Amplification and linear summation of synaptic effects on motoneuron firing rate

The aim of this study was to measure the effects of synaptic input on moton euron firing rate in an unanesthetized cat preparation, where activation of voltage-sensitive dendritic conductances may influence synaptic integratio n and repetitive firing. In anesthetized cats, the change in firing rate pr oduced by a steady synaptic input is approximately equal to the product of the effective synaptic current measured at the resting potential (I-N) and the slope of the linear relation between somatically injected current and m otoneuron discharge rate (f-I slope). However, previous studies in the unan esthetized decerebrate cat indicate that firing rate modulation may be stro ngly influenced by voltage-dependent dendritic conductances. To quantify th e effects of these conductances on motoneuron firing behavior, we injected suprathreshold current steps into medial gastrocnemius motoneurons of decer ebrate cats and measured the changes in firing rate produced by superimpose d excitatory synaptic input. In the same cells, we measured I-N and the f-I slope to determine the predicted change in firing rate (DeltaF = I-N * f-I slope). In contrast to previous results in anesthetized cats, synaptically induced changes in motoneuron firing rate were greater-than-predicted. Thi s enhanced effect indicates that additional inward current was present duri ng repetitive firing. This additional inward current amplified the effectiv e synaptic currents produced by two different excitatory sources, group Ia muscle spindle afferents and caudal cutaneous sural nerve afferents. There was a trend toward more prevalent amplification of the Ia input (14/16 cell s) than the sural input (11/16 cells). However, in those cells where both i nputs were amplified (10/16 cells), amplification was similar in magnitude for each source. When these two synaptic inputs were simultaneously activat ed, their combined effect was generally very close to the linear sum of the ir amplified individual effects. Linear summation is also observed in media l gastrocnemius motoneurons of anesthetized cats, where amplification is no t present. This similarity suggests that amplification does not disturb the processes of synaptic integration. Linear summation of amplified input was evident for the two segmental inputs studied here. If these phenomena also hold for other synaptic sources, then the presence of active dendritic con ductances underlying amplification might enable motoneurons to integrate mu ltiple synaptic inputs and drive motoneuron firing rates throughout the ent ire physiological range in a relatively simple fashion.