Your senses and nervous system
The sense organs contain receptors that are sensitive to stimuli.
Human vision is binocular, which gives us a good perception of depth. Vision defects include short-sightedness, long-sightedness and colour blindness.
The nervous system consists of the central nervous system - brain and spinal cord -and peripheral nerves. Nerve impulses travel through the axons of neurones - nerve cells. Reflex actions are fast, automatic and protective responses.
Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.
|sense organs||receptors sensitive to|
|touch, pressure, pain and temperature|
|chemicals in food|
|chemicals in the air|
|sound and position of the head|
The central nervous system - CNS - in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.
You need to be able to name and locate the main parts of the eye.
You should also be able to describe the functions of the main parts of the eye.
|Cornea||Front part of the tough outer coat, the sclera. It is convex and transparent.||refracts light - bends it as it enters the eye|
|Iris||Pigmented - decides the colour of your eyes - so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil.||controls how much light enters the pupil|
|Lens||Transparent, bi-convex, flexible disc behind the iris attached by the suspensory ligaments to the ciliary muscles.||focuses light onto the retina|
|Retina||The lining of the back of eye containing two types of photoreceptor cells - rods - sensitive to dim light and black and white - and cones - sensitive to colour. A small area called the fovea in the middle of the retina has many more cones than rods.||contains the light receptors|
|Optic nerve||Bundle of sensory neurones at back of eye.||carries impulses from the eye to the brain|
Because our eyes sit side by side, each eye captures a slightly different view. This is called stereoscopic vision. When signals from the two eyes reach the brain, they are superimposed and processed into a single picture with depth. As a result, we get a 3D picture and are able to judge distances well.
Most birds and lizards have monocular vision - their eyes are on each side of their head. This gives them a greater field of view, which is useful for spotting predators. However, they have poor depth perception.
Light passes through the eyeball to the retina. It is refracted (its rays are bent) by the cornea and lens, so that the light is brought to a focus on the retina.
Someone with short-sightedness can see near objects clearly, but cannot focus properly on distant objects. This is because the lens focuses the sharpest image in front of the retina, instead of on it.
Someone with long-sightedness can see distant objects clearly, but cannot focus properly on near objects. This is because the lens focuses the sharpest image behind the retina, instead of on it.
Colour blindness is an inherited condition. The retina contains cells that are sensitive to red, green or blue light. People with colour blindness have a lack of receptors, or defects in them. People with red-green colour blindness, for example, have difficulty distinguishing shades of red and green.
The main parts of the nervous system are the central nervous system - the CNS - the brain and spinal cord - and the peripheral nervous system. Nerve impulses are mainly electrical and carried in nerve cells, or neurones.
The diagram shows the main parts of a motor neurone - which carries nerve impulses to effectors such as muscles.
The nerve impulse is carried in the axon.
The sheath acts as an insulator - rather like the plastic coating of an electrical wire. Neurones are adapted to their function by being long, so that they communicate with distant parts of the body. They also have branched endings called dendrites. These connect with many other neurones.
The gap between two neurones is called a synapse. The nerve impulse passes across this gap through chemicals that diffuse across the gap. The electrical signal travels along an axon and triggers the release of chemical transmitters from the nerve ending of the first neurone. These diffuse across the gap and make the second neurone re-transmit the electrical signal. The animation shows how this works:
When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.
Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.
This is what happens:
The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.
Light is focused onto the retina by refraction at the cornea, and by the lens. The lens changes shape to make the fine adjustments needed to produce a sharp image. This is called accommodation. As we get older, accommodation becomes slower and weaker.
|object||ciliary muscles||suspensory ligaments||shape of lens|
|distant||relax||stretch||thin – less convex|
|near||contract||slacken||fat – more convex|
The animation shows how this works:
Someone with short-sightedness can see near objects clearly, but cannot focus properly on distant objects. This is caused by the eyeball being elongated, so that the distance between the lens and the retina is too great. It can be corrected by placing a concave lens in front of the eye.
Someone with long-sightedness can see distant objects clearly, but cannot focus properly on near objects. This is because the lens focuses the sharpest image behind the retina, instead of on it. This defect is often age-related, and due to a loss of elasticity in the lens. It is corrected by putting a convex lens in front of the eye.