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 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 dialeted

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