The Contractile Apparatus of Skeletal Muscle

The Contractile Apparatus of Skeletal Muscle

The spatial relation between the filaments that make up the myofibrils within skeletal muscle fibres is highly regular. This regular organisation of the myofilaments gives rise to the cross-striation, which characterises skeletal and cardiac muscle. Sets of individual "stria" correspond to the smallest contractile units of skeletal muscle, the sarcomeres. Rows of sarcomeres form the myofibrils (), which extend throughout the length of the skeletal muscle fibre.

Depending on the distribution and interconnection of myofilaments a number of "bands" and "lines" can be distinguished in the sarcomeres:

I-band - actin filaments, A-band - myosin filaments which may overlap with actin filaments, H-band - zone of myosin filaments only (no overlap with actin filaments) within the A-band, Z-line - zone of apposition of actin filaments belonging to two neighbouring sarcomeres (mediated by a protein called alpha-actinin), M-line - band of connections between myosin filaments (mediated by proteins, e.g. myomesin, M-protein).

The average length of a sarcomere is about 2.5 µm (contracted ~1.5 µm, stretched ~3 µm).

The protein titin extends from the Z-line to the M-line. It is attached to the Z-line and the myosin filaments. Titin has an elastic part which is located between the Z-line and the border between the I- and A-bands. Titin contributes to keeping the filaments of the contractile apparatus in alignment and to the passive stretch resistance of muscle fibres.
Other cytoskeletal proteins interconnect the Z-lines of neighbouring myofibrils. Because of this connection, the A- and I-bands of neighbouring myofibrils lie side-by-side in the muscle fibre. These cytoskeletal proteins also connect the Z-lines of the peripheral myofibrils to the sarcolemma.

Muscle-Tendon Junction

At the muscle-tendon junction, the end of a muscle fibre forms deep invaginations, which increase its surface area. The basement membrane of the muscle fibre extends into these invagination and, so do the collagen fibrils of the tendons. The actin filaments of the last sarcomeres extend into cytoplasmic specialisations associated with zonula adherens-like membrane specialisations. Instead of interconnecting two cells, the cell membrane is here anchored to the basement membrane of the muscle cell. The basement membrane is, in turn, connected to the collagen fibrils of the tendons. 

Title: The Contractile Apparatus of Skeletal Muscle
by:om at 2013-02-15T03:08:00+07:00
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The Contractile Apparatus of Skeletal Muscle