Neurology of the Visual System Flashcards
Describe the visual pathway anatomy or neural pathway, including the retinal pathway, and identifying the first, second, third and fourth order neurones along the way
- Rod and Cone Photoreceptors - first order neurones
- Retinal Bipolar Neurones and other intermediate neurones - second order neurones
- Retinal Ganglion Cells - third order neurones
- Retinal Ganglion Cells continue along the optic nerve bilaterally, becoming myelinated
- 50% of the ganglion nerve fibres decussate at the optic chiasm - these are fibres from the nasal retina, while 50% continue ipsilaterally - these are fibres from the temporal retina
- Continues along the optic tracts
- Terminates, synapsing at the lateral geniculate nucleus (LGN)
- Optic radiation - fourth order neurones then begin
- They travel to and terminate at the Primary Visual Cortex (aka Striate Cortex)
- The Primary Visual Cortex (Striate Cortex) can then communicate with the Extra-Striate Cortex for further visual processing
What does the receptive field of a neurone describe?
- Retinal space within which incoming light can alter the firing pattern of a neuron
1) How does the receptive field of neurones vary as you get from lower to higher order neurones?
2) Describe the receptive field of a photoreceptor
3) Describe the receptive field of a bipolar neurone cell
4) Describe the receptive field of a retinal ganglion cell
1)
- The receptive field increases
2)
- Small space surrounding the photoreceptors
3)
- Neighbouring photoreceptors converging on the bipolar neurone cells
4)
- Neighbouring photoreceptors converging on the retinal ganglion cells
- Therefore, also the neighbouring photoreceptors converging on all of these bipolar neurone cells
What does the convergence of a receptive field describe?
Convergence – Number of lower order neurons field synapsing on the same higher order neuron
Describe the properties that low convergence of receptive fields confers
- Fine visual acuity
- Low light sensitivity
Describe the properties that high convergence of receptive fields confers
- Coarse visual acuity
- High light sensitivity
How does the convergence of receptive fields of cones and rods vary, and thus how does it affect their properties they confer to vision?
- Convergence of cones < convergence of rods
- Therefore there is more fine visual acuity and low light sensitivity in cones (photopic vision)
- And coarse visual activity and high light sensitivity in rods (scotopic vision)
1) What are on-centre and off-centre retinal ganglion cells?
2) What do these principles allow?
1)
- On-centre: stimulated by light at the centre of the receptive field. Inhibited by light on the edge of the receptive field
- Off-centre: Inhibited by light at the centre of the receptive field. Stimulated by light on the edge of the receptive field
2)
- Contrast sensitivity
- Enhanced edge detection
Which retinal ganglion fibres cross contralaterally at the optic chiasm and which continue ipsilaterally?
- Nasal retinal ganglion cells cross contralaterally
- Temporal retinal ganglion cells continue ipsilaterally
What visual field defects will occur with lesions at the optic chiasma?
- Damages crossed ganglion fibres from nasal retina in both eyes
- Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
What visual field defects will occur with lesions posterior (note this is in reverse to the direction you think it is - I mean closer to the eye not along the optic tract) to the optic chiasma, mention for each side?
- Right sided lesion – Left Homonymous Hemianopia in Both Eyes
- Left sided lesion – Right Homonymous Hemianopia in Both Eyes
- N.B. Homonymous Hemianopia = visual field loss on the same side of both eyes
Give a condition that causes bitemporal hemianopia and how it causes it
- Pituitary adenoma - enlargement of pituitary
- Compresses at the optic chiasm just above it
- Thus causing bitemporal hemianopia
What conditions typically give rise to homonymous hemianopia (with macular sparing)?
- Stroke or cerebrovascular accidents
- Lesion posterior to the optic chiasm
What neuro-anatomical landmark can help identify the location of the primary visual cortex?
The primary visual cortex lies along the calcarine fissure
What is a distinctive feature upon examination of the primary visual cord, which also gives rise to its alternate name, the ‘striate cortex’?
- Characterized by a distinct stripe derived from the myelinated fibre of the Optic Radiation projecting into the Visual Cortex
Describe the representation of various aspects of the visual field in the primary visual cortex (PVC)
- Disproportionately large section representing the macula
- Superior visual field projects to below the calcarine fissure
- Inferior visual field projects to above the calcarine fissure
- Right hemifield from both eyes projects onto left side of PVC
- Left hemifield from both eyes projects onto right side of PVC
What characteristic visual deficits does stroke cause, and explain how it causes this?
- Contralateral homonymous hemianopia with macular sparing
- Causes injury posterior to the optic chiasm therefore causing homonymous hemianopia, and due to the decussation of the fibres, the visual field deficit will be contralateral
- However, since there is dual blood supply from right and left posterior cerebral arteries perfusing the macula, it is spared of injury and therefore of defect
1) Where is the extrastriate cortex?
2) What is the function of the extrastriate cortex?
1)
- Surrounds the striate cortex but contained within the occipital lobe
2)
- Higher visual processing
1) What is the dorsal pathway from the primary visual cortex (PVC) - what is the neural pathway?
2) What is the function of this pathway?
3) Thus what would damage to this pathway cause?
1)
- PVC → projects to the posterior parietal cortex
2)
- Motion detection
3)
- Motion blindness
1) What is the ventral pathway from the primary visual cortex (PVC) - what is the neural pathway?
2) What is the function of this pathway?
3) What condition could damage to this pathway cause?
1)
- PVC → projects to the inferiotemporal cortex (temporal visual cortex)
2)
- Object representation facial recognition
- Detailed fine central vision
- Colour vision
3)
- Cerebral achromatopsia - inability to perceive colour despite normal functioning cone photoreceptors
Describe the pupillary responses to light and dark environments, how it does this mechanically and neurally (neural stimulation) and why it does this (what adaptational benefits does it confer)?
In light:
- PUPIL CONSTRICTION: iris muscle contracts to constrict the pupillary aperture
- Mediated by parasympathetic nerve within CN III (oculomotor nerve)
- Decreases spherical aberrations and glare
- Increases depth of field
- Reduces light entering the eye
- Reduces bleaching of photo-pigments
In dark:
- PUPIL DILATION: Radial muscle contracts to cause this
- Mediated by sympathetic nerve
- Allows more light into the eye
Outline the neural afferent and efferent pathways from and to the pupil
Afferent
- Follows the optic pathway initially, then breaks away at a point
- So Rod and Cone photoreceptors → Bipolar cells → Retinal Ganglion cells → along Optic Tract → some pupil-specific ganglion cells exit the Optic Tract before reaching the Lateral Geniculate Nucleus (LGN) → instead synapses at the pretectal nucleus in the brainstem → then to the Edingar-Westphal Nucleus bilaterally from each eye
Efferent
- Edinger-Westphal Nucleus → Oculomotor Nerve (CN III) efferent → synapses at Ciliary Ganglion → Short Posterior Ciliary Nerve → Pupillary Sphincter
1) What is the direct light reflex?
2) What is the consensual light reflex?
3) What is the physiological basis for the consensual light reflex?
1)
- Pupillary constriction of the light-stimulated eye
2)
- Pupillary constriction of fellow eye of the light-stimulated eye - i.e. stimulating one eye with light will cause pupillary constriction in both
3)
- Afferent pathway from pupils on either side alone will stimulate efferent pathway on both sides
Keeping in mind the direct and consensual pupillary reflexes, consider what will happen when you stimulate the right and left eyes in right afferent defect e.g. damage to the optic nerve
- No direct pupillary constriction in right eye when shone in right eye
- No consensual pupillary constriction when shone in right eye
- Intact direct pupillary constriction in left eye when shone in left eye
- Intact consensual pupillary constriction in both eyes when shone in left eye
