The daily routine of an average person is loaded with lots of activities like walking, running, writing, typing and so on. We know that muscles in our body help us doing all these activities, but how? In simple terms, we could say that muscle contraction and relaxation is the principle. Let us see the structure of contractile proteins followed by muscle contraction mechanism.
Skeletal muscle is composed of muscle fibers which have smaller units called myofibrils. There are three types of proteins make up each myofibril; they are contractile, regulatory and structural proteins.
By contractile proteins, we mean actin (thin filament) and myosin (thick filament). Each actin filament is composed of two helical “F” actin (filamentous actin) and each ‘F’ actin is made up of multiple units of ‘G’ actin. Along with the ‘F’ actin, two filaments of regulatory proteins tropomyosin and troponin at regular intervals are present. During muscle relaxation, troponin covers the binding sites for myosin on actin filaments.
Each myosin is composed of multiple units of meromyosin which has two important parts- a globular head known as heavy meromyosin with a short arm and a tail known as light meromyosin. The head and arms project at regular distance and angle from each other from the surface of myosin filament and are known as the cross arm. The head bears binding sites for ATP and active sites for actin. Let us now try to understand the muscle contraction mechanism.
During contraction, the thin filaments slide over the thick filaments. A signal sent by the central nervous system via motor neuron initiates muscle contraction. The neuromuscular junction is the junction between a motor neuron and sarcolemma. Acetylcholine is released when a neural signal reaches this junction and action potential is generated in the sarcolemma. When this spreads through the muscle fibre, calcium ion is released in the sarcoplasm. Calcium then binds to troponin on actin filaments and exposes the active sites for myosin. Myosin binds to the exposed active site on actin using energy from the hydrolysis of ATP. This pulls the actin towards the centre. The Z lines attached to these are also pulled, and contraction occurs. Myosin is in a relaxed state.
Consequently, the hydrolysis of ATP at the myosin head continues and this leads to further sliding. This is repeated till calcium ions are pumped back to the sarcolemma and results in covering of the actin sites again. The Z lines move back to their original positions. This causes relaxation. Muscle fatigue occurs due to repeated activation of the muscles leading to accumulation of lactic acid.
Muscles appear in red colour due to a pigment called myoglobin. Myoglobin rich muscles are called red fibers. They also contain lots of mitochondria which they may use for energy production. Muscles which lack myoglobin and white in colour are called white fibers.
Also Read: Sliding Filament Theory
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