According to the book ‘Mechanism of Muscular Contraction’ (Jack A. Rall, 2014), the myofibrils are made up of three kinds of proteins: structural, regulatory, and contractile proteins. The research is mostly focused on the last two types of proteins, contractile and regulatory proteins. For starters, actin and myosin are two contractile proteins present in myofibrils. Actin contributes to the forming of the thin filament, while myosin is responsible for the formation of the thick yarn.
Each has nearly 300 molecules of myosin under the thick filament and the form of golf clubs woven together. The tails of the threads point to the sarcomere which is toward the M line while the head whereas the head projections face toward the thin filaments and have an ATPase, actin-binding site and the ATP binding site. Whenever an individual is at rest, he ATP binds with the ATPase, and a single myosin molecule hence is hydrolyzed at a slow rate. Moreover, in the relaxation time, the actin-binding sites of the particles of myosin are blocked by the troponin-tropomyosin complex. The fig one below shows the myosin heads while fig 2 shows the A and me junction formation. The A-I junction is discussed under the triad structure.
Fig 1Fig 2
The regulatory protein has the troponin and the tropomyosin that form part of the thin filament and are also involved in the on and off contractions of the muscles. During the relaxation of tissue, tropomyosin forms a blockage to the myosin-binding sites on the actin proteins to prevent the attachment of the myosin heads. By the latter, the muscles are unable to contract. The primary work of troponin is to hold the tropomyosin proteins in place. In conditions where calcium goes in the cytoplasm of the muscle fiber, the calcium can bind to the troponin molecule that later changes the shape of the of troponin molecule, and the tropomyosin is pulled away from the myosin-binding site on every actin molecule.
Myosin is the primary functional unit that is involved in the contraction muscle contraction mechanism. The myosin heads pull the actin at the binding point, re-rock and attach to more binding sites. The above stated repeated movement is referred to as the cross-bridge cycle. The flow above is simulated to the movement of the oars in a boat. Every cycle movement requires energy that is in the form of ATP and myosin heads action in the sarcomere.
The figure shows how the repeated myosin movements happen systematically.
The Triad Structure and its Functions
The structure of the triad is formed by the interface between T-tubule and two portions of sarcoplasmic reticulum. In most mammals, the triads are located on the A-I junction. It is the junction between the A and I bands found in the sarcomere that is the smallest unit of the muscle fiber as seen in fig 2. If observed under the electron microscope of long sections, the triad is seen as triplet structures between myofibrils and moderately offset from the Z-line. In the cases of tauopathies like myotubular myopathy, the composition may be thought to be disorganized or even absent.
One of the significant roles of the triad structure is the excitation-contraction coupling. Excitation of the muscle makes it contract due to the stimulus above. The contractions can travel to the membrane and later to the T-tubules. The release of calcium from the sarcoplasmic reticulum to the sarcoplasm is brought about by the interaction between the dihydropyridine receptors in the T-t tubule and the ryanodine receptor.
The above fig is the structure of the triad. As seen, it is made up of two portions of sarcoplasmic reticulum and a single T-tubule.
The excitation-contraction coupling mechanism is the link needed for the muscle excitation for the sarcoplasmic reticulum to release the Ca++(Judy and Meissner, 2012). Muscle cell contraction is triggered by the action potential in the skeletal muscle, and the calcium ions are the ones responsible for the regulation whenever any diminution comes up or not. The significant structures required in the excitation-contraction coupling are referred to as the T-tubules. They have a tube shape hence have the more relaxed penetration of the muscle fiber. Furtherly, there is direct mechanical of interaction between the sarcoplasmic reticulum calcium ions channel and the T-tubule voltage sensor that gives a specified excitation-contraction coupling in the skeletal muscle. The mechanism can be classified into three main phases namely the spread of depolarization, the binding of calcium troponin and the generation force. Depolarization begins and spreads to the sarcolemma then AP is propagated into the T-tubules causing the release of calcium from the sarcoplasmic reticulum sacs. In the second phase, calcium binds with troponin molecules on the thin filament. The binding causes configurational changes to the troponin are hence removing tropomyosin from its blocking position on the actin filament. The last phase is about the cross-bridging cycle. The stage describes the cyclic events required in the force generated by the myosin heads during muscle contractions.
Sliding Filament Theory
Secondly, according to the sliding filament theory, both the actin and myosin remain at same length when they slide past each other during the muscle contractions. For the muscles to experience contractions, stimulation in the form of impulse from a motor neuron must take place. Hence, the theory tries to give an explanation on how the muscles contract for the force production. The sliding approach is divided into four distinct stages. One, there is the muscle activation that involves stimulation of an action potential by the motor nerve, for it to pass a neuron to the neuromuscular junction. The latter results in the sarcoplasmic reticulum stimulation to give out calcium to the muscle cell. The muscle contraction stage gives account on how the flooded calcium in the muscle cell provides the allowance to the myosin and actin binding. Later, the bond formed crosses the bridges and contract by use of energy that is in the ATP form. The third stage is referred to recharging step, and it entails the synthesization of ATP to allow healthy maintenance of the bond formed between myosin and actin. Lastly, the in the relaxation stage, the stimulation of nerves stops, there is sufficient pumping of sarcoplasmic reticulum hence the link between actin and myosin is broken making the muscles to relax.
Mechanism of Muscle Relaxation
Muscle relaxation is signaled by the motor neuron that causes a halt in the release of chemical signal known as ACh to the synapse. It leads to the depolarization of the muscle fiber that results in the closure of the sarcoplasmic reticulum doors. The calcium ions are released, and the ATP-driven pumps will get the ions out of the sarcoplasm back to the reticulum. The latter gives rise to the defense of the actin-binding sites that are between the thick and thin filament. Lastly, the tension in the muscles is lost making to have a relaxation.
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