S8) Functional Anatomy of the Eye Flashcards
Describe the structure of the orbital cavity
The orbital cavity is pyramidal shaped with its apex pointing posteriorly and base anteriorly
Three openings at its apex transmitting nerves and blood vessels into and out of orbit.
Identify them
Identify 3 important anatomical relations of the orbital cavity
- Paranasal air sinuses (maxillary and ethmoid)
- Nasal cavity
- Anterior cranial fossa
Identify two important implications of the anatomical relations of the orbital cavity
- Orbital trauma
- Spread of infection
Identify the different boundaries of the orbit
- Roof (superior wall)
- Floor (inferior wall)
- Medial wall
- Lateral wall
- Apex
- Base
Describe the structures forming the roof of the orbital cavity
- The roof is formed by the frontal bone and the lesser wing of the sphenoid
- The frontal bone separates the orbit from the anterior cranial fossa
Describe the structures forming the floor of the orbital cavity
- The floor is formed by the maxilla, palatine and zygomatic bones
- The maxilla separates the orbit from the underlying maxillary sinus
Describe the structures forming the medial wall of the orbital cavity
- The medial wall is formed by the ethmoid, maxilla, lacrimal and sphenoid bones
- The ethmoid bone separates the orbit from the ethmoid sinus
Describe the structures forming the lateral wall of the orbital cavity
The lateral wall is formed by the zygomatic bone and greater wing of the sphenoid
Describe the structures forming the apex and base of the orbital cavity
- The apex is located at the opening to the optic canal (optic foramen)
- The base (aka orbital rim) opens out into the face, and is bounded by the eyelids
Identify the weakest parts of the orbital cavity and explain why they are most easily fractured
Medial wall and floor as they are thinner and contain air cavities
What is an orbital blowout fracture?
An orbital blowout fracture is a fracture leading to the partial herniation of the orbital contents through the the orbital wall due to blunt force trauma to the eye
How does an orbital blow out fracture present?
- Periorbital swelling (painful)
- Double vision (especially on vertical gaze)
- Impaired vision
- Anaesthesia over affected cheek (infraorbital nerve damage)
How do orbital blow out fractures occur?
Sudden increase in intra-orbital pressure from retropulsion of eye ball e.g. by fist/ball fractures floor of orbit
What is the result of an orbital blow out fracture?
- Orbital contents and blood can prolapse into maxillary and ethmoid sinuses respectively
- The fracture site can ‘trap’ structures e.g. soft tissue such as the extra orbital muscle located near floor or orbit
Describe the structure and function of the eyelids (palpabrae)
- Structure: consists of tarsal plates and muscles and have glands at their edges
- Function: protect the eye when palpebral fissure is closed
Identify 5 contents of the orbital cavity
- Lacrimal apparatus
- Neurovascular structure
- Orbital fat
- Globe of the eye (eyeball) and its internal structures
- Extra-ocular muscles
What do the tarsal plates do?
Tarsal plates provide a connective tissue skeleton to the eyelid for firmness and shape
What is the orbital septum?
Orbital septum is a thin sheet of fibrous tissue originating from orbital rim and blends with the tendon of LPS and tarsal plates
What does the orbital septum do?
The orbital septum separates intra-orbital contents from eyelid fat and orbicularis oculi
What is the clinical signficance of the orbital septum?
The orbital septum acts as a barrier against infection spreading from the pre-septal space to post-septal (orbital cavity proper)
What is periorbital cellulitis?
Periorbital cellulitis is the cellulitis of orbital structures which occurs secondary to infection from bites, periorbital trauma, sinuses (fronto-ethmoidal sinuses)
Identify 2 complications of periorbital cellulitis
- Abscess formation
- Intracranial spread of infection → cavernous sinus thrombosis
Describe the arterial supply of the orbit cavity
Arterial supply via ophthalmic artery and its branches
Describe the venous drainage of the orbital cavity
The veins of orbit drain to cavernous sinus, pterygoid venous plexus and facial veins
Describe the general sensory, special sensory and motor innervation of the orbital cavity
- General sensory: opthalmic nerve (CN Va)
- Special sensory: optic nerve (CN II)
- Motor: oculomotor (CN III), trochlear (CN IV), abducens (CN VI)
Identify the glands of the eye
What is the Meibomian gland and what does it do?
Meibomian glands secrete a lipid-rich substance to lubricate the edge of the eyelids and mix the tear film over the surface of the eye to prevent tears from evaporating to quickly
What is a Meibomian cyst?
A Meibomian cyst (aka chalazion) is a lump in the eyelid caused by the blockage and resultant inflammation of the Meibomian gland
What is a stye?
A stye is a small painful lump in the inside of the eyelid due to the blockage of eyelash follicles
The lacrimal apparatus is also involved in secretion of tears into conjunctival sac.
Identify the specific structures involved
- Lacrimal gland
- Lacrimal caniculi
- Lacrimal sac
- Nasolacrimal duct
What does the lacrimal gland do, where is it found and how is it controlled?
- The lacrimal gland secretes lacrimal fluid (tears)
- It lies in the lacrimal fossa on the superolateral part of the orbit
- It is under parasympathetic control via the facial nerve
Describe the process of secretion from the lacrimal gland
- Lacrimal fluid enters the conjunctival sac through the lacrimal canaliculi and passes into the lacrimal lake at the medial angle of the eye
- The fluid then drains into the lacrimal sac before passing into the nasal cavity via the nasolacrimal duct
What are the small holes on the medial side of the eyelid and what purpose do they serve?
There are two lacrimal puncta in the medial portion of each eyelid which function to collect tears produced by the lacrimal glands
What is the conjunctiva and what does it do?
Conjunctiva is a transparent, secretory mucous membrane which produces mucous and tears to lubricate the conjunctival and corneal surfaces
Describe the location and structure of the conjuctiva
- Location: covers sclera and lines inside of eyelids, forming a conjunctival sac (does not cover over cornea)
- Structure: highly vascular with small blood vessels within the membrane
What is conjunctivitis?
Conjunctivitis (aka pink eye) is an eye condition that causes redness and inflammation of the conjuctiva due to infection or allergy
What is a subconjunctival haemorrhage?
A subconjunctival haemorrhage (aka hyposphagma) is bleeding underneath the conjunctiva due to the rupture of one of the blood vessels in this mucosal layer
What is the physiological effect of blinking?
Blinking washes tear film across front of eye, rinsing and lubricating the conjunctivae and cornea
Which 3 structures maintain the position of the eyeball?
- Suspensory ligament (sits underneath like a sling)
- Rectus muscles
- Orbital fat
The eyeball has three layers.
Identify and describe them
- Outer: tough, fibrous sclera which continues anteriorly as transparent cornea
- Middle: choroid which continues anteriorly as ciliary body and iris (vascular)
- Inner: retina (inner photosensitive layer)
What does the sclera do?
- Provides attachment for the extra-ocular muscles
- Gives shape to the eyeball
- Continous with the dural sheath covering the optic nerve
Describe the structure and function of the ciliary body
- Structure: vascular, muscular, consists of a ciliary process and muscle
- Function: connects the choroid with the iris, produces aqueous humour (ciliary processes)
Describe the general structure of the retina
The retina consists of an inner neurosensory layer and an outer pigmented epithelial cell layer
What does the retina do?
The photosensitive layer of retina generates action potentials in response to light
The pigmented layer lies between the choroid and neurosensory layer of the retina and its cells contain melanin.
What is the role of melanin?
Melanin absorbs scattered light that has passed into the eye, reducing reflection and allowing us to focus images appropriately on to the retina
The neurosensory layer is the area of the retina that senses light and is where the photoreceptors (rods and cones) are found.
What is the role of the rod cells?
The rods are cells found in the peripheral parts of the retina, responsible for vision in low intensity light (night vision) and do not discern colours
The neurosensory layer is the area of the retina that senses light and is where the photoreceptors (rods and cones) are found.
What is the role of the cone cells?
The cones are responsible for high visual acuity and colour vision and many are concentrated in an area of the retina called the macula
What is the macula?
The macula is an area of the retina upon which acts as the centre of your vision and is visible on fundoscopy as a slightly darker area of the retina, lateral to the optic disc
What is the fovea?
The fovea is the area at the very centre of the macula where the only photoreceptors are cones
What is the palpebral fissure?
The palpebral fissure is the opening between the eye lids
Label the structures 1-3 below in the eye:
Label the structures 4-6 below in the eye:
Label the structures 7-11 below in the eye:
The eyeball has three chambers. Identify them
Describe the contents of the different chambers of the eyeball
- Vitreous chamber is filled with a transparent, jelly-like vitreous humour
- Anterior and posterior chambers are filled with transparent aqueous humour
What is aqueous humour and what does it do?
Aqueous humour is a substance secreted by the ciliary processes within ciliary body and nourishes the lens and cornea
Describe the flow and drainage of aqueous humour
- Flows from posterior chamber, through pupil into anterior chamber
- Drains through iridocorneal angle (between iris and cornea) via trabecular meshwork into canal of Schlemm
What is the canal of Schlemm?
The canal of Schlemm is a circumferential venous channel draining into venous circulation
What is glaucoma?
- Glaucoma is an eye condition due to the obstruction of the drainage of aqueous humour, leading to a rise in intra-ocular pressure
- If left untreated glaucoma leads to irreversible damage and death of the optic nerve, causing impairment of vision or even blindness
Drainage of aqueous humour from anterior chamber can be blocked.
Explain 2 possible ways this can occur
- Trabecular meshwork deteriorates → open angle glaucoma (chronic – most common)
- Narrowing of irido corneal angle → closed angle glaucoma (acute – less common)
What is the iris?
The iris is a thin contractile diaphragm, located anteriorly to the lens, with a central aperture (the pupil) for transmission of light
The iris gives the colour to the eye.
Apart from this, what else does it do?
Two muscles (sphincter and dilator pupillae) form the iris and control the size of the pupil, both of which are under the control of the ANS
What is the lens?
The lens is a transparent biconvex structure enclosed in a capsule, lying posterior to the iris and attached to the ciliary body by the suspensory ligaments
Describe the neurovascular supply to the lens
The lens is without nerve innervation or blood supply, receiving its nutrients entirely from the aqueous humour that surrounds and bathes it
In four steps, describe the mechanism in which the retina responds to light
⇒ Light (photons) reaches the photosensitive retina
⇒ Photoreceptors (rods/cones) detect light
⇒ Action potentials are generated in response to light
⇒ Action potentials pass via ganglion cells whose axons collect in area of optic disc
What is refraction?
Refraction is the change in direction of light on passing through boundary of two different mediums
Light will be refracted as it passes through a number of structures and ‘fluids’ from outside eye towards retina.
Identify these structures
- From air into liquid tear film → refract
- Through cornea → refract
- Through lens and vitreous humour before reaches retina
Where does most refraction of light occur?
The most refraction of light occurs at the air-cornea interface
Focusing near objects requires a greater refraction of light.
Why is this?
Light rays from near-objects are more divergent and require greater refraction to bring them into focus on retina
Outline the accommodation reflex
- Pupil constricts
- Eyes converge & image is brought to focus on same point of retina
- Lens becomes more biconcave (fatter)
What are cataracts?
Cataracts are progressive opacities (clouding) occurring within lens leading to decreased vision
What is presbyopia?
Presbyopia is the age-related inability to focus near-object as the lens becomes stiffer and less able to change shape
What is papilloedema?
Papilloedema is a condition in which increased pressure in or around the brain causes the part of the optic nerve inside the eye to swell
What is blepharitis?
Blepharitis is a common eye condition wherein the eyelids become red and inflamed often due to bacterial infection
There are six extra-ocular muscles that attach to the globe of the eye, whose contraction causes movement and therefore control our direction of gaze.
Identify them
- Lateral Rectus
- Medial Rectus
- Superior Rectus
- Inferior Rectus
- Inferior oblique
- Superior oblique
State the origin and attachment of the superior rectus muscle
- Origin: common tendinous ring
- Attachment: superior and anterior aspect of the sclera
State the function and innervation of the superior rectus muscle
- Function: elevation (slightly adducts and intorts)
- Innervation: oculomotor nerve (CN III)
State the origin and attachment of the inferior rectus muscle
- Origin: inferior part of the common tendinous ring
- Attachment: inferior and anterior aspect of the sclera
State the function and innervation of the inferior rectus muscle
- Function: depression (slight adduct and externally rotate)
- Innervation: oculomotor nerve (CN III)
State the origin and attachment of the medial rectus muscle
- Origin: medial part of the common tendinous ring
- Attachment: antero-medial aspect of the sclera
State the function and innervation of the medial rectus muscle
- Function: adduction
- Innervation: oculomotor nerve (CN III)
State the origin and attachment of the lateral rectus muscle
- Origin: lateral part of the common tendinous ring
- Attachment: anterio-lateral aspect of the sclera
State the function and innervation of the lateral rectus muscle
- Function: abduction
- Innervation: abducens nerve (CN VI)
State the origin and attachment of the superior oblique muscle
- Origin: body of the sphenoid bone
- Attachment: sclera of the eye, posterior to the superior rectus
its functional pull is from the trochlear
State the function and innervation of the superior oblique muscle
- Function: depression, abduction and medial rotation
- Innervation: trochlear nerve (CN IV)
most powerful when the eye is already positioned medially
State the origin and attachment of the inferior oblique muscle
- Origin: anterior aspect of the orbital floor
- Attachment: sclera of the eye, posterior to the lateral rectus
State the function and innervation of the inferior oblique muscle
- Function: elevation, abduction and lateral rotation
- Innervation: oculomotor nerve (CN III)
How does one test the function of the different extra-ocular muscles clinically?
Label the following structures on this image of the retina:
- Artery
- Vein
- Optic disc
- Macula
- Fovea
What are the different steps involved in examining the eye?
- I = Inspection
- VA = Visual acuity
- F = Visual fields
- R = Reflexes
- O = Opthalmoscope
- M = Movements
why do we need two eyes?
- binocular vision
- wider field of vision and depth perception
- both eyes just be aligned so light hits same spot on both eyes
what is conjugate eye movement
coordination of both eye movement
what is diplopia
double vision due to misalignment of two eyes
LR6 SO4
- Lateral rectus innervated by abducens
- Superior oblique innervated by trochlear
- rest are innervated by occulormotor
what are the two axis involved in the eye
- axis of the orbit
- visual axis (where extra ocular muscles attach at an oblique angle)
- they confer several actions of the eye
which extra ocular muscles are stronger elevators and depressors when the eye is in the adducted position
- strongest elevator: inferior oblique
- strongest depressor: superior oblique
which extra ocular muscles are stronger elevators and depressors of the eyeball when the eye is in the abducted position
strongest elevator: superior rectus
strongest depressor: inferior rectus
why might the abnormality of one muscle make a big different in gaze
- muscles all contribute to an equal but opposite pull
- if one muscle is weakened the other one is no longer antagonised
- resting position of eyeball may deviate
how to test a persons rectus muscles
- move eye laterally (lateral rectus)
- look up (superior rectus)
- look down (inferior rectus)
how to test person oblique
- move eye medially (medial rectus)
- Look up (inferior oblique)
- look down (superior oblique)
cranial nerve 3 palsies
Cranial nerve 4 palsies
cranial nerve 6 palsises
- horizontal diplopia worse on the side that is effected
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