Mechanism of Muscle Contraction (NCERT) - Free MCQ Practice Test with solutions,

Muscles contain a red coloured pigment called myoglobin that stores oxygen. Some muscles have high content of myoglobin which gives them reddish appearance. Such muscles are called red muscles. Such muscles have abundant mitochondria and show slow rate of contraction for long periods that's why they are called slow twitch muscles.

In the given figure A, the length of the two sarcomeres is normal i.e., the muscle is in relaxed state. In figure B. the length of sarcomeres, shortens, H-zone narrows and size of l-band decreases i.e., the muscle is contracting. In figure C, the length of sarcomere further shortens and H-zone disappears and l-band further decreases i.e., the muscle is maximally contracted.

The contractile protein of a muscle is myosin. Myosin constitutes 55% of muscle protein by weight. The thick filaments consist mainly of myosin protein. A myosin molecule consists of six polypeptide chains, two identical heavy chains and four light chains.

A muscle fibre would contract only when it receives stimulation of certain intensity called threshold stimulus. Response of a muscle fibre to a stimulus is not proportional to its intensity. It is absent when the intensity is subliminal. Muscle fibre contracts to the maximum whether the stimulus has threshold value or supraliminal value.

In muscle contraction, both Ca²⁺ and Mg²⁺ are used. Ca²⁺ helps in the formation of actomyosin by combining actin and myosin proteins. This will provide contraction in muscle. ATPase activity of myosin requires both Ca²⁺ and Mg²⁺. This helps in release of energy. This energy is used up in the contraction of muscle.

Sarcoplasmic reticulum of muscle cells store Ca²⁺ ions. During contracting of muscle, Ca²⁺ ions are released from sarcoplasmic reticulum to sarcoplasm and vice versa occurs during relaxation. Therefore, if a muscle undergoes rapid contraction and relaxation it requires constant plugging in and out of Ca2+ ions from sarcoplasmic reticulum.

During muscle contraction, chemical energy is converted into mechanical energy.

Myoglobin (red coloured oxygen storing pigment) is present in all muscle fibres. Red muscle fibres have abundant myoglobin while white muscle fibres have less myoglobin

During muscle contraction, the cross-bridge of the myosin of thick myofilaments connect with portions of actin of the thin myofilaments. The myosin and the thin and thick myofilaments slide past each other. Length of A-band remains constant and actin filaments meet in the centre of the sarcomere. The Z-line comes closer and width of l-band decreases.

When the muscle is at rest, the tropomyosin molecule covers the binding site of the actin molecule where interaction with myosin occurs. During contraction of the Tuscte the active site is uncovered, allowing the interaction of actin With myosin.

The figure in the question shows formation of cross bridge between myosin head and actin filament. Next stage would be the figure showing the sliding/rotation of the head of myosin filament of the actin filament.

Muscle contraction is brought about by sliding movement of actin filaments over myosin filaments. When a muscle fibril contracts, ita A band remain constant and I band shortens. H zone also disappears as the actin filaments of both sides in each sarcomere overlap each other at M-line. M-line and Z-line also come closer.

Potassium is the most abundant mineral in muscles. It works closely with sodium in muscle function. All other minerals are present only in traces. So, the correct option is 'potassium'.

a. As a nerve impulse( action potential) from a motor neuron arrives via its axon at the neuromuscular junction, the vesicles in the endplate release a neurotransmitter, the acetylcholine, into the nerve-muscle gap (synaptic cleft).
b. Acetylcholine depolarizes the postsynaptic muscle cell's sarcolemma and set up an action potential in the latter.
c. The action potential spreads deep into the interior of the muscle fibre via T- tubules and Z lines.
d. The incoming action potential changes the permeability of the SR, causing to release calcium ions.
e. The calcium ions bind to troponin molecules.
f. The latter change shape and shift position.
So, the correct answer is '(ii) →(iv)→(vi)→(i)→(vii)→(iii)→(v)'.

The energy for muscle contraction is obtained from ATP.