Muscle Tissue and Myofilaments

• Muscle tissue is responsible for movement of the body and its parts as well as changes in the size and shape of internal organs.
• Aggregates of specialized, elongated cells arranged in parallel arrays characterize this tissue, in which contraction is the primary function.

Myofilaments

• Two types of myofilaments associated with cell contraction: thin and thick.
• Thin filaments are 6 to 8 nm in diameter and 1.0 µm long, composed primarily of the protein actin.
• F-actin forms each thin filament as a polymer primarily formed from G-actin molecules.
• Thick filaments are ~15 nm in diameter and 1.5 µm long, composed primarily of the protein myosin II.
• Each thick filament consists of 200 to 300 myosin II molecules.
• The long, rod-shaped tail portion of each molecule aggregates in a regular parallel arrangement, but the head portions project out in a regular helical pattern.
• Myofilaments form the bulk of the sarcoplasm in muscle cells.
• Although actin and myosin are also present in most other cell types, muscle cells contain a large number of aligned contractile filaments to facilitate mechanical work.

Muscle Classification

• Muscle is classified according to the appearance of contractile cells.
• Two principal types of muscle are recognized: striated and smooth.
• Cells in striated muscle possess cross-striations at the light microscope level.
• Cells in smooth muscle do not exhibit cross-striations.
• Striated muscle is further subclassified on the basis of location: skeletal, visceral, and cardiac.

Skeletal Muscle

• Attached to bone, responsible for movement of the axial and appendicular skeleton, and for maintenance of body position and posture.
• Provides precise eye movement as extraocular muscles of the eye.

Visceral Striated Muscle

• Morphologically identical to skeletal muscle but is restricted to the soft tissues.
• Specifically, it can be found in the tongue, pharynx, lumbar part of the diaphragm, and upper part of the esophagus, where it plays an essential role in speech, breathing, and swallowing.

Cardiac Muscle

• Striated muscle found in the wall of the heart and in the base of the large veins that empty into the heart.

Skeletal Muscle Syncytium

• Skeletal muscle cells are multinucleated syncytium.
• Each muscle cell, or fiber, is formed during development by the fusion of small, individual muscle cells called myoblasts.
• Mature multinucleated muscle fibers reveal a polygonal shape with a diameter of 10 to 100 µm when viewed in cross-section.
• Fiber lengths vary from millimeters, as in the stapedius muscle of the middle ear, to almost a meter, as in the sartorius muscle of the lower limb.
• Nuclei of a skeletal muscle fiber are located in the cytoplasm immediately beneath, the sarcolemma.
• Sarcolemma consists of the plasma membrane of the muscle cell, its external lamina, and the surrounding reticular lamina.
• Skeletal muscles consist of striated muscle fibers held together by connective tissue.

Muscle Connective Tissue

• Essential for force transduction.
• At the end of the muscle, the connective tissue continues as a tendon or some other arrangement of collagen fibers that attaches the muscle, usually to bone.
• A rich supply of blood vessels and nerves travels in the connective tissue.
• Connective tissue associated with muscle includes: endomysium, perimysium, and epimysium.

Endomysium and Perimysium

• The delicate layer of reticular fibers that immediately surrounds individual muscle fibers.
• Only small-diameter blood vessels and the finest neuronal branches are present within the endomysium, running parallel to the muscle fibers.
• The thicker connective tissue layer that surrounds a group of fibers to form a bundle or fascicle.
• Fascicles are functional units of muscle fibers that tend to work together to perform a specific function.
• Larger blood vessels and nerves travel in the perimysium.
• The sheath of dense connective tissue that surrounds a collection of fascicles that constitutes the muscle.
• Major vascular and nerve supply of the muscle penetrates the epimysium.

Skeletal Muscle Fiber Types

• Three types: red, white, and intermediate.
• The color differences are not apparent in hematoxylin and eosin (H&E)-stained sections.
• Histochemical reactions, based on oxidative enzyme activity, reveal several types of skeletal muscle fibers.

Muscle Fiber Characteristics

• Classified by contractile speed, enzymatic velocity of the fiber's myosin ATPase reaction, and metabolic profile.
• Contractile speed determines how fast the fiber can contract and relax.
• Velocity of the myosin ATPase reaction determines the rate at which this enzyme can break down ATP molecules during the contraction cycle.
• The metabolic profile indicates the capacity for ATP production by oxidative phosphorylation or glycolysis.
• Fibers characterized by oxidative metabolism contain large amounts of myoglobin and increased numbers of mitochondria, with their constituent cytochrome electron transport complexes.

Myoglobin

• Small, globular, 17.8 kDa oxygen-binding protein that contains a ferrous form of iron (Fe+2).
• Resembles hemoglobin in the erythrocytes.
• Functions primarily to store oxygen in muscle fibers and provides a readily available source for muscle metabolism.
• Indicative of muscle injury when detected in the blood.
• The three types of skeletal muscle fibers are type I (slow oxidative), type lla (fast oxidative glycolytic), and type Ilb (fast glycolytic) fibers.

Type I Fibers

• Small fibers that appear red in fresh specimens and contain many mitochondria and large amounts of myoglobin and cytochrome complexes.
• Demonstrated by strong succinic dehydrogenase and NADH-TR histochemical staining reactions.
• Show high levels of mitochondrial oxidative enzymes.
• Slow-twitch, fatigue-resistant motor units but generate less tension.
• The fiber's myosin ATPase reaction velocity is the slowest of all of the fiber types.
• Type I fibers are typically found in the limb muscles of mammals and in the breast muscle of migrating birds.
• Type I fibers adapted to the long, slow contraction needed to maintain erect posture are the principal fibers of the long erector spinae muscles of the back in humans.

Type Ila Fibers

• These intermediate fibers appear red in fresh tissue.
• Contain large amounts of glycogen and are capable of anaerobic glycolysis.
• Fast-twitch, fatigue-resistant motor units and generate high peak muscle tension.
• Athletes who have these fibers include 400- and 800-m sprinters, middle-distance swimmers, and hockey players.

Type IIb Fibers

• They have low levels of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen.
• fast-twitch fatigue-prone motor units which generate high peak tensions
• Have the fastest myosin ATPase velocity of all the other fiber types.
• Short distance sprinters, weight lifters, and other field athletes have a high percentage of type IIb fibers.