The Neurology of the Visual System Flashcards
Describe the route of the visual pathway
- Eye
- Optic Nerve – Ganglion Nerve Fibres
- Optic Chiasm – Half of the nerve fibres cross here
- Optic Tract – Ganglion nerve fibres exit as optic tract
- Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus
- Optic Radiation – 4th order neuron
- Primary Visual Cortex or Striate
Cortes – within the Occipital Lobe - Extrastriate Cortex
What are the first, second and third order neurons form the retina
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second Order Neurons – Retinal Bipolar Cells
Third Order Neurons – Retinal Ganglion Cells
Describe the retinal ganglion cells
Optic Nerve (CN II) Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline – Optic Tract Travels to. Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
What is the receptive field
Retinal space within which incoming light can alter the firing pattern of a neuron
Photoreceptor – a small circular space surrounding the photoreceptor
Retinal Ganglion Cell
– Inputfrom neighbouring photoreceptors (convergence)
What is convergence
Number of lower order neurons field synapsing on the same higher order neuron
Compare the convergence of cone and rod systems and central vs peripheral retina
Cone System Convergence > Rod System Convergence
Central Retina Convergence > Peripheral Retina Convergence
Describe low convergence
Small receptive field
Fine visual acuity
Low light sensitivity
Describe high convergence
Large receptive field
Coarse visual acuity
High light sensitivity
Describe the on-centre ganglion cells
Stimulated by light at the centre of the receptive field
Inhibited by light on the edge of the receptive field
Describe the off-centre ganglion cells
Inhibited by light at the centre of the receptive field
Stimulated by light on the edge of the receptive field
What is the importance of retinal ganglion cells
Contrast sensitivity
Enhanced edge detection
Describe the optic chiasm
53% of ganglion fibres cross here
Crossed fibres originate from the nasal retina, responsible for the temporal visual field.
Uncrossed fibres originate from the temporal retina, responsible for nasal visual field
Describe the affects of lesions o the optic chiasm
Anterior to chiasm - one eye only
At the chiasm - Causes bitemporal hemianopia
Posterior to chiasm - visual field in both eyes affected
Right side lesion - left homonymous hemianopia in both eyes
Describe bitemporal hemianopia
BitemporalHemianopia
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
Homonymous Hemianopia
Stroke (Cerebrovascular Accident)
Describe the primary visual cortex
Situated along Calcarine Sulcus within Occipital Lobe
Also known as Striate Cortex
Characterized by a distinct stripe derived from the myelinated fibre of the Optic Radiation projecting into the Visual Cortex
What projects to the different parts of the primary visual cortex
Below calcarine fissure - superior visual field
Above calcarine fissure - inferior visual field
Left primary visual cortex - right hemifield
Right primary visual cortex - left hemifield
What are the functions of the primary visual cortex
Organised as columns with unique sensitivity to visual stimulus or a particular orientation
Right eye and left dominant columns intersperse each other
Describe the common cause of primary visual cortex damage and what it leads to
Often due to stroke
Leads to Contralateral Homonymous Hemianopia with Macula Sparing
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides
What is the function of the pupil and what is contraction and dilation mediated by
Regulates light input to the eye
contraction - parasympathetic (CN III)
dilation - sympathetic
What are the effects of pupil constriction
Decreases spherical aberrations and glare
Increases depth of field
Reduces bleaching of photo-pigments
What are the effects of pupil dilations
Increases light sensitivity in the dark by allowing more light in
Explain the pupillary reflex
- Rod and cone photoreceptors synapse on bipolar cells synapsing on retinal ganglion cells
- Pupil-specific ganglion cells exist at the posterior 1/3 of the optic tract before entering the lateral geniculate nucleus
- Synapses at the brain stem (pretectal nucleus)
- Afferent pathway from each eye synapses on the Edinger-Westphal nuclei on both sides in the brainstem
- Path to oculomotor nerve efferent
- Synapse at ciliary ganglion
- Short posterior ciliary nerve -> pupillary sphincter
Compare direct to consensual light reflex and explain why this occurs
Direct - constriction of the pupil that is light-stimulated
Consensual - constriction of the other eye
Afferent pathway on either side will stimulate efferent pathway on both sides
Describe what occurs in a right afferent defect
Normal pupil constriction in both eyes when left eye is stimulated with light
No pupil constriction in both eyes when the right eye is stimulated with light
e.g. damage to optic nerve
