Test: Movement and Locomotion - Muscle & Mechanism of Muscle Contraction (September 12) - NEET MCQ

Sarcomere is the functional unit of myofibril. It contains two types of protein filaments called actin and myosin. These filaments slide upon each other to bring about the contraction of the muscles.

A myofibril has dark and light bands. The dark bands are called A bands (Anisotropic bands). Light bands are called I bands (isotropic bands).

The type of muscle fibre present in the wall of alimentary canal is smooth muscle fibre. Smooth muscle fibres are non-striated and involuntary.

A myofibril has dark and. light bands. The dark band called A-band has at the centre, a comparatively less dark zone called H-zone. In the centre of H-zone is the M-line (Mittleschebe line). Each l-band has at its centre a dark membrane called Z-line (Zwischenschiebe line). The part of the myofibril between two successive Z-lines is called sacromere. In fact, each sacromere is a bundle of thick and thin myofilaments.

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.

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.

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, 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.

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.

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)'.