The Eye and the Nervous System BiologyMad.com

The Retina

BiologyMad.com The Retina Contains 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 cones Form synapses with special interneurones called bipolar neurones These 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

BiologyMad.com Visual Transduction This 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 disks These disks contain thousands of molecules of rhodopsin (photoreceptor molecule)

Visual Transduction Rhodopsin consists of: BiologyMad.com Visual Transduction Rhodopsin consists of: Opsin (membrane bound protein) Retinal (covalently-bound prosthetic group) sensitive part Retinal is made from vitamin A Retinal is the light sensitive part - exists in 2 forms: cis and trans forms Vitamin deficiency causes night blindeness

Rhodopsin with cis retinal Rhodopsin with trans retinal BiologyMad.com Visual Transduction In 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 bleaching Light- fast (ms) Rhodopsin with cis retinal Rhodopsin with trans retinal

Rhodopsin with cis retinal Rhodopsin with trans retinal BiologyMad.com Visual Transduction The 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 resynthesised This 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 retinal Rhodopsin with trans retinal Dark - slow (mins)

Visual Transduction Bleaching of the rhodopsin in a rod cell i BiologyMad.com Visual Transduction Bleaching of the rhodopsin in a rod cell i Alters the permeability of the membrane to Na+ nerve impulse sensory neurone in the optic nerve to the brain

Visual Transduction Rhodopsin controls sodium channels BiologyMad.com Visual Transduction Rhodopsin controls sodium channels Rhodopsin with cis retinal opens sodium channels (absence of light) Rhodopsin with trans retinal closes sodium channels (light)

BiologyMad.com In 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 polarised Therefore rod cells release neurotransmitter in the dark However the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse

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BiologyMad.com In the Light… As cis retinal is converted to trans retinal, the Na+ channels begin to close i less neurotransmitter is produced. If the threshold is reached, the bipolar cell will be depolarised i forms an impulse which is then passed to the ganglion cells and then to the brain

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Rods and Cones Rods Cones Outer segment is rod shaped BiologyMad.com Rods and Cones Rods Cones Outer segment is rod shaped Outer segment is cons shaped 109 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 sensitivity Poor sensitivity Only 1 type  monochromatic vision 3 types (R, G & B)  colour vision Many rods connected to one bipolar cell  poor acuity = poor resolution Each cone is connected to one bipolar cell  good acuity = good resolution

BiologyMad.com Colour Vision 3 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 spectrum 10% red cones 45% blue cones

BiologyMad.com Colour Vision Coloured light will stimulate these 3 cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colour Red light  stimulates R cones Yellow light  stimulates R and G cones equally Cyan light  stimulates B and G cones equally White light  stimulates all 3 cones equally Called the trichromatic theory of colour vision The 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.

Colour Vision When 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

BiologyMad.com Accommodation Refers 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 retina The lens shape can be altered by suspensory ligaments and the ciliary muscles. This adjusts the focus

Accommodation Distant objects: BiologyMad.com Accommodation Distant 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.

Accommodation Close objects: BiologyMad.com Accommodation Close 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.

BiologyMad.com The Iris Regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them Composed of 2 sets of muscles: Circular and radial  have opposite affects (antagonistic)

BiologyMad.com The Iris By contracting and relaxing these muscles the pupil can be constricted and dialeted

The Iris Is under control of the autonomic nervous system BiologyMad.com The Iris Is under control of the autonomic nervous system Sympathetic Nerve  pupil dilation Parasympathtic Nerve  pupil constriction The drug atropine inhibits the parasympathetic nerve, causing pupil to dilate

The Iris The iris is a good example of a reflex arc: stimulus BiologyMad.com The Iris The iris is a good example of a reflex arc: stimulus More Light receptor Rods and Cones Sensory neurone coordinator Brain Motor neurone effector Iris muscles response Pupil constricts