The myosin filaments possess some hook like heads that bind with some active sites of the actin filaments to form cross-bridges. The cross-bridges are alternately broken and reformed very quickly in such a way that each head of myosin leaves one active site of actin and binds with the next site so that the actin filament is pulled towards the center of the sarcomere.
In a relaxed muscle, the active sites of actin remain covered by tropomyosin, and troponin helps in it. In this state, the myosin heads cannot bind with actin because the active sites of actin are not exposed. A skeletal muscle contraction is initiated by excitation of the motor nerve supplying the muscle. When a motor nerve is excited, electrical impulses are generated in the sarcolemma of the muscle fiber by the process of neuromuscular transmission. The impulses spread all over the sarcoiemma, enter into the muscle fiber through the T-tubules and finally release Ca++ from the sarcoplasmic reticulum. The Ca++ binds with the troponin to displace the tropomyosin so that the active sites of actin become exposed and bind with the myosin heads forming the cross-bridges.
In the mean time, the Ca ions also activate the enzyme adenosine-triphosphatase (ATP-ase) present in the myosin heads. The myosin ATP-ase splits ATP into ADP and Pi to release the energy stored in it. This energy helps in repeated breakdown and renovation of the cross-bridges so that the actin filaments are pulled inward by the myosin filaments.
After the muscle contraction, the Ca ions are returned back into the sarcoplasmic reticulum by an active pump mechanism driven by the energy released from ATP breakdown. With the sequestrations of Ca++ from the contact of myofilaments, the changes that have taken place in the muscle fibre are stopped and reversed as a result of which the muscle fiber is relaxed. Thus, we see that both the processes of muscle contraction and relaxation require energy derived from ATP. The mechanism of muscle contraction and relaxation may be summarized as follow :
Contraction :Excitation of motor nerve ►► Arrival of nerve impulse at themotor nerve terminal►► Generation of impulse in the sarcolemma through neuromuscular transmission and its conduction over the sarcolemma and T-tubules►► Release of Ca++ from sarcoplasmic reticulum►► Combination of Ca++ with troponin►► Displacement of tropomyosin►► Exposure of active sites of actin►► Formation of cross-bridges between actin and myosin►► Activation of myosin ATP-ase►► Breakdown of ATP►► Pulling of actin filaments toward the center of the myosin filaments►►
Myofibril and the muscle contraction.
Relaxation :Cessation of excitation of motor nerve and neuromuscular transmission►► Reuptake of Ca++ by the sarcoplasmic reticulum►► Covering of active sites of actin by the joint action of tropomyosin and troponin and inactivation of myosin ATP-ase►► Cessation of cross bridge formation►►
Restoration of the original relaxed state.
Structural changes during a muscle contractions :
Contraction of a muscle, some structural (histological) changes are observed in it. Those are as follow:
i) The I-band is reduced.
ii) Although the A-band remains unchanged whole, the H-band present within the A-band is reduced
iii) The adjacent Z-come closer.
Chemical changes due to muscle contraction :
During muscular exercise and in the post exercise recovery period, a numt chemical changes or processes occur in the muscle. The main purpose of processes is to supply energy for the muscular work. These processes are div into two categories, those are:
1. Breakdown of ATP : ATP (adenosine triphosphate) is the energy cut of a cell i.e., it is the immediate source of energy. During muscle contractio myosin ATP-ase becomes active and hydrolyzes the ATP stored in the muscle resulting, the terminal high energy phosphate bond (~P) of ATP is split (inorganic phosphate or phosphoric acid) and ADP (adenosine di-phosphate produced with liberation of energy. This energy causes muscle contraction or relaxation. The reaction is as under: -
ATP + H2O -------Myosin ATP-ase----► ATP + Pi + Energy
2. Regeneration of ATP : The amount of ATP stored in the muscle is limited. So, with the onset of muscle contraction and ATP breakdown, resynthisis of ATP also begins in order to maintain a steady supply of ATP in the muscles, resynthesis of ATP occurs in two ways—[i] From creatine phosphate [ii] From oxidation of food.