The Nervous System

 
There are three main conceptual areas to the nervous system - the autonomic, the central and the peripheral systems.

The autonomic nervous system.

This is the system which controls and energises involuntary systems such as the lungs and the heart. As its name suggests, it is autonomous. It is a complex of control systems, which in the absence of outside influences, work independently. It is divided into the sympathetic and parasympathetic nervous systems. Generally increased activity in one leads to a decrease in the other.

 
Although autonomous, it is not of course, isolated from the rest of the body. It influences, and is influenced by, the outside nervous and endocrine systems according to need. Using the terminology we used when discussing the endocrine system, 'set points' are altered. Thus, under stress, the action of the heart and lungs changes.

The Peripheral Nervous System.

Radiating from the spine is the Peripheral Nervous System. These are the sensory nerves, which receive sensations, and the motor nerves that energise the muscles and so on.

Most form what are known as reflex arcs, whereby a sensory nerve will connect directly to a motor nerve through a junction in the spine.

I burnt my hand on the oven the other day and, almost instantly, snatched my hand away. The sensory nerves on my fingers triggered a withdrawal motor action in my arm, long before my brain had worked out what was going on.

Evolution has preserved the mechanism because it is extremely fast, efficient and usually effective. The knee jerk action is well-known, but there are many others. If you hold your hand out to receive an object, its weight stretches the muscles in the arm slightly. Sensory neurons in the muscles, called spindle receptors, connect to the motor neurons in the spine, which cause extra contraction in the muscle to compensate for the weight.

The Central Nervous System.

The brain and the spine are strictly speaking one entity, the Central Nervous System, largely divided into the sensory and motor systems.

Nerves of the Central Nervous System in the spinal cord, connect to reflex arcs, and transmit signals to and from the brain, which then uses other senses, such as sight, to work out what has happened and why, whether there was any further danger, and to keep a record not to touch the oven again when it is switched on.

These connections, properly known as synapses, can sometimes inhibit or stimulate the peripheral system.

By far the most noticeable feature of the human brain is the cerebral cortex. It consists of a sheet of folded nervous tissue extending over almost the whole of the inner part of the brain. It consists of a number of areas whose names have mainly derived from their physical appearance. Increasing knowledge over recent decades has identified various functional areas.

Towards the front are areas concerned with smell, speech and hearing, while towards the rear is the association cortex. Visual information from the eyes passes via two bundles of nerves to the visual cortex, right at the rear.

The two areas that concern us at this point are the primary motor cortex and the primary somatosensory cortex, which lie in two adjacent curved areas in the centre. Communication with sensory and motor systems of the spine is 'mapped' onto specific areas. The largest areas with greatest numbers of nerves are those for the hands and the face, including those concerned with vocalisation.

NEXT Nerves.