Visual systems pathways

Question 1

What is the main function of the visual pathway?

Answer 1

transmits signal from eye to the visual cortex

Question 2

What are the visual pathway landmarks?

Answer 2

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

Question 3

What are the first order neurone?

Answer 3

Rod and Cone Retinal Photoreceptors

Question 4

What are the second order neurons?

Answer 4

Retinal Bipolar Cells

Question 5

What are the third order neurons?

Answer 5

Retinal Ganglion Cells

Question 6

What are the features of the optic nerve?

Answer 6

Optic Nerve (CN II)

Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline

Optic Tract

Destinations
Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex

Question 7

What is the optic chiasm?

Answer 7

Important Landmark in Visual Pathway

Question 8

What do lesion of the optic chiasma result in?

Answer 8

Lesions anterior to Optic Chiasma affect visual field in one eye only
Lesions posterior to Optic Chiasma affect visual field in both eyes

Question 9

What are the crossed fibres responsible for?

Answer 9

originating from nasal retina, responsible for temporal visual field

Question 10

What are uncrossed fibres responsible for?

Answer 10

originating from temporal retina, responsible for nasal visual field

Question 11

Give examples of disorders of the visual pathway

Answer 11

Monocular blindness
Bitemporal hemianopia
Right nasal hemianopia
Homonymous hemianopia
Quadrantanopia
Macular sparing

Question 12

What is bitemporal hemianopia caused by?

Answer 12

Typically caused by enlargement of Pituitary Gland Tumour

Pituitary Gland sits under Optic Chiasma

Question 13

What causes homonymous hemianopia?

Answer 13

Stroke (Cerebrovascular Accident)

Question 14

What are the features of homonymous hemianopia with macular sparing?

Answer 14

Damage to Primary Visual Cortex

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

Question 15

What happens to the pupil in light?

Answer 15

Constriction

decreases spherical aberrations and glare
increases depth of field

reduces bleaching of photo-pigments

Question 16

What is pupillary constriction mediated by?

Answer 16

parasymapthetic nerve (within CN III)

Question 17

What happens to the pupil in dark?

Answer 17

Dilatation

increases light sensitivity in the dark by allowing more light into the eye

pupillary dilatation mediated by sympathetic nerve

Question 18

What are the two pathways involved in pupillary reflex?

Answer 18

Afferent

Efferent

Question 19

What are the features of the afferent pathway?

Answer 19

Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells

Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus

Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem

Question 20

What are the features of the efferent pathway?

Answer 20

Edinger-Westphal Nucleus -> Oculomotor Nerve
Efferent ->

Synapses at Ciliary ganglion ->

Short Posterior Ciliary Nerve -> Pupillary Sphincter

Question 21

What is the direct light reflex?

Answer 21

Constriction of Pupil of the light-stimulated eye

Question 22

What is the consensusual light reflex?

Answer 22

Constriction of Pupil of the other (fellow) eye

Question 23

What is the neurological basis of the consensual light reflex?

Answer 23

Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides

Question 24

What is the right afferent defect?

Answer 24

E.g. damage to optic nerve
No pupil constriction in both eyes when right eye is stimulated with light
Normal pupil constriction in both eyes when left eye is stimulated with light

Question 25

What is the right efferent defect?

Answer 25

E.g. Damage to Right 3rd Nerve
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated with light

Question 26

What is the unilateral afferent defect?

Answer 26

Difference response pending on which eye is stimulated

Question 27

What is the unilateral efferent defect?

Answer 27

Same unequal response between left and right eye irrespective which eye is stimulated

Question 28

What is the swinging torch test?

Answer 28

Partial pupillary response still present when the damaged eye is stimulated

Elicited by the swinging torch test – alternating stimulation of right and left eye with light

Both Pupils constrict when light swings to left undamaged side

Both Pupils paradoxically dilate when light swings to the right damaged side