Membrane Potentials

Membrane Potentials

1. Resting Membrane Potential

2. Graded Potential

3. Action Potential

   • occurs in the membrane of excitable tissues (nerve or muscle) in response to graded potentials that reach threshold
   • a large, rapid depolarization occurs in which the membrane potential becomes positive for a breid time.
   • the membrane potential changes from -70mV to +30mV!
   • is capable of being propagated long distances along the length of an axo without any decrease in strength.
   • generation of an AP is based on the selective permeability of the plasma membrane and the Na+ and K+ electrochemical gradients
     • changes in permeability result from the opening and closing of gated ion channels
   • Phases of an Action Potential
     • 1. Depolarization
       • Occurs when the membrane potential reaches threshold. Results in a change in membrane potential from -70mV to +30mV.
       • Is caused by a sudden and dramatic increase in Na+ permeability due to the opening of voltage-gated Na channels that open at threshold, moving Na+ INTO the cell
       • 2. Repolarization
         • Is caused by the rapid reduction to Na permeability and the increase in K permeability
         • The movement of K down its electrochemical gradient OUT of the cell repolarizes the membrane to bring it back to -70mV
         • +30mV to -70mv
         • 3. Hyperpolarization
           • Is caused by the permeability of K remaining elevated for a brief time after resting membrane potential is reached, going slightly below -70mV (overshoot)
           • This happens because the closing of K gated channels is slower than the closing of Na gated channels
           • -70mV to -85mV
       • -70mV to +30mV
   • Propagation of Action Potentials
     • action potentials set up electrochemical gradients in extracellular and intracellular fluids
       • fluids have low resistance to current flow & cations move from the area where the membrane was depolarized to an adjacent area, and weill depolarize that area.
         • Propagation of Action Potentials in Unmyelinated Axons
           • Entire axon doesn't depolarize at once - sequencial depolarization of mini sections because of fluids
             • Change in membrane potential in one region creates a difference in potential in fluids.
               • Each region sets up depolarization for the next region.
                 • LARGE DIAMETER of axon = LESS RESISTANCE TO CURRENT FLOW and action potential propagates FASTER
         • Propagation of Action Potentials in Myenlinated Axons = SALTATORY CONDUCTION
           • Myelin provides high resistance to ion movement/current flow across the membrane, but longitudinal resistance is still low
             • NODES OF RAVIER = gaps in the myelin where membrane is not insulated and is exposed to intersitial fluid. Membrane has a greater concentration of voltage-gated channels.
               • Action potential will be produced at nodes and will not be produced where myelin is present.
                 • Flow of current is from one node to the next - "jumping" of action potential until action potential reaches axon terminal
           • Conduction velocity is greater in myelinated axons vs. unmyelinated