Hole's Human Anatomy & Physiology - Chapter 09 Muscular System

Overview of Muscle Tissues

• The muscular system contains three distinct types of muscle tissue:

  • Skeletal Muscle:

  • Attached primarily to the bones of the skeleton and the skin of the face.

  • Operates under conscious, voluntary control.

  • Contains striations.

  • Cardiac Muscle:

  • Forms the majority of the heart wall.

  • Operates under involuntary (non-conscious) control.

  • Responsible for the rhythmic pumping action of the heart.

  • Contains striations and intercalated discs.

  • Smooth Muscle:

  • Located in the walls of hollow internal organs (such as the digestive tract, blood vessels, and urinary bladder).

  • Operates under involuntary (non-conscious) control.

  • Non-striated.

Structure of Skeletal Muscle

General Composition and Connective Tissue Coverings

• The human body contains over $600$ individual skeletal muscles, each functioning as an organ of the muscular system.

• Skeletal muscles consist of multiple tissue types: skeletal muscle tissue, nervous tissue, blood, and connective tissue.

• Connective Tissue Coverings:

  • Fascia: A thin layer of dense connective tissue that surrounds individual muscles and holds them in position.

  • Tendon: A cord-like mass of dense connective tissue that attaches a muscle to a bone.

  • Aponeurosis: A broad, sheet-like fibrous tissue sheet that attaches a muscle to a bone, skin, or adjacent muscle tissue.

• Internal Connective Tissue Layers:

  • Epimysium: Connective tissue layer surrounding the entire muscle organ; situated directly beneath the deep fascia.

  • Perimysium: Connective tissue layer extending inward from the epimysium to surround individual bundles of muscle fibers called fascicles.

  • Endomysium: Delicately structured connective tissue surrounding each individual muscle cell (muscle fiber) within a fascicle.

Muscle Compartments and Fascial Layers

• Compartment: An enclosed space containing a functional group of muscles, blood vessels, and nerves, wrapped within a layer of deep fascia.

• Compartment Syndrome:

  • Pathological condition triggered by fluid accumulation (edema or hemorrhage) within a closed fascial compartment.

  • Elevates internal compartment pressure, compressing blood vessels and leading to ischemia (deficient oxygen and nutrient delivery).

  • Results in severe, persistent pain and tissue necrosis if unaddressed.

• Fascia Organization:

  • Deep Fascia: Encloses and surrounds skeletal muscles.

  • Subcutaneous Fascia: Positioned directly beneath the skin, connecting deep fascia to cutaneous structures.

  • Subserous Fascia: Connects to the serous membranes lining internal body cavities.

Microscopic Anatomy of Skeletal Muscle Fibers

• Skeletal Muscle Fiber (Cell):

  • A long, cylindrical, multinucleated cell.

  • Sarcolemma: The plasma membrane of the muscle fiber.

  • Sarcoplasm: The cytoplasm of the muscle fiber, containing abundant mitochondria and parallel myofibrils.

  • Myofibrils: Cylindrical organelles running parallel along the entire length of the cell, composed of two main types of myofilaments:

  • Thin Filaments: Composed predominantly of the protein actin.

  • Thick Filaments: Composed predominantly of the protein myosin.

Sarcomere Structural Anatomy

• Sarcomere: The fundamental functional unit of contraction within a myofibril, repeating end-to-end.

  • I Band (Light Band): Composed exclusively of thin actin filaments; anchored directly to the Z line.

  • A Band (Dark Band): Spans the entire length of thick myosin filaments; includes regions where thick and thin filaments overlap.

  • H Zone: Central region of the A band composed exclusively of thick myosin filaments.

  • Z Line (Z Disc): Structural boundary separating adjacent sarcomeres; located at the center of the I band and anchors actin filaments in place.

  • M Line: Structural line at the exact center of the sarcomere and A band; anchors adjacent thick filaments.

  • Titin: Elastic protein strands extending from the Z line to the M line, anchoring thick filaments to Z discs and maintaining sarcomere alignment.

Molecular Composition of Filaments

• Thick Filaments:

  • Formed from molecules of myosin.

  • Each myosin molecule consists of two twisted protein strands with globular projection heads extending outward.

  • Myosin heads bind to active sites on actin to form cross-bridges.

• Thin Filaments:

  • Formed primarily from a double-stranded helix of globular actin proteins.

  • Contain specific binding sites for myosin heads.

  • Regulatory Proteins:

  • Tropomyosin: Rod-shaped proteins occupying the grooves of actin filaments; physically block myosin-binding sites when the muscle fiber is relaxed.

  • Troponin: Complex of three subunits attached to tropomyosin; binds calcium ions ($Ca^{2+}$) to induce a conformational shift that exposes active sites on actin.

Sarcoplasmic Reticulum and Transverse Tubules

• Sarcoplasmic Reticulum (SR): Specialized smooth endoplasmic reticulum that forms a network surrounding each myofibril; actively stores and releases calcium ions ($Ca^{2+}$).

• Transverse Tubules (T Tubules): Deep invaginations of the sarcolemma that extend transversely across the cell; open to the extracellular space to conduct muscle action potentials deep into the sarcoplasm.

• Triad: A structural grouping consisting of one central T tubule flanked on both sides by dilated terminal cisternae of the sarcoplasmic reticulum.

Skeletal Muscle Contraction

Neuromuscular Junction

• Neuromuscular Junction (NMJ / Myoneural Junction): A specialized chemical synapse between the axon terminal of a motor neuron and the sarcolemma of a skeletal muscle fiber.

• Components of the NMJ:

  • Motor Neuron: Somatic nerve cell that extends from the central nervous system to stimulate a muscle fiber.

  • Motor End Plate: Highly folded region of the sarcolemma opposite the synaptic terminal, densely packed with neurotransmitter receptors.

  • Synaptic Cleft: Narrow extracellular space separating the axon terminal from the motor end plate.

  • Synaptic Vesicles: Membrane-bound sacs in the axon terminal containing the neurotransmitter acetylcholine (ACh).

  • Acetylcholine (ACh): The specific neurotransmitter synthesized and released by somatic motor neurons to stimulate muscle contraction.

Contraction Stimulus and Action Potential Generation

• A nerve impulse traveling down a motor neuron axon reaches the terminal, causing synaptic vesicles to fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft via exocytosis.

• Acetylcholine diffuses across the synaptic cleft and binds to specific acetylcholine receptors on the motor end plate.

• ACh binding increases membrane permeability to sodium ions ($Na^+$) and potassium ions ($K^+$), generating a localized depolarization known as an end-plate potential.

• This local potential triggers a self-propagating muscle impulse (action potential) that sweeps across the entire sarcolemma and travels deep into the fiber along T tubules.

• The arrival of the muscle impulse at T tubules stimulates voltage-gated calcium channels in adjacent terminal cisternae of the sarcoplasmic reticulum, causing calcium ions ($Ca^{2+}$) to diffuse rapidly out of the SR into the cytosol.

Excitation-Contraction Coupling

• Excitation-Contraction Coupling: The complex sequence of events connecting electrical stimulation of the sarcolemma to mechanical contraction of the sarcomeres.

• Sequence of Events:

  • In a relaxed muscle fiber, cytosol $Ca^{2+}$ concentration is extremely low, and tropomyosin blocks myosin-binding sites on actin.

  • When released from the sarcoplasmic reticulum, $Ca^{2+}$ diffuses into the sarcoplasm and binds directly to troponin molecules.

  • Calcium binding alters the three-dimensional conformation of troponin, pulling tropomyosin aside.

  • Tropomyosin displacement exposes active myosin-binding sites on actin.

  • High-energy myosin heads attach to these exposed sites, forming cross-bridges.

Sliding Filament Model

• Sliding Filament Mechanism: Muscle contraction occurs as thin actin filaments slide inward past thick myosin filaments toward the center of the sarcomere.

• Sarcomere Dimensional Changes During Contraction:

  • H zones narrow and may disappear completely.

  • I bands narrow significantly.

  • Z lines are pulled closer together, shortening the overall sarcomere length.

  • Filament Length: Thick filaments and thin filaments do not shorten; the degree of filament overlap increases.

Cross-Bridge Cycle

• The cross-bridge cycle proceeds through six recurring stages:

  1. Cross-Bridge Formation: Energized myosin head (holding $ADP + P_i$) binds to an exposed active site on actin.

  2. Power Stroke: Myosin head bends forward, pulling the thin actin filament toward the center of the sarcomere; stored energy is expended, releasing $ADP$ and inorganic phosphate ($P_i$).

  3. Cross-Bridge Detachment: A new molecule of adenosine triphosphate ($ATP$) binds to the myosin head, causing it to release its hold on actin.

  4. ATP Hydrolysis: Myosin ATPase enzyme hydrolyzes the bound $ATP$ into $ADP$ and $P_i$; energy released re-cocks the myosin head into its original high-energy configuration.

  5. Re-attachment: If $Ca^{2+}$ remains bound to troponin, the re-cocked myosin head attaches to the next actin binding site down the filament.

  6. Contraction Maintenance: Cycling continues repeatedly as long as $ATP$ is available and elevated cytosol $Ca^{2+}$ levels persist.

Muscle Relaxation Process

• Steps of Relaxation:

  1. Neuronal stimulation terminates, stopping acetylcholine release.

  2. Acetylcholinesterase (AChE): An enzyme present in the synaptic cleft rapidly hydrolyzes remaining acetylcholine into choline and acetate, preventing continuous stimulation.

  3. The sarcolemma and T tubules re-establish their resting electrical potential.

  4. Active transport pumps ($Ca^{2+}\,text{-ATPase}$) pump cytosol $Ca^{2+}$ back into the lumen of the sarcoplasmic reticulum against a steep concentration gradient (requiring $ATP$).

  5. Loss of bound $Ca^{2+}$ causes troponin to revert to its original shape, shifting tropomyosin back over the myosin-binding sites on actin.

  6. Cross-bridge interactions are prevented, and the muscle fiber passively relaxes to its initial resting length.

Energy Sources for Muscle Contraction

• Stored ATP: Free $ATP$ present in muscle cytoplasm provides initial energy for only a few seconds of continuous maximal contraction.

• Creatine Phosphate:

  • High-energy molecule stored in muscle cells containing high-energy phosphate bonds.

  • Stores excess energy released from mitochondria when cellular $ATP$ levels are high.

  • When cellular $ATP$ levels drop ($ATP \nrightarrow ADP + P_i$), creatine phosphokinase transfers a phosphate group directly from creatine phosphate to $ADP$, rapidly re-synthesizing $ATP$.

  • Combined stored $ATP$ and creatine phosphate fuel approximately $10\,\text{seconds}$ of maximal muscular effort.

• Cellular Respiration:

  • Required for sustained activity beyond $10\,\text{seconds}$.

  • Muscle fibers store glucose internally in the form of the polysaccharide glycogen.

  • Anaerobic Phase (Glycolysis):

  • Occurs in the cytoplasm; does not require oxygen.

  • Cleaves glucose into two pyruvic acid molecules, yielding a net $2\,\text{ATP}$.

  • Aerobic Phase:

  • Occurs in mitochondria in the presence of adequate oxygen.

  • Includes the Citric Acid Cycle ($2\,\text{ATP}$) and Electron Transport Chain ($28\,\text{ATP}$), synthesizing up to total aerobic energy output.

  • Myoglobin: A red iron-containing protein synthesized in muscle cells that binds oxygen with high affinity, storing it locally to supply mitochondria during contraction.

Oxygen Debt and Anaerobic Metabolism

• Anaerobic (Lactic Acid) Threshold: During strenuous exercise, respiratory and cardiovascular systems cannot supply oxygen fast enough to meet mitochondrial demand, forcing cells to rely on anaerobic glycolysis.

• Pyruvic acid is converted directly into lactic acid, which diffuses out of muscle cells into the bloodstream.

• Oxygen Debt Definition: The precise volume of oxygen required following intense physical exertion to enable liver cells to convert accumulated lactic acid back into glucose, as well as to restore skeletal muscle concentrations of $ATP$, creatine phosphate, and oxygen bound to myoglobin.

Muscle Fatigue and Cramping

• Muscle Fatigue: The loss of structural ability to contract despite continued neural stimulation.

  • Primary Causes: Reduced local blood supply, extracellular/intracellular ion imbalances across the sarcolemma (e.g., loss of $K^+$), accumulation of lactic acid, and loss of psychological drive.

• Muscle Cramp: A severe, painful, involuntary, sustained muscle contraction triggered by localized shifts in extracellular fluid electrolyte concentrations.

Heat Production

• Active skeletal muscle cells are the principal source of body heat.

• Over $50\%$ of total energy released during cellular respiration is lost as heat; less than $50\%$ is successfully captured within $ATP$ bonds.

• Blood circulates through active skeletal muscle tissue to transfer generated heat throughout the body core.

Clinical Pathologies of the Muscular System

• Myasthenia Gravis (MG):

  • An autoimmune pathology where production of autoantibodies attacks functional nicotinic acetylcholine receptors at motor end plates.

  • Receptors may be reduced to $1/3$ of normal density.

  • Symptoms: Severe muscular weakness, ptosis, progressive fatigue.

  • Therapeutic interventions: Acetylcholinesterase inhibitors (preventing ACh degradation), immunosuppressant medications, therapeutic plasma exchange (plasmapheresis), and monoclonal antibody administration.

• Muscular Dystrophy:

  • Group of inherited muscle-wasting conditions caused by mutations in the gene encoding dystrophin.

  • Dystrophin normally anchors structural proteins on the inner sarcolemma to extracellular matrix structures, protecting membranes during mechanical strain.

  • In dystrophy, absence of functional dystrophin leads to sarcolemma tears, cellular degradation, muscle cell death, and fibrosis.

• Botulism (Clostridium botulinum):

  • Anaerobic bacterial toxin that blocks pre-synaptic exocytosis of acetylcholine from axon terminals.

  • Ingestion leads to severe paralysis of skeletal muscles, including respiratory distress.

  • Therapeutic application: Highly diluted injections (Botox) are utilized clinically to paralytically smooth facial hyperfunctional wrinkles, control strabismus, or suppress chronic migraines.

Muscular Responses and Mechanics

Threshold Stimulus and Twitch Dynamics

• Threshold Stimulus: The minimum electrical or chemical strength required to evoke an action potential across a muscle fiber sarcolemma.

• Twitch: The isolated, brief contractile response of a single muscle fiber to a single threshold stimulus.

• Phases of a Muscle Twitch:

  1. Latent Period: Brief delay between stimulus application and the onset of force generation (time required for excitation-contraction coupling, $Ca^{2+}$ release, and cross-bridge engagement).

  2. Period of Contraction: Phase during which tension develops and the fiber shortens.

  3. Period of Relaxation: Phase during which cross-bridges detach, $Ca^{2+}$ returns to the SR, and tension declines.

Length-Tension Relationship

• The maximal force a muscle fiber generates depends directly on its initial fiber length prior to stimulation.

  • Optimal Length: Normal resting length provides maximum overlap between thick myosin heads and thin actin active sites, generating peak force.

  • Overly Shortened: Sarcomeres are compressed; Z lines collide with thick filaments, severely restricting contraction force.

  • Overly Stretched: Minimal overlap between thick and thin filaments; myosin heads cannot engage actin, yielding reduced force output.

Summation and Tetanus

• Summation: Process where high-frequency electrical stimuli deliver successive action potentials before a fiber fully relaxes, causing twitch forces to combine and generate progressively higher mechanical tension.

• Partial Tetanus (Incomplete Tetany): Higher stimulus frequencies shorten relaxation intervals, causing forceful sustained contractions with visible oscillation periods.

• Complete Tetanus (Complete Tetany): Extremely high stimulus frequencies abolish relaxation periods completely, generating a maximum sustained contraction force plateau (demonstrated experimentally in laboratory preparations).

Motor Units and Recruitment

• Motor Unit: A single motor neuron together with every individual muscle fiber it innervates.

  • Small Motor Units: Innervate as few as $2\,\text{to}\,3$ muscle fibers per neuron; located in muscles requiring fine, precise control (e.g., extraocular eye muscles, intrinsic hand muscles).

  • Large Motor Units: Innervate hundreds to thousands of muscle fibers per neuron; located in large postural or power muscles (e.g., gastrocnemius, gluteus maximus).

• Recruitment: Sequential activation of additional motor units within a muscle organ to produce greater mechanical force.

  • Smaller motor units (driven by small-diameter motor axons) possess lower thresholds and are recruited first.

  • Larger motor units (driven by larger-diameter motor axons) possess higher thresholds and are recruited later under intense demand.

• Muscle Tone (Tonus): A continuous state of low-level partial contraction present in resting skeletal muscles, maintained by alternating, involuntary motor unit activations to maintain posture.

Isotonic and Isometric Contractions

• Isotonic Contractions: Muscle develops constant tension while changing length.

  • Concentric Contraction: Muscle shortens as generated force exceeds resistive load (e.g., lifting a weight).

  • Eccentric Contraction: Muscle lengthens while maintaining tension because external resistance exceeds generated force (e.g., controlled lowering of a weight).

• Isometric Contractions: Muscle develops mechanical tension without altering its external length; internal structural attachments remain stationary against immovable resistance (e.g., holding a weight static in mid-air or pushing against a wall).

Skeletal Muscle Fiber Types

• Skeletal muscle fibers are categorized into three major functional categories:

  • Slow-Twitch Fibers (Type I, Slow Oxidative - SO):

  • High myoglobin content ("red fibers"), dense capillary networks, and abundant mitochondria.

  • High aerobic oxidative capacity, slow myosin ATPase speed, resistant to fatigue.

  • Examples: Postural back muscles, soleus muscle.

  • Fast-Twitch Glycolytic Fibers (Type IIb, Fast Glycolytic - FG):

  • Low myoglobin content ("white fibers"), reduced capillary networks, sparse mitochondria, extensive sarcoplasmic reticulum.

  • High anaerobic glycolytic capacity, rapid myosin ATPase speed, generate massive force rapidly, highly susceptible to fatigue.

  • Examples: Intrinsic hand muscles, extrinsic eye muscles.

  • Fast-Twitch Oxidative Fibers (Type IIa, Fast Oxidative - FO / Intermediate):

  • Intermediate myoglobin content ("pink fibers"), good blood supply, high mitochondrial density.

  • Rapid myosin ATPase speed with intermediate resistance to fatigue.

Exercise Adaptations (Use and Disuse)

• Hypertrophy: Enlargement of exercised muscle tissue.

  • Forceful, high-intensity resistance exercise stimulates fast-twitch fibers to synthesize new actin and myosin myofilaments, increasing myofibril cross-sectional area and overall muscle mass.

  • Aerobic endurance exercise stimulates slow-twitch fibers to synthesize additional mitochondria, myoglobin, and surrounding capillary networks without significant structural fiber enlargement.

• Atrophy: Decrease in muscle size, fiber diameter, and overall strength resulting from disuse, casting, or loss of neural stimulation.

Smooth Muscle Tissue

Structural Morphology

• Smooth muscle cells are shorter than skeletal muscle fibers, possess tapering elongated ends, and contain a single centrally located nucleus.

• Myofilaments are distributed in an unorganized pattern throughout the cytoplasm, lacking visible striations.

• Cells lack transverse tubules and possess a poorly developed sarcoplasmic reticulum.

Subtypes of Smooth Muscle

• Multi-Unit Smooth Muscle:

  • Muscle fibers exist as separate structural units, operating independently of adjacent cells.

  • Typically lacks spontaneous rhythmicity; activated independently by autonomic nerves or specific hormones.

  • Locations: Iris of the eye, walls of large blood vessels.

• Visceral Smooth Muscle (Single-Unit Smooth Muscle):

  • Spindle-shaped fibers arranged in closely packed sheets.

  • Membranes joined via gap junctions, allowing ions to flow freely so cells contract together as a single functional unit.

  • Exhibits spontaneous rhythmicity and self-excitation, driving peristalsis.

  • Locations: Walls of hollow visceral organs (stomach, intestines, urinary bladder, uterus).

Smooth Muscle Contraction Physiology

• Similarities to Skeletal Muscle: Contraction relies on actin-myosin cross-bridge interactions, triggered by intracellular $Ca^{2+}$ elevation, powered by $ATP$ breakdown.

• Differences from Skeletal Muscle:

  • Smooth muscle lacks troponin; calcium ions bind instead to the regulatory protein calmodulin.

  • $Ca^{2+}\,text{-calmodulin}$ complex activates an enzyme called myosin light-chain kinase (MLCK), which phosphorylates myosin heads to initiate cross-bridge binding.

  • Governed by two neurotransmitters: Acetylcholine (ACh) and Norepinephrine (NE) (which can either excite or inhibit depending on receptor sub-types).

  • Highly responsive to circulating hormones, local tissue pH, oxygen/carbon dioxide fluctuations, and direct mechanical stretch.

  • Slower to initiate contraction and relaxation, highly resistant to fatigue, and can alter overall length while maintaining structural tautness.

Cardiac Muscle Tissue

• Located exclusively within the heart wall.

• Composed of striated, branching cells containing a single central nucleus.

• Interconnected by specialized cell junction complexes termed intercalated discs, which contain desmosomes for mechanical adhesion and gap junctions for rapid electrical ion diffusion.

• Networks of interconnected cardiac cells contract together as an all-or-none functional syncytium.

• Self-exciting, inherently rhythmic, and possesses an exceptionally prolonged absolute refractory period that prevents sustained tetanic contractions.

Comparison of Muscle Tissue Characteristics

Skeletal Muscle Actions and Levers

Lever Systems in Movement

• Movement of bones around joints operates via mechanical lever systems comprising four components:

  1. Rigid Bar or Rod: Represented by bones.

  2. Fulcrum or Pivot: Represented by joints.

  3. Resistance (Object Weight): The load lifted against gravity or force.

  4. Force (Effort): Mechanical force provided by muscle contraction.

• Classes of Levers:

  • First-Class Lever: Sequence is Resistance - Fulcrum - Force (e.g., scissors; extension of elbow via triceps brachii; head extension at atlas).

  • Second-Class Lever: Sequence is Fulcrum - Resistance - Force (e.g., wheelbarrow; standing on toes via gastrocnemius contraction).

  • Third-Class Lever: Sequence is Resistance - Force - Fulcrum (e.g., forceps/tweezers; flexion of forearm at elbow via biceps brachii).

Muscle Attachment Sites and Interactions

• Origin: The immovable or stationary attachment point of a skeletal muscle.

• Insertion: The movable attachment point pulled toward the origin during muscle contraction.

• Functional Muscle Roles:

  • Agonist: A muscle causing a specific anatomical movement.

  • Prime Mover: The agonist muscle bearing primary functional responsibility for producing a movement.

  • Synergists: Helper muscles contracting alongside the prime mover to assist action or stabilize intermediate joints.

  • Antagonist: Muscle whose contraction directly opposes the action of a prime mover.

Anatomic Breakdown of Major Skeletal Muscles

Muscles of Facial Expression

Muscles of Mastication

Muscles That Move the Head and Vertebral Column

Muscles That Move the Pectoral Girdle

Muscles That Move the Arm

Muscles That Move the Forearm

Muscles That Move the Hand

Muscles of the Abdominal Wall

Muscles of the Pelvic Floor and Perineum

Muscles That Move the Thigh

Muscles That Move the Leg

Muscles That Move the Foot

Lifespan Changes in the Muscular System

• Progressive age-related changes begin during the decade of the 40s:

  • Intracellular levels of myoglobin, $ATP$, and creatine phosphate begin a steady decline.

  • Adipose cells and fibrous connective tissue progressively replace lost skeletal muscle fibers.

  • By age $80$, approximately $50\%$ of total skeletal muscle mass has succumbed to age-related atrophy (sarcopenia).

  • Muscle strength declines steadily, muscle contraction speed slows, and reflex responses become significantly delayed.

  • Regular physical exercise (both resistance training and aerobic conditioning) helps preserve structural muscle mass, capillary density, and overall metabolic function into late adulthood.