The relationship of afterhyperpolarizations of extraocular motoneurons to membrane potential.

The effect of membrane potential displacements caused by external current injection on the amplitudes of two antidromically evoked afterhyperpolarizations (AHP1 and AHP2) was measured in extraocular motoneurons. AHP1 had a reversal potential of-70mV and an average compensation for displacements from the reversal potential ("compensation gain") of -0.36. AHP2 had a reversal potential of-80mV and a compensation gain of -0.08. Measurements of input resistance of these motoneurons demonstrate a 30-50% decrease at depolarized membrane potentials. This rectification could account for the apparent lack of summation of AHP2. The functional role of AHP1 and AHP2 in the regulation of discharge frequencies of extraocular motoneurons during nromal eye movements is discussed.