A: I. Structure of a skeletal muscle:
Skeletal muscle is formed by a number of bundles of muscle fibres called
fascicles. The fascicles of a muscle are held together by collagenous connective
tissue layer called fascia.
a. Muscle fibre: Muscle fibre of a skeletal muscle is a syncitium because it is
formed by the fusion of many uninucleate myocytes of embryo. It is bound by
a sarcolemma (plasmalemma) and the cytoplasm is called sarcoplasm. It
contains many, peripheral nuclei. Endoplasmic reticulum of muscle fibre is
known as sarcoplasmic reticulum (it is the store house of Ca++ ions).
Sarcoplasm contains many parallel filaments called myofibrils.
b. Myofibril: A myofibril of striated muscle fibre has alternate dark (A - band /
Anisotropic band) and light (I - band/ Isotropic band) bands. A - band
contains thick filaments formed by a protein called myosin. I - band contains
thin filaments formed by a protein called actin besides two regulatory proteins,
troponin and tropomyosin. The thin filaments extend into A - band among
thick filaments and therefore, overlap in certain parts of A-band giving more
darker appearance.
Hence, the part of A - band which contains only thick
filaments is little bit lighter and is known as H - zone or Hensen's disc. The
thick filaments in A - band are held together in the middle by a thin membrane
called M - line. In the centre of I - band, there is an elastic fibre called Z - line
(Krause's membrane or Dobie's line). The portion of myofibril between two
successive Z - lines is known as sarcomere. It is the functional unit of
contraction.
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