Download MBBS Physiology Presentations 68 Structure Skeletal Muscle Lecture Notes

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Skeletal Muscle

n Human body contains over 400 skeletal

muscles

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n 40-50% of total body weight

n Functions of skeletal muscle

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n Force production for locomotion and

breathing

n Force production for postural support

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n Heat production during cold stress
Structure of Skeletal Muscle:

Connective Tissue Covering

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n Epimysium

n Surrounds entire muscle

n Perimysium

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n Surrounds bundles of muscle fibers

n Fascicles

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n Endomysium

n Surrounds individual muscle fibers
Structure of Skeletal Muscle:

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Microstructure

n Sarcolemma

n Muscle cel membrane

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n Myofibrils

n Threadlike strands within muscle fibers

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n Actin (thin filament)

n Troponin

n Tropomyosin

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n Myosin (thick filament)
Structure of Skeletal Muscle:

The Sarcomere

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n Further divisions of myofibrils

n Z-line

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n A-band

n I-band

n Within the sarcoplasm

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n Sarcoplasmic reticulum

n Storage sites for calcium

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n Transverse tubules

n Terminal cisternae

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The Neuromuscular Junction

n Site where motor neuron meets the muscle

fiber

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n Separated by gap cal ed the neuromuscular cleft

n Motor end plate

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n Pocket formed around motor neuron by

sarcolemma

n Acetylcholine is released from the motor

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neuron

n Causes an end-plate potential (EPP)

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n Depolarization of muscle fiber

Illustration of the

Neuromuscular Junction

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Motor Unit

n Single motorneuron & muscle fibers it

innervates

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n Eye muscles ? 1:1 muscle/nerve ratio
n Hamstrings ? 300:1 muscle/nerve ratio

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Muscular Contraction

n The sliding filament model

n Muscle shortening occurs due to the

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movement of the actin filament over the

myosin filament

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n Formation of cross-bridges between actin

and myosin filaments

n Reduction in the distance between Z-lines

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of the sarcomere

The Sliding Filament Model of

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Muscle Contraction


Cross-Bridge Formation in

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Muscle Contraction
Sliding Filament Theory

n Rest ? uncharged ATP cross-bridge complex
n Excitation-coupling ? charged ATP cross-

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bridge complex, "turned on"

n Contraction ? actomyosin ? ATP > ADP & Pi +

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energy

n Recharging ? reload cross-bridge with ATP
n Relaxation ? cross-bridges "turned off"

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Muscle Function

n Al or none law ? fiber contracts

completely or not at al

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n Muscle strength gradation

n Multiple motor unit summation ? more

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motor units per unit of time

n Wave summation ? vary frequency of

contraction of individual motor units

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Energy for Muscle Contraction

n ATP is required for muscle contraction

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n Myosin ATPase breaks down ATP as fiber

contracts

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n Sources of ATP

n Phosphocreatine (PC)

n Glycolysis

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n Oxidative phosphorylation

Sources of ATP for Muscle

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Contraction
Properties of Muscle Fibers

n Biochemical properties

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n Oxidative capacity

n Type of ATPase

n Contractile properties

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n Maximal force production

n Speed of contraction

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n Muscle fiber efficiency

Individual Fiber Types

Fast fibers

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Slow fibers

n Type IIb fibers

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n Type I fibers

n Fast-twitch fibers

n Slow-twitch fibers

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n Fast-glycolytic fibers

n Slow-oxidative fibers

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n Type IIa fibers

n Intermediate fibers

n Fast-oxidative

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glycolytic fibers

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Comparison of Maximal

Shortening Velocities Between

Fiber Types

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Histochemical Staining of Fiber

Type
Fiber Types and Performance

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n Power athletes

n Sprinters
n Possess high percentage of fast fibers

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n Endurance athletes

n Distance runners
n Have high percentage of slow fibers

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n Others

n Weight lifters and nonathletes
n Have about 50% slow and 50% fast fibers

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Alteration of Fiber Type by

Training

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n Endurance and resistance training

n Cannot change fast fibers to slow fibers

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n Can result in shift from Type IIb to IIa

fibers

n Toward more oxidative properties

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Training-Induced Changes in

Muscle Fiber Type
Hypertrophy and Hyperplasia

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n Increase in size

n Increase in number

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Age-Related Changes in

Skeletal Muscle

n Aging is associated with a loss of muscle

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mass

n Rate increases after 50 years of age

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n Regular exercise training can improve

strength and endurance

n Cannot completely eliminate the age-

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related loss in muscle mass


Types of Muscle Contraction

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n Isometric

n Muscle exerts force without changing length

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n Pul ing against immovable object

n Postural muscles

n Isotonic (dynamic)

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n Concentric

n Muscle shortens during force production

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n Eccentric

n Muscle produces force but length increases

Isotonic and Isometric

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Contractions


Illustration of a Simple Twitch

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Force Regulation in Muscle

n Types and number of motor units recruited

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n More motor units = greater force

n Fast motor units = greater force

n Initial muscle length

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n "Ideal" length for force generation

n Nature of the motor units neural stimulation

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n Frequency of stimulation

n Simple twitch, summation, and tetanus

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Relationship Between Stimulus

Frequency and Force

Generation

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Length-Tension Relationship in

Skeletal Muscle

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Simple Twitch, Summation,

and Tetanus

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Force-Velocity Relationship

n At any absolute force the speed of

movement is greater in muscle with

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higher percent of fast-twitch fibers

n The maximum velocity of shortening is

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greatest at the lowest force

n True for both slow and fast-twitch fibers

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Force-Velocity Relationship

Force-Power Relationship

n At any given velocity of movement the

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power generated is greater in a muscle

with a higher percent of fast-twitch

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fibers

n The peak power increases with velocity

up to movement speed of 200-300

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degrees?second-1

n Force decreases with increasing movement

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speed beyond this velocity


Force-Power Relationship

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Receptors in Muscle

n Muscle spindle

n Detect dynamic and static changes in muscle

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length

n Stretch reflex

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n Stretch on muscle causes reflex contraction

n Golgi tendon organ (GTO)

n Monitor tension developed in muscle

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n Prevents damage during excessive force

generation

n Stimulation results in reflex relaxation of muscle

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Muscle Spindle

Golgi Tendon Organ

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