vision and the visual pathways Flashcards
what are the 4 regions of the retina?
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
what are the 4 regions of photoreceptors?
synaptic terminal
inner segment
cell body
outer segment (with photosensitive disks)
explanation of rod cells
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
explanation of cone cells
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
what it the principle visual pathway?
photoreceptors
bipolar cells of retina
ganglion cells of retina
lateral geniculate nucleus
primary visual cortex
importance of partial decussation at the optic chiasm
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
where is the optic chiasm?
rostral to the tuber cinereum of the hypothalamus and between the terminating internal carotid arteries
activity of rods and cones in the dark
both steadily release glutamate (depolarisation)
activity of rods and cones in the light
they are hyperpolarised which results in the inhibition of the release of glutamate from photoreceptors
this affects bipolar cells
what are the first-order neurons in the central visual pathway?
bipolar cells in the retina
what are the second-order neurons in the central visual pathway?
ganglion cells that form the optic nerve
where is glutamate released from photoreceptors?
the synaptic terminal
glutamate hyperpolarises what bipolar cells?
on bipolar cells
glutamate depolarises what bipolar cells?
off bipolar cells
what happens to an on bipolar cell in light?
depolarise
what happens to an off bipolar cell in light?
hyperpolarise
what are the receptors of an on bipolar cell?
metabotropic receptors
what are the receptors of an off bipolar cell?
ionotropic receptors
what do on bipolar cells synapse with?
on ganglion cells
what are the only cells that fire action potentials?
ganglion cells
what ganglion cells increase firing rate in the dark?
off
what ganglion cells increase firing rate in the light?
on
how are photoreceptors in rhe retina organised?
antagonistic concentric-centre surround receptive fields
what are horizontal cells?
inhibitory interneurons
what happens if light hits the centre of the receptive field?
activates the center cone which transmits information to the bipolar cell and then the ganglion
what happens if light hits the surround of the receptive field?
activates the surround cone which activates the horizontal cell which switches off activaton in the bipolar cell
why is the organisation of the receptive fields important?
detection of contrasts and changes to the receptive field- esepcially edges
where does the optic tract mainly diverge away from the chiasm to?
passes around the cerebral peduncle to terminate mainly in the lateral geniculate nucleus within the lateral geniculate body of the thalamus
where do a small number of fibres leave the optic nerve to?
the pretectal areal and the superior colliculus
pathway of third-order neurons from the LGN
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)
