What is a reflex arc?

A reflex arc describes what controls a reflex reaction in humans. When something happens to an organism that needs a quick response a reflex action occurs. It allows the action to be 'unthinking'. A stimuli (such as heat) causes a receptor to send an impulse along the sensory neurone to the spinal cord (central nervous system/CNS). The signal crosses the synapse between the sensory neurone and relay neurone using chemicals. The relay neurone transmits the singal through the CNS and across a synapse to a motor neurone, this then allows the movement of the muscle or effector to move the body out of harms way.