There are three neuron types: motor, sensory and relay. When these neurons act together they make a reflecx action: that is, an involuntary not a planned move. Any organism depends on its ability to respond to threat or change in the environment. The nervous system responds fast but with short lived responses.
The nerve pathway is called an arc, the receptor detects a stimulus, sensory neurons send an impulse to relay neurons. These connect with motor neurons which send electrical impulses to an effector. That produces a response such as a muscle contraction.This is the order of a reflex arc.
Sensory receptor receives stimulus from outside world. Sensory neuron sends message to central nervous system. Interneurons process information by communicating with brain. Motor neurons send message back to the muscle. The effector (muscle) reacts by contracting.