Abstract
Our understanding of the behaviour of motoneurons (MNs) in mammals partly relies on our knowledge of the relationships between MN membrane properties, such as MN size, resistance, rheobase, capacitance, time constant, axonal conduction velocity and afterhyperpolarization period. Based on scattered but converging evidence, current experimental studies and review papers qualitatively assumed that some of these MN properties are related. Here, we reprocessed the data from 27 experimental studies in cat and rat MN preparations to empirically demonstrate that all experimentally measured MN properties are associated to MN size. Moreover, we expanded this finding by deriving mathematical relationships between each pair of MN properties. These relationships were validated against independent experimental results not used to derive them. The obtained relationships support the classic description of a MN as a membrane equivalent electrical circuit and describe for the first time the association between MN size and MN membrane capacitance and time constant. The obtained relations indicate that motor units are recruited in order of increasing MN size, muscle unit size, MN rheobase, unit force recruitment thresholds and tetanic forces, but underlines that MN size and recruitment order may not be related to motor unit type.
Significance statement This study processed all available experimental data to date to provide the first mathematical and empirical proof that all motoneuron (MN) properties – rheobase, resistance, capacitance, membrane time constant, axonal conduction velocity, afterhyperpolarization period – are directly predictable from MN size. Mathematical relationships between each of these MN properties are derived and validated, and best reproduce the current knowledge in MN properties. For the first time, MN profiles of inter-consistent and motoneuron-specific properties can be built from these equations.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
CONFLICT OF INTEREST STATEMENT: The authors declare no competing financial interests.