Table: Using the Brain to Move a Muscle - Merck Manuals Consumer Version

Using the Brain to Move a Muscle

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: The sensory nerve pathway to the brain The motor nerve pathway to the muscle
	If sensory receptors in the skin detect pain or a change in temperature, they transmit an impulse (signal), which ultimately reaches the brain. The impulse travels along a sensory nerve to the spinal cord. The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord. The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord. The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information, located deep in the brain. The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors). The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse. The nerve carrying the impulse crosses to the opposite side at the base of the brain. The impulse is sent down the spinal cord. The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord. The impulse travels out of the spinal cord along the length of the motor nerve. At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where the impulse stimulates the muscle to move.
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.