Neuronal Control of Muscle Contraction

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Neuronal Control of Muscle Contraction

• Sliding filament theory of muscle contraction
• Energetics of muscle contraction
• Motoneuronal control of muscle contraction

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• Muscle Fiber

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Composition of Individual Fibers
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• Sarcomere

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• Proper Alignment of Filaments Ensured by Two
  Proteins

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• Titan has an elastic component which helps the
  stretched sarcomere to return to its resting
  length and stabilizes the position of the
  contractile porteins.
• Nebulin, an inelastic protein helps titin. It
  helps align the actin filaments.

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• Sarcomere Length Changes During Contraction

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• The I-band and H-zone shorten but the A-band
  remains constant.
• Sliding-filament theory of contraction.
  Overlapping filaments slide past each other.

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• Molecular Basis of Contraction

• Myosin is a motor protein which converts the
  chemical energy of ATP into mechanical energy of
  motion.

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Power Stroke
- molecular basis of muscle contraction.

• Slides about 0.06 mm.
• After death, rigor mortis sets in as crossbridges
  freeze, since no more ATP is produced to bind
  to the myosin heads.

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Ca2 is a Trigger For Contraction

• Troponin and tropomyosin prevent myosin heads
  from completing the power stroke like a safety
  latch on a gun.
• Tropomyosin partially blocks the binding site for
  myosin during rest. Contraction is initiated when
  Ca2 binds to troponin causing tropomyosin to
  change its shape and expose the rest of the
  myosin binding site to complete the power stroke.

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Exciting-Contraction (E-C) Coupling

• APs resulting from transmitter release triggers
  muscle contraction.
• The combination of electrical and mechanical
  events in the muscle fiber is called
  excitation-contraction coupling.

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• Muscle APs in the t-tubules activates
  dihydropyridine (DHP) receptors, which open Ca2
  channels. Ca2 binds to troponin to initiate the
  power stroke.

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Sources of Energy For Contraction
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• Aerobic metabolism is very efficient but requires
  an adequate supply of oxygen to the muscles.
• Anaerobic metabolism of glucose is not efficient
  and makes cells acidic through the production of
  lactic acid.
• As a backup energy source phosphocreatine which
  contains high-energy phosphate bonds.

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Muscle Fatigue

• Muscle fatigue is a condition in which the
  muscle is no longer able to generate or sustain
  the expected power output.
• Its thought to mainly arise from failure in
  excitation-contraction coupling within the muscle
  than from presynaptic factors.
• Central fatigue include subjective feelings of
  tiredness and a desire to cease activity. Its
  thought that central fatigue precedes
  physiological fatigue in the muscle. Acidosis of
  lactic acid dumped into the bloodstream may
  influence the sensation of fatigue perceived in
  the brain.

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• However, other factors which may contribute to
  fatigue may arise from
• A) Depletion of glycogen stores within the
  muscle.
• B) Accumulation of H from the buildup of lactic
  acid and the increased production of inorganic
  phosphate from ATP breakdown. Both H and
  inorganic phosphates interfere with crossbridge
  function.
• C) Increased production of extracellular K
  production with maximal exercise depolarizes the
  membrane potential and decreases release of Ca2
  from the SR.
• D) Neuronal causes result from failure of
  transmission at the neuromuscular junction.

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Speed of Contraction and Resistance to Fatigue
Determine the Fiber-Type Composition of Muscle

• The speed of contraction with repeated
  stimulation is determined by the isoforms of
  myosin present in the thick filaments. Different
  isoforms have different ATPase activity.
• Fast fibers split ATP more rapidly and complete
  more contraction cycles than slow fibers. This
  speed translates into fast tension development.
• The duration of contraction is determined by the
  rate at which Ca2 is removed from the cytosol by
  the SR.
• Resistance to fatigue with repeated stimulation
  is thought to result from preventing buildup of
  lactic acid.
• Skeletal muscle fibers can be classified into
  three groups
• A) Fast-twitch glycolytic fibers.
• B) Fast-twitch oxidative fibers.
• C) Slow-twitch (oxidative) fibers.

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Tension Developed in a Muscle is a Function of
Sarcomere Length

• The resting length of the muscle needs to be
  optimum to produce maximal tension.
• Sarcomere has to form optimum number of
  crossbridges to generate maximal force.

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The Somatic Motor Neuron and the Muscle Fibers it
Innervates is Called a Motor Unit

• An AP in the motor neuron causes all the muscle
  fibers it innervates to contract.
• The number of fibers in a motor unit varies (e.g.
  small number of fibres in motor units which exert
  fine control, like in eye muscles) , but the
  fiber-type composition of the motor unit remains
  the same.
• Inheritance in part determines the fiber-type
  composition, however it can also be changed by
  altering the fibers metabolic characteristics.

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A Muscle is Composed of Many Motor Units

• A motor unit contracts in an all-or-none manner.
• In a muscle, the tension and its duration can be
  varied by
• (a) Changing the number of motor units responding
  at one time.
• (b) Changing the type of motor unit which is
  active.

• Tension could be increased by recruitment of
  additional motor units. Recruitment is controlled
  by the nervous system and proceeds in a fixed
  order.

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Nervous Control of Recruitment

• Order of recruitment is highly correlated with
  the diameter and conduction velocity of the axon,
  the size of the motor neuron cell body and the
  size and strength of the muscle fibres in the
  motor unit.
• Small motor neurons fire first and the largest
  fire last. This is the size principle of motor
  neuron recruitment.
• The size principle serves two purposes A) allows
  the most fatigue-resistant fibers to be recruited
  first and keeps the most fatigable fibers in
  reserve until higher forces need to be generated.
  B) the increment of force generated by
  successively activated motor units will be
  roughly proportional to the level of force at
  which each individual unit is recruited.
• As the highest threshold for motor neurons are
  recruited, the muscle contractions are reaching a
  maximum. Motor units drop out in the order
  opposite from their recruitment.
• Slow-twitch oxidative fibers have the lowest
  threshold for recruitment.
• Fast-oxidative fibers have a medium threshold for
  recruitment.
• Fast-twitch glycolytic fibers have a high
  threshold for recruitment.

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• Small cell bodies have a high transmembrane
  resistance (Rhigh)because they have a smaller
  surface area and fewer channels. Thus, according
  to Ohms law (V IRhigh), the synaptic currents
  produce large excitatory graded potentials
  (EPSPs) which readily fire APs. However, the
  velocity of the APs as they travel towards the
  axon terminals are slow because of the small
  diameter axons.
• In contrast, in large motor neurons, the cell
  bodies have a larger surface area and more
  channels thus, a lower transmembrane resistance
  (Rlow). The synaptic currents therefore produce
  subthreshold EPSPs (V IRlow), making it harder
  to trigger APs. However, if triggered, they
  travel down the large diameter axons faster.