Organization and role of the autonomic nervous system

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The autonomic nervous system (ANS) is that part of our nervous system which controls all the vital functions and other physiological processes which mostly do not come under voluntary control.As the word autonomy indicates it is a self regulating part of the nervous system.

Without the proper functioning of the autonomic nervous system the body will not be able to adjust to changing circumstances and survival of the organism will be in danger.

For example when we get up after lying down for some time blood tends to accumulate in the blood vessels of the lower limbs due to gravity. This will significantly reduce the blood flow to the brain resulting in impairment of its function.

But the ANS immediately swings into action causing constriction of the blood vessels of the lower limbs and ensuring proper blood supply to all the organs.

When the atmospheric temperature is very high blood vessels in the skin dilate allowing more blood to flow into the skin causing perspiration and heat loss so that the core temperature remains stable.

If the atmospheric temperature is very low the blood vessels of the skin constrict conserving heat. Shivering is induced so that the muscular contractions produce more heat.

There are two divisions of the ANS called sympathetic nervous system and parasympathetic nervous system. The two have opposite effects on various organs and a balance between the two is essential for proper functioning of the body. Some of the actions of the two divisions of the ANS on various organs are as follows.

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autonomic nervous system

Target organSympathetic systemParasympathetic system
Eyes Dilates pupils Constricts pupils
Lungs Dilates bronchi Constricts bronchi
Heart Heart rate and force of contraction increased Heart rate and force of contraction decreased
Salivary glands Saliva production reduced Saliva production increased
Urinary bladder bladder wall relaxed sphincter closedbladder wall contracted sphincter opened

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autonomic nervous system summary

The sympathetic nervous system supplies the glands of the body, all the smooth muscles (smooth muscles are found in blood vessels, intestines and bronchi and are not under voluntary control unlike skeletal muscles) and the muscles of the heart.

sympathetic nervous system

The fibers leave the central nervous system through cranial and spinal nerves. After leaving the central nervous system they end in masses of neurons called ganglia (sympathetic ganglia). These fibers are called pre-ganglionic fibers. From these ganglia post-ganglionic fibers arise and supply the organs.

The pre-ganglionic fibers of the sympathetic system secrete acetylcholine as neurotransmitter while the post-ganglionic fibers secrete noradrenaline except in sweat glands where they secrete acetylcholine.

Sympathetic fibers of the oculomotor nerve (III cranial nerve) arise in the cells of the oculomotor nerve in the tegmentum of the midbrain. They leave the oculomotor nerve and join the ciliary ganglion forming synapses with the neurons of the ganglion.

From the ganglion post-ganglionic fibers arise and reach the eye ball as short ciliary nerves. These fibers supply the muscles responsible for accommodation and causing dilation of the pupil.

Sympathetic fibers of facial nerve (VII cranial nerve) start from the nucleus in the pons and passing in the facial nerve leave through its branches and end in submaxillary and sphenopalative ganglia.

Post-ganglionic fibers arise from these nuclei and supply the salivary glands, mucous membrane of the mouth and nose.

The sympathetic fibers of the glossopharyngral nerve (IX cranial nerve) start in the nucleus in the medulla and end in the otic ganglion. Post-ganglionic fibers arise in the otic ganglion and supply the parotid salivary gland and glands of the tongue.

Thoraco-lumbar sympathetic fibers: They arise in the lateral column of the spinal cord and pass through the anterior roots of the thoracic and upper two or three lumbar nerves.

The pre-ganglionic fibers leave the anterior roots and join the ganglia in the sympathetic chains. They are called white rami communicantes as they are myelinated and look white.

sympathetic nerve white rami

There are two sympathetic chains one on either side of the spine. As the name indicates they are elongated structures made up of bundles of nerve fibers interspersed with masses of neurons called ganglia.

Each chain has about 23 ganglia. The upper end of the chain continues into the skull forming a network on the internal carotid artery. The lower end joins that of the other chain and ends in a ganglion called ganglion impar which lies in front of the coccyx.

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sympathetic chains

From the neurons in the ganglia post-ganglionic fibers arise from the neurons in these ganglia and pass through gray rami communicantes to all the spinal nerves. They are called gray because they are not covered with myelin and look gray. Through the spinal nerves they supply the glands in the skin and the blood vessels.

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sympathetic fibers postsynaptic

The thoracolumbar sympathetics have two types of ganglia-central and collateral. The central ganglia are found along the sympathetic chains. The collateral ganglia are found in three networks of nerves in front of the spine called the prevertebral plexuses in thorax, abdomen and pelvis. They are called the cardiac, celiac and hypogastric plexuses respectively.

The cardiac plexus is situated near the base of the heart and has superficial and deep parts. It supplies the heart and the great blood vessels.

The celiac plexus (solar plexus) is the largest of the three and is situated at the level of the first lumbar vertebra. It has two large ganglia called the ciliac ganglia and a network of nerve fibers. It is situated behind the stomach and supplies the abdominal organs.

The hypogastric plexus is situated in front of the last lumbar vertebra and the sacral promontory. It supplies the organs of the pelvis.

The parasympathetic division of the ANS also follows the scheme of pre and post-ganglionic fibers. In contrast to the sympathetic division the ganglia of the parasympathetic division are situated close to the target organs and the post-ganglionic fibers are short.

The parasympathetic system is responsible for conservation of energy and assimilation of ingested food. It slows heart rate and increases peristaltic movements in the intestines. In contrast the sympathetic system goes into overdrive whenever a person is exposed to any kind of stress.

The pre-ganglionic fibers of the parasympathetic division arise from the neurons in the nuclei of III, VII, IX and tenth cranial nerves and also from the second, third and fourth sacral segments of the spinal cord. Hence it is also called cranio-sacral outflow.

Parasympathetic Nervous System

The parasympathetic fibers of the III cranial nerve are responsible for causing constriction of the pupils of the eyes. Those in the VII nerve affect the secretion of the salivary glands.

The vagus nerve has very extensive distribution. Its parasympathetic fibers supply the heart (slowing of heart rate and force of contraction), lungs (cause narrowing of airways), stomach and upper intestine (increase peristaltic movements) and ureters (help in expulsion of urine).

The sacral fibers form network of fibers called pelvic plexus and supply lower intestine, bladder and reproductive organs.

The parasympathetic fibers use only acetylcholine as neurotransmitter.

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Page last reviewed on 2nd January 2011

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