The Eye and the Nervous System

The Eye and the Nervous System

The Eye and the Nervous System

The Retina

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

Rods and Cones The eye is made of cells that are called Rods and Cones. Cone cells are coned shaped and Rod cells are rod shaped.

Rods and cones Light path

Inside the rod and the cone

Visual Acuity The Rod cells are more sensitive than the Cone cells but the Cone cells have a higher acuity than Rod cells.

Colour Vision Rods cells have monochromatic vision and Cone cells have trichromatic vision. Cone cells see in bright light and Rod cells see in black and white and in dark light. There are three different coloured Cone cells. These are red, green and blue. There are an equal amount of coloured Cone cells in the eye. The Cone cells are all situated at the fovea.

Bipolar Cells Three Rod cells are connected to one bipolar cell which means that when only one of the Rod cells are activated an impulse is sent to the brain. One Cone cells is connected to one bipolar cell which means that the light needs activate each Cone cell to send an impulse. This is why the Cone cells have a higher acuity and why they cant function in the dark.

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: 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 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 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 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 Rhodopsin with cis retinal opens sodium channels (absence of light) Rhodopsin with trans retinal closes sodium channels (light)

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

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

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

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

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

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: 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: 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.

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)

The Iris By contracting and relaxing these muscles the pupil can be constricted and dilated

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 More Light receptor Rods and Cones Sensory neurone coordinator Brain Motor neurone effector Iris muscles response Pupil constricts

There are TWO basic simple lens types: Concave and Convex What are lenses? Lenses bend light in useful ways. Most devices that control light have one or more lenses in them (some use only mirrors, which can do most of the same things that lenses can do). There are TWO basic simple lens types: Concave and Convex

CONVEX or POSITIVE lenses will CONVERGE or FOCUS light and can form an IMAGE.

CONCAVE or NEGATIVE lenses will DIVERGE (spread out) light rays

Convex lens The correct name for farsightedness is Hyperopia. The shape of your eye does not bend light correctly, resulting in a blurred image. A convex lens is usually used to correct this problem.

Concave lens The correct name of nearsightedness is myopia. Myopia occurs when the eyeball is slightly longer than usual from front to back. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. A concave lens is usually used to correct this problem.

Vision http://www.aoa.org/x6024.xml Vision begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye.

The cornea bends or refracts the rays that pass through a round hole called the pupil.

The iris, or colored portion of the eye that surrounds the pupil, opens and closes.

The pupil gets bigger or smaller to regulate the amount of light passing through.

The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye.

The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells. The images that we see are projected onto the retina upside down.  Our brain quite simply, flips the images over so that we see things upright.

The optic nerve transmits information to the brain.

The vitreous body gives the eye its shape.