Muscle Contraction

The neuromuscular junction or motor end plate is the site where motor neurons form synapses with skeletal muscle fibers. It is here where action potentials traveling along the motor neuron trigger the events leading to muscle contraction. Study the images below and this slide to see the association of motor neurons with skeletal muscle fibers.

Arrival of an action potential at the axon terminus triggers the release of acetylcholine from synaptic vesicles into the synaptic cleft. Acetylcholine binds acetylcholine receptors on the muscle fiber sarcolemma, triggering its depolarization. This depolarization travels along the T tubules, resulting in release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum into the sarcoplasm. Ca2+ binds troponin C, unmasking the myosin-binding site on the actin thin filaments to initiate the contraction cycle.

Study the electron micrographs below showing the ultrastructural features of the neuromuscular junction and the muscle fiber with its triads containing the T tubules and terminal cisternae of the sarcoplasmic reticulum in close association with the sarcomere. Recognizing the ultrastructural features of the neuromuscular junction and skeletal muscle fibers will help you understand the events involved in muscle contraction.

Moving right along to cardiac muscle.