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 –70m V 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 normal eye movements is discussed.