Page 70 - Physiology and Pathophysiology MNU 2024-2025 نظرى
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Clinical pharmacy 2024/2025             Level 2 Pharm D                       Physiology and Pathophysiology (MD303)
 The Action Potential


   At the resting membrane potential (–70 mV)  most ion channels are in their

 resting state (closed but capable of opening)
                                                                          +
   When the neuron is stimulated  the activation gates of the voltage-gated Na
 channels open  permitting the influx of Na  ions  further depolarization toward
                                 +
 threshold

                                             +
   At the threshold potential  all voltage-gated Na  channels are open  the
 “spike” of the action potential

   Approximately 1m sec after the activation gates open  the inactivation gates
                                                         +
 +
 of the Na  channels close; in addition, the activation gates of the K  channels open,
 resulting in the repolarization of the neuron.

   The increase in K  ion permeability results in the after-hyperpolarization [the
        +
 cell is in its relative refractory period (RRP)]

 The absolute refractory period (ARP) begins when the voltage-gated Na+ channels
 become activated & continues through the inactivation phase.
 During  this  time    no  further  Na+  ion  influx  &  no  new  action  potentials  can  be
 generated.













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