Muscle Types and Structure

Classification of Muscle Types

• Skeletal muscle: long, multinucleated cells with cross-striations, voluntary, and forceful
• Cardiac muscle: branched cells with cross-striations, intercalated discs, involuntary, and rhythmic
• Smooth muscle: fusiform cells without striations, involuntary, and slow

Specialized Terms

• Sarcolemma: the cell membrane
• Sarcoplasm: the cytoplasm
• Sarcoplasmic reticulum (SER): a type of smooth endoplasmic reticulum

Organization of Skeletal Muscle

• Epimysium: dense connective tissue surrounding the entire muscle
• Perimysium: thin connective tissue surrounding bundles of muscle fibers (fascicle)
• Endomysium: reticular fiber and fibroblast surrounding individual muscle fibers

Light Microscopic Structure of Skeletal Muscle Fibers

• Long, cylindrical cells with multiple nuclei beneath the sarcolemma
• Cross-striations: alternating dark and light bands
• Myofibrils: thread-like structures arranged in parallel in the muscle fibers

Myofibrils and Sarcomere

• Myofibrils: long, cylindrical filamentous bundles parallel to the long axis of the muscle fibers
• Sarcomere: the segment between two Z lines (½ I band + 1 A band + ½ I band), the unit of structure and function of muscle fibers
• I band: bisected by a dark transverse line, Z line
• A band: a light zone present, H band
• H band: bisected by M line

Myofilaments

• Thick filaments: composed of myosin molecules with a rod and two globular heads, occupying A band
• Thin filaments: composed of actin, tropomyosin, and troponin, inserting into the Z line and extending into the A band

Transverse Tubules and Sarcoplasmic Reticulum

• Transverse (T) tubules: sarcolemma tubular infoldings at A- and I- band boundaries, responsible for rapid conduction of impulses
• Sarcoplasmic reticulum: storing calcium ions, located between two adjacent T tubules
• A triad: a T tubule + 2 terminal cisternae, triggering Ca2+ release

Mechanism of Contraction

• Sliding filament hypothesis: myosin heads combine with actin to form cross-bridges, then pivot, moving thin filaments toward the sarcomere center

Cardiac Muscle

• Found in the heart and proximal portions of the aorta and venae
• Involuntary, striated muscle, contracting spontaneously and displaying a rhythmic beat
• Short, columnar, and branched cells with one or two centrally placed nuclei
• Cross-striations and myofibrils, but less distinct
• Intercalated disks link cells to one another

Ultrastructure of Cardiac Muscle

• Discrete myofibril bundles are not present
• Larger T tubules at Z line level
• Sarcoplasmic reticulum: not well-developed, small terminal cisternae
• Dyads: T tubule and terminal cisternae on one side
• More mitochondria, glycogen, and lipid droplets

Intercalated Disks

• Transverse and lateral portions
• Desmosomes: bind cardiac cells together
• Fascia adherens: supply anchoring sites for actin filaments
• Gap junctions: provide ionic continuity between adjacent cells

Smooth Muscle

• Seen in blood vessels and hollow viscera (visceral muscle)
• Arranged in layers
• Enclosed by a network of type I and type III collagen fibers
• Nonstriated cells

Light Microscopic Structure of Smooth Muscle

• Spindle-shaped cells with an oval, centrally located nucleus
• A range of diameters
• Without striations

Electron Microscopic Structure of Smooth Muscle

• Gap junctions
• Caveolae
• A rudimentary sarcoplasmic reticulum
• Myofibrillar arrays: thick filaments, thin filaments, dense bodies, and intermediate filaments (desmin)

Other Functions of Smooth Muscle

• Supplement fibroblast activity: synthesizing collagen, elastin, and proteoglycans
• Be similar to other contractile cells, such as myofibroblasts and pericytes

Comparative Features

• Skeletal muscle: long, cylindrical, multinucleated, peripheral
• Cardiac muscle: branching, central, single nucleus
• Smooth muscle: spindle, central, single nucleus