Seeley\'s Essentials of Anatomy & Physiology Chapter 7 PDF

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Ch. 7: Muscular System ● Functions of the Muscular System ○ Movement of body ■ Contraction of skeletal muscles is responsible for the overall movements of body ○ Posture ■ Skeletal muscles constantly maintain tone ○ Respiration ■ Muscles of thorax carry out movements for respiration ○ Body Heat Production ■ Skeletal muscles contract to give off heat as a by-product ○ Communication ■ Involved in all aspects of communications like speaking and writing ○ Constriction of Organs & Vessels ■ Contraction of smooth muscle within walls of internal organs causes them to constrict ○ Contraction of Heart ■ Contraction of cardiac muscle causes heart to beat ● Characteristics of Skeletal Muscle ○ 40% of body weight ○ Striated muscle → transverse bands, striations, can be seen in muscle ○ 4 Functions of Skeletal Muscle: ■ Contractility ● Ability to shorten w/ force, causing movement ■ Excitability ● Ability to respond to stimulus ■ Extensibility ● Ability to stretch ■ Elasticity ● Ability to recoil after stretching ○ Skeletal Muscle Structure (Striated) ■ Connective Tissue Coverings of Muscle ● Epimysium (muscular fascia): connective tissue sheath that surrounds entire muscle ● Perimysium: loose connective tissue surrounding each bundle of fasciculi (visible bundles) ● Endomysium: surrounds muscle fiber ■ Muscle Fiber Structure ● Single, cylindrical fiber ● Multi-nuclear ● Sarcolemma: cell membrane of a muscle fiber

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○ Has many tubelike invaginations (transverse tubules, “T tubules”) along the surface ○ Associated with the sarcoplasmic reticulum (endoplasmic reticulum of a muscle fiber) ■ T tubules connect sarcolemma to sarcoplasmic reticulum ● Sarcoplasmic reticulum stores high concentration of calcium ions → crucial for muscle contraction ● Sarcoplasm: cytoplasm of muscle fiber ○ Contains many myofibrils (threads that extend from one end of the muscle fiber to the other) ■ Myofibrils consist of 2 protein fibers: ● Actin Myofilaments: thin ● Myosin Myofilaments: thick ■ Actin & Myosin myofilaments are arranged into sarcomeres (basic structural & functional unit of a muscle → extend from Z disk to Z disk)

■ Actin & Myosin Myofilaments ● Actin ○ Thin ○ Consists of actin, troponin, and tropomyosin ■ Actin strands: attachment sites for myosin myofilaments ■ Troponin: binding sites for calcium ions ■ Tropomyosin: block myosin myofilament binding sites on actin myofilament in an unstimulated muscle ● If no calcium ions are present, tropomyosin filaments cover attachment sites on actin myofilament ● If calcium ions are present, they bind to troponin so that tropomyosin filaments

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expose the attachment sites on actin myofilaments ● Myosin ○ Thick ○ Myosin head properties: ■ Heads can bind to attachment sites on actin myofilaments ■ Heads can bend and straighten during contraction ■ Heads can break down ATP, releasing energy ■ Sarcomeres ● Basic structural and functional unit of skeletal muscle ● When sarcomeres shorten, myofibrils shorten, causing contraction of muscle fiber during contraction ● Extends from Z disk to Z disk ○ Z disk → network of protein fibers forming an attachment site for actin myofilaments ● I band: ○ Light ○ Consists of only actin myofilaments ○ Spans each Z disk & ends at myosin myofilaments ○ Decrease in size during contraction ● A band ○ Dark ○ Consists of an overlap between actin and myosin myofilaments ○ Extends length of myosin myofilaments ○ Stays same size during contraction ● H zone: ○ Light ○ At center of sarcomere → when contracting → decrease in size during contraction ○ Consists of only myosin myofilaments ● M line: ○ Dark ○ Myosin myofilaments are anchored here ● I band & A band are responsible for striations in skeletal muscle fiber

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● Excitability of Muscle Fibers ○ Muscle fibers have electrical properties, similar to neuron ■ Inside of the membrane → NEGATIVE (-) charge ■ Outside of membrane → POSITIVE (+) charge ○ Cell membrane is polarized ○ Resting Membrane Potential: charge difference across the membrane of a resting cell ■ Uneven distribution of ions across the cell membrane ■ Develops for 3 reasons: ● Concentration of potassium ions inside cell is higher than outside of cell ● Concentration of sodium ions outside the cell is higher than inside of cell ● Cell membrane is more permeable to potassium ions than sodium ions ○ Excitable cells have many leak channels for potassium ■ Potassium leaks out of cell faster than sodium leaks into cell ○ Potassium channels = open ○ Sodium channels (and other ion channels) = closed ○ Other negatively-charged molecules like proteins are also trapped inside cell due to being impermeable → negative inside (Ion Channels & the Action Potential, Diagram)

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○ Outward concentration gradient & inward electrical gradient for potassium ions ■ Since potassium is positively charged, moving from inside of cell to outside of cell makes inside of cell membrane even more negatively charged ○ Resting membrane potential is an equilibrium in which the tendency for potassium to diffuse out of cell is opposed by negative charges inside of cell (attract potassium into cell) ■ Active transport of sodium and potassium by the sodium-potassium pump maintains this uneven distribution across the cell membrane ○ A change in resting membrane potential is done by changes in membrane permeability to potassium/sodium ions ■ Stimulation in a muscle fiber causes sodium channels to open ● Depolarization! → excitation → action potential ○ After inside of cell membrane is more positive than outside due to increased sodium permeability, these sodium channels close and additional potassium channels to reach equilibrium again ■ Repolarization! ○ Action Potential: results in muscle contraction in a muscle fiber; consists of the rapid depolarization and repolarization by the cell membrane ● Nerve Supply & Muscle Fiber Stimulation (Control of Movement) ○ Motor Neurons: specialized nerve cells that stimulate muscles to contract after being activated by the brain ■ Generate action potentials (travel to skeletal muscle fibers) ● Axons of these neurons enter muscles and send branches to several muscle fibers ○ Neuromuscular Junction: synaptic junction between nerve axon and a muscle fiber ■ (also known as a synapse) ○ Motor Unit: single motor neuron and all skeletal muscle fibers it innervates (all-or-none law → once stimulated, all will contract) ■ The fewer fibers in a motor unit (e.g. hand), the greater control you have over that muscle ■ Large motor unit vs. smaller motor unit ○ Neuromuscular Junction (NMJ) ■ Connection between the motor neuron and muscle fiber ○ Motor End Plate ○ Presynaptic Terminal → enlarged axon terminal ○ Synaptic Cleft → space between presynaptic terminal and muscle fiber membrane ○ Postsynaptic Membrane → muscle fiber membrane ■ Contains many synaptic vesicles ● Contain acetylcholine (ACh) → neurotransmitter

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○ When action potential reaches presynaptic terminal, calcium-ion channels open ○ Calcium ions enter presynaptic terminal and cause synaptic vesicles to release ACh into synaptic cleft via exocytosis ○ ACh diffuses across synaptic cleft and binds to ACh receptor sites (nicotinic ACh receptors, nAChRs) on sodium-ion channels in postsynaptic membrane ■ ACh binding to ACh receptor site opens sodium-ion channels → inflow of sodium ions ■ Inflow of sodium ions causes action potential once threshold is reached ○ Action potential travels along length of muscle fiber and causes contraction (sliding-filament model) ○ ACh left in the synaptic cleft is broken down by an enzyme, acetylcholinesterase (AChe) ■ This breakdown ensures that one action potential in neuron equals one action potential in skeletal muscle fibers which equals one contraction of each muscle fiber ● Predict the Effect ○ Curare → blocks nAChRs → ACh can’t bind, no action potential nor contraction ○ Sarin → inhibits AChe → contraction constantly ○ Nicotine → stimulates nAChRs → contractions easier, faster heart beat ● Muscle Contraction (Sliding Filament Model) ○ Contraction of skeletal muscle tissue occurs when actin and myosin myofilaments slide past one another → sarcomeres shorten! ○ Sliding Filament Model: actin and myosin myofilaments slide over one another during muscle contraction ■ Neither actin nor myosin fibers shorten during contraction! ■ H zones & I bands shorten during contraction, but A bands do NOT change in length ○ During relaxation, sarcomeres lengthen (requires opposing force) ○ After a person dies, ATP is not available, and cross-bridges that have formed are not released → muscles become rigid ■ “Rigor mortis” ○ Calcium essentially causes contraction! ■ If low to no calcium, trouble with contraction

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○ Some energy from ATP is required for muscle contraction, some is released as heat → increase in body temperature ■ Explains why someone becomes warmer during exercise ■ Shivering is another mechanism to increase body temperature ○ Muscle relaxation occurs as calcium ions are actively transported back into sarcoplasmic reticulum (requires ATP) ■ Attachment sites on actin molecules are then covered again by tropomyosin to disable cross-bridging ● Muscle Twitch, Summation, Tetanus, and Recruitment ○ Muscle Twitch: contraction of a muscle fiber in response to single stimulus ■ Involves all muscle fibers in a motor unit (entire unit!) ○ All-or-None Law ■ Applies to individual myofibrils NOT the entire muscle ■ Strength of muscle contraction has to do w/ summation of an individual myofibril’s contraction as well as the number of myofibrils that contract ○ 3 Phases of Contraction: ● Lag (Latent) phase: time between stimulus and contraction ○ Action potentials are produced in 1+ motor neurons ■ Must result in release of calcium ions to form cross-bridges or contraction phase will NOT occur ● Contraction phase: time during muscle contraction ○ Force of contraction increased in 2 ways: ■ Summation: increased force by rapidly stimulating them (increased stimulus frequency) ● Tetany: sustained contraction, no relaxation due to constant stimulation ■ Recruitment: number of muscle fibers stimulated increased by increasing number of motor units stimulated → more force ● Relaxation phase: time during muscle relaxation ● Energy for Contraction ○ Muscle fibers are very energy-demanding → requires either aerobic (w/ O2) or anaerobic (without O2) ATP production ○ Two sources for ATP: ■ Cellular Respiration ● Aerobic ● Anaerobic ■ Creatine Phosphate ● Energy instantly available

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● Stores are depleted quickly ● 1 Creatine phosphate = 1 ATP + 1 Creatine ATP is derived from 4 processes in skeletal muscle: ■ Aerobic production of ATP during most exercise and normal conditions ■ Anaerobic production of ATP during intensive short-term work ■ Conversion of creatine phosphate to ATP ■ Conversion of 2 ADP to 1 ATP & 1 AMP during heavy exercise Aerobic Respiration: breakdown of glucose in presence of oxygen to produce CO2, H2O, and approx. 36 ATP molecules ■ e.g. TCA/Krebs Cycle, Glycolysis, Citric Acid Cycle, electrontransport chain ■ Can also process lipids or amino acids to make ATP ■ More efficient than anaerobic respiration, more flexible than anaerobic respiration Anaerobic Respiration: does NOT require oxygen; breaks down glucose to make 2 ATP & lactate (less efficient and accumulates toxins) Muscle cells store creatine phosphate (high-energy molecule), since they cannot store ATP ■ Creatine phosphate provides a means of storing energy which can be quickly used to maintain proper ATP in contracting muscle fibers ■ Excess ATP is used to synthesize creatine phosphate

■ After intense exercise, respiratory rate and volume stay elevated for a time ● Increased respiratory activity provides oxygen to pay back oxygen deficit ● Lactic Acid ○ Must get rid of it (toxic to cells in high concentration) ■ Requires oxygen ○ Converted back to glucose to pay back “oxygen debt” ● Recovery oxygen consumption is the amount of oxygen needed in chem. rxns that occur to: ○ Convert lactate (lactic acid) to glucose ○ Replenish depleted ATP & creatine phosphate stores in muscle fibers ○ Replenish oxygen stores in lungs, blood, and muscles ● Endurance Training ○ Increases number of mitochondria ■ More energy available ○ Increases ability to get oxygen (e.g. lung volume, hemoglobin, etc.) ○ Increases myoglobin ● Fatigue

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○ Temporary state of reduced work capacity ■ Without fatigue, muscle fibers would work to the point of structural damage ○ Prevents exhaustion of ATP reserves! ○ Mechanisms: ■ Acidosis & ATP depletion due to increased ATP consumption or decreased ATP production ● Anaerobic respiration lowers pH (acidosis) because of breakdown of glucose to lactate & protons ○ Lowered pH decreases effectiveness of calcium ions on actin and less calcium-ion release from sarcoplasmic reticulum ■ Oxidative stress (buildup of excess reactive oxygen species, ROS/Free Radicals) ● During intense exercise, increases in ROS production cause a breakdown of proteins, lipids, and nucleic acids ● Also, ROS trigger immune system chem. (interleukin → mediator of inflammation → cause of muscle soreness) ■ Inflammation ● T lymphocytes migrate into heavily worked muscles ○ Increase perception of pain to protect from further damage ● Physiological Contracture: muscle is incapable of contracting or relaxing → occurs when too little ATP bind to myosin myofilaments ● Psychological Fatigue: involves CNS rather than the actual muscle → individual “perceives” that muscle contraction is impossible ● Effect of Fiber Type on Activity Level ○ Myoglobin: stores & releases oxygen in a muscle ○ Large postural muscles have more slow-twitch fibers ○ Muscles of upper limb have more fast-twitch fibers ○ Exercise increases blood supply to muscles, # of mitochondria per muscle fiber, # of myofibrils and myofilaments → hypertrophy ■ With weight training, type IIb myosin can be replaced with type IIa myosin ■ Vigorous exercise can cause type I myofilaments to be replaced with type IIa myofilaments ○ Satellite Cells: undifferentiated cells below endomysium ■ Can differentiate when stimulated and develop into new, functional muscle fibers ● Stimulated by destruction of existing muscle fibers (e.g. injury, disease, strength training)

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● Types of Muscle Contractions ○ Isometric contractions: length of muscle does NOT change ■ Amount of tension increases during contraction (i.e. back muscles) ○ Isotonic contractions: equal tension throughout contraction ■ Length of muscle decreases during contraction (i.e. arm/finger muscles) ■ Concentric: muscle tension increases as muscle shortens ■ Eccentric: tension does NOT change, but opposing resistance causes muscle to lengthen ● (slowly lowering a heavy weight) ● Substantial force is produced → muscles can be injured ● Muscle Tone ○ Partial contraction in resting muscles ■ Maintains posture ○ Keeps muscles ready to respond optimally ○ Excitation of alternating/different motor units ○ Requires input from central nervous system (CNS)! ● Smooth muscle cells contain less actin and myosin than skeletal muscles ○ Smooth muscle cells do not have organized myofilaments either ○ Contract more slowly than skeletal muscle cells (do not develop an oxygen deficit) ● Autorhythmicity: periodic, spontaneous contraction (involuntary) ○ Hormones can stimulate smooth & cardiac muscle to contract ● Cardiac muscle cells have organized myofilaments (sarcomeres) because they’re striated, but unorganized myofilaments ○ Autorhythmic contraction (involuntary) ○ No oxygen deficit, no fatigue ○ Intercalated Disks: include tight & gap junctions

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● Skeletal Muscle Anatomy ○ General Principles ■ Tendon: connective tissue connecting muscle to bone ● Broad, sheetlike tendons are called aponeuroses ● Retinaculum: dense regular connective tissue sheath holding down tendons of wrist, ankle etc. ■ 2 points of attachment for each muscle: ● Origin (head): most stationary end of muscle ○ Typically proximal or medial to insertion points ○ Some muscles have multiple origins (i.e. bicep, tricep) ● Insertion: mobile attachment point of muscle ■ Agonist: muscle contracting ■ Antagonist: muscle opposing contraction; works in opposition to another muscle ■ For example, when flexing elbow, biceps is agonist while triceps relax to allow elbow to bend (act as the antagonist) ● Role is reversed when extending elbow! ■ Synergists: members of a group of muscles working together to cause a movement ● Prime Mover: muscle among group of synergists that plays the major role in accomplishing the movement ○ Contributes most ■ Fixators: muscles that hold 1 bone in place relative to body while a more distal bone is moved ● i.e. scapula muscles holding scapula in place while humerus is moved...