The major advance in our understanding of supernormality is the following hypothesis. Focal accumulations of extruded potassium ion in unstirred-clefts (invaginated extensions of the extracellular space in Purkinje fibers) cause threshold to fall by virtue of the early effects of hyperkalemia: lowering of threshold and acceleration of conduction. These effects rapidly dissipate in diastole as ionic equilibration takes place; threshold rises and assumes its late diastolic value. It is not known if this mechanism is applicable to the supernormality of specialized atrial fibers. Comparing the strength interval curves of supernormality and triggered automaticity the fall in threshold during the upstroke of the late after-depolarisation occurs later than the supernormal phase and is followed by a threshold rise (during the downstroke of the after-potential). Supernormality does not correspondingly show this biphasic change in the strength interval curve.
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