Neurology Of Visual Pathway

Question 1

What are the visual landmarks from eye to visual cortex and summarise the visual pathway

Answer 1

Eye, optic nerve, optic chiasm, optic tract, lateral genital age nucleus, optic radiation, primary visual cortex

1st order neurones (rod and cone photoreceptors), 2nd order neurones (retinal bipolar cells), 3rd order neurones (retinal ganglion cells - optic nerve, partial decussation, optic chiasma to optic tract terminate at LGN).

Question 2

Describe what is the receptive field of a neurone is?

Answer 2

Retinal space within which oncoming light can alter the firing pattern of a neurone. Photoreceptor a small circular space surrounding the photoreceptor. Retinal ganglion cell - input from neighbouring.

Small receptive field (fine visual acuity and low light sensitivity - 1:1:1 centre of fovea). Whereas large receptive field is opposite. The further away the more photoreceptors feed the one ganglion and therefore easy to spot sudden movement

Question 3

Describe the types of retinal ganglion cells

Answer 3

1. On-centre ganglion: stimulated by light at the centre of the receptive field and inhibited by light on the edge of the receptive field
2. Off centre ganglion: inhibited by light at the centre of the receptive field and stimulated by light on the edge of the receptive field.

Question 4

Decline the visual field defects

Answer 4

1. Lesions at optic chiasma damages crossed ganglion fibres from the nasal resting in both eyes. Temporal field defect in both eyes (bitemporal hemianopia).
2. Lesion posterior to optic chiasma: right sided lesion = left homonymous, hemianopia in both eyes and the left sided is right homonymoys

Question 5

Describe the structure and role of the primary visual cortex and extrastriate cortex

Answer 5

Situated along calcarine sulcus within occipital lobe (striate cortex) - characterises by distinc stripe derived from the myelinated fibre of the optic radiation into the visual cortex, abov the calcarine fissure - inferior visual field, below the calcarine fissure - superior visual field.

It is organised as coloumns with unique sensitivity to visual stimulus of a particular orientation, right eye and left dominant coloumns interspace each other = depth perception and gauge distance.

Extrastriate cortex - around the primary cortex within the occipital lobe > converts basic visual information, orientation and position into complex information

Question 6

Explain the dorsal and ventral pathway

Answer 6

Dorsal pathway - primary visual cortex > posterior parietal cortex > motion detection > visually guided action = damage results in motion blindness

Ventral pathway - primary visual cortex > inter temporal cortex> object representation, detailed fine central vision and colour vision > damage may result in cerebral achromatopia

Question 7

Explain the pupillary function and reflex

Answer 7

Pupillary function: regulates light input to the eye. In light pupil constricts and decreases spherical abbreviations and glove, increases depth of field, reduces bleaching of photopigmented and mediated by parasympathetic nerve CNIII and in absence of light is the opposite

Pupillary reflex: afferent pathway and efferent pathway

Question 8

What are the right efferent and afferent defects, unilateral afferent and efferent defects

Answer 8

Right afferent defect - damage to optic nerve, no pupil construction in both eyes when stumulated with light.

Right efferent defect: damage to the right CNIII nerve = no right pupil constriction whether right or left eye. Left pupil contracts not right

Unilateral afferent: difference response pending in which eye is stimulated

Unilateral efferent: some unequal response between left and right eye irrespective which eye is stimulated

Question 9

Summarise the different speed eye movements

Answer 9

1. Saccade - short fast burst (reflexive, scanning, memory guided)
2. Smooth pursuit - sustain slow movements, driven by motion of a moving target across the retina.

Question 10

Which nerves innervate which muscles, and the effects of the defects of these nerves on eye movement

Answer 10

SR3, IR3, MR3, IO3, SO4, LR6

Third nerve palsy: affected eye down and out, droopy eyelid,

Fourth nerve palsy: eyes look to the right?

Sixth nerve palsy: affected eye unable to abduct and deviates inwards (double vision on affected eye).