BIOL243 Human Anatomy & Physiology I - Lecture Notes PDF

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 9 – Muscle Tissue MUSCLE TISSUE Males (AMAB) = ~42% total body mass; Females (AFAB) = ~36% total body mass Muscle-related tissues denoted by prefixes myo-, mys-, and sarco- e.g., s...

BIOL243 – HUMAN ANATOMY & PHYSIOLOGY I Charles Smith, PhD CSCS HUMAN ANATOMY & PHYSIOLOGY Ch. 9 – Muscle Tissue MUSCLE TISSUE Males (AMAB) = ~42% total body mass; Females (AFAB) = ~36% total body mass Muscle-related tissues denoted by prefixes myo-, mys-, and sarco- e.g., sarcoplasm = cytoplasm for a muscle cell 3 Types of Muscle Tissue: Skeletal Attach to bone & skin Striated Voluntary control Cardiac Only found in heart (myocardium) Striated Involuntary control Smooth Walls of hollow organs (i.e., GI, urinary bladder, blood vessels, airways) Non-striated Involuntary control MUSCLE TISSUE 4 Main Characteristics: 1. Excitability: responsiveness; ability to receive & respond to stimuli 2. Contractility: ability to shorten forcibly when stimulated 3. Extensibility: ability to be stretched 4. Elasticity: ability to recoil to resting length without being disfigured 4 Main Functions 1. Produce Movement: locomotion & joint manipulation 2. Maintain Body Posture & Position: keep body upright & everything in place 3. Joint Stabilization: keep joints from dislocating or overextending in any direction 4. Heat: from contraction; help maintain thermoregulation & metabolism SKELETAL MUSCLE MAKEUP Skeletal muscle are multilayered Muscle Belly: all fascicles surrounded by epimysium Fascicle: bundle of myofibers surrounded by perimysium Myofiber: bundle of myofibrils surrounded by sarcolemma deep to endomysium Aka “muscle cell” Sarcoplasm fills space between myofibers Myofibril: bundle of sarcomeres connected in series & parallel 80% of myofiber volumen Sarcomere: series of myofilaments “Functional” contractile unit of muscle MYOFIBER STRUCTURES Each fiber has a variety of structures which aid in its function Outside myofibrils Mitochondria Energy (ATP) production Sarcoplasmic Reticulum Release and reuptake calcium (Ca2+) ions Necessary for Sliding Filament Model T-Tubules Carry action potential from sarcolemma deep into myofiber Opens calcium (Ca2+) channels from SR into sarcoplasm Myonuclei Contain DNA for creation of new myofibrils Necessary for growth & repair Satellite Cells “Muscle stem cells”; can differentiate into whatever tissue necessary Necessary for growth & repair SARCOMERE STRUCTURE Sarcomeres made of 3 myofilaments: Actin (Thin) Filaments Subunits of Troponin: Calcium (Ca2+) binding protein Tropomyosin: strand-like structure covering myosin-binding sites; moves to reveal sites when calcium binds to Troponin Myosin (Thick) Filaments “contractile” filaments Bind to & pull actin closer together Titin (Elastic) Filaments Give sarcomere its elasticity Key demarcations: Z-Disc: ends of Actin filaments; demarcate ends of sarcomere M-Line: midline of sarcomere H-zone: area on either side of M-Line with no actin- myosin overlap (myosin only) SKELETAL MUSCLE CONTRACTION When muscle is relaxed: Sarcomeres (and myofibers) are at full length Some actin-myosin overlap Distinct M-Line & H-Zone When muscle contracts: Sarcomeres (and myofibers) shorten Sliding Filament Model Calcium binding to troponin & ATP hydrolysis causes myosin to bind to actin forming a cross-bridge Thin filaments slide past thick filaments causing actin- myosin overlap Actin filaments overlap M-Line; H-Zone shrinks and disappears SLIDING FILAMENT MODEL Rest Myosin head in “low energy” state ATP bonded Not yet attached to actin SLIDING FILAMENT MODEL Cross-Bridge ATP Hydrolysis Myosin head energized & attaches to binding site on actin SLIDING FILAMENT MODEL Power Stroke ADP & Pi released Myosin tail “contract, cocking & pulling the actin-myosin bridge Moves Actin filament closer to M- Line Sarcomere shortens SLIDING FILAMENT MODEL Cleavage New ATP binds to myosin head Myosin head releases from binding site on actin Return to low-energy state Cross-Bridge Cycling Cycle will repeat so long as Calcium (Ca2+) is available Keeps binding to troponin opening myosin-binding sites ATP available Keep energizing myosin head CROSS-BRIDGE CYCLING EXCITATION-CONTRACTION COUPLING Muscle does not just contract on its own Requires an action potential (stimulus) from brain or spinal column Voluntary movements; reflexes Motor Unit Motor neuron and all the myofibers it innervates Neuromuscular Junction Where the motor neuron “meets” its fiber group 3 Main Components: Synaptic Vesicles: hold neuron’s neurotransmitter (acetycholine, ACh) in axon Synaptic Cleft: space between neuron’s axon terminal and the sarcolemma End-Plate Receptors: binding proteins for acetylcholine (ACh) MEMBRANE POTENTIAL The sarcolemma is polarized Holds a resting, negative charge Charge is carefully maintained by balancing ions on either side of sarcolemma Motor neuron stimuli seek to depolarize sarcolemma “Flip” the charge Generate an Action Potential (stimulus) by causing ion shuttling across sarcolemma membrane EXCITATION-CONTRACTION COUPLING Goal of E-C Coupling: Convert an electrical signal (from brain) to chemical signal (neurotransmitter release) Chemical signal (ACh binding to end-plate receptors) becomes electrical signal (sarcolemma depolarization; action potential) Electrical signal (action potential) becomes chemical signal (calcium release) which stimulates Sliding Filament Model EXCITATION-CONTRACTION COUPLING 1. Neuron Releases ACh ACh binds to end-plate receptors Ligand-Gated Channels open Ion channels which only open when specific chemical messenger binds to them (i.e., ACh) 2. Sarcolemma Depolarizes Open ion channels facilitate sodium (Na+) influx & potassium (K+) efflux Membrane’s charge “increases” toward threshold When threshold achieve, Action Potential generated Voltage-Gated Channels open Ion channels which only open in response to change in membrane potential 3. Action Potential Propagates along Sarcolemma Sodium (Na+) & potassium (K+) ions continue to shuttle in and out “pushing” potential down entire sarcolemma EXCITATION-CONTRACTION COUPLING 4. T-Tubules Action potential eventually reaches T-tubules Triggers voltage-sensitive proteins Open calcium (Ca2+) channels from sarcoplasmic reticulum Calcium (Ca2+) ions flood sarcoplasm Bind to Troponin 5. Sliding Filament Model 6. Calcium Reuptake After neuronal action potential ceases: Sodium-potassium channels close Membrane potential normalizes back to resting Calcium goes back into SR Tropomyosin covers myosin-binding sites Contraction ceases SIZE PRINCIPLE Size Principle: motor units are recruited in the order of their size Smaller motor units first Lower forces, but used for finer motor tasks Largest motor units last Higher forces Facilitates an orderly recruitment of muscle fibers Training/exercise can allow us to selectively recruit larger motor units earlier MUSCLE CONTRACTIONS Generally, 3 Types of Muscle Contractions Isometric: external load (or muscle tension) changes, but muscle length is constant Commonly used in rehab Characterizes many stabilization contractions in our body e.g., pressing against a brick wall Isotonic: external load (or muscle tension) is constant, but muscle length changes Most of our daily movement/activities 2 Actions Concentric: muscle shortening Muscle does work (i.e., lifting something) Eccentric: muscle lengthening Muscle produces force (i.e., resisting something) To help keep joints stable during movement, when one muscle acts concentrically the muscle opposite it will act eccentrically (i.e., biceps & triceps) Isokinetic: external load (or muscle tension) varies as muscle length changes such that the movement speed is constant Not seen much in day-to-day More common in rehab and research settings to study force-velocity relationship & joint torque ENERGY FOR CONTRACTIONS Muscle needs ATP to contract Without it: Myosin won’t detach from actin Myosin can’t get excited and attach to actin However, raw ATP stores only last for ~ 4 – 6 secs Need to replenish & restore ATP for muscle to act throughout the day Fatigue sets in when energy use rates exceed the rate we use energy Can also be related to ion imbalances disrupting membrane depolarization ENERGY SYSTEMS 1. Phosphagen (PCr) System ADP gets phosphorylated (gains a phosphate) by phosphocreatine Most immediate source of ATP Anaerobic Lasts ~15 secs +1 ATP per cycle 2. Glycolysis Conversion of glucose into ATP Fueled “sugar” stores in muscle (and from blood) Anaerobic results in Lactate production; Aerobic results in Pyruvate Lasts ~60 secs (anaerobically) or until glucose depleted (aerobically) Net +2 ATP per cycle (initial -1 ATP energy investment) 3. Oxidative Phosphorylation Pyruvate fuels Krebs Cycle which largely generates electron carriers Electron carriers then go through Electron Transport Chain which shuttle H+ ions to make ATP Aerobic Lasts until all fuel sources depleted (or death) Net + 32 ATP per cylce SKELETAL MUSCLE FIBER TYPING 3 Types of Fiber Classed largely based upon primary energy system Type IIx (Fast-glycolytic) Rely largely on PCr & anaerobic glycolysis High force & power; highly fatigable i.e., sprinting, jumping, throwing Type IIa (Fast-oxidative) Use combination of anaerobic glycolysis & oxidative phosphorylation Moderate force & power; moderately fatigable i.e., sustained sprints, most weight-based exercise Type I (Slow-oxidative) Rely largely on oxidative phosphorylation Low force & power; fatigue-resistant i.e., maintaining posture, endurance running, most day-to-day activities MUSCLE DEVELOPMENT & DECAY As we age, muscle grows & develops differently In infancy, this reflects changes in neuromuscular control Muscle develops from head to toe, proximal to distal Why babies lift their heads first, reach before grasping, crawl before walking Control peaks during mid-adolescence, however exercise & training can continue this later on After age 30, muscle mass begins to decline (sarcopenia) Muscle fibers decrease and are replaced with connective tissue fibers Training & exercise can abate or reverse this If paralyzed, immobilized, or bedridden, muscle tissue will irreversibly degenerate (disuse atrophy) Muscle mass not only lost but so is functionality (~5% decrease in strength per day) Paralyzed muscle can shrink to 25% initial size While genders have differences in muscle masses, muscle strength relative to body mass is similar SMOOTH MUSCLE Smooth muscle is a little different from skeletal muscle Has fewer myosin filaments than skeletal muscle Actin has calmodulin which binds calcium instead of troponin Still has tropomyosin Less forceful & powerful, but wildly more efficient Slower contractions & relaxations Maintains contractions for prolonged periods of time Most holding a constant, moderate contraction without fatiguing (smooth muscle tone) Very low energy cost Will regenerate throughout the lifespan Skeletal muscle is limited in its regenerative capacity MUSCLE TISSUE SUMMARY Muscles are multilayered structures which require all units innervated to contract in an organized, coordinated manner Contractions are carried out via Excitation-Contraction Coupling Motor neuron innervates its motor unit Generates an action potential on the sarcolemma triggering Ca2+ release Ca2+ binds to troponin causing tropomyosin to open the myosin binding sites for Sliding Filament Model The strength of the action potential, and force of the contraction are regulated by the frequency & intensity of the stimulus along with the size principle We have different types of contractile fibers which are classified based upon what systems they predominantly use to replenish ATP In turn, have varying fatigabilities Skeletal muscles in particular will perform either isometric, isotonic, or isokinetic contractions depending on whether the load, muscle length, and/or movement speed change during the contraction SAMPLE QUESTIONS 1. The binding of myosin to actin causing a decrease in the distance between Z-lines describes what model for muscle contraction? 2. To what on actin must calcium bind for myosin to be able to cross- bridge? 3. What type of contraction is characterized by the load remaining constant but the length of the muscle changing? 4. Which skeletal muscle fiber type is the most fatigable? 5. Which energy system is capable of generating the greatest overall amount of ATP?  COPYRIGHT © Pearson Edited by Charles Smith, PhD CSCS 2024