During skeletal muscle contraction, the thick filament slides past the thin filament (actin) via repeated binding, releasing myosin along the filament. This entire process takes place in sequential order.
(i) Step 1: Muscle contraction is initiated by a signal that is sent by a CNS (Central nervous system) through a motor neuron. A neuromuscular junction or motorend
plate is a junction between a motor neuron and the sarcolemma of the muscle fibre. When a signal reaches the neuromuscular junctions, Acetylcholine (a neurotransmitter) is released which results in the generation of an action potential in the sarcolemma.
(ii) Step 2: This spreads through the muscle fibres, resulting in the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.
(iii) Step 3: The increased calcium ions \( (Ca ^{2+})\) in the sarcoplasm leads to the activation of actin sites. Calcium ions \( (Ca ^{2+})\) bind with the subunit of troponin on actin filaments and thus remove the masking of active sites for myosin. Hence, active sites on actin are exposed and this allows myosin heads to attach to this site.
(iv) Step 4: The myosin head now attaches to the exposed site of actin to form a cross-bridge by utilizing energy from ATP hydrolysis. The actin filaments are pulled towards the centre of the A-band. The Z line attached to the actin filaments is also pulled, causing shortening of the sarcomere, i.e., the contraction of the muscle occurs. It is clear from the above steps, that during contraction, the length of the A-band or anisotropic band remains constant as its original length whereas the I-band or isotropic band gets reduced.
(v) Step 5: After muscle contraction, the myosin head goes back to its relaxed state, releasing ADP and inorganic phosphate (Pi). A new ATP molecule binds and
detaches myosin, thus the cross-bridges are broken.
(vi) Step 6: This process of formation and breaking down is repeated causing further sliding. This process continues until the calcium ions are pumped back to the
sarcoplasmic cisternae. Hence, the calcium ions concentration decreases. This results in masking the actin filaments and leading to muscle relaxation.
Answered by Pragya Singh | 1 year agoFill in the blank spaces:
(a) All mammals (except a few) have __________ cervical vertebra.
(b) The number of phalanges in each limb of human is __________
(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely____ and _____.
(d) In a muscle fibre \( (Ca^{2+} )\) is stored in __________
(e) __________ and __________ pairs of ribs are called floating ribs.
(f) The human cranium is made of __________ bones
Name the type of joint between the following:-
(a) atlas/axis
(b) carpal/metacarpal of thumb
(c) Between phalanges
(d) femur/acetabulum
(e) Between cranial bones
(f) Between pubic bones in the pelvic girdle
How do you distinguish between a skeletal muscle and a cardiac muscle?
What are the different types of movements exhibited by the cells of the human body?
Match Column I with Column II:
| Column I | Column II |
| (a) Smooth Muscle | (i) Myoglobin |
| (b) Tropomyosin | (ii) Thin Filament |
| (c) Red muscle | (iii) Sutures |
| (d) Skull | (iv) Involuntary |