The autonomic nervous system comprises:
- Sensory neurons that convey information from the body’s internal organs to the
central nervous system. The resulting sensations are not normally consciously perceived
(they relate, for example, to the level of carbon dioxide in the blood), although pain is
an exception to this rule.
- Motor neurons, that regulate visceral activities (those of the internal organs) by
either increasing (exciting) or decreasing (inhibiting) organ function – for example,
autonomic motor neurons can either increase, or decrease, heart rate.
The output (motor) component of the ANS can be divided further – into the
sympathetic and parasympathetic nervous systems. Generally speaking, the two systems
innervate (or provide a nerve supply to) exactly the same organs – they simply
regulate them in different – mainly opposite – ways:
- the sympathetic nerves increase visceral activity (e.g. heart rate up)
- the parasympathetic nerves decrease activity (e.g. heart rate down)
As a result of this, the sympathetic nervous system is most active during periods of
exercise – conversely, the parasympathetic nervous system is most active during
periods of rest and relaxation.
Sympathetic Nervous System
