Muscle & Nerve Tissue I

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Introduction to Muscles "Multicellular contractile units" Common feature: Presence of contractile proteins (actin & myosin) Single-Cell Contractile Units Myoepithelial cells (Glandular tissue) Myofibroblasts (Normal CT/scar tissue) Pericytes (Blood vessels) Skeletal Muscles - Microscopic Structure Muscle cells/myocytes ("fibres"): Long, thin, form bundles (fascicles). A muscle is composed of many fascicles. Connective Tissue Coverings: Endomysium: surrounds each muscle fibre. Perimysium: surrounds bundles of muscle fibres/fascicles. Epimysium: surrounds the entire muscle. Fascicle size reflects function: Small fasciculi: fine, highly controlled movements. Large fasciculi: gross movements. Skeletal Muscle Cells (Fibres) - Features Extremely elongated, un-branched, cylindrical. Multinucleated: nuclei are flattened, located at regular intervals beneath the cell membrane. Presence of cross striations (visible by H&E). Capillaries run along the length of the fibres. Skeletal Muscle - Ultrastructure Sarcolemma: Cell/plasma membrane. Sarcoplasm: Cytoplasm (mitochondria, other organelles, glycogen). T system: Extensions of the sarcolemma into the cell. Myofibrils: Contractile structures within fibres, elongated, cylindrical, parallel to long axis. Myofilaments: Long protein structures within myofibrils (actin, myosin, titin). Sarcoplasmic Reticulum: Smooth ER, embraces each myofibril in a network of membranes. Other important proteins: Desmin (intermediate filament protein). Dystrophin: links myofibrils to sarcolemma, transmits contractile forces. Sarcomere - Contractile Unit Composed of actin (thin) & myosin (thick) filaments. Distance between two successive Z-lines. Bands & Zones: I band: Isotropic, lighter, only thin filaments. A band: Anisotropic, darker, contains thick filaments. H zone: "Clear disc," no overlap of filaments. M line: "Middle disc," anchors thick filaments. Z disc: "Between disc," anchors thin filaments, pulled closer during contraction. Titin (Connectin): Anchored to Z-band and M-line, maintains precise structural arrangement. Motor End Plate & Motor Unit Motor End Plate: Terminal branches of a motor axon ending in irregular, club-shaped synaptic end formations, embedded in the sarcolemma. Motor Unit: Group of muscle fibres supplied by an individual motor nerve fibre. Skeletal Muscle - Functional Adaptations Sarcomere: Functional unit with arrangement of contractile proteins. T system: Allows simultaneous contraction of all sarcomeres. Sarcoplasmic Reticulum: Concentrates $\text{Ca}^{2+}$ within its lumen. Capillary network, mitochondria, glycogen: Meet high energy requirements. Connective tissue framework: Anchors muscle fibres, continuous with tendons. Smooth Muscles - General Function: Continuous rhythmic contractions. Contract as a whole muscle (not motor units). Influenced by ANS, hormones, local metabolites. Smooth Muscle - Microscopic Anatomy Cells/Fibres: Short, elongated spindle-shaped with tapered ends (may bifurcate). Bound together in irregular branching fasciculi (functional units). Cytoplasm: Elongated central single nucleus, few mitochondria. Contractile proteins: Not arranged as myofibrils (no cross striations). Actin and myosin criss-cross the cell. Following contraction, cells become globular. Cardiac Muscles - General Functional requirement: Strong, continuous contractions. Features intermediate between skeletal and smooth muscle. Cardiac Muscle - Microscopic Anatomy Muscle fibres/cells: Long, cylindrical, branched. 1-2 central nuclei. Contractile proteins arranged like skeletal muscle (cross striations seen). T Tubules and sarcoplasmic reticulum present. Intercalated discs: Characteristic feature. Highly modified cells form pacemaker cells & conducting system. Connective tissue (endomysium) between muscle fibres, rich capillary network. Intercalated Discs Specialized intercellular junctions between adjacent cells. Points of adherence of myofibrils. Transmit forces of contraction. Areas of low resistance, permitting rapid spread of contractile stimuli. Clinical Conditions Myocardial Infarction: Affects cardiac muscle. Duchenne Muscular Dystrophy: Affects skeletal muscle, gene defects of the dystrophin gene. Comparison of Muscle Types Feature Skeletal Muscle Cardiac Muscle Smooth Muscle Cell Shape Long, cylindrical Branched, cylindrical Spindle-shaped Nuclei Many, peripheral 1-2, central Single, central Striations Present Present Absent Control Voluntary Involuntary Involuntary Intercalated Discs Absent Present Absent T Tubules Well-developed Present Rudimentary/Absent Regeneration Limited (satellite cells) Very limited Good