Muscle structure and Function

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Muscle structure and Function

• Chapter 9 10

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Muscle Overview

• The three types of muscle tissue are skeletal,
  cardiac, and smooth
• These types differ in structure, location,
  function, and means of activation

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What are some muscle similarities?

• Skeletal and smooth muscle cells are elongated
  and are called muscle fibers
• Muscle contraction depends on two kinds of
  myofilaments actin and myosin
• Muscle terminology is similar
• Sarcolemma muscle plasma membrane
• Sarcoplasm cytoplasm of a muscle cell
• Prefixes myo, mys, and sarco all refer to muscle

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What is skeletal muscle tissue?

• Packaged in skeletal muscles that attach to and
  cover the bony skeleton
• Has obvious stripes called striations
• Is controlled voluntarily (i.e., by conscious
  control)
• Contracts rapidly but tires easily
• Is responsible for overall body motility
• Is extremely adaptable and can exert forces over
  a range from a fraction of an ounce to over 70
  pounds

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What is cardiac muscle tissue?

• Occurs only in the heart
• Is striated like skeletal muscle but is not
  voluntary
• Contracts at a fairly steady rate set by the
  hearts pacemaker
• Neural controls allow the heart to respond to
  changes in bodily needs

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What is smooth muscle tissue?

• Found in the walls of hollow visceral organs,
  such as the stomach, urinary bladder, and
  respiratory passages
• Forces food and other substances through internal
  body channels
• It is not striated and is involuntary

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What are the functions of muscles?

• Skeletal muscles are responsible for all
  locomotion
• Cardiac muscle is responsible for coursing the
  blood through the body
• Smooth muscle helps maintain blood pressure, and
  squeezes or propels substances (i.e., food,
  feces) through organs
• Muscles also maintain posture, stabilize joints,
  and generate heat

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What are the functional characteristics of
muscles?

• Excitability, or irritability the ability to
  receive and respond to stimuli
• Contractility the ability to shorten forcibly
• Extensibility the ability to be stretched or
  extended
• Elasticity the ability to recoil and resume the
  original resting length

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What is the anatomy of a skeletal muscle?

• Each muscle is a discrete organ composed of
  muscle tissue, blood vessels, nerve fibers, and
  connective tissue
• The three connective tissue wrappings are
• Epimysium an overcoat of dense regular CT that
  surrounds the entire muscle
• Perimysium fibrous CT that surrounds groups of
  muscle fibers called fascicles
• Endomysium fine sheath of CT composed of
  reticular fibers surrounding each muscle fiber

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Figure 9.1
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How do skeletal muscles get nutrients?

• Each muscle is served by one nerve, an artery,
  and one or more veins
• Each skeletal muscle fiber is supplied with a
  nerve ending that controls contraction
• Contracting fibers require continuous delivery of
  oxygen and nutrients via arteries
• Wastes must be removed via veins

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How are skeletal muscles attached to the skeleton?

• Muscles span joints and are attached to bone in
  at least two places
• When muscles contract, the movable bone (the
  muscles insertion) moves toward the immovable
  bone the muscles origin
• Muscles attach
• Directly epimysium of the muscle is fused to
  the periosteum of a bone
• Indirectly CT wrappings extend beyond the
  muscle as a tendon or aponeurosis

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How do muscle fibers contract?

• See laser disk.

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What is a muscle twitch?

• A muscle twitch is the response of a muscle to a
  single brief threshold stimulus
• The three phases of a muscle twitch are
• Latent period first few milliseconds after
  stimulation when excitation-contraction coupling
  is taking place

Figure 9.12a
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Muscle Twitch

• Period of contraction cross bridges actively
  form and the muscle shortens
• Period of relaxation Ca2 is reabsorbed into
  the SR, and muscle tension goes to zero

Figure 9.12a
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What are graded muscle responses?

• Graded muscle responses are
• Variations in the degree of muscle contraction
• Required for proper control of skeletal movement
• Responses are graded by
• Changing the frequency of stimulation
• Changing the strength of the stimulus

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How do muscles respond to varying stimuli?

• A single stimulus results in a single contractile
  response a muscle twitch
• Frequently delivered stimuli (muscle does not
  have time to completely relax) increases
  contractile force wave summation

Figure 9.13
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Muscle Response to Varying Stimuli

• More rapidly delivered stimuli result in
  incomplete tetanus
• If stimuli are given quickly enough, complete
  tetanus results

Figure 9.13
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What controls the force of contraction?

• Threshold stimulus the stimulus strength at
  which the first observable muscle contraction
  occurs
• Beyond threshold, muscle contracts more
  vigorously as stimulus strength is increased
• Force of contraction is precisely controlled by
  multiple motor unit summation
• This phenomenon, called recruitment, brings more
  and more muscle fibers into play

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animations

• skeletal muscle physiology
• sarcomere shortening
• twitches
• sliding filaments
• action potential propagation
• structure of a muscle...

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What is treppe (staircase effect)?

• Staircase increased contraction in response to
  multiple stimuli of the same strength
• Contractions increase because
• There is increasing availability of Ca2 in the
  sarcoplasm
• Muscle enzyme systems become more efficient
  because heat is increased as muscle contracts

Figure 9.14
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What is muscle tone?

• Muscle tone
• The constant, slightly contracted state of all
  muscles, which does not produce active movements
• Keeps the muscles firm, healthy, and ready to
  respond to stimulus
• Spinal reflexes account for muscle tone by
• Activating one motor unit and then another
• Responding to activation of stretch receptors in
  muscles and tendons

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What are isotonic contractions?

• In isotonic contractions, the muscle changes in
  length (decreasing the angle of the joint) and
  moves the load
• The two types of isotonic contractions are
  concentric and eccentric
• Concentric contractions the muscle shortens and
  does work
• Eccentric contractions the muscle contracts as
  it lengthens

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Isotonic Contractions
Figure 9.15a
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What are isometric contractions?

• Tension increases to the muscles capacity, but
  the muscle neither shortens nor lengthens
• Occurs if the load is greater than the tension
  the muscle is able to develop

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Isometric Contractions
Figure 9.15b
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Muscle Metabolism Energy for Contraction

• ATP is the only source used directly for
  contractile activity
• As soon as available stores of ATP are hydrolyzed
  (4-6 seconds), they are regenerated by
• The interaction of ADP with creatine phosphate
  (CP)
• Anaerobic glycolysis and the Cori cycle
• Aerobic respiration

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Muscle Metabolism Energy for Contraction
Figure 9.16
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Muscle Metabolism Cori Cycle

• When muscle contractile activity reaches 70 of
  maximum
• Bulging muscles compress blood vessels
• Oxygen delivery is impaired
• Pyruvic acid is converted into lactic acid

Figure 9.17
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Muscle Metabolism Cori Cycle

• The lactic acid
• Diffuses into the bloodstream
• Is picked up and used as fuel by the liver,
  kidneys, and heart
• Is converted back into pyruvic acid by the liver

Figure 9.17
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What is muscle fatigue?

• Muscle fatigue the muscle is in a state of
  physiological inability to contract
• Muscle fatigue occurs when (read pg. 300)
• ATP production fails to keep pace with ATP use
• There is a relative deficit of ATP, causing
  contractures (states of continuous contraction)
• Lactic acid accumulates in the muscle
• Ionic imbalances are present

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What is oxygen debt?

• Vigorous exercise causes dramatic changes in
  muscle chemistry
• For a muscle to return to a resting state
• Oxygen reserves must be replenished
• Lactic acid must be converted to pyruvic acid
• Glycogen stores must be replaced
• ATP and CP reserves must be resynthesized
• Oxygen debt the extra amount of O2 needed for
  the above restorative processes

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What happens to extra heat produced by muscle
contraction?

• Only 40 of the energy released in muscle
  activity is useful as work
• The remaining 60 is given off as heat
• Dangerous heat levels are prevented by radiation
  of heat from the skin and sweating

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What determines the force of contraction?

• The force of contraction is affected by
• The number of muscle fibers contracting the
  more motor fibers in a muscle, the stronger the
  contraction
• The relative size of the muscle the bulkier
  the muscle, the greater its strength

Figure 9.19a
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Force of Contraction

• Series-elastic elements the noncontractile
  structures in a muscle
• Degree of muscle stretch muscles contract
  strongest when muscle fibers are 80-120 of their
  normal resting length

Figure 9.19a
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How do different muscle fibers control the speed
of contraction?

• Slow oxidative fibers contract slowly, have slow
  acting myosin ATPases, and are fatigue resistant
• Fast oxidative fibers contract quickly, have fast
  myosin ATPases, and have moderate resistance to
  fatigue
• Fast glycolytic fibers contract quickly, have
  fast myosin ATPases, and are easily fatigued

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What is peristalsis?

• Smooth muscles double line hollow tube-like
  organs
• When the longitudinal layer contracts, the organ
  dilates and contracts
• When the circular layer contracts, the organ
  elongates and pinches the lumen
• Peristalsis alternating contractions and
  relaxations of smooth muscles that mix and
  squeeze substances through the lumen of hollow
  organs

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Angular Movement

• Flexion
• Extension
• Dorsiflexion and plantar flexion of the foot
• Abduction
• Adduction
• Circumduction

Figure 8.5a
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Angular Movement
Figure 8.5b
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Angular Movement
Figure 8.5c, d
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Angular Movement
Figure 8.5e, f
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Rotation

• The turning of a bone around its own long axis
• Examples
• Between first two vertebrae
• Hip and shoulder joints

Figure 8.5g
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Special Movements

• Supination and pronation
• Inversion and eversion
• Protraction and retraction
• Elevation and depression
• Opposition

Figure 8.6a
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Special Movements
Figure 8.6b
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Special Movements
Figure 8.6c
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Special Movements
Figure 8.6d
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Special Movements
Figure 8.6e
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Interactions of Skeletal Muscles

• Skeletal muscles work together or in opposition
• Muscles only pull (never push)
• As muscles shorten, the insertion generally moves
  toward the origin
• Whatever a muscle (or group of muscles) does,
  another muscle (or group) undoes

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Muscle Classification Functional Groups

• Prime movers provide the major force for
  producing a specific movement
• Antagonists oppose or reverse a particular
  movement
• Synergists
• Add force to a movement
• Reduce undesirable or unnecessary movement
• Fixators synergists that immobilize a bone or
  muscles origin

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Naming Skeletal Muscles

• Location of muscle bone or body region
  associated with the muscle
• Shape of muscle e.g., the deltoid muscle
  (deltoid triangle)
• Relative size e.g., maximus (largest), minimus
  (smallest), longus (long)
• Direction of fibers e.g., rectus (fibers run
  straight), transversus, and oblique (fibers run
  at angles to an imaginary defined axis)

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Naming Skeletal Muscles

• Number of origins e.g., biceps (two origins)
  and triceps (three origins)
• Location of attachments named according to
  point of origin or insertion
• Action e.g., flexor or extensor, as in the
  names of muscles that flex or extend, respectively