Factors determining the variation of the afterhyperpolarization duration in cat lumbar alpha-motoneurones.

The duration of the afterhyperpolarization (AHP) in cat spinal alpha-motoneurones varies systematically with motoneurone type, being shorter in motoneurones projecting to fast-contracting muscle units. Recent experiments have shown that the AHP duration is correlated with the amount of sag found in the voltage response to injected constant current pulses. Using a model of the sag process, the present study shows that this correlation is likely to be causal to a substantial extent. Short AHP durations in fast motoneurones may thus be as much, or more, a consequence of a more developed sag process than of faster kinetics of the K conductance process underlying the AHP. This notion is also supported by the experimental observation of a decreased amount of sag and a prolonged AHP duration after axotomy.