Question

Differentiate between A-band and I-band.

Answer 1

Hint: The muscle fibres contain parallelly arranged myofibrils which have alternate light and dark bands. The dark present on the myofibril is called ‘A- band’ or anisotropic band and the light band present on the myofibril is called I-band or isotropic band.

Complete answer:
Skeletal muscle consists of many fibres, anatomical units of muscle. The muscle fibre is enclosed by a plasma membrane called sarcolemma. The sarcolemma surrounds the sarcoplasm which contains many nuclei. Light and dark bands are present on the muscle fibre. The sarcolemma invaginates to form T-tubules. These T-tubules are formed over the myofibrils. A single myofibril is made up of two types of myofilaments-thick myofilament and thin myofilament. The thick myofilament contains myosin protein and the thin myofilament consists mainly of actin protein. The endoplasmic reticulum in the sarcoplasm is called the sarcoplasmic reticulum. It is the storehouse of calcium required during muscle contraction. A-band is a dark band on the myofibril and I-band is called a light band also present on myofibril.

S.No.	A-Band	I-Band
1.	It appears dark under the microscope.	It has a light appearance under the microscope.
2.	The middle part contains a variable light zone.	The middle part contains a densely stained multilayered membrane called Z-line.
3.	It has both myosin and actin filaments.	It consists of only actin filaments.
4.	Length of A-band does not change during contraction.	I- band shortens during contraction of muscle fibre.

Note: At the centre of the dark A-band, a comparatively light band called ‘H-band’ or Hensen’s zone is present. A dark M-line passes through the centre of the H-band. The light I-band has a dark line that passes through the centre called Z-line. The area of myofibril between the two successive Z-line is called a sarcomere.