Chapter 11: Muscle Tissue – Structure, Function, and Physiology

Muscle Tissue

Introduction

Muscle tissue is essential for movement, a defining characteristic of all living organisms. There are three primary types of muscle tissue: skeletal, cardiac, and smooth muscle. Understanding muscle requires knowledge at the molecular, cellular, and tissue levels.

• Movement is a fundamental property of life, enabled by muscle contraction.

• Muscle cells convert the chemical energy of ATP into mechanical energy.

• Muscle tissue is one of the four primary tissue types, alongside epithelial, connective, and neural tissue.

I. Introduction to Muscle

Overview of Muscle Types

• Skeletal Muscle: Voluntary, striated, usually attached to bones.

• Cardiac Muscle: Involuntary, striated, found in the heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

Muscle physiology underlies concepts such as warm-up, strength, endurance, and fatigue.

II. Skeletal Muscle

A. Anatomy of Skeletal Muscles

1. Connective Tissue Components

• Epimysium: Outermost layer, surrounds the entire muscle.

• Perimysium: Divides muscle into compartments called fascicles (bundles of muscle fibers).

• Endomysium: Innermost layer, surrounds individual muscle fibers (cells).

• Tendons: Attach muscle to bone; composed of dense regular connective tissue.

• Fascia: Fibrous membrane covering, supporting, and separating muscles.

• Collagen: Extensible and elastic, stretches under tension and recoils, protecting muscle from injury and returning it to resting length.

• Elastic components: Contribute to muscle elasticity and recoil.

2. Microscopic Anatomy of Skeletal Muscle Tissue

• Muscle fibers are long, multinucleated cells formed by the fusion of myoblasts during development.

• Satellite cells (myosatellite cells) are unfused myoblasts that aid in muscle repair.

• Sarcolemma: The plasma membrane of a muscle fiber.

• Transverse (T) tubules: Infoldings of the sarcolemma that transmit contraction signals deep into the fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber, containing glycogen (energy storage) and myoglobin (oxygen binding).

• Myofibrils: Bundles of myofilaments (actin and myosin) responsible for contraction.

• Sarcoplasmic reticulum (SR): Smooth ER network around each myofibril, stores calcium in terminal cisternae.

• Triad: A T tubule and two terminal cisternae, crucial for excitation-contraction coupling.

Structural Hierarchy of Skeletal Muscle

Structural Level	Description
Muscle	Contractile organ, attached to bones by tendons, composed of fascicles, surrounded by epimysium.
Fascicle	Bundle of muscle fibers within a muscle, surrounded by perimysium.
Muscle Fiber	Single muscle cell, multinucleated, surrounded by endomysium and sarcolemma.
Myofibril	Bundle of protein myofilaments within a muscle fiber, surrounded by SR and mitochondria.
Sarcomere	Segment of myofibril from one Z disc to the next, functional contractile unit.
Myofilaments	Protein strands (thick and thin filaments) that carry out contraction.

B. Muscle Fiber Structure

• Myosin (Thick Filaments): Composed of 200–500 myosin molecules, each with two entwined polypeptides. Heads project outward in a spiral array; central bare zone has no heads.

• Actin (Thin Filaments): Two intertwined protein strands with active sites. Tropomyosin covers active sites; troponin binds calcium and regulates tropomyosin position.

• Regulatory Proteins: Tropomyosin and troponin regulate contraction by controlling access to actin's active sites.

Organization of Filaments (Sarcomere Structure)

• H band: Thick filaments only (center of A band).

• M line: Middle of H band, where thick filaments are linked.

• I band: Thin filaments only.

• A band: Contains M line, H band, and zone of overlap.

• Z line (disc): Boundary of sarcomere, where thin filaments attach.

A sarcomere is the segment from one Z disc to the next and is the functional contractile unit of muscle.

C. Nerve-Muscle Relationships

• Skeletal muscle requires stimulation by a nerve to contract.

• Somatic motor neurons originate in the brainstem or spinal cord and innervate skeletal muscle fibers.

Neuromuscular Junction (NMJ)

• Connection between a nerve fiber and a muscle cell.

• Acetylcholine (ACh) is the neurotransmitter released to stimulate muscle contraction.

• Key components:

  • Synaptic knob: Swollen end of nerve fiber containing ACh.

  • Junctional folds: Increase surface area for ACh receptors; contain acetylcholinesterase to break down ACh.

  • Synaptic cleft: Tiny gap between nerve and muscle cells.

Neuromuscular Toxins

• Pesticides (cholinesterase inhibitors): Prevent breakdown of ACh, causing spastic paralysis.

• Tetanus (Clostridium tetani): Blocks glycine release, causing overstimulation and spastic paralysis.

• Curare: Competes with ACh, causing flaccid paralysis and possible respiratory arrest.

D. Contraction of a Muscle (Sliding Filament Theory)

1. Electrically Excitable Cells

• Muscle fibers have a resting membrane potential (RMP) due to ion distribution: Na+ outside, K+ and anions inside.

• Typical RMP: $-90\ \text{mV}$

• Stimulation opens ion gates, causing Na+ influx and K+ efflux, generating an action potential.

2. Muscle Contraction and Relaxation Phases

• Excitation: Nerve action potentials lead to muscle action potentials.

• Excitation-contraction coupling: Action potentials on sarcolemma activate myofilaments.

• Contraction: Muscle fiber develops tension and may shorten.

• Relaxation: Muscle fiber returns to resting length when stimulation ends.

3. Steps of Excitation and Contraction

• Excitation (Steps 1–5):

  • Nerve signal opens voltage-gated Ca2+ channels; Ca2+ triggers ACh release into synaptic cleft.

  • ACh binds to receptors, opening Na+ and K+ channels; RMP shifts from $-90\ \text{mV}$ to $+75\ \text{mV}$ (end-plate potential).

  • Voltage change opens nearby channels, producing an action potential.

• Excitation-Contraction Coupling (Steps 6–9):

  • Action potential spreads over sarcolemma and into T tubules.

  • Ca2+ released from SR binds to troponin, causing tropomyosin to expose actin's active sites.

• Contraction (Steps 10–13):

  • Myosin ATPase hydrolyzes ATP, "cocking" the myosin head.

  • Myosin binds actin, forming a cross-bridge.

  • Power stroke: Myosin head releases ADP and Pi, pulling thin filament past thick.

  • ATP binds myosin, releasing it from actin; cycle repeats as long as Ca2+ and ATP are present.

• Relaxation (Steps 14–18):

  • Nerve stimulation ceases; acetylcholinesterase removes ACh.

  • Active transport pumps Ca2+ back into SR (requires ATP).

  • Troponin-tropomyosin complex covers actin's active sites; muscle returns to resting length.

4. Adjusting Tension in Whole Muscle

• Myogram: Chart of muscle contraction timing and strength.

• Twitch: Single contraction-relaxation cycle.

• Treppe: Repeated stimulation after relaxation phase increases tension.

• Summation: Increased frequency leads to greater tension.

• Incomplete tetanus: Rapid cycles of contraction and relaxation.

• Complete tetanus: No relaxation between stimuli; maximum tension.

5. Rigor Mortis

• Postmortem stiffening of muscles due to lack of ATP for relaxation.

• Peaks at ~12 hours after death, then diminishes as proteins decay.

E. ATP Sources for Muscle Contraction

• All muscle contraction depends on ATP.

• Anaerobic fermentation: Produces ATP without oxygen, yields lactic acid.

• Aerobic respiration: Requires oxygen, produces more ATP, yields CO2 and H2O.

F. Types of Skeletal Muscle Fibers

• Fast fibers: Contract quickly, large diameter, few mitochondria, fatigue rapidly.

• Slow fibers: Contract slowly, small diameter, many mitochondria, high endurance, rich in myoglobin.

• White muscles: Mostly fast fibers, pale appearance.

• Red muscles: Mostly slow fibers, dark appearance.

• Most human muscles are mixed (pink).

• Hypertrophy: Muscle growth from training; increases fiber size, myofibrils, mitochondria, and glycogen.

• Atrophy: Muscle wasting from inactivity; reduces size, tone, and power.

III. Cardiac Muscle

Cardiac muscle is found only in the heart and is responsible for pumping blood throughout the body. It is involuntary and striated, with unique features for endurance and coordinated contraction.

• Cells are thick, branched, and connected by intercalated discs (contain gap junctions and desmosomes).

• Autorhythmic due to pacemaker cells.

• Relies almost exclusively on aerobic respiration; large mitochondria make it resistant to fatigue.

• Vulnerable to oxygen supply interruptions.

IV. Smooth Muscle

Smooth muscle is found in the walls of hollow organs and is responsible for involuntary movements such as peristalsis and regulation of blood vessel diameter.

• Fusiform (spindle-shaped) cells with a single nucleus.

• Lacks striations; contracts in a slow, sustained manner.

• Forms layers in organs (e.g., digestive tract, blood vessels).

• Can provide fine control (e.g., iris of the eye, piloerector muscles).

V. Muscle Disorders

Muscular Dystrophy

• Group of hereditary diseases causing degeneration and weakness of skeletal muscles, replaced by adipose and fibrous tissue.

• Duchenne Muscular Dystrophy: X-linked recessive, primarily affects males, onset in childhood, fatal by age 20.

• Caused by mutation in the gene for dystrophin, leading to membrane tears and necrosis.

• Facioscapulohumeral MD: Autosomal dominant, affects facial and shoulder muscles.

• Limb-girdle dystrophy: Affects shoulder, arm, and pelvic muscles; intermediate severity.

Myasthenia Gravis

• Autoimmune disease where antibodies attack ACh receptors at neuromuscular junctions.

• Leads to muscle weakness, especially in facial muscles (drooping eyelids, double vision, difficulty swallowing).

• Treatments include cholinesterase inhibitors, immunosuppressive agents, thymectomy, and plasmapheresis.