The human nervous system may be conveniently subdivided into two divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves extending to and from the central nervous system).
The central nervous system is the control centre of the nervous system. The peripheral nervous system link receptors and effectors with the central nervous system. Nerves from the brain are called cranial nerves, wheres those from the spinal cord are called spinal nerves.
The spinal cord of the central nervous system is a white cord of tissue passing through the bony tunnel made by the vertebrae. The spinal cord extends from the base of the brain to the bottom of the backbone. Three membranes called meninges surround the spinal cord and protect it. The outer tissue of the spinal cord is white (white matter), while the inner tissue is gray (gray matter). A central canal in the spinal cord carries cerebrospinal fluid, which provides for the nutrition and gaseous needs of the cord tissue. The neurons of the spinal cord serve as a coordinating center for the reflex arc and a connecting system between the peripheral nervous system and the brain.
The peripheral nervous system is a collection of nerves that connect the brain and spinal cord to other parts of the body and the external environment. It is subdivided into the sensory somatic system and the autonomic nervous system.
The sensory somatic system carries impulses from the external environment and the senses. It consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves. The sensory somatic system permits humans to be aware of the outside environment and react to it voluntarily.
The autonomic nervous system works on an involuntary basis. It consists of two groups of motor neurons and a set of knotlike groups of cell bodies called ganglia. Motor neurons extend to and from the ganglia to the body organs. One subdivision of the autonomic nervous system is the sympathetic nervous system. Impulses propagated in this system prepare the body for an emergency. They cause the heartbeat to increase, the arteries to constrict, the pupils to dilate, and other changes to take place. The other subdivision is the parasympathetic nervous system. Impulses in this system return the body to normal after an emergency has occurred.