Chapter 01 Power Points
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Chapter 01 Power Points Chapter 01 Power Points Presentation Transcript

  • 1 Structure and Function of Exercising Muscle chapter
  • Learning Objectives
    • Learn the structure of skeletal muscle, the muscle fiber, and the myofibril
    • Learn the cellular events leading to a basic muscle contraction
    • Discover how muscle functions during exercise
    • Consider the differences in fiber types, and how they are recruited, and their impact on physical performance
    • Learn how muscles generate force and movement
  • Types of Muscles
    • Smooth
    • Involuntary muscle; controlled by the autonomic nervous system
      • Located in the walls of blood vessels and throughout internal organs
    • Cardiac
    • Controlled by the autonomic nervous and endocrine systems
      • Located only in the heart
    • Skeletal
    • Voluntary muscle; controlled consciously by the somatic nervous system
      • More than 600 different skeletal muscles located throughout the body
  • Microscopic Images of Muscle Microscopic photographs of (a) skeletal, (b) cardiac, and (c) smooth muscle a b c
  • The Basic Structure of Skeletal Muscle
  • Structure of a Single Muscle Fiber
  • Skeletal Muscle Fiber Structure
    • Key Points
    • An individual muscle cell is called a muscle fiber
    • A muscle fiber is enclosed by a plasma membrane called the sarcolemma
    • The cytoplasm of a muscle fiber is called a sarcoplasm
    • Within the sarcoplasm, the extensive T-tubules allow transport of substances throughout the muscle fiber
    • The sarcoplasmic reticulum (SR) stores calcium
  • An Electron Micrograph of Myofibrils © Custom Medical Stock Photo Myofibrils
  • Sarcomere: The Functional Unit of the Myofibril
    • The sarcomere contains the contractual elements between each pair of Z-disks
      • An I-band (light zone)
      • An A-band (dark zone)
      • An H-zone (in the middle of the A-band)
      • An M-line in the middle of the H-zone
      • The rest of the A-band
      • A second I band
  • Structure of the Sarcomere
  • The Myofibril
    • Key Points
    • Myofibrils are made up of sarcomeres
    • A sarcomere is composed of protein filaments of myosin and actin
    • Interactions between the filaments is responsible for muscle contraction
    • Myosin, the thick filament, is composed of two protein strands, each folded into a globular head at one end
    • The thin filament is composed of actin, tropomyosin, and troponin, with one end attached to a Z-disk
  • Alpha Motor Neurons
    • One  -motor neuron innervates many muscle fibers, collectively called the motor unit
    • The action potential arrives at the dendrites and travels down the axon to the axon terminal
  • Events Leading to Muscle Fiber Contraction
    • A motor neuron releases acetylcholine (ACh) at the neuromuscular junction
    • ACh binds to receptors on the sarcolemma
    • If enough ACh binds to receptors, an action potential is transmitted the full length of the muscle fiber
    • The action potential triggers the release of Ca 2+ from the sarcoplasmic reticulum
    • Ca 2+ binds to troponin on the actin filament, and the troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament
  • Sequence of Events Leading to Muscle Contraction
  • Sliding Filament Theory
    • With Ca 2+ activation, myosin heads bind to actin, resulting in a conformational change in the cross-bridge
    • The myosin head tilts toward the arm of the cross-bridge and drags the actin toward the center of the sarcomere
    • The tilt of the myosin head is known as a power stroke
    • The pulling of the actin filament past the myosin filament results in shortening of the sarcomere and the generation of muscle force
  • Sliding Filament Theory
  • Molecular Events of Cross-Bridge Cycling in Skeletal Muscle
    • Fig. 12.9, p. 405 from HUMAN PHYSIOLOGY, 4th ed. By Dee Unglaub Silverthorn. Copyright © 2007 by Pearson Education, Inc. Adapted by permission.
  • Muscle Fiber Contraction
    • Key Points
    • Muscle contraction is initiated by an  -motor neuron action potential
    • The motor neuron releases ACh, which opens up ion gates in the muscle cell membrane
    • Sodium enters the muscle cell depolarizing the plasmalemma
    • The action potential travels throughout the plasmalemma and through the T-tubules, which releases stored Ca 2+ ions from the sarcoplasmic reticulum
    • (continued)
  • Muscle Fiber Contraction (continued)
    • Key Points
    • Ca 2+ ions bind with troponin, lifting the tropomyosin molecules off the active sites on the actin filament
    • The myosin head binds to the active actin site
    • The myosin head binds ATP, and ATPase on the myosin heads splits ATP into ADP and P i , releasing energy to fuel the muscle contraction
    • The myosin head tilts, pulling the thin filament past the thick filament — power stroke
    • Muscle contraction ends when Ca 2+ is actively pumped out of the sarcoplasm back to the sarcoplasmic reticulum
  • Muscle Biopsy
    • The muscle biopsy allows us to study muscle fibers and the effects of acute exercise and chronic training on muscle fiber composition
      • The skin is first anesthetized and then a small incision is made through the skin
      • A hollow Bergstrom needle is inserted into the muscle belly to take the sample
      • The sample is mounted, frozen, thinly sliced, and examined under a microscope
  • Muscle Biopsy
  • Muscle Fiber Types
    • Slow-twitch fibers, Type I (~50%), oxidative
    • Fast-twitch fibers, Type II
      • Type IIa (25%), fast oxidative/glycolytic (FOG)
      • Type IIx in humans (~25%) ~ IIb in animals, fast glycolytic (FG)
      • Type IIc (1-3%)
    • The percentage of each fiber type is variable among muscles, among individuals, and with exercise training
  • A Photomicrograph Showing Type I, Type IIa, and Type IIx Muscle Fibers
    • Type I (black), type IIa (white), and type IIx (gray) muscle fibers
  • Determination of Myosin Isoforms Myosin isoforms are determined by electrophoretic separation of the proteins and by staining. Determining the myosin isoform helps to identify the fiber type.
  •  
  •  
  • Single Muscle Fiber Physiology
    • Peak power is different between muscle fiber types
    • All fiber types tend to reach their peak power at ~20% peak force
  • Muscle Fiber Types
    • Key Points
    • Skeletal muscle contains type I and type II fibers
    • Different fiber types have different myosin ATPase activities
    • Type II fibers have a more highly developed SR, delivering more Ca 2+
    • Type II motor units are larger compared to type I motor units
    • Type II motor units have more muscle fibers to contract and produce more force than type I motor units
    • The proportion of type I and type II fibers in an individual’s arm and leg muscles are usually similar
    • (continued)
  • Muscle Fiber Types (continued)
    • Key Points
    • Type I fibers have higher aerobic endurance and are well suited to low-intensity endurance activities
    • Type II fibers are better suited for anaerobic activity
      • Type IIa fibers play a major role in high intensity exercise
      • Type IIx fibers are activated when the force demanded of a muscle is high
  • Determination of Fiber Type
    • Fiber type is genetically determined (twin studies)
    • Fiber type is determined by the  -motor neuron that innervates the muscle fibers
    • Endurance training, strength training, and muscular inactivity may cause a shift in myosin isoforms
      • Exercise training ↓ type IIx and ↑ type IIa
    • Aging may shift the relative distribution of type I and type II fibers
      • ↓ type II and ↑ type I
  • Motor Unit Recruitment
    • Principle of orderly recruitment
    • Motor units are activated on the basis of a fixed order
      • type I -> type IIa -> type IIx
  • Size Principle
    • Size principle
    • The order of motor unit recruitment is directly related to
    • the motor neuron size
  • Motor Unit Recruitment
    • Key Points
    • Motor units give an all-or-none response
    • Activating more motor units and thus more muscle fibers produces more force
    • Motor units are recruited in an orderly way to generate force or for long duration events
      • type I -> type IIa -> type IIx
  •  
  • Athletes and Fiber Type
    • Key Points
    • Muscle fiber composition differs in athletes by sport and event
    • Speed and strength events are characterized by a higher percentage of type II fibers
    • Endurance events are characterized by a higher percentage of type I fibers
  • Types of Muscle Contraction
    • Concentric contraction: Force is developed while the muscle is shortening
    • Isometric contraction: Force is generated but the length of the muscle is unchanged
    • Eccentric contraction: Force is generated while the muscle is lengthening
  • Variation in Force Production with Frequency of Stimulation
    • Adapted, by permission, from G.A. Brooks, et al., 2005, Exercise Physiology: Human bioenergetics and its applications, 4th ed. (New York: McGraw-Hill), 388. With permission of the McGraw-Hill Companies.
  • Force Variation with Changes in Sarcomere Length
    • Adapted, by permission, from B.R. MacIntosh, P.F. Gardiner, and A.J. McComas, 2006, Skeletal Muscle: Form and function, 2nd ed. (Champaign, IL: Human Kinetics), 156.
  • Relationship Between Muscle Lengthening and Shortening Velocity and Force Production
  • Muscle Force Generation
    • Key Points
    • 3 types of muscle contraction
      • Concentric
      • Isometric
      • Eccentric
    • Force production is increased by recruitment of more motor units and through increased frequency of stimulation
    • Force production is maximized at the muscle’s optimal length
    • Speed of contraction also affects the amount of force produced