It is an offshoot of the visceral motor division of the central nervous system, it has visceral motor neurons which differ from somatic motor neurons of the cranial and spinal nerves in that they are not connected directly with effector organs but through two neurons, one from the brain or spinal cord to an autonomic ganglion where it forms synapse with second neuron which goes to an effector organ (muscle, visceral organ, or gland).
Because of these synapses the visceral motor fibres are of two types; the first neurons are preganglionic whose cell bodies are located in the gray matter of the brain or spinal cord, their fibres are modulated and go through ventral roots of spinal nerves and white rami communicates to the autonomic ganglia.
The second neurons are postganglionic which form synapses by their dendrites with the first neurons in autonomic ganglia; they go to the effector organs.
The postganglionated neurons have non-medullated axons and go through gray rami communicantes and spinal nerves.
Langley distinguished two divisions of the vertebrate autonomic nervous system, the sympathetic and the parasympathetic nervous systems.
In mammals these two divisions are distinct anatomically physiologically and pharmacologically. The sympathetic arises from the thoracico-Iumbar region of the central nervous system, the parasympathetic from the cranio-sacral region.
Typically, each visceral organ receives both sympathetic and parasympathetic fibres ; one is excitatory, while the other is inhibitory.
1. Sympathetic system is also called thoraco-lumbar out-flow because its preganglionic fibres join the spinal cord only in the thoracic and lumbar regions.
It consists of two sympathetic nerves or chains running from the head to the end of the sacral region, one on each side of the vertebral column.
Each sympathetic chain bears several ganglia called lateral or chain ganglia, some chain ganglia fuse to form three ganglia in the neck known as superior, middle, and inferior cervical ganglia, after which there is a linear series of ganglia in the thoracic and lumbar regions.
Other sympathetic ganglia (coe- liac, superior and inferior mesenteric) lie in the viscera and are collectively known as prevertebral ganglia.
All these ganglia are connected to spinal nerve through rami communicantes. Preganglionic fibres arise from the spinal cord and pass through ventral roots of spinal nerves and white rami communicates of all the thoracic spinal nerves and first three lumbar spinal nerves and then go to the chain ganglia.
Some preganglionic fibres pass through the sympathetic chain of each side and go to the head to communicate with some cranial nerves.
Some other preganglionic fibres arise from lateral ganglia and go to prevertebral ganglia in the viscera.
From the lateral or prevertebral ganglia arise non-medullated post-ganglionic fibres going to visceral organs under the involuntary contrr’ (nose, eye muscles, salivary glands, heart, larynx, trachea, bronchi, lungs, alimentary canal, liver pancreas, adrenal glands, kidneys; bladder, and gonads).
The postganglionic fibres form plexuses in collateral ganglia, such as solar plexus in the coeliac ganglion. Some postganglionic fibres go back through spinal nerves and gray rami communicantes to the skin, small blood vessels, erector hair muscles, and sweat glands.
The postganglionic fibres of the sympathetic system secrete sympathin which generally stimulates these organs, e.g., the fibres dilate the pupils and bronchi, increase heart beat, decrease secretion of saliva and digestive juices, temporarily reduce peristalsis, contract hair muscles causing hairs to stand up; cause sweat glands to secrete.
All these reactions are usually associated with fear, anger, and pain, they cause expenditure of energy.
2. Parasympathetic system also called craniosacral out-flows because some cranial nervps-and sacral spinal nerves are involved.
It consists of ganglia, preganglionic, and postganglionic fibres. The preganglionic fibres are joined to cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal), X (vagus), and to the second, third, and fourth sacral spinal nerves.
The preganglionic fibres having their cell bodies in the mid-brain and medulla go to four ganglia in the head, those of the III cranial nerve to a ciliary ganglion, those of the VII nerve to sphenopalatine and submaxillary ganglia, those of the IX nerve to an otic ganglion, and those of the X nerve to very small ganglia situated in the organs concerned.
The preganglionic fibres having their cell bodies in the gray matter of spinal cord pass through the sacral spinal nerves 2 to 4 and go directly to a pelvic ganglion without passing through the sympathetic chain.
Thus the preganglionic fibres of cranial and sacral nerves terminate in ganglia located in or close to the organ supplied by them, hence these fibres are rather long.
From the ganglia arise very short postganglionic fibres, those from the ciliary ganglion go to eye muscles, those from sphenopalatine ganglion to lacrimal glands and nose, those from submaxillary ganglion to salivary glands, those from otic ganglion to salivary glands, other very short postganglionic fibres arise from small ganglia located in the organs (innervated by the vagus) and go to the heart, larynx, trachea, bronchi, lungs, alimentary canal, liver, pancreas, adrenal glands, kidneys, bladder, and gonads.
The postganglionic fibres from the pelvic ganglion go to the colon, kidneys, bladder, and gonads, thus the postganglionic fibres are very short.
The preganglionic parasympathetic fibres secrete acetylcholine, it has an inhibitory effect on organs which is antagonistic to that of the sympathetic system, e.g., they constrict the pupils and bronchi, decrease the rate of heart beat, increase secretion of saliva and digestive juices, increase peristaltic movements, and stop secretions of glands.
All these reactions are associated with comfortable sensations in the body, they also conserve energy.
Another peculiarity of the autonomic system is a double innervation of organs from the sympathetic and parasympathetic systems, the two systems work antagonistically in ord6r to control the functions of all involuntary mechanisms in the body.
A fibre of the sympathetic system is generally stimulatory and it starts an action in an organ, then the fibre of the parasympathetic system to the same organ stops that action after a time, hence the sympathetic and parasympathetic systems are antagonistic, e,g., the iris is dilated by stimulation of a sympathetic nerve and is contracted by stimulation of its parasympathetic nerve.
The autonomic nervous system regulates the functions of those organs which are not under the control of the will, though its actions are very slow.
This system controls many involuntary actions, such as heart beat, respiration, digestion, secretions of giands, and excretion, it controls processes continued over long periods of time.
The nerve centres controlling the autonomic nervous system are located in the hypothalamus through which the system forms connections with other nervous tissues.
Some neurons of the autonomic nervous system form the medulla of the adrenal glands, these are called chromaffin cells. Secretion of the medulla of adrenal glands called adrenaline or epinephrine stimulates the sympathetic but not the parasympathetic nerves.
The nerve endings of sympathetic fibres secrete sympathin which is very similar to adrenaline, and reactions caused by sympathin are similar to those produced by adrenaline, either substance can stimulate the organs supplied by sympathetic fibres.
Similarly stimulation of the parasympathetic fibres produces acetylcholine, the acetylcholine from the vagus slows the heart beat, it can be neutralized by adrenaline. Hence the stimulation of the autonomic nervous system produces its effects by releasing some chemical substance in the effector organs, as also in the case of endocrine glands, thus the nervous and chemical stimulations appear to be the same, though the effects of nervous stimulation are very rapid than to those of hormones.