Neuro 13 - Neurology of the Visual System Flashcards
Describe the visual pathway anatomy
Landmarks:
Eye –> optic nerve –> optic chasm –> optic tract —> lateral geniculate nucleus (ganglion fibres synapse here) —> optic radiation (4th order neuron) —> primary visual cortex (also extra striate cortex)
Where is the primary visual cortex located?
Occipital lobe
Describe the visual pathway in the retina
- First order neurones = rod and cone retinal photoreceptors
- 2nd order neurones = retinal bipolar cells
- 3rd order neurones = retinal ganglion cells (i.e. Optic nerve - which partially decussates at optic chiasm (53% crossing midline), optic tract)
Describe the retinal field of a neuron
Retinal space within which incoming light can alter firing pattern of a neurones
Photoreceptors feed into bipolar cells which feed into ganglion cells (+/- signals). Retinal ganglion cells get input from neighbouring photoreceptors (convergence)
What is convergence
Number of lower order neurones fields synapsing on same higher order neuron
Cones have lower convergence than rods. Central convergence is …. than peripheral convergence
Central convergence < peripheral convergence
Contrast high and low convergence
Low convergence (cones):
- Small receptive field
- Fine visual acuity
- Low light sensitivity
High convergence (rods):
- Large receptive field
- Coarse visual acuity
- High light sensitivity
Describe the 2 types of retinal ganglion cells
On centre:
- Stimulated by light at centre of receptive field
- Inhibited by light on edge of receptive field
Off centre:
- inhibited by light in centre of receptive field
- stimulated by light on edge of receptive field
On/off centre ganglion cells are important for contrast Sensitivity and enhanced edge detection
Describe crossing and uncrossing of fibres in the optic chiasm
Crossing fibres = originate from nasal retina, responsible for temporal visual field
Uncrossing fibres = originate from temporal retina, responsible for nasal visual field
Describe how lesions may affect the visual field
Lesions anterior to optic chiasm = affects visual field in one eye only
Lesions posterior to optic chiasm = affects vision in both eyes
When might one get left anopia (i.e. blindness in left side)
If left optic nerve damaged/lesioned
When might one get bitemporal hemianopia (i.e. outside temporal visual fields not present)
If lesion at optic chiasm
When might one get right homonymous hemianopia (i.e. can’t see in Lt nasal and Rt temporal fields)
Left sided lesion posterior to optic chiasm
Homonymous hemianopia is typically due to?
Stroke
Bitemporal hemianopia is typically due to?
Pit gland tumour - most common
When might right nasal hemianopia occur
If only half of the optic chiasm is affected
Most visual field defects affect the vertical line and not the horizontal line (except in quadrant anopia). What eye condition can affect the horizontal field line?
Glaucoma
Where is the primary visual cortex located
Along the cal calcarine sulcus in the occipital lobe (has distinct stripe from myelinated fibre of optic radiation projecting into visual cortex)
How is the primary visual cortex represented
- Disproportionately large area for macula
- Superior visual field –> projects BELOW calcarine fissure
- Inferior visual field –> projects ABOVE calcarine fissure
- Rt hemifield (stuff seen from R side of both eyes) —> L primary visual cortex
- Lt hemifield (stuff seen from L side of both eyes) —> R primary visual cortex
Describe the arrangement of the primary visual cortex in relation to its function
Arranged in columns —> R eye and L eye dominant columns intersperse each other
Each ganglion goes to a specific nucleus. This is what makes rewiring difficult.
Each column has unique sensitivity to visual stimulus of a particular orientation
How does macular sparing homonymous hemianopia arise
- Damage to primary VC due to stroke
- This leads to contralateral homonymous hemianopia (with macula sparing)
- The macula is spared because the area representing the macula receives dual blood supply from posterior cerebral arteries of both sides
Describe the extra striate cortex pathways
(Extrastriate cortex = area around primary VC in occipital lobe - it converts basic visual info, orientation fn position into complex information)
2 pathways-
- Dorsal pathway - Primary VC to posterior parietal cortex. Needed for motion detection, visually guided action. Damage results in motion blindness
- Ventral pathway - Primary VC to inferotemporal cortex. Needed for object representation, face recognition, detailed fine central vision and colour vision. Damage may lead to cerebral achromatopsia
Pupillary function is to regulate light input into the eye (less than 2 log unit change). How does this differ in light and dark
In light: constriction (circular)
- Decreases spherical aberrations and glare, increases depth of field, reduces bleaching of photo-pigments (less light enters eye)
- Constriction mediated by PNS (CN3)
In dark: dilatation (radial)
- increases light sensitivity by allowing more light into eye
- mediated by SNS
Describe the pathways of the pupillary reflex
Afferent:
- Rod and cone photoreceptors synapse on bipolar cells, which synapse on retinal ganglion cells
- Retinal ganglion cells exit at posterior third of optic tract–> then enter lateral geniculate nucleus
- Synapses at brainstem (in pretectal nucleus)
- Synapses on Edinger-Westphal nuclei on both sides in brainstem
Efferent pathway:
1. Edinger-westphal —> oculomotor nerve —> synapses at ciliary ganglion —-> short posterior ciliary nerve —> pupillary sphincter
