Chapter 9: Muscles & Muscle Tissue PDF

Chapter 9: Muscles & Muscle Tissue 3 Types of Muscle Terminologies: myo, mys and sarco are prefixes for muscle voluntary muscles – consciously controlled involuntary muscles – cannot be consciously contolled 1. skeletal muscle – voluntary 2. cardiac muscle – involuntary 3. smooth...

Chapter 9: Muscles & Muscle Tissue 3 Types of Muscle Terminologies: myo, mys and sarco are prefixes for muscle voluntary muscles – consciously controlled involuntary muscles – cannot be consciously contolled 1. skeletal muscle – voluntary 2. cardiac muscle – involuntary 3. smooth muscle – involuntary Comparison of Muscle Types Comparison of skeletal, cardiac and smooth muscle. Comparison of Muscle Types All muscle types share four main characteristics: 1. Excitability (responsiveness): ability to receive and 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 Muscle Functions Muscle has four important functions: 1. Produce movement: responsible for all locomotion and manipulation Example: walking, digesting, pumping blood 2. Maintain posture and body position 3. Stabilize joints 4. Generate heat as they contract Skeletal Muscle Anatomy Skeletal (voluntary) muscle is made up of different tissues with 3 features: 1. Nerve and blood supply 2. Connective tissue sheaths 3. Attachments Skeletal Muscle Anatomy – Nerve and Blood Supply Each muscle receives a nerve, artery and veins – consciously controlled skeletal muscle has nerves supplying every fiber to control activity Contracting muscle fibers require HUGE amounts of oxygen and nutrients – waste (CO2) must be removed quickly Skeletal Muscle Anatomy – Connective Tissue Sheaths Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue Support cells and reinforce whole muscle Sheaths from external to internal: – Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia – Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) – Endomysium: fine areolar connective tissue surrounding each muscle fiber Skeletal Muscle Anatomy – Connective Tissue Sheaths Figure 9.1 Comparison of tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Skeletal Muscle Anatomy – Attachments Muscles span joints and attach to bones Muscles attach to bone in at least two places – Insertion: attachment to movable bone – Origin: attachment to immovable or less movable bone Attachments can be direct or indirect: – Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage intercostal muscles Skeletal Muscle Anatomy – Attachments Attachments can be direct or indirect: – Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis most common type of attachment more durable – tendons are comprised of tough, collagen fibers which can withstand the abrasion of rough bony projects (which would tear apart the more delicate muscle tissues) Levels of Organization of Skeletal Muscle Table 9.1 Structure and organization levels of skeletal muscle. Microscopic Anatomy of Skeletal Muscle Fiber. Microscopic Anatomy of Skeletal Muscle Fiber Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei Sarcolemma: muscle fiber plasma membrane Sarcoplasm: muscle fiber cytoplasm Contains many glycosomes for glycogen storage, as well as myoglobin for O2 storage Modified organelles – Myofibrils – Sarcoplasmic reticulum – T tubules Microscopic Anatomy of Skeletal Muscle Fiber - Myofibrils Myofibrils are densely packed, rodlike elements – Single muscle fiber can contain 1000s – Accounts for ~80% of muscle cell volume Myofibril features – Striations – Sarcomeres – Myofilaments – Molecular composition of myofilaments Microscopic Anatomy of Skeletal Muscle Fiber - Myofibrils Striations: stripes formed from repeating series of dark and light bands along length of each myofibril – A bands: dark regions H zone: lighter region in middle of dark A band – M line: line of protein that bisects H zone vertically – I bands: lighter regions Z disc (line): coin-shaped sheet of proteins on midline of light I band Microscopic Anatomy of Skeletal Muscle Fiber - Myofibrils Sarcomere – Smallest contractile unit (functional unit) of muscle fiber – Contains A band with half of an I band at each end Consists of area between Z discs – Individual sarcomeres align end to end along myofibril, like boxcars of train Microscopic Anatomy of Skeletal Muscle Fiber - Myofibrils Myofilaments – Orderly arrangement of actin and myosin myofilaments within sarcomere – Actin myofilaments: thin filaments Extend across I band and partway in A band Anchored to Z discs – Myosin myofilaments: thick filaments Extend length of A band Connected at M line Levels of Organization of Skeletal Muscle Table 9.1 Structure and organization levels of skeletal muscle. Myosin – Molecular Composition Figure 9.3 Composition of thick and thin filaments. Actin – Molecular Composition Figure 9.3 Composition of thick and thin filaments. Myosin and Actin – Molecular Composition Myosin – heads contain an actin-binding site Actin – contains active sites for myosin head attachment – regulatory proteins control availability of active sites tropomyosin and troponin Cross-bridge – linking of mysoin head to actin Sliding Filament Model of Contraction In the relaxed state, thin and thick filaments overlap only slightly at ends of A band Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more – Neither thick nor thin filaments change length, just overlap more Sliding Filament Model of Contraction When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcome in a ratcheting action – causes shortening of muscle fiber Sliding Filament Model of Contraction Z discs are pulled toward M line I bands shorten Z discs become closer H zones disappear A bands move closer to each other Sliding Filament Model of Contraction Z discs are pulled toward M line I bands shorten Z discs become closer H zones disappear A bands move closer to each other Microscopic Anatomy of Skeletal Muscle Fiber - Sarcoplasmic Reticulum Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril – Most run longitudinally – Terminal cisterns form perpendicular cross channels at the A–I band junction – SR functions in regulation of intracellular Ca2+ levels – Stores and releases Ca2+ Microscopic Anatomy of Skeletal Muscle Fiber – T Tubules T tubules – Tube formed by perpendicular protrusion of sarcolemma deep into cell interior (T= transverse!) Increase muscle fiber’s surface area greatly Lumen continuous with extracellular space Allow electrical nerve transmissions to reach deep into interior of each muscle fiber – Tubules penetrate cell’s interior at each A–I band junction between terminal cisterns Sarcoplasmic Reticulum and T Tubules Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Sarcoplasmic Reticulum and T Tubules Electrical impulses pass through the sarcolemma  T tubule proteins change shape  SR proteins also change shape  Triggers release of calcium into muscle cell cytoplasm Neuromuscular Junction Decision to move is activated by brain, electrical signal is transmitted down spinal cord to motor neurons which then chemically activate muscle fibers Neuromuscular junction (NMJ) is a small space (synapse) between a motor neuron and a muscle fiber (motor end plate) – Each muscle fiber has one neuromuscular junction with one motor neuron Axons Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle – Each axon divides into many branches as it enters muscle Axons Axon terminal is the end of the axon – storage of membrane-bound synaptic vesicles containing the chemical neurotransmitter acetylcholine (ACh) – Action potential is the electrical signal that causes ACh release into the neuromuscular junction Action potential causes a rapid shift away the highly negative resting membrane potential voltage-dependent proteins open or close – Voltage-gated Ca channels open! Neuromuscular Junction Figure 9.7 Neuromuscular junction Events at the NMJ 1.Action potential arrives at axon terminal 2.Voltage-gated calcium channels open, calcium enters motor neuron 3.Calcium entry causes release of ACh neurotransmitter into NMJ synapse 4.ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma 5.ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate potential 6.Acetylcholinesterase degrades ACh – stops the chemical signal from reaching muscle Excitable cells Neurons and muscle cells are both types of excitable cells capable of action potentials – Excitable cells are capable of changing resting membrane potential voltages Polarized: a voltage exists across membrane At rest, inside of cell is negative compared to outside Action potential causes massive changes in electrical charge across plasma membrane – action potential causes cell to get less negative, i.e. become depolarized – action potential ceases when cell repolarizes Motor End Plate Motor end plate is the muscle portion of the NMJ – contain chemically-gated ion channels (receptors) which open in the presence of ACh (chemical) open ACh receptors allow for a net influx positive charge (Na+ influx >>>> K+ efflux) into the muscle fiber causing the motor end plate to depolarize – this local depolarziation is called the end plate potential Motor End Plate If end plate potential causes enough of a change in the membrane potential, then a new action potential will be generated in the muscle fiber – threshold is the critical, minimal level of depolarizing change a membrane must undergo to generate new action potential – if threshold is attained, then voltage-gated Na channels in the sarcolemma will open which triggers a massive influx of Na+ and a huge depolarization in the muscle fiber this large depolarization is the generation of the action potential which will spread along sarcolemma Motor End Plate Figure 9.8 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber Motor End Plate Repolarization of the muscle fiber restores resting conditions – Na+ voltage-gated channels close, and voltage- gated K+ channels open – K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage – Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete – Ionic conditions of resting state are restored by Na+-K+ pump Excitation-Contraction (E-C) Coupling Excitation-contraction (E-C) coupling: sequence of events that transmit action potential along sarcolemma (excitation) leads to (coupling) sliding of myofilaments (contraction) AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR – Ca2+ release leads to contraction AP is brief and ends before contraction is seen Sarcoplasmic Reticulum and T Tubules Electrical impulses pass through the sarcolemma  T tubule proteins change shape  SR proteins also change shape  Triggers release of calcium into muscle cell cytoplasm Why Ca2+ necessary for E-C Coupling? Actin – Molecular Composition Figure 9.3 Composition of thick and thin filaments. Actin – Molecular Composition Ca2+ binds to troponin Actin – Molecular Composition troponin changes shape altering the position of rope-like tropomyosin tropomyosin moves away exposing the myosin-binding sites on actin Actin – Molecular Composition Myosin heads are then allowed to bind to actin, forming cross bridge Cross Bridge Cycling Four steps of the cross bridge cycle 1. Cross bridge formation: high-energy myosin head attaches to actin thin filament active site 2. Working (power) stroke: myosin head pivots and pulls thin filament toward M line 3. Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach 4. Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state This energy will be used for power stroke in next cross bridge cycle Cross Bridge Cycle Focus Figure 9.3 Cross Bridge Cycle Cross Bridge Cycle As long as Ca2+ is available, myosin-binding sites on actin will be exposed and contraction will continue – Ca2+ remains available as long as ACh is present in the NMJ Ca2+  Focus Figure 9.3 Composition of thin filaments. Cross Bridge Cycle When nervous stimulation ceases, Ca2+ is pumped back into SR, and contraction ends – moving Ca2+ from low concentration (cytosol) to high concentration (SR) is primary active transport: more ATP needed! Ca2+  Focus Figure 9.3 Composition of thin filaments. Check Your Understanding 9-9 From the time an action potential reaches the axon terminal of a motor neuron until cross bridge cycling begins several sets of ion channels are activated. List these channels in the order that they are activated and what causes each to open. The Big Picture The Big Picture - Four steps must occur for skeletal muscle fiber contraction: 1. Events at neuromuscular junction 2. Muscle fiber excitation 3. Excitation-contraction coupling 4. Cross bridge cycling But how does this translate into entire muscles contracting? Whole Muscle Contraction Same principles apply to contraction of both single fibers and whole muscles Motor unit: the motor neuron and the muscle fibers it supplies – each motor neuron supplies four to several hundred fibers small the fiber number, the greater the fine control Whole Muscle Contraction muscle fibers from a motor unit are spread throughout the whole muscle, so stimuation of a single motor unit causes on weak contraction of the entire muscle Motor Unit Figure 9.10 A motor unit consists of one motor neuron and all the muscles fibers it innervates Whole Muscle Contraction Muscle twitch: simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron – Muscle fiber contracts quickly, then relaxes Three phases of muscle twitch 1. latent period (E-C coupling) 2. contraction (cross bridge formation; tension increases) 3. relaxatin (Ca2+ reuptake into SR; tension zeros) Whole Muscle Contraction Muscle twitch: simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron – differences in strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles Example: eye muscles’ contraction is rapid and brief calf muscles contract more slowly and for longer Graded Muscle Reponses Normal muscle contraction is relatively smooth, and strength varies with needs – A muscle twitch is seen only in lab setting or with neuromuscular problems, but not in normal muscle Graded muscle responses vary strength of contraction for different demands – Required for proper control of skeletal movement Responses are graded by: – Changing frequency of stimulation – Changing strength of stimulation Muscle Twitch - Individual Figure 9.13a Individual muscle twitch Muscle Twitch – Multiple twitches Figure 9.13a A second stimulus delivered after relaxation is complete does not produce summation Graded Muscle Reponse A second stimulus delievered after relaxation is complete does not produce any additive effects on muscle contraction Temporal (wave) summation results if 2 stimuli are received by a muscle in rapid sucession Muscle fibers do not have time to completely relax between stimuli, so twitches increase in force with each stimulus Additional Ca2+ that is released with second stimulus stimulates more shortening Muscle Twitch – Temporal Summation Figure 9.13b Additional stimuli delievered before relaxatin is complete produce temporal summation Graded Muscle Reponse If stimuli frequency increases, muscle tension reaches near maximum – Produces smooth, continuous contractions that add up (summation) – Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus Muscle Twitch – Unfused Tetanus Figure 9.13c Higher stimuation frequency results in unfused tetanus Graded Muscle Reponse If stimuli frequency further increase, muscle tension reaches maximum – Referred to as fused (complete) tetanus because contractions “fuse” into one smooth sustained contraction plateau – Prolonged muscle contractions lead to muscle fatigue Muscle Twitch – Fused Tetanus Figure 9.13d At even higher stimuation frequencies, there is no relaxation at all between stimuli. This is fused tetanus Graded Muscle Reponse Recruitment (or multiple motor unit summation): stimulus is sent to more muscle fibers, leading to more precise control Motor units in muscle usually contract asynchronously to help prevent fatigue Figure not in textbook Figure 9.15 Size principle of recruitment Figure 9.14 Recruitment Relationship between stimulus intensity (top), motor unit recruitment (middle), and muscle tension (bottom) Types of Contractions Isotonic contractions: muscle changes in length (myosin head pivots and actin slides towards H zone) moving load – Isotonic contractions can be either concentric or eccentric: Concentric contractions: muscle shortens and does work – Example: biceps contract to pick up a book Eccentric contractions: muscle lengthens and generates force – Example: laying a book down causes biceps to lengthen while generating a force Types of Contractions Isometric contractions – Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens Electrochemical and mechanical events are same in isotonic or isometric contractions, but results are different – In isometric contractions, cross bridges generate force, but actin filaments do not move Myosin heads “spin their wheels” on same actin- binding site Figure 9.16 Isotonic (concentric) and isometric contractions Summary of ATP Usage in Contraction ATP supplies the energy needed for the muscle fiber to: – move and detach cross bridges – pump Ca2+ back into SR – pump Na+ out of and pump K+ back into the cell after E-C coupling Available store of ATP depleted in 4-6 seconds ATP is the only source of energy for contractile activities and must be regenerated quickly! ATP Energy for Contraction ATP is regenerated quickly by three mechanisms: 1. Direct phosphorylation of ADP by creatine phosphate (CP) 2. Anaerobic pathway: glycolysis and lactic acid formation 3. Aerobic pathway ATP Energy for Contraction Direct phosphorylation of ADP by creatine phosphate (CP) – Creatine phosphate is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP Creatine kinase is enzyme that carries out transfer of phosphate Muscle fibers have enough ATP and CP reserves to power cell for about 15 seconds Creatine phosphate + ADP → creatine + ATP ATP Energy for Contraction Anaerobic pathway: glycolysis and lactic acid formation – ATP can also be generated by breaking down and using energy stored in glucose (30-40 seconds) Glycolysis: first step in glucose breakdown – Does not require oxygen – Glucose is broken into 2 pyruvic acid molecules – 2 ATPs are generated for each glucose broken down Low oxygen levels prevent pyruvic acid from entering aerobic respiration phase ATP Energy for Contraction Anaerobic pathway: glycolysis and lactic acid formation – Normally, pyruvic acid enters mitochondria to start aerobic respiration phase; however, at high intensity activity, oxygen is not available Bulging muscles compress blood vessels, impairing oxygen delivery – In the absence of oxygen, referred to as anaerobic glycolysis, pyruvic acid is converted to lactic acid ATP Energy for Contraction Aerobic Respiration – Produces 95% of ATP during rest and light-to- moderate exercise Slower than anaerobic pathway (hours) – Consists of series of chemical reactions that occur in mitochondria and require oxygen Breaks glucose into CO2, H2O, and large amount ATP (32 can be produced per 1 molecule of glucose) – Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, and free fatty acids Fatty acids are main fuel after 30 minutes of exercise Muscle Fatigue Fatigue is the physiological inability to contract despite continued stimulation Possible causes include: – Ionic imbalances can cause fatigue Levels of K+, Na+ and Ca2+ can change disrupting membrane potential of muscle cell – Increased inorganic phosphage (Pi) from CP and ATP breakdown may interfere with calcium release from SR or hamper power Muscle Fatigue – Decreased ATP and increased magnesium As ATP levels drop, magnesium levels increase and this can interfere with voltage sensitive T tubule proteins – Decreased glycogen Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles – muscle fatigue serves to prevent complete depletion of ATP which would result in muscle cell death (rigor mortis) Excess Postexercise Oxygen Consumption For a muscle to return to its pre-exercise state: – Oxygen reserves are replenished – Lactic acid is reconverted to pyruvic acid – Glycogen stores are replaced – ATP and creatine phosphate reserves are resynthesized All replenishing steps require extra oxygen, so this is referred to as excess postexercise oxygen consumption (EPOC) – Formerly referred to as “oxygen debt” Check Your Understanding 9-16 Chris joined the cross-country team partway through the season and has just completed another gruelling session of trying to keep up with their teammates. They are breathing heavily, their legs are weak, and they are sweating profusely. Why is Christ breathing heavily? Which ATP-generating pathway have their working muscles been using? What metabolic products might account for their muscle weakness? Factors of Muscle Contractions Force of Muscle Contractions Force of contraction depends on number of cross bridges attached, which is affected by four factors: 1. Number of muscle fibers stimulated (recruitment): the more motor units recruited, the greater the force. 2. Relative size of fibers: the bulkier the muscle, the more tension it can develop Muscle cells can increase in size (hypertrophy) with regular exercise Factors of Muscle Contractions Force of Muscle Contractions Force of contraction depends on number of cross bridges attached, which is affected by four factors: 3. Frequency of stimulation: the higher the frequency, the greater the force Stimuli are added together Factors of Muscle Contractions Force of Muscle Contractions Force of contraction depends on number of cross bridges attached, which is affected by four factors: 4. Degree of muscle stretch: muscle fibers with sarcomeres that are 80–120% their normal resting length generate more force If sarcomere is less than 80% resting length, filaments overlap too much, and force decreases If sarcomere is greater than 120% of resting length, filaments do not overlap enough so force decreases Degree of Muscle Stretch Figure 9.20 Length-tension relationships of sarcomeres in skeletal muscles Figure 9.19 Factors that increase the force of skeletal muscle contraction. Factors of Muscle Contractions Velocity and Duration of Contraction How fast a muscle contracts and how long it can stay contracted is influenced by: – Muscle fiber type – Load – Recruitment (activating more motor units) Factors of Muscle Contractions Velocity and Duration of Contraction Muscle fiber type – Classified according to two characteristics 1. Speed of contraction – slow or fast fibers according to: – Speed at which myosin ATPases split ATP – Pattern of electrical activity of motor neurons 2. Metabolic pathways used for ATP synthesis – Oxidative fibers: use aerobic pathways – Glycolytic fibers: use anaerobic glycolysis Factors of Muscle Contractions Velocity and Duration of Contraction Muscle fiber type – Based on these two criteria, skeletal muscle fibers can be classified into three types: Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers – Most muscles contain mixture of fiber types, resulting in a range of contractile speed and fatigue resistance All fibers in one motor unit are the same type Genetics dictate individual’s percentage of each Factors of Muscle Contractions Velocity and Duration of Contraction Muscle fiber type – Different muscle types are better suited for different jobs Slow oxidative fibers: low-intensity, endurance activities – Example: maintaining posture Fast oxidative fibers: medium-intensity activities – Example: sprinting or walking Fast glycolytic fibers: short-term intense or powerful movements – Example: hitting a baseball Glycolytic fibers may convert to oxidative fibers (vice versa) with prolonged useage Factors of Muscle Contractions Muscles must be active to remain healthy Disuse atrophy (degeneration and loss of mass) – Due to immobilization or loss of neural stimulation – Can begin almost immediately. Muscle strength can decline 5% per day Paralyzed muscles may atrophy to one-fourth initial size Fibrous connective tissue replaces lost muscle tissue – Rehabilitation is impossible at this point Check Your Understanding 9-18 Diego called several friends to help him move. Would he prefer to have those with more slow oxidative muscle fibers or those with more fast glycolytic fibers as his helpers? Why? 9-19 There are no slow glycolytic fibers. Explain why it would not make sense to have these kinds of fibers? Smooth Muscle Muscle found in the walls of hollow organs (except heart) – alternating contractions/relaxations of smooth muscle layers mix and squeeze substances through the lumen of hollow organs Smooth vs. Skeletal Muscle In addition to differences in cell shape and appearance, smooth muscle: – Lacks connective tissue sheaths Contains endomysium only – Contain varicosities (bulbous swellings) of nerve fibers instead of neuromuscular junctions Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a diffuse junction Innervated by the autonomic nervous system Smooth vs. Skeletal Muscle – Smooth muscle has less elaborate SR, and no T tubules SR is less developed than in skeletal muscle – SR does store intracellular Ca2+, but most calcium used for contraction has extracellular origins – Sarcolemma contains pouchlike infoldings that contain numerous Ca2+ channels that open to allow rapid influx of extracellular Ca2+ Smooth vs. Skeletal Muscle – Smooth muscle fibers are usually electrically connected via gap junctions whereas skeletal muscle fibers are electrically isolated Gap junctions are specialized cell connections that allow depolarization to spread from cell to cell – There are no striations and no sarcomeres, but they do contain overlapping thick and thin filaments Smooth vs. Skeletal Muscle – Thick filaments are fewer and have myosin heads along entire length Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2) Thick filaments have heads along entire length, making smooth muscle as powerful as skeletal muscle – No troponin complex Does contain tropomyosin, but not troponin Protein calmodulin binds Ca2+ Smooth vs. Skeletal Muscle – Thick and thin filaments arranged diagonally Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner – Intermediate filament–dense body network Contain lattice-like arrangement of non contractile intermediate filaments that resist tension Dense bodies: proteins that anchor filaments to sarcolemma at regular intervals – Correspond to Z discs of skeletal muscle During contraction, areas of sarcolemma between dense bodies bulge outward – Make muscle cell look puffy Smooth Muscle Relaxation and Contraction Figure 9.26 Intermediate filaments and dense bodies of smooth muscle fibers harness the pull generated by myosin cross bridges. Smooth Muscle Relaxation and Contraction Mechanism of contraction – Slow, synchronized contractions – Cells electrically coupled by gap junctions Action potentials transmitted from fiber to fiber – Some cells are self-excitatory (depolarize without external stimuli) Act as pacemakers for sheets of muscle Rate and intensity of contraction may be modified by neural and chemical stimuli Smooth Muscle Relaxation and Contraction Mechanism of contraction – Contraction in smooth muscle is similar to skeletal muscle contraction in following ways: Actin and myosin interact by sliding filament mechanism Final trigger is increased intracellular Ca2+ level ATP energizes sliding process Contraction stops when Ca2+ is no longer available Smooth Muscle Relaxation and Contraction Mechanism of contraction – Contraction in smooth muscle is different from skeletal muscle contraction in following ways: Some Ca2+ still obtained from SR, but mostly comes from extracellular space Ca2+ binds to calmodulin, not troponin Smooth Muscle Relaxation and Contraction Mechanism of contraction – Contraction in smooth muscle is different from skeletal muscle contraction in following ways: Activated calmodulin then activates myosin kinase (myosin light chain kinase) Activated myosin kinase phosphorylates myosin head, activating it – Leads to crossbridge formation with actin Smooth Muscle Relaxation and Contraction Mechanism of contraction – Stopping smooth muscle contraction requires more steps than skeletal muscle Relaxation requires: – Ca2+ detachment from calmodulin – Active transport of Ca2+ into SR and extracellularly – Dephosphorylation of myosin to inactive myosin Smooth Muscle Relaxation and Contraction – Response to stretch Stress-relaxation response: responds to stretch only briefly, then adapts to new length – Retains ability to contract on demand – Enables organs such as stomach and bladder to temporarily store contents Types of Smooth Muscle All smooth muscle is categorized as either: – Unitary: all hollow organs except heart aka visceral muscle, commonly found in body and possess all common characteristic of smooth muscle – Multiunit: located in large airways in lungs, larger arteries, pili muscles of skin, iris of eye similar to skeletal muscle in some features: – independent muscle fibers, motor unit recruitment, graded contractions in response to neural stimuli, very few gap junctions with rare spontaneous depolarization like unitary smooth muscle: – controlled by autonomic nervous system and hormones Comparative summary of the 3 types of muscle Table 9.2 Structural and functional characteristics of the 3 types of muscle Check Your Understanding 9-22 Calcium is the trigger for contraction of all muscle types. How does its binding differ in skeletal muscle and smooth muscle fibers?