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.