The Neuromuscular Junction

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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
• Acetylcholine is released from the motor neuron
• Causes an end-plate potential (EPP)
• Depolarization of muscle fiber

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Depolarization of a muscle fiber causes
contraction Latent period
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Distinct differences between skeletal and cardiac
muscle
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Tension varies with muscle potential A muscle
fiber displays an all-or-non twitch in vivo Huh?
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Sarcoplasmic reticulum
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Crab muscle- T tubules align with A-band
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Free calcium rises in stimulated
muscle Dye-furapta fluoresces in absence of
calcium
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So how does this work?
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Calcium- induced calcium release Cardiac muscle
DHPR-Dihydropyridine receptor RyR- Ryanodyne
receptor
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DHPR-Dihydropyridine receptor RyR- Ryanodyne
receptor
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Depolarization induced calcium release Skeletal
muscle
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Several types of ion channel
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DHPR
RyR
DHPR-Dihydropyridine receptor RyR- Ryanodyne
receptor
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Calcium release protein- in SR membrane
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Regulation of muscle contraction
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Depolarization of a muscle fiber causes
contraction Latent period
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Contractile and elastic components
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Time course of active state differs from the time
course of tension
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Tetanus
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Relationship Between Stimulus Frequency and Force
Generation

• Summation of forces / tetanus

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Muscle types

• Skeletal muscle
• Striated
• Voluntary
• Smooth muscle
• Non-striated
• Involuntary
• Cardiac muscle
• Electrically coupled cells
• Molluscan Catch muscle
• Insect flight muscle

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Smooth Muscle

• Little or no SR
• No T-tubules
• Some are myogenic (single unit)
• Some are neurogenic (multiunit)

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• Smooth muscle regulated indirectly by Caldesmon
  binding to actin
• Low calcium results in high caldesmon binding
• Caldesmon phosphorylation causes contraction

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• Myosin light chain kinase activity
• Phosphorylation of myosin
• by MLCK results in actin binding
• By PKC blocks actin binding

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Molluscan Catch Muscle
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Molluscan Catch Muscle Mussel Muscle!
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Insect flight muscle

• Synchronous-low wing beat frequency
• Dragonflies, moths locusts
• Each beat driven by a nerve impulse
• Asynchronous-high frequency (100-1000/s)
• Mosquitoes, flies, bees, beetles
• Each nerve impulse drives up to 40 beats

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Synchronous
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Asynchronous
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Insect flight muscle (asynchronous)
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Types of Muscle Contraction

• Isometric
• Muscle exerts force without changing length
• Pulling against immovable object
• Postural muscles
• Isotonic (dynamic)
• Muscle shortens during force production

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Force Regulation in Muscle

• Frequency of stimulation
• Simple twitch, summation, and tetanus
• Number and types of motor units recruited
• More motor units greater force
• Fast motor units greater force
• Initial muscle length (Sarcomere)

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Tetanus
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Relationship Between Stimulus Frequency and Force
Generation

• Summation of forces / tetanus

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Force Regulation in Muscle

• Frequency of stimulation
• Simple twitch, summation, and tetanus
• Number and types of motor units recruited
• More motor units greater force
• Fast motor units greater force
• Initial muscle length (Sarcomere)

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Motor unit strength

• Tension depends on number of muscle fibers per
  nerve

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Relationship Between Stimulus Strength and Force
Generation

• Related to the number of motor units recruited

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Energy for Muscle Contraction

• ATP is required for muscle contraction
• Sources of ATP
• Phosphocreatine (PC)
• Glycolysis
• Oxidative phosphorylation
• Different fiber types use different ATP systems

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Properties of Muscle Fibers

• Biochemical properties
• Oxidative capacity
• Type of ATPase
• Contractile properties
• Maximal force production
• Speed of contraction (Vmax)
• Muscle fiber efficiency

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Force tension (load or mass)
Work force x distance Power work/time
For a given rate of shortening, high Vmax muscle
produces more force
For a given rate of shortening, high Vmax muscle
produces more power.
When data is combined, low Vmax muscles are more
efficient at low velocities, and high Vmax
muscles are more efficient at high velocities
High power muscles use more energy (ATP)
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For a given rate of shortening, high Vmax muscle
produces more force Force tension (load)
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For a given rate of shortening, high Vmax muscle
produces more power. Work force x distance
Power work/time
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High power muscles use more energy (ATP)
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When data is combined, low Vmax muscles are more
efficient at low velocities, and high Vmax
muscles are more efficient at high velocities
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Fiber Types and athletic performance

• Power athletes
• Sprinters
• Possess high percentage of fast fibers (Type IIA
  and IIB)
• Endurance athletes
• Distance runners
• Have high percentage of slow fibers (Type I)
• Others
• Weight lifters and nonathletes
• Have about 50 slow and 50 fast fibers

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Alteration of Fiber Type by endurance training

• Endurance and resistance training
• Cannot change fast fibers (II) to slow fibers
  (I)
• Can result in shift from Type IIb (glycolytic) to
  IIa (oxidative) fibers

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• Preferential recruitment of fibers by exercise
  type

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• Higher of type I fibers elevates oxygen usage
  (VO2max)
• True of both athletes and non-athletes

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Fiber distribution in fish
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Fatigue
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Age-Related Changes in Skeletal Muscle

• Aging is associated with a loss of muscle mass.
  -(
• Rate increases after 50 years of age. -(
• Regular exercise training can improve strength
  and endurance -)
• Cannot completely eliminate the age-related loss
  in muscle mass -(

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