Chapter 1.pptx

Chapter 1 STRUCTURE AND FUNCTION OF EXERCISING MUSCLE  CHAPTER 1 Overview Distinguish the unique anatomy of skeletal muscle in comparison to smooth and cardiac muscle Demonstrate concentric, eccentric, and isometric contraction Illustrate the sliding filament theory and winding filament...

Chapter 1 STRUCTURE AND FUNCTION OF EXERCISING MUSCLE  CHAPTER 1 Overview Distinguish the unique anatomy of skeletal muscle in comparison to smooth and cardiac muscle Demonstrate concentric, eccentric, and isometric contraction Illustrate the sliding filament theory and winding filament theories Differentiate the different skeletal muscle types and their effects on performance Three Types of Muscle Tissue Smooth: Skeletal: Cardiac: involunta voluntary involunta ry, hollow , skeleton ry, heart organs  Anatomy of Skeletal Muscle Entire muscle ◦ Surrounded by epimysium ◦ Made of many bundles (fasciculi) Fasciculi ◦ Surrounded by perimysium Figure 1.2 ◦ Made of individual muscle cells (muscle fibers) Muscle fiber ◦ Surrounded by endomysium ◦ Made of myofibrils divided into sarcomeres Figure 1.3 Structure of Muscle Fibers Plasmalemma (cell membrane) ◦ Fuses with tendon. ◦ Conducts action potential. ◦ Maintains pH, transports nutrients. Satellite cells ◦ Involved in muscle growth and development. ◦ Aids response to injury, immobilization, training. Structure of Muscle Fibers Sarcoplasm ◦ Serves as cytoplasm of muscle cell. ◦ Has unique features: glycogen storage, myoglobin. Transverse tubules (T-tubules) ◦ Serve as extensions of plasmalemma. ◦ Carry action potential deep into muscle fiber. Sarcoplasmic reticulum (SR): Ca2+ storage  Myofibrils and Sarcomeres Myofibrils ◦ Muscle  fasciculi  muscle fiber  myofibril (large to small) ◦ Hundreds to thousands per muscle fiber Sarcomeres ◦ Basic contractile element of skeletal muscle ◦ End to end for full myofibril length  Sarcomeres Distinctive striped appearance (striations) ◦ A-bands: dark stripes ◦ I-bands: light stripes ◦ H-zone: middle of A-band ◦ M-line: middle of H-zone Common boundary structure: Z-disk Herzog, W. (2014). The role of titin in eccentric muscle contraction. The Journal of Experimental Biology, 217, 2825- 2833. https://doi.org/10.1242/jeb.099127  Sarcomere: Protein Filaments Muscle contraction Actin (thin filaments) ◦ Show up lighter under microscope. ◦ I-band contains only actin filaments. Myosin (thick filaments) ◦ Show up darker under microscope. ◦ A-band contains both actin and myosin filaments. ◦ H-zone contains only myosin filaments. In Review  Skeletal muscle is striated, voluntary, and multi-nucleate  An individual muscle cell is called a muscle fiber  Muscle fibers have a cell membrane (plasmalemma) and the same organelles as other cells  The cytoplasm of a muscle fiber is called sarcoplasm full of glycogen and myoglobin  T-tubules and sarcoplasmic reticulum are found in the sarcoplasm and store calcium  The sarcomere is the smallest functional unit of a muscle  Myosin (Thick Filaments) Two intertwined filaments Eddinger, T. J. (1998). Myosin heavy chain isoforms and dynamic contractile properties: Skeletal versus smooth muscle. Biological Sciences Faculty Research and Publications, 119 Globular heads (3), 425-434. https://doi.org/10.1016/S0305- 0491(98)00003-0 ◦ Protrude 360° from thick filament axis ◦ Will interact with actin filaments for contraction Titin as stabilizer  Composed of three proteins ◦ Actin: contains myosin-binding site ◦ Tropomyosin: covers active site at rest ◦ Troponin (anchored to actin): moves tropomyosin Actin (Thin Filaments) Anchored at Z-disk Equally spaced out by titin Nebulin is an anchoring protein  Titan (Third Myofilament) Acts like a spring (stiffness increases with muscle activation and force development). ◦ Extends from Z-disk to M-band ◦ Ca2+ binds to titan, increasing muscle force when stretched ◦ Addressed by “winding filament theory” Monroy, J. A., Powers, K. L., Gilmore, L. A., Uyeno, T. A., Lindstedt, S. L., & Nishikawa, K. C. (2012). What is the role of titin in active muscle? Exercise And Sport Sciences Reviews, 40(2), 73–78. Stabilizes sarcomeres and centers https://doi.org/10.1097/JES.0b013e31824580c6 myosin Prevents overstretching  Figure 1.5 Eddinger, T. J. (1998). Myosin heavy chain isoforms and dynamic contractile properties: Skeletal versus smooth muscle. Biological Sciences Faculty Research and Publications, 119 (3), 425-434. https://doi.org/10.1016/S0305-0491(98)00003-0 A. Skeletal muscle—bipolar filament B. Smooth muscle—side polar filament  Motor Units Motor unit ◦ Consists of a single a-motor neuron + all fibers it innervates. ◦ More operating motor units = more contractile force. Neuromuscular junction ◦ Consists of synapse between a-motor neuron and muscle fiber. ◦ Serves as site of communication between neuron and muscle. Figure 1.7 Muscle Fiber Contraction: Excitation–Contraction Coupling 1. Action potential (AP) starts in the brain. 2. AP arrives at axon terminal, releases acetylcholine (ACh). 3. ACh crosses synapse, binds to ACh receptors on plasmalemma. 4. AP travels down plasmalemma and T-tubules. 5. Triggers Ca2+ release from sarcoplasmic reticulum (SR). 6. Ca2+ enables actin–myosin contraction. Figure 1.8 Neuromuscular Junction  The sarcomere is the function unit of the muscle—many sarcomeres together create a myofibril—each muscle fiber contains several thousand myofibrils Each sarcomere is made of proteins: myosin, actin, titin, troponin, and tropomyosin The action potential is initiated at the Review brain as an electrical signal ◦ Signal becomes a chemical—Ach released ◦ Crosses the synaptic cleft at the neuromuscular junction ◦ Binds receptor on the cell membrane (plasmalemma) ◦ Signal becomes electrical and spreads across membrane ◦ Ca++ released and enables the actin- myosin contraction  Role of Ca2+ in Muscle Fiber AP arrives at SR from T-tubule ◦ SR is sensitive to electrical charge ◦ Causes mass release of Ca2+ into sarcoplasm Ca2+ binds to troponin on thin filament ◦ At rest, tropomyosin covers myosin-binding site, blocking actin– myosin attraction ◦ Troponin–Ca2+ complex moves tropomyosin ◦ Myosin binds to actin; contraction can occur  Relaxed state ◦ No actin–myosin interaction occurs at Sliding binding site Filament ◦ Myofilaments overlap a little Theory: How Contracted state Muscles ◦ Myosin head pulls actin toward sarcomere center (power stroke) Create ◦ Filaments slide past each other Movement ◦ Sarcomeres, myofibrils, muscle fiber : Relaxed all shorten and Contracted  After the Power Stroke: Sliding ADP is released from the Filament myosin binding site Myosin returns to the Theory: ready state How Able to bind a new ATP Muscles to continue to the Create contraction Process continues Movement until... : Power Z-disk reaches myosin Stroke filaments, or AP stops and Ca2+ gets pumped back into SR  Figure 1.9 For audio description use this link: https://players.brightcove.net/901973548001/kplGlX8REA_default/index.html? videoId=5990663523001 Figure 1.10 Animation: https://players.brightcov e.net/901973548001/kpl GlX8REA_default/index.h tml?videoId=599194249 3001  Energy for Muscle Contraction Adenosine triphosphate (ATP) is necessary for muscle contraction Binds to myosin head ◦ ATPase on myosin head ◦ ATP  ADP + Pi + energy  Muscle Relaxation AP ends; electrical stimulation of SR stops Ca2+ is pumped back into SR ◦ Is stored until next AP arrives ◦ Requires ATP Without Ca2+, troponin and tropomyosin return to resting conformation ◦ Cover myosin-binding site ◦ Prevent actin–myosin cross-bridging   The action potential travels down the plasmalemma causing the release of Ca2+ from the T-tubules  Ca2+ binds to troponin and moves the tropomyosin off of the myosin-binding site of the actin molecules  Myosin binds to actin, ATP binds to myosin allowing the myosin to release the actin  ATP bound to myosin is split into ADP + Pi In Review  The myosin head moves 90º and binds a the adjacent actin molecule  Pi is release and contributing energy to the power stroke, moving myosin to 45º toward the middle  The myosin head discharges the ADP and enters the ready state  Process continues until complete contraction occurs or calcium is pumped back into the T-tubules—ATP required  Muscle Fiber Types ~50% of fibers in an an Type average muscle Peak tension tension in 110 110 ms (slow twitch) twitch) I Peak tension tension in 50 ms ms (fast twitch) Type Type Type IIa IIa (~25% (~25% of fibers fibers in an average muscle) Type Type IIx IIx (~25% of of fibers II in an average muscle) FIGURE 1.11 A PHOTOMICROGRAPH SHOWING TYPE I (BLACK), TYPE IIA (WHITE), TYPE IIX (GRAY)  Variable speed of myosin ATPase Fast myosin ATPase = fast contraction cycling Slower myosin ATPase = Type I slower contraction cycling Versus Muscle biopsy Type II: Small (10-100 g) piece of ATPase muscle is removed Difference Biopsy is frozen, sliced, s examined with microscope Gel electrophoresis Type I and II fibers have different types of myosin isoforms Process separates types of myosin by size Figure 1.12a & b  Type I Versus Type II:SR and Motor Unit Differences Sarcoplasmic reticulum ◦ Type II fibers have a more highly developed SR. ◦ Ca2+ release is faster, and Vo is 3 to 5 times faster. Motor units ◦ Type I motor unit: smaller neuron, 300 fibers  Fiber classification System 1 (preferred) Type I Type IIa Type IIx System 2 Slow twitch (ST) Fast twitch a (FTa) Fast twitch x (FTx) System 3 Slow oxidative (SO) Fast Fast glycolytic (FG) oxidative/glycolytic (FOG) Characteristics of fiber types Oxidative capacity High Moderately high Low Glycolytic capacity Low High Highest Contractile speed Slow Fast Fast Fatigue resistance High Moderate Low Motor unit strength Low High High Table 1.1 Classification of Muscle Fiber Types  Type I Versus Type II: Peak Power Type IIx > type IIa > type I ◦ Effects of various factors (e.g., SR, motor units) ◦ Single-fiber recording All muscle fibers reach peak power at ~20% of peak force Figure 1.13c Distribution of Fiber Types (Type I : Type II Ratios) Each person has unique ratios Arm and leg ratios are similar in one person ◦ Type I predominates in endurance athletes ◦ Type II predominates in power athletes Soleus is type I in everyone Characteristic Type I Type IIa Type IIx Fibers per motor neuron ≤300 ≥300 ≥300 Motor neuron size Smaller Larger Larger Motor neuron Slower Faster Faster conduction velocity Contraction speed (ms) 110 50 50 Type of myosin ATPase Slow Fast Fast Sarcoplasmic reticulum Low High High development Table 1.2 Structural and Functional Characteristics of Muscle Fiber Types In Review Most skeletal muscle contains both type I and type II muscle fibers Different fiber types have different myosin ATPases, different developmental levels of sarcoplasmic reticulum, and α-motor unit innervation Type I Type II ATPase Slow ATPase Fast ATPase SR Simpler SR Highly developed Motor unit SR A single motor unit A single motor unit innervation innervates ≤ 300 innervates ≥ 300 muscle fibers muscle fibers  Possess high aerobic Recruited for endurance Can maintain Require low-intensity exercise for oxygen for aerobic prolonged ATP exercise and periods production daily Type I activities Fibers During Exercise Efficiently produce ATP from fat and carbohydrate  Type II fibers in general ◦ Fatigue quickly (poor aerobic endurance) ◦ Produce ATP anaerobically Type II Type IIa Fibers ◦ Produce more force, fatigue faster than type I During ◦ Used for short, intense endurance Exercise (1,600 m run) Type IIx ◦ Seldom used for everyday activities ◦ Used for short, explosive sprints (100 m) Fiber Type Determinants Determine which a-motor neurons innervate fibers Genetic factors Fibers differentiate based on a- motor neuron Endurance training, strength training, and detraining shifts Training factors myosin isoforms Training can induce small (10%) change in fiber type Aging: loss of type II motor units  Muscle Fiber (Motor Unit) Recruitment Method for altering force production ◦ Less force production: fewer or smaller motor units ◦ More force production: more or larger motor units ◦ Type I motor units smaller than type II Recruitment order: type I, type IIa, type IIx Orderly Recruitment and Size Principle Recruit minimum number of motor units needed ◦ First: smallest (type I) motor units ◦ Next: midsize (type IIa) motor units ◦ Last: largest (type IIx) motor units Recruit in same order each time Size principle: Order of recruitment relates directly to size of a-motor neuron Fiber Type and Athletic Success Type I predominates in endurance athletes Type II predominates in sprinters Success is also affected by other factors: ◦ Cardiovascular function ◦ Motivation ◦ Training habits ◦ Muscle size  Static (isometric) Muscle produces force but does not change length Types of Joint angle does not Muscle change Contractio Myosin cross-bridges n form and recycle (no Dynamic sliding) Muscle produces force and changes length Joint movement is produced Dynamic Contraction Subtypes Concentric ◦ Muscle shortens while producing force. ◦ This is the most familiar type of contraction. ◦ Sarcomere shortens; filaments slide toward center. Eccentric ◦ Muscle lengthens while producing force. ◦ Cross-bridges form, but sarcomere lengthens. ◦ One example is lowering heavy weight.  Motor unit recruitment Type II motor units = more force Type I motor units = less force Fewer small fibers versus more large fibers Frequency of stimulation (rate coding) Twitch Generatio Summation Tetanus n of Force Length–tension relation Optimal sarcomere length equals optimal overlap. If too short or too stretched, little or no Speed–force force develops.relation Concentric: Maximal force development decreases at higher speeds. Eccentric: Maximal force development increases at higher speeds. Figure 1.14 Figure 1.15 Length–tension relationship Optimal sarcomere length equals optimal overlap If too short or too stretched, little or no force develops Figure 1.16 Speed-force relationship: Concentric: Maximal force development de creases at higher speeds​ Eccentric: Maximal force development increas es at higher speeds​ Review  Type I = aerobic  Type II =anaerobic  Motor units supply and all-or-none response recruited in order—Type I, Type IIa, and Type Iix—principle of orderly recruitment  Different athletes’ have different muscle types, which can translate to optimal success  Contractions: concentric (shortening)—the slower, the more force generated, isometric (no movement), eccentric (lengthening)—the quicker movement, the more force generated  Force maximized at the optimal length of the muscle  Force can be maximized by recruiting more motor units and/or frequency of stimulation