Question

During skeletal muscle contractions following events can occur:
i. I-band shortens
ii. A- band shortens
iii. H- zone shortens
iv. Sarcomere contract
v. ATP changes to ADP and ${P_i}$
Choose the option with incorrect events.
a) only i.
b) only iii
c) iv and v
d) only ii

Answer 1

: In the skeletal muscle there is presence of I band and A band. The I band has thin filaments present in it and the A band has both thin and thick filaments present in it. The H zone of the muscle consists of the thick filament only.

Complete answer:
The skeletal muscle contraction can be studied by the sliding filament theory. In this theory the thick myosin filaments slide and pull the thin myosin filaments repeatedly so that they slide over to one another. In the events which follow the muscle contraction there is release of the calcium ions which is caused due to the transmission of the nerve impulse to the sarcolemma. These ions are released into the sarcoplasmic reticulum and bind to the troponin for the rotation of the tropomyosin. The myosin then binds to the tropomyosin and ATP is hydrolysed. The ATP further releases the bond of tropomyosin and myosin and muscle relaxes. In this whole process the H zone shortens along with the I band and leads to the contraction of the muscle. But the A band does not contract and remains in its original shape.

So, the answer is ‘A band shortens’.

Note:
In the skeletal muscle there is presence of Z discs which consists of the actin filament. These Z discs are the end point of each sarcomere. When the muscle contracts it pulls the Z discs closer to each other and this leads to the muscle contraction. During muscle contraction the H zone also shortens and in the relaxed state it expands.