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Section 03:
The Muscular System
Chapter 18 – Skeletal Muscle: Structure and Function
Chapter 19 – Neural Control of Human Movement
Chapter 22 – Muscular Strength: Training Muscles to Become Stronger (Part 2 – Structural and Functional Adaptations to Resistance Training, pp. 539‐553)
HPHE 6710 Exercise Physiology II
Dr. Cheatham
Chapter 18
Skeletal Muscle: Structure and Function
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Chapter Objectives
• Be able to identify the different overall structures/level of organization of muscle
• Identify the different components of a muscle fiber
• Understand the influence of muscle pennation
on force production and velocity of contraction
• Understand the Sliding‐Filament Model
• Understand excitation‐contraction coupling
• Understand the different muscle fiber types and the influence on muscle performance
Gross Structure of Skeletal Muscle
• Levels of Organization
– Connective tissue layers
• Endomysium
– Wraps each muscle fiber, separates from other fibers
• Perimysium
– Surrounds a bundle of fibers (up to 150) called a fasciculus
• Epimysium
– Surrounds all the bundles to form the entire muscle
– Tapers at both proximal and distal ends to form tendons (connect muscle to bone)
» Origin: More stable bone
» Insertion: Moving bone
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Gross Structure of Skeletal Muscle
• Levels of Organization (cont’d)
– Sarcolemma
• Thin elastic membrane that surrounds the muscle fiber
• Plasma membrane
– Bilayer lipid structure that conducts the electrochemical wave of depolarization over the surface of the muscle fiber
• Basement membrane
– Proteins and strands of collagen fibrils that fuse with the outer covering of the tendon
• Satellite Cells
– Between the plasma and basement membranes
– Cellular growth, recovery, hypertrophy
Gross Structure of Skeletal Muscle
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Gross Structure of Skeletal Muscle
• Levels of Organization (cont’d)
– Sarcoplasmic Reticulum
• Extensive latticelike network of tubules and vesicles
• Provides structural integrity
• Stores, releases, and reabsorbs Ca2+
Gross Structure of Skeletal Muscle
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Gross Structure of Skeletal Muscle
• Chemical Composition
– 75% water
– 20% protein
• Myosin, actin, tropomyosin, troponin, myoglobin
– 5% salts, phosphates, ions, macronutrients
• Blood Supply
– Skeletal muscle has a rich vascular network
• 200 to 500 capillaries per mm2 of muscle
– Flow is rhythmic
• Vessels compress during contraction phase
• Vessels open during relaxation phase
Gross Structure of Skeletal Muscle
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Skeletal Muscle Ultrastructure
• Myofibrils contain myofilaments.
– Actin (thin filament)
– Myosin (thick filament)
• Also of functional importance
– Troponin
– Tropomyosin
• In addition, there are several other structural proteins.
Skeletal Muscle Ultrastructure
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Skeletal Muscle Ultrastructure
• The Sarcomere
– Most basic, functional unit of a muscle fiber
– Runs from Z line to Z line
Skeletal Muscle Ultrastructure
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Skeletal Muscle Ultrastructure
• Sarcomere (cont’d)
– A Band (Dark)
– Overlap of thick and thin filaments
• H Zone
– Lower density, lack of actin
• M Band
– Sarcomeres center
– I Band (Light)
• Lower density
Skeletal Muscle Ultrastructure
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Muscle Fiber Alignment
• Long axis of a muscle
– From origin to insertion
– Determines fiber arrangement
• Fusiform: Fibers run parallel to long axis
– Rapid muscle shortening
• Pennate: Fibers are at an oblique angle to long axis
– Bipennate, Multipennate
– Greater force production
• Fiber arrangement influences
– Force‐generating capacity
– Physiologic cross sections
Muscle Fiber Alignment
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Muscle Fiber Alignment
Actin‐Myosin Orientation
• Actin filaments lie in a hexagonal pattern around myosin.
• Crossbridges spiral around the myosin where actin and myosin overlap.
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Actin‐Myosin Orientation
• Tropomyosin
– Lies along actin in the groove formed by the double helix
• Inhibits actin–myosin interaction
• Troponin
– Embedded at regular intervals along actin
• Interacts with Ca2+
• Moves tropomyosin, uncovering active actin sites
Actin‐Myosin Orientation
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Actin‐Myosin Orientation
• Intracellular Tubule Systems
– The T tubular system is distributed around the myofibrils such that each sarcomere has two triads.
– Each triad contains
• 2 vesicles
• 1 T tubule
Actin‐Myosin Orientation
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Chemical and Mechanical Events During Muscle Action and Relaxation
• Sliding Filament Model
– Contraction occurs as myosin and actin
slide past one another.
– Myosin crossbridges
cyclically attach, rotate, and detach from actin filaments.
– Energy is provided by ATP hydrolysis.
Chemical and Mechanical Events During Muscle Action and Relaxation
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Chemical and Mechanical Events During Muscle Action and Relaxation
• Sarcomere Length‐Isometric Tension
Chemical and Mechanical Events During Muscle Action and Relaxation
• Link Between Actin, Myosin, and ATP
– Myosin head bends around ATP molecule, becomes cocked
– Myosin interacts with actin.
– ATP is hydrolyzed.
– Energy release forces power stroke.
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Chemical and Mechanical Events During Muscle Action and Relaxation
Chemical and Mechanical Events During Muscle Action and Relaxation
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Chemical and Mechanical Events During Muscle Action and Relaxation
http://youtu.be/0kFmbrRJq4w
Chemical and Mechanical Events During Muscle Action and Relaxation
• Excitation‐
Contraction Coupling
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Muscle Fiber Types
• Fast‐Twitch Fibers (Type II)
–
–
–
–
–
–
High capacity to transmit AP
High myosin ATPase activity
Rapid Ca2+ release and uptake by SR
High rate of crossbridge turnover
Capable of high force generation
Rely on anaerobic metabolism
• Slow‐Twitch Fibers (Type I)
–
–
–
–
Low myosin ATPase activity
Slower Ca2+ release and uptake by SR
Low glycolytic capacity
Large and numerous of mitochondria
Muscle Fiber Types
• Fast‐Twitch Subdivisions
– IIa fibers
• Fast shortening speed
• Moderately well‐developed capacity for both anaerobic and aerobic energy production
– IIb fibers
• Most rapid shortening velocity
• Rely on anaerobic energy production
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Muscle Fiber Types
Muscle Fiber Types
http://youtu.be/6Ts_INXzhzM
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Muscle Fiber Types
http://youtu.be/ewKFGVFtnII
Muscle Fiber Types
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Muscle Fiber Types
Muscle Fiber Types
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Chapter 19
Neural Control of Human Movement
Chapter Objectives
Review the CNS and peripheral nervous system
Understand motor unit anatomy
Review the action potential
Understand the concept of excitation‐
contraction coupling
• Understand the activities of the motor unit as they relate to force production
• Be familiar with the characteristics of different muscle fiber types
• Understand proprioception
•
•
•
•
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Neuromotor System Organization
• Human nervous system has two major parts:
– Central Nervous System
– Peripheral Nervous System
Neuromotor System Organization
• CNS – The Brain
– Six main areas:
•
•
•
•
•
•
Medulla oblongata
Pons
Midbrain
Cerebellum
Diencephalon
Telencephalon
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Neuromotor System Organization
• CNS – The Brain (cont’d)
– Cerebellum
• Receives motor output signals from the central command in the cortex
• Obtains sensory input from peripheral receptors in muscles, tendons, joints, skin and from auditory, visual, and vestibular end organs
• Serves as the major comparing, evaluating, and integrating center for postural adjustments, locomotion, maintenance of equilibrium, perceptions of speed of body movement
Neuromotor System Organization
• CNS – The Brain (cont’d)
– Diencephalon
• Thalamus
• Hypothalamus
– Regulates metabolic rate and body temperature
– Influences activity of the ANS
– Regulates and maintains the body’s internal milieu
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Neuromotor System Organization
• CNS – The Brain (cont’d)
– Telencephalon
• Contains two hemispheres of cerebral cortex
• 4 lobes
–
–
–
–
Frontal
Parietal
Temporal
Occipital
Neuromotor System Organization
• CNS – The Brain (cont’d)
– Brainstem
• Medulla oblongata
– Serves as bridge between the two hemispheres of the cerebellum
• Pons
• Midbrain
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Neuromotor System Organization
• CNS – The Spinal Cord
– 45 cm long, 1 cm diameter
– Encased by 33 vertebrae
– Provides for two‐way flow of communication between brain and periphery via nerve tracts and sensory receptors
– Spinal cord contains three types of neurons
• Motor neurons
– Efferent fibers run to the skeletal muscle fibers
• Sensory neurons
– Afferent fibers that enter the spinal cord from the periphery
• Interneurons
Neuromotor System Organization
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Neuromotor System Organization
• Peripheral Nervous System
– 31 pairs of spinal nerves
– 12 pairs of cranial nerves
– Two types of efferent neurons
• Somatic neurons: innervate skeletal muscle
• Autonomic neurons: activate smooth muscle, cardiac muscle, sweat and salivary glands, and some endocrine glands
– Sympathetic and Parasympathetic Nervous Systems
Neuromotor System Organization
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Neuromotor System Organization
Neuromotor System Organization
• The Reflex Arc
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Nerve Supply to Muscle
• One nerve innervates at least one of the body’s approximately 250 million muscle fibers
• Individuals possess approximately 420,000 motor neurons
• The number of muscle fibers per motor neuron generally relates to a muscles movement pattern
• Thinking question: Which neurons would only innervate a few muscle fibers? Which neurons would innervate several hundred muscle fibers?
Nerve Supply to Muscle
• Motor Unit Anatomy
– Motor Unit: The functional unit of movement
• Motor neuron and innervated muscle fibers
– Motor Neuron Pool: The collection of alpha motor neurons that innervate a single muscle
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Nerve Supply to Muscle
• Motor Unit Anatomy (cont’d)
– The Anterior Motor Neuron
Nerve Supply to Muscle
• Motor Unit Anatomy (cont’d)
– The Anterior Motor Neuron
• Neuromuscular Junction
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Action Potential Review
Action Potential Review
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Action Potential Review
Nerve Supply to Muscle
• Motor Unit Anatomy (cont’d)
– The Anterior Motor Neuron
• Excitation
– Facilitation
» Temporal summation
» Spatial Summation
– Inhibition
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Nerve Supply to Muscle
Nerve Supply to Muscle
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Motor Unit Functional Characteristics
• Twitch Characteristics
Motor Unit Functional Characteristics
• Twitch Characteristics (cont’d)
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Motor Unit Functional Characteristics
• Tension Characteristics
• “All or none principle”
– Gradation of force
• Motor unit recruitment
– To increase force, more motor units are recruited
– Motor units are recruited from smaller axons to larger axons (size principle)
• Discharge frequency
Motor Unit Functional Characteristics
• Tension Characteristics (cont’d)
– Neuromuscular Fatigue
• Fatigue is the decline in muscle tension or force capacity with repeated stimulation
• Voluntary muscle contractions are the result of a chain of events/systems:
– CNS
» Alterations in neurotransmitters, ammonia, cytokins (central fatigue)
– Peripheral Nervous System
» Fatigue ??
– Neuromuscular Junction
» Fatigue ??
– Muscle Fiber
» Glycogen depletion, PO2, H+, reduced enzyme activity, etc. (p. 409)
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Receptors in Muscles, Joints, and Tendons: The Proprioceptors
• Muscle Spindles
– Provide sensory feedback about changes in muscle fiber length and tension
Receptors in Muscles, Joints, and Tendons: The Proprioceptors
• Muscle Spindles (cont’d)
– Stretch Reflex
• Spindle responding to stretch
• Afferent nerve fiber
• Efferent spinal cord motor neuron
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Receptors in Muscles, Joints, and Tendons: The Proprioceptors
• Golgi Tendon Organs
– Located at musculotendonous junction
– Detect difference in tension generated by active muscle
– Respond to tension generated by
• Muscle contraction
• Passive stretch
– Protect muscle from excessive load
Receptors in Muscles, Joints, and Tendons: The Proprioceptors
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Receptors in Muscles, Joints, and Tendons: The Proprioceptors
• Pacinian Corpuscles
– Small ellipsoidal bodies
– Located near Golgi tendon organs
– Embedded in a single unmyelinated nerve fiber
– Detect changes in movement or pressure
Chapter 22
Muscular Strength: Training Muscles to Become Stronger (Part 2 – Structural and Functional Adaptations to Resistance Training, pp. 539‐547)
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Section Objectives
• Understand the mechanisms relating to muscle hypertrophy
• Understand the mechanisms relating to muscle fiber type alteration
Introduction
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Factors That Modify the Expression of Human Strength
• Psychologic‐Neural Factors
Factors That Modify the Expression of Human Strength
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Factors That Modify the Expression of Human Strength
Journal of Applied Physiology, 1961.
Factors That Modify the Expression of Human Strength
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Factors That Modify the Expression of Human Strength
Factors That Modify the Expression of Human Strength
• Psychologic‐Neural Factors (cont’d)
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Factors That Modify the Expression of Human Strength
• Muscular Factors
Factors That Modify the Expression of Human Strength
• Muscular Factors – Muscle Hypertrophy
– The primary stimulus for muscle hypertrophy is an increase in muscular tension/force
– Mechanical stress “triggers” signaling proteins to activate genes that activate translation of mRNA and stimulate protein synthesis
– What, though, “signals” this response to occur
• Will cover in a few slides (Muscle Cell Remodeling)
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Factors That Modify the Expression of Human Strength
Factors That Modify the Expression of Human Strength
• Significant Metabolic Adaptations Occur
– Fiber type alteration
• I ↔ IIA ↔ IIX ↔ IIB
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Factors That Modify the Expression of Human Strength
Factors That Modify the Expression of Human Strength
• Muscle Cell Remodeling: Current Thinking
– Muscle is a dynamic tissue in that it can change
– It is well established that muscle hypertrophy can occur and most likely fiber types can change as well
– What changes actually occur?:
• Fiber Type
– Most likely changes in the myosin ATPase isoform expression
• Increase in muscle size
– Hypertrophy: the addition of more muscle proteins to existing muscle fibers
– Hyperplasia: new muscle fibers
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Factors That Modify the Expression of Human Strength
• Muscle Cell Remodeling: Current Thinking
– These changes to muscle have one thing in common:
• There is a change in gene expression or mRNA which will lead to an increase (or decrease) in protein synthesis
– The underlying question:
• What “signals” cause gene expression to be turned on or turned off for a specific protein
• The answer to this question is not entirely clear
Factors That Modify the Expression of Human Strength
• Fiber Type Changes
– Again, the change really is a change in the isoform
of myosin ATPase
• Intracellular calcium may play a role
• Hypertrophy
– Autocrine signals (IFG‐1, Prostaglandins)
– Acute immune response (IL‐6)
– Growth factors
– Early response genes
– Regulation of myostatin
– Activation of satellite cells
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Factors That Modify the Expression of Human Strength
Factors That Modify the Expression of Human Strength
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Factors That Modify the Expression of Human Strength
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