Nervous system

A single celled organism carries out all the functions of life within its plasma cell membrane. However once organisms become multicellular they need to coordinate what happens in all of their different cells.

As multicellular organisms get bigger and bigger, cells differentiate and form tissues and organs which carry out specific functions. It is important that the different body systems work together. Animals have the biggest problem because they need to move around to find food and mates. A nervous system gives them a coordination system that works fast.

The human nervous system

The human body is made up of around 10 billion cells which all have to work together to keep you alive. People need to move about, avoid danger, find food, water and mates, control their body temperature and coordinate the activities of all the main body systems (for example the digestive system and circulatory system). This coordination is brought about by the nervous system and the endocrine system. The nervous system provides rapid coordination using electrical impulses which travel around the body fast – from about 1-120 metres per second.

In humans the nervous system is made up of the following parts:

• Receptors – these are specialised cells which are sensitive to changes in the environment (stimuli). Receptor cells are adapted to respond to different stimuli such as light or sound. Lots of receptor cells are often found together in sense organs for example the eye contains light sensitive cells; the nose contains cells which are sensitive to taste.
• Neurones – these are cells which are adapted to carry information in the form of electrical impulses. They have long extensions which mean impulses can travel all around the body. Sensory neurones carry information from sensory receptors to the central nervous system. Motor neurones carry information from the central nervous system to the effector organs. These are the muscles and glands which bring about changes in response to stimuli.

A nerve is a bundle of neurones. Sensory nerves contain only sensory neurones, motor nerves contain only motor neurones and mixed nerves carry both.

• The central nervous system (CNS) – this is made up of the brain and the spinal cord. Impulses from sensory neurones feed into the CNS. Here the sensory information is processed. The CNS coordinates the response to the incoming information and then sends out impulses along the motor neurones. As a result the body responds to the original stimulus.

The way the nervous system works to control the responses of your body can be summed up as:

receptor → sensory neurone → coordinator (CNS) → motor neurone → effector