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The nervous system controls the smooth functioning of all systems in the body as well as all interactions between the human being and the environment. The nervous system is comprised of millions of neurons that are interconnected and form a communications network within the body that governs many vital functions including:

• The five senses (sight, hearing, touch, smell, and taste)
• Voluntary functions (e.g. walking, holding an object)
• Involuntary functions (e.g. breathing, blood pressure)
• Cognitive reasoning

The human nervous system has two major components:

• Central nervous system - includes the brain and spinal cord

• Peripheral nervous system - includes the nerves that lead from the brain and spinal cord to all parts of the body. An extensive system of specialized nerves makes up the peripheral nervous system which is responsible for a variety of important functions. These specialized nerves include:

  • motor nerves carry messages from the brain to the body and are responsible for the ability to move any part of the body (e.g., hands, feet)
  • sensory nerves carry information from organs (e.g. the skin) to the central nervous system where it is processed into sensation (e.g., touch, temperature changes, and vibrations)
  • nerves that control autonomic (involuntary) functions including heart rate, blood pressure, breathing, digestion, and bladder function

Each nerve cell (neuron) in the human body has three parts:

• Cell body - also called soma which is similar to the cell body of all other cells.
• Dendrites - Fibers of varying sizes which extend from the cell body and are the sensory terminals of the neuron. They receive messages from neighboring cells and transmit them to the cell body.
• Axon - also called nerve fiber that extends from the cell body and transfers a signal from the cell body to another nerve or muscle cell. Axons can be either myelinated (insulated by the myelin sheath made up of specialized cells) or unmyelinated. The presence or absence of myelin affects the speed of transmission of impulses; conduction speed is significantly faster in myelinated cells. Nerve fibers may be either long or short.