The activity of leech motoneurons during motor patterns is regulated by intrinsic properties and synaptic inputs

The activity of motoneurons during motor patterns depends on their intrinsic properties and on synaptic inputs. This study analyzed the properties of two leech motoneurons: the excitors of dorsal longitudinal muscles (DE-3) and of dorsal and ventral longitudinal muscles (MN-L) in basal conditions (normal and high Mg²⁺ saline) and during crawling. The voltage-current relationships in DE-3 and MN-L were similar. The curves exhibited the largest slope around resting potential, showed marked inward and outward rectification, and were not affected by high Mg²⁺. In response to 5-s pulses, DE-3 exhibited a fast initial adaptation, a slow recovery and a very slow late adaptation. High Mg²⁺ abolished the initial high frequency. The frequency-voltage relationship for the rest of the response was highly similar in normal and in high Mg²⁺ saline. MN-L exhibited a minor initial adaptation and then fired steadily. High Mg²⁺ diminished the frequency-voltage relationship. During crawling DE-3 and MN-L fired in phase and their frequency-voltage curves overlapped with the lower end of the curves obtained in basal conditions. The results suggest that the activity of these motoneurons during crawling was regulated, to a large extent, by synaptic inputs.