3 BiologyMad.comThe RetinaContains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones.The photoreceptor cells are at the back of the retina, and the light has to pass through several layers of neurones to reach them.2 kinds of photoreceptors – rods and conesForm synapses with special interneurones called bipolar neuronesThese in turn synapse with sensory neurones called galglion cells.The axons of the ganlion cells cover the inner surface of the retina and form the optic nerve (about a million axons) that lead to the brain
4 BiologyMad.comVisual TransductionThis is the process by which light initiates a nerve impulse.The structure of a rod cell:Detection of light is carried out on the membrane disksThese disks contain thousands of molecules of rhodopsin (photoreceptor molecule)
5 Visual Transduction Rhodopsin consists of: BiologyMad.comVisual TransductionRhodopsin consists of:Opsin (membrane bound protein)Retinal (covalently-bound prosthetic group) sensitive partRetinal is made from vitamin ARetinal is the light sensitive part -exists in 2 forms: cis and trans formsVitamin deficiency causes night blindeness
6 Rhodopsin with cis retinal Rhodopsin with trans retinal BiologyMad.comVisual TransductionIn the dark retinal is in the cis form.When it absorbs a photon of light it quickly switches to the trans form.This changes the shape of the opsin protein – a process called bleachingLight- fast (ms)Rhodopsin with cis retinalRhodopsin with trans retinal
7 Rhodopsin with cis retinal Rhodopsin with trans retinal BiologyMad.comVisual TransductionThe reverse reaction (trans to cis) requires an enzyme reaction and is very slow (taking a few minutes)This process requires ATP, as rhodopsin has to be resynthesisedThis explains why you are initially blind when you walk from sunlight to a dark room:in the light almost all your retinal was in the trans form, and it takes some time to form enough cis retinal to respond to the light indoors.Light- fast (ms)Rhodopsin with cis retinalRhodopsin with trans retinalDark - slow (mins)
8 Visual Transduction Bleaching of the rhodopsin in a rod cell i BiologyMad.comVisual TransductionBleaching of the rhodopsin in a rod celliAlters the permeability of the membrane to Na+nerve impulsesensory neurone in the optic nerveto the brain
9 Visual Transduction Rhodopsin controls sodium channels BiologyMad.comVisual TransductionRhodopsin controls sodium channelsRhodopsin with cis retinal opens sodium channels (absence of light)Rhodopsin with trans retinal closes sodium channels (light)
10 BiologyMad.comIn the Dark…In the dark the channel is open Na+ flow in can cause rod cells to depolarise.Therefore in total darkness, the membrane of a rod cell is polarisedTherefore rod cells release neurotransmitter in the darkHowever the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse
12 BiologyMad.comIn the Light…As cis retinal is converted to trans retinal, the Na+ channels begin to close iless neurotransmitter is produced. If the threshold is reached, the bipolar cell will be depolarisediforms an impulse which is then passed to the ganglion cells and then to the brain
14 Rods and Cones Rods Cones Outer segment is rod shaped BiologyMad.comRods and ConesRodsConesOuter segment is rod shapedOuter segment is cons shaped109 cells per eye, distributed throughout the retina, so used for peripheral vision.106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.Good sensitivityPoor sensitivityOnly 1 type monochromatic vision3 types (R, G & B) colour visionMany rods connected to one bipolar cell poor acuity = poor resolutionEach cone is connected to one bipolar cell good acuity = good resolution
15 BiologyMad.comColour Vision3 different cone cells. Each have a different form of opsin (they have the same retinal)3 forms of rhodopsin are sensitive to different parts of the spectrum10% red cones45% blue cones
16 BiologyMad.comColour VisionColoured light will stimulate these 3 cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colourRed light stimulates R conesYellow light stimulates R and G cones equallyCyan light stimulates B and G cones equallyWhite light stimulates all 3 cones equallyCalled the trichromatic theory of colour visionThe red, green and blue opsin proteins are made by three different genes. The green and red genes are on the X chromosome, which means that males have only one copy of these genes (i.e. they’re haploid for these genes). About 8% of males have a defect in one or other of these genes, leading to red-green colour blindness. Other forms of colour blindness are also possible, but are much rarer.
17 Colour VisionWhen we look at something the image falls on the fovea and we see it in colour and sharp detail.Objects in the periphery of our field of view are not seen in colour, or detail.The fovea has high density of cones.Each cone has a synapse with one bipolar cell and one ganglion each cone sends impulses to the brain about its own small area of the retina high visual acuity
18 BiologyMad.comAccommodationRefers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retinaThe lens shape can be altered by suspensory ligaments and the ciliary muscles. This adjusts the focus
19 Accommodation Distant objects: BiologyMad.comAccommodationDistant objects:Light rays are almost parallel so do not need much refraction to focus onto the retina.The lens therefore needs to be thin and “weak” (i.e. have a long focal length).To do this the ciliary muscles relax, making a wider ring and allowing the suspensory ligaments (which are under tension from the pressure of the vitreous humour) to pull the lens out, making it thinner.
20 Accommodation Close objects: BiologyMad.comAccommodationClose objects:Light rays are likely to be diverging, so need more refraction to focus them onto the retina.The lens therefore needs to be thick and “strong” (i.e. have a short focal length).To do this the ciliary muscles contract, making a smaller ring and taking the tension off the suspensory ligaments, which allows the lens to revert to its smaller, fatter shape.
21 BiologyMad.comThe IrisRegulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage themComposed of 2 sets of muscles:Circular and radial have opposite affects (antagonistic)
22 BiologyMad.comThe IrisBy contracting and relaxing these muscles the pupil can be constricted and dialeted
23 The Iris Is under control of the autonomic nervous system BiologyMad.comThe IrisIs under control of the autonomic nervous systemSympathetic Nerve pupil dilationParasympathtic Nerve pupil constrictionThe drug atropine inhibits the parasympathetic nerve, causing pupil to dilate
24 The Iris The iris is a good example of a reflex arc: stimulus BiologyMad.comThe IrisThe iris is a good example of a reflex arc:stimulusMore LightreceptorRods and ConesSensory neuronecoordinatorBrainMotor neuroneeffectorIris musclesresponsePupil constricts