Effects of morphine and meperidine on action potential production in frog's skeletal muscle fibers.

Morphine (3.3 times 10-minus 4 M) and meperidine (8.8 times 10-minus 5 M) inhibited action potential production in frog's skeletal muscle fibers. Over these concentration ranges, neither the resting membrane potentials nor the resting membrane electric properties of the fibers appeared to be modified. Both drugs depressed excitability and the rising phase of the action potential by inhibiting the specific increase in sodium conductance which normally follows an adequate stimulus. Both drugs also seemed to inhibit the secondary rise in potassium conductance which normally occurs during an action potential, causing a prolongation of the action potential duration.