THE INCIDENCE OF INITIAL DOUBLETS IN THE DISCHARGES OF MOTONEURONS OF2 DIFFERENT INSPIRATORY MUSCLES IN THE CAT

1. Trains of action potentials in motoneurones frequently commence wit h an initial doublet; i.e. a uniquely short interspike interval. Previ ous authors have speculated on the functional importance of initial do ublets. Here we test the hypotheses that these doublets are associated with particular classes of motoneurones or particular physiological c onditions. 2. Discharges of inspiratory motoneurones were recorded ext racellularly in the thoracic ventral horn of anaesthetized, paralysed cats. Seventy units (35 each with axons in the internal and external i ntercostal nerves) were classified on the basis of their maximum firin g rates, start times in the respiratory cycle and axonal destination. 3. Initial doublets were defined by an interspike interval < 14 ms. Of seventeen units firing initial doublets, fifteen had axons in the ext ernal intercostal nerve and two had axons in the internal intercostal nerve. Neither maximum firing rate nor start time during the respirato ry cycle predicted the occurrence of doublets. 4. The chemical drive t o breathe was manipulated by altering the CO2 content of the inspired gas or by briefly stopping the respiratory pump. Varying the chemical drive to breathe had no consistent effect on the occurrence of initial doublets. 5. These results support the view that initial doublets are part of the normal pattern of discharge of motoneurones. However, bec ause the incidence of doublets does not consistently support previous functional hypotheses, we argue that the occurrence of doublets may no t necessarily be dictated by the CNS, but in some circumstances it is an epiphenomenon dependent on the state of the motoneurone, in particu lar on the statistical properties of its synaptic inputs.