Muscular System Notes

Introduction

  • Three muscle types: skeletal, smooth, and cardiac.
  • Focus: skeletal muscle.

Structure of a Skeletal Muscle

  • Muscle as an organ: skeletal muscle tissue, connective tissues, nervous tissue, and blood.
  • Connective Tissue Coverings:
    • Fascia surrounds and separates muscles.
    • Tendons attach muscle to bone periosteum.
    • Aponeuroses: broad connective tissue sheets.
    • Connective tissue layers:
      • Epimysium: around whole muscle.
      • Perimysium: around fascicles (bundles).
      • Endomysium: around each muscle fiber.
  • Skeletal Muscle Fibers:
    • Muscle fiber: single, long, cylindrical cell.
    • Sarcolemma (cell membrane), sarcoplasm (cytoplasm) with mitochondria and nuclei.
    • Myofibrils in sarcoplasm.
      • Thick filaments: myosin.
      • Thin filaments: actin.
      • Organization creates striations.
  • Sarcomere Structure:
    • Sarcomere: Z line to Z line.
    • I bands: actin filaments, anchored to Z lines (light bands).
    • A bands: overlapping thick and thin filaments (dark bands).
    • H zone: myosin filaments only (center of A bands).
  • Sarcoplasmic reticulum (endoplasmic reticulum) lies beneath the sarcolemma.
  • Sarcoplasmic reticulum associates with transverse (T) tubules (sarcolemma invaginations).
    • T tubule between sarcoplasmic reticulum cisternae.
    • Sarcoplasmic reticulum and T tubules activate muscle contraction.

Neuromuscular Junction

  • Motor neuron and muscle fiber meet here.
    • Motor end plate: folded sarcolemma with nuclei and mitochondria.
    • Motor neuron cytoplasm: mitochondria and synaptic vesicles (neurotransmitters).

Motor Units

  • Motor neuron and controlled muscle fibers.
  • Muscle fibers contract together when stimulated.

Skeletal Muscle Contraction

  • Sarcomeres shorten, muscle pulls against attachments.
  • Myosin and Actin Role:
    • Myosin: twisted strands with cross-bridges.
    • Actin: globular protein with myosin binding sites; includes tropomyosin and troponin.
  • Sliding Filament Theory:
    • Myosin cross-bridge binds to actin, bends, and pulls.
    • Myosin releases, reattaches, and pulls again.
    • ATP provides energy via ATPase for cross-bridge “cocking”.
  • Stimulus for Contraction:
    • Motor neuron releases acetylcholine into synaptic cleft.
    • Motor end plate receptors detect neurotransmitters.
    • Muscle impulse spreads over sarcolemma and into T tubules.
    • Sarcoplasmic reticulum releases stored calcium into sarcoplasm.
    • Calcium interacts with troponin and tropomyosin, exposing myosin binding sites.
    • Myosin binds and pulls actin, shortening sarcomeres.
    • Acetylcholinesterase decomposes acetylcholine.
    • Calcium returns to sarcoplasmic reticulum, breaking myosin-actin linkages.

Energy Sources for Contraction

  • ATP is the direct energy source.
  • Creatine phosphate stores excess energy and regenerates ATP from ADP.
  • Creatine phosphokinase synthesizes creatine phosphate when ATP is sufficient.
  • Creatine phosphate energy converts ADP back to ATP.

Oxygen Supply and Cellular Respiration

  • Glycolysis yields limited ATP; muscles need oxygen for glucose breakdown in mitochondria.
  • Hemoglobin carries oxygen to muscles.
  • Myoglobin stores oxygen in muscle tissue.

Oxygen Debt

  • Rest/moderate activity: sufficient oxygen for aerobic respiration.
  • Strenuous exercise: oxygen deficiency, lactic acid accumulation (anaerobic respiration).
  • Lactic acid diffuses into bloodstream to liver.
  • Oxygen debt:
    • Oxygen needed by liver cells to convert lactic acid to glucose.
    • Oxygen needed by muscle cells to restore ATP and creatine phosphate.
  • Repaying oxygen debt can take hours.

Muscle Fatigue

  • Muscle loses ability to contract during strenuous exercise.
  • Lactic acid accumulation lowers pH.
  • Muscle cramp: lack of ATP prevents calcium return to sarcoplasmic reticulum.

Heat Production

  • Muscle contraction.
  • Energy from cellular respiration.

Muscular Responses

  • Studied by stimulating single fiber and recording responses.
  • Threshold Stimulus: minimum stimulus for response.
  • All-or-None Response: muscle fiber contracts fully or not at all.

Recording a Muscular Contraction

  • Myogram: recording of stimulated muscle contraction.
  • Twitch: single, short contraction involving few motor units.
  • Latent period: delay between stimulus and contraction (less than 0.01 second).
  • Followed by contraction and relaxation periods.

Summation

  • Force of twitches combine without full relaxation.
  • Tetanic contraction (tetanus): sustained contraction without relaxation.

Recruitment of Motor Units

  • Increase in activated motor units with higher stimulation intensities.

Sustained Contractions

  • Summation and recruitment produce sustained contraction of increasing strength.
  • Muscle tone: continuous sustained contraction of motor units.

Smooth Muscles

  • Elongated, tapered, lack striations, undeveloped sarcoplasmic reticulum.
  • Types:
    • Multiunit: in blood vessels and iris; fibers act separately.
    • Visceral: in sheets in hollow organ walls; stimulate each other (rhythmicity); peristalsis.
  • Smooth Muscle Contraction:
    • Myosin-actin mechanism similar to skeletal muscle.
    • Acetylcholine and norepinephrine stimulate or inhibit.
    • Hormones stimulate or inhibit.
    • Slower contraction/relaxation, longer contraction with same ATP.

Cardiac Muscle

  • Contraction similar to skeletal/smooth.
  • Transverse tubules provide extra calcium for longer contraction.
  • Intercalated discs join cells, transmit contraction force, and aid impulse transmission.
  • Self-exciting and rhythmic; contracts as a unit.

Skeletal Muscle Actions

  • Origin and Insertion:
    • Origin: immovable end.
    • Insertion: movable end.
    • Contraction pulls insertion toward origin.
  • Interaction of Skeletal Muscles:
    • Prime mover: main muscle doing work.
    • Synergists: helper muscles.
    • Antagonists: opposing muscles.

Major Skeletal Muscles

  • Named by size, shape, location, action, number of attachments, or fiber direction.
  • Muscles of Facial Expression: attach to bones and skin (see Table 7.4).
  • Muscles of Mastication: chewing muscles (see Table 7.4).
  • Muscles that Move the Head: neck and back muscles (see Tables 7.4, 7.5).
  • Muscles that Move the Pectoral Girdle: chest and shoulder muscles (see Table 7.8).
  • Muscles that Move the Arm: connect arm to pectoral girdle, ribs, and vertebral column (see Table 7.9).
  • Muscles that Move the Forearm: connect humerus to ulna and radius (see Table 7.10).
  • Muscles that Move the Wrist, Hand, and Fingers: originate from distal humerus, radius, and ulna (see Table 7.10).
  • Muscles of the Abdominal Wall: connect rib cage and vertebral column to pelvic girdle (see Table 7.6).
  • Muscles of the Pelvic Outlet: urogenital and pelvic diaphragms (see Table 7.7).
  • Muscles that Move the Thigh: attached to femur and pelvic girdle (see Table 7.11).
  • Muscles that Move the Leg: connect tibia/fibula to femur/pelvic girdle (see Table 7.12).
  • Muscles that Move the Ankle, Foot, and Toes: attached to femur, fibula, or tibia (see Table 7.13).