Skeletal Muscle Physiology

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Skeletal Muscle Physiology

• How do contractions occur?
• Remember that muscles are excitable

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Electrochemical gradient

• Both neurons and muscle cells maintain
  electrochemical gradients across their plasma
  membranes
• Intracellular fluid is negatively charged

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2 Electrochemical gradients
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Remember that skeletal muscle

• Is under voluntary controlso we need a stimulus
  to begin the process of contraction
• Where does the stimulus come from?
• Motor neurons!
• One motor neuron may innervate many muscle cells

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What do you need to produce a contraction?

• Must transfer message (action potential) from the
  neuron throughout the muscle cell (via transverse
  tubules)
• Thick and thin filaments must interact
• What ions play a role? Na, Ca2
• Where does the Energy to contract come from?
• ATP

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What is an Action Potential (AP)?

• A propagated change in the transmembrane
  potential of excitable cells
• This is the message telling the cell to contract!
• initiated by a change in the membranes
  permeability to Na

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• Cell _at_ rest Gated channels closed
• Stimulus arrives! Na channels open Na rushes
  IN Depolarization
• Slow moving K channels open K rushes OUT
  Repolarization

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What connection?
How is a signal transferred from neuron to muscle
cell?
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Signal transduction

• AP arrives _at_ presynaptic terminal causes Ca2
  channels to open
• Ca2 ions enter stimulate neurotransmitter
  release (ACh) from synaptic vessicles into
  synaptic cleft

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Signal transduction

• ACh diffuses across synaptic cleft binds to ACh
  receptors on Na channel proteins in sarcolemma
  of muscle cell

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Signal transduction

• Influx of Na ions results in depolarization of
  postsynaptic membrane when threshold is
  reached, postsynaptic cell (muscle cell) fires an
  AP

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Ca2 ions released

• Ca2 binds to troponin of thin filaments
• Allows interaction of thick and thin filaments
• Causing a contraction

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Exposure of attachment sight Ca2 binds to
troponin allows tropomyosin to move, exposing
myosin attachment sight
Cross-bridge formation Myosin heads attach to
actin subunits. P released
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Power Stroke Stored E in myosin heads used to
pull actin filament toward M line. ADP released
from myosin head
ATP regenerated attached to myosin head Could
be new ATP or phosphorylated ADP from previous
step
Cross-bridge release ATP broken down to ADP P.
Myosin head releases
Recovery Stroke Myosin heads return to resting
position. E still stored in myosin head
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Recruitment Summation

• We know single muscle cells contract when AP
  arrives
• One single AP stimulus produces a single Twitch
• Twitches produce muscle tension
• How long does one twitch take?
• How do twitches achieve whole muscle contraction?
• By building tension
• Multiple motor units are stimulated (recruited)
• APs arrive more frequently

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Twitch Contraction

• Three phases
• Latent AP reaches sarcolemma SR releases Ca2
  2ms
• Contraction Cross-bridge formation Ca2,
  troponin 15ms
• Relaxation Ca2 uptake tropomyosin covers
  actin 25ms

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What happens when APs arrive?
relaxation phase Complete
relaxation phase Incomplete
relaxation phase Eliminated
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Motor units control tension

• 1 motor unit all the muscle fibers controlled
  by a single motor neuron
• Can the size (of motor units) vary?
• Yes! Why would it vary?
• Level of control required
• Muscles of the eye - precise control 4-6 fibers
• Muscles of the leg - gross control 1-2k fibers

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Motor Units
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Motor unit recruitment
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What ultimately controls muscle tension?

• Presence of Ca2 ions
• More Ca2 ions present more to potentially bind
  to troponin
• Stronger contraction (more tension produced)

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Cardiac muscle

• Heart muscle
• Cells directly connected via intercalated discs
  (pores through which ions pass)
• Allows all connected cells to contract as one
• Cardiac muscle is autorhythmic (spontaneous
  generation of AP)
• Involuntary (influenced by hormones)
• Metabolism is always aerobic

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Smooth muscle

• Less actin myosin, no sarcomeres
• Contracts slowly
• No O2 debt
• Autorhythmic
• Involuntary control