The Action Potential

The Action Potential

1. 2 types of Voltage Gated Ion Channels produces Action Potentials

   • Voltage Gated channels not present on dendrites or soma. Concentrated on axon hillock
     • Axon hillock is the site of excitatory and inhibitory signal summation
   • Depolarizaton
     • Vg(Na+)
       • Opens first, when axon hillock is sufficently depolarized. Na+ conductance increases 5000 fold
         • Na+ rushes in due to low intracellular concentration/negative charge
       • Activation gate
         • Opens in response to depolarization
       • Inactivation gate
         • Closes in response to depolarization
         • Slightly slower to close than activation is to open
         • Remains closed until membrane potential returns to resting potential
   • Repolarization
     • Vg(K+)
       • Opens second, when axon hillock is depolarized. Right when Na+ channels are inactivated. K+ conductance increases 30 fold
         • K+ rushes out due to concentration
         • Repolarization phase begins
       • Closes slowly as membrane returns to negative state
   • Resting Potential
     • K+ conductance is 50 times greater than Na+. Due to leak channels
   • Afterhyperpolarization
     • Absolute Refractory Period
       • Na+ inactivation gates are still closed
       • Prevents backpropogation
     • Relative Refractory Period
       • K+ conductance is higher than normal while Vg(K+) channels slowly close--> Makes it harder to depolarize again

2. Myelin Sheath

   • Provided by glial cells
   • Prevents leakage of current through axon membrane
   • Nodes of Ranvier
     • Unmylelinated area concentrated with voltage gated channels
     • Allows for leaping of current called saltatory conduction
     • Allows for fewer ions to conduct which means less ATP is used for Na+/K+ ATPase pump
   • Unmylinated Axons
     • Smaller diameter due to greater energy expenditure on more Na+/K+ATPase

3. Speed is governed by

   • Thickness of diameter. Thicker is faster
     • Density of obstacles(vesicles, etc) that Na+ can run into is smaller, leading to faster propagation of action potential
   • Degree of myelination. Higher is faster
     • Decreases capacitance of the membrane due to more surface area of the membrane
       • When there are less ions on the membrane, they are able to disperse faster, allowing faster propagation of action potential