The nervous system provides a high-speed communication system to and from almost every part of the body.

Two major parts comprise the human nervous system:

  1. The central nervous system (CNS)
  2. The peripheral nervous system (PNS)

Human Nervous System

The central nervous system (CNS) is made up of the brain and spinal cord.

The peripheral nervous system (PNS) includes the nerves that lead into and out of the central nervous system. The peripheral nervous system consists of the autonomic nervous system and the somatic nervous system.

The autonomic nervous system is not consciously controlled. The autonomic nervous system is made up of the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system sets off the “fight-or-flight” reaction. The parasympathetic nervous system has an effect opposite to that of the sympathetic nervous system.

Sympathetic and parasympathetic nervous systems

The somatic nervous system is made up of sensory nerves that carry impulses from the body’s sense organs to the central nervous system. This system also consists of motor nerves that transmit commands from the central nervous system to the muscles.

Neurons and Reflex Responses

90 percent of the body’s neurons are found in the CNS.

A neuron consists of three parts:

  1. the cell body,
  2. dendrites,
  3. an axon.


A reflex arc is the nerve pathway that leads from stimulus to reflex action.

Reflex arc

The three major types of neurons:

  1. a sensory neuron requires a strong stimulus to activate it.
  2. an interneuron acts as a link between a sensory neuron and a motor neuron.
  3. a motor neuron triggers the contraction of the muscles.