• Ohms Law: V = I x R

  (Pressure = Flow times Resistance)
  (Positive current is the outward flow of + ions)

• The concept of measurable versus virtual potentials:

  Membrane potentials are measurable quantities. Nernst (Equilibrium) potentials are virtual potentials.

  (If you are at the Nernst potential, then, the electrical force = the chemical force)

• The 1-dimensional plot of the specific potentials:

  1. The Nernst potentials for Na+, K+:

     --VK----------------------0 mV------------VNa-----
     (-91 mV)                               (+72 mV)
     

  2. The typical nerve membrane potential:

     ----VM------------------ 0 mV----------------------
      (-85 mV)  
     

  3. The driving force on an ion (for instance sodium):

            <------------(VM - VNa)----------->
                     (-85 minus +72 = -157 mV)  
     

After completing the lab, you should know the following:

• Modules 1-4 deal with electrochemical forces:

  • Diffusion forces derive from unequal concentrations of ions across a membrane.
  • Electrical forces derive from unequal "+" (or "-") charges across a membrane.
  • u: Electrical forces can combine with -- or oppose -- chemical forces.
  • An increase in an ion's conductance tends to drive the membrane potential towards that ion's equilibrium potential.

• The Action Potential is an example of positive feedback: a depolarization causes an increase in the number of open Na channels, which in turn causes a further depolarization, which in turn causes a further increase in the number of open Na channels.............

• The following characteristics of the action potential are primarily due to the changing number of sodium channels that are in the inactive state.

  • A Refractory Period follows on the tail of an action potential.

    • No action potential can be elicited during the absolute refractory period, regardless of how large the stimulus is, because a large fraction of the sodium channels have become inactivated.
    • A larger than normal stimulus is required to generate an action potential during the relative refractory period (or the action potential is smaller and slower), because many of the sodium channels are still in the inactive state.

  • The nerve shows Accommodation when the membrane potential is more positive than -50 mV.

    • No action potential can be elicited if the membrane potential ventures too close to zero because all of the sodium channels have become inactivated.

• The Threshold for the firing of an action potential varies. For a nerve to cross threshold:

  • Pacing currents reduce or abolish the need for a stimulus.
  • Inhibitory currents increase the minimum required stimulus.