Algebraical summation in synaptic activation of motoneurones firing within the 'primary range' to injected currents.

1. In intracellular studies of cat lumbar motoneurones constant synaptic stimuli such as stretch, contraction or a high-frequency stimulation of a cut afferent nerve have been superimposed on firing in response to injected currents.2. As long as the slope relating spike frequency to injected current remained constant, which by definition is the ;primary range' of firing, algebraical summation of superimposed synaptic stimulation prevailed. Added quantities in these experiments were then between 2.0 and 56.2 impulses/sec for excitation, between -2.8 and -21.8 impulses/sec for inhibition.3. Data were obtained correlating firing rates with amount of synaptic potential and current respectively.4. Theoretical implications are dealt with in the Discussion.