vision and the visual pathways

Question 1

what are the 4 regions of the retina?

Answer 1

retina- innermost layer of the eyeball, has a laminar structure (3 main layers of cells)
macula- lack of blood vessels
fovea- thinnest region and contains only cones
optic disk- blood vessels originate and optic nerve fibres exit

Question 2

what are the 4 regions of photoreceptors?

Answer 2

synaptic terminal
inner segment
cell body
outer segment (with photosensitive disks)

Question 3

explanation of rod cells

Answer 3

20 times more rods than cones
monochromatic
able to detect a single photon
function only in dim light
found mainly in peirphery of the retina

Question 4

explanation of cone cells

Answer 4

maximally sensitie to red, green or blue light
found primarily in fovea
crucial for visual detail (high visual acuity) and colour vision
directly exposed to light

Question 5

what it the principle visual pathway?

Answer 5

photoreceptors
bipolar cells of retina
ganglion cells of retina
lateral geniculate nucleus
primary visual cortex

Question 6

importance of partial decussation at the optic chiasm

Answer 6

the retina can be divided into two halves- the nasal
portion, and the temporal portion
at the optic chiasm axons originating from the medial (nasal) retina decussate and continue as the
contralateral optic tract
axons from the lateral (temporal) retina remain ipsilateral - do not cross at the chiasm
the result is that the left hemisphere processes information from the right visual field, and the right
hemisphere from the left visual
this gives us binocular vision

Question 7

where is the optic chiasm?

Answer 7

rostral to the tuber cinereum of the hypothalamus and between the terminating internal carotid arteries

Question 8

activity of rods and cones in the dark

Answer 8

both steadily release glutamate (depolarisation)

Question 9

activity of rods and cones in the light

Answer 9

they are hyperpolarised which results in the inhibition of the release of glutamate from photoreceptors
this affects bipolar cells

Question 10

what are the first-order neurons in the central visual pathway?

Answer 10

bipolar cells in the retina

Question 11

what are the second-order neurons in the central visual pathway?

Answer 11

ganglion cells that form the optic nerve

Question 12

where is glutamate released from photoreceptors?

Answer 12

the synaptic terminal

Question 13

glutamate hyperpolarises what bipolar cells?

Answer 13

on bipolar cells

Question 14

glutamate depolarises what bipolar cells?

Answer 14

off bipolar cells

Question 15

what happens to an on bipolar cell in light?

Answer 15

depolarise

Question 16

what happens to an off bipolar cell in light?

Answer 16

hyperpolarise

Question 17

what are the receptors of an on bipolar cell?

Answer 17

metabotropic receptors

Question 18

what are the receptors of an off bipolar cell?

Answer 18

ionotropic receptors

Question 19

what do on bipolar cells synapse with?

Answer 19

on ganglion cells

Question 20

what are the only cells that fire action potentials?

Answer 20

ganglion cells

Question 21

what ganglion cells increase firing rate in the dark?

Answer 21

off

Question 22

what ganglion cells increase firing rate in the light?

Answer 22

on

Question 23

how are photoreceptors in rhe retina organised?

Answer 23

antagonistic concentric-centre surround receptive fields

Question 24

what are horizontal cells?

Answer 24

inhibitory interneurons

Question 25

what happens if light hits the centre of the receptive field?

Answer 25

activates the center cone which transmits information to the bipolar cell and then the ganglion

Question 26

what happens if light hits the surround of the receptive field?

Answer 26

activates the surround cone which activates the horizontal cell which switches off activaton in the bipolar cell

Question 27

why is the organisation of the receptive fields important?

Answer 27

detection of contrasts and changes to the receptive field- esepcially edges

Question 28

where does the optic tract mainly diverge away from the chiasm to?

Answer 28

passes around the cerebral peduncle to terminate mainly in the lateral geniculate nucleus within the lateral geniculate body of the thalamus

Question 29

where do a small number of fibres leave the optic nerve to?

Answer 29

the pretectal areal and the superior colliculus

Question 30

pathway of third-order neurons from the LGN

Answer 30

third-order thalamocortical neurones project through the retrolenticular part of the internal capsule and form the optic radiation, which terminates in the primary visual cortex of the occipital lobe (primarily on medial surface of the hemisphere above the calcerine sulcus)