-Structure of Skeletal Muscle: Skeletal muscles are usually attached to bone by tendons composed of connective tissue. This connective tissue also ensheathe the entire muscle & is called epimysium. Skeletal muscles consist of numerous subunits or bundles called fasicles (or fascicles). Fascicles are also surrounded by connective tissue (called the perimysium) and each fascicle is composed of numerous muscle fibers (or muscle cells which are multi-nucleated).
Fascicles are visible to the naked eye in fresh muscle. Muscle cells, ensheathed by endomysium, consist of many fibrils (or myofibrils), and these myofibrils are made up of long protein molecules called myofilaments. Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions. The basic unit of muscle contraction is the sarcomere. Skeletal and cardiac muscle are striated (smooth muscle has a similar contractile mechanism but is not so highly organized and does not show striations).
The striations are caused by alignment of bands: the most prominent are the A (anisotropic) and I (isotropic) bands and the Z line. Each muscle fiber contains contractile units called myofibrils which are visible under the light microscope. However, when viewed under electron microscope, each myofibril is composed of smaller units, the myofilaments.
The myofilaments are of two kinds, differing in dimension and chemical composition, and their arrangement is responsible for the cross-banded pattern seen in striated muscle. The A band is principally formed by the parallel arrays of myosin filaments.
The myosin filaments are slightly thicker in the middle and taper towards both ends. The myosin filaments are held together at the midpoint of the A band by cross connections to form the M line. The thinner actin filaments constitute the I band and they extend in either direction from the Z line.
They are not limited to the I band, but extend some distance into the adjacent A band to occupy the spaces between the thicker myosin filaments.
The H band is the central region of the A band that is not penetrated by the actin filaments. In stretched myofibrils the H band is broad whereas in the contracted state is very narrow or entirely missing. The unit between 2 Z lines/discs is called the sarcomere. The sarcomere is the basic unit of contraction.
Githiari answered the question on November 8, 2017 at 05:51
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