Copyright © 2007 Dorling Kindersley
The nervous system is the body’s main control system. It is made up of the CENTRAL NERVOUS SYSTEM (or CNS) and a network of NERVES that extend from the CNS to all parts of the body. The nervous system regulates both voluntary activities, such as walking and talking, and involuntary activities, such as breathing, which you make no conscious decisions about.
The CNS consists of the brain and spinal cord. The rest of the nervous system, called the peripheral nervous system, consists of nerves. These include 12 pairs of nerves that branch from the brain (cranial nerves) and 31 pairs that branch from the spinal cord (spinal nerves).
The nervous system contains billions of neurons (nerve cells). A neuron has a cell body, arms called dendrites, and a long projecting fibre, the axon. Electrical signals – up to 2,500 per second – can pass along axons. They can also jump between neurons by means of chemicals that pass across the gaps in synapses (neuron junctions).
The main parts of the brain are the large folded cerebrum, the brainstem, which forms a stalk at the foot of the brain, the cerebellum behind it, and central structures, such as the thalamus.
Nerves are made up of bundles of the axons of nerve cells. Some of these carry information picked up by sensory receptors around the body to the CNS for processing. Other axons carry messages from the CNS to muscles, causing movement, or to the body’s glands, causing the release of hormones. Many axons are surrounded by a protective sheath containing a fatty substance called myelin. This acts to insulate the axons electrically.
The CNS has two main tasks. It has to process information, both about the outside world (obtained by organs such as the eyes) and about the inside of the body (obtained by internal receptors). It also has to generate responses such as movement that will protect and maintain the body. Some activity within the CNS is quite simple REFLEX (automatic) activity. But much of its activity, particularly in the brain’s cerebrum, is complex and conscious.
The cortex (outer layer) of the cerebrum has many functions. Different areas of the cortex are involved in processing or analysing sensory information, sending signals to direct muscle movements, or in other activities such as reasoning, memory, or creative thought.
The spinal cord’s main function is to transmit information between the brain and spinal nerves. It is also involved in some reflex activity. Its grey matter is made up of the cell bodies of neurons. Its white matter contains axons (neuronal fibres). These are arranged into groups called tracts and carry signals up and down the cord.
In its simplest sense, a reflex is an emergency reaction of the nervous system to a threat such as a hot object touching the skin. In a wider sense, reflexes are automatic responses to a wide range of situations in the body and are key to many internal activities, such as the heart beat. A division of the nervous system called the autonomic nervous system is in overall control of these internal activities.
In a simple reflex, information passes from the area affected, in this case the finger, to the central nervous system (red pathway). This triggers an immediate response, in this case the contraction of a muscle (blue pathway) to withdraw the finger. Here, the reflex action involves only two nerves and the spinal cord. However, a signal also passes to the brain, which registers the pain.