Associations between the passive, transitional, and active properties of turtle motoneurons

The literature on the associations between the properties of vertebrate spinal motoneurons (MNs) is dominated by studies on the deeply anesthetized cats, with the measurements made with low-resistance microelectrodes, and limited largely to passive (cell at rest) vs. transitional (rest-to-rheobase action potential) properties. There has been far less consideration of active (repetitive-firing) firing properties, like the parameters of the stimulus current-spike frequency (I-f) relation. The present study shows that several conventionally accepted associations among MN properties, including those between the determining and defining parameters of the I-f relation, are either absent or weak when the measurements are made with high-resistance microelectrodes, and using unanesthetized slices of the adult turtle's spinal cord. The strength of such associations is even further diminished when the MNs are exhibiting modulator-induced plateau potentials. These novel and potentially controversial findings invite consideration of their relation to literature on MN repetitive firing properties, as recorded with sharp microelectrodes in spinal and brainstem slices of several other vertebrate species, including neonatal and adult mammals.