Autonomic nervous system

All our conscious movements and activities are made possible by the nervous system. Messages are sent from the brain along nerves to the muscles, to tell them when and how strongly to contract, so that we can walk, run, jump, lift a heavy weight, play a musical instrument – or blink our eyes. A large area of the brain, the cortex, is in overall control of all these conscious activities. But there are many other bodily functions involving muscle-powered movements that are vital to everyday living, yet over which we have no conscious, thinking control. For example breathing, heartbeat, digestive movements and various other activities are – indeed , have to be – controlled subconsciously. These are all processes we rely on for our bodies to function, yet which are far too numerous and complex for us to think about controlling them. These subconscious activities are regulated by two closely related systems: hormones, from the endocrine system, and the autonomic nervous system. One small part of the brain, the hypothalamus, is the master controller of these automatic functions. It is tucked away under the large parts of the brain, and has links with both the endocrine glands (via the pituitary gland, which lies just below it) and the autonomic nerves. The hypothalamus can be thought of as an integrating centre. It receives information from other parts of the brain, such as the cortex, and sends instructions, in the form of nerve impulses, along autonomic fibres to many parts of the body. Although both endocrine and autonomic systems are under the same master control, they have different roles and different ways of exerting their influence on the body. The main difference between them is the speed of reaction. The endocrine system is relatively slow-acting because the hormones have to be released from their gland, transported round the body in the bloodstream, then infiltrate their target organ before an effect is seen. On the other hand, the automatic nervous system exerts its control much more rapidly. Messages are flashed to target organs as quick-fire electrical impulses moving at speeds of up to several metres per second.

There are two main parts to the autonomic nervous system: the parasympathetic and the sympathetic system. Many tissues and organs are innervated by both systems. Such a dual nerve supply is present for the heart, lungs, intestines, bladder, sex organs and for the adjustment of the diameter of the pupils in the eyes. The two systems differ very much in structure, the chemicals (neurotransmitters) they release when stimulated and in their effect on the target organs. Their action is generally to oppose and balance each other’s functions.

Parasympathetic system

The nerves of this system arise in the brain stem and in the sacral part of the spinal cord. When stimulated, those nerves release a neurotransmitter called acetylcholine which directly affects the target tissue. The most important parasympathetic nerve from the brain stem is the tenth, vagus nerve. Nearly all the internal organs are influenced by this one nerve. Only the organs in the pelvis are innervated by the sacral parasympathetic nerves. When the parasympathetic nervous system is activated, the body is allowed to recover from activity. The heart rate is slowed, for example, and the flow of blood is directed from the muscles to the digestive system.

The sympathetic system

The nerves of this system arise in the middle regions of the spinal cord – the thoracic (chest) and lumbar (lower back) regions. Many of the nerves relay in the sympathetic trunk, a bundle of nerves lying on both sides of the vertebral column. Instead of acetylcholine, at the end of a sympathetic nerve fibre adrenaline and noradrenaline are released. In general the effects of the sympathetic system are opposed to those of the parasympathetic system.

Fight or flight?

The sympathetic nervous system has an important role in ‘crisis’ situations. In these the sympathetic nerves stimulate the adrenal glands, which lie over the kidneys, to secrete adrenaline and noradrenaline. Adrenaline is a powerful stimulant and causes a series of responses that drive the body into action. It causes hair to stand on end, the heart to beat faster and more powerfully, and the blood to be shunted away from the skin and abdominal organs to the skeletal muscles. The body is thus ready for action – for ‘fight or flight’. Adrenaline also speeds up general metabolism and liberates energy stores into the blood. Because of this important link with the adrenal glands, and hence the endocrine system, the sympathetic nervous system has wide-ranging effects on parts of the body not directly connected to the autonomic nerve supply.

The effects of drugs

Both parts of the autonomic system can be manipulated by certain drugs, either to block or to enhance one part selectively. Atropine, for example, blocks the parasympathetic system by preventing acetylcholine working on the target organs; this drug is sometimes used to open wide the pupils, normally constricted by the parasympathetic nerves, for eye examination. It may also be used before surgery to ‘dry up’ secretions such as saliva. Drugs similar to noradrenaline can be used to enhance the actions of the sympathetic nervous system; to open tightened air passages if a person suffers a bad asthma attack, for instance. Beta-blocker drugs prevent noradrenaline from stimulating the heartbeat and are used to treat heart conditions.