The Human Nervous System

Organisms need to detect changes in their surroundings and respond to them so they can find food, avoid danger and keep conditions inside the body stable.

A change in the environment is a stimulus. Examples include light, sound, touch, temperature, chemicals and pressure.

Receptors are cells or organs that detect stimuli and convert them into electrical impulses. Different receptors respond to different stimuli, as in the table.

Effectors are the parts of the body that respond to the stimulus. Muscles contract and glands secrete hormones or other substances, such as sweat or saliva. A response can therefore be a movement or a secretion.

A neurone has a cell body with a nucleus, branching dendrites that receive signals, and a long fibre called an axon that carries impulses.

The sequence for a voluntary response is: stimulus, receptor, sensory neurone, CNS (brain and spinal cord), motor neurone, effector, response. For example, if a person hears a shout, sound receptors in the ear detect the vibrations, sensory neurones carry impulses to the brain, the brain processes the information and decides what to do, and motor neurones carry impulses to muscles in the neck so the head turns.

Nervous responses are fast because impulses travel in milliseconds, and they are usually short-lived and precise, acting on a very specific effector. This contrasts with hormonal responses, which are slower, longer-lasting and often widespread, carried in the blood. You should compare these two systems in an examination.

Voluntary responses are under conscious control and involve the cerebral cortex, for example deciding to pick up a pen. Involuntary (automatic) responses happen without conscious thought, such as the control of heart rate by the medulla, or reflex actions. Reflexes are especially fast because the impulse passes through the spinal cord or lower brain without waiting for a decision from the higher brain, as the topic on the reflex arc explains.

Damage to nerves or to the spinal cord can cut off the pathway between the brain and body, which is why spinal injuries may cause paralysis or loss of sensation below the injury.

A nerve is a bundle of many axons, from sensory and motor neurones, wrapped together in a protective sheath.

The brain and spinal cord are also protected by the meninges and cerebrospinal fluid, which acts as a shock absorber and carries nutrients. This matters because neurones that die are generally not replaced, so protecting the CNS is more important than for tissues that can regrow.