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Characteristics of Muscle Fibers:  Type I and Type II fibers

 
Type I fibers:
-  Smaller.
-  Produce less force than type II fibers.
-  Much more energy efficient. 
Less myosin ATPase
-  Slow myosin molecules.
-  Underdeveloped Sarcoplasmic Reticulum
          Fewer Voltage Dependent Gates - that means the SR has fewer doors for calcium to pass through,
          and therefore it takes longer for calcium to hit a threshold level in the sarcoplasm. 
-  Troponin has a lower affinity for calcium. 
-  Innervated by slower alpha motor neurons. 
          The speed that an alpha motor neuron can transmit a signal is determined by the thickness of the 
          axon.  The thicker the axon, the faster the speed of transmission.  So, slower alpha motor neurons
          have thinner axons. 
-  Better capacity for energy production. 
          Larger mitochondria, more mitochondria, and more blood vessels (important for oxidative work). 
-  More geared towards oxidative work, although it also performs nonoxidative work. 
 
Type II fibers: 
-  Rapid response, due to a higher concentration of myosin ATPase and fast myosin molecules.
-  High force production.
-  Fast rate of fatigue.
-  Hypertrophy (increase in size / cross-sectional area) moreso than Type I fibers. 
-  Several types.  Type IIa and IIb are the types we know the most about. 
-  Type IIa fibers may work both oxidatively and nonoxidatively. 
-  Type IIb may work only by nonoxidative metabolism. 
  
 

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