Muscular System: Histology and Physiology Study Guide

Chapter 9: Muscular System: Histology and Physiology Lecture Outline

9.1 Functions of the Muscular System

  • Types of Muscle Tissue:

    • Skeletal Muscle:
    • Responsible for locomotion, facial expressions, posture, respiratory movements, and other types of body movement.
    • Voluntary control.
    • Smooth Muscle:
    • Located in walls of hollow organs, blood vessels, eyes, glands, and skin.
    • Functions include:
      • Propel urine,
      • Mix food in the digestive tract,
      • Dilation/constriction of pupils,
      • Regulation of blood flow.
    • In some locations, smooth muscle is autorhythmic.
    • Controlled involuntarily by the endocrine and autonomic nervous systems.
    • Cardiac Muscle:
    • Found only in the heart.
    • Major source of blood movement.
    • Autorhythmic.
    • Controlled involuntarily by the endocrine and autonomic nervous systems.

  • Key Functions of the Muscular System:

    1. Movement of the body.
    2. Maintenance of posture.
    3. Respiration.
    4. Production of body heat.
    5. Communication.
    6. Constriction of organs and vessels.
    7. Contraction of the heart.

9.2 General Properties of Muscle Tissue

  • 1. Contractility: Ability to shorten with force.
  • 2. Excitability: Capacity to respond to a stimulus (usually from nerves).
  • 3. Extensibility: Ability to be stretched beyond its normal resting length while still being able to contract.
  • 4. Elasticity: Ability to recoil to original resting length after being stretched.

9.3 Skeletal Muscle Anatomy

  • Connective Tissue Coverings:
    • Epimysium:
    • Connective tissue surrounding a whole muscle, merging with muscular fascia (CT layer between adjacent muscles and between muscles and skin).
    • Perimysium:
    • Loose CT surrounding a group of muscle fibers; serves as a passage for blood vessels and nerves.
    • Bundles of muscle cells are called fascicles.
    • Endomysium:
    • Loose CT separating individual muscle fibers within each fascicle.
    • Collagen from CT layers merges to form tendons or aponeuroses, which attach muscle to bone.
  • Nerves and Blood Vessels:
    • Motor neurons stimulate skeletal muscle contraction; each motor neuron controls several muscle fibers.
    • An artery and 1-2 veins extend with a nerve through CT layers; extensive capillary beds surround muscle fibers.

9.4 Whole Skeletal Muscle Fiber Structure

  • Key Components:
    • Muscular fascia surrounds individual muscles and groups of muscles.
    • Epimysium surrounds muscles.
    • Perimysium surrounds fascicles.
    • Endomysium surrounds muscle fibers.
    • Axon of motor neuron connects to muscle fibers at the neuromuscular junction.

9.5 Skeletal Muscle Fiber Anatomy

  • Develop from fusion of myoblasts, resulting in large, multinucleated muscle cells.
    • Average length: 1-4 mm (can reach up to 1 foot).
    • Average diameter: 10-100 microns.
    • Have a striated appearance due to organized arrangement of myofilaments.
    • Number of fibers remains relatively constant after birth; muscle size increases due to hypertrophy of muscle fibers.

9.6 Cardiac Muscle Characteristics

  • Present only in the heart; striated muscle with intercalated disks and gap junctions.
  • Each cardiac muscle cell typically has one nucleus.
  • Autorhythmic cells generate action potentials of longer duration and longer refractory period, regulated by Ca²⁺.

9.7 Smooth Muscle Characteristics

  • Not striated, with fibers smaller than those in skeletal muscle.
  • Spindle-shaped with a single central nucleus.
  • Contains more actin than myosin.
  • Features caveolae (indentations in the sarcolemma that may act like T tubules) and dense bodies instead of Z disks.
  • Contraction regulated by calcium, which binds to calmodulin that in turn regulates myosin kinase.
  • Relaxation occurs through the action of myosin phosphatase.

9.8 Histology of Muscle Fibers

  • Electrical Components:
    • Sarcolemma: Plasma membrane surrounding sarcoplasm (cytoplasm) of a muscle cell.
    • Transverse Tubules (T tubules): Inward folds of the sarcolemma projecting into the muscle cell interior.
    • Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that stores calcium.
    • Enlarged portions called terminal cisternae lie adjacent to T tubules.
  • Triad Formation: Two terminal cisternae and their associated T tubule form a triad.
  • Mechanical Components:
    • Myofibrils: Bundles of protein filaments responsible for contraction, containing the actin (thin) and myosin (thick) myofilaments, arranged into functional units known as sarcomeres.

9.9 Sarcomere Structure

  • Sarcomere: Basic functional unit of muscle fiber; the smallest part capable of contraction.
  • Regions within Sarcomere:
    • Z disk: Filamentous network of protein serving as an attachment for actin.
    • I bands: Lighter staining regions extending to the ends of myosin myofilaments, containing Z disks.
    • A bands: Central dark regions containing overlapping actin and myosin myofilaments except at the center.
    • H zone: Region in the A band where actin and myosin do not overlap.
    • M line: Middle of the H zone, holding myosin in place with delicate filaments.

9.10 Sliding Filament Model

  • Actin myofilaments slide over myosin, leading to sarcomere shortening.
    • Neither actin nor myosin change length during this process.
    • Sarcomere shortening causes skeletal muscle contraction; during relaxation, sarcomeres lengthen from external forces, such as the contraction of antagonistic muscles.

9.11 Muscle Contraction Physiology

  • Nervous System Control:
    • Muscle contractions are controlled via action potentials; resting membrane potentials must exist for action potentials to occur.
    • Resting Membrane Potential: Inside the cell, there is a higher concentration of K⁺ ions compared to the outside, which has a higher concentration of Na⁺ ions, maintained by the Na⁺/K⁺ pump.
  • Action Potentials Phases:
    • Depolarization: Inside membrane becomes less negative; if threshold is reached, depolarization occurs.
    • Repolarization: Return of resting potential; can drop below original potential.
    • All-or-None Principle: Action potentials either occur fully or not at all and propagate from one location to another, not moving along the membrane itself.

9.12 Function of the Neuromuscular Junction

  • Neuromuscular Junction Structure:
    • Points of contact between motor neuron and muscle fiber; consists of presynaptic terminal, synaptic cleft, and postsynaptic membrane (motor end-plate).
  • Neuromuscular Junction Function:
    1. Action potential arrives at presynaptic terminal, opening voltage-gated Ca²⁺ channels.
    2. Ca²⁺ ions enter and initiate release of acetylcholine (ACh).
    3. ACh diffuses across synaptic cleft and binds to ligand-gated Na⁺ channels on postsynaptic membrane.
    4. Na⁺ enters postsynaptic cell, causing depolarization; if threshold is passed, an action potential is generated.
    5. Acetylcholinesterase breaks down ACh to prevent accumulation.

9.13 Excitation-Contraction Coupling

  • Links electrical and mechanical components of contraction:
    • Action potential produced on sarcolemma → propagates into T tubules → opens calcium channels on SR terminal cisternae → calcium enters sarcoplasm → binds troponin → muscle contraction occurs.

9.14 Summary of Muscle Contraction Process

  1. Action potential travels along the axon to a neuromuscular junction.
  2. Voltage-gated Ca²⁺ channels open, allowing Ca²⁺ entry into the presynaptic terminal.
  3. ACh is released from vesicles in the presynaptic terminal.
  4. ACh binds to ligand-gated Na⁺ channels, facilitating Na⁺ entry into the fiber.
  5. Action potential is generated and travels along the sarcolemma and T tubules.
  6. Ca²⁺ release from the sarcoplasmic reticulum occurs.
  7. Ca²⁺ binds to troponin, allowing cross-bridge formation.
  8. ATP is broken down, leading to power strokes that pull actin past myosin.
  9. The cycle continues while Ca²⁺ is present.

9.15 Muscle Relaxation

  • Requires active transport of Ca²⁺ back into the sarcoplasmic reticulum.
  • Absence of Ca²⁺ allows the troponin-tropomyosin complex to block binding sites on actin, leading to muscle relaxation.

9.16 Whole Skeletal Muscle Physiology

  • Muscle Twitch:
    • Contraction in response to a stimulus causing an action potential in muscle fibers.
    • Phases include lag (latent), contraction, and relaxation.

9.17 Types of Muscle Contractions

  • Isometric: No change in length but tension increases (e.g., postural muscles).
  • Isotonic: Change in length while maintaining constant tension.

9.18 Motor Units

  • Comprise a single motor neuron and all muscle fibers it innervates.
  • Motor Unit Numbers:
    • Large muscles have many fibers per unit.
    • Small muscles, responsible for delicate movements, have fewer fibers per unit.

9.19 Force of Contraction in Individual Muscle Fibers

  • Treppe: Graded response in rested muscles, stronger contractions following because of higher Ca²⁺ concentrations in sarcoplasm.

9.20 Wave Summation and Tetanus

  • Increased action potential frequency leads to stronger contractions:
    • Wave Summation: Muscle tension increases with contraction frequencies.
    • Incomplete Tetanus: Muscle fibers relax partially between contractions.
    • Complete Tetanus: No relaxation occurs between contractions.

9.21 Muscle Length at Time of Contraction

  • Active Tension: Force from lifting an object during muscle contraction.
    • Length affects active tension: too stretched or not stretched at all diminishes cross-bridge formation.
  • Passive Tension: Tension when muscle is stretched without stimulation.
  • Total Tension: Sum of active and passive tension.

9.22 Recruitments and Motor Units

  • **Motor Unit Recruitment: **
    • Strength of contraction graded by recruiting more motor units.
    • Threshold Stimulus: Triggers contraction.
    • Maximal Stimulus: Engages all motor units of a muscle.
  • The Size Principle: Smaller motor units recruited first during muscle activation.
  • Muscle Tone: Constant tension in muscles due to asynchronous contraction of a small percentage of motor units.

9.23 Types of Isotonic and Isometric Contractions

  • Isotonic Contraction: Length changes but tension remains constant.
    • Concentric: Muscle shortens while overcoming resistance.
    • Eccentric: Muscle lengthens while maintaining tension.

9.24 Muscle Fiber Types

  • Slow-twitch (Type I) Fibers:
    • Contract slowly, smaller diameter, better blood supply, more mitochondria, more fatigue-resistant.
    • Found in postural muscles, concentrated more in lower limbs.
  • Fast-twitch (Type II) Fibers:
    • Contract rapidly, have more extensive glycolytic capacity, less blood supply.
    • Found in lower limbs of sprinters, upper limbs of most individuals.
    • Comes in oxidative and glycolytic forms.

9.25 Effects of Exercise on Muscle Fiber Size

  • Hypertrophy: Increase in muscle size due to increased myofibrils and nuclei, along with improved coordination and metabolic enzyme production.
  • Atrophy: Decrease in muscle size due to inactivity.

9.26 Heat Production During Muscle Activity

  • During exercise, metabolic rate and heat production increase; excess heat can be lost through vasodilation and sweating.
  • Shivering: Uncoordinated contractions of muscle fibers lead to heat production.

9.27 Energy Sources for Muscle Contraction

  • ATP provides energy through:
    1. Conversion of two ADP molecules to one ATP and AMP through adenylate kinase.
    2. Phosphate transfer from creatine kinase to ADP, forming ATP.
    3. Anaerobic Respiration: Breakdown of glucose in absence of oxygen leads to ATP and lactic acid.
    4. Aerobic Respiration: Breakdown of glucose using oxygen, producing ATP, CO₂, and water.

9.28 Muscle Fatigue and Soreness

  • Physiological Fatigue: Results from ATP depletion, oxidative stress, and inflammation; leads to decreased performance.
  • Psychological Fatigue: Dependent on individual’s emotional state.
  • Muscle soreness attributed to muscle fiber damage and inflammation.

9.29 Physiological Contracture and Rigor Mortis

  • Physiological Contracture: Fatigue state where lack of ATP prevents muscle relaxation and contraction.
  • Rigor Mortis: Postmortem condition where Ca²⁺ leaks into sarcoplasm, causing muscle stiffness until tissues decay.

9.30 Oxygen Deficit and Recovery

  • Oxygen Deficit: The lag between the start of exercise and increased breathing rate.
  • Excess Postexercise Oxygen Consumption: The time it takes for breathing to return to normal post-exercise, necessary for restoring homeostasis.

9.31 Effects of Aging on Skeletal Muscle

  • Reduced muscle mass leading to sarcopenia (muscle atrophy), prolonged contraction time in response to stimuli, decreased stamina, and longer recovery times due to loss of muscle fibers and reduced capillary density.