The function of motor neurons is to release the neurotransmitter acetylcholine, which are released at a junction known as the neuromuscular junction, that bind to postsynaptic receptors, causing the muscle fiber to respond and result into a muscle movement. For example, the alpha motor neurons, also known as the lower motor neurons, usually innervate skeletal muscle, causing contractions of the muscles that result in movement.
The acetylcholine binds to the receptors on the fiber of the muscle, causing an action potential to be propagated along the fiber of the muscle in both directions. The contraction of the muscle is then triggered by this action potential. If both ends of the muscle are rigid, the contraction leads to an increase in the force on the supports, and this is referred to as the isometric contraction. If the ends of the muscle are not fixed and the muscle is shortened without any resistance, the contraction causes a constant force known as the isotonic contraction.
The motor neurons responsible for the body and limb movements are found in the anterior horn of the spinal cord, while those responsible for facial and head movements are found in the motor nuclei of the brain stem. The motor neurons are the only way the muscles can communicate with the motor system in the body.