9.7 detection of light by mammals Flashcards
what are the two types of photoreceptors in the retina?
rods and cones
what is the fovea?
area of the retina containing only cones, region of highest visual activity
which photoreceptor has their own sensory neurones?
cones
what is the visual pigment in rods?
rhodopsin
what is rhodopsin made up of?
opsin and retinal
what happens when a photon of light hits a rhodopsin molecule?
rhodopsin is bleached and breaks down into opsin and retinal
what happenes to rods in dim light?
1- rhodpsin breaks down to retinal+opsin
2- opsin, acting as an enzyme, closes the Na+ channels in the outer segment so there is no facilitated diffusion
3- Na+ is actively pumped out making the inside more negative
4- neurone is now hyperpolarised
5- causing the bipolar neurone to become depolarised
6- bipolar neurones release neurotransmitter to the ganglion cells and an action potential is initiated
what happened to rod cells in the dark?
1- retinol+opsin reform rhodopsin
2- Na+ channels are open so Na+ can diffuse back into the outer segment
3- Na+ actively pumped out of the inner segment using ATP which maintains the concentration gradient between the outer and inner segments
4- inner segment slightly depolarises
5- neurotransmitter glutamate is released binding to the bipolar cell
6- bipolar cell is not depolarised and therefore there is no action potential initiated in ganglion cells
what happens when light hits cones?
1- iodopsin is broken down but needs more light than rhodopsin so colours are not easily distinguished in dim light
2- each type of cone has it’s own iodopsin