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Autonomic nervous system

While the somatic nervous system[?] regulates body functions which can be affected by our will like the motor functions of the skeletal muscle[?] or the perception of sensory stimuli, the autonomic nervous system concentrates on controlling the involuntary functions of the organs like digestion, breathing, metabolism or blood pressure. Although these actions cannot be controlled by our will, they are influenced by our state of mind, and we can become aware of them, mainly when something does not work properly, causing us to feel pain.

The autonomic nervous system is divided into three elements which act together, either in a synergistic[?] or an antagonistic way[?]. They are called the sympathetic, the parasympathetic and the enteric nervous system.

The sympathetic nervous system is responsible for the provision of energy needed for emergency situations like hunger, fear or extreme physical activity. Therefore the sympathetic neurons induce glycogenolysis[?] in liver cells and lipolysis[?] in the adipose tissue[?], raise blood pressure, heart rate[?] and the blood supply in the skeletal muscle at the expense of the gastrointestinal tract and the skin, dilate both the pupils of the eyes[?] and the bronchioles[?], providing the person with sufficient visibility and oxygen.

After having mastered such a challenging situation the body is exhausted and needs to rest, recover and gain new energy. These tasks are under the control of the parasympathetic nervous system, which lowers the heart rate and the blood pressure, diverts a great amount of blood back to the skin and the gastrointestinal tract, contracts the pupils of the eyes as well as the bronchioles, stimulates secretion in the salivary glands and accelerates peristalsis.

The enteric nervous system is situated in the intestinal wall. Although being able to operate on its own, it is modulated by sympathetic and parasympathetic nerve fibres which are connected to its intramural plexus[?], the submucous and the myenteric plexus. It regulates secretion in the enteric glands, regeneration of the intestinal epithelium[?] and the contraction of the smooth muscle[?] cells, thus motility[?].

In contrast to the somatomotoric nerve fibres, which consist of only one neuron, the sympathetic and parasympathetic fibres have two, a preganglionic and a postganglionic nerve cell connected by a ganglion. Here the nerve impulse is transferred from the preganglionic cell to the postganglionic by the chemical transmitter acetylcholine, which is released from the first neuron and binds to a nicotinic receptor[?] of the second. This neuron, then, transfers the impulse to an effector cell, in most cases a smooth muscle cell or a gland cell by using a second neurotransmitter, which is acetylcholine again in parasympathetic fibres and noradrenalin[?] in the sympathetic nervous system. Preganglionic sympathetic fibres also end in the adrenal medulla[?], which can be regarded as the biggest sympathetic ganglion, but as there are no postganglionic cells the adrenal gland releases the second neurotransmitter noradrenalin or more often adrenalin[?] into the blood stream.

All somas of preganglionic autonomic nerve cells are situated in the central nervous system. As regards the sympathetic nervous system they lie in the thoracal and lumbal segments of the spinal cord, the preganglionic parasympathetic cell bodies are situated in the brain stem (cranial parasympathetic) and in the sacral spinal cord (sacral parasympathetic). The axons of the sympathetic and sacral parasympathetic fibres leave the spinal cord through its ventral root, enter the spinal nerve and then the white communicating branch, from where they run to their further destination.

The sympathetic axons build a chain of 22 ganglions, the so-called trunk of the sympathetic nerve[?], on both sides of the spinal column[?], from where the splanchnic nerves run to the prevertebral ganglia, which lie in front of the aorta at the place where its unpaired visceral branches leave the large vessel. The left and right trunk of the sympathetic nerve fuse to an unpaired ganglion in the pelvic area. The organs innervated by sympathetic fibres are numerous and manifold, heart, lungs, esophagus, stomach, small and large intestine, liver, gallbladder and genital organs.

All these organs are also innervated by the parasympathetic nervous system. The digestive system from the lower part of the colon downwards is regulated by the sacral parasympathetic fibres which form pelvic ganglia, the upper parts of it are controlled by the vagus nerve, the biggest element of the cranial parasympathetic system, whose fibres, after having left the brain stem, accompany four cerebral nerves before they build the cranial parasympathetic ganglia, which innervate eye muscles and salivary glands.

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