The sympathetic and parasympathetic nervous system

The autonomic nervous system is actually two separate systems: the sympathetic nervous system and the parasympathetic nervous system. It is mostly through increased sympathetic activity that the stress response is activated. The parasympathetic nervous system also has a role to play here because of the corresponding decrease in its activity.

The role of the parasympathetic nervous system is to conserve energy, to aid digestion and to defend the body from the invasion of foreign material such as bacteria. Increasing activity in the parasympathetic nervous system results, for example, in increased secretion of the eyes (tears), mouth (saliva), nose and lungs (mucus) and stomach (acid gastric juice). These secretions trap and destroy foreign material (Figure 7).

Figure 7 summary of autonomic nervous system activity

The action of the sympathetic nervous system is largely brought about by a neurotransmitter called noradrenaline. Parasympathetic action is mainly brought about by a neurotransmitter called acetylcholine.

Most body organs receive messages from both divisions of the autonomic nervous system. However, there are some which receive messages from only one of the branches. For example, the adrenal medulla, spleen and sweat glands receive messages from the sympathetic division only, whilst some of the salivary glands receive only a parasympathetic supply. This pattern of nerve supply distribution is important in determining body organ activity and allows a variety of different responses to be made by the body depending on the circumstances.

The table on page 44 shows the effect on body organ activity of stimulating the parasympathetic and sympathetic nerves.

You will notice that sympathetic stimulation causes increased activity in some organs, for example heart muscle, but reduced activity in others, such as gut muscles. Similarly, parasympathetic stimulation increases activity in some organs, such as gut muscles, but decreases activity in others, for example heart muscle. Where an organ has both sympathetic and parasympathetic nerve supplies, sympathetic stimulation increases the activity whereas parasympathetic stimulation decreases activity. For example, an increase in sympathetic activity while parasympathetic activity is low will result in a CD higher heart rate than normal. This is what happens if we increase our level of muscular activity, for example in exercising.

On the other hand, an increase in parasympathetic activity while sympathetic activity is low will result in a lower heart rate; this happens when we are rested and relaxing.

Most organs of the body have a dominant control by either the sympathetic or parasympathetic systems. For example, the heart and blood vessels are controlled predominantly by the sympathetic nervous system.

Blood flow throughout the body is regulated almost entirely by increasing or decreasing sympathetic activity. The directing of blood to the muscles during exercise is an important aspect of the stress response as we shall see later.

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