Question

Which statement is correct for muscle contraction?
A. Length of I- Band increases
B. Length of H- band increases
C. Length of Z- band increases
D. Length of A- band remains constant

Answer 1

Hint: Muscle cells contain protein filaments of actin and myosin that slide past on one another and produce a contraction that changes both the length and the shape of the cell.
Each muscle fiber has an alternate light and dark band.
The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments and which shortens the myofibril.

Complete answer:
To solve this question, we need to know more about muscles.
Muscle: Muscle is a soft tissue found in most of the animals.
Muscle cells contain protein filaments of actin and myosin that slide past on one another and produce a contraction that changes both the length and the shape of the cell.
Muscles function to produce force and motion.
They are primarily responsible for maintaining and changing posture of the body, helping in locomotion as well as movement of internal organs, such as the contraction of the heart and digestion.
The sliding filament theory explains the process of muscle contraction during which the thin filaments slide over the thick filaments and which shortens the myofibril.
The important points are:
Each muscle fiber has an alternate light and dark band.
This contains a special contractile protein called actin and myosin respectively.
Actin is a thin contractile protein present in the light band and is known as the I-band whereas myosin is a thick contractile protein present in the dark band is called A-band.
There is an elastic fiber known as Z line which helps in bisecting each I-band.
The thin filament is firmly joined to the Z line.
The central part of the thick filament is not covered by the thin filament is known as the H-zone.
During muscle contraction, the myosin heads come in close contact with the thin filaments.
As a result, the thin filaments are pulled towards the sarcomere.
The Z line attached to the actin filaments is pulled which leads to the shortening of the sarcomere.
So, the length of the band remains constant and the I-band shortens and the H-zone disappears.

So, the correct answer is option D. length of A- band remains constant.

Note: There are three types of muscle tissue recognized in vertebrates:
Skeletal muscle is used to affect skeletal movement such as locomotion and helps in maintaining posture.
Smooth muscle is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels,
Cardiac muscle is also an involuntary muscle but is more akin in structure to skeletal muscle, and is found only in the heart.