Neurology 13 - Neurology of the Visual System Flashcards
Describe the visual pathway anatomy
- Eye
- Optic nerve - ganglion nerve fibres
- Optic chiasm - half the nerve fibres cross here
- Optic tract - glanglion nerve fibres exit
- Lateral geniculate nucleus (synapse)
- Optic radiation (4th order neuron)
- Primary visual cortex or striate cortex within the occipital libe
- Extrastriate cortex
Describe the visual pathway in the retina
- First order neurons are rod and cone retinal photoreceptors
- Second order neurons are the retinal bipolar cells
- Third order neurons are the retinal ganglion cells (optic nerve, decussation at chiasm, optic tract)
Describe the receptive field
- Retinal space within which incoming light can alter the firing pattern of a neuron
- Photoreceptors are a small circular space surrounding the photoreceptor
- Retinal ganglion cells have input from neighbouring photoreceptors (convergence)
Describe the convergence of receptive field in the eyes
- Number of lower order neurons field synapsing on the same higher order neuron
- Cone system convergence is larger than in rod system
- Central retina convergence is larger than peripheral retina convergence
- Low convergence results in small receptive field, fine visual acuity and low light sensitivity
- High convergence results in large receptive field, with high light sensitivity and coarse visual acuity
What are the on-centre and off-centre ganglion cells?
- On centre are stimulated by light at the centre of the receptive field
- Off centre ganglion are stimulated by lack of light in the centre of the receptive field
- Important for contrast sensitivity and enhanced edge detection
What is the optic chiasma?
- 53% of ganglion fibres cross at the optic chiasma
- Uncrossed fibres are responsible for temporal visual field
- Crossed fibres are responsible for nasal visual field
What do lesions anterior and posterior to the optic chiasma cause?
- Lesions anterior to the optic chiasm affect one eye only
- Lesions at the optic chiasm affect visual field in both eyes
- Posterior cause left homonymous hemianopia in both eyes (right sided) or right homonymous hemianopia in both eyes (left sided)
List the disorders of visual pathway
- Monocular blindness (one eye)
- Bitemporal hemianopia
- Nasal hemianopia
- Homonymous hemianopia
- Quadrantopia
- Macular sparing (central sparing)
List the causes of bitemporal hemianopia
- Enlargement of pituitary gland tumour
- Pituitary sits under optic chiasma
What causes homonymous hemianopia?
Stroke (cerebrovascular accident)
Describe the structue of the primary visual cortex
- Situated alone the calcarine sulcus within the occipital lobe
- Known as striate cortex
- Characterised by a sidtinct shape derived from the myelinated fibre of the optic radiation projecting into the visual cortex
Describe the representation in the primary visual cortex
- Disproportionately large area represents the macula
- Superior visual field projects below the calcarine fissure
- Inferior visual field projects to above the calcarine fissure
- Right hemifield from both eyes projects to left primary visual cortex
- Left hemifield from both eyes projects to the right primary visual cortex
What is the function of the primary visual cortex?
- Organised in columns with unique sensitivity to visual stimulus of a particular ortientation
- Right eye and left dominant columns intersperse each other
List the causes of macular sparing homonymous hemianopia
- Damage to primary visual cortex, often due to stroke
- Area representing the macula receives dual blood supply from the posterior cerebral arteries
What is the extrastriate cortex?
- Area around the primary visual cortex within the occipital lobe
- Converts basic visual information, orientation and oposition into complex information
What is the ventral pathway of the extrastriate cortex?
- Primary visual cortex to inferiotemporal cortex
- Object representation, face regognition, detailed fine central vision and colour vision
- Damage may result in cerebral achromatopsia
What is the dorsal pathway of the extrastriate cortex?
- Primary visual cortex to posterior parietal cortex
- Involved in motion detection, visually guided action
- Damage results in motion blindness
Describe pupillary funtion
- Regulates light input to the eye
- In light, there is pupil constriction to decrease glare and spherical aberrations, increases depth of field, mediated by the oculomotor nerve
- In the dark, there is dilation to increase light sensitivity, mediated by sympathetic nerve
Describe the afferent pathway of the pupillary reflex
- Rode and cone photoreceptors synapse on bipolar cells which synapse on retinal ganglion cells
- Pupil specific ganglion cells exit at posterior third of optic tract before entering the lateral genticulate nucleus
- Synapses at brainstem
- Synapses on the edinger westphal nuscli on either side
Describe the efferent pathway of the pupillary reflex
- Edinger westphal nuclus to ocular motor nerve efferent
- Synapses at the ciliary ganglion
- Short posterior ciliary nerve to pupillary sphincter
What is the direct light reflex?
- Constriction of pupil of stimulated eye
What is the consensual light reflex?
- Constricion of the other eye
- Afferent pathway on either side stimulates the efferent pathway on both sides
What is the result of a right afferent defect?
- Eg. damage to optic nerve
- No constriction in both eyes when right eye stimulates
- Normal constriction when left eye is stimulated with light
What is the result of a right efferent defect?
- Damage to right oculomotor nerve
- No right pupil constriction whether right or left eye is stimulated with light
- Left pupil constricts whether the right or left eye is stimulated