Muscles and Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is one of the four primary tissue types in the human body, alongside epithelial, connective, and nervous tissues. Muscle tissue is specialized for contraction, enabling movement and various physiological functions.

• Three types of muscle tissue: Skeletal, Cardiac, and Smooth (see Table 9.3).

• Muscle cell: Also called a muscle fiber, especially in skeletal and smooth muscle, characterized by elongation.

• Prefixes indicating muscle: myo-, mys-, sarco-.

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and specialized structures.

• Sarcoplasmic reticulum (SR): Specialized smooth endoplasmic reticulum that stores and releases calcium ions for muscle contraction.

Types of Muscle Tissue

Skeletal Muscle: Structure and Function

Skeletal muscle is responsible for voluntary movements and is highly adaptable, capable of generating a wide range of forces.

• Functions: Locomotion, manipulation, maintaining posture, stabilizing joints, generating heat, regulating entrances/exits of digestive and urinary tracts.

• Histology: Striations due to actin and myosin proteins; multinucleate fibers; nuclei are peripheral and flattened; fibers are long (10-30 cm).

• Formation: Fibers formed by fusion of embryonic myoblasts; satellite cells aid regeneration.

Cardiac Muscle: Structure and Function

Cardiac muscle is found only in the heart and is specialized for rhythmic, involuntary contractions.

• Location: Heart wall.

• Function: Pumps blood throughout the body.

• Control: Involuntary; pacemaker cells initiate contraction; neural controls modulate heart rate.

• Histology: Striated, bifurcated fibers, uninucleate, intercalated discs (desmosomes for strength, gap junctions for communication).

Smooth Muscle: Structure and Function

Smooth muscle is found in the walls of hollow organs and blood vessels, responsible for slow, sustained, involuntary contractions.

• Functions: Movement of substances, regulation of vessel diameter, arrector pili function, valve operation.

• Histology: Spindle-shaped, arranged in sheets, non-striated, oval central nuclei, uninucleate.

Functional Characteristics of Muscle

Muscle tissue exhibits four key properties essential for its function.

• Excitability (Responsiveness): Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched without damage.

• Elasticity: Ability to return to original shape after stretching.

Muscle Functions

• Movement

• Maintaining posture and body position

• Stabilizing joints

• Generating heat

Skeletal Muscle Arrangement

Each skeletal muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue sheaths.

• Each muscle has one nerve, one artery, and one or more veins.

• Nerve endings control contraction; arteries deliver oxygen/nutrients; veins remove waste.

• Connective tissue sheaths:

  • Epimysium: Dense regular CT around entire muscle, connects to deep fascia.

  • Perimysium: Fibrous CT surrounding groups of muscle fibers (fascicles).

  • Endomysium: Fine sheet of CT surrounding each muscle fiber.

• Tendon: Combination of epimysium, perimysium, and endomysium at muscle end, attaches to bone.

• Aponeurosis: Tendon in the form of a broad sheet.

Types of Muscle Attachment

• Direct attachment: Epimysium fuses to periosteum of bone.

• Indirect attachment: CT wrappings extend beyond muscle as tendon or aponeurosis.

• Origin: Point of attachment on the immovable bone.

• Insertion: Point of attachment on the movable bone.

Histology of Skeletal Muscle

• Fiber: Long cylindrical cell with 100+ nuclei beneath sarcolemma.

• Diameter: 10-100 micrometers; Length: 10-30 cm.

• Formation: Fusion of myoblasts; satellite cells for regeneration.

• Sarcoplasm: Contains glycosomes (store glycogen for energy) and myoglobin (stores oxygen).

• Myofibrils: Densely packed, rod-like contractile elements (hundreds per fiber).

• Myofibril content: Makes up ~80% of muscle fiber content.

• Bands: A band (dark, myosin/thick), I band (light, actin/thin), M line (center of thick filaments), H zone (only thick filaments), Z discs (anchor for actin).

• Sarcomere: Smallest contractile unit; region between two Z discs.

Molecular Detail of Muscle Contraction

• Thick filament: Composed of myosin (dimer protein with rod-like tail and two globular heads).

• Myosin heads: Form cross bridges; ATPases break down ATP.

• Thin filament: Composed of actin (two helical polymers twisted together).

• Tropomyosin: Covers myosin binding site on actin in relaxed muscle.

• Troponin: Binds calcium, moves tropomyosin to expose binding site for contraction.

Sarcoplasmic Reticulum and T Tubules

• Sarcoplasmic reticulum (SR): Smooth ER surrounding each myofibril; stores calcium ions.

• Terminal cisternae: Perpendicular cross channels.

• T tubules: Extensions of sarcolemma penetrating cell interior at A-band/I-band junctions; conduct electrical impulses.

• Triad: Two terminal cisternae + one T tubule; site of communication.

Sliding Filament Theory of Muscle Contraction

The sliding filament theory explains how muscles contract by the sliding of actin (thin) filaments past myosin (thick) filaments, increasing their overlap.

• Myosin heads bind to actin and pull, causing muscle shortening.

• ATP is required for contraction.

• During contraction:

  • H zone and I band decrease in size.

  • Z lines move closer together.

  • A band remains constant.

Neuromuscular Junction and Excitation-Contraction Coupling

Muscle contraction is initiated at the neuromuscular junction, where motor neurons communicate with muscle fibers.

• Motor neuron axons branch and form neuromuscular junctions with muscle fibers.

• Arrival of nerve impulse opens voltage-gated calcium channels; calcium triggers exocytosis of synaptic vesicles containing acetylcholine (ACh).

• ACh diffuses across synaptic cleft, binds to receptors on sarcolemma.

• ACh is degraded by acetylcholinesterase after signaling.

• Sarcolemma propagates action potential to T tubules, triggering SR to release calcium.

• Calcium binds troponin, moving tropomyosin and allowing myosin-actin interaction.

• Contraction stops when calcium is reabsorbed into SR.

Crossbridge Cycle

• Attach: Calcium present, myosin binds actin.

• Pivot: Release of ADP + Pi, myosin head pivots.

• Detach: ATP binds, myosin detaches from actin.

• Return: ATP hydrolyzes to ADP + Pi, myosin head returns to original position.

• Rigor mortis: Occurs when no ATP is available; myosin remains attached to actin.

Depolarization and Repolarization

• Resting state: More K+ inside, more Na+ outside; maintained by sodium-potassium ATPase pump.

• ACh binding opens chemically gated channels; Na+ enters, causing depolarization.

• Action potential propagates along sarcolemma.

• Repolarization: Na+ channels close, K+ channels open; K+ exits, restoring resting potential.

Equation for sodium-potassium pump:

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates.

• Small motor units: Fine movements (e.g., fingers, eyes).

• Large motor units: Gross movements (e.g., thighs, hips).

• Muscle fibers are distributed throughout the muscle for coordinated contraction.

Muscle Twitch and Summation

• Twitch: Response to a single, brief threshold stimulus.

• Phases: Latent period (excitation-contraction coupling), contraction (crossbridge formation), relaxation (calcium reabsorbed).

• Summation: Frequent stimuli increase contractile force; muscle does not relax fully.

• Unfused tetanus: Quivering contraction due to frequent stimuli.

• Tetanus: Smooth, sustained contraction; stimuli are so frequent that relaxation does not occur.

Muscle Tone

• Constant, slightly contracted state of all muscles.

• No active movement; motor units alternate activation.

• Keeps muscles firm, healthy, and ready to respond.

• Regular exercise increases muscle tone and metabolic rate.

Types of Muscle Contraction

Muscle Metabolism

ATP is the sole energy source for muscle contraction. Muscles store only a small amount of ATP and must regenerate it rapidly.

• ATP regeneration:

  • Interaction of ADP with creatine phosphate (CP):

  • Enzyme: Creatine kinase (CK/CPK); presence in blood indicates muscle damage.

  • CP increases maximum muscle power for ~10 seconds.

• Glycolysis: Anaerobic, releases 2 ATP per glucose; pyruvic acid converts to lactic acid under anaerobic conditions.

• Lactic acid: Travels in bloodstream, used by heart, liver, kidneys; increases activity duration to ~40 seconds.

• Aerobic respiration: Occurs in mitochondria, requires oxygen; produces 32 ATP per glucose.

• Equation:

• At rest/light exercise: Uses glucose, pyruvic acid, free fatty acids; later, mostly fatty acids.

• Heat production: 40% of energy moves muscle; 60% released as heat.

Effects of Exercise

• Aerobic exercise: Increases capillaries, mitochondria, myoglobin synthesis, metabolism efficiency, endurance, strength, resistance to fatigue.

• Resistance exercise: Increases muscle fiber size, mitochondria, myofilaments, glycogen storage, connective tissue.

• Disuse atrophy: Degeneration and loss of muscle mass; up to 5% loss per day with bedrest/paralysis.

Muscle Relaxation

• Elastic recoil

• Opposing muscle contraction (quicker than recoil)

• Gravity assists relaxation

Muscle Fatigue

• Depleted metabolic reserves (glycogen, lipids, amino acids, ATP)

• Decreased pH in muscle fiber and whole muscle

• Weariness: Decreased pH and pain signals sent to brain

• Physical damage to sarcolemma and SR

Example:

During a sprint, skeletal muscle rapidly uses stored ATP and creatine phosphate, then switches to glycolysis and lactic acid production. Prolonged activity relies on aerobic respiration for sustained energy.

Additional info: Table 9.3 and Figures referenced in the notes provide visual representations of muscle types, histology, and contraction mechanisms. Key terms and concepts are expanded for clarity and completeness.