Structure of skeletal muscle:
(a) Each skeletal muscle fibre contains many myofilaments (myofibrils) that have characteristic striations.
(b) A myofibril has alternate dark and light striations. The dark bands are also known as A-band or Anisotropic bands. The light bands are also called I band or Isotropic band. This is due to the presence of two fibrous contractile proteins- thin Actin filament and thick Myosin filament. I-bands contain actin while A-bands contain both actin and myosin. They are arranged parallel to each other.
(c) At the centre of A-band, a comparatively less dark zone called H-zone (= Hensen zone) is present. It is formed of only myosin.
(d) Myosin filament (thick filament) in the ‘A’ Band is also held together in the middle of ‘I’ Band by thin fibrous membrane called ‘M’ line.
(e) Each I-band is bisected by a dense dark band called Z-line. Actin filament (thin filament are firmly attached to the “Z” lines.
(f) The region of the myofibril between two successive Z-lines is considered as functional unit of contraction and is called a sarcomere.
(g) Thus, the sarcomere comprises A band and half of each adjacent I-band. They are the structural and functional units of a myofibril.
Mechanism of muscle contraction:
(a) Muscle contraction is initiated by signals from the CNS that travel along the axon and reach the neuromuscular junction or motor end plate via motor neuron.
(b) As a result, Acetylcholine (a neurotransmitter) is released into the synaptic cleft and binds to the receptors (nicotinic receptors) on the motor end plate.
(c) This binding stimulates the opening of sodium ion channels allowing the movement of sodium ions into the muscle cell and the generation of an action potential.
(d) The action potential spreads over the surface of the muscle fibre along the sarcolemma, traveling into the muscle cell at the T tubules.
(e) Immediately after the action potential is generated sodium ions are being pumped back muscle cell.
(f) The action potential travelling down the T tubules triggers the release of calcium ions from the sarcoplasmic reticulum into the muscle fibre.
(g) The increased calcium ions react with the troponin molecules of the thin filaments causing it to change shape.
(h) This change in shape allows the tropomyosin molecules to uncover the binding sites on the actin molecules.
(i) In this stage, the myosin head attaches to the exposed site of actin and forms cross bridges by utilizing energy from ATP hydrolysis. This pulls actin filaments on both sides towards the centre of A-band.
(j) Once the binding sites on the actin molecules are exposed, the myosin heads interact with the actin forming a cross bridge by utilizing energy from ATP hydrolysis. (k) Once a cross bridge is formed, the myosin head uses the stored energy from the breakdown of ATP to do a power **.
(l) The power ** is a swivelling action that pulls the actin toward the center of the sarcomere.
(m) After muscle contraction, the myosin head pulls the actin filament and releases ADP along with inorganic phosphate and goes back to its relaxed state. A new ATP molecule binds and the cross-bridge is broken.
(n) The ATP is again hydrolysed by the myosin head and the above process is repeated causing further sliding. This cycle is repeated until the actin meets in the middle of the sarcomere.
(o) The contraction cycle continues as long as ATP and calcium ions are available.
(p) During the contraction cycle the calcium ions are actively being pumped back into the sarcoplasmic reticulum.
(q) As the level of calcium ions fall, the calcium ions begin to dissociate from the troponin.
(r) This allows the troponin to go back to its original shape causing the tropomyosin molecule to cover the active sites on the actin molecule preventing the myosin heads from binding.
(*) This causes the return of Z-lines back to their original position, i.e., relaxation. The reaction time of the fibres varies in different muscles.
