When the resting membrane potential, vm, is suddenly changed from -90 mv to the threshold level of about -65 mv the properties of the cell membrane change dramatically Nat enters the myocyte Action through specific fast Na channels that exist in the membrane conductances These channels can be blocked by the puffer fish toxin, tetrodotoxin FIGURE 16-9. Changes in gNa gca, and gx during phases o to 4 of the action potential (A)of a fast-response cardiac cell. The conductance diagrams(B)are qualita tive, not quantitativeWhen the resting membrane potential, Vm, is suddenly changed from -90 mV to the threshold level of about -65 mV the properties of the cell membrane change dramatically. Na+ enters the myocyte through specific fast Na+ channels that exist in the membrane. These channels can be blocked by the puffer fish toxin, tetrodotoxin