The Sub-Primary Range of firing is present in both cat and mouse spinal motoneurones and its relationship to force development is similar for the two species.

In the motor system force gradation is achieved by recruitment of motoneurones and rate modulation of their firing frequency. Classical experiments investigating the relationship between injected current to the soma during intracellular recording and the firing frequency (the I-f-relation) in cat spinal motoneurones identified two clear ranges - a primary range and secondary range. Recent work in mice, however, has identified an additional range proposed to be exclusive to rodents, the Sub-Primary Range (SPR), due to the presence of mixed-mode oscillations of the membrane potential. Surprisingly, fully summated tetanic contractions occurred in mice during SPR frequencies. With the mouse now one of the most popular models to investigate motor control, it is crucial that such discrepancies between observations in mice and basic principles that have been widely accepted in larger animals are resolved.To do this, we have reinvestigated the I-f-relation using ramp current injections in spinal motoneurones in both barbiturate anaesthetised and decerebrate (non-anaesthetised) cats and mice. We demonstrated the presence of the SPR and mixed mode oscillations in both species and that the SPR is enhanced by barbiturate anaesthetics. Our measurements of the frequency-force relation in both cats and mice support the classical opinion, i.e. that firing frequencies in the higher end of the primary range are necessary to obtain a full summation. By systematically varying the leg oil pool temperature (from 37°C to room temperature) we found that only at lower temperatures can maximal summation occur at SPR frequencies due to prolongation of individual muscle twitches.SIGNIFICANCE STATEMENT:This work investigates recent revelations that mouse motoneurones behave in a fundamentally different way to motoneurones of larger animals with respect to the importance of rate modulation of motoneurone firing for force gradation. The current studies systematically addresses the proposed discrepancies between mice and larger species (cats) and demonstrates that mouse motoneurones, in fact, use rate modulation as a mechanism of force modulation in a similar manner to the classical descriptions in larger animals.