Response properties of motoneurones in a slice preparation of the turtle spinal cord.

1. Motoneurones in transverse sections of the turtle spinal cord were investigated in vitro with intracellular recording techniques. 2. Turtle motoneurones had a resting membrane potential of -60 to -80 mV, spike height of 90-110 mV and were able to maintain rhythmic firing during depolarization. In agreement with the size variation of the cells the input resistance and time constant ranged from 18 M omega and 12 ms to 55 M omega and 61 ms. 3. The active response properties of motoneurones included time-dependent inward rectification in response to hyperpolarizing current pulses. The action potential had an initial segment (IS) and a soma-dendritic (SD) component and was followed by a fast and a slow after-hyperpolarization (AHP) with different sensitivity to membrane potential. 4. The relation between firing rate and injected current was sigmoid when determined for the first few interspike intervals during depolarizing current pulses. Adaptation was biphasic with an early phase lasting 0.5-1 s and a late phase lasting 10-20 s. 5. The ionic conductances responsible for the active membrane properties included a tetrodotoxin (TTX)-sensitive Na+ conductance generating the action potential and a Ca2+ conductance transiently activated during the action potential. A tetraethylamonium (TEA)-sensitive K+ conductance was responsible for spike repolarization and the fast AHP. A Ca2+-dependent K+ conductance, blocked by Mn2+ and apamin, accounted for only part of the slow AHP. The time-dependent inward rectification was selectively blocked by extracellular Cs+ at concentrations below 1 mM. 6. The results show that the response properties of spinal motoneurones of the turtle are closely similar to those known from mammals in vivo. The experiments confirm and extend the identification of the ionic conductances underlying the active response properties of spinal motoneurones.