The human nervous system

The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

Information from receptors passes along cells called neurones as electrical impulses to the central nervous system (CNS), which is the brain and spinal cord. The CNS coordinates the response of effectors, which may be muscles contracting or glands secreting hormones.

The sequence to learn, and to write out whenever a question describes a response:

stimulus → receptor → coordinator → effector → response

A reflex arc is the fast version of that sequence, and you should be able to explain how each structure in it relates to its function.

• The receptor detects the stimulus. • The sensory neurone carries the impulse to the CNS. • A synapse is the gap between two neurones. The impulse arrives, a chemical is released across the gap, and a new impulse starts in the next neurone — which is why synapses slow the journey slightly. • The relay neurone, in the spinal cord, passes it straight across. • The motor neurone carries the impulse out to the effector. • The effector — a muscle or gland — carries out the response.

Reflex actions are automatic and rapid, and they do not involve the conscious part of the brain. That is exactly why they matter: pulling your hand off something hot has to happen before you have decided anything, so bypassing conscious thought protects the body from damage.

Reaction-time data comes with its own skills. You should be able to extract and interpret data from graphs, charts and tables about the functioning of the nervous system, and to translate information about reaction times between numerical and graphical forms.

In practice that means three things. Reading a table: compare like with like — the same person, the same hand — and quote the figures. Turning a table into a graph: the condition goes on the x-axis, the reaction time on the y-axis, and a bar chart suits conditions that are categories while a line graph suits a numerical factor such as number of hours of sleep. Reading a graph back: give the value with its unit, and say what it means about the response, not just that the bar is taller.

The brain controls complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions. Three regions must be identifiable on a diagram, with their functions.

The eye is a sense organ containing receptors sensitive to light intensity and colour. Seven structures must be identifiable on a diagram, with their functions.

Accommodation is the process of changing the shape of the lens to focus on near or distant objects. Learn both sequences as three steps each, in order.

To focus on a near object: the ciliary muscles contract, the suspensory ligaments loosen, and the lens is then thicker and refracts light rays strongly.

To focus on a distant object: the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is then pulled thin and only slightly refracts light rays.

Adaptation to dim light is a different mechanism, done by the iris. In dim light the pupil widens to let more light in; in bright light it narrows to protect the retina. The iris muscles do that automatically — it is a reflex.

Two common defects are myopia (short sightedness) and hyperopia (long sightedness), in which rays of light do not focus on the retina. In myopia the rays focus in front of the retina, so distant objects are blurred; in hyperopia they would focus behind it, so near objects are blurred.

Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina — a concave lens for myopia, a convex lens for hyperopia. New technologies include hard and soft contact lenses, laser surgery to change the shape of the cornea, and a replacement lens in the eye.

In a ray diagram, find where the rays cross: in front of the retina is myopia, behind it is hyperopia, and the corrected diagram should show them meeting exactly on the retina.

Reading a ray diagram is a skill in itself, and the specification asks for it directly. Work through it in the same order every time.

First, find where the rays cross. In myopia they cross in front of the retina and are already spreading apart again by the time they reach it, so the image is blurred — the eye refracts the light too strongly. In hyperopia they have not crossed by the time they reach the retina and would only meet somewhere behind it, so the eye refracts too weakly.

Then work out what the lens has to do. Myopia needs the rays spread apart before they enter, so that the eye's own focusing brings them together later — that is a concave (diverging) lens. Hyperopia needs them brought together sooner — a convex (converging) lens. In a corrected diagram the rays should meet exactly on the retina, and if they do not, the diagram is showing you the uncorrected case.

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. The centre contains receptors sensitive to the temperature of the blood, and the skin contains temperature receptors that send nervous impulses to it. So the body is sampling temperature in two places: the blood inside and the surface outside.