The extensive dendritic tree of the adult spinal motoneuron generates a powerful persistent inward current (PIC). We investigated how this dendritic PIC influenced conversion of synaptic input to rhythmic firing. A linearly increasing, predominantly excitatory synaptic input was generated in triceps ankle extensor motoneurons by slow stretch (duration: 2-10 s) of the Achilles tendon in the decerebrate cat preparation. The firing pattern evoked by stretch was measured by injecting a steady current to depolarize the cell to threshold for firing. The effective synaptic current (IN, the net synaptic current reaching the soma of the cell) evoked by stretch was measured during voltage clamp. Hyperpolarized holding potentials were used to minimize the activation of the dendritic PIC and thus estimate stretch-evoked IN for a passive dendritic tree(IN.PASS). Depolarized holding potentials that approximated the average membrane potential during rhythmic firing allowed strong activation of the dendritic PIC and thus resulted in marked enhancement of the total stretch-evoked IN (IN.TOT). The net effect of the dendritic PIC on the generation of rhythmic firing was assessed by plotting stretch-evoked firing (strong PIC activation) versus stretch-evoked IN.PASS (minimal PIC activation). The gain of this input-output function for the neuron (I-ON) was found to be ∼2.7 times as high as for the standard injected frequency current (F-I) function in low-input conductance neurons. However, about halfway through the stretch, firing rate tended to become constant, resulting in a sharp saturation in I-ON that was not present in F-I. In addition, the gain of I-ON decreased sharply with increasing input conductance, resulting in much lower stretch-evoked firing rates in high-input conductance cells. All three of these phenomena (high initial gain, saturation, and differences in low- and high-input conductance cells) were also readily apparent in the differences between stretch-evoked IN.TOT and IN.PASS and thus could be accounted for by the activation of the dendritic PIC. As a result, stretch-evoked IN.TOT and F-I provided an accurate prediction of the overall change in stretch-evoked firing. However, in about half of the low-input conductance cells, the rate of rise of firing in response to stretch was not smoothly graded but instead consisted of a rapid surge. Stretch-evoked IN.TOT was always smoothly graded. This suggests that although stretch-evoked IN.TOT can be used to predict the overall change in firing, prediction of the dynamics of firing may be less accurate.
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