Question

A-bands of the muscle are dark and contain myosin.
B. I- bands are the light bands and contain actin.
C. During muscle contraction the A - band contracts.
D. The part between the two Z - lines are called a sarcomere.
E. The central part of the thin filament, not overlapped by thick filament is called H-zone.

Which of the above statements are correct and which are incorrect?
A) A, B and C are correct, while D and E are incorrect.
B) A, C, E are correct, while B, D are incorrect.
C) A and B are correct, while C, D and E are incorrect.
D) A, B, C and E are correct, while D is incorrect.
E) A, B and D are correct, while C and E are incorrect.

Answer 1

Hint: Myosin is ATP-dependent and is responsible for actin-based motility. Actin is a multi-functional protein produced by microfilaments. The complex unit of striated muscle tissue is the sarcomere.

Complete answer: Electron micrographs of groups of filaments aligned with their axes parallel to the length of the fibres reveal thin sections of muscle fibres. There are two types of filaments; thick and thin filaments. There is a light band of low density called the I band in a longitudinal section through a group of myofibrils. There is a prominent dense line called the Z line in the centre of the I band, but in fact, given the myofibril's three-dimensional structure, it is more fitting to speak of Z discs. The primary structural and functional unit is directly responsible for muscle contraction and can be assumed to be the region between two Z lines. Thus, myofibril can be seen as a stack of sarcomeres. The A-band appears black, comprising thick filaments that are partially overlapped with thin filaments.

The thick and thin filaments overlap contain myosin to form dark bands called A bands. Alone, thin filaments form light bands, called I bands, and are made from actin. The spacing between Z-lines decreases and the width of the H and I-bands decreases also during muscle contraction. The unit of muscle contraction is delimited by two Z-lines and is called the sarcomere. So, option D is true.
The H band, where there is no overlap is within the A band.
Hence, the correct answer is option D.

Note: Via rapid, intense contractions called phasic contractions, skeletal muscles respond to a single electric shock of sufficient magnitude. If the ends of the muscle of the frog sartorius (at ) are set to stop shortening, the stress increases for approximately 200 milliseconds and then starts to decrease, at first very quickly and then more slowly. During this mechanical reaction to a single stimulus, called a twitch, more happens than the tension record indicates.