Question

Krause’s membrane or Z-line is a myofibril, which separates two adjacent
(a) Sarcomeres
(b) H-zones
(c) I-bands
(d) A-bands

Answer 1

Hint: Muscular fibre, in a skeletal muscle, is composed of myofibrils. They are made up of two types of filaments whose arrangement gives a characteristic light and dark bands. Their sliding over one another causes the contraction of a muscle.

Complete answer:
Skeletal muscles are made up of a number of muscular bundles or fascicle which are held together by a layer of connective tissue ‘fascia’. Muscular bundles, in turn, are composed of muscle fibre. Muscle fibres are formed by parallelly arranged myofibrils or myofilaments. Each myofibril has alternate light and dark bands known as I-band and A-band. I-bands or Isotropic bands include actin and A-band includes myosin. Each I- band at its centre has a dark membrane known as ‘Z-line’ or ‘Z-disc’.Actin filaments attach at Z-disc by a protein named as ‘alpha-actinin’. Z-disc separates two I-band at its middle. The area between the two Z-discs is considered a sarcomere.

Additional Information: -Actin filaments are thinner than myosin filaments, hence, are also termed as thin and thick filaments respectively.
- I-band consists of only thin filaments while A-band consists of both thick and thin filaments.
- At a resting state, the edges of thin filaments on either side of the thick filament partially overlap the free ends of thick filaments leaving the central part of the thick filaments. This free central portion of the thick filaments which is not overlapped by any thin filament is called the H-zone. This region is comparatively lighter.
- In the middle of the H-zone, a vertical ‘M-line’ is visible which contains accessory proteins to hold thick filaments.
- In the middle of the I-band, Z-disc is located. It is also known as Krause’s membrane or Dobeie’s line. - The area between two successive Z-line or ‘sarcomere’ is considered as the functional unit of contraction. It consists of an A-band in the middle and half of an I-band from either side.
So, the correct answer is ‘ Sarcomeres.’

Note: Skeletal muscle or striated muscle has a striated appearance due to the arrangement of thin(actin) and thick(myosin) myofilaments. Actin filaments are thinner but shorter than myosin filaments. It is made up of three proteins- actin, troponin, and tropomyosin while myosin filament is made of myosin and meromyosin proteins.