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(Autonomous: Gr. Autos= self + nomos, law)

Autonomic nervous system (ANS) is the part of nervous system concerned with the innervation of involuntary structures such as cardiac muscle, smooth muscles and exocrine glands.
ANS is distributed throughout the central and peripheral nervous systems.
Autonomic nervous system (ANS) is concerned with maintaining a constant internal environment (homeostasis).
It does so by making fine adjustments in certain bodily functions.
It does so by controlling smooth muscle, the secretion of glands and modulation of cardiac rhythm.

The term ‘autonomous’ is deceptive. Autonomic nervous system (ANS) responds quickly to changes in bodily activities. Its functions are organized and regulated in central nervous system (CNS). The concept of autonomy is mainly functional.
The activities of ANS normally do not impinge upon the conscious of the individual

 


Autonomic nervous system has
► Afferent neuron
► Connector neuron
► Efferent neuron

Afferent impulses originate in visceral receptors of cardiac muscle, smooth muscles and exocrine glands
Visceral receptors are
Chemo-receptors [Gr. chemeia chemistry + L. recipere to receive, accept]
A receptor adapted for excitation by chemical substances e.g., olfactory and gustatory receptors, or a sense organ, as the carotid body or the aortic bodies, which is sensitive to chemical changes in blood, especially reduced oxygen content, and reflexly increases both respiration and blood pressure
Baro-receptors [Gr. baros weight + L. recipere to receive, accept]
A sensory nerve ending that is stimulated by changes in pressure, as those in the walls of blood vessels
Osmo-receptors [Gr. osmos impulse + L. recipere to receive, accept]
A specialized sensory nerve ending that is stimulated by changes in osmotic pressure of the surrounding medium

These impulses travel through afferent neurons to central nervous system
In CNS these impulses are integrated through connector neurons
Finally these impulses leave CNS via efferent neurons to visceral effector organs

Anatomically, ANS is formed by a collection of nerve cells located in CNS through cranial and spinal nerves, and ganglia located in the paths of these nerves.

Efferent pathways of ANS are made up of 2 neurons. The 1st neuron is located in CNS. Its axon synapses with a 2nd multipolar neuron, located in a ganglion of the peripheral nervous system.
The axons of the 1st neuron are called preganglionic fibers
The axons of the 2nd neuron to muscle or gland are called postganglionic fibers

One preganglionic neuron, through its axon, may synapse with the dendrites of many postganglionic neurons
This makes the control, exerted by ANS, very rapid

The chemical secreted at all preganglionic endings and parasympathetic postganglionic endings is acetylcholine, which is released from nerve endings by nerve impulses.

The only organ that receives preganglionic fibres is the adrenal medulla and it behaves like a ganglion, whose cells have changed into secretory cells rather ganglion cells



Autonomic nervous system is composed of
Sympathetic
Parasympathetic

These differ in morphology and function
Sympathetic nervous system is concerned with emergency situation (“fight or flight” reaction)
Parasympathetic nervous system is concerned with conservation of energy

Sympathetic preganglionic fibers pass out through thoracic and lumbar spinal nerves and this is known as ‘thoracolumbar outflow’. The cell bodies of these fibers lie in lateral gray horns of the spinal cord from thoracic1 to lumbar2

 

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Cross section of spinal cord at the level of fifth thoracic segment
The H-shaped grey matter shows three horns. Look at the lateral horn. This is the site for the cell bodies of sympathetic connector neurons.

The cell bodies of postganglionic sympathetic neurons are mostly located in the ganglia of the sympathetic trunk or ganglia in more peripheral plexuses. They almost always lie closer to the spinal cord than to the areas innervated. This, however, is not true for those that innervate viscera of the pelvis

Parasympathetic preganglionic efferent fibers pass out via certain cranial & sacral nerves; this is known as ‘craniosacral’ outflow. The cell bodies of these fibers lie in the motor nuclei of third, seventh, ninth and tenth cranial nerves and in the gray matter of sacral 2 to 4 segments of spinal cord
The cell bodies of postganglionic parasympathetic lie close to the structures supplied or often distributed in the walls of the innervated viscus

The motor part of somatic nervous system is concerned with the innervation of skeletal muscles.
The cell bodies, in somatic nervous system, are in the motor nuclei of cranial nerves and in the anterior horn cells of spinal cord
The nerve fibers which leave the central nervous system run uninterruptedly to the muscle fibres. There is single neuron

The great difference between autonomic nervous system (ANS) and the somatic system is that the pathway from nerve cells in central nervous system (CNS) to the target organ is interrupted by synapses in a ganglion.
There are two neurons in ANS with preganglionic and postganglionic fibers

The cell bodies of preganglionic fibers are always in central nervous system (CNS)
In sympathetic system, they are in lateral horn of spinal cord of all thoracic and the upper two lumbar segments (sometimes third lumbar segment).
In parasympathetic system, they are in third, seventh, ninth and tenth cranial nerves’ nuclei and in lateral horn of spinal cord of second, third and forth sacral segments.

The cell bodies of postganglionic fibers are in ganglia outside CNS
In sympathetic system, the ganglia are either in sympathetic trunk or in collateral ganglia (such as coeliac ganglia).
In parasympathetic system, the ganglia are terminal ganglia. They are usually within the walls of the viscera concerned.
There are four ganglia which are some little distance from the structures innervated and they are ciliary, pterygopalatine, submandibular and otic ganglia.

Autonomic nervous system (ANS) has two components
Sympathetic nervous system
It prepares the body for an emergency. It accelerates the heart rate, causes constriction of peripheral blood vessels and raises the blood pressure. It brings about a redistribution of the blood, so that blood leaves the areas of the skin and intestine and becomes available to brain, heart, and skeletal muscle. At the same time it inhibits peristalsis of intestinal tract and closes the sphincters.

Parasympathetic nervous system
It conserves and restores energy. It slows the heart rate, increases peristalsis of intestines, increases glandular activity and opens the sphincters.