Visual physiology

Question 1

Photoreceptors

Answer 1

• inner segment (contains nucleus and other protein-making machinery) attached to synaptic terminal that contacts many different dendrites
• axon links inner segment and synaptic terminal (doesnt fire APs)
• outer segment is packed with layers of membrane discs
• Rod = night vision
• Cone = day vision

Question 2

Response to increased light

Answer 2

• membrane contains leak potassium channels - allows K to move down its gradient and carry positive charge out with it - gives RMP of -40mV
• In the outer segment there are open Na channels - depolarises the cell slightly to -40mV
• when light hits the receptor, it closes some of the Na channels, causing the membrane to hyperpolarise
• the repolarisation then comes from a decrease in light hitting the outer segment

Question 3

Transduction - initiation of light response

Answer 3

• Sodium channels are being help open by 2 cGMP molecules (cGMP levels are relatively high in IC space)
• different photoreceptors have different types of opsins, however they all have in common binding sites for 11-cis retinal
• The binding of opsin and retinal is what makes the photopigment
• Retinal has a carbon ring and C chain - all links are trans except one at 11C, which is a weaker cis bond
• when light hits this bond it breaks, and when it is reformed, a trans bond is made - removes the kink in the tail and produces the active form

Question 4

Transduction - amplification

Answer 4

• produces a biochemical cascade, activating an enzyme decreasing cGMP > Na channels close
• the more light, the bigger the change will be and the more Na channels will close
• As soon as the response happens, you need to be able to terminate it so that it doesnt linger

Question 5

Transduction - termination

Answer 5

• cascade terminated at several levels, including the phosphorylation of rhodopsin by rhodopsin kinase, and the subsequent binding of phosphorylated rhodopsin by arrestin.

Question 6

Adaptation

Answer 6

• photoreceptors adapt to new light conditions
• at any given light level, they can adapt to have a RMP of -40mV, therefore can respond strongly in either direction around that level
• they adapt using multiple mechanisms that slow or speed up mechanisms in the cascade or termination and regeneration procedures

Question 7

Rapid response

Answer 7

• If light flickers at 60Hz, the receptor can hyperpolarise and repolarise fast enoguh to see the flicker
• the rapid response to light results in a very high metabolic rate - need a rapid supplt of o2 and nutrients
• the metabolic activity in the retina is about 7x higher than in the rest of the CNS
• mostly due to very high activity in the photoreceptor outer segments

Question 8

How do photoreceptor outer segments support high resolution sampling of the image?

Answer 8

• packed in a neat hexagonal array - no capillaries
• special bloody supply - choroid
• outer segments point towards the outside of the eye ball - therefore light has to pass all the way through the segment of retina
• photoreceptors need to be adjacent to choroid to get a good enough blood supply
• the barrier is RPE - blood retinal barrier

Question 9

RPE

Answer 9

• cell makes long sheet-like processes that enfold outer segments, and pumps fluid out, creating suction - holds retina in place
• regenerates 11-cis retinal
• helps to renew the outer segment membranes
• acts as the blood-retinal barrier between outer-segments and the choroid

Question 10

Degeneration of RPE

Answer 10

• retinoids (retinals and retinols) damage membranes
• electromagnetic radiation and high oxygen concentrations causes photo-oxidation
• phospholipids and proteins are easily photo-oxidised
• with becomes clogged with IC debris (lipofuscon) and fatty plaques (drusen) - cant get rid of it - builds up between photoreceptors and their oxygen supply

Question 11

Different types of ganglion cell

Answer 11

• off cells - excited by decreased illumination
• on cells - excited by increased illumination
• parvocellular - specialised for high resolution and colour (small cells)
• magnocellular - specialised to detect fast moving and low contrast - larger

Question 12

Seeing fine detail

Answer 12

• ganglion cell gets input from single photoreceptor in central retina - very small receptive field
• horizontal cells (inhibitory interneurons) gather input for the surrounding cones and inhibit vertical pathway

Question 13

Ganglion cells respond to contrast

Answer 13

• in a receptive field there are excitatory fields and inhibitory fileds (inhibited by lateral inhibition via inhib. inter. Gather info from surrounding photoreceptors)
• ganglion cells will only respond when there is an uneven illumination in the inhibitory surround and excitatory centre
• if the light covers the entire receptive field, there will be a balance of inputs - no response

Question 14

parvo vs magno

Answer 14

• P = fine details, 1:1 cone:ganglion, smaller receptive field
• M = for contrast and fast moving objects, many rods to one ganglion (convergent and much more sensitive), 4x larger receptive field than parvocellular

Question 15

Night vision

Answer 15

• rods only function at low light, will have lots of rods converging on one ganglion
• very faint stimulus in the darkness will actually still produce a respnse in this cell
• some people have degenerative diseases that wipe out tjeir cones - very poor resolution vision as they have to use massively convergent receptive fields

Question 16

Seeing colour

Answer 16

• we see colour using 3 different types of cone - all have broad response
• each will respond to light over a broad range of wavelengths
• retina compares output and the ratio of their response - determines colour
• genes for green and red are on X chromosone - hend lots of colour blind males

Question 17

Primary visual cortex

Answer 17

• retina and LATERAL geniculate nucleus encode: contrast and wavelength
• primary visual cortical cells: the orientation of edges, presence of corners, direction of motion, binocular disparity

Question 18

Visual pathway

Answer 18

• inputs from the eyes are separate up to the primary visual cortex
• info from eyes terminate in alternating stripes of cortex in layer 4
• retina > optic nerve > chiasm > tract > lateral geniculate nucleus > primary visual cortex

Question 19

Higher visual cortex - shape and form

Answer 19

• a string of visual areas that runs along the base of the visual cortex
• extract colour, face info, identity
• what shape/colour is it?
• what is it?
• what does it mean?
• Starts at visual cortex, goes to base of temporal cortex and up to frontal cortex
• damage to inferior temporal area = visual agnosia
  > can see things or faces, but wont be able to recognise what or who they are
  >can draw a picture of the things you see without knowing what they are

Question 20

Higher visual cortex - location and movement

Answer 20

• primary visual cortex > superior temporal areas > parietal cortex
• spatial info - where is it and where is it going?, how does it relate to other objects? are we moving, or is the object?
• refceives lots of magnocellular input

Question 21

Saccadic movements

Answer 21

• quick reflex by both eyes in same direction (new thing on screen, eyes jump towards it)
• driven by retinal input to SUPERIOR colliculus - tells you where things are and gives appropriate signals to CN nuclei to move eyes to it
• uses vertical and horizontal gaze centres > CNII, IV and VI
• exploratory = eyes flicker between important features
• voluntary = driven by frontal eye fields, jump to right sort of location, then parietal takes over to correct movements

Question 22

Pursuit movements

Answer 22

• following a moving object, keeping it on fovea
• driven by frontal eye fields
• circuits detect changes in movement for correction come from temporal areas
• pontine nuclei > cerebellum > vestibular nucleus > eye muscles

Question 23

Disconjugate - convergence

Answer 23

• visual cortex sends signal to vergence centre > CNIII > medial rectus contraction
• also activates nuclei to be able to accommodate
• visual cortex > vergence centre > CNII > medial rectus
• visual cortex > vergence centre > Edinger-westphal nucleus > ciliary ganglion > ciliary muscles