2/19/2018. Learn and Understand:

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1 Muscular System with Special Emphasis on Skeletal Muscle Anatomy and Physiology Learn and Understand: The definition of cell changes again The contractile unit of muscle is the sarcomere. ATP and Ca 2+ must be available for muscle to contract and relax. Skeletal muscle is stimulated to contract by neurons of the CNS. Nervous system controls force applied. Ion movement and changes in electrical potential across the sarcolemma is the ultimate signal for contraction. Muscle cells vary in their ability to use sources of energy and their speed of contraction. Muscle Functions Four important functions Movement of bones or fluids (e.g., blood) Maintaining posture and body position Stabilizing joints Heat generation (especially skeletal muscle) Additional functions Protects organs, forms valves, controls pupil size, causes "goosebumps" Special Characteristics of Muscle Tissue Excitability (responsiveness): ability to receive and respond to stimuli Contractility: ability to shorten forcibly when stimulated Elasticity: ability to stretch beyond resting length and recoil Muscle Tissue Nearly half of body's mass Female skeletal muscle makes up 36% of body mass Male skeletal muscle makes up 42% of body mass, primarily due to testosterone Transforms chemical energy (ATP) to directed mechanical energy exerts force Three types Skeletal Cardiac Smooth Myo, mys, and sarco - prefixes for muscle 1

2 Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Bone Tendon Epimysium Epimysium Perimysium Endomysium Muscle fiber in middle of a fascicle Blood vessel Perimysium wrapping a fascicle Endomysium (between individual muscle fibers) Muscle fiber Fascicle Perimysium Skeletal Muscles Each muscle served by one artery, one nerve, and one or more veins Connective tissue sheaths of skeletal muscle External to internal Epimysium: dense irregular connective tissue Perimysium: fibrous connective tissue surrounding fascicles Endomysium: fine areolar connective tissue Skeletal Muscle Fibers: Anatomy Long, cylindrical cell up to 30 cm long Multiple nuclei Sarcolemma Sarcoplasm Glycosomes for glycogen storage, myoglobin for O 2 storage amount of each dependent on muscle type Modified structures: myofibrils, sarcoplasmic reticulum, and T tubules 2

3 Figure 9.2b Microscopic anatomy of a skeletal muscle fiber. Sarcolemma Mitochondrion Dark A band Light I band Nucleus Myofibril Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band A band I band M line Sarcomere Figure 9.2d Microscopic anatomy of the sarcomere Z disc Sarcomere M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament. Thin filament Each thick filament consists of many myosin A thin filament consists of two strands of actin molecules whose heads protrude at oppositeends subunits twisted into a helix plus two types of of the filament. regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites Heads ATPbinding site Flexible hinge region Tail Actin subunits Active sites for myosin attachment Myosin molecule Actin subunits 3

4 Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils and sarcomeres of skeletal muscle. Part of a skeletal muscle fiber (cell) I band A band I band Z disc H zone Z disc M line Myofibril Sarcolemma Sarcolemma Triad: T tubule Terminal cisterns of the SR (2) Tubules of the SR Myofibrils Mitochondria Triad Relationships T tubules conduct impulses deep into muscle fiber; every sarcomere Integral proteins protrude into intermembrane space from T tubule and SR cistern membranes and connect with each other T tubule integral proteins act as voltage sensors and change shape in response to voltage changes SR integral proteins are channels that release Ca 2+ from SR cisterns when voltage sensors change shape Sliding Filament Model of Muscle Contraction In relaxed state, thin and thick filaments overlap only at ends of A band Actin myofilaments are pulled (slide) over myosin to shorten sarcomeres Actin and myosin do not change length Occurs when myosin heads bind to actin Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening 4

5 Tension (percent of maximum) 2/19/2018 Figure 9.6 Sliding filament model of contraction. Slide 2 1 Fully relaxed sarcomere of a muscle fiber Z H Z I A I Figure 9.6 Sliding filament model of contraction. Slide 3 2 Fully contracted sarcomere of a muscle fiber Z I A Z I Figure 9.22 Length-tension relationships of sarcomeres in skeletal muscles. Sarcomeres greatly shortened Sarcomeres at resting length Sarcomeres excessively stretched 75% 100% 170% Optimal sarcomere operating length (80% 120% of resting length) Percent of resting sarcomere length 5

6 Stimulus for Contraction: Upsetting Ion Concentrations at the Sarcolemma Resting membrane potential (RMP) maintained by active transport Just outside the sarcolemma: high Na + concentration, some Cl -, some K + Just inside the sarcolemma: high K + and negativelycharged proteins Action potential (AP) stimulates contraction changes to membrane permeability resulting in ion movement Voltage change is the stimulus Resting potential re-established almost immediately Polarized Membrane: Resting Membrane Potential -90 mv potential across membrane Explanation of Resting Membrane Potential at Sarcolemma Unequally-distributed ions Plasma membrane Membrane is POLARIZED 6

7 Ion Channel Role in Maintaining/Upsetting Potential Types Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site Example ligand: neurotransmitters Voltage-gated Open and close in response to small voltage changes across plasma membrane Each is specific for one ion Resting Potential Activation Gate What s missing: Open K + channels Na + /K + pump Inactivation gate Action Potentials -50 to -55 mv RMP Phases Graded (end plate) potential at NMJ Threshold Depolarization Repolarization All-or-none principle Propagation 7

8 Membrane potential (mv) 2/19/2018 Action Potential Resting Depolarization Repolarization Figure 9.10 Action potential tracing indicates changes in Na + and K + ion channels Depolarization due to Na + entry Na + channels open Na + channels close, K + channels open Repolarization due to K + exit K + channels closed Time (ms) The Nerve Stimulus and Events at the Neuromuscular Junction Skeletal muscles stimulated by somatic motor neurons Axons of motor neurons travel from central nervous system via nerves to skeletal muscle Each axon forms several branches as it enters muscle Each axon ending forms neuromuscular junction with single muscle fiber Usually only one per muscle fiber Situated midway along length of muscle fiber 8

9 Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released. Action potential (AP) Myelinated axon of motor neuron Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Axon terminal of motor neuron Synaptic vesicle containing ACh Synaptic cleft Fusing synaptic vesicles ACh Junctional folds of sarcolemma Sarcoplasm of muscle fiber Postsynaptic membrane ion channel opens; ions pass. ACh Degraded ACh Ion channel closes; ions cannot pass. Acetylcholinesterase Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. + Closed K + Open Na channel channel Na ACh-containing synaptic vesicle Ca 2+ Ca 2+ Axon terminal of neuromuscular junction K + Action potential Synaptic cleft Wave of depolarization 1. Nerve impulse arrives at axon terminal acetylcholine released by synaptic terminal into synaptic cleft 2. ACh diffuses across cleft and binds with nicotinic (excitatory) receptors on sarcolemma opening sodium ion gates 3. Sodium influx depolarizes sarcolemma to threshold 4. Propagation of AP away from NMJ along fiber sarcolemma Closed Na + channel Na + K + Open K + channel Action Potential Propagation Propagation in one direction only due to refractory period 9

10 Excitation-Contraction (E-C) Coupling Steps in E-C Coupling: Sarcolemma Voltage-sensitive T tubule tubule protein 1 Ca 2+ release channel 2 Synaptic cleft Axon terminal of motor neuron at NMJ Terminal cistern of SR Muscle fiber Action potential is generated ACh T tubule Terminal cistern of SR Triad Sarcolemma Actin Troponin Tropomyosin blocking active sites Myosin 3 Active sites exposed and ready for myosin binding One sarcomere 4 One myofibril Myosin cross bridge Events that transmit AP along sarcolemma lead to sliding of myofilaments AP brief; ends before contraction Causes rise in intracellular Ca 2+ which initiates contraction Role of Calcium (Ca 2+ ) in Contraction At low intracellular Ca 2+ concentration Tropomyosin blocks active sites Myosin heads cannot attach to actin Muscle fiber relaxed At higher intracellular Ca 2+ concentrations Ca 2+ binds to troponin Troponin changes shape and moves tropomyosin away from myosin-binding sites Myosin heads bind to actin When nervous stimulation ceases, Ca 2+ pumped back into SR and contraction ends Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Actin Ca 2+ Thin filament PLAY A&P Flix : The Cross Bridge Cycle Myosin cross bridge Myosin 1 Thick filament ATP hydrolysis 4 2 In the absence of ATP, myosin heads will not detach, causing RIGOR MORTIS. *This cycle will continue as long as ATP is available and Ca 2+ is bound to troponin. 3 10

11 Relaxation Ca 2+ moves away from troponin-tropomyosin complex causing relaxation Ca 2+ diffuses out of the myofibril Transported back into sarcoplasmic reticulum by active transport. Troponin-tropomyosin complex re-establishes its position and blocks binding sites. Myosin cannot form cross bridges, filaments cannot slide Muscle recoil Sarcomere elements, connective tissue, antagonistic muscle action and opposing forces, gravity Figure 9.7 The phases leading to muscle fiber contraction. Action potential (AP) arrives at axon terminal at neuromuscular junction ACh released; binds to receptors on sarcolemma Phase 1 Motor neuron stimulates muscle fiber (see Figure 9.8). Ion permeability of sarcolemma changes Local change in membrane voltage (depolarization) occurs Local depolarization (end plate potential) ignites AP in sarcolemma AP travels across the entire sarcolemma AP travels along T tubules Phase 2: Excitation-contraction coupling occurs (see Figures 9.9 and 9.11). SR releases Ca 2+ ; Ca 2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins Principles of Muscle Mechanics Same principles apply to contraction of single fiber and whole muscle Contraction produces muscle tension, force exerted on load or object to be moved 11

12 Percentage of maximum tension 2/19/2018 Latent Period of period contraction Figure 9.14a The muscle fiber twitch. Period of relaxation Latent period Time when E-C coupling events occur Time between AP initiation and beginning of contraction Time (ms) Single stimulus Myogram showing the three phases of an isometric twitch Motor Unit: The Nerve-Muscle Functional Unit Each muscle served by at least one motor nerve Motor nerve contains axons of up to hundreds of motor neurons Axons branch into terminals, each of which NMJ with single muscle fiber Motor unit = motor neuron and all (four to several hundred) muscle fibers it supplies Smaller number = fine control Figure 9.13 A motor unit consists of one motor neuron and all the muscle fibers it innervates. Spinal cord Motor Motor unit 1 unit 2 Axon terminals at Branching axon neuromuscular junctions to motor unit Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. Branching axon terminals form neuromuscular junctions, one per muscle fiber (photomicrograph 330x). 12

13 Tension 2/19/2018 Motor Unit Muscle fibers from motor unit spread throughout muscle so single motor unit causes weak contraction of entire muscle Motor units in muscle usually contract asynchronously; helps prevent fatigue Graded Muscle Responses Graded muscle responses Varying strength of contraction for different demands Required for proper control of skeletal movement Responses graded by 1. Changing frequency of stimulation 2. Changing strength of stimulation Figure 9.15a A muscle's response to changes in stimulation frequency. Single stimulus single twitch Contraction Maximal tension of a single twitch Relaxation 0 Stimulus 100 Time (ms)

14 Tension Tension 2/19/2018 Figure 9.15b A muscle's response to changes in stimulation frequency. Low stimulation frequency unfused (incomplete) tetanus Partial relaxation 0 Stimuli 100 Time (ms) If another stimulus is applied before the muscle relaxes completely, then more tension results. This is wave (or temporal) summation and results in unfused (or incomplete) tetanus Figure 9.15c A muscle's response to changes in stimulation frequency. High stimulation frequency fused (complete) tetanus 0 Stimuli 100 Time (ms) At higher stimulus frequencies, there is no relaxation at all between stimuli. This is fused (complete) tetanus. Response to Change in Stimulus Strength Recruitment (multiple motor unit summation) controls force of contraction Subthreshold stimuli no observable contractions Threshold stimulus: stimulus strength causing first observable muscle contraction Maximal stimulus strongest stimulus that increases contractile force 14

15 Tension Stimulus voltage 2/19/2018 Figure 9.16 Relationship between stimulus intensity (graph at top) and muscle tension (tracing below). Stimulus strength Maximal stimulus Threshold stimulus Stimuli to nerve Proportion of motor units excited Strength of muscle contraction Maximal contraction Time (ms) Frog Gastrocnemius Muscle Tone Constant, slightly contracted state of all muscles Due to spinal reflexes Groups of motor units alternately activated in response to input from stretch receptors in muscles Keeps muscles firm, healthy, and ready to respond Less active when lying down or asleep 15

16 Muscle Metabolism: Energy for Contraction ATP only source used directly to move and detach cross bridges, calcium pumps in SR, return of Na + & K + after excitation-contraction coupling Available stores of ATP depleted in 4 6 seconds ATP regenerated by: Direct phosphorylation of ADP by creatine phosphate (CP) Anaerobic pathway (glycolysis lactic acid) Aerobic respiration Direct phosphorylation Coupled reaction of creatine Phosphate (CP) and ADP Energy source: CP Anaerobic pathway Glycolysis and lactic acid formation Energy source: glucose Creatine Creatine kinase Glucose (from glycogen breakdown or delivered from blood) Glycolysis in cytosol 2 net gain Released to blood Pyruvic acid Lactic acid Oxygen use: None Products: 1 ATP per CP, creatine Duration of energy provided: 15 seconds Oxygen use: None Products: 2 ATP per glucose, lactic acid Duration of energy provided: seconds, or slightly more Anaerobic Pathway Glycolysis does not require oxygen At 70% of maximum contractile activity Bulging muscles compress blood vessels; oxygen delivery impaired Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2½ times faster Anaerobic threshold Point at which muscle metabolism converts to anaerobic 16

17 Aerobic pathway Aerobic cellular respiration Energy source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism Glucose (from glycogen breakdown or delivered from blood) Pyruvic acid Fatty acids Amino Aerobic respiration acids in mitochondria 32 net gain per glucose Oxygen use: Required Products: 32 ATP per glucose, CO 2, H 2O Duration of energy provided: Hours Produces 95% of ATP during rest and light to moderate exercise; slow Series of chemical reactions that require oxygen Fuels: 1. stored glycogen 2. then bloodborne glucose 3. pyruvic acid from glycolysis 4. and free fatty acids Aerobic endurance Length of time muscle contracts using aerobic pathways Figure 9.20 Comparison of energy sources used during short-duration exercise and prolonged-duration exercise. Short-duration exercise Prolonged-duration exercise 6 seconds 10 seconds seconds End of exercise Hours ATP stored in muscles is used first. ATP is formed from Glycogen stored in muscles is broken down to glucose, creatine phosphate which is oxidized to generate ATP (anaerobic pathway). and ADP (direct phosphorylation). ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway. Muscle Fatigue Physiological inability to contract despite continued stimulation Occurs when Ionic imbalances (K +, Ca 2+, P i ) interfere with E-C coupling Prolonged exercise may damage SR and interferes with Ca 2+ regulation and release Total lack of ATP occurs rarely, during states of continuous contraction, and causes contractures (continuous contractions) Includes rigor mortis 17

18 Excess Postexercise Oxygen Consumption To return muscle to resting state Oxygen reserves replenished Lactic acid converted to pyruvic acid Glycogen stores replaced ATP and creatine phosphate reserves replenished All require extra oxygen; occurs post exercise Muscle Fiber Type Most muscles contain mixture of fiber types Classified according to two characteristics Speed of contraction: slow or fast fibers according to Speed at which myosin ATPases split ATP Pattern of electrical activity of motor neurons Metabolic pathways for ATP synthesis Oxidative fibers use aerobic pathways Slow oxidative fibers; Fast oxidative fibers; Glycolytic fibers use anaerobic glycolysis Fast glycolytic fibers All fibers in one motor unit same type Genetics dictate individual's percentage of each Training can aid in development of what you currently have 18