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Muscle Tissue Flashcards

Three Types of Muscular Tissue

  • Skeletal, cardiac, and smooth muscle.
  • Skeletal: moves bones, multinucleated, striated, voluntary.
  • Cardiac: pumps blood, one nucleus, striated, involuntary, intercalated discs.
  • Smooth: various organs (e.g., GI tract), various functions (e.g., peristalsis), one nucleus, no striations, involuntary.

Functions of Muscular Tissue

  • Create motion, stabilize body positions, store and move substances, generate heat (thermogenesis).
  • Muscles make up about 40%-50% of body weight.

Properties of Muscular Tissue

  • Electrical excitability: responds to stimuli, creating muscle action potentials.
  • Contractility: shortens in length, creating tension.
  • Extensibility: extends or stretches without damage.
  • Elasticity: returns to original length and shape.

Skeletal Muscle Structure

  • Body contains over 600 skeletal muscles.
  • Connective tissue coverings: fascia, epimysium, perimysium, and endomysium.

Levels of Organization within a Skeletal Muscle

  • Skeletal muscle: organ made of muscle fascicles.
  • Muscle fascicle: bundle of muscle fibers wrapped in perimysium.
  • Muscle fiber: long cylindrical cell covered by endomysium and sarcolemma.
  • Myofibril: threadlike contractile elements within sarcoplasm.
  • Filaments (myofilaments): contractile proteins (actin, myosin, troponin, tropomyosin).

Skeletal Muscle Fibers

  • Each fiber is a long, cylindrical muscle cell.
  • Sarcolemma: cell membrane.
  • Sarcoplasm: cytoplasm containing mitochondria, nuclei, and myofibrils.
  • Myofibrils contain thick (myosin) and thin (actin, troponin, tropomyosin) filaments.
  • Sarcoplasmic reticulum (SR): membranous channels.

Myofibrils and Sarcomeres

  • Sarcomeres: units joined end-to-end, extending from one Z line to the next.
  • Striations: alternating pattern of light (I bands, actin) and dark (A bands, overlapping actin and myosin) bands.
  • H zone: myosin filaments only.
  • M line: proteins holding myosin filaments in place.

Components of a Sarcomere

  • Z discs: separate sarcomeres.
  • A band: entire length of thick filaments, including overlapping thin filaments.
  • I band: remainder of thin filaments, no thick filaments.
  • H band: thick filaments only.
  • M line: proteins holding thick filaments together.

Muscle Proteins

  • Contractile: myosin, actin
  • Regulatory: troponin, tropomyosin
  • Structural: titin, nebulin, alpha-actinin, myomesin, dystrophin

Neuromuscular Junction (NMJ)

  • Skeletal muscle fibers contract only when stimulated by a motor neuron.
  • Synapse: functional connection between neuron and muscle fiber.
  • Neurotransmitters: chemicals released at synapse to communicate.
  • Motor end plate: specialized region of sarcolemma with neurotransmitter receptors.

Skeletal Muscle Contraction

  • Sarcomeres shorten, pulling on muscle attachments.
  • Myosin binds to actin, filaments slide past each other.

Role of Myosin and Actin

  • Myosin: thick filaments with globular heads.
  • Actin: thin filaments with myosin-binding sites, associated with troponin and tropomyosin.

Sliding Filament Model

  • Myosin head attaches to actin, forming a cross-bridge.
  • Head bends, pulling actin toward the center of the sarcomere.
  • Head releases, reattaches, and repeats.
  • ATP provides energy; ATPase breaks down ATP.

Stimulus for Contraction

  • Acetylcholine: neurotransmitter at NMJ.
  • Sarcoplasmic reticulum releases calcium, which interacts with troponin and tropomyosin.
  • Myosin binding sites on actin are exposed.
  • Cross-bridges form and pull on actin filaments, shortening the sarcomere.

Stimulus for Contraction (Continued)

  • Contraction continues as long as the nerve impulse continues.
  • Acetylcholinesterase decomposes acetylcholine.
  • Calcium returns to the sarcoplasmic reticulum (ATP).
  • ATP binds to myosin heads, breaking linkages between myosin and actin.
  • Actin returns to its original position; muscle relaxes.

Sliding Filament Mechanism

  • Myosin pulls on actin, thin filament slides inward.
  • Z discs move toward each other, sarcomere shortens.
  • Force is transmitted throughout the entire muscle.

Rigor Mortis

  • Rigidity of muscles 3-4 hours after death.
  • Calcium leaks out, myosin binds to actin, cross-bridges form.
  • ATP synthesis ceases, cross-bridges can't detach.
  • After 24 hours, proteolytic enzymes digest the cross- bridges.

Muscle Metabolism

  • Muscles produce ATP via: creatine phosphate, anaerobic glycolysis, and aerobic respiration.

Creatine Phosphate

  • Creatine kinase transfers phosphate from CP to ADP to rapidly yield ATP.

Anaerobic Glycolysis

  • When CP stores are depleted, glucose is converted to pyruvic acid to generate ATP.

Aerobic Respiration

  • Pyruvic acid enters mitochondria, undergoes oxygen-requiring reactions to generate ATP.

Muscle Fatigue

  • Inability to maintain force of contraction after prolonged activity.
  • Due to inadequate Ca2+ release, depletion of CP/O2/nutrients, lactic acid/ADP buildup, insufficient ACh release.

Central Fatigue

  • Changes in the central nervous system lead to cessation of exercise..

Oxygen Consumption After Exercise

  • Extra oxygen after exercise replenishes CP stores, converts lactate to pyruvate, and reloads O2 onto myoglobin.

Cardiac Muscle

  • Same arrangement as skeletal muscle, but has intercalated discs.
  • Intercalated discs contain desmosomes and gap junctions.
  • Cells have more mitochondria, contractions last longer than skeletal muscle.

Smooth Muscle

  • Not striated, tapered ends.
  • Can be single-unit or multi-unit fibers.
  • Contractions start slower, last longer than skeletal and cardiac.
  • Can shorten and stretch more than skeletal and cardiac.

Summary of Three Types of Muscular Tissues

  • Skeletal: long, cylindrical, striated, voluntary.
  • Cardiac: branched, striated, involuntary.
  • Smooth: tapered, not striated, involuntary.

Regeneration of Muscle Tissue

  • Mature skeletal muscle fibers cannot undergo mitosis.
  • Hypertrophy, hyperplasia, smooth muscle, and pericytes.

Aging and Muscle Tissue

  • Muscle tissue replaced by fibrous connective and adipose tissue.
  • Muscle strength and flexibility decrease, reflexes slow.

Control of Muscle Tension

  • Strength depends on the number of activated motor units.
  • Motor unit: somatic motor neuron and the muscle fibers it innervates.

Motor Unit Recruitment

  • Process in which the number of active motor units increases.
  • Weakest units recruited first, followed by stronger units.

Twitch Contraction

  • Brief contraction of all fibers in a motor unit in response to a single action potential - Includes latent, contraction, relaxation, and refractory periods.

Muscle Tone

  • Small amount of tension even at rest, established by alternating, involuntary activation of small motor units.

Isotonic vs. Isometric Contractions

  • Isotonic: tension is constant, muscle length changes (concentric, eccentric).
  • Isometric: muscle contracts, but length does not change.