Anatomy Flashcards
What is the orbit?
The bony cavity which holds the eyeball
Name the 7 bones that make up the orbit
Frontal bone Sphenoid bone Zygomatic bone Maxilla Palatine Ethmoid Lacrimal
What is meant by the orbital plates?
The flat sections of bone found within the orbit which are thin-walled and so susceptible to fractures
Which 3 bones have an orbital plate?
Frontal
Ethmoid
Maxilla
What are the functions of the following bony features of the orbit:
-Optic canal in the sphenoid bone
-Superior orbital fissure in the sphenoid bone
-Supraorbital foramen/notch in the frontal bone
-Infraorbital foramen in the maxilla
?
- Optic canal in the sphenoid bone: entry of the optic nerve and ophthalmic artery
- Superior orbital fissure in the sphenoid bone: entry of CN III, IV and VI
- Supraorbital foramen/notch in the frontal bone: passage of the supraorbital NVB
- Infraorbital foramen in the maxilla: passage of the infraorbital NVB
The orbit is described as a pyramid shape. Where are the apex and base of the ‘pyramid’ found?
Apex: posteriorly at the optic canal
Base: the orbital rim
The 4 margins of the orbit are the lateral wall, medial wall, orbital floor and orbital roof. Which 2 of these are most susceptible to fracture? Why?
Medial wall + orbital floor
They are formed by thin orbital plates
What is a blowout fracture?
When pressure from a blow to thicker orbital bones fractures the medial wall/ orbital floor, which are formed by thinner orbital plates
What is a trap door fracture?
What is a possible consequence?
When a fracture in part of a bone in the orbital floor allows fat and other optic contents to slip through
If a nerve becomes trapped in the trap door, this can lead to parasthesia in the part of the face supplied by that nerve
Name the external sphincter muscle which completely encircles the eyeball
Orbicularis oculi
What are the 2 parts of the orbicularis oculi and what are their functions?
Orbital part: completely surrounds the orbit proper and is responsible for tight squeezing shut of the eyes
Palpebral part: lies directly on the eyelid and is responsible for gentle closing of the eye
What is the nerve supply to orbicularis oculi?
CN VII - the facial nerve (as it is a muscle of facial expression)
Name the 6 parts of the fibrous skeleton found deep to the orbicularis oculi muscle
- Superior tarsus
- Inferior tarsus
- Medial palpebral ligament
- Lateral palpebral ligament
- Tendon of levator palpebrae superioris (LPS)
- Orbital septum
The orbital septum extends from the orbital rim to the tarsi around the eyelids. What is the function of the orbital septum?
To prevent spread of infection between superficial and deep parts of the orbit
What is levator palpebrae superioris (LPS)?
A muscle responsible for elevating the upper eyelid
levator = elevate, palpebrae = eyelid, superioris = upper
Which muscle connects to LPS in the eyelid? Why is it special?
Superior tarsus muscle (Mueller’s muscle)
It is a smooth muscle so receives sympathetic innervation
What is the function of the superior tarsus muscle (Mueller’s muscle)?
Widens the parpebral fissure (i.e., opens the eyes wide) during times of sympathetic stimulation
What gland is found in the eyelid and what is its function?
The tarsal glands
Secrete lipids that line the eyelids to prevent overflow of lacrimal fluid (tears)
The lacrimal gland is found superolaterally to the orbit and its function is to secrete lacrimal fluid. What nerve innervates the lacrimal gland and what is its modality?
CN VII (facial nerve)
Parasympathetic secretomotor innervation
Describe the route of lacrimal fluid from the lacrimal gland to the nasal cavity
- Fluid washes over eye from lacrimal gland towards the medial angle
- Passes into the lacrimal lake at the medial angle
- Passes through the lacrimal puncta and into the 2 canaliculi
- Passes into the lacrimal sac - the first part of the nasolacrimal duct
- Nasolacrimal duct drains into the nasal cavity via the inferior meatus
There are 7 extraocular muscles. Name the 4 rectus muscles
Superior rectus
Inferior rectus
Medial rectus
Lateral rectus
Why are the extraocular rectus muscles called ‘rectus’?
‘Rectus’ means straight and the rectus muscles run from the posterior part of the orbit in a straight line and attach to the eyeball relatively anteriorly
What is the common origin and the common insertion of the rectus muscles?
Origin: common tendinous ring
Insertion: sclera of the eyeball
There are 7 extraocular muscles. Name the 2 oblique muscles
Superior oblique
Inferior oblique
Why are the extraocular oblique muscles called ‘oblique’?
They attach to the more posteriorly at an oblique angle
What are the separate origins and common insertion of the oblique muscles?
SO origin: sphenoid bone
IO origin: orbital plate of maxilla
Insertion: sclera of the eyeball
What is the trochlea?
A loop of connective tissue which the superior oblique runs through before changing direction to attach to the posterior eyeball
What is the last extraocular muscle?
Levator palpebrae superioris (LPS)
What is the origin and the insertion of LPS?
Origin: lesser wing of sphenoid
Insertion: skin and tarsus of superior eyelid
How can we remember the innervation to the extraocular muscles?
LR6 SO4 AO3
LR6: lateral rectus = CN 6 = abducens nerve
SO4: superior oblique = CN 4 = trochlear nerve
AO3: all others = CN 3 = oculomotor nerve
Movements of the eye are described according to movement of the X from its Y position?
Movement of the pupil from its primary position (i.e., gaze straight forward)
Clinical testing of extraocular muscles:
Which muscle abducts the eye only so that the superior rectus and inferior rectus can be tested?
Lateral rectus
Clinical testing of extraocular muscles:
When the eye has been abducted by the lateral rectus, what actions of…
-Superior rectus
-Inferior rectus
… are being tested?
- Superior rectus: elevation
- Inferior rectus: depression
Clinical testing of extraocular muscles:
Which muscle adducts the eye only so that the superior oblique and inferior oblique can be tested?
Medial rectus
Clinical testing of extraocular muscles:
When the eye has been adducted by the medial rectus, what actions of…
-Superior oblique
-Inferior oblique
… are being tested?
- Superior oblique: depression
- Inferior oblique: elevation
What are ‘yoke muscles’?
Primary muscles working together to achieve a common position of gaze
What 2 extraocular muscles work together to purely elevate the eyes from the primary position?
Superior rectus
Inferior oblique
What 2 extraocular muscles work together to purely depress the eyes from the primary position?
Inferior rectus
Superior oblique
Glands and smooth muscles of the head and neck, including the orbit, receive autonomic innervation.
Describe the route of pre-synaptic sympathetic axons to the orbit
- Originate from autonomic centres in the brain
- Pass down the spinal cord and exit at T1 spinal level (thoracolumbar outflow)
- Ascend into the neck via the sympathetic chain
- Synapse in the superior cervical sympathetic ganglion (just behind the mandible)
Describe the route of post-synaptic sympathetic axons to the orbit
- Leave the superior cervical sympathetic ganglion
- Enter the peri-arterial sympathetic plexus on the internal and external carotid arteries
- Follow the internal carotid artery into the head and neck
- Follows the ophthalmic artery (a branch of the internal carotid) to the orbit
Describe the route of pre-synaptic parasympathetic axons to the orbit
- CN’s 3, 7, 9 and 10 leave the CNS to travel to the head and neck (craniosacral outflow)
- The inferior division of CN III travels to supply the orbit
- CN III fibres synapse in the ciliary ganglion in the bony orbit
Describe the route of post-synaptic parasympathetic axons to the orbit
- Leave the ciliary ganglion via the short ciliary nerve
- Supply smooth muscles and glands of the eye
What is the
-Sympathetic
-Parasympathetic
autonomic ganglion involved in innervation to the eye?
- Sympathetic: superior cervical ganglion
- Parasympathetic: ciliary ganglion
The inferior division of CN III (oculomotor) is the only nerve to synapse in the ciliary ganglion. What other nerves pass through it?
- Post-synaptic sympathetic fibres
- General sensory fibres
Describe the route of CN III (oculomotor nerve) to the eye
- Connects with CNS at junction of midbrain and pons
- Passes through the cavernous sinus
- Exits cranial cavity via the superior orbital fissure in the sphenoid bone
- Enters the bony orbit and splits into a superior and an inferior division
- Both divisions give somatic motor supply to the extraocular muscles
- The inferior division gives pre-synaptic parasympathetic axons to the ciliary ganglion
What is the difference between the cornea and the conjunctiva?
Cornea = a clear outer layer overing the iris and pupil
Conjunctiva = a clear membrane which covers the sclera (white part) of the eye and the inner surface of the eyelids
Name the place where the cornea and conjunctiva meet
The limbus (corneoscleral junction)
The blood vessels seen in the sclera are called…
Conjunctival vessels
What is the conjunctival fornix?
The fold at the junction of the conjunctiva covering the sclera and the conjunctiva covering the inner eyelid
What are the 3 layers of the eye?
- Fibrous layer (outer layer)
- Uvea i.e., vascular layer (middle layer)
- Retina (inner layer)
What are the 2 parts of the fibrous outer layer of the eye?
Cornea (involved in refraction)
Sclera (involved in muscle attachment)
What are the 3 parts of the uvea (middle vascular layer) of the eye?
Iris (pupil diameter) Ciliary body (controls iris, lens, and aqueous secretion) Choroid (nutrition + gas exchange)
The eye is divided into 2 segments. What are they and what divides them?
Anterior segment (1/3rd of eye in front of lens) Posterior segment (2/3rds of eye behind lens)
The anterior segment is split up into the anterior and posterior chamber. What are the boundaries of these?
Anterior chamber: between cornea and iris
Posterior chamber: between iris and suspensory ligaments of the lens
What does the posterior segment (2/3rds) of the eye contain?
A jelly-like mesh called the vitreous body/humor which helps maintain the position of the retina and the shape of the globe
It is a common site for ‘floaters’
What does the anterior segment (1/3rd) of the eye contain?
Aqueous humor
Describe the production and circulation of the aqueous humor in the anterior segment of the eye
- Ciliary processes in the ciliary body produce and secrete aqueous
- Aqueous circulates within the posterior chamber and nourish the lens
- It then passes through the pupil into the anterior chamber to nourish the inner surface of the cornea
- Aqueous is reabsorbed into the scleral venous sinus (Canal of Schlemm) at the iridocorneal angle
What is the iridocorneal angle?
The angle between the cornea and the iris in the anterior chamber of the anterior segment
(best seen in a cross section of the eye, not straight on)
The majority of the blood supply to the eye comes from the X artery.
The Y artery gives off the X artery and passes through the cavernous sinus.
X = ophthalmic Y = internal carotid
The majority of the blood supply to the eye comes from the ophthalmic artery.
The internal carotid artery gives off the ophthalmic artery and passes through the cavernous sinus.
Once it has branched from the internal carotid, how does the ophthalmic artery reach the eye?
It follows the optic nerve through the optic canal
Which two branches of the ophthalmic artery supply the eye?
Ciliary arteries (supply choroidal vessels) Central artery of the retina (supplies the retina)
State 2 significant features of the central artery of the retina
- It pierces the optic nerve
- It is an end artery, so is the only source of blood supply to the retina
What is the only vein draining the retina?
The central vein of the retina
What are the 2 main veins that drain the whole eye?
Superior ophthalmic vein
Inferior ophthalmic vein
The ophthalmic veins both drain into the…
Cavernous sinus (via the superior orbital fissure in the sphenoid bone)
What is meant by the danger triangle of the face?
The area of the face where infections can be passed back to the orbit due to connections between the ophthalmic veins and the facial vein
The retina is the inner, photosensitive layer of the eye. Define these 4 features of the retina:
- Fundus
- Optic disc
- Macula
- Fovea
Fundus = the posterior surface of the eye, opposite the lens, where light is focused
Optic disc = area of the retina where retinal ganglion cells exit the eye to form the optic nerve (CN II), also the only point of entry/exit for blood vessels
Macula = a small oval area near the centre of the retina where the greatest density of cones are found (photoreceptors that pick up coloured light)
Fovea = tiny depression at the centre of the macula where density of cones is the highest, so is the area of most acute vision
What is meant by the ‘blind spot’ of the eye?
A small portion of the visual field that corresponds to the position of the optic disc
The optic disc has no photoreceptors so it is insensitive to light, and so light that hits this area will not be converted into images
The retina has many layers but we just need to know 4. What are these?
- Photoreceptors
- Bipolar cells
- Ganglion cells
- Ganglion cell axons
Light coming into the eye must first pass through the ganglion axon, ganglion cell, and bipolar cell layers before reaching the photoreceptor layer. T/F?
True
The photoreceptor layer then sends the signals back to the bipolar cells
The retinal arteries and veins lie anterior/interior/posterior to the retina?
Anterior
During fundoscopy, the retina is divided into 4 quadrants. What are they? What can help us figure out what side we are looking at?
Upper temporal quadrant
Lower temporal quadrant
Upper nasal quadrant
Lower nasal quadrant
The optic disc is found in the nasal quadrants
What are the 2 main vessels seen coming out of the optic disc on fundoscopy?
The central artery of the retina and the central vein of the retina
What would be the result if…
-The central artery/vein of the retina
-A branch of the central artery/vein
… was blocked?
- The central artery/vein of the retina blockage: monocular blindness
- A branch of the central artery/vein blockage: loss of an area of the visual field corresponding to the area of ischaemia
Light from the right visual field is processed in the upper/lower/left/right primary visual cortex?
Light from the lower visual field is processed in the upper/lower/left/right primary visual cortex?
Light from the RIGHT visual field is processed in the LEFT primary visual cortex (and vice versa)
Light from the LOWER visual field is processed in the UPPER primary visual cortex
What ganglion is found in the visual pathway?
The lateral geniculate nucleus (LGN)
CN V1 and CN V2 of the trigeminal nerve provide sensory innervation to which parts of the eye?
CN V1 (ophthalmic nerve): upper eyelid, cornea, conjunctiva CN V2 (maxillary nerve): lower eyelid
Describe the corneal (blink) reflex
Sensory component:
- Highly sensitive cornea is irritated
- Action potentials conducted from cornea via CN V1 (ophthalmic division of trigeminal)
- AP’s travel along CN V1, to the trigeminal ganglion, along CN V, and to the pons
- Pass along CNS interneurons between CN V and CN VII (facial nerve)
Motor component:
- Action potentials conducted via CN VII (facial nerve)
- AP’s reach the palpebral part of orbicularis oculi and trigger closing of the eye in a quick blink
Name 6 autonomic reflexes of the eye
Sympathetic vs parasympathetic stimulation Pupillary light reflex Accommodation reflex Lacrimation reflex Vestibulo-ocular reflex Oculocardiac reflex
Briefly describe the vestibulo-ocular reflex
Turns the eyes in the opposite direction to head movement to stabilise the gaze on an object during head movement
(involves CN III, IV, VI and VIII)
Briefly describe the oculocardiac reflex
Reflex bradycardia to reduce pressure on the eye in response to tension on the extraocular muscles or pressure on the eye
(involves CN V1 and CN X)
Which 2 muscles cause the eyelids to widen the eyes during times of sympathetic stimulation?
Levator palpebrae superioris
Mueller’s muscle/superior tarsus muscle
(both in the upper eyelid)
Describe the difference in innervation between levator palpebrae superioris (LPS) and Mueller’s muscle
LPS = skeletal muscle = CN III (oculomotor)
Mueller’s muscle = smooth muscle = sympathetic innervation
Why does parasympathetic stimulation constrict the pupil during times of bright light and ‘rest and digest’?
- There is no need to take in extra bright light into the eye
- There is no need to look for far away sources of danger so eyes are better suited for near vision
Why does parasympathetic stimulation constrict the pupil during times of bright light and ‘rest and digest’?
- There is no need to take in extra bright light into the eye
- There is no need to look for far away sources of danger so eyes are better suited for near vision
Which muscle constricts the pupils during parasympathetic stimulation?
Where is it found?
Sphincter pupillae fibres
These encircle the pupil around the internal circumference of the iris
Why does sympathetic stimulation dilate the pupil during times of low light and ‘fight or flight’?
- To take in extra light
- To better suit the eyes for far away vision so to assess for approaching danger
Which muscle dilates the pupils during sympathetic stimulation?
Where is its origin and insertion?
Dilator pupillae fibres
- Fixed at their point of origin around the external circumference of the iris
- Mobile at their point of insertion around the internal circumference of the iris
What is meant by a…
-Mydriatic pupil
-Miotic pupil
?
Mydriatic pupil: abnormally dilated pupil
Miotic pupil: abnormally constricted pupil
A fixed pin point pupil can be physiological. T/F?
False
A fixed pin point pupil or a fixed dilated (‘blown’) pupil is usually a sign of serious pathology
e.g., opiate drugs (pin point), CN III pathology (blown)
Describe the pupillary light reflex
When a light is shined in one pupil, both pupils should rapidly constrict
In the pupillary light reflex…
- Constriction of the stimulated eye is known as the X light reflex
- Constriction of the stimulated eye is known as the Y light reflex
?
X - direct light reflex
Y - consensual light reflex
Why does the consensual response occur in the pupillary light reflex?
The motor component of the reflex is bilateral via CN’s III (oculomotor)
Describe the reflex pathway of the pupillary light reflex
Sensory component:
- Light shined in eye stimulates action potentials in the ipsilateral CN II (optic nerve)
- Stimuli is in both visual fields so crosses over in the optic chiasma to reach both the right and left primary visual cortexes of the brain
Motor component:
- Both the right and left Edinger-Westphal nuclei (containing parasympathetic cell bodies) are stimulated
- Action potentials are generated bilaterally in CN III (oculomotor)
- These travel to the sphincter pupillae fibres and constrict both pupils
What does the lens accommodation reflex do?
Flattens the lens to focus on far away objects and causes the lens to become more spherical to focus on closer objects
What 2 important structures control the shape of the lens?
The ciliary muscle in the ciliary body
The suspensory ligaments of the lens which connect the circumference of the lens to the ciliary muscle
Describe the changes that occur in the lens accommodation reflex to focus on a distant object
- Ciliary muscle relaxes
- This tenses the suspensory ligaments
- The lens is pulled and flattens to allow for better focus on distant objects
Describe the changes that occur in the lens accommodation reflex to focus on a close up object
- Ciliary muscle contracts
- This relaxes the suspensory ligaments
- Pull on the lens is slackened and so the lens becomes more spherical and can allow for better focus on close up objects
Which nerve innervates the ciliary muscle and with what modality?
Parasympathetic innervation via CN III (oculomotor nerve)
Lacrimation (production of tears) is another autonomic reflex of the eye. Name the 3 types of tears produced by the lacrimal gland
Basal tears
Reflex tears
Emotional tears
Describe the function of each of the types of tears:
Basal tears
Reflex tears
Emotional tears
Basal tears:
- continuously present to clean, nourish and hydrate the avascular cornea
- contain lysozyme to hydrolyse bacterial cell walls
Reflex tears:
-produced in response to mechanical or chemical irritation to protect the eye
Emotional tears:
-produced in response to strong emotions (but not sure why!)
What is the sensory and the motor nerve involved in reflex tear production?
CN V1 (ophthalmic branch of trigeminal) senses irritation to the cornea/conjunctiva
Parasympathetic axons from CN VII (facial nerve) provide secretomotor supply to the lacrimal gland
What are the meninges?
The three protective membranes that are continuous around the brain and the spinal cord
Name the 3 meninges from outer to inner
Dura mater
Arachnoid mater
Pia mater
Describe the dura mater
“Hard mother”
- It is the tough outer layer of connective tissue
- Made up of two layers - the periosteal and meningeal layers - which enclose the dural venous sinuses
- Innervated by CN V (trigeminal)
What are the dural venous sinuses found in the dura mater?
Venous channels that drain deoxygenated blood from the cranium to the internal jugular vein
Describe the arachnoid mater
“Spidery mother”
- It is the middle, avascular connective tissue layer
- Small extensions reach down to the pia and give it a spider-web appearance
- Outpouchings of arachnoid mater into the dura mater are known as arachnoid granulations
What is the function of arachnoid granulations?
To allow CSF to re-enter circulation via the dural venous sinuses
What is the subarachnoid space? What does it contain?
The space between the arachnoid mater and the pia mater
Contains CSF to cushion the brain and also blood vessels
Describe the pia mater
“Faithful mother”
- It is the innermost, thin, and highly vascularised connective tissue layer
- It tightly adheres to the brain and follows its contours e.g., gyri, sulci
CSF is produced in the brain and completely surrounds the brain and spinal cord in the subarachnoid space.
How is a sample of CSF obtained?
Via lumbar puncture
At what IV disc level should a lumbar puncture be taken to obtain a CSF sample? Why?
L3/4 or L4/5 IV disc level
The spinal cord ends at L2 level so the needle should be inserted below L2 to avoid damage to the spinal cord
At what vertebral level does the subarachnoid space end?
S2
The subarachnoid space is in communication with CSF-filled spaces in the brain called…?
The ventricles
What are the 4 ventricles of the brain?
Right lateral ventricle
Left lateral ventricle
(Midline) 3rd ventricle
(Midline) 4th ventricle
What is the function of the ventricles?
They produce CSF
How do the ventricles produce CSF?
CSF is secreted by the choroid plexus - a secretory tissue formed by modified pia - within each ventricle
Name the structure that connects the…
- Lateral ventricles to the 3rd ventricle
- 3rd ventricle to the 4th ventricle
- Lateral ventricles to the 3rd ventricle: Foramen of Munro
- 3rd ventricle to the 4th ventricle: Cerebral aqueduct
Describe the production and circulation of CSF
- CSF is produced by the choroid plexus in the left and right lateral ventricles
- It flows through the foramen of Munro and into the 3rd ventricle
- It joins CSF produced by the choroid plexus in the 3rd ventricle
- CSF flows through the cerebral aqueduct to the 4th ventricle
- It joins CSF produced by the choroid plexus in the 4th ventricle
- A small amount of CSF enters the central spinal canal to cushion the spinal cord but the majority moves into the subarachnoid space
- CSF circulates in the subarachnoid space and is reabsorbed into the dural venous sinuses via the arachnoid granulations into the dura mater
Name the 2 foramen that allow the fourth ventricle to communicate with the subarachnoid space
Foramen Magendie
Foramen Luschka
