The conduction velocity along the fiber varies directly with the action potential amplitude and the rate of change of the potential (dVm/dt) during phase 0. The action potential amplitude is the potential difference between the fully depolarized and the fully polarized regions of the cell interior. The magnitude of the local current is proportional to this potential difference (see ). Because these local currents shift the potential of the resting zone toward the threshold value, they are local stimuli that depolarize the adjacent resting portion of the fiber to its threshold potential. The greater the potential difference between the depolarized and polarized regions (i.e., the greater the action potential amplitude), the more effectively local stimuli can depolarize adjacent parts of the membrane, and the more rapidly the wave of depolarization is propagated down the fiber.
The dVm/dt during phase 0 is also an important determinant of conduction velocity. If the active portion of the fiber depolarizes gradually, the local currents between the resting region and the neighboring depolarizing region are small. The resting region adjacent to the active zone is depolarized gradually, and more time is therefore required for each new section of the fiber to reach threshold. This allows some sodium channels to inactivate.
The resting membrane potential is another important determinant of conduction velocity. Changes in the resting membrane potential influence both the amplitude of the action potential and dVm/dt, which in turn alter the conduction velocity (Fig. 16.13 ). Depolarization of Vm inactivates the fast sodium channels, which in turn decreases the amplitude of the action potential and the dVm/dt, and as a consequence conduction velocity is slowed. In addition to changes in extracellular [K+], premature excitation of a cell that has not completely repolarized also results in a •Fig. 16.13 Effect of Changes in Extracellular Potassium Concentration (Extracellular [K+]) on the Action Potentials Recorded From a Purkinje Fiber. Thestimulusartifact(StintracingD)appearsasabiphasicspiketotheleftoftheupstrokeoftheactionpotential.Thehorizontal dashed lines nearthepeaksoftheactionpotentialsdenote0mV.Whenextracellular[K+]is3mmol/L(“mM”;tracingsAandF),therestingmembranevoltage(Vm)is−82mV,andtheslopeofphase0issteep.Attheendofphase0,theovershootattainsavalueof+30mV.Hence,theactionpotentialamplitudeis112mV.Thedistancefromthestimulusartifacttothebeginningofphase0isinverselyproportionaltotheconductionvelocity.Whenextracellular[K+]isincreasedgraduallyto16mmol(tracingsBtoE),therestingVmbecomesprogressivelylessnegative.Atthesametime,theamplitudesanddurationsoftheactionpotentialsandthesteepnessoftheupstrokesalldiminish.Asaconsequence,conductionvelocitydecreasesprogressively.Atextracellular[K+]levelsof14and16mmol(tracingsDandE),therestingVmattainslevelssufficienttoinactivateallthefastsodiumchannelsandleadtothecharacteristicslow-responseactionpotentials.(FromMyerburgRJ,LazzaraR.In:FischE[ed].Complex Electrocardiography. Philadelphia:FADavis;1973.) decrease in conduction velocity. This too reflects the fact that when Vm is depolarized, more fast sodium channels are inactivated, and thus only a fraction of the sodium channels are available to conduct the inward Na+ current during phase 0.
Conduction of the Slow Response