The duration of after-hyperpolarization in hindlimb alpha motoneurones of different sizes in the cat.

In the present intracellular investigation, direct measurements were made of the size of the cell body and the duration of after-hyperpolarization (AHP) among 76 hindlimb motoneurones of the cat. Following the penetration and intracellular recording of action- and after-potentials, each one of the cells was individually labelled by an injection of Procion yellow, Procion red or horseradish peroxidase from a microelectrode. In accordance with expectations, a significant negative correlation was found between the size of a motoneuronal cell body and the duration of its AHP. Motoneurones with a mean soma diameter exceeding about 50 microns usually had an AHP shorter than 110 msec. Cells smaller than 50 microns could, however, have AHPs of widely varying durations (range from 55 to 265 msec).