Muscle Anatomy and Physiology Notes

Microscopic Anatomy of Skeletal Muscle

• Skeletal muscle fibers are long, cylindrical cells.

• Each fiber has multiple oval-shaped nuclei under the sarcolemma (plasma membrane).

• Diameter: 10-100 micrometers.

• Length: up to 30 centimeters (due to fusion of embryonic cells).

• Sarcoplasm: cytoplasm with glycosomes and myoglobin.

• Organelles: typical cell organelles, myofibrils, sarcoplasmic reticulum (SR), and T-tubules.

Myofibrils

• Hundreds to thousands per muscle fiber.

• Run parallel to the muscle fiber.

• Occupy 80% of cell volume.

• Contain sarcomeres (responsible for contraction).

• Sarcomeres contain three types of myofilaments.

Sarcoplasmic Reticulum (SR)

• Smooth endoplasmic reticulum surrounding myofibrils.

• Most tubules run along the myofibril's long axis.

• Terminal cisterns run perpendicularly in pairs.

• Function: Regulate calcium ion (Ca^{2+}) levels; store and release Ca^{2+} to signal contraction.

T-Tubules

• Elongated tubes between paired terminal cisterns, forming triads.

• Triads consist of a T-tubule and two terminal cisterns.

• T-tubules are extensions of the sarcolemma.

• Function: Conduct electrical impulses to sarcomeres, triggering Ca^{2+} release.

Sliding Filament Model of Contraction

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

• Cross-bridges repeatedly form and break, sliding thin filaments toward the sarcomere's center.

• Sarcomere shortens as thin filaments slide.

• Z discs are pulled toward the M line.

• I bands shorten, Z discs move closer, H zones disappear, A bands move closer.

Myofilaments

• Thick filaments: Primarily myosin, located in the center.

• Thin filaments: Primarily actin, located towards the sides; also contain tropomyosin (blocks myosin-binding sites) and troponin (binds to actin, tropomyosin, and Ca^{2+}).

• Elastic filaments: Composed of titin, a giant protein that holds thick filaments in place.

Proteins in Muscle Contraction

• Myosin, actin, tropomyosin, troponin, and titin play roles in the cross-bridge cycle.

• Other proteins: Dystrophin, nebulin, myomesin, and C proteins also contribute.

Physiology of Skeletal Muscle

Neuromuscular Junction

• Somatic motor neurons activate skeletal muscle fibers.

• Axons extend to muscle cells and form neuromuscular junctions.

• Axon terminal is separated from the muscle fiber by the synaptic cleft (filled with gel-like substance).

• Synaptic vesicles in the axon terminal contain acetylcholine (ACh).

• Junctional folds on the sarcolemma increase surface area for ACh receptors.

Action Potential and Muscle Contraction

• Action potential reaches axon terminal, releasing ACh into the synaptic cleft.

• ACh diffuses across the cleft and binds to ACh receptors on the sarcolemma.

• Acetylcholinesterase breaks down ACh, preventing continuous contraction.

• ACh binding opens ligand-gated ion channels, increasing Na^{+} influx and decreasing K^{+} efflux, causing depolarization.

• Endplate potential (localized depolarization) spreads and opens voltage-gated Na^{+} channels.

• If threshold potential is reached, an action potential is generated.

• Depolarization wave opens voltage-gated Na^{+} channels along the sarcolemma.

Repolarization

• Voltage-gated Na^{+} channels close, and voltage-gated K^{+} channels open.

• K^{+} efflux causes repolarization.

• Muscle fibers enter a refractory period until repolarization is complete.

Classification of Skeletal Muscle Fibers

• Based on contraction velocity: Slow fibers and fast fibers.

• Based on ATP production pathways: Oxidative fibers and glycolytic fibers.

Slow Oxidative (SO) Fibers

• Contract slowly due to slow myosin ATPases.

• Dependent on oxygen delivery and aerobic pathways.

• High myoglobin (red), low glycogen, small diameter, many mitochondria, many capillaries.

• Best suited for endurance activities.

Fast Oxidative (FO) Fibers

• Contract quickly due to fast myosin ATPases.

• Dependent on aerobic pathways but also use glycolytic reserves.

• High myoglobin (red to pink), moderate glycogen, moderate diameter, many mitochondria, many capillaries.

• Best suited for sprinting and walking.

Fast Glycolytic (FG) Fibers

• Contract rapidly due to fast myosin ATPases.

• Independent of oxygen; use glycolytic reserves.

• Low myoglobin (white), low glycogen, large diameter, few mitochondria, few capillaries.

• Best suited for short, rapid, intense movements.

Muscle Tension and Load

• Muscle tension: Force exerted by a muscle on an object.

• Load: Force exerted on the muscle by the weight of the object.

Types of Contractions

• Isometric: Muscle exerts force, but the load doesn't move; muscle fiber length doesn't change.

• Isotonic: Muscle force overcomes the load; muscle fiber length changes.

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

Muscle Tone

• Slight contraction even when relaxed.

• Keeps muscles firm, healthy, and ready to respond.

• Assists in joint stabilization and posture maintenance.

Metabolism in Muscle

ATP Regeneration

• Muscles store ATP for only 4-6 seconds of activity.

• ATP must be regenerated quickly.

• Three pathways regenerate ATP after hydrolysis to ADP and inorganic phosphate.

Direct Phosphorylation

• Creatine phosphate (CP) regenerates ATP.

• CP + ADP → ATP (catalyzed by creatine kinase).

• Muscles store 2-3 times more CP than ATP; provides ~15 seconds of power.

• CP is replenished during rest.

Anaerobic Glycolysis

• Used when ATP and CP are exhausted.

• ATP generated by catabolizing glucose (from blood or muscle glycogen).

• Does not require oxygen.

• Glucose → 2 pyruvic acid molecules.

• Pyruvic acid converted to lactic acid during rigorous activity.

• Harvests 5% of ATP compared to aerobic respiration but is 2.5 times faster.

• Useful for 30-40 seconds of strenuous activity.

Aerobic Respiration

• Produces 95% of ATP during rest, light exercise, and moderate exercise.

• Requires oxygen.

• Glucose + Oxygen → Carbon dioxide + Water + ATP.

• Glycogen becomes pyruvic acid, then fatty acids are used.

• Produces a large quantity of ATP but is slow.

Smooth Muscle

Microscopic Anatomy

• Located in the walls of most hollow organs (except the heart).

• Spindle-shaped cells with a central nucleus.

• 1/10th the width and thousands of times shorter than skeletal muscle fibers.

• Fine connective tissue (endomysium) between cells.

• Organized into two sheets:

  • Longitudinal layer (outer): Fibers run parallel to the organ's long axis; contraction dilates or shortens the organ.

  • Circular layer (inner): Fibers run around the organ's circumference; contraction constricts or elongates the organ.

• Peristalsis: Continuous involuntary contraction and relaxation of the two layers, propelling and mixing substances.

Innervation and Excitation-Contraction Coupling

• Innervated by autonomic (involuntary) nervous system fibers.

• Nerve fibers have varicosities that form diffuse junctions.

• Varicosities release neurotransmitters into the synaptic cleft.

• Less developed sarcoplasmic reticulum compared to skeletal muscles.

• Lacks T-tubules but has caveolae (pouchlike infoldings) that sequester extracellular Ca^{2+}.

• Extracellular Ca^{2+} is primarily responsible for excitation-contraction coupling.

Filament Arrangement

• Not striated; lacks sarcomeres.

• Myosin to actin ratio is 1:13 (vs. 1:2 in skeletal muscles).

• Surplus of myosin heads along the entire length of the muscle.

• Lacks troponin complexes; calmodulin is the Ca^{2+}-binding site.

• Contracts in a corkscrew-like manner due to diagonal arrangement of filaments.

Physiology

• Contracts slowly and synchronously due to gap junctions.

• Pacemaker cells set the pace of contraction.

• Sliding filament model and ATP utilization apply.

• Contraction triggered by rises in intracellular Ca^{2+} levels.

• Ca^{2+} activates myosin by interacting with calmodulin.

• Calmodulin activates myosin kinase (myosin light chain kinase), which phosphorylates myosin.

• Contracts 30 times slower than skeletal muscles; maintains tension longer with less energy.

Regulation of Contraction

• Regulated by neural stimuli, hormones, and localized chemical changes.

• Neural stimuli generate action potentials and increase cytosolic Ca^{2+} levels.

• Nerve endings release various neurotransmitters that stimulate different smooth muscle fibers.

• Receptor molecules (ACh receptors, norepinephrine receptors, etc.) determine which fibers are stimulated.

• Some smooth muscles contract through spontaneous depolarization or in response to chemical stimuli that bind to G protein-linked receptors.

• Hormones, histamine, excess CO_2, low pH, and lack of oxygen can cause contraction or relaxation.

Special Features

• Maintains muscle tone without fatigue.

• Stretching causes initial contraction followed by adaptation and relaxation.

• Allows hollow organs to contract and relax slowly without expelling all contents.

• Smooth muscle stretching generates more tension than skeletal muscle stretching.

• Smooth muscles can shorten to half their length and lengthen to twice their length, while skeletal muscles can only undergo length changes of 30% to remain functioning.

Hypertrophy and Hyperplasia

• All muscle fibers can hypertrophy (increase in size).

• Some smooth muscle fibers undergo hyperplasia (divide and increase in number).

• Example: Uterus growth during puberty and pregnancy.

Types of Smooth Muscle

Unitary Smooth Muscle

• Found in the walls of all hollow organs except the heart (visceral muscle).

• Displays the characteristics listed above.

Multi-Unit Smooth Muscle

• Found in arrector pili muscles and internal eye muscles.

• Gap junctions and spontaneous depolarization are rare.

• More similar to skeletal muscles with independent muscle fibers and neuromuscular junctions.

• Served by the autonomic nervous system and responds to hormones.

Cardiac Muscle

• Found only in the walls of the heart.

• Striated and involuntary.

• Pacemaker cells set the tempo for contraction.

• ATP produced via aerobic pathways.

• Branched cells forming chains.

• Typically has a single nucleus (may have two during late fetal development).

• Has endomysium attached to the fibrous skeleton of the heart.

• Similar sarcoplasmic reticulum to smooth muscles.

• Ca^{2+} for contraction comes from both the sarcoplasmic reticulum and extracellular fluid.

• Contraction speed is slower than skeletal but faster than smooth muscles.

• Has myofibrils like skeletal muscle but with irregular thickness.

• Gap junctions present at intercalated discs; neuromuscular junctions absent.

• Troponin complexes present (similar to skeletal muscle).

• Contains one large T-tubule per sarcomere (rather than two smaller ones).