Two nervous systems are involved in nervous control: the somatic nervous system and the autonomic nervous system. Both systems have the spinal cord (medulla spinalis) and the brain (cerebrum) at their center. The peripheral pathways, which are the neurites (nerve fibers) of the nerve cells, run from the centers outward. If these neurites conduct an impulse from the periphery to the center, they are esthesodic fibers (ie, they convey sensory impulses). If an impulse is transmitted from the center to the effector organ, they are fibers that stimulate movement.
Somatic Nervous System
The processing of stimuli by conscious action is the characteristic of the somatic nervous system. Stimuli from the outside world received by sensory organs are responded to with deliberate reactions.
The spinal cord is the first and primary center of the somatic and the autonomic nervous systems. It lies in a canal formed by the spinal column and is encased in a very firm covering.The spinal cord is able to respond to certain stimuli itself in the form of rapid but unconscious reflexes.
A reflex is the simplest form of response to a stimulus by the central nervous system. In this process, there are reflex arcs (or paths) that comprise at least two neurons (nerve cells with processes). The reflex arc also includes afferent conduction toward the receptor and efferent conduction away to the effector organ. A distinction can be drawn between innate and conditioned reflexes. People possess innate (or inborn) reflexes from birth; in medical examinations, these reflexes can provide information about the state of health of the spinal cord.
Conditioned reflexes are learned sequences of movement. Conditioned reflexes are triggered by unconnected impressions that are linked to each other by frequent associations. Reflexes can be influenced by medicines or by a person's will.