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ELECTRICAL ACTIVITY OF NERVESMuch of our early knowledge of nerve electrical activity came from the study of giant squid axons. Because of their size, it was easy to insert electrodes into them for accurate measurements of the membrane potential.
These animals live in sea water, so the concentrations are all higher than the corresponding concentrations for mammals, but the ratios of inside to outside concentrations are what matter, and these are similar to mammalian values and similar to the ratios I chose for the hypothetical cell. RESTING POTENTIALThe resting potential of squid nerves is near -70 mV. EXCITATIONElectrical activity is produced in nerves by a variety of stimuli. Pressure, stretch, chemical transmitters such as acetylcholine, and the passage of electrical current across the nerve membrane all can initiate nerve activity. In each case, the stimulus produces a change in the membrane potential that leads to an "explosive" response of the nerve. Because it is easy, experimenters most often produce nerve activity by passing an electric current into the cells. MEASURE THRESHOLDThe threshold stimulus current, and the corresponding voltage threshold may be measured experimentally by altering the stimulus until an action potential is obtained:
SUPRATHRESHOLD STIMULATIONIf a stimulus brings the membrane potential above threshold, the membrane voltage abruptly depolarizes, and an action potential results:
WHAT CAUSES THE ACTION POTENTIAL?A hypothesis can be made by comparing the action potential with the responses of the hypothetical cell. The nerve cell contents and surrounding fluid have compositions similar to those chosen for the hypothetical cell. The resting potential of nerve is slightly positive to the potential we would calculate using the Nernst equation and the concentrations of potassium outside and inside (assuming the membrane is permeable only to potassium). At the peak of the action potential, the cell voltage reaches a value near that we would calculate (using the Nernst eq. and the sodium concentrations) if we assumed the membrane were permeant only to sodium at the peak of the action potential. For example, compare the appearance of a series of nerve action potentials to the record obtained from the hypothetical cell with the switch that allowed the membrane to be changed from potassium selective to sodium selective:
Researchers hypothesized that the membrane of a resting nerve cell is much more permeable to potassium than to other ions, so it is close to EK. Depolarization to a voltage above threshold somehow dramatically changes the membrane properties so that the membrane becomes much more permeant to sodium than to potassium, so the voltage approaches ENa. It is useful to put this hypothesis in terms of conductance, rather than permeability. Let gK represent potassium conductance, and gNa represent sodium conductance. Then the hypothesis is:
This is a reasonable general explanation of the basis of the nerve action potential, but one must go deeper to explain the complex electrical behavior of the cells. |
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Copyright 1998, Joe Patlak and Ray Gibbons, Department of
Physiology, University of Vermont. |
This
is an all-or-none response. If the stimulus is adequate, the response is independent of
the strength of the stimulus. The energy for the action potential does not come from the
stimulus; it comes from the cell. 
