Neurology of the visual system Flashcards
Where does the visual pathway transmit a signal from and to?
From the eye to the optic chiasm
Which cranial nerve is the optic nerve?
2
What happens at the optic chiasm?
Half of the optic nerve fibres cross
What do ganglion nerve fibres exit the optic chiasm as?
The optic tract
Where do ganglion nerve fibres originate?
Retina
Where do ganglion nerve fibres synapse with the next order neurone?
Lateral geniculate nucleus
Where is the lateral geniculate nucleus?
In the thalamus
What is the optic radiation?
4th order neurone that relays signals to the primary visual cortex in the occipital lobe for visual processing
Where does the primary visual cortex (striate cortex) relay information to?
Extra striate cortex
In the retina part of the visual pathway, what are the first order neurones?
Rod and cone retinal photoreceptors
What are second order neurones?
Bipolar cells
What are third order neurones?
Retinal ganglion cells
What is the receptive field of a retinal neurone?
Retinal space within which incoming light can alter the firing pattern of a neurone
Do rods or cones have bigger receptive fields?
Rods- high sensitivity
Do rods or cones synapse less upon the same ganglion cells?
Cones- high visual acuity
What is an on-centre ganglion cell?
It is stimulated by light falling on the centre of its receptive field and inhibited by light falling on the edge of its receptive field
What is an off-centre ganglion cell?
It is inhibited by light on centre of receptive field and stimulated by light on edge
What does this have an important role in?
Contrast sensitivity and enhanced edge detection
What is the difference between lesions anterior or posterior to the optic chiasm?
Lesions anterior to the optic chasm affect one eye
Posterior affects both eyes
Where do the 53% of fibres that cross at the optic chasm originate from?
Nasal retina
What is it called where lesions lead to problems in both eyes?
Bitemporal hemianopia
What are the two divisions of the optic radiation and where do they travel?
Upper division - through the parietal lobe (inferior visual quadrants)
Lower division- through the temporal lobe (superior visual quadrants)
What is the Meyer’s loop formed from?
Lower division loops
What will a Meyer’s loop lesion lead to?
Vision loss in one of the superior quadrants (superior homonymous quadrantopia)
What is the most common cause of bitemporal hemianopia?
Enlargement of the pituitary gland which sits within the sella turcica beneath the optic chiasm
Where is the primary visual cortex situated?
Along the calcarine sulcus within the occipital lobe
What part of vision is PVC above the calcarine sulcus responsible for?
Inferior visual field
What is the main cause of damage to the primary visual cortex?
Strokes
What does the extrastriate cortex do?
Converts basic visual information to position and orientation
How does the pupil regulate light input?
In light, the iris circular muscle contracts and there is constriction of the pupillary aperture which reduces light entering the eye and reduces the amount of photopigment bleaching which increases the depth of field
Where do the retinal ganglion cells involved in the pupillary light reflex leave the optic tract and synapse?
They exit the posterior 1/3 before the lateral geniculate nucleus and synapse upon the dorsal brainstem- pretectal nucleus
What happens at the Edinger-Westphal nucleus?
The afferent pathway from each eye synapses there on both sides of the brainstem
What provides the efferent pathway of the pupillary reflex?
Parasympathetic nerve that originates at the Edinger-Westphal nucleus and synapses at the ciliary ganglion with the short posterior ciliary nerve which then directly innervates the iris pupillary sphincter
Will light in the left eye stimulate the pupillary reflex in the right eye?
Yes- both eyes
What is an example of an afferent deficit?
Damage to the optic nerve
What happens in an afferent deficit?
No constriction in either eye when affected eye is stimulated but normal constriction in both eyes when unaffected eye is stimulated
What happens in an efferent deficit?
There is no constriction in affected eye when either eye is stimulated and the unaffected eye constricts both times
How do you test for a relative afferent pupillary deficit>
Swinging torch test
What is duction?
Movement in one eye
What is version?
Simultaneous movement of both eyes in same direction
What is vergeance?
Simultaneous movement of eyes in opposite directions
What is convergence?
Simultaneous adduction movement of both eyes when viewing a near object
What is a saccade?
Short fast burst
What is smooth pursuit?
Sustained slow movement
What are the six extra-ocular muscles?
Superior rectus Inferior rectus Superior oblique Inferior oblique Lateral rectus Medial rectus
Which muscle does the trochlear nerve innervate?
Superior oblique
Which muscle does the abducens innervate?
Lateral rectus
Which muscles does the oculomotor innervate
Inferior oblique, superior rectus, inferior rectus and medial rectus
Where do all the rectus muscles originate from?
Common tendinous ring behind the eye at the orbital apex
Where do the rectus muscles insert?
Sclera of the anterior globe- they pull the eye backwards
In relation to rectus muscles, which direction do the obliques rotate the eye?
The opposite direction
What effect does the superior rectus have on the eye?
It elevates the eye when in an abducted position
What effect does the superior oblique have on the eye?
It depresses the eye when in an adducted position
In 3rd nerve palsy what are the only two eye muscles that are working?
Lateral rectus and superior oblique
What does this mean happens to the eye in 3rd nerve palsy?
The eye moves down and out
What else do you get in 3rd nerve palsy?
Ptosis (drooping eyelid) because the oculomotor also innervates lid levator. There is also pupil dilation due to loss of parasympathetic innervation from oculomotor
What muscle is no longer innervated in 6th nerve palsy?
Lateral rectus
What does this means happens to the eye in 6th nerve palsy?
A deficit in abduction of the affected eye
What is Hering’s law?
There is equal innate innervation to both muscles from both eyes involved in conjugate or pair movements
What does the medial longitudinal fasciculatus do?
It acts as a link synchronising the 6th nerve nucleus on the right side and third nerve nucleus on left side so when the eyes looks to the left, both eyes move at the same time
In what condition is Hering’s law violated?
Internuclear opthalmoplegia
What is Sherrington’s law of reciprocal innervation?
Agonist muscles contract whilst antagonist muscles relax
In what condition is Sherrington’s law violated?
Duane’s syndrome
What is Duane’s syndrome?
Congenital absence of abducens- both medial and lateral rectus are innervated by oculomotor now so both agonist and antagonist contract at same time
