Autonomic nerves regulates the functions the viscera, including the heart, stomach and intestines. We are often unaware of the autonomic functions because they are involuntary - generally speaking not under the control of the will. we are generally unaware of our heart beat or the contractions of the stomach. However some autonomic functions do involve voluntary control, such as the emptying of the bladder.
Some individuals can train themselves to influence their heart beat, but in the main, the autonomic system remains an involuntary system, dependent on reflexes to modulate its effects. |
One of the important functions of autonomic nerves is in emergency, frightening, situations that require us either to stand up and fight, or flee, i.e.to take flight. This fright, flight or fight behaviour is dependent on massive activation of the sympathetic nerves.
The mass activation of sympathetic nerves increases heart rate and cardiac output to supply the muscle with oxygen, takes blood away from organs not involved in musclar exercise and sends that blood to the muscles involved in fight or flight. The adrenal medulla is also activated releasing adrenaline into the blood stream, which aids the changes already mentioned, but also releases glucose form the liver and fat from adipose tissue, to the metabolic substrated for muscle become redily available.
The behaviour also includes obvious signs of aggression - piloerection, changes in skin colour, dilatation of the pupils, sweating etc. |
At REST however, the sympathetic system does not produce the massive autonomic activity of the flight or fight behaviour.
The sympathetic controls skin blood flow and the activity of sweat glands, both means of losing heat from the body, and these activities go on without us thinking about them.
Similarly, blood pressure is controlled largely by the sympathetic system in an involuntary manner.
At REST, parasympathetic activities include digestion and achieve an orderly sequence of events associated with relaxation. |
Neurotransmitters
The functional differences between sympathetic and parasympathetic post-ganglionic neurones occur because they use different neurotransmitters.
The main transmitter produced and released by sympathetic post-ganglionic neurones is noradrenaline (sometimes called norepinephrine), whereas in parasympathetic post-ganglionic neurones the neurotransmitter is acetylcholine.
In contrast, the neurotransmitter release by the pre-ganglionic neurones, sympathetic and parasympathetic is acetylcholine in both sorts of ganglion.
In addition some autonomic nerves contain other axons that release transmitters that are neither noradrenaline nor acetylcholine; these are non-adrenergic non-cholinergic (NANC) nerves.
One example of the activity of NANC nerves is in the receptive relaxation of the stomach, during which the stomach relaxes in order to receive food from the oesophagus. |
* |
Generally speaking, internal organs receive a nerve supply from both divisions of the autonomic nervous system, but two important exceptions are the smooth muscle of blood vessels and the ventricles of the heart, which are only innervated by the sympathetic nerves.
Reciprocal Actions
When both divisions of the autonomic nervous system innervate an internal organ, they commonly have antagonistic, opposing functions. |
Adrenal Medulla
The adrenal medulla consists of modified post-ganglionic sympathetic neurones that secrete their transmitter, mainly adrenaline, into the blood stream.
Adrenaline is a hormone that is secreterd into the bloodstream and circulates to reach its target organs. The actions are therefore slower (because of the time taken for blood to circulate) and generalised (because blood reaches all tissues of the body).
|