Muscles are made up of contractile fibers which are formed of myofibrils. Muscles exhibit the property of excitability, contractibility, extensibility and elasticity. Muscles are responsible for movements of limbs and internal organs and thus coordinate locomotion in humans. This type of muscular movement is based on the contractile capacity of the muscle fibers.
During muscle contraction, the actin filaments on either of the myosin filaments slide over and move towards the Centre. The actin filaments move deeper into the A-band. The Z-lines limiting the sarcomere are pulled closer together and the length of I-band decreases and ultimately disappears. The myosin cross bridge has an acting –binding site and as ATPase binding site.
The cross bridges on the myosin filament act as hooks and pull the actin filament further deeper into the A-band.Actuall, the I-bands reduce in length and A-band remains unchanged.
Excitation of a muscle fiber:
Muscle contraction is initiated by a signal from central nervous system through motor neuron. A motor neuron along with the muscle fibers connected to it constitutes a motor unit. The junction between motor neuron and the sarcolemma of the muscle fiber is called the neuromuscular junction.
A nerve signal arrives at the neuromuscular junction. This impulse spreads rapidly due to depolarization of the surface of the sarcomere. Soon a neurotransmitter is released at the neuromuscular junction and it enters into the sarcomere through its membrane channel. The sarcolemma suddenly becomes more permeable to Na+.it generates an action potential in the muscle fibre.The action potential travels all along the length of the musclefibre.As a result Ca++ stored in the sarcoplasmic reticulum are released. This Ca++ binds with troponin of the actin filaments. As a result conformational change occurs in the troponin molecule and the active sites of the F-actin molecules become exposed. These active sites are specific to the myosin head. The myosin head and active sites join to form a cross bridge which pulls the actin filaments towards the center of the sarcomere.