Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate in the brain, as when a person consciously decides to move a muscle—for example, to pick up a book.
Or the impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to sense pain or a change in temperature. This sensory information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways:
If the sensation occurs suddenly and is severe (as when stepping on a sharp rock or picking up a cup of very hot coffee), the impulse may travel to the spinal cord and directly back to the motor nerve, bypassing the brain. The result is a quick response of a muscle—by immediately withdrawing from whatever is causing the pain. This response is called a spinal reflex.