Somatic Nervous System: The somatic nervous system includes both motor neurons and sensory neurons. The fibres of motor and sensory neurons are bundled together into nerves, which are of two types. i. Cranial nerves connected directly to the brain, such as the optic nerve (for eye), auditory nerve (for ears), mixed nerves (for face), etc. There are 12 pairs of cranial nerves.
ii. Spinal nerves emerging from spinal cord. There are 31 pairs of spinal nerves.
Every spinal nerve is a mixed nerve having both sensory and motor nerves. The somatic nervous system regulates voluntary activities while the autonomic nervous system performs a variety of functions which are not under the control of an individual. Let us study more about the autonomic nervous system.
2. Autonomic Nervous System:
The autonomic nervous system (ANS) includes a chain of 22 pairs of ganglia which lie close to the spinal cord and are associated with the organs they control. ANS is primarily a motor system consisting of neurons that control the functioning of many organs. i.
Heart muscles ii. Glands iii. Smooth muscles (muscles of blood vessels, digestive, respiratory and reproductive tracts) The ANS can stimulate or inhibit the activity of its target organs. The ANS is divided into two divisions—sympathetic and parasympathetic nervous systems. These two divisions are anatomically and functionally distinct. The sympathetic fibres arise from the thoracic (chest) and lumbar (waist) region of the spinal cord, whereas the parasympathetic fibres arises from the brain and the sacral (pelvic) region of the spinal cord. The effect of the two systems is antagonistic.
In general, the sympathetic system stimulates a particular function and prepares the body for violent actions against unusual emergency conditions, while the parasympathetic system has inhibitory or calming down effect, i.e. it reestablishes normal conditions after the violent action is over.