THE NERVOUS SYSTEM

meaningThe nervous system is a complex collection of nerves and specialized cells known as neurons that transmit signals between different parts of the body.

Structure of the nervous systemPNSNervous systemCNSparasympatheticAutonomic NS (efferent)Somatic NS (afferent)Spinal cordbrainsympathetic31 pairs of spinal nerves12 pairs of cranial nervesmedullacerebellumcerebrumStructure : All neurons havethree parts:DENDRITE: conduct nerve impulsesCELL BODY and AXON (conducts nerve impulses away from the cell body). Dendrites and axons are sometimes calledFIBERS Most long fibers are covered by a MYELIN SHEATH

The sheath has spaces in it exposing the axon called NODES OF RANVIER .The sheath is secreted by SCHWANN CELLS each of which has a nucleus.

There are three types of neurons:

SENSORY NEURON : Afferent neuron) takes a message from a sense organ to CNS. has long dendrite and short axonMOTOR NEURON: efferent neuron) takes Message Away From CNS to a Muscle Fiber orgland. Short dendrites, long axonINTERNEURON: association neuron or Connector neuron: completely contained within CNS. Conveys messages between parts of the system. Dendrites, axons, may be long or short.