The autonomic nervous system is divided into two parts. These are: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system originates in the spinal cord at the levels T1 to L3 where T stands for the thoracic region of the spinal cord and L stands for the lumbar region of the spinal cord.
The parasymathetic division of the autonomic nervous system has its origin in two parts. These are: the nuclei of the brain stem and in the sacral part of the spinal cord.
The neurotransmitter that mediates the action of the parasympathetic nervous system is nicotinic acetylcholine in preganglionic neurons and muscarinic acetylcholine in postganglionic neurons.
In the sympathetic division of the autonomic nervous system the neurotransmitter that mediates its action is nicotinic acetylcholine in preganglionic neurons and norepinephrine in postganglionic neurons.
The part of the parasympathetic nervous system that is of unusual importance is the vagus nerve or cranial nerve X. This nerve innervates the gastrointestinal tract in addition to the larynx with parasympathetic nerves which stimulates peristaltic action and also stimulates digestion and secretion of digestive juice.
The intestine has an intrinsic nervous system which is called the enteric nervous system. It synapses with neurons from the vagus nerve that innervate the stomach and the intestine with parasymathetic nerves. The sympathetic neurons also synapse with neurons in the enteric division. Thus providing symathetic innervation to the gastrointestinal tract also.
Both the sympathetic and parasympathetic nervous systems are regulated by centers in the central nervous system that are located in the hypothalamus and the brain stem.
Organs in the body and which they are innervated by sympathetic nerves include the adrenal medulla which upon stimulation by the sympathetic neurons stimulates the release of norpinephrine and epinephrine to the blood circulation which mediate stress situation and cause blood vessels constriction. In addition it causes increased heart rate and increased sweating. There is no apparent role to the parasympathetic division on the adrenal medulla.
Tears secretion is mostly affected by the parasympathetic nervous system. The sympathetic division causes few secretion from the lacrimal glands.
The pancreas is stimulated by the parasympathetic nervous system to secrete insulin which in turn leads to the polymerization of glucose into the polymer glycogen. In addition the pancreas is stimulated by the parasympathetic division to secrete pancreatic juice. This is in contrast to the sympathetic division which inhibits the secretion of insulin and pancreatic juice. It stimulates the release of glucagon which stimulates the production of glucose from the polymer glycogen.
The parasympathetic nervous system stimulates the constriction of the pupils of the eyes through the action of the oculomotor cranial nerve. While the sympathetic nervous system stimulates the widening of the pupils of the eyes by contracting the radial muscle of the iris of the eyes.
Sweat secretion is predominantly mediated by the sympathetic nervous system. There is no known effect of the parasympathetic division on sweat secretion.
The skin hair and skin hair muscle are mediated by the symathetic division of the nervous system in which hair erects in respone to norepinephrine secretion and hair muscle contract also in response to the action of the sympathetic nervous system.
Salivation is predominantly stimulated by the action of the parasympathetic nervous system. Vasodilatation of the salivary gland arterioles lead to water secretion. The sympathetic division leads to the stopping of salivation.
Skeletal muscles are contracted by the stimulation of the sympathetic neurons accompanied by vasocnstriction. There is no apparent role to the parasympathetic nervous system in muscles contraction.
The function of the two kidneys is mediated by the sympathetic nervous system in which constriction of the kidney blood vessels lead to decreas in the amount of urine excreted in the kidney tubules. There is no known function to the parasympathetic division in kidney filtration function.
The urinary bladder function is mediated by both sympathetic and parasympathetic neurons in which parasympathetic neurons stimulate the contraction of the bladder muscle wall and relaxes its sphincter. Thus promoting urination. The sympathetic neurons mediate the opposite function. Thus promoting urine retention.