Lecture 10: Vision 1 Flashcards
What fundamentally limits visual acuity?
neural and optical factors
What are the optical factors which affect visual acuity?
pupil size
clarity of optical medial (cataracts, corneal opacities)
refractive errors -> blur (myopia, hypermetropia, astigmatism, presbyopia)
How does light trigger a neural response in the eye?
light travels through to the back of the eye where photoreceptors are located
light activates the photoreceptors which kicks off the process of seeing
What are the characteristics of rods?
useful for night vision very sensitive one type only no colour vision absent from fovea
What are the characteristics of cones?
useful for day vision less sensitive three types allow colour vision densest in fovea
What is the ratio of rods to cones?
20:1
What are the photopigments in rods and cones and what is their role?
rods = rhodopsin
cones = cone-opsin
their role is to be activated by light which results in the conformational change of a protein
What do opsins bind to?
vitamin A
What happens when light hits 11-cis retinal?
all-trans retinal is formed
this molecular change in shape kicks off the process which allows us to see
How do photoreceptors respond to light?
photoreceptors are hyperpolarised by light and respond with graded changes in membrane potential (not action potentials)
What do photoreceptors use as their neurotransmitter?
glutamate
What happens to photoreceptors in the dark?
cGMP gates a sodium channel causing continuous influx of sodium ions -> depolarisation of the cell
Explain the sequence of events that occurs when photoreceptors are exposed to light.
in the light cGMP breaks down to GMP -> cGMP no longer gates the sodium channels -> flow of Na ions ceases -> cell is hyperpolarised
What happens when light activates rhodopsin?
initiates a cascade of events that ultimately leads to the closure of cGMP gated sodium channels which results in hyperpolarisation
Rh -> transducin -> PDE -> breaks down cGMP
How is information about light passed through the retina?
there is a “through” pathway (Ph - BC - GCs) and there are lateral interactions (horizontal cells / amacrine cells)
What are the second order neurons (bipolar cells)?
important neurons in “through” pathway
important for spatial vision and colour vision
What do bipolar cells receive input from and what do they form output synapses with?
receive input
from photoreceptors and
form output synapses with
retinal ganglion cells (and amacrine cells)
What are the 10 different types of second order neurons?
1x rod bipolar cell
9x cone-bipolar cells
What are the many different types of ganglion cells? What is their role?
ON, OFF, M and P
release glutamate and fire action potentials
How do ganglion cells respond to light?
by either increasing or decreasing their action potential firing rate
What is the receptive field of a ganglion cell?
the area of retina that when stimulated with light changes the cell’s membrane potential
What are the responses of ganglion cells?
increase or decrease in firing
transient or sustained response
visual information is passed to higher cortical centres in parallel
What is the role of horizontal cells?
provide output onto photoreceptors and use inhibitory neurotransmitter GABA to induce lateral inhibition
respond to light by hyperpolarising
What is the role of amacrine cells?
important for lateral inhibition as well
for the most part are considered inhibitory cells (release glycine and GABA)
