Motor nerves carry signals from the brain and the spinal cord to muscles and glands throughout the body. They activate the voluntary muscles, the muscles that are controlled through conscious effort. They also are responsible for reflex actions. When motor nerves function abnormally, the results can be seen as spasms or as a deterioration of muscular ability.

These nerves are efferent nerves, meaning that they carry impulses from the nervous center outward. They are distinct from sensory nerves, which supply the brain with information from other parts of the body about matters concerning touch, temperature and pain. They are also distinct from cranial nerves, which can be either sensory, motor or both. Motor nerves pair with sensory nerves in the spinal column to form 31 pairs of mixed nerves.

The brain and spinal cord are known together as the central nervous system (CNS). Motor nerves originate in the brain and take a two-part path down the spinal column. Upper motor neurons are called first order neurons, meaning that they are contained completely within the CNS.

Signals from the first order neurons cross a gap called a synapse to reach the lower motor neurons, the second order neurons that extend from the brain stem down to the body muscles. Motor neurons terminate in short dendrites and transmit their messages through a long axon. They operate oppositely to sensory nerves, which have short axons and receive their messages from long dendrites.

Motor nerves can respond to inputs from the sensory nerves without input from the brain. The spinal column allows motor nerves to move muscles in response to sensory messages. These automatic responses are called reflexes, and they do not require input from the brain. Reflex actions allow the body to react to dangerous situations quickly. For instance, they cause muscles to pull back from a heat source before the brain has had time to process the pain.

Damaged motor nerves can often be detected when they cause muscles to atrophy or when they produce twitching movements. Neurologists will often observe how a patient walks in order to obtain clues about his or her nervous system functioning. Motor nerve function can be monitored by recording the electromyograph potentials of the muscles they control. Amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, is a degenerative condition of motor nerves. Lesions to the upper motor neurons are responsible for cerebral palsy.