In vertebrates it consists of two main parts:
1- the central nervous system (CNS)
The CNS consists of the brain and spinal cord.
2-the peripheral nervous system (PNS).
The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.
-The autonomic nervous system is further subdivided into:
1- the sympathetic
The sympathetic nervous system is activated in cases of emergencies to mobilize energy.
2- the parasympathetic nervous systems.
the parasympathetic nervous system is activated when organisms are in a relaxed state.
-The nervous system contains two main categories or types of cells: neurons and glial cells.