On factors determining orderly recruitment of motor units: a role for intrinsic membrane properties

Abstract

A commonly held notion is that motoneurone size is a key factor underlying the orderly recruitment of motor units according to their tension. According to this ‘size principle’ originally proposed by Henneman, this comes about as a result of the belief that motoneurone excitability is a function of motoneurone size. In this article we argue that there exists a considerable variation in excitability among motoneurones, due to a variation in intrinsic membrane properties, and that these are largely uncorrelated with motoneurone size. It is proposed that this systematic variation in intrinsic membrane properties is a substantial factor underlying the orderly recruitment of motor units.