The nervous system is what allows living things to gain and process information from the external environment and convert this information into instructions. Your five basic senses – touch, small, taste, vision and hearing – are rooted in your nervous system.
There are many ways to divide the nervous system up for purposes of study; for example, "afferent nerves of the right lower limb" would refer specifically to the afferent (sensory) nerves of your right thigh, calf and shin, and exclude the efferent (motor) nerves of those regions.
Divisions of the Human Nervous System
The nervous system can be divided into portions on the basis of anatomy, on the basis of function or using a combination of both. Most schemes begin by distinguishing between the central nervous system or CNS, which includes the brain and spinal cord, and the peripheral nervous system, which includes all other nervous-system tissue. The PNS in turn is divided into the somatic and autonomic nervous systems (SNS and ANS), with these terms translating to "voluntary" and "involuntary" respectively. Finally, the ANS can be divided into the parasympathetic and sympathetic nervous systems on the basis of the type of involuntary responses generated within each.
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The Somatic Nervous System
The somatic nervous system includes everything under your voluntary control as well as one involuntary function, the somatic reflex arc (this is what a doctor tests for when tapping the tendon under your knee with rubber hammer). The SNS includes both afferent (sensory) nerves that transmit various types of information (e.g., smells, pressure and pain) to the the brain for processing and efferent (motor) nerves that direct the muscles under your control, such as those in your legs and arms, to execute certain movements, such as throwing or running.
The nerves of the SNS are classified on the basis of location. For example, there are 12 pairs of cranial nerves, which originate in the head and supply the muscles of the eyes, throat and other areas within the head with both motor and sensory fibers; and 31 pairs of spinal nerves, all of which service the voluntary muscles of the trunk, pelvis, arms and legs. The neurotransmitter chemical acetylcholine is an excitatory neurotransmitter in the SNS, meaning that it tends to stimulate movements.
The Autonomic Nervous System
The autonomic nervous system vs somatic nervous system distinction is functional: While the somatic nervous system is under your conscious control, none of the autonomic nervous system is. Of course, the two systems interact, with involuntary nervous-system responses permitting more energetic purposeful movements and so on. The neurotransmitter chemical acetylcholine is an inhibitory neurotransmitter in the SNS, meaning that its presence tends to damp out movements. Digestion, the beating of your heart and various internal secretions results from activities of the ANS.
The sympathetic branch of the ANS has CNS components in the chest, abdomen and back. Its signals are processed in structures called peripheral ganglia (singular: ganglion) that lie close to the spinal cord.
The parasympathetic branch of the ANS has its CNS portion in the head and the lower end of the spinal cord. It also has peripheral ganglia, but these are close to the target organs of nervous signals rather than close to the spine.
The Autonomic Reflex Arc
Like the SNS, the ANS has its own kind of reflex arc. The sensory sides of the somatic and autonomic reflex arcs are essentially the same, but the motor sides are different. In a somatic reflex arc, the motor information passes unimpeded from the spinal cord to the target muscle. In an autonomic reflex arc, however, the efferent signal from the spinal cord passes through a peripheral ganglion and then to the target tissue, which is often the smooth muscle of internal organs.