Muscles and Muscle Tissue

Types and Functional Characteristics of Muscle Tissue

• Muscle Types:
  • Skeletal Muscle: Packaged onto the bony skeleton, striated, voluntary, contracts rapidly but tires easily, responsible for overall body motility, and adaptable enough to exert forces ranging from a fraction of an ounce to over $70\,\text{pounds}$.
  • Cardiac Muscle: Located exclusively in the heart, striated, involuntary, and contracts at a steady rate set by the heart's pacemaker with neural controls accommodating bodily changes.
  • Smooth Muscle: Located in walls of hollow visceral organs (including the stomach, urinary bladder, and respiratory passages), non-striated, involuntary, and propels substances through internal channels.
• Shared Characteristics & Terminology:
  • Skeletal and smooth muscle cells are elongated and termed muscle fibers.
  • Muscle contraction relies on two types of myofilaments: actin and myosin.
  • Muscle prefixes include myo-, mys-, and sarco- (e.g., sarcolemma refers to the plasma membrane, sarcoplasm to the cytoplasm).
• Core Functional Properties:
  • Excitability (Irritability): Ability to receive and respond to stimuli.
  • Contractility: Ability to shorten forcibly.
  • Extensibility: Ability to be stretched or extended.
  • Elasticity: Ability to recoil and resume original resting length.

Skeletal Muscle Organization and Attachments

• Connective Tissue Sheaths:
  • Endomysium: Fine sheath of reticular fibers surrounding each muscle fiber.
  • Perimysium: Fibrous connective tissue surrounding bundles of fibers called fascicles.
  • Epimysium: Overcoat of dense regular connective tissue surrounding the entire muscle organ.
• Nerve and Blood Supply:
  • Each muscle is served by one nerve, one artery, and one or more veins.
  • Every muscle fiber possesses an individual nerve ending controlling its contraction.
  • Continuous oxygen and nutrient delivery is handled by arteries, while metabolic wastes exit through veins.
• Muscle Attachments:
  • Skeletal muscles span joints; during contraction, the movable bone (insertion) pulls toward the immovable or less movable bone (origin).
  • Direct Attachment: Epimysium fuses directly to the periosteum of a bone.
  • Indirect Attachment: Connective tissue wrappings extend beyond the muscle as a cord-like tendon or flat sheet-like aponeurosis.

Microscopic Anatomy and the Sarcomere

• Muscle Fiber Structure:
  • Elongated cylindrical syncytium formed by fusion of embryonic cells, containing multiple nuclei situated immediately beneath the sarcolemma.
  • Dimensions span $10\text{ to }100\,\text{mm}$ in diameter and extend up to hundreds of centimeters in length.
  • Sarcoplasm is packed with glycosomes for glycogen storage and myoglobin for oxygen binding.
• Myofibrils & Banding Structure:
  • Rodlike contractile elements that make up the vast majority of muscle volume.
  • Characterized by aligned, repeating alternating stripes: dark A bands and light I bands.
  • Sarcomere: The fundamental contractile unit of skeletal muscle, demarcated between two successive Z discs.
  • Z disc: Coin-shaped sheet of connectin proteins that anchors thin filaments and interconnects neighboring myofibrils.
  • H zone: Lighter midregion of the A band where thin filaments do not overlap thick filaments.
  • M line: Dark strip in the center of the H zone containing the protein desmin.
• Ultrastructure of Myofilaments:
  • Thick Filaments: Composed of myosin; each molecule consists of a rodlike tail made of two interwoven heavy polypeptide chains and two globular heads made of light polypeptide chains that form cross bridges.
  • Thin Filaments: Composed primarily of actin; helical polymer of globular G actin subunits bearing binding sites for myosin heads, paired with the regulatory subunits troponin and tropomyosin.
  • Elastic Filaments: Composed of titin, stabilizing thick filament positioning.

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic Reticulum (SR):
  • Elaborate smooth endoplasmic reticulum that runs longitudinally around individual myofibrils and controls intracellular $Ca^{2+}$ concentrations.
  • Forms perpendicular cross channels called paired terminal cisternae.
• T Tubules:
  • Elongated invaginations continuous with the sarcolemma that penetrate into the interior of the muscle fiber at every A band–I band junction.
  • Propagate electrical impulses deeply to trigger $Ca^{2+}$ release from the flanking terminal cisternae.
• Triads:
  • Three-part structures consisting of a central T tubule flanked by two terminal cisternae of the sarcoplasmic reticulum.
  • T tubule integral proteins function as voltage sensors, communicating with SR foot protein receptors to regulate $Ca^{2+}$ liberation.

Sliding Filament Model and Neuromuscular Junction

• Sliding Filament Mechanism:
  • Muscle contraction occurs when thin filaments slide past thick filaments, increasing the degree of overlap.
  • Myosin heads repeatedly attach to actin active sites, pivot in a ratchet-like stroke, and detach, pulling thin filaments toward the center of the sarcomere.
• Events at the Neuromuscular Junction:
  • Motor neurons from the somatic nervous system extend axons that form neuromuscular junctions with individual muscle fibers.
  • Axon terminals contain synaptic vesicles loaded with the neurotransmitter acetylcholine (ACh), separated from the motor end plate by the synaptic cleft.
  • Action potential arrival at the axon terminal triggers the opening of voltage-regulated $Ca^{2+}$ channels, driving $Ca^{2+}$ influx.
  • $Ca^{2+}$ influx stimulates exocytosis of ACh into the synaptic cleft.
  • ACh diffuses across the cleft and binds to ligand-gated receptors at the junctional folds of the motor end plate.
  • Receptor binding opens chemically gated channels, permitting simultaneous diffusion of $Na^+$ inward and $K^+$ outward, depolarizing the sarcolemma and generating a propagated action potential.
• Acetylcholine Breakdown:
  • Acetylcholinesterase rapidly splits ACh into acetic acid and choline in the synaptic cleft, halting stimulation and preventing sustained, unwanted contractions.