Introduction to Autonomic Nervous System
The autonomic nervous system (visceral, vegetative or involuntary nervous system) is widely distributed throughout the body. It controls tissues, smooth muscles, heart and glands, which are not under voluntary controls. The autonomic nervous system is dependent on the central nervous system with which it is connected by effarent and afferent nerves.
The autonomic nervous system is divided functionally into two parts:
A. The sympathetic system (thoraco-lumber out flow) – Which lies in front of the vertebra column and is associated and connected with the spinal cord by nerve fibers.
B. The parasympathetic system (cranio-sacral out flow) – Which is divided into two parts composed of the cranial and sacral autonomic nerves.
The sympathetic and parasympathetic mostly exhibit mutual antagonism.
The sympathetic system is concerned with preparing the organisms for emergency (fight or flight) the sympathetic is not essential for normal existence.
The parasympathetic is mostly concerned with the vegetative functions e.g. motility and secretion of gastro intestinal tract. This system is essential for the normal existence of the organisms.
Both the sympathetic and parasympathetic essentially consist of:
1. A preganglionic nerve
2. A ganglion
3. A post ganglionic nerve
4. An effectors organ
A. Parasympathetic System:
The cranial autonomic are the third, seventh, ninth and tenth cranial nerves. This form the means by which the parasympathetic fibers pass cut from the brain the organs partly controlled by them.
The transmitter in preganglionic parasympathetic nerve is acetyl choline (ACH) which is liberated at the ganglion.
Again acetyl choline is the transmitter in postganglionic parasympathetic nerve endings.
By means of the third cranial nerve, the oculomotor nerve fibers rich the circular muscular fibers of the iris, stimulating the movements which determine the size of the pupil of the eye. By the seventh nerve, the facial and the ninth, the glossopharyngeal nerve, motor secretary fibers reach the salivary glands. The vagus or tenth cranial nerve is the largest autonomic nerve. It has a very wide distribution and sends fibers to a number of glands an organ.
The sacral parasympathetic nerves pass out from the sacral region of the spinal cord. These form the nerve to the pelvic viscera and together which the sympathetic nerve from the plexuses which supply the colon, rectum and bladder.
B. Sympathetic system:
The sympathetic system consist of double chain of ganglionated cords extending from the base of the skull, lying in front of the vertebral column to end the pelvis opposite the coccyx. These ganglia are arrange in pains and distributed from the follow regions:
In the neck – Three pairs of cerebral ganglia
In the chest – Eleven pairs of thoracic ganglia
In the loins – Four pairs of lumber ganglia
In the pelvis – Four pairs of sacral ganglia
Front of coccyx – The ganglion impar block
These ganglia are intimately connected with the CNS through the spinal cord by means of communicating braches, which pas outwards from the cord to ganglia and inwards from ganglion to cord. Other sympathetic ganglia are placed in relation to those-
The cardiac plexus is placed near the base of the heart and send braches to it and to the lungs.
The coelise plexus lies in front of the sacrum and supplies organs in the pelvis.
The transmitter in preganglionic sympathetic nerve is acetyl choline. But in the post ganglionic sympathetic nerves, noradrenalin is the transmitter. It is liberated at the post ganglionic sympathetic nerve ending.
Sympathetic nerves supply innervations to the muscle of the heart, the involuntary muscle of all blood vessels, and of viscera such as the stomach, pancreas and intestines. It supplies motor secretary fibers to the sweat glands.
A system of dual control (Sympathetic & Parasympathetic):
The majority of organs have a dual supply receiving some fibers from the sympathetic system and some from the cranial or sacral autonomic nerves the activity of the organ being stimulated by one set of nerves and inhibited by the other set each-acting in antagonism to the other.
The heart receives accelator fibers from the sympathetic nerves and inhibitory fibers from the vagi.
The blood vessels have their vasoconstrictors and vasodilators.
The elimentary canal has accelator and inhibitory nerves, which increase and decrease peristaltic movements respectively.