Skeletal Muscle - Explained
A type of striated muscle tissue and a major muscle type, the controls of this muscle are with the somatic nervous system. Somatic nervous system, simply put, is associated with the voluntary control of the movements of the body and also with the sensory reception of external stimuli. Coming back to striated muscle though, it has muscle fibers which are combined in parallel fibers. If you see a cross section of a skeletal muscle, the muscle is covered in a layer of Epimysium or simply put, the connective tissue. This tissue acts as a protective layer for the muscle from friction against other bones and muscles. There are bundles of muscle fibers too which are called the Fasciculi. This is in short and the structure of a skeletal muscle, and that is how skeletal tissue is made up.
How Does Skeletal Muscle Contraction Happen?
The process of muscle contraction is a complex one. Unless one has a good idea of muscles and their functioning and the whole biology behind it, it is difficult to understand that. Still, here is making an attempt to make it easier. This is how the contraction happens, which most often than not, happens in the course of muscle building.
- Nerve Impulse happens first. The action potentials arrive at synaptic end bulb - the axon terminal. Then the calcium ions diffuse into synaptic bulb through voltage-gated calcium channels.
- In the next level of the contraction, Acetylcholine is released by vesicles through exocytosis into synaptic cleft and then it binds to ACh receptors on the sarcolemma.
- Na+ ion enters through these ion channels (and K+ leaves).
- Na+ entrance creates an action potential on the sarcolemma. The sarcolemma is then depolarized and muscle fiber is excited
- Excitation of the muscle is followed by the action potential traveling to the T-tubules. There, the terminal cisternae release Ca2+ into the sarcoplasm and the Ca2+ binds to the troponin, causing a shift of the tropomyosin.
- The myosin binding sites (on the actin) are consequently exposed .
- A power stroke follows then, followed by the ATP molecule action, which is bound to the myosin head, being hydrolyzed to ADP + Pi.
- ADP remains for the moment on the myosin head.
- Finally, the energy released by this process results in the swiveling of the myosin head. But it straightens out, in reality.
- Taking ahead from the last step, related to myosin head, what happens now is that activated myosin head binds to the actin, forming a cross bridge, which is a spontaneous reaction.
- In this process, the ADP + Pi is released and the head flexes, leading the thin filament to be pulled along the myosin.
- Myosin head will be bound till a new ATP molecule binds to it. The new ATP will lead the head to be released from the actin and hence permit the myosin head to gear up again for another power stroke.
- The stimulus ends here and then ACh is destroyed by AchEsterase or is diffused throughout the synaptic cleft. Membrane is no longer permeable to Na+.
- Ca2+ transported (active, ATP used) back into the sarcoplasmic reticulum.
- Low [Ca2+] in sarcoplasm makes the Ca2+ leave the troponin. Eventually, the tropomyosin shifts around to cover the myosin binding sites. The sarcomere will lengthen as the filaments slide back to their original position.
Whoa! All this is too complicated, isn't it? What makes this process even complex is the fact there are 3 to 4 ways in which a muscle can contract, for instance there is isometric muscle contraction! However, we need not remember the plethora of chemical reactions happening in there. A better idea is to just be aware that there is an external stimulus to which our muscles respond and then the contraction happens. Of course we do not realize that there are so many things which take place during skeletal muscle contraction! Fascinating, huh?