Discuss the microscopic structure of skeletal muscle tissue.

(N/A) Each organised skeletal muscle in our body is made of a number of muscle bundles or fascicles held together by a common collagenous connective tissue layer called fascia. Each muscle bundle contains a number of muscle fibres.
- Each muscle fibre is lined by the plasma membrane called sarcolemma enclosing sarcoplasm.
- Muscle fibre is a syncytium as the sarcoplasm contains many nuclei.
- The endoplasmic reticulum of the muscle fibres (sarcoplasmic reticulum) is the storehouse of calcium ions.
- $A$ characteristic feature of the muscle fibre is the presence of a large number of parallelly arranged filaments in the sarcoplasm called myofilaments or myofibrils.
- Each myofibril has alternate dark and light bands on it. It is due to the distribution pattern of two important proteins - Actin and Myosin.
- The light bands contain actin and is called $I$-band (Isotropic band).
- The dark band is called $A$-band (Anisotropic band) and contains myosin protein.
- Both the proteins are arranged as rod-like structures,parallel to each other and to the longitudinal axis of the myofibrils.
- Actin filaments are thinner as compared to the myosin filaments.
- In the centre of each $I$-band is an elastic fibre called $Z$-line which bisects it. The thin filaments are firmly attached to the $Z$-line.
- The thick filaments in the $A$-band are also held together in the middle of this band by a thin fibrous membrane called $M$-line.
- The $A$ and $I$ bands are arranged alternately throughout the length of the myofibrils.
- The portion of the myofibril between two successive $Z$-lines is considered as the functional unit of contraction and is called a sarcomere.
- In a resting state,the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part of the thick filament,not overlapped by thin filaments,is called $H$-zone.