VPB 121: Veterinary Physiology - II (2+1)

STRUCTURE OF MUSCLE

• A muscle has an origin (less movable), belly and the insertion (highly movable) parts.Muscles are surrounded by a sheet of connective tissue called epimysium (site for fat deposition).
• From epimysium smaller sheets of connective tissue called perimysium arise and divide muscle into several small bundles. This perimysium contains blood vessels, nerves and muscles spindles. Perimysium envelopes a bundle of 50 to 200 fibres. At the end of each fibre, the sarcolemmal sheath fuses to form tendon fibre. Bundle of tendon fibres in turn forms the muscle tendon, which aids the attachment of the muscle into the bones. Each muscle fiber is enclosed by endomycium which is a fine sheet of connective tissue arising from perimysium.
• A muscles fiber is termed as a muscle cell- which is elongated tubular structure with tapering ends. Muscle cell is covered by the plasma membrane called sarcolemma (Gk: Sarkos – flesh). Each muscle cell is around 20-30 mm in length with a diameter of 10-100 μm. Length and diameter varies with exercise, plane of nutrition and maturity. In a muscle, the muscle fibers are arranged as number of muscle bundles known as "fasciculus".

Muscle cells can be grouped into 3 categories as per its metabolism

• FG cells (fast twitch glycolytic cells) – Type IIIb (white) – absent in canines
• FOG cells (fast twitch oxidative glycolytic cells) – Type IIa
• SO cells (slow twitch oxidative cells) – Type I (red)

Sub cellular components

• Muscle cell contains organelles like sarcolemma, nucleus, Golgi apparatus, lysosomes, mitochondria or sarcosomes, endoplasmic or sarcoplasmic reticulum, perioxisomes and ribosomes and myofibrils.

• Sarcolemma is about 70 – 100 nm in thickness composed of 60% proteins, 20% phospholipids, 15% cholesterol and 5% collagen fibrils, can propagate an action potential.
• Skeletal muscle cells are multinucleated and an average of 100 - 200 nuclei per cell. Because skeletal muscle cell in a mature muscle originates form fusion of 100 – 200 different embryonic muscle cells called myoblasts, with each myoblast contributing one nucleus. They lie peripherally just under sarcolemma in mature vertebrates skeletal muscle. Nuclei of skeletal muscle lack mitotic ability where as smooth and cardiac muscle cell contains only one nucleus, located in the interior of the cell.
• Cardiac muscles cells and FOG and SO types of skeletal muscle cells contains numerous mitochondria and they are concerned with aerobic metabolism for energy and are capable of prolonged and sustained contractions. FG skeletal muscle cells contain a low no of sarcosomes.
• Muscle cells contain small spherical glycogen granules which are numerous in cells of well fed rested animals than in muscle cells of starved or exhausted animals (1% of the muscle mass).