Neural Control and Coordination

For the body to work as one unit, its many parts must be coordinated — made to work together at the right time. In animals this is done in two ways: rapidly by the nervous system (using electrical signals) and more slowly by the endocrine system (using chemical hormones). This chapter is about neural (nervous) control.

The basic unit of the nervous system is the neuron (nerve cell) — a special cell built to carry messages quickly. A neuron has three parts:

• The cell body (cyton) — contains the nucleus.
• Many short branches called dendrites — which receive signals.
• A long fibre called the axon — which carries the signal away to the next cell.

The message that travels along a neuron is the nerve impulse — an electrical signal. It moves along the axon and reaches the end, where the neuron meets the next neuron (or a muscle). This tiny gap between two neurons is the synapse. At the synapse the electrical signal cannot jump the gap directly, so the neuron releases chemical messengers called neurotransmitters, which carry the signal across to the next neuron. In this way impulses pass from neuron to neuron, always in one direction. A bundle of many axons wrapped together forms a nerve.

The human nervous system has two main divisions: the central nervous system (CNS) — the brain and the spinal cord, which process information and give commands — and the peripheral nervous system (PNS) — the network of nerves that connects the CNS to the rest of the body.

The brain is the control centre, protected inside the skull. It has three main parts:

• Cerebrum — the large, folded front part; the centre of thinking, memory, intelligence, the senses and voluntary actions.
• Cerebellum — below and behind the cerebrum; controls balance and the coordination of muscular movements.
• Medulla oblongata (brain stem) — connects to the spinal cord; controls involuntary actions like heartbeat, breathing and digestion.

The spinal cord runs down the back inside the backbone; it carries messages between the brain and the body and controls reflexes.

A reflex action is a quick, automatic response to a stimulus that does not need conscious thought — for example, pulling your hand away from a hot object. The path the signal follows is the reflex arc: a receptor detects the stimulus → a sensory neuron carries the message to the spinal cord → the spinal cord processes it → a motor neuron carries the command to a muscle (effector), which acts. Because the action is decided in the spinal cord (not the brain), it is very fast and protects us from harm.

To respond to the world, the body must first sense it. Sense organs (receptors) are special structures that detect changes (stimuli) in the surroundings and send signals to the brain. Humans have five main sense organs:

• Eyes — detect light and let us see. Light enters through the cornea and pupil, is focused by the lens onto the light-sensitive retina at the back, which sends signals through the optic nerve to the brain. The iris controls how much light enters.
• Ears — detect sound and also help with balance. Sound makes the eardrum vibrate; tiny ear bones pass the vibrations to the inner ear (cochlea), which sends signals to the brain.
• Nose — detects smell (chemicals in the air) using olfactory receptors.
• Tongue — detects taste using taste buds that sense sweet, sour, salty, bitter and umami.
• Skin — detects touch, pressure, pain, heat and cold.

Each sense organ has receptors suited to a particular kind of stimulus, and all of them work by converting the stimulus into nerve impulses that travel to the brain, where the sensation is finally "felt" and understood. Caring for the sense organs — for example, protecting the eyes from very bright light and the ears from very loud noise — helps keep them healthy.