Describe the contraction of a muscle fiber, starting with what events take place at the neuromuscular junction, explaining how depolarization along the length of the sarcolemma occurs, the role of calcium (where it is released, what it binds to, etc.), and the events that occur after calcium binds to troponin. Draw a few simple, labelled diagrams of a neuromuscular junction and sarcomere and outline the steps of excitation-contraction coupling, including power stroke, recovery stroke, formation of cross bridges, role of ATP, etc.

Introduction: Muscle is a mesodermal contractile connective tissue. Muscles contain unique characteristics such as elasticity, excitability, extensibility, and contractility. Muscles are classified into three types based on their location: Smooth Skeleton Cardiac Answer: The contraction of muscle fiber has been explained in the bullets below: Muscle contraction begins when a neural signal from the Central Nervous System travels from the motor neurons to the neuromuscular junction. The neuromuscular junction connects the muscle fiber to the nerve fiber. These impulses then go to the neuromuscular junction's axon terminals. As a result, neurotransmitter-containing vesicles (acetylcholine) fuse with axon membranes. Acetylcholine is therefore released. This acetylcholine goes across the synaptic cleft (the area between the axon membrane and the motor endplate) and causes an action potential to be generated within the sarcolemma due to the creation of a local voltage potential known as depolarization. This results in the transformation of stimuli from one form to the other. This action potential travels through the T-tubules and then to the sarcoplasmic reticulum. This relay is aided by a protein called dystrophin. When the action potential is received, the sarcoplasmic reticulum releases calcium ions into the sarcoplasm. These calcium ions attach to troponin C, which exposes the actin's myosin-binding sites. This happens because the binding of troponin C with the calcium causes troponin I to move from its original place and shift to other places. This exposes the myosin-binding locations on the actin filament. Simultaneously, ATP is attached to the ATP-binding site on the head of the myosin filament. As soon as the ATP binds on the ATP binding site, it is hydrolyzed to ADP, Pi (inorganic phosphate), and energy. Next, the ADP moves out. The moving out of the ADP will cause the myosin head to get attached to the exposed myosin binding site on the action. This is termed power-stroke or actomyosin bridge formation, or cross-bridge formation. Now when Pi moves out, the head of the myosin (which is bound with the actin) moves backward, causing the actin to move backward (actin slides over myosin). This is a recovery stroke or backward stroke (only the head of the thick filament moves while the remaining does not). This complete process is referred to as the sliding-filament theory. #Note Excitation/depolarization of the muscle fiber results in forming an action potential, which initiates muscular contraction via a series of events. Calcium ions link these two processes. Excitation-contraction coupling is the process through which the muscle fiber's depolarization initiates contraction.Diagrammatic illustration of a sarcomere: Diagrammatic representation of the formation of cross-bride, head rotation, and breaking of cross-bridge: