The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body.

          Neurons send signals to other cells as              electrochemical waves travelling along thin fibers called axons, which cause chemicals called neurotransmitters to be released at junctions called synapses. 

Sensory neurons are activated by physical stimuli impinging on them, and send signals that inform the central nervous system of the state of the body and the external environment

 Motor neurons,

situated either

in the central nervous system or in peripheral ganglia, connect the nervous system to muscles or other effector organs.

The peripheral nervous system

consists of

sensory neurons,

clusters of neurons

called ganglia

 and nerves connecting them

to each other

and to the

central nervous system.

                               The central nervous system                        

      of vertebrates  contains

the brain, spinal cord

 and retina.

The interactions of

all these types of neurons

 form neural circuits

 that generate

an organism's perception

 of the world

    ...and determine its behavior.