Divisions of the Nervous System
We only have one nervous system but because of its complexity it is difficult to consider all the parts at the same time. So, to simplify
its study, we divide it into parts in terms of its structures (structural classification) or in terms of its activities (functional
classification). Each of the classification schemes is described below.
STRUCTURAL CLASSIFICATION:
If the nervous system is considered as a whole, there are two main divisions ---- the central nervous system
(CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord and occupies the
dorsal cavity. They interpret sensory information and serve as an integrating and command center to issue instructions
based on past experiences and current conditions. The PNS is the part of the nervous system outside of the central
nervous system. It consists mainly of the nerves that extend from the brain and spinal cord. Spinal nerves carry impulses
to and from the spinal cord. Cranial nerves carry information to and from the brain. These nerves serve as
communication lines and link all parts of the body by carrying impulses from the sensory receptors to the CNS and from
the CNS to appropriate glands and muscles.
FUNCTIONAL CLASSIFICATION
The functional classification scheme is only concerned with PNS structures.
The sensory, or afferent division consists of nerve fibers that convey impulses to the central nervous system from
sensory receptors located in various parts of the body. Sensory fibers delivering impulses from the skin, skeletal muscles,
and joints are called somatic sensory fibers, whereas those transmitting impulses from the visceral (internal) organs are
called visceral sensory fibers, or visceral afferents. The sensory division keeps the CNS constantly informed of events
going on both inside and outside the body.
The motor, or efferent division carries impulses from the CNS to effector organs, the muscles, and glands. These
impulses activate muscles and glands; that is, they effect or bring about a motor response.
The motor division in turn has two subdivisions:
The somatic nervous system allows us to consciously, or voluntarily, control our skeletal
muscles. Hence, this system is also called the voluntary nervous system. (One exception
to this is reflexes….While reflexes are controlled by skeletal muscles, they are involuntary)
The autonomic nervous system (ANS) regulates events that are automatic, or involuntary, such as the
activity of smooth and cardiac muscles and glands. This subdivision, commonly called the involuntary
nervous system, itself has two parts: the sympathetic and parasympathetic nervous systems. Both of
these systems serve the same organs but cause essentially opposite effects, counterbalancing each other’s
activities to keep body systems running smoothly.
The sympathetic nervous system mobilizes the body during extreme situations (such as fear,
exercise, or rage). A pounding heart, deep breathing, cold, sweaty skin, and dilate eye pupils are
signs of the sympathetic nervous system. It is often referred to as the “fight or flight system.”
The parasympathetic division is most active when the body is at rest and not threatened in any
way. This system is sometimes called the “resting and digesting” system because it is concerned
with normal digestion , elimination of wastes and decreasing demands on the cardiovascular
system.