9.7 - Detection of light by mammals Flashcards
What is the structure of the retina?
two parts: top = rods + cone cells, bottom = bipolar nerve cells which synapse to optic nerve
filled w rods + cones each split into four parts:
Outer segment-light sensitive and contains flattened membranous vesicles with photosensitive pigments
Constriction- a narrow region between segments
Inner segment- packed with mitochondria and ribosomes
Synaptic region
What is the difference between rod and cone cells?
Cone found at centre, rod found at periphery
cones conc at fovea; rods distributed equally
rods more numerous
Cone has iodopsin, rod has rhodopsin
Iodopsin degrades with high intensity light, rhodopsin degrades with any light
Cone sensitive to red, green or blue light, rod is insensitive to light
What happens when Rhodopsin is stimulated by light?
- The pigment within rods is degraded into opsin and retinal
- Opsin stimulates a series of enzyme-catalysed reactions that cause the cell surface membrane of the rod cell to become hyperpolarised
- Rod cells no longer release neurotransmitters
- The cell surface membrane of the bipolar cell becomes depolarised and release neurotransmitters
- The ganglion cell becomes depolarised and transmits an impulse to the optic nerve
- Rhodopsin is resynthesised in the dark
What happens when Rhodopsin is not stimulated by light?
- Cell surface membrane is depolarised
- Rod cells release a neurotransmitter onto the bipolar cell
- This has an inhibitory synapse so a neurotransmitter is not released to the ganglion cell
- No impulses are sent from the ganglion cell to the optic nerve
why is convergence a useful adaptation + feature of light sensitive cells of retina in eye?
- bc often several receptor cells often synapse w single sensory neurone
- so if generator potential from individual receptor cell = insufficient to set up synapse - potentials may add up to trigger an AP
- increases sensitivity of sensory system in low level stimuli
what are the main structures of the human eye? 14
- conjunctiva (outermost layer
- cornea = clear layer - plays role in focusing light + protecting eye
- pupil = circular jole in iris in which light enters eye
- iris = circular sheet of miscue controlling amount of light entering eye
- lens = imp for fine focusing of light onto retina
- ciliary muscles - change SHAPE OF LENS
- suspensor ligament
- vitreous humour = clear jelly maintaining shape of eye
- sclera = tough membrane all around
- choroid = pigmented cells preventing internal reflection of eye - in front of sclera at back of eye
- retina = layer of light sensitive cells ( rods+cones) + neurones leading from these photoreceptors -> optic nerve; in front of choroid in back
- fovea = part of retina w only ones = region of HIGHEST VISUAL ACUITY
- optic nerve = carry impulses from retina to brain
- blind spot = point where optic nerve leaves eye - NO photoreceptors cells - no vision
what is accommodation and how does it happen?
- focusing of rays from near/diatant objects by changing shape of lens
- light/photons travel thru transparent media in light ray - these rays reflect at predictable angles when striking object
- rays passing thru mediums of Dif density refract by lens + cornea
- rays from image in retina
what happens at near accommodation?
- ciliary muscles = contracted - constricting space around lens
- suspensory ligaments = slack - low tension
- lens = fat + round
- refraction of light = increased refraction to focus DIVERGING rays
how do rod cells work?
- photon of light hits molecule of rhodopsin + if big enough light energy absorbed - rhodopsin broken down
- enough pigment has to be broken down for TP to be met in bipolar cell
- then AP occurs
- doesn’t req a lot of light energy to break down rhodopsin
- even if low light - many rod cells connected to single bipolar cell (retinal convergence) - collective broken down rhodopsin gives big enough stimulus for AP (summation)
what is the downside of retinal convergence?
- low visual acuity as brain cannot distinguish bw Dif light sources
what happens in cone cells?
- 3 types of cone cells w Dif types of iodopsin pigment (red, green, blue) which absorb Dif wavelengths of light
- depending on prop of each cone cell stimulated we perceive colour images
- iodopsin only broken down if HIGH LIGHT INTENSITY & only 1 cone cell for each bipolar cell (no retinal convergence) so no spatial summation} dont see colours when dark -> THEREFORE HIGH VISUAL ACUITY
why are most cone cells at fovea?
- fovea is directly opposite lens so receives highest LI - therefore most cone cells
