A motor nerve is a nerve which carries impulses from the central nervous system which trigger muscles to contract. All of the voluntary muscles in the body are controlled with motor nerves, which means that any time someone decides to move, a motor nerve is involved. All vertebrate animals use this highly effective system for controlling their voluntary muscles. Involuntary muscles such as the heart move using a different system.
These nerves are made up of motor neurons, neurons which specialize in carrying signals which will result in muscle contraction. Where a motor nerve meets a muscle, the neuron releases chemicals which stimulate muscle contraction. Once these chemicals are broken down, the muscle relaxes again. Motor nerves only have an excitatory action, meaning that they can only signal contractions, not relaxation of the muscle.
These nerves are among a group of nerves known as efferent nerves because they carry data from the central nervous system to the rest of the body. When information moves in the opposite direction, it is known as afferent. As an example of the difference between these groups of nerves, efferent nerves would carry the signal to tell the muscles of the hand to grip a pot, but afferent nerves would carry the information that the pot is hot and the hand has been burned.
It is possible to administer medications which interfere with the activity of motor nerves. These drugs are known as muscle relaxants because they make it difficult or impossible for motor nerves to send signals which will cause the muscles to contract, thereby leaving the muscles in a relaxed state. An example of a use for muscle relaxants is a medical procedure in which it is critical for the patient to stay relaxed, such as the insertion of a urinary catheter.
The network of motor nerve pathways in the body allows people to perform a variety of tasks, from simple to complex. Damage to these nerves or to the parts of the brain which communicate with them can lead to difficulty in making movements, or to confused and irregular movements. Some people who experience brain damage, for instance, have difficulty walking in the wake of the damage because the part of their brain which communicates with the motor nerves in the legs has been disturbed. In these cases, they must learn to walk all over again, teaching their brain how to communicate with the motor nerve pathways.