Chapter 10 – Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is essential for movement, posture, and various physiological functions. There are three main types: skeletal, cardiac, and smooth muscle, each with distinct structural and functional properties.

• Skeletal muscle: Voluntary, striated, multinucleated fibers; responsible for body movement.

• Cardiac muscle: Involuntary, striated, branched fibers; found only in the heart.

• Smooth muscle: Involuntary, non-striated, spindle-shaped cells; found in walls of hollow organs.

Properties of muscle cells:

• Excitability: Ability to respond to stimuli.

• Contractility: Ability to shorten forcefully.

• Extensibility: Ability to stretch without damage.

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

Muscle tension refers to the force generated by muscle contraction. It is distinct from conductivity, which is the ability to transmit electrical signals.

Structural Anatomy of Skeletal Muscle

Skeletal muscle structure is organized from macroscopic to microscopic levels, each contributing to its function.

Gross Anatomy

• Connective tissue wrappings:

  • Epimysium: Surrounds entire muscle.

  • Perimysium: Surrounds fascicles (bundles of fibers).

  • Endomysium: Surrounds individual muscle fibers.

• Tendon vs. Aponeurosis: Tendons are cord-like, aponeuroses are sheet-like attachments to bone.

• Blood vessels: Supply nutrients and oxygen to muscle tissue.

Microscopic Anatomy

• Muscle fiber: A single muscle cell, multinucleated and elongated.

• Satellite cell: Stem cell involved in muscle repair.

• Myofibril: Rod-like unit within muscle fiber, composed of sarcomeres.

• Myofilament: Protein filaments (actin and myosin) within myofibrils.

• Sarcomere: Functional unit of contraction, defined by Z-discs.

Relationship between myofilaments and sarcomeres: Sarcomeres are composed of overlapping thick (myosin) and thin (actin) filaments, enabling contraction.

• Sarcolemma: Plasma membrane of muscle fiber.

• T-tubules: Invaginations of sarcolemma for signal transmission.

• Sarcoplasmic reticulum: Stores and releases calcium ions for contraction.

Thick filament: Composed mainly of myosin protein, which binds to actin during contraction.

Levels of Organization within Skeletal Muscle

Physiology of Skeletal Muscle Contraction

Muscle contraction involves electrical and chemical events leading to the sliding of filaments within sarcomeres.

Neuromuscular Junction (NMJ)

• Resting membrane potential: Electrical charge difference across the sarcolemma, typically -70 mV.

• Action potential: Rapid change in membrane potential that triggers contraction.

• NMJ structure: Includes axon terminal, synaptic knob, synaptic cleft, and motor end plate.

• Neurotransmitter: Acetylcholine (ACh) is released from synaptic vesicles, binds to receptors on motor end plate.

Steps in Skeletal Muscle Contraction:

1. Excitation: Action potential arrives at NMJ, ACh released, binds to receptors, initiates muscle action potential.

2. Excitation-Contraction Coupling: Action potential travels along sarcolemma and T-tubules, triggers Ca2+ release from sarcoplasmic reticulum.

3. Sarcomere: Crossbridge Cycling: Ca2+ binds to troponin, exposing binding sites on actin; myosin heads bind to actin, perform power stroke, ATP binds to myosin, detachment and re-cocking occur.

Key Equations:

• Resting membrane potential:

• ATP hydrolysis:

Refractory period: Time during which muscle fiber cannot respond to a new stimulus.

Calcium release: Ca2+ is released from sarcoplasmic reticulum, essential for contraction.

Energy Supply for Muscle Contraction

Muscle contraction requires ATP, which is supplied by several metabolic pathways.

• Creatine kinase: Transfers phosphate from creatine phosphate to ADP to form ATP.

• Glycolysis: Anaerobic breakdown of glucose to produce ATP and pyruvate.

• Aerobic respiration: Uses oxygen to produce ATP in mitochondria.

Glycolysis equation:

ATP sources: Creatine phosphate, glycolysis, and aerobic respiration.

Excess postexercise oxygen consumption (EPOC): Increased oxygen intake after exercise to restore metabolic conditions.

Muscle Twitch and Contraction Types

A muscle twitch is a single contraction response to a stimulus. Muscle contractions can be classified based on tension and movement.

• Isometric contraction: Tension increases, muscle length remains constant.

• Isotonic contraction: Muscle changes length; includes concentric (shortening) and eccentric (lengthening) contractions.

Length-tension relationship: Optimal overlap of actin and myosin produces maximal tension.

Classification of Muscle Fiber Types

Muscle fibers are classified based on contraction speed and metabolic properties.

Distribution of fiber types varies with muscle function and training.

Muscle Response to Exercise

Muscle tissue adapts to different types of exercise.

• Endurance exercise: Increases oxidative capacity, mitochondrial density, and capillary supply.

• Resistance exercise: Increases muscle fiber size (hypertrophy) and strength.

• Disuse: Leads to muscle atrophy and decreased strength.

Age-related changes: Muscle mass and strength decrease with age.

Cardiac and Smooth Muscle Tissue

Cardiac and smooth muscle tissues have unique anatomical and physiological features.

• Cardiac muscle: Contains intercalated discs, gap junctions, and autorhythmic pacemaker cells for coordinated contraction.

• Smooth muscle: Non-striated, contracts via latchbridge and cross-bridge mechanisms; can be arranged as single-unit or multi-unit types.

Latchbridge mechanism: Allows smooth muscle to maintain tension with minimal ATP consumption.

Single-unit smooth muscle: Cells contract together due to gap junctions; found in walls of hollow organs.

Multi-unit smooth muscle: Cells contract independently; found in large arteries and airways.

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Example: During running, skeletal muscles contract rapidly and repeatedly, cardiac muscle increases heart rate and output, and smooth muscle in blood vessels regulates blood flow.

Additional info: Some details, such as specific figure references and deeper molecular mechanisms, were inferred or expanded for completeness and clarity.