SYNAPTIC ACTIVATION OF PLATEAUS IN HINDLIMB MOTONEURONS OF DECEREBRATE CATS

Intracellular recordings were made from hindlimb motoneurons in decere brate cats to study how synaptic inputs could affect the threshold at which plateau potentials are activated with current injections through the recording microelectrode in the cell body. This study was prompte d by recent evidence that the noninactivating inward currents that reg eneratively produce the plateau potentials arise (partly) from dendrit ic conductances, which may be relatively more accessible to synaptic i nput than to current injected into the soma. Initially, cells were stu died by injecting a slow triangular current ramp intracellularly to de termine the threshold for activation of the plateau. In cells where th e sodium spikes were blocked with intracellular QX314, plateau activat ion was readily seen as a sudden jump in membrane potential, which was not directly reversed as the current was decreased. With normal spiki ng, the plateau activation (the noninactivating inward current) was re flected by a steep and sustained jump in firing rate that was not dire ctly reversed as the current was decreased. Importantly, the threshold for plateau activation (at 34 Hz on average) was significantly above the recruitment level(13 Hz on average). When tonic synaptic excitatio n [excitatory postsynaptic potentials (EPSPs)] was provided either by stretching the triceps surae muscle or by stimulating its nerve at a h igh frequency, the threshold for plateau activation by intracellular c urrent injection was significantly lowered (by 12 Hz or 5.8 mV on aver age, without and with QX314, respectively). Conversely, tonic synaptic inhibition [inhibitory postsynaptic potentials (IPSPs)], provided by appropriate nerve stimulation, significantly raised the plateau thresh old (by 19 Hz or 7.6 mV on average). These effects were graded with th e intensity of tonic EPSPs and IPSPs. Strong enough EPSPs brought the plateau threshold down sufficiently that it was activated by the intra cellular current soon after recruitment. A further increase in tonic E PSPs recruited the cell directly, and in this case the plateau was act ivated at or before recruitment. The finding that synaptic excitation can produce plateau activation below the recruitment level is of impor tance for the interpretation of its function. With this low-threshold activation, the plateau potentials are likely important in securing an effective recruitment to frequencies that produce significant force g eneration and would subsequently have no further affect on the frequen cy modulation, other than to provide a steady depolarizing bias that w ould help to sustain firing (cf. self-sustained firing). Additional ju mps in frequency after recruitment (i.e., bistable firing) would not b e expected.