Different species of alpha motoneurons in same pool: further evidence from effects of inhibition on their firing rates.

1. Single units of the plantaris pool were isolated in ventral root filaments of decerebrate cats and their critical firing levels (CFLs) were determined. Motoneurons of similar size were compared in firing rate (FR) during repetitive stimulation of the plantaris nerve to establish control values and also during added stimulation of various inhibitory nerves (sural, hamstrings, or peroneal). 2. Criteria, based on maximal firing rate, were developed whereby certain pairs of units of similar size could be reliably classified into different types. 3. A second, independent set of criteria was formulated by which the same pair of units could be classified according to their responses to added inhibitory inputs. 4. The ability to distinguish motoneurons consistently by more than one set of criteria reinforces the evidence that different physiological types of units exist within a single motoneuron pool. 5. The findings indicate that different types of cells either receive different densities of input from certain inhibitory sources or that they react differentially to the same amounts of these inputs.