The peripheral nervous system This consists of the 31 pairs of spinal nerves (see diagram overleaf). A nerve is a bundle of neuronal fibres, mainly axons, travelling together enclosed in a protective sheath. The spinal nerves in humans and other primates contain many hundreds of thousands of fibres, which can be classified as either sensory or motor. I mentioned in the last chapter how the function of sensory receptors is to convert stimuli into a neural code that the brain can deal with: the stimulus, such as a pin prick, acts on a specific receptor in the skin which converts the physical stimulus into a train of nerve impulses. These nerve impulses then travel from the receptor towards the spinal cord and brain along a sensory axon. There are many types of sensory receptor in the skin, dealing with touch, pain, pressure, heat, and cold, i.e. stimuli in the environment. The highly specialised receptors of the eyes and ears respond to distant stimuli in the environment such as objects, people, sounds etc. All of this sensory information travels along sensory axons into the central nervous system. Many of these stimuli require a response. The brain analyses the input, and organises any necessary response. Where this involves movement, the brain sends messages down motor pathways to the spinal cord, and then out through the spinal nerves along motor axons which eventually synapse onto the mus-

}cranial nerves (12 pairs)

The combination of sensory pathways handling information from the external environment and motor pathways carrying commands out to the muscles of the skeleton is called the somatic nervous system. It can be seen as vital to our interactions with the outside world.