The nervous system in our body is mainly classified as central and peripheral nervous system. The former comprises the brain, spinal cord and nerve network. The latter is composed of network of nerves and tissues all over the body. The peripheral nervous system in turn is divided in to somatic and autonomic nervous systems. The somatic nervous system is responsible for allowing us to feel the external stimuli and for the voluntary movements of our body. The five sense organ nerves are part of and controlled by the somatic nervous system. The autonomic nervous system controls involuntary processes in our body.
The messages from the different sensory nerves in the sense organs like ears, eyes, nose, skin and tongue are inputs of the somatic system. The muscles which are voluntary are those attached to the skeleton or bones in our body. These muscles are controlled by the network or somatic nerves.
The sensory inputs are transferred from the sensory organ to the brain. In response, the brain gives the input impulse which is then transferred by the somatic nerves to the targeted muscles to be moved.
The two parts of the somatic nerves that are related to this cause-effect relation are grouped under afferent nerves and efferent or motor nerves. The former carry input signals from the sense organ to the central nervous system, while the latter carry response signals from the CNS to the muscles.
The neurons on the spine or brain are directly connected to the sense organ. The neuron which is the building block of the nerves comprises cell body, and axon. The former is located in the central nervous system while the later is attached to the sense organ, skin or muscle. The axon transfers the electromagnetic signal from one cell body to another.
In the autonomic nervous system, the target organ is connected to the central nervous system through two neurons. The somatic nervous system has one neuron making this connection.
Most of the muscle contractions controlled by SNS are voluntary while the reflexive movements of some skeletal muscles are involuntary and termed as reflex arcs. The movement of the muscle in response to an input stimulus is without voluntary control or activity in our brain. These movements occur since the signal travel through the afferent and efferent nerves are limited only till the spine and not to the central nervous system. A typical example of this movement is when one pulls their finger back from a hot surface.
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