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ОглавлениеCHAPTER 2 How Our Nervous System Works
Our nervous system consists of two main parts, known as the voluntary nervous system and the involuntary nervous system.
THE VOLUNTARY NERVOUS SYSTEM
This part directs the movement of the limbs, head and trunk, and we control it more or less as we wish, hence its name. It consists of the brain and spinal cord, from which a number of paired nerves arise, each ending in the muscle it supplies.
THE INVOLUNTARY NERVOUS SYSTEM
This second part controls the internal organs – heart, blood-vessels, lungs, intestines, etc., even the flow of saliva and sweat. It has its headquarters in a brain centre connected with a delicate network of fibres lying on either side of the spinal column (backbone), from which numerous threadlike branches pass to the internal organs. This second part is not under our immediate control but – and this is of paramount importance in understanding ‘nerves’ – it responds to our moods. For example, when we are afraid our cheeks blanch, our pupils dilate, our heart beats quickly and our hands may sweat. We do not consciously react like this and we have no power to stop these reactions other than to change our mood. We therefore call this part of the Involuntary Nervous System.
The involuntary nervous system itself consists of two parts, the sympathetic and the parasympathetic. Of these, the sympathetic ‘sympathizes’ more demonstratively with our moods, hence its name. The action of the sympathetic nerves strengthens an animal’s defences against the various dangers which beset it, such as extremes of temperature, deprivation of water, attack by its enemies.
Have you ever seen a frightened animal standing stock-still from fear before taking flight? Its nostrils and its pupils dilate, its heart races, it breathes quickly. The sympathetic division of the involuntary nervous system has prepared it for fight or flight.
The pattern of fear
We human beings react in the same way when afraid. Fear begins as an impulse in our brain which excites the sympathetic nerves to stimulate various regions (skin) and organs (heart, lungs, intestines) to produce the signs and symptoms of fear – the sweating hands, racing heart, quick breathing, dry mouth, etc. The sympathetic nerves do this by means of a substance called adrenalin, which is released at the nerve terminals in the organs concerned. Also, our two adrenal glands, themselves under sympathetic nervous stimulation, secrete additional adrenalin into our bloodstream to enhance the action of the sympathetic nerves.
When we are afraid we also feel a horrible sensation in the ‘pit of the stomach’. This is the most distressing component of fear. However, the complete picture of fear includes all the symptoms induced by adrenalin, the sweating hands, racing heart, heaving chest, etc., as well as the spasm of fear felt in our ‘middle’.
Normally we do not feel our body functioning, because parasympathetic nerves hold the sympathetic nerves in check. It is only when we are overwrought (angry, afraid, excited) that the sympathetic nerves dominate the parasympathetic and we are conscious of certain organs functioning. A healthy body without stress is a peaceful body.
Most of us associate kindness and understanding with the word sympathetic, and as the reactions of the sympathetic nerves can be anything but kind, some people find it difficult to reconcile themselves to the term ‘sympathetic nerves’. Therefore, to avoid any bewilderment, I shall henceforth refer to the sympathetic nerves as the adrenalin-releasing nerves – which, after all, is what they are.
So, briefly, we have a Voluntary Nervous System by means of which we move our body; an Involuntary Nervous System, consisting of adrenalin-releasing and parasympathetic divisions which control the functions of our internal organs, each part balancing the other. Normally we do not feel our involuntary nervous system working, but when we are overwrought the adrenalin-releasing nerves are especially stimulated and we may feel our heart beat quickly, our hands may sweat and our mouth may feel dry. In addition, our stomach may ‘churn’, we may feel breathless, giddy, and may have an urgent need to retire to the toilet.
