The Neurology of the Visual System Flashcards
What are some of the landmarks in the vestibular system
Eye Optic nerve (CNII) Optic chiasm Optic tracts Lateral geniculate nucleus Optic radiation Primary visual cortex
Describe the pathway of the visual system
- Optic nerve fibres consist of the axons of retinal ganglion nerve cells
- Half of the fibres cross at the optic chiasm
- The new fibre tracts are the optic tracts as they exit the optic chiasm
- Fibres synapse in the lateral geniculate nucleus of the thalamus
- 4th order neuron as the optic radiation
- Relays signals to the primary visual cortex in the occipital lobe
What is the different between a lesion anterior and posterior to the optic chiasm
Anterior to the optic chiasm - One eye affected
Posterior to optic chiasm - Both eyes affected
What occurs in the eyes I there is a lesion behind the optic chiasm on the right and left sides
Right side - left homonymous hemianopia in both eyes (contralateral)
Left side - right homonymous hemianopia in both eyes (contralateral)
What can bitemporal hemianopia be caused by
Typically caused by the enlargement of a pituitary gland tumour, which sits under the optic chiasma.
What is homonymous hemianopia caused by
Caused by stroke
Describe macular sparing homonymous hemianopia
Damage to primary visual cortex
Often due to stroke
Leads to contralateral homonymous hemianopia with macula sparing
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides = well protected
Where is the primary visual cortex
In the occipital lobe along the calcarine sulcus
What is the function of the primary visual cortex and its portions
Final destination of all visual information.
Superior portion - Upper portion of retina
Inferior portion - Lower portion of retina
Information from the macula takes up a larger segment of the cortex than any other part of the retina
Describe the afferent pathway of the pupillary reflex
- Pupil-specific retinal ganglion cells travel along the optic nerve
- Exits at the posterior 1/3 of the optic tract
- Synapses at the Pretectal nucleus in the brainstem
- Afferents from each eye will synapse on the Edinger-westphal nucleus on both sides of the brainstem
Describe the efferent pathway of the pupillary reflex
- Leaves from the Edinger-westphal nucleus via the oculomotor nerve (CNIII)
- Synapse at the ciliary ganglion
- Short posterior ciliary nerve (from ganglion) will go to the pupillary sphincter (and cause constriction)
What happens in the relative afferent pupillary defect and how is it tested for
Partial pupillary response is still present when the damaged eye is stimulated
Swinging torch test - alternating stimulation of right and left eye with the light.
Both pupils constrict when the light swings to the left undamaged side
Both pupils dilate when the light swings to the right damaged side
Define duction
Eye movement in one eye
Define vergence
Simultaneous movement of both eyes in the opposite direction
Define convergence
Simultaneous adduction movement in both eyes while viewing a near object
Define version
Simultaneous movement of both eyes in the same direction
What movements are brought about by the rectus muscles
moves eye maximally in the ABDUCTED POSITION
Superior rectus: Moves the eye up.
Inferior rectus: Moves the eye down.
Lateral Rectus: Moves the eye toward the outside of the head
Medial Rectus: Moves the eye toward the middle of the head
What movements are brought about by the oblique muscles
Moves the eye maximally in the ADDUCTED POSITION
Superior Oblique:
Passes under the Superior Rectus.
Travels through the trochlea
Depresses and abducts.
Inferior Oblique:
Passes over the Inferior Rectus.
Elevates and Abducts
What muscles are supplied by the superior branch of the third cranial nerve
Superior Rectus – elevates eye
Lid Levator – raises eyelid
Which muscles are supplied by the inferior branch of the third cranial nerve
Inferior Rectus – depresses eye
Medial Rectus – adducts eye
Inferior Oblique – elevates eye
Parasympathetic Nerve – constricts pupil
Where is the eye in a third nerve palsy and why
Outwards (unopposed muscle tone from the lateral rectus, CN VI)
Downwards (unopposed muscle tone from the superior rectus, CN IV)
What occurs in third nerve palsy other than eye movement
There is also ptosis/drooping of eyelid due to lack of innervation to the levator palpebrae superioris
Mydriasis (pupil widening) also occurs
Describe what occurs in sixth nerve palsy
Eye is unable to move outwards and deviates inwards (lateral rectus innervation impaired
Diplopia (double-vision) possible
Describe how you would test for individual movements of the extrinsic eye muscles
Lateral Rectus - Move finger LATERALLY
Medial Rectus - Move finger MEDIALLY
Superior Rectus - Move finger LATERALLY + UP
Inferior Rectus - Move finger LATERALLY + DOWN
Superior Oblique - Move finger MEDIALLY + DOWN
Inferior Oblique - Move finger MEDIALLY + UP
