Muscular System: Structure, Function, and Physiology (Chapters 9 & 10.1-10.3)

Muscular System Overview

Muscle Functions

The muscular system is essential for movement, stability, and various physiological processes. Muscles perform several key functions in the human body:

• Movement: Muscles contract to produce movement of body parts and locomotion.

• Stability & Posture: Muscles help maintain posture and stabilize joints.

• Strengthen Joints: Muscles reinforce and support joints, preventing dislocation.

• Communication: Facial muscles and others enable speech, writing, and nonverbal communication.

• Heat Production: Muscle contractions generate heat, helping maintain body temperature.

• Control Openings: Sphincter muscles regulate the opening and closing of body passages.

• Support Soft Tissues: Muscles provide support to internal organs and soft tissues.

Muscle Tissue Characteristics

Properties of Muscle Tissue

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

• Conductivity: Ability to conduct electrical impulses along the cell membrane.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched without being damaged.

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

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, multinucleated; attached to bones for movement.

• Cardiac Muscle: Involuntary, striated, single nucleus, intercalated discs; found in the heart.

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

Muscle Structure and Connective Tissue Sheaths

Connective Tissue Sheaths

• Endomysium: Surrounds individual muscle fibers.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Epimysium: Surrounds the entire muscle.

• Fascicles: Bundles of muscle fibers within a muscle.

Muscle Attachments

• Origin: Fixed attachment point (sometimes called "heads").

• Insertion: Movable attachment point.

• Direct (Fleshy) Attachments: Muscle fibers attach directly to bone or cartilage.

• Indirect Attachments: Muscle connects to bone via tendons or aponeuroses.

• Tendon: Cord-like connective tissue connecting muscle to bone.

• Aponeurosis: Broad, flat connective tissue sheet.

Muscle Microanatomy

Muscle Fiber Anatomy

• Sarcolemma: Plasma membrane of a muscle fiber.

• Sarcoplasm: Cytoplasm of a muscle fiber, containing organelles and myofibrils.

• Multiple Nuclei: Skeletal muscle fibers are multinucleated.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions (Ca2+).

• T-tubules: Invaginations of the sarcolemma that transmit action potentials into the cell.

• Myofibrils: Rod-like structures containing contractile proteins.

• Myoblasts: Embryonic cells that fuse to form muscle fibers.

• Satellite Cells: Stem cells involved in muscle repair.

Striations and Sarcomere Structure

• A-bands: Dark bands containing thick filaments (myosin).

• H-zone: Lighter region in the middle of the A-band; only thick filaments.

• M-line: Center of the H-zone; holds thick filaments together.

• I-bands: Light bands containing thin filaments (actin).

• Z-disc: Boundary of a sarcomere; anchors thin filaments.

• Sarcomere: Functional contractile unit of muscle, from Z-disc to Z-disc.

Myofilaments

• Myosin (Thick Filaments): Motor protein with heads that bind to actin.

• Actin (Thin Filaments): Protein with binding sites for myosin.

• Tropomyosin: Blocks myosin-binding sites on actin at rest.

• Troponin: Binds Ca2+ and moves tropomyosin to expose binding sites.

Neuromuscular Junction and Muscle Contraction

Neuromuscular Junction (NMJ)

• Synaptic Knob: End of the motor neuron; releases neurotransmitter.

• Synaptic Cleft: Gap between neuron and muscle fiber.

• Acetylcholine (ACh): Neurotransmitter that initiates muscle contraction.

Sliding Filament Model

Describes how muscles contract by the sliding of actin and myosin filaments past each other, shortening the sarcomere.

Membrane Potentials and Action Potentials

• Sodium-Potassium Pump: Maintains resting membrane potential by moving 3 Na+ out and 2 K+ in (requires ATP).

• Resting Membrane Potential (RMP): The electrical charge difference across the sarcolemma at rest.

• Depolarization: Na+ enters the cell, making the inside more positive.

• Repolarization: K+ exits the cell, restoring negative charge inside.

• End Plate Potential (EPP): Local depolarization at the NMJ.

• Action Potential (AP): Electrical impulse that travels along the sarcolemma.

Muscle Contraction Phases

• Excitation Phase: Nerve impulse leads to ACh release and muscle fiber depolarization.

• Excitation-Contraction Coupling: AP triggers Ca2+ release from SR, leading to contraction.

• Myosin-Actin Interaction (4-Step Cycle): Cross-bridge formation, power stroke, detachment, and reactivation.

• Muscle Relaxation: Ca2+ reabsorbed into SR, muscle returns to resting state.

Clinical and Pathological Aspects

• Myasthenia Gravis: Autoimmune disease affecting ACh receptors, causing muscle weakness.

• Effects of Toxins: Botox blocks ACh release; tetanus causes sustained contraction; curare blocks ACh receptors.

• Rigor Mortis: Stiffening of muscles after death due to lack of ATP.

Motor Units and Muscle Twitch

Motor Units

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• Muscle Twitch: Response of a muscle to a single stimulus.

• Phases of Twitch:

  • Latent Period: Time between stimulus and contraction.

  • Contraction Period: Muscle shortens.

  • Relaxation Period: Muscle returns to resting length.

Causes of Twitch Variation

• Frequency of stimulus

• Ca2+ concentration

• Muscle stretch

• Temperature

• pH

• Hydration

Effects of Stimulus Frequency

• Treppe: Gradual increase in contraction strength with repeated stimulation.

• Wave Summation: Increased force with rapid stimuli.

• Incomplete (Unfused) Tetanus: Partial relaxation between stimuli.

• Complete Tetanus: No relaxation; sustained contraction.

Effects of Stimulus Intensity

• Threshold: Minimum stimulus needed for contraction.

• Recruitment: Increasing number of active motor units.

• Multiple Motor Unit (MMU) Summation: Adding more motor units increases force.

Whole Muscle Physiology

Length-Tension Relationship

The force a muscle can generate depends on its length at the time of stimulation. Optimal overlap of actin and myosin produces maximal tension.

Muscle Tone and Graded Response

• Muscle Tone: Continuous, partial contraction of muscles.

• Graded Response: Variation in muscle contraction strength based on stimulus.

Types of Contractions

• Isometric Contraction: Muscle tension increases, but length does not change.

• Isotonic Contraction: Muscle changes length while tension remains constant.

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

Energy Sources for Muscle Contraction

• At Rest: Aerobic respiration (mainly fatty acids).

• Immediate: Stored ATP and direct phosphorylation by creatine phosphate.

• Short Term: Anaerobic fermentation (glycolysis).

• Long Term: Aerobic respiration (glucose, fatty acids).

Muscle Fatigue and Oxygen Debt

• Muscle Fatigue: Inability to contract despite stimulation, often due to ATP depletion.

• Oxygen Debt: Extra oxygen required after exercise to restore metabolic conditions.

Heat Production

Muscle activity produces heat as a byproduct, contributing to body temperature regulation.

Classification of Muscle Fibers

Fiber Type	Metabolism	Contraction Speed	Fatigue Resistance
Slow Oxidative	Aerobic	Slow	High
Fast Oxidative	Mostly Aerobic	Fast	Intermediate
Fast Glycolytic	Anaerobic (Fermentation)	Fast	Low

Strength Factors

• Muscle size

• Fascicle arrangement

• Size of active motor units

• Multiple motor unit summation

• Temporal summation

• Length-tension relationship

• Fatigue

Effects of Exercise

• Resistance Exercise: Increases muscle size and strength.

• Aerobic (Endurance) Exercise: Increases stamina and fatigue resistance.

Muscle Shapes and Actions (Ch. 10.1-10.3)

Muscle Shapes (Fascicle Arrangements)

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).

• Convergent: Broad origin, fascicles converge toward a single tendon (e.g., pectoralis major).

• Parallel: Fascicles run parallel to the long axis (e.g., sartorius).

• Fusiform: Spindle-shaped with expanded belly (e.g., biceps brachii).

• Unipennate: Fascicles insert into one side of tendon (e.g., extensor digitorum longus).

• Bipennate: Fascicles insert into both sides of tendon (e.g., rectus femoris).

• Multipennate: Fascicles attach to multiple tendons (e.g., deltoid).

Muscle Actions

• Prime Mover (Agonist): Main muscle responsible for movement.

• Synergist: Assists the prime mover.

• Antagonist: Opposes the prime mover.

• Fixator: Stabilizes the origin of the prime mover.

Naming of Muscles

• Location (e.g., temporalis)

• Shape (e.g., deltoid)

• Size (e.g., gluteus maximus)

• Direction of fibers (e.g., rectus abdominis)

• Number of origins (e.g., biceps brachii)

• Location of attachments (e.g., sternocleidomastoid)

• Action (e.g., flexor carpi radialis)

Smooth and Cardiac Muscle

Cardiac Muscle

• Striated & Involuntary: Found only in the heart.

• Intercalated Discs: Specialized junctions for synchronized contraction.

• Structural Differences: Less developed SR, larger T-tubules.

• Repair: Only by fibrosis; no regeneration (lacks satellite cells).

• Metabolism: Almost exclusively aerobic; high mitochondrial content (up to 25%).

• Fatigue Resistance: Not prone to fatigue; cannot ferment, so susceptible to damage if oxygen is cut off.

• Contraction: Autorhythmic (spontaneous); rate influenced by autonomic nervous system.

Smooth Muscle

• Non-striated & Involuntary: Found in walls of hollow organs (e.g., intestines, blood vessels).

• Structure: Small, spindle-shaped cells with single, central nucleus; lack coarse connective tissue sheaths.

• SR & T-tubules: SR less developed; T-tubules absent; sarcolemma has Ca2+-containing infoldings.

• Intermediate Filaments: Extensive network attached to dense bodies.

• Regeneration: Capable of cell division and growth.

• Filament Organization: No sarcomeres; thin filaments attach to dense bodies; thick:thin ratio is 1:13; filaments arranged diagonally.

• Organization: Usually two sheets (circular and longitudinal); responsible for peristalsis.

Smooth Muscle Physiology

• Contractions: Slow, synchronized; entire sheet contracts in unison.

• Stimuli: Neuronal, hormonal, or physical (stretch).

• Neuromuscular Junctions: Lacks structured NMJs; uses diffuse junctions.

• Contraction Mechanism: Based on sliding filament model, but Ca2+ comes from extracellular fluid; no troponin (uses calmodulin and light-chain myokinase).

• Contraction Sequence: Myosin-actin-dense bodies-cell membrane; cell shortens with a twisting action.

Example: Peristalsis in the digestive tract is produced by coordinated contraction of smooth muscle layers.

Additional info: Smooth muscle's ability to regenerate is important for healing after injury, unlike cardiac muscle, which heals by fibrosis.