Tuesday, August 17, 2010

Alpha Motor Neurons

Alpha motor neurons (a-MNs) are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles.

While their cell bodies are found in the central nervous system (CNS), alpha motor neurons are also considered part of the somatic nervous system—a branch of the peripheral nervous system (PNS)—because their axons extend into the periphery to innervate skeletal muscles. An alpha motor neuron and the muscle fibers it innervates is a motor unit. A motor neuron pool contains the cell bodies of all the alpha motor neurons involved in contracting a single muscle.

Alpha motor neurons which contract the head and neck muscles are located in the brainstem. The remaining alpha motor neurons innervate the rest of the body muscles and are situated in the anterior horn of the spinal cord. Because there are fewer muscles in the head and neck than in the rest of the body, there are more a-MNs in the spinal cord than in the brainstem. Alpha motor neurons on one side of the brainstem or spinal cord innervate muscles on that same side of body. The one exception is the trochlear nucleus in the brainstem, which innervates the superior oblique muscle of the eye on the opposite side of the face.