Question

During muscle contraction:
A. Size of A-band remains the same
B. Size of H-zone becomes smaller
C. Size of I-band decreases
D. All of the above

Answer 1

Hint:
Our skeletal muscles are made up of a number of muscle bundles which contain a number of muscle fibres. The characteristic feature of each muscle fibre is the parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils. These myofibrils have a striated appearance due to the presence of actin and myosin proteins which are thin and thick filaments respectively

Complete answer:
Actin filaments are thinner and appear light in colour, this light band is called I-band or isotropic band whereas the myosin filaments are thicker and appear darker in colour, this dark band is called A-band or anisotropic band. Z-line is an elastic fibre present in the middle of the I-band and is firmly attached to the actin filaments. M-line is a thin fibrous membrane present in the middle of A-band. The A and I bands are arranged in an alternative manner within the muscle fibre. Sarcomere is the functional unit of contraction present between two successive Z-lines. H-zone is the central part of the thick filament which is not overlapped by the thin filament during the resting stage.
Muscle contraction takes place by sliding thin filaments over the thick filaments.
Step1:- The motor neurons bring the signals to the sarcolemma of the muscle fibre where a neurotransmitter is released into the synaptic cleft by generating an action potential.
Step2:- This action potential results in the release of calcium ions into the sarcoplasm by the sarcoplasmic reticulum.
Step3:- The calcium ions activate the actin filaments and bind with the troponin and remove the masking of the active sites for myosin.
Step4:- Now, the actin and myosin filaments bind together to form a cross ridge by utilising the energy from the ATP hydrolysis.
Step5:- Due to this, the actin filaments are pulled inwards and that causes the H-zone to reduce. This is the step where contraction occurs.
Step6:- During the contraction; that is the shortening of the muscle I-band reduces but the A-band retains the length.
Thus, the correct answer is All of the above

Option ‘D’ is correct

Note:
Some muscles appear to be red in colour which is because of the high myoglobin content. Such muscle fibres are called red fibres. The muscle fibres which have less myoglobin content appear to be white in colour and are called white fibres.