The Motor Unit
The skeletal muscles that move the limbs are made up of many motor units which consist of one alpha motor neuron and the muscle fibers it innervates. An action potential from the brain travels down this neuron and stimulates every fiber of the motor unit equally, causing them to contract. Specifically, when an action potential reaches the axon terminal of the alpha motor neuron, voltage-gated calcium channels in the membrane open and calcium ions trigger the release of acetylcholine-containing vesicles. These vesicles diffuse across the synapse to the muscle fiber and bind nicotinic receptors, which are chemically gated ion channels, on the muscle cell membrane. This event causes the channels to open and allow sodium ion entry which depolarizes the muscle fiber and triggers and action potential that contracts the muscle cell[40]. |
"Use It or Lose It"
The alpha motor neuron does more than cause contractions, it is also vital to muscle cell health. In the absence of communication between the motor neuron and the muscle, the cell gets a signal to begin apoptosis and dies causing muscle atrophy[40].
Picture References
course notes