Question

Define the following structure:
Muscle, Muscle fibre, Myofibril, sarcomere, Filaments.

Answer 1

Hint: An animal body is broadly composed of four types of tissues. One of them contributes most to the total weight of the body. These are concerned with the movement and locomotion of the body of an animal.

Complete answer:
Muscles: Muscles or muscular tissue are the soft tissues in the body which generates the physical force needed to make the body structures move.
Muscle fibre: 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.
Myofibril: Muscle fibres are formed by parallelly arranged myofibrils of myofilaments.
Filaments: Myofilament is further composed of thin and thick filaments, thin filaments are made up of actin proteins while thick filaments are made up of myosin protein. 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.

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.