Structure and Function of Skeletal Muscle

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Structure and Function of Skeletal Muscle
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Three Muscle Types

• Skeletal- striated
• Cardiac- striated, intercalated discs
• Smooth- not striated
• All muscle tissue exhibit
• Responsiveness - The ability to receive and
  respond to a stimulus
• Conductivity the ability of the impulse to
  travel along the plasma membrane of the muscle
  cell.
• Contractility - The ability to shorten
• Elasticity - The ability to recoil and resume
  original length

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Skeletal Muscle

• Human body contains over 400 skeletal muscles
• 40-50 of total body weight
• Functions of skeletal muscle
• Force production for locomotion and breathing
• Force production for postural support
• Heat production during cold stress

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The Neuromuscular Junction

• Site where motor neuron meets the muscle fiber
• Separated by gap called the neuromuscular cleft
• Motor end plate
• Pocket formed around motor neuron by sarcolemma
• Neurotransmitter is released from the motor
  neuron
• Causes an end-plate potential (EPP)
• Depolarization of muscle fiber
• Each axonal terminal forms a junction with a
  single muscle fiber (cell)
• Single neuron has several connections and thus
  stimulate several muscle fibers

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Motor Unit

• Consist of one nerve fiber and all the muscle
  fibers it innervate
• Behaves as a single functional unit and contract
  as one

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Behavior of the whole muscle

• Threshold a minimal voltage necessary to
  produce a muscle contraction
• Muscle twitch a single contraction of skeletal
  muscle. It is composed of latent, contraction and
  relaxation periods
• Latent period the time between stimulation and
  the beginning of contraction
• Includes all time required for excitation,
  excitation-contraction coupling, and stretching
  of the series elastic components
• Refractory period a phase after a twitch during
  which the muscle can not respond to another
  stimulus

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Force Regulation in Muscle

• 2 ways to stimulate variable contraction strength
  of a muscle
• Multiple motor unit summation - activate more
  motor units. Increased stimulus force
• Wave summation - stimulate a muscle at a higher
  rate of stimuli delivered to the muscle
• Second stimulus is administered before complete
  relaxation of muscle
• Treppe a response to series of stimuli of the
  same strength given before the muscle completely
  relaxed
• Fatigue results from a prolonged period of
  contraction. Muscle loose the ability to contract

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An initial stimulation and resulting twitch all
by itself. Then we have 2 stimuli in somewhat
rapid succession. The 2nd twitch has added on to
the first. This is known as wave or temporal
summation. It occurs because there is still
calcium from the 1st twitch in the sarcoplasm at
the time of the 2nd twitch.
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Muscle Fatigue

• Physiological inability to contract
• Results primarily from a relative deficit of ATP.

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Types of Contractions

• Contractions can be
• Isometric
• Iso same, metrmeasure
• Isotonic
• Isosame, tontension

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Isotonic Contractions

• In isotonic contractions, the muscle changes in
  length (decreasing the angle of the joint) and
  moves the load (Isosame, tontension)
• The two types of isotonic contractions are
• Concentric contractions the muscle shortens and
  does work
• Eccentric contractions the muscle contracts as
  it lengthens
• Example someone pulls your arm straight while
  at the same time you try to keep the arm locked
  in one position.

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Isometric Contractions

• Tension increases to the muscles capacity, but
  the muscle neither shortens nor lengthens (Iso
  same, metrmeasure) postural muscles
• Occurs if the load is greater than the tension
  the muscle is able to develop
• Example - carrying an object - your hands and
  arms would be opposing the gravity with equal
  force going upwards. Since your arms are neither
  raising or lowering, your biceps will be
  isometrically contracting.

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Isotonic and Isometric Contractions