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General Organization of the Vertebrate Nervous System

The nervous system is composed almost exclusively of nervous tissue, as you would expect. The system is divided into two major subdivisions:
  • Central nervous system (CNS): consists of the brain and spinal cord.
  • Peripheral nervous system (PNS): consists of all nerves that carry information to and away from the CNS.
Neurons in peripheral nerves can be classified functionally:
  • sensory neurons  carry signals towards the CNS.
  • motor neurons carry signals from the CNS to an effector organ.
Sensory pathways begin with neurons containing specialized sensory receptors, which are able to gather information about both the external and internal environments of the body. This information is sent to the CNS where it is processed and integrated, and a decision is made about an appropriate response. That response is carried out through motor pathways that stimulate an effector, usually a muscle or gland.
As noted previously, a nerve is simply a bundle of neurons held together by connective tissue. If a nerve consists exclusively of sensory neurons, it is referred to as a sensory nerve; likewise, a motor nerve contains only motor neurons. Many nerves contain both types and are referred to as mixed nerves. Peripheral nerves can also be classified anatomically, depending upon where they are connected to the central nervous system: 
  • spinal nerves originate from the spinal cord.
  • cranial nerves are connected directly to the brain.
Motor pathways in the peripheral nervous system can be further subdivided:
  • somatic (voluntary) nerves control the movement of voluntary skeletal muscles.
  • autonomic (involuntary) nerves control those functions which occur “automatically”, such as the regulation of breathing and heartrate, and the stimulation of glands and smooth muscles.
the sympathetic division has the opposite effect, and is often cited for its ability to contribute to the “fight or flight” reaction.

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