Smooth Muscle

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Smooth Muscle

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Lecture Note
University of Southern California
Bisc 307l - General Physiology
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BISC 307L 1st Edition Lecture 12 Current Lecture  Motor Units o Every one of these qualities vary continuously across fiber types o 3 types of muscle fibers o type 2b – fast fatigue able o type 2a- fast fatigue resistant o type 1- slow twitch fibers o muscle tend to be predominant in one type or the other although these fibers do intermix o Size  Motor neurons larger for type 2b>2a>1  Everything about them bigger not just soma  Means that a given amount of excitation delivered to both types of neurons will excite the small ones first because the smaller neuron has less total membrane so they have a higher total resistance between inside and outside (input resistance higher for small neuron). Since there’s less membrane it also has less capacitance  Same excitation will depolarize this small neuron more  1a afferent: initiates stretch reflex  if you give a tap to the tendon, the units that respond tend to be the slow units not the fast ones o Muscle units  Fast twitch ones produce more force  Should show that there are more muscle fibers in the fast twitch ones  Big powerful motor units in fast twitch  ON the right : slow – force is much weaker, fewer and smaller fibers, the twitch is longer in duration (build up and decay) o Fatigue  Stimulate continuously, type 2b can maintain initial level of force for shorter time than type 1  Slow can maintain for an hour  Type 2a is in between these two o IF you record form a living animal, you see that the type 2B fibers fire at much higher frequencies. o Type one would be lower frequencies (action potential has longer refractory period) o Thresholds: relatively the same though takes less excitation for the type 1 to get to that level o Type 1= dark meat

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