Neurology of the visual pathway Flashcards
Visual pathway Landmarks
- 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
Neurones in the visual pathway
First Order Neurones – Rod and Cone Retinal Photoreceptors
Second Order Neurones – Retinal Bipolar Cells
Third Order Neurones – Retinal Ganglion Cells
Fourth Order Neurones- Optic Nerve
Optic Chiasm
53% of ganglion fibres cross over
Receptive Field
Retinal space within which incoming light can alter the firing pattern of a neurone
Receptive field of photoreceptors vs ganglion cells
The Receptive Field of Ganglion Cells covers a much larger area than that of a single photoreceptor.
It includes the Receptive Fields of all the photoreceptors, that synapse upon the Ganglion Cells indirectly via Bipolar Cells.
Convergence of Receptive fields
Convergence – Number of lower order neurons field synapsing on the same higher order neuron
Cone System Convergence > Rod System Convergence Central Retina Convergence > Peripheral Retina Convergence
Low convergence results in
- Small Receptive Field
- Fine Visual Acuity
- Low Light Sensitivity
High convergence results in
- Large Receptive Field
- Coarse Visual Acuity
- High Light Sensitivity
On-centre and Off-centre ganglion cells
On-centre Ganglion: Stimulated by light at the centre of the receptive field. Inhibited by light on the edge of the receptive field
Off-centre Ganglion: Inhibited by light at the centre of the receptive field. Stimulated by light on the edge of the receptive field.
Important for: Contrast Sensitivity and Enhanced Edge Detection
Lesions anterior and posterior to Optic Chiasma
Lesions anterior to Optic Chiasm affect visual field in one eye only
Lesions posterior to Optic Chiasm affect visual field in both eyes
What visual defect are caused by each lesion
Lesion in one optic nerve anterior to the Optic Chiasma- causes unilateral field loss
Lesion at Optic Chiasma- Bitemporal hemianopia
Lesion posterior to Optic Chiasma- Homonymous hemianopia (right lesion causes left homonymous hemianopia and visa versa)
Cause of bitemporal hemianopia
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
Cause of Homonymous hemianopia
Stroke
Primary Visual Cortex location
Situated along Calcarine Sulcus within Occipital Lobe
Representation of primary visual cortex(PVC)
Disproportionately large area representing the macula
Superior Visual Field projects to below the Calcarine Fissure
Inferior Visual Field projects to above the Calcarine Fissure
The right Hemifield from both eyes projects onto Left PVC
The left Hemifield from both eyes projects onto Right PVC
Organisation of primary visual cortex
Organized as columns with unique sensitivity to visual stimulus of a particular orientation
Right eye and left dominant columns intersperse each other
Extrastriate Cortex
Area around PVC within the Occipital Lobe
Converts basic visual information, orientation and position into complex information
Pathways from PVC- their function and what results if damaged
Dorsal Pathway
- PVC to Posterior Parietal Cortex
- Motion Detection
- Visually-Guided Action
- Damage results in Motion Blindnes
Ventral Pathway
- Primary Visual Cortex to Inferiotemporal Cortex
- Object Representation, Face Recognition
- Detailed fine central vision and colour vision
- Damage may result in Cerebral Achromatopsia
Pupil constriction- what muscle causes it and whats the purpose
Iris muscle contracts to constrict the pupillary aperture
- Decreases spherical aberrations and glare
- Increases depth of field and reduces amount of light entering the eye
- Reduces bleaching of photo-pigments
- Pupillary constriction mediated by parasymapthetic nerve (within CN III)
Pupil dilation- what muscle causes it and whats the purpose
Increases light sensitivity in the dark (allows more light into the eye) – radial muscle contracts
Pupillary dilatation mediated by sympathetic nerve
Afferent pathway of Pupillary reflex
Pupil-specific ganglion cells exits at posterior third of optic tract before entering the LGN (lateral geniculate nucleus)
Synpases at Brain Stem (Pretectal Nucleus)
Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Efferent pathway of Pupillary reflex
Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent
Synapses at Ciliary ganglion
Short Posterior Ciliary Nerve -> Pupillary Sphincter
Direct vs consensual pupil reflex
Direct Light Reflex – Constriction of Pupil of the light-stimulated eye
Consensual Light Reflex – Constriction of Pupil of the fellow (other) eye
Neurological basis of both pupils constructing even if only one is stimulated
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
