Muscular System Notes

Skeletal Muscle

  • Approximately 600 skeletal muscles in the body.

  • Attached to bones and facial skin.

  • Under voluntary or conscious control.

  • Composed of skeletal muscle tissue, nerves, blood, and connective tissue.

Connective Tissue Coverings

  • Fascia: Thin covering around a muscle.

  • Tendon: Cord-like mass connecting muscle to bone.

  • Aponeurosis: Sheet-like mass connecting muscle to bone, skin, or another muscle.

Connective Tissue Surrounding Muscle

  • Epimysium: Surrounds the whole muscle, beneath the fascia.

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

  • Endomysium: Surrounds individual muscle fibers within a fascicle.

Muscle Fiber Structure

  • Muscle fibers are multinucleated cells.

  • Sarcolemma: Cell membrane of muscle fibers.

  • Sarcoplasm: Cytoplasm of muscle fiber.

  • Myofibrils: Composed of actin (thin filaments) and myosin (thick filaments).

  • Sarcomeres: Units connected end-to-end to make up myofibrils.

  • Sarcoplasmic Reticulum (SR): Endoplasmic reticulum that stores calcium.

  • T Tubule: Relays electrical impulses to the SR.

  • Triad: Unit consisting of one T tubule and two SR cisternae.

Thick and Thin Filaments

  • Thick filaments: Made of myosin protein; heads form cross-bridges with thin filaments.

  • Thin filaments: Made of actin protein; associated with troponin and tropomyosin (regulatory proteins).

  • Troponin and tropomyosin prevent cross-bridging when the muscle is not contracting.

Muscle Contraction

  • Involves shortening of muscle fiber as it pulls on attachment points.

  • Requires cellular and chemical interactions.

  • Actin and myosin filaments slide past each other, shortening sarcomeres.

Neuromuscular Junctions

  • Synapse where a motor neuron axon interacts with a skeletal muscle fiber.

  • Motor neuron must stimulate muscle fiber to contract.

  • Consists of motor neuron, motor endplate, synaptic cleft, synaptic vesicles, and neurotransmitters.

  • Acetylcholine: Neurotransmitter used to stimulate skeletal muscle fibers.

Acetylcholine and Muscle Impulse

  • Action potentials cause the release of acetylcholine from synaptic vesicles.

  • Acetylcholine travels through the synaptic cleft and binds to receptors on the muscle fiber membrane.

  • Changes membrane permeability to sodium and potassium ions, generating a muscle impulse or action potential.

  • Impulse causes the release of calcium ions from the sarcoplasmic reticulum, leading to muscle contraction.

Sarcomere Structure

  • I band: Light band composed of thin actin filaments.

  • A band: Dark band composed of thick myosin filaments with overlapping actin filaments.

  • H zone: Center of A band, composed of thick myosin filaments.

  • Z line (Z disc): Sarcomere boundary, center of I band, anchors filaments.

  • M line: Center of sarcomere and A band, anchors thick filaments.

Muscle Contraction Mechanism

  • Thick and thin filaments slide past each other.

  • H zones and I bands narrow.

  • Z lines move closer together.

  • Filament lengths do not change; overlap increases.

Excitation-Contraction Coupling

  • Connection between muscle fiber stimulation and contraction.

  • During relaxation, calcium ions are stored in the sarcoplasmic reticulum.

  • Troponin-tropomyosin complexes cover binding sites on actin filaments.

Muscle Stimulation and Calcium's Role

  • Muscle impulses cause the SR to release calcium ions into the cytosol.

  • Calcium ions bind to troponin, changing its shape.

  • Tropomyosin alters position, exposing binding sites on actin.

  • Myosin heads bind to actin, forming cross-bridges.

Cross-Bridge Cycling

  1. Myosin head attaches to actin binding site forming cross bridge.

  2. Myosin cross bridge pulls thin filaments toward center of sarcomere.

  3. ADPADP and phosphate are released from myosin.

  4. A new ATPATP binds to myosin and the linkage between actin and myosin crossbridge breaks.

  5. ATPATP splits. Myosin crossbridge goes back to original position ready to bind to another binding site on actin.

Muscle Relaxation

  • Acetylcholinesterase: Enzyme that breaks down acetylcholine.

  • Muscle impulse stops when acetylcholine is decomposed.

  • Calcium pumps bring calcium back into the sarcoplasmic reticulum.

  • Troponin-tropomyosin complex recovers binding sites on actin, preventing actin-myosin binding.

Energy Sources for Muscle Contraction

  • ATP reserves: Limited initial source.

  • Creatine phosphate: Regenerates ATP from ADP and phosphate, fueling approximately 10 seconds of intense contraction.

  • Cellular respiration: Breaks down glucose to produce ATP.

Cellular Respiration Phases

  • Anaerobic phase (Glycolysis): Occurs in the cytoplasm, produces little ATP.

  • Aerobic phase (Citric acid cycle and electron transport system): Occurs in mitochondria, produces the most ATP.

  • Myoglobin stores extra oxygen in muscle.

  • Anaerobic respiration creates lactic acid, causing muscle pain and soreness.

Muscle Fatigue

  • Occurs with overuse, oxygen depletion, and lactic acid buildup.

  • Causes pain and soreness.

  • Body signals the need to repay oxygen debt and rest.

  • Muscle cramping can occur with electrolyte depletion.

Types of Muscle Fibers

  • Slow-twitch fibers (Type I): Slow oxidative, use aerobic respiration, many mitochondria, abundant myoglobin, slow to contract, for steady exercise.

  • Fast-twitch fibers (Type IIa): Intermediate fibers, contract quickly, intermediate oxygen supply and myoglobin, fatigue-resistant, for transitioning paces.

  • Fast-twitch fibers (Type IIb): Fast glycolytic fibers, poor blood supply, less myoglobin, best at anaerobic respiration, susceptible to fatigue, for sprinting.

Smooth Muscle

  • Lacks striations.

  • Involuntary control.

  • Shorter muscles, found in blood vessels and alimentary canal.

Types of Smooth Muscle

  • Multiunit: Found in blood vessels, independent function, stimulated by hormones and neuron impulses.

  • Visceral: Found in the gut, act as a unit, exhibit rhythmicity, conduct peristalsis.

Smooth Muscle Contraction

  • Similar to skeletal muscle: actin-myosin interaction, calcium, ATP.

  • Triggered by membrane impulses.

  • Differs from skeletal muscle: lacks troponin, uses calmodulin.

  • Uses norepinephrine and acetylcholine as neurotransmitters.

  • Hormones can stimulate or inhibit contraction.

  • Slower to contract and more fatigue-resistant than skeletal muscle.

Cardiac Muscle

  • Found only in the heart.

  • Striated, like skeletal muscle.

  • Involuntary control, like smooth muscle.

  • Fibers form intercalated discs.

  • Contracts as a unit (syncytium).

  • Self-exciting, rhythmic, longer refractory period than skeletal muscle.

  • Does not fatigue, more mitochondria.