Flashcards in Vestibular pathways Deck (44):
Outline the visual pathway
Optic nerve consists of axons from retinal ganglion cells -> optic chiasm -> optic tract -> from lateral geniculate nucleus, optic radiation projects -> primary visual cortex
What is the extra striate cortex?
Region adjacent to the primary visual cortex that is necessary for higher visual processing
What is a receptive field?
It is the space in the retina within which light falling upon it will alter the firing rate of a given neuron.
What is convergence?
It is the number of lower order neurones that synapse with one higher order neurones
What is the difference in convergence between rod and cone cells?
Cone cells: Low convergence, few photoreceptors synapse on one ganglion cell. Smaller receptive field.
Rod cells: Higher convergence but low near the macula than the peripheral retina
What is the difference and significance of low and high convergence?
Low convergence: small receptive field, fine visual acuity, low light sensitivity
High convergence: large receptive field, course visual acuity, high light sensitivity
What is the difference between on and off centre retinal ganglion cells?
On-centre: Stimulated by light falling at the centre of its receptive field and inhibited by light falling on its edge
Off-centre: Inhibited by light falling on its centre and stimulated by light falling on the edge of its receptive field
-Important in enhanced edge detection
How does a lesion anterior and posterior to the optic chiasm affect vision?
Anterior lesion: Affects one eye
Posteiror lesion: Affects both eyes
Where do crossed and uncrossed fibres arise from?
Crossed: Nasal retina responsible for the temporal half of the visual field
Uncrossed: Temporal retina responsible for nasa half of visual field
What occurs to your vision with a lesion at the optic chiasm?
Affects crossed fibres e.g. those from nasal retina
What occurs to your vision with a lesion posterior to the optic chiasm?
Right sided lesion: left homonymous hemianopia in both eyes
Left sided lesion: right homonymous hemianopia in both eyes
What are the different parts of optic radiation?
Upper division (parietal lobe): inferior visual quadrants
Lower division (temporal lobe): superior visual quadrants. Loops back anteriorly and forms Meyer's loop.
What is Meyer's loop lesion?
Loss of vision in one of the superior quadrants.
Superior homonymous quadrantopia
What happens if there is a lesion to the parietal lobe?
Inferior homonymous quadrantopia
What is a cause of a bitemporal hemianopia?
Pituitary gland tumour as it sits below the optic chiasm
What can cause a homonymous hemianopia?
Stroke or cerebrovascular accident
Where is the primary visual cortex and how do you recognise it?
Situated along the calcarine sulcus in the occipital lobe
Characterised by a distinct white myelinated fibre of the optic radiation
What does the primary visual cortex function as?
Processes visual info for static and moving objects
Large area represents macular central vision
Where do the inferior and superior visual fields project to?
Inferior visual field: projects above calcarine fissure
Superior visual field: projects below calcarine fissure
Where do the right and left hemifields project to?
Right hemifield projects to left primary visual cortex
Left hemifield projects to right primary visual cortex
How is the primary visual cortex organised?
In functional columns, each sensitive to visual stimuli at different orientations
What is meant by macular sparing and when would this present?
Presents in contralateral homonymous hemianopia
Macular is spared as it receives dual blood supply from right and left posterior cerebral arteries
What is the extrastriate cortex?
It is the area around the primary visual cortex in the occipital lobe.
Converts basic visual information, orientation into complex
What is the dorsal pathway?
Primary visual cortex -> parietal lobe
Damage results in motion blindness
What is the ventral pathway?
Primary visual cortex-> inferiotemporal cortex
Object and facial recognition
Detailed and fine visual acuity
Damage results in cerebral achromatopsia
What is the pupillary response in light?
Ciliary muscle contraction
Decreases size of pupillary aperture
Reduces photopigment bleaching
Increases depth of field
Mediated by parasympathetic nerve of CNIII
What is the pupillary response in darkness?
Pupil dilation due to sympathetic nerve (ophthalmic nerve)
Iris radial muscle contracts
Increases light sensitivity
Describe the afferent pathway of the pupillary light reflex
Pupil specific retinal ganglion cells exit the posterior third of the optic tract
Synapse at the pretectal nucleus of the brainstem and relay to the Edinger-westphal nucleus
Constricts the pupil via pupillary sphincter via the ciliary ganglion
What is the consensual light response?
Only one eye needs to be stimulated to elicit the pupillary constriction response
Describe the efferent pathway of the pupillary light reflex
Parasympathetic nerve from Edinger-westphal nucleus
Synapses at ciliary ganglion -> short posterior ciliary nerve -> innervates pupillary sphincter
Describe the response with a right afferent defect
No pupil constriction of either eye when right eye is stimulated
Pupil constriction occurs in both when left is stimulated
Describe the response with a right efferent defect
Pupil constriction on the right does not occur when right eye is stimulated but does on the left with either stimulation.
No constriction of the right eye when either eye is stimulated.
What would you expect to see in a swinging torch test when there is right afferent damage?
If the torch is swung to the right; the pupils will paradoxically dilate due to a reduced parasympathetic drive for pupillary constriction in both eyes.
What is the sympathetic innervation for pupil dilation?
Post synaptic neurons travel down the brainstem and synapse at the superior cervical ganglion.
Third order neurons travel through the carotid plexus and enter the orbit through V1 of CNV
What is the difference between smooth pursuit and saccade?
Smooth pursuit is involuntary (up to 60 degrees per second).
Saccade is voluntary or involuntary (up to 900 degrees/sec)
Where do the extra ocular muscles originate?
The rectus muscles originate in the common tendinous ring and insert into the sclera of the anterior globe.
The inferior oblique comes in nasally (maxillary bone).
What is the difference between the superior oblique and superior rectus?
Superior oblique: maximal depression when eye is in adducted position.
Superior rectus: maximal elevation when eye is in abducted position.
Key difference between rectus and oblique muscles?
Rectus: Abduction, attach to the anterior of the globe and pull backwards
Oblique: Adduction, attach to the posterior of the globe and pull forwards
Why is it that only when the eye is fully abducted can only the superior and inferior rectus muscles elevate or depress it?
This is because at this position, the anterior-posterior axis of the eye is aligned with the insertion of the vertical rectus muscles.
What does the superior and inferior branch of CNIII supply?
Superior branch: superior rectus, lid levator palpebrae
Inferior branch: inferior rectus, medial rectus, inferior oblique, parasympathetic nerve
Describe what is third nerve palsy?
Only extra ocular muscles not supplied by CNIII are working i.e. superior oblique and lateral rectus.
Eye moves down and out.
Describe what is sixth nerve palsy?
Lateral rectus muscle of the affected eye is unable to carry out abduction.
Affected eye is unable to abduct and will deviate inwards.
Will have double vision when asked to look at side of the affected eye.
Common: microvascular disease can cause nerve damage
What is optokinetic nystagmus and how is this reflex important?
Smooth pursuit (tracking) and fast reset saccade
Useful to test using moving grating pattern in pre-verbal children to assess their visual acuity.