridional rows of cuboidal preequatorial cells
- Here, cells undergo mitotic division, elongate anteriorly and posteriorly, and form the differentiated fiber cells of the lens
- In the human lens, cell division continues throughout life and is responsible for the continued growth of the lens
What happens if germinative cells are left behind afetr phaco?
they can give rise to posterior capsular opacification as a result of aberrant proliferation and cell migration
When are parts of the lens formed?
The nucleus is the part of the fiber mass that is formed at birth, and the cortex forms as new fibers are added postnatally
shape and connections between the fiber sells of the lens
hexagonal in cross section, have a spindle shape, and possess numerous interlocking, fingerlike projections
What does the cytoplasm of the lens fiber cells contain?
It contains lens crystallins.
- Apart from the most superficial cortical fibers, the cytoplasm is homogeneous and contains few organelles
Why does the lens have a high refractive index?
The high refractive index of the lens results from the high concentration of lens crystallins (α, β, and γ) in the fiber cytoplasm
Layout of the lens fibers
The lens sutures are formed by the interdigitation of the anterior and posterior tips of the spindle-shaped fibers. In the fetal lens, this interdigitation forms the anterior Y-shaped suture and the posterior inverted Y-shaped suture. As the lens ages, further branches are added to the sutures; each new set of branch points corresponds to the appearance of a fresh optical zone of discontinuity.
Origin of the zonular fibers
basal lamina of the nonpigmented epithelium of the pars plana and the pars plicata o the ciliary body
Attachment of the zonular fibers
chiefly to the lens capsule anterior and posterior to the equator
Histology of the zonular fibers
Each zonular fiber is made up of multiple filaments of fibrillin that merge with the equatorial lens capsule. In Marfan syndrome, mutations in the fibrillin gene lead to weakening of the zonule and subluxation of the lens.
Diameter of the macula
5-6 mm
Why is the color of the fundus reddish?
due to the transmission of light reflected from the posterior sclera through the capillary bed of the choroid
Developmental origin ofthe retina
Inner and outer layers of the optic cup
10 layers of the neurosensory retina from inner to outer retina
- internal limiting membrane
- nerve fiber layer
- ganglion cell layer
- inner plexiform layer
- inner nuclear layer
- middle limiting membrane
- outer plexiform layer
- outer nuclear layer
- external limiting membrane
- rod and cone inner and outer segments
What does the RPE consist of and where does it lie?
The RPE consists of a monolayer of hexagonal cells that extends anteriorly from the optic disc to the ora serrata, where it merges with the pigmented epithelium of the ciliary body
Functions of the RPE
- vit A metabolism
- maintenance of the outer blood-retina barrier
- phagocytosis of the photoreceptor puter segments
- absorption of light (reduction of scatter)
- heat exchange
- formation of the basal lamina of the Bruch membrane
- production of the mucopolysaccharide matrix surrounding the outer segments
- active transport of materials into and out of the RPE
Aret thr RPE cells polarized?
yes, like other epithelial and endothelial cells
Basal aspect of the RPE
The basal aspect is intricately folded and provides a large surface of attachment to the thin basal lamina that forms the inner layer of the Bruch membrane
Apices of the RPE cells
The apices have multiple villous processes that engage with the photoreceptor outer segments
What is retinal detachment?
Separation of the RPE from the neurosensory retina
How are contiguous RPE cells attached to eachother?
- firmly, by a series of lateral, intercellular junctional complexes.
- zonulae occludentes: they consist of fused plasma membranes forming a circular band of belt between adjacent cells
- a small intercellular space is present between zonulae adherentes
- both of them provide structural stability and also plays an important role in maintaining the outer blood-retina barrier
Thickness of the retina
- in the papillomacular bundle near the optic nerve: 0.23 mm
- foveola: 0.10 mm
- ora serrata: 0.11 mm
Size of the RPE cells
10-60 um
RPE cells in the fovea are taller and thinner, contain more melanosomes, and have larger melanosomes.
These characteristics account in part for the decreased transmission of choroidal fluorescence observed during fundus fluorescein angiography. Cells in the periphery are shorter, broader, and less pigmented.
How many RPE cells doesthe eye of a fetus or an infant have?
4-6 billion
Does the number of RPE cells increase with age?
yes, but only a little
Are mitotic figures apparent in the RPE of a normal adult eye?
no
What are melanosomes of the RPE?
- multiple round and ovoid pigment granules in the cytoplasmn of the RPE cells
Development of themelanosomes of the RPE?
These organelles develop in situ during formation of the optic cup and first appear as nonmelanized premelanosomes.
(Their development contrasts sharply with that of the pigment granules in uveal melanocytes, which are derived from the neural crest and later migrate into the uvea. )
Development of the pigment granules of the uveal melannocytes
they are derived from the neural crest and later migrate into the uvea
What are lipofuscin granules of the RPE?
- probably arise from the discs of photoreceptor outer segments and represent residual bodies arising from phagosomal activity.
- This so-called wear-andtear pigment is less electron dense than the melanosomes, and its concentration increases gradually with age.
- Clinically, these lipofuscin granules are responsible for the signal observed with fundus autofluorescence
What are phagosomes of the RPE?
- membrane-enclosed packets of disc outer segments that have been engulfed by the RPE
- Several stages of disintegration are evident at any given time
- In some species, shedding and degradation of the membranes of rod and cone outer segments follow a diurnal rhythm synchronized with daily fluctuations of environmental light
What does the cytoplasm of an RPE cell contain?
- melanosomes
- lipofuscin granules
- phagosomes
- numerous mitochondria (involved in the aerobic metabolism)
- rough-surfaced endoplasmic reticulum
- Golgi
- large and round nucleus
What is a drusen?
Throughout life, incompletely digested residual bodies, lipofuscin pigment, phagesomes, and other material are excreted beneath the basal lamina of the RPE. These contribute to the formation of drusen, which are accumulations of this extracellular material.
Typical location of drusen?
between the basement membrane of the RPE cells and the inner collagenous zone of the Bruch membrane.
how are drusen classified?
by fundoscopic appearance as either hard or soft
elements of the neurosensory retina
neuronal, glial, and vascular elements
How are the photoreceptor cells connected to the RPE?
The outer segments, surrounded by a mucopolysaccharide matrix, make contact with the apical processes of the RPE. Tight junctions or other intercellular connections do not exist between the photoreceptor cell outer segments and the RPE. The factors responsible for keeping these layers in apposition are poorly understood but probably involve active transport.
Rod photoreceptor outer segment
- contains multiple laminated discs resembling a stack of coins
- and a central connecting cilium
- The microt ubules of the cilium have a 9-plus-0 cross-sectional configuration rather than the 9-plus-2 configuration found in motile cilia
Rod photoreceptor inner segment
- outer ellipsoid
- contains numerous mitochondria - inner myoid
- contains large amount of glycogen
- continuous with the main cell body (where the nucleus is located)
- The inner portion of the cell contains the synaptic body, or spherule, of the rod, which is formed by a single invagination that accommodates 2 horizontal cell processes and 1 or more central bipolar dendrites
Outer segments of the cones
different morphology depending on their location in the retina
Inner segments of the cones
- he extrafoveal cone photoreceptors of the retina have conical ellipsoids and myoids, and their nuclei tend to be closer to the external limiting membrane than are the nuclei of the rods
- The cone synaptic body, or pedicle, is more complex than the rod spherule. Cone pedicles synapse with other rods and cones as well as with horizontal and bipolar cell processes
- Foveal cones have cylindrical inner segments similar to rods but otherwise are cytologically identical to extrafoveal cones
Difference between the outer segments of the rods and cones
- their structire is similar, but:
Rod discs are not attached to the cell membrane; they are discrete structures. Cone discs are attached to the cell membrane and are thought to be renewed by membranous replacement
Horizontal cells - synaptic connections
with many rod spherules and cone pedicles; they also extend cell processes horizontally throughout the outer plexiform layer
Bipolar cells - synaptic connections and orientation
- vertical orientation
- Their dendrites synapse with either rod or cone synaptic bodies, and their axons make synaptic contact with ganglion cells and amacrine cells in the inner plexiform layer
Orientation of the axons of the ganglion cells
The axons of the ganglion cells bend to become parallel to the inner surface of the retina, where they form the nerve fiber layer and later the axons of the optic nerve.
How many optic nerve fibers does each optic nerve have?
more than 1 million
Orientation of the nerve fibers of the retina
- The nerve fibers from the temporal retina follow an arcuate course around the macula to enter the superior and inferior poles of the optic disc
- The papillomacular fibers travel straight to the optic nerve from the fovea
- The nasal axons also pursue a radial course
How are the erve fibers visible ophthalmoscopically?
green (red-free) illumination
number of rods
120 million
number of cones
6 million
Glial elements of the retina
- fibrous and protoplasmic astrocytes
- microglia
- Müller cells
- they provide structural support and nutrition to the retina and are crucial to normal physiology.
Müller cells - location
- extend vertically from the external limiting membrane inward to the internal limiting membrane.
- Their nuclei are located in the inner nuclear layer
Blood supply of the inner portion of the retina
branches of the central retinal artery
In addition, a cilioretinal artery can branch from the ciliary circulation to supply the macula
Occurance of a cilioretinal artery? (%)
18-32% of eyes
Blood-retina barrier is due to…?
the single layer of nonfenestrated endothelial cells, whose tight junctions are impervious to tracer substances such as fluorescein and horseradish peroxidase
Histology of retinal blood vessels
- single layer of nonfenestrated endothel cells connected ny tight junctions
- basal lamina covers the surfface of the endothelium
- basement membrene contains an interrupted layer of peric)rtes, or mural cells, surrounded by their own basement membrane material
- Retinal blood vessels lack an internal elastic lamina and the continuous layer of smooth muscle cells found in other vessels in the body
- Smooth muscle cells are occasionally present in vessels near the optic nerve head. They become a more discontinuous layer as the retinal arterioles pass farther out to the peripheral retina
To which part of the retinal vessels are Müller cells and other glial elements attached?
basal lamina
How deep do retinal blood vessels extend?
do not ordinarily extend deeper than the middle limiting membrane
Where are venous occlusive disorders common?
at arteriovenous crossings
Where venules and arterioles cross, they share a common basement membrane
What is the external limitig membrane?
- located next to the RPE
- not a true membrane
- formed by the attachment sites of adjacent photoreceptors and Müller cells
- highly fenestrated
What is the outer plexiform layer?
- composed of the interconnections between the photoreceptor synaptic bodies and the horizontal and bipolar cells
- In the macular region, the OPL is thicker and contains more fibers, because the axons of the rods and cones become longer and more oblique as they deviate from the fovea (Henle fiber layer)
- At the edge of the foveola, it lies almost parallel to the internal limiting membrane
What is the inner nuclear layer?
It contains nuclei of bipolar, Müller, horizontal, and amacrine cells
What is the outer nuclear layer?
It contains nuclei of rods and cones
What is the Henle fiber layer?
OPL in the macular region
What is the middle limiting membrane?
formed by a zone of desmosome-like attachments in the region of the synaptic bodies of the photoreceptor cells. The retinal blood vessels ordinarily do not extend beyond this point.
What is the inner plexiform layer?
- consists of axons of the bipolar and amacrine cells
- and the dendrites of the ganglion cells and their synapses
What is the ganglion cell layer?
- it is made up of the cell bodies of the ganglion cells that lie near the inner surface of the retina
What is the nerve fiber layer?
it is formed by axons of the ganglion cells
Normally, these axons do not become myelinated until after they pass through the lamina cribrosa of the optic nerve.
What is the internal limiting membrane?
- not a true membrane
- formed by the footplates of the Müller cells and attachments to the basal lamina
Basal lamina of the retina
smooth on the vitreal side but appears undulating on the retinal side, where it follows the contour of the Müller cells. The thickness of the basal lamina varies
Orientation of the cells in the retina?
Overall, cells and their processes in the retina are oriented perpendicular to the plane of the RPE in the middle and outer layers but parallel to the retinal surface in the inner layers
For this reason, deposits of blood or exudates tend to form round blots in the outer layers (where small capillaries are found) and linear or flame-shaped patterns in the nerve fiber layer
At the fovea, the outer layers also tend to be parallel to the surface (Henle fiber layer). As a result, radial or star-shaped patterns may arise when these extracellular spaces are filled with serum and exudate.
What is the macula histologically?
the region with more than 1 layer of ganglion cell nuclei
size of the fovea
1500 um
size of the foveolar avascular zone
250-600 um
Why is the macula yellow?
due to the presence of carotenoid pigments, which are located chiefly in the Henle fiber layer
Major (caritenoid) pigments of the macula
zeaxantin and lutein
Lutein to zeaxantin ratio
In the central area (0.25 mm from the fovea), the lutein-to-zeaxanthin ratio is 1:2.4, and in the periphery (2.2-8.7 mm from the fovea), the ratio is greater than 2:1.
This variation in pigment ratio corresponds to the rod-to-cone ratio. Lutein is more concentrated in rod-dense areas of the retina; zeaxanthin is more concentrated in cone-dense areas
Where is lipofuscin?
Lipofuscin, the yellow age pigment, has been observed in the cytoplasm of the perifoveal ganglion cells by electron microscopy.
What is the parafovea? Size?
0.5 mm wide, where the GCL, the INL, and the OPL are thickest
What is the perifovea? Size?
- most peripheral region of the macula
- sorrounds the parafovea
- 1.5 mm wide
Cause of the masking of choroidal fluorescence observed in the macula during fundus fluorescein angiography
caused partly by xanthophyll pigment and partly by the higher melanin pigment content of the foveal RPE
where is the foveola?
approximately 4.0 mm temporal and 0.8 mm inferior to the center of the optic disc
Size of the foveola?
It is approximately 0.35 mm across and 0.10 mm thick at its center
Borders of the foveola
The borders of the foveola merge imperceptibly with the fovea
Orientation of the photoreceptor cells in the region of the foveola
The nuclei of the photoreceptor cells in the region of the foveola bow forward toward the ILM to form the fovea externa
Cells in the region of the foveola
- Usually, only photoreceptors, Muller cells, and other glial cells are present in this area
- Occasionally, light microscopy reveals ganglion cell nuclei just below the ILM
Photoreceptor layer of the foveola
composed entirely of cones, whose close packing accounts for the high visual acuity for which this small area is responsible
Characteristics of foveolar cones
- shaped like rods but possess all the cytologic characteristics of extramacular cones.
- The outer segments are oriented parallel to the visual axis and perpendicular to the plane of the RPE
( In contrast, the peripheral photoreceptor cell outer segments are tilted toward the entrance pupil)
Lcation of the foveolar avascular zone
is an important clinical landmark in the treatment of subretinal neovascular membranes by laser photocoagulation. Its location is approximately the same as that of the foveola, and its appearance in fundus fluorescein angiograms varies greatly
What is the ora serrata?
- boundary between the retina and the pars plana
- Topographically, the ora serrata is relatively smooth temporally and serrated nasally
- The ora serrata is in a watershed zone between the anterior and posterior vascular systems, which may in part explain why peripheral retinal degeneration is relatively commoo. The peripheral retina in the region of the ora serrata is markedly attenuated. The photoreceptors are malformed, and the overlying retina frequently appears cystic in paraffin sections
Distance of the ora serrata from the Schwalbe line
between 5.75 mm nasally and 6.50 mm temporally
- In myopia, this distance is greater; in hyperopia, it is shorter
Does the Bruch membrane extend beyond the ora serrata?
The Bruch membrane extends anteriorly, beyond the ora serrata, but is modified because there is no choriocapillaris in the ciliary body
Diamater and circumference of the eye at the ora serrata and the equator
At the ora serrata, the diameter of the eye is 20 mm and the circumference is 63 mm; at the equator, the diameter is 24 mm and the circumference is 75 mm
Do retinall blood vessels reach the ora serrata?
no, they end in loops before the ora serrata
What are Blessig-Iwanoff cysts?
Retina appears cystic in paraffin sections in the region of the ora serrata
why is the vitreus important?
the transparent vitreous humor is important to the metabolism of the intraocular tissues because it provides a route for metabolites used by the lens, ciliary body, and retina
volume of the vitreus
close to 4.0 mL
Percentage of water in the vitreus
99%
Viscosity of the vitreus
Its viscosity is approximately twice that of water, mainly due to the presence of the mucopolysaccharide hyaluronic acid
What type of collagen is in the vitreus?
chiefly type II
Ultrastructure of the vitreus
At the ultrastructural level, fine collagen fibrils (chiefly type II) and cells have been identified in the vitreous
Origin and function of hyalocytes
The origin and function of these cells, termed hyalocytes, are unknown, but they probably represent modified histiocytes, glial cells, or fibroblasts
with which structures do the fibrils at the vitreus base merge?
with the basal lamina of the nonpigmented epithelium of the pars plana and the ILM of the retina
Where does the vitreus adhere to the retina?
peripherally at the vitreous base, which extends from 2.0 mm anterior to the ora serrata to approximately 4.0 mm posterior to it. Additional attachments exist at the disc margin, at the perimacular region, along the retinal vessels, and at the periphery of the posterior lens capsule
What is the Cloquet canal?
During embryonic development, regression of the hyaloid vasculature results in the formation of an S-shaped channel (the Cloquet canal), which passes sinuously from a point slightly nasal to the posterior pole of the lens (Mittendorf dot) to the margin of the optic nerve head
What is the Mittendorf dot?
A point slightly nasal to the posterior pole of the lens from which the CLoquet canal passes
Remnnants of the fetal vasculature on the nerve head
Vascular loops and Bergmeister papilla
Origin of CN I
small olfactory receptors in the mucous membrane of the nose
Pathway of CN I fibers
Unmyelinated CN I fibers pass from these receptors in the nasal cavity through the cribriform plate of the ethmoidal bone and enter the ventral surface of the olfactory bulb, where they form the nerve
Where does the olfactors tract run?
- posteriorly from the bulb beneath the frontal lobe of the brain in a groove (or sulcus) and lateral to the gyrus rectus
The gyrus rectus forms the anterolateral border of the suprasellar cistern. Meningiomas arising from the arachnoid cells in this area can cause important ophthalmic signs and symptoms associated with loss of olfaction.
How many axons does the optic nerve have?
more than 1 million
4 topographic areas od the optic nerve
intraocular region of the optic nerve: optic disc, or nerve head; prelaminar area; and laminar area
intraorbital region (located within the muscle cone)
intracanalicular region (located within the optic canal)
intracranial region (ending in the optic chiasm)
Length of the intraocular part of the optic nerve
1.0 mm
Diameter of the intraocular part of the optic nerve
1.76 x 1.92 mm
Blood supply of the intraocular part of the optic nerve
- retinal arterioles
- branches of posterior ciliary arteries
Length of the intraorbital part of the optic nerve
25-30 mm
it is greater than greater than the distance between the back of the globe and the optic canal (18 mm)
Diameter of the intraorbital part of the optic nerve
3-4 mm
Blood supply of the intraorbital part of the optic nerve
Intraneural branches of central retinal artery CRA; pial branches from CRA and choroid
Length of the intracanalicular part of the optic nerve
4-10 mm
Diameter of the intracanalicular part of the optic nerve
3-4 mm ?
Blood supply of the intracanalicular part of the optic nerve
ophthalmic artery
Length of the intracranial part of the optic nerve
10 mm
Diameter of the intracranial part of the optic nerve
4-7 mm
Blood supply of the intracranial part of the optic nerve
Branches of internal carotid and ophthalmic arteries
Organization of the optic nerve
- similar to that of the white matter of the brain
- Developmentally, the optic nerve is part of the brain
What sheats sorround the optic nerve fibers?
glial cell sheats (not Schwann!)
distance between the back of the globe and the optic canal
18 mm
Course of the optic nerve when the eye is in primary position
sinuous
Axial proptosis secondary to thyroid-related eye disease or a retrobulbar tumor can lead to stretching of the optic nerve, which may cause chronic nerve injury and optic neuropathy.
Size of the optic disc
The size of the optic disc varies widely, averaging 1.76 mm horizontally and 1.92 mm vertically
What is the physiologic cup?
- cup-shaped depression of the optic nerve head
- located slightly temporal to the disc’s geometric center
- represents an axon-free region
the main branches of the central retinal artery (CRA) and the central retinal vein (CRV) pass through the center of the cup
4 parts of the optic nerve HEAD
- superficial nerve fiber layer
- prelaminar area
- laminar area
- retrolaminar area
Superficial nerve fiber layer of the optic nerve head
- nonmyelinated ganglion cell axons retain their retinotopic organization ( fibers from the upper retina above and those from the lower retina below; from the temporal lateral and from the nasal medial)
- macular fibers are laterally placed
- In the nerve head, foveal fibers are located peripherally, and peripapillary fibers are located centrally
Macular fibers constitute approximately ?? of the optic nerve?
1/3
How do the ganglion cell axons enter the optic nerve head?
- in the prelaminar area
- The ganglion cell axons that enter the nerve head are supported by a “wicker basket” of astrocytic glial cells and are segregated into bundles, or fascicles, that pass through the lamina cribrosa
Role of astrocytes in the optic nerve head
- astrocytes invest the optic nerve and form continuous circular tubes that enclose groups of nerve fibers throughout their intraocular and intraorbital course, separating them from connective tissue elements at all sites
- At the nerve head, the Muller cells that make up the internal limiting membrane (ILM) are replaced by astrocytes.
- astrocytes form an ILM that covers the surface of the nerve head and is continuous with the Muller cell-derived ILM of the retina
What % of the nerve head volume do astrocytes constitute?
10%
Cause of a narrow pigmented crescent at the temporal marcin of the disc
the pigment epithelium may be exposed at the temporal margin of the disc
Cause of oartial or absent pigmentation at the temporal margin of the disc
pigment epithelium and choroid fail to reach the temporal margin
Cause of the staining of the disc normally observed in late phases of fluorescein fundus angiography
- The relationship between the choroid and the prelaminar portion of the optic nerve partly accounts for the staining
- The disc vessels do not leak, but the choroidal capillaries are freely permeable to fluorescein, which can therefore diffuse into the lamina
What could be the first objective sign of damage from glaucoma or other forms of optic neuropathy?
When the optic nerve is damaged, axons and supporting glial elements can be lost, causing pathologic enlargement of the optic cup. This cupping may be the first objective sign of damage
How many connective tissue plates does the lamina cribrosa comprise?
~10
Role of the connective tissue plates of the lamina cribrosa?
- they are integrated with the sclera
- their pores transmit the axon bundles
Are the Lamina Cribrosa openings larger at top or bottom
The openings are wider above than below, which may imply less protection from the mechanical effects of pressure in glaucoma
Histology of the lamina cribrosa
type I and III collagens
- abundant elastin
- laminin
- fibronectin
- astrocytes surround the axon bundles
- small blood vessels are present
Functions of the lamina cribrosa
(1) scaffold for the optic nerve axons
(2) point of fixation for the CRA and CRV
(3) reinforcement of the posterior segment of the globe.
where does the diameter of the optic nerve increase and why?
- behind the lamina cribrosa in the retrolaminar area
- it increases to 3 mm
- cause:
- myelination of the nerve fibers
- presence of oligodendroglia
- presence of the surrounding meningeal sheaths (pia, arachnoid, and dura)
What does the axoplasm of the neurons of the retrolaminar area contain?
- neurofilments
- microtubules
- mitochondria
- smooth endoplasmatic reticulum
Where does the retrolaminar nerve run?
The retrolaminar nerve continues proximally (as the intraorbital part of the optic nerve) to the apex of the orbit
which rectus muscles share a connective tissue sheath partially with the optic nerve?
The superior and medial rectus muscles partially share a connective tissue sheath with the optic nerve
This connection may partly explain why patients with retrobulbar neuritis report symptoms of pain on eye movement
Why is the optic nerve susceptible to shearing forces from trauma transmitted to the orbital apex via the bony buttresses of the orbit?
At the optic canal, the dural sheath of the nerve fuses to the periosteum, completely immobilizing the nerve and rendering it susceptible to shearing forces from trauma transmitted to the orbital apex via the bony buttresses of the orbit.
Innermost layer of the optic nerve sheath
pia mater
What is the pia mater?
It is a nscular connective tissue coat, covered with meningothelial cells, that sends numerous septa into the optic nerve, dividing its axons into bundles.
Where are the septaof the pia mater?
- the pia mater sends septa into the optic nerve, dividing axons into buldles
- The septa continue throughout the intraorbital and intracanalicular regions of the nerve and end just before the chiasm
What do the septa of the pia mater contain?
- collagen
- elastic tissue
- fibroblasts
- nerves
- small arterioles and venules
Role of the septa of the pia mater
- They provide mechanical support for the nerve bundles
- and nutrition to the axons and glial cells
Do te pia and septa have a direct connection with nerve axons?
no.
A mantle of astrocytic glial cells prevents the pia and septa from having direct contact with nerve axons.
What is the arachnoid mater composed of?
- collagenous tissue
- small amounts of elastic tissue
- small amounts of meningothial cells
Location of the arachnoid mater
lines the dura mater and is connected to the pia across the subarachnoid space by vascular trabeculae
Where is the subarachnoid space?
- it ends anteriorly at the level of the lamina cribrosa
- Posteriorly, it is usually continuous with the subarachnoid space of the brain
What is the border tissue of Elschnig?
the dense connective tissue that joins the sclera with the Bruch membrane, enclosing the choroid and forming the scleral ring that defines the margin of the optic disc
- At the posterior termination of the choroid on the temporal side, the border tissue of Elschnig lies between the astrocytes surrounding the optic nerve canal and the stroma of the choroid
- On the nasal side, the choroidal stroma is directly adjacent to the astrocytes surrounding the nerve (This collection of astrocytes surrounding the canal is known as the border tissue, which is continuous with a similar glial lining at the termination of the retina)
The nerve fibers of the retina are segregated into ??? fascicles by astrocytes
approximately 1000
How are nerve fibers and their surrounding astrocytes connected at the lamina cribrosa?
On reaching the lamina cribrosa, the nerve fascicles and their surrounding astrocytes are separated from each other by connective tissue
What is the lamina cribrosa?
- an extension of scleral collagen and elastic fibers through the nerve
- The external choroid also sends some connective tissue to the anterior part of the lamina
Where do the nerve fibers of the optic nerva become myelinated?
at the external part of the lamina cribrosa.
columns of oligodendrocytes and a few astrocytes are present within the nerve fascicles
What is the central connective tssue strand?
- The central retinal vessels are surrounded by a perivascular connective tissue throughout its course in the nerve
- this connective tissue blends with the connective tissue of the lamina cribrosa
What can you see on fundus with elevated ICP with respect to central retinal vessels
Because the central retinal vessels cross subarachnoid space, a rise in intracranial pressure can compress the retinal vein and raise the venous pressure within the retina above the intraocular pressure. This situation causes intraocular venous dilatation and the loss of spontaneous venous pulsation at the nerve head. Such an absence of pulsation may clinically indicate raised intracranial pressure
thickness of the dura mater
0.3-0.5 mm
What does the dura mater consist of?
- dense bundles of collagen and elastic tissue that fuse anteriorly with the outer layers of the sclera
Why do patients feel pain in case of retrobulbar neuritis and other inflammatory optic nerve diseases?
The meninges of the optic nerve are supplied by sensory nerve fibers, which account IN PART for the pain experienced by patients with retrobulbar neuritis and other inflammatory optic nerve diseases.
What is the blood supply of the ON in the optic canal
derived from pial vessels originating from the ophthalmic artery.
What is indirect traumatic optic neuropathy?
The optic nen e and surrounding arachnoid are tethered to the periosteum of the bony canal within the intracanalicular region. Blunt trauma, particularly over the eyebrow, can transmit the force of injury to the intracanalicular region, causing shearing and interruption of the blood supply to the nerve in this area.
What can optic nerve edema in the intracanalicular region lead to?
compartment syndrome
Path of the ON intracranially
- After passing through the optic canals, the 2 optic nerves lie above the ophthalmic arteries, above and medial to the internal carotid arteries
- The anterior cerebral arteries cross over the optic nerves and are connected by the anterior communicating artery, which completes the anterior portion of the circle of Willis
- The optic nerves then pass posteriorly over the cavernous sinus to join in the optic chiasm
- The chiasm then divides into right and left optic tracts, which end in their respective lateral geniculate bodies
- From these bodies arise the geniculocalcarine pathways (or optic radiations), which pass to each primary visual cortex
What 2 vessels branch off from the ophthalmic artery once it enters the muscle cone at the annulus of Zinn?
The CRA and, usually, 2 long posterior ciliary arteries branch off from the ophthalmic artery once it enters the muscle cone at the annulus of Zinn
arterial supply of the optic nerve head - retrolaminar nerve
- the retrolaminar nerve is supplied chiefly by pial vessels and short posterior ciliary vessels, with some help from the CRA and recurrent choroidal arteries
arterial supply of the optic nerve head - laminar part
The lamina is supplied by short posterior ciliary arteries or by branches of the arterial circle of Haller and Zinn (circle of Zinn-Haller). This circle arises from the paraoptic branches of the short posterior ciliary arteries and is usually embedded in the sclera around the nerve head. It is often incomplete and may be divided into superior and inferior halves. The CRA does not supply this region
arterial supply of the optic nerve head - prelaminar nerve
The prelaminar nerve is supplied by the short posterior ciliary arteries ( cilioretinal arteries, if present) and recurrent choroidal arteries, although their relative contribution is debated
arterial supply of the optic nerve head - nerve fiber layer
The nerve fiber layer is supplied by the CRA
are posterior ciliary arteries terminal arteries?
The posterior ciliary arteries are terminal arteries, and the area where the respective capillary beds from each artery meet is termed the watershed zone. When perfusion pressure drops, the tissue lying within this area is the most vulnerable to ischemia. Consequences can be significant when the entire optic nerve head or a part of it lies within the watershed zone.
arterial supply of the intraorbital region of the optic nerve
- proximally by the pial vascular network and by neighboring branches of the ophthalmic artery
- Distally, it is also supplied by intraneural branches ofthe CRA
- Most anteriorly, it is supplied by short posterior ciliary arteries and occasional peripapillary choroidal arteries
Arterial supply of the intracanalicular region on the optic nerve
almost exclusively by the ophthalmic artery.
Arterial supply of the intracranial region of the optic nerve
primarily by branches of both the ICA and the ophthalmic artery
Endothelium of the CRA
The lumen of the CRA is surrounded by nonfenestrated endothelial cells with typical zonulae occludentes that are similar to those in retinal vessels
Difference between the CRA and the retinal arterioles in their histology
- The CRA contains a fenestrated internal elastic lamina and an outer layer of smooth muscle cells surrounded by a thin basement membrane
- the retinal arterioles have no internal elastic lamina, and they lose their smooth muscle cells shortly after entering the retina
Histology of the CRV
- endothelial al cells
- thin basal lamina
- thick collagenous adventitia
where is the optic chiasm
The optic chiasm makes up part of the anterior inferior floor of the third ventricle. It is surrounded by pia and arachnoid and is richly vascularized
Dimensions of optic chiasm
12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick
What passes through the Wilbrand knee
- The extramacular fibers from the inferonasal retina cross anteriorly in the chiasm at the “Wilbrand knee” before passing into the optic tract
- Extramacular superonasal fibers cross directly to the opposite tract
- Extramacular temporal fibers remain uncrossed in the chiasm and optic tract
Where are the macular projections in the optic nerve?
centrally
how much of the total volume of the optic nerve and chiasmal fibers do the macular projections constitute?
80-90%
Where do the temporal macular fibers cross the chiasm?
pursue a direct course through the chiasm as a bundle of uncrossed fibers
Where do the nasal macular fibers cross the chiasm?
cross in the posterior part of the chiasm
What % of the optic nerve fibers are crossed and uncrosed?
Approximately 53% of the optic nerve fibers are crossed, and 47% are uncrossed.
Orientation of fibers in the optic tract
- Each optic tract contains ipsilateral temporal and contralateral nasal fibers from the optic nerves
- Fibers (both crossed and uncrossed) from the upper retinal projections travel medially in the optic tract
- lower projections move laterally
- The macular fibers adopt a dorsolateral orientation as they course toward the lateral geniculate body
What is the LGN
- The lateral geniculate body, or nucleus, is the synaptic zone for the higher visual projections
- It is an oval, caplike structure that receives approximately 70% of the optic tract fibers within its 6 alternating layers of gray and white matter
- The 6 layers, numbered consecutively from below upward, give rise to the optic radiations.
What % of the optic tract fibers does the lateral geniculate body recieve?
~ 70%
How many layers does the lateral geniculate body have?
6 alternating layers of gray and white matter
Which layers of the lateral geniculate body contains axons from the contralateral optic nerve?
1,4,6
Which layers of the lateral geniculate body contains axons from the ipsilateral optic nerve?
2,3,5
Rola of the optic radiations
they connect the lateral geniculate body with the cortex of the occipital lobe
Path of the optic radiations
- The fibers of the optic radiations leave the lateral geniculate body and wind around the temporal horn of the lateral ventricle, approaching the anterior tip of the temporal lobe, or loop of Meyer.
- They then sweep backward toward the visual area of the occipital lobe
Damage to the optic radiation in the anterior temporal lobe causes…?
wedge-shaped, upper homonymous “pie in the sky” visual field defect
where is the visual cortex?
occupies the superior and inferior lips of the calcarine fissure on the posterior and medial surfaces of the occipital lobes
How many layers does the visual cortex have?
The visual cortex, the thinnest area of the human cerebral cortex, has 6 cellular layers
Where are the macular fibers in the visual cortex
Macular function is extremely well represented in the visual cortex and occupies the most posterior position at the tip of the occipital lobe
What is the most anterior portion of the calcarine fissure occupied by?
contralateral nasal retinal fibers only
Arterial supply of the visual cortex
The posterior cerebral artery, a branch of the basilar artery, supplies the visual cortex almost exclusively. However, the blood supply to the occipital lobe does show anatomical variation; in some individuals, the middle cerebral artery contributes
How many fibers does CN III contain?
24 000
Function of CN III?
- it supplies all the extraocular muscles except the superior oblique and the lateral rectus
- It also carries cholinergic innervation to the pupillary sphincter and the ciliary muscle
Origin of CN III in brain
arises from a complex group of cells in the rostral midbrain, or mesencephalon, at the level of the superior colliculus. This nuclear complex lies ventral to the periaqueductal gray matter, is immediately rostral to the CN IV nuclear complex, and is bounded inferolaterally by the medial longitudinal fasciculus
What does the CN III nucleus consist of?
- several distinct, large motor cell subnuclei, each of which subserves the extraocular muscle it innervates
- Except for a single central caudal nucleus that serves both levator palpebrae superioris muscles, the cell groups are paired
What’s special about CN III subnuclei for superior rectus?
Fibers from the dorsal nucleus cross, or decussate, in the caudal aspect of the nucleus and therefore supply the contralateral superior rectus muscles
What’s special about CN III subnuclei for levator palpebrae superioris?
One central caudal nucleus supplies both muscles
- Notably, the shared innervation of both levator muscles is responsible for Hering’s law of equal innervation.
Where is the Edinger-Westphal nucleus?
cephalad and dorsomedial in location
Role of the Edinger-Westphal nucleus?
It provides the parasympathetic preganglionic efferent innervation to the ciliary muscle and pupillary sphincter
Which subnuclei of CN III supply the medial rectus uscles?
the most ventral
Is there a subnucleus for ocular convergence?
A subnucleus for ocular convergence has been described but is not consistently found in primates
Pathway of CN III
- The fascicular portion of CN III travels ventrally from the nuclear complex, through the red nucleus, between the medial aspects of the cerebral peduncles, and through the corticospinal fibers
- It exits in the interpeduncular space
- In the subarachnoid space, CN III passes below the posterior cerebral artery and above the superior cerebellar artery, the 2 major branches of the basilar artery
- The nerve travels forward in the interpeduncular cistern lateral to the posterior communicating artery and penetrates the arachnoid between the free and attached borders of the tentorium cereb elli
- pierces the dura on the lateral side of the posterior clinoid process, initially traversing the roof of the cavernous sinus
- It runs along the lateral wall of the cavernous sinus and above CN IV and enters the orbit through the superior orbital fissure.
Common ssite of aneurysms that affect CN III
the junction of the posterior communicating artery and the ICA.
Where does CN III divide?
usually separates into superior and inferior divisions after passing through the annulus of Zinn in the orbit. Alternatively, it may divide within the anterior cavernous sinus
Does CN III have a topographic organization?
The nerve maintains a topographic organization even in the midbrain, so lesions almost anywhere along its course may cause a divisional nerve palsy
What does the superior division of CN III innervate?
the superior rectus and levator palpebrae muscles
What does the inferior division of CN III innervate?
The larger inferior division splits into 3 branches to supply the medial and inferior rectus muscles and the inferior oblique muscle
What’s the course of the parasympathetic fibers of CN III?
- wind around the periphery of the nerve, enter the inferior division, and course through the branch that supplies the inferior oblique muscle
- They join the ciliary ganglion, where they synapse with the postganglionic fibers, which emerge as many short ciliary nerves
Course of short ciliary nerves
pierce the sclera and travel through the choroid to innervate the pupillary sphincter and the ciliary muscle
The superficial location of these fibers makes them more vulnerable to compression, such as from an aneurysm, than to ischemia. However, a pupil-sparing oculomotor nerve palsy, even in the context of systemic vascular disease, is not a perfect indicator of the absence of an enlarging aneurysm, and a growing number of neuro-ophthalmologists recommend emergent imaging (by computed tomography/computed tomography angiography or magnetic resonance imaging/magnetic resonance angiography) for anyone with new-onset CN III palsy with incomplete ptosis
Describe the light reflex
- The afferent pupillary pathway coincides with that of the visual pathway and includes a decussation of nasal fibers in the chiasm
- At the posterior part of the optic tract, the pupillary fibers leave the visual fibers and pass to the lateral side of the midbrain to reach the pretectal nuclei at the level of the superior colliculus
- Here, efferent fibers arise and pass to the Edinger-Westphal nuclei, decussating partially (both ventral to the aqueduct and dorsally, in the posterior commissure)
- Preganglionic parasympathetic fibers leave each Edinger-Westphal nucleus and run in the oculomotor nerve as it leaves the brainstem. The fibers spiral downward to lie medially in the nerve at the level of the petroclinoid ligament and inferiorly in the inferior division of CN III as it enters the orbit
- These fibers synapse in the ciliary ganglion and give rise to postganglionic myelinated short ciliary nerves
What % of the short ciliary nerves are pupillomotor? What about the rest?
~ 3-5 %
The rest are designated for the ciliary muscle and are concerned with the near reflex
What is the near reflex?
The near reflex is a synkinesis that occurs when attention is changed from distance to near. This reflex includes accommodation, pupil constriction, and convergence
Pathway of the near reflex
- initiated in the occipital association cortex, from which impulses descend along corticofugal pathways to relay in pretectal and possibly tegmental areas
- From these relays, fibers pass to the Edinger-Westphal nuclei, the motor nuclei of the medial rectus muscles, and the nuclei of CN VI
- Fibers for the near reflex approach the pretectal nucleus from the ventral aspect; thus, compressive dorsal lesions of the optic tectum spare the near pupil reflex relative to the light reflex (light-near dissociation)
- Efferent fibers for accommodation follow the same general pathway as do those for the light reflex, but their final distribution (via the short ciliary nerves) is to the ciliary muscle.
What is light-near dissociation?
Fibers for the near reflex approach the pretectal nucleus from the ventral aspect; thus, compressive dorsal lesions of the optic tectum spare the near pupil reflex relative to the light reflex (light-near dissociation).
How many fibers does CN IV contain?
~ 3400
the fewest nerve fibers of any cranial nerve
Length of the intracranial course of CN IV
75 mm (longest of the CNs)
Where is the nucleus of CN IV
- in the caudal mesencephalon at the level of the inferior colliculus near the periaqueductal gray matter, ventral to the aqueduct of Sylvius
- It is continuous with the caudal end of the CN III nucleus and differs histologically from that nucleus only in the smaller size of its cells
- Like the CN III nucleus, it is bounded ventrolaterally by the medial longitudinal fasciculus
Which CN is the only cranial nerve that is completely decussated and the only motor nerve to exit dorsally from the nervous system?
CN IV
Course of CN IV
- The fascicles of CN IV curve dorsocaudally around the periaqueductal gray matter and decussate completely in the superior medullary velum
- The nerves exit the brainstem just beneath the inferior colliculus. Thus, CN IV is the only cranial nerve that is completely decussated and the only motor nerve to exit dorsally from the nervous system
- As it curves around the brainstem in the ambient cistern, CN IV runs beneath the free edge of the tentorium, passes between the posterior cerebral and superior cerebellar arteries, and then pierces the dura mater to enter the cavernous sinus
- Cranial nerve IV travels beneath CN III and above the ophthalmic division of CN V in the lateral wall of the cavernous sinus
- It enters the orbit through the superior orbital fissure outside the annulus of Zinn and runs superiorly to innervate the superior oblique muscle
- Because of its location outside the muscle cone, CN IV is usually not affected by injection of retrobulbar anesthetics
Which is the largest cranial nerve?
CN V(trigeminal)
Divisions of CN V
sensory and motor
What does the sensory portion of the CN V innervate?
the greater part of the scalp, forehead, face, eyelids, eyes, lacrimal glands, extraocular muscles, ears, dura mater, and tongue
What does the motor portion of the CN V innervate?
muscles of mastication through branches of the mandibular division
Extent of the CN V nuclear complex
from the midbrain to the upper cervical segments, often as caudal as C4
Nucleai of the CN V nuclear complex
- mesencephalic nucleus
- main sensory nucleus
- spinal nucleus and tract
- motor nucleus located in the pons
Important interconnections exist between the different subdivisions of the CN V sensory nuclei and the reticular formation
What does the CN V mesencephalic nucleus do?
The mesencephalic nucleus mediates proprioception and deep sensation from the masticatory, facial, and extraocular muscles
Where is the CN V mesencephalic nucleus?
The nucleus extends inferiorly into the posterior pons as far as the main sensory nucleus
What does the CN V main sensory nucleus do?
It receives its input from ascending branches of the sensory root, and it serves light touch from the skin and mucous membranes
Where is the CN V main sensory nucleus?
in the pons, lateral to the motor nucleus. It is continuous with the mesencephalic nucleus (above) and with the spinal nucleus (below)
Sensory root of CN V
upon entering the pons, divides into an ascending tract and a descending tract.
- The ascending tract terminates in the main sensory nucleus
- the descending tract ends in the spinal nucleus
What does the CN V spinal nucleus and tract do?
- The nucleus receives pain and temperature afferents from the descending spinal tract, which also carries cutaneous components of CN VII, CN IX, and CN X that serve sensations from the ear and external auditory meatus
where is the CN V spinal nucleus and tract?
extend through the medulla to C4
- The sensory fibers from the ophthalmic division of CN V (V 1) terminate in the most ventral portion of the spinal nucleus and tract
- CN V (V 1) terminate in the most ventral portion of the spinal nucleus and tract
- The fibers from the mandibular division (V3) end in the dorsal parts of the nucleus
Where is the cutaneous territory of each of the CN V divisions represented?
in the spinal nudeus and tract in a rostral-caudal direction
- Fibers from the perioral region are thought to terminate most rostrally in the nucleus; fibers from the peripheral face and scalp end in the caudal portion. The zone between them, the midfacial region, is projected onto the central portion of the nucleus
What does damage to the trigeminal sensory nucleus at the level of the brainstem cause?
bilateral sensory loss in concentric areas of the face, with the sensory area surrounding the mouth in the center
- If a patient verifies this distribution of sensory loss, the lesion is in the brainstem
Conversely, sensory loss that follows the peripheral distribution of the trigeminal sensory divisions (ophthalmic, maxillary, and mandibular) indicates that the lesion lies in CN V after it exits the brainstem
Pathway of the axons from the main sensory, spinal, and portions of the mesencephalic nuclei
- they relay sensory information to higher sensory areas of the brain
- The axons cross the midline in the pons and ascend to the thalamus along the ventral and dorsal trigeminothalamic tracts
- They terminate in the nerve cells of the ventral posteromedial nucleus of the thalamus
- These cells, in turn, send axons through the internal capsule to the postcentral gyrus of the cerebral cortex.
The oculocardiac reflex is mediated by what CN?
The afferent limb of the oculocardiac reflex is mediated by the trigeminal nerve. Although the mechanism underlying the oculocardiac reflex is not known, the size and length of the trigeminal nucleus may suggest a physical etiology for the interaction between the trigeminal nerve and the vagus nerve ( CN X).
Where is the motor nucleus of the trigeminal nerve?
medial to the main sensory nucleus in the pons
From where does the CN V motor nucleus recieve fibers?
- both cerebral hemispheres
- reticular formation
- red nucleus
- tectum
- medial logitudinal fasciculus
- mesencephalic nucleus
A monosynaptic reflex arc is formed by cells from the mesencephalic nucleus and the motor nucleus
Where does the motor nucleus on the CN V send axons?
The motor nucleus sends off axons that form the motor root, which eventually supplies:
- the muscles of mastication (pterygoid, masseter, and temporalis)
- tensor tympani muscle
- tensor veli palatini muscle
- mylohyoid muscle
- anterior belly of the digastric muscle
Intracranial pathway of CN V
- emerges from the upper lateral portion of the ventral pons
- passes over the petrous apex
- forms the trigeminal ganglion, and then divides into 3 branches
What cells does the trigeminal ganglion contain?
The trigeminal ganglion, also called the gasserian or semilunar ganglion, contains the cells of origin of all the CN V sensory axons
What is the gasserian ganglion?
trigeminal ganglion
What is the semilunar ganglion
trigeminal ganglion
Shape of the trigeminal ganglion
crescent
Where is the trigeminal ganglion?
- The crescent -shaped ganglion occupies a recess in the dura mater posterolateral to the cavernous sinus
- This recess, called the Meckel cave, is near the apex of the petrous part of the temporal bone in the middle cranial fossa
- Medially, the trigeminal ganglion is close to the ICA and the posterior cavernous sinus.
What is the Meckel cave?
The trigeminal ganglion occupies a recess in the dura mater posterolateral to the cavernous sinus. This recess, called the Meckel cave, is near the apex of the petrous part of the temporal bone in the middle cranial fossa
Divisions of CN V
The 3 divisions ofCN V are the ophthalmic (V1), the maxillary (V2), and the mandibular (V3).
In which sinus does CN V1 run?
The ophthalmic division enters the cavernous sinus lateral to the ICA and courses beneath CN III and CN IV
Branches of CN V1 in the cavernnous sinus
- Within the sinus, it gives off a tentorial-dural branch
- which supplies sensation to the cerebral vessels, dura mater of the anterior fossa, cavernous sinus, sphenoid wing, petrous apex, Meckel cave, tentorium cerebelli, falx cerebri, and dural venous sinuses
CN V1 passes into orbit Via what and divides into what?
CN V1 passes into the orbit through the superior orbital fissure and divides into 3 branches: frontal, lacrimal, and nasociliary
divisions and role of the frontal nerve
- supraorbital and supratrochlear nerves
- which provide sensation to the medial portion of the upper eyelid and the conjunctiva, forehead, scalp, frontal sinuses, and side of the nose
Where does the supratrochlear and the supraorbital nerve exit the orbit?
According to common teaching, the supratrochlear nerve exits the orbit 17 mm from midline, whereas the supraorbital nerve exits at 27 mm from midline, through either a notch or a true foramen
What dos the lacrimal nerve innervate?
- lacrimal gland and the neighboring conjunctiva and skin
Parasympathetic supply of the lacrimal gland
It was formerly suggested that postganglionic parasympathetic lacrimal secretory fibers, arising in the pterygopalatine ganglion, were carried to the lacrimal gland via a zygomaticotemporal connection with the lacrimal nerve. However, it is now thought more likely that the gland receives its parasympathetic supply directly from the retro-orbital plexus
Innervation of the lateral forhead
Occasionally, the lacrimal nerve exits the orbit via a lacrimal foramen to supply the lateral forehead. Otherwise, that area is supplied by branches of the supraorbital nerve
Branches of the nasociliary nerve
- nasal branches
- infratrochlear branch
- long ciliary nerves.
- short ciliary nerves
- The CN V fibers pass through the ciliary ganglion to join the nasociliary nerve
( The ciliary nerves also contain postganglionic parasympathetic fibers from the ganglion to the pupillary sphincter and the ciliary muscle)
Cause of neutrotropic keratopathy
The trigeminal nerve, like other sensory nerves, interacts with its innervated tissues. In the cornea, sensory innervation is important for corneal homeostasis, and loss of sensation leads to neurotrophic keratopathy.
What do the nasal branches of the nasociliary nerve supply?
middle and inferior turbinates, septum, lateral nasal wall, and tip of the nose
What does the infratrochlear branch of the ciliary nerve supply?
lacrimal drainage system, the conjunctiva, and the skin of the medial canthal region
Function of the long ciliary nerves
carry sensory fibers from the ciliary body, the iris, and the cornea and provide sympathetic innervation to the dilator muscle of the iris
Which nerve carry sensation from the globe?
short ciliary nerves
Cn V2 leaves skull via
The maxillary division leaves the trigeminal ganglion to exit the skull through the foramen rotundum, which lies below the superior orbital fissure
Cn V2 course and innervation after foramen rotundum
- courses through the pterygopalatine fossa into the inferior orbital fissure
- then passes through the infraorbital canal as the infraorbital nerve
- After exiting the infraorbital foramen, CN V2 divides into an inferior palpebral branch supplying the lower eyelid, a nasal branch for the side of the nose, and a superior labial branch for the upper lip.
- The teeth, maxillary sinus, roof of the mouth, and soft palate are also innervated by branches of the maxillary division
Divisions of CN V2
After exiting the infraorbital foramen, CN V2 divides into an inferior palpebral branch supplying the lower eyelid, a nasal branch for the side of the nose, and a superior labial branch for the upper lip
- The teeth, maxillary sinus, roof of the mouth, and soft palate are also innervated by branches of the maxillary division
Lacrimal reflex arc
- The afferent pathway is provided by the first and second divisions of CN V.
- The efferent path proceeds from the lacrimal nucleus (close to the superior salivary nucleus) via CN VII (nervus intermedius), through the geniculate ganglion, the greater superficial petrosal nerve, and the nerve of the pterygoid canal (where it is joined by sympathetic fibers from the deep petrosal nerve).
- The nerve then passes to the pterygopalatine ganglion, where it synapses with postganglionic fibers. These fibers reach the lacrimal gland directly, via the retro-orbital plexus of nerves (particularly CN V1 ). The fibers carry cholinergic and vasoactive intestinal polypeptide (VIP)-ergic fibers to the gland.
Type of fibers of CN V3
sensory and motor
Where does CN V3 exit the skull?
foramen ovale
Function of CN V3
- provides motor input for the masticatory muscles
- Sensation is supplied to the mucosa and skin of the mandible, lower lip, tongue, external ear, and tympanum
Where is the nucleus of CN VI?
in the floor oft he fourth ventricle, beneath the facial colliculus, in the caudal pons. Fibers of CN VII pass over or loop around the CN VI nucleus and exit in the cerebellopontine angle. The medial longitudinal fasciculus lies medial to the CN VI nucleus
Course of the fascicular portion of CN VI
The fascicular portion of the nerve runs ventrally through the paramedian pontine reticular formation and the pyramidal tract and leaves the brainstem in the pontomedullary junction
Course and function of CN VI
Cranial nerve VI takes a vertical course along the ventral face of the pons and is crossed by the anterior inferior cerebellar artery. It continues through the subarachnoid space along the surface of the clivus, surrounded by the Batson venous plexus, to perforate the dura mater below the crest of the petrous portion of the temporal bone, approximately 2 em below the posterior clinoid process. It then passes intradurally through or around the inferior petrosal sinus and beneath the petroclinoid (Gruber) ligament through the Dorello canal, where it enters the cavernous sinus. This long route, especially along the surface of the clivus and beneath the petroclinoid ligament, is responsible for this nerve’s susceptibility to stretch injury leading to paresis in the context of increased intracranial pressure. In the cavernous sinus, CN \Ir uns below and lateral to the carotid artery and may transiently carry sympathetic fibers from the carotid plexus. It passes through the superior orbital fissure within the annulus of Zinn and innervates the lateral rectus muscle on its ocular surface.
Developmental origin of CN VII
second branchial arch
IS CN VII a sensory of a motor nerve?
It is a complex mixed sensory and motor nerve. The motor root contains special visceral efferent fibers that innervate the muscles of facial expression
Sensory root of CN VII
nervus intermedius
- contains special visceral afferent, general somatic afferent, and general visceral efferent fibers
Special visceral afferent fibers of the nervus intermedius
- convey the sense of taste from the anterior twothirds of the tongue
- terminate centrally in the nucleus of the tractus solitarius
General somatic afferent fibers of the nervus intermedius
- convey sensation from the external auditory meatus and the retroauricular skin
- centrally, they enter the spinal nucleus of CN V.
General somatic efferent fibers of the nervus intermedius
- provide preganglionic parasympathetic innervation by way of the sphenopalatine and submandibular ganglia to the lacrimal, submaxillary, and sublingual glands
Where is the motor nucleus of CN VII?
The motor nucleus of CN VII is a cigar-shaped column, 4 mm long, located in the caudal third of the pons. It is ventrolateral to the CN VI nucleus, ventromedial to the spinal nucleus of CN V, and dorsal to the superior olive
Role of the four subgroups of the motor nucleus of CN VII
- Four distinct subgroups within the nucleus innervate specific facial muscles;
- the ventral portion of the intermediate group probably supplies axons to the orbicularis oculi
- The part of the nucleus supplying the upper half of the face receives corticobulbar input from both cerebral hemispheres
- The lower half of the face is influenced by corticobulbar fibers from the opposite cerebral hemisphere
Course of the fibers from the motor nucleus of CN VII
Fibers from the motor nucleus course dorsomedially to approach the floor of the fourth ventricle and then ascend immediately dorsal to the CN VI nucleus. At . the rostral end of the CN VI nucleus, the main facial motor fibers arch over its dorsal surface (forming the internal genu of CN VII) and then pass ventrolaterally between the spinal nucleus of CN V and the CN VII nucleus to exit the brainstem at the pontomedullary junction.
What is the facial colliculus?
The bulge formed by the CN VII genu in the floor of the fourth ventricle is the facial colliculus
Where is the sensory nucleus of CN VII?
The sensory nucleus ofC N VII is the rostral portion of the tractus solitarius, sometimes known as the gustatory nucleus. It lies lateral to the motor and parasympathetic nuclei in the caudal pons
Function of the sensory nucleus of CN VII
Sensations of taste from the anterior two-thirds of the tongue are carried by special visceral afferent fibers to this nucleus. The impulses travel along the lingual nerve and chorda tympani; the cell bodies for these impulses are located in the geniculate ganglion. The impulses eventually reach the brain through the nervus intermedius
With which nerves does CN VII travel together through the lateral pontine cistern?
Cranial nerve VII, the nervus intermedius, and CN VIII (the acoustic nerve) pass together through the lateral pontine cistern in the cerebellopontine angle and enter the internal auditory meatus in a common meningeal sheath. Cranial nerve VII and the intermedius nerve then enter the fallopian canal, the longest bony canal traversed by any cranial nerve (30 mm).
What is the longest bony canal traversed by any cranial nerve?
fallopian canal (30 mm)
Segments of CN VII in its course through the fallopian canal
- After passing anterolaterally for a short distance known as the labyrinthine segment
- the nerves bend sharply at the geniculate ganglion and are then directed dorsolaterally past the tympanic cavity. This 90° bend, known as the tympanic segment, is the external genu of CN VII
- The third segment of the nerve, the mastoid segment, is directed straight down toward the base of the skull.
What is the external genu of CN VII?
the nerves bend sharply at the geniculate ganglion on the fallopian canal and are then directed dorsolaterally past the tympanic cavity. This 90° bend, known as the tympanic segment, is the external genu of CN VII
What branches leave the tympanic segment of CN VII?
Two parasympathetic branches from the superior salivatory and lacrimal nuclei leave the nerve at the tympanic segment: the greater superficial petrosal nerve and a small filament that joins the inferior petrosal nerve.
What nerve leaves the CN VII in the mastoid segment?
stapedius nerve
What nerve joins the CN VII in the mastoid segment?
chorda tympani
Where does the CN VII trunk leave the skull?
at the stylomastoid foramen
What is the pes anserinus?
after CN VII exits the skull at the stylomastoid foramen, it separates into a large temporofacial division and a small cervicofacial division between the superficial and deep lobes of the parotid gland This area of branching is known as the pes anserinus.
Branches of the temporofacial division of CN VII
- temporal (frontal ?)
- zygomatic
- buccal
Branches of the cervicofacial division of the CN VII
The cervicofacial division is the origin of the marginal mandibular and calli branches
anastomoses and branching patterns are numerous
What does the temporal branch of the temporofacial division of the CN VII supply?
- upper half of the -orbicularis oculi
- frontalis, corrugator supercilii, and pyramidalis muscles
What does the zygomatic branch of the temporofacial division of the CN VII supply?
- lower half of the orbicularis oculi
Course of the temporal branch of the temporofacial division of the CN VII
- The temporal (or frontal) branch of the facial nerve crosses the zygomatic arch as one or multiple “twigs” inside the deep layers of the temporoparietal fascia.
- The nerve is fairly superficial as it crosses the zygomatic arch at the junction of the anterior one-third and posterior two-thirds of the arch
- It then enters the more superficial layer of the temporoparietal fascia while staying below the superficial musculoaponeurotic system (SMAS)
- A good approximation of the course of the nerve across the zygomatic arch follows the point at which a line between the tragus and the lateral eyelid commissure is bisected by a line that begins at the earlobe
In what surgical approaches can the temporal branch of the temporofacial division of the CN VII be injured?
The nerve can be injured in the context of perizygomatic or temple surgical approaches, such as Tenzel or Mustarde semicircular flap reconstruction of the eyelid, temporal artery biopsy, and cosmetic forehead and midface surgery.
Parasympathetic outflow of the CN VII
originates in the superior salivatory nucleus and the lacrimal nucleus, both of which lie posterolateral to the motor nucleus and probably receive afferent fibers from the hypothalamus
These preganglionic parasympathetic fibers pass peripherally as part of the nervus intermedius and divide into 2 groups near the external genu of CN VII. The lacrimal group of fibers passes to the pterygopalatine ganglion in the greater superficial petrosal nerve. The salivatory group of fibers projects through the chorda tympani nerve to the submandibular ganglion to innervate the submandibular and sublingual salivary glands.
Where does the superior salivatory nucleus recieve input from?
- probably from the hypothalamus
- olfactory system
What mediates emotional tearing?
The hypothalamic fibers reaching the lacrimal nucleus may mediate emotional tearing, and there is supranuclear input from the cortex and the limbic system
What controls reflex lacrimaion?
- Reflex lacrimation is controlled by afferents from the sensory nuclei of CN V.
Course of the greater superficial petrosal nerve
- extends forward on the anterior surface of the petrous temporal bone to join the deep petrosal nerve (sympathetic fibers) and form the nerve of the pterygoid canal
- This nerve enters the pterygopalatine fossa; joins the pterygopalatine ganglion; and gives rise to unmyelinated postganglionic fibers that innervate the globe, lacrimal gland, glands of the palate, and nose
- Those parasympathetic fibers destined for the orbit enter it via the superior orbital fissure, along with branches of the ophthalmic nerve (CN V 1)
- Here, they are joined by sympathetic fibers from the carotid plexus and form a retro-orbital plexus of nerves, whose rami oculares supply orbital vessels or enter the globe to supply the choroid and anterior segment structures. Some of these fibers enter the globe directly; others enter via connections with the short ciliary nerves. The rami oculares also supply the lacrimal gland
What is the cavernous sinus?
an interconnected series of venous channels located just posterior to the orbital apex and lateral to the sphenoidal air sinus and pituitary fossa
What structures are located within the cavernous sinus?
- the ICA surrounded by the sympathetic carotid plexus
- CN III, CN IV, and CN VI
- the ophthalmic and maxillary divisions of CN V
Venous sinuses of the skull
cavernous, superior sagittal, transverse, straight, sigmoid, petrosal
Where do venous sinuses of the skull drain?
internal jugular veins
Circle of Willis
The major arteries supplying the brain are the right and left I CAs (which distribute blood primarily to the rostral portion of the brain) and the right and left vertebral arteries (which join to form the basilar artery). The basilar artery distributes blood primarily to the brainstem and the posterior portion of the brain. These arteries interconnect at the base of the brain at the circle of Willis
