UNLABELLED: Intense synaptic transmission during scratch network activity increases conductance and induces irregular firing in spinal motoneurons. It is not known whether this high-conductance state is a select feature for scratching or a property that goes with spinal motor network activity in general. Here we compare conductance and firing patterns in spinal motoneurons during network activity for scratching and swimming in an ex vivo carapace-spinal cord preparation from adult turtles (Trachemys scripta elegans). The pattern and relative engagement of motoneurons are distinctly different in scratching and swimming. Nevertheless, we found increased synaptic fluctuations in membrane potential, irregular firing, and increased conductance in spinal motoneurons during scratch and swim network activity. Our finding indicates that intense synaptic activation of motoneurons is a general feature of spinal motor network activity.

SIGNIFICANCE STATEMENT: Neurons embedded in active neural networks can enter high-conductance states with irregular firing. This was previously shown for spinal motoneurons during scratching. Because scratching is highly specialized rhythmic behavior, it is not known whether high-conductance states and irregular firing are a peculiarity for motoneurons during scratching. Here, using intracellular recordings from motoneurons in an ex vivo carapace-spinal cord preparation from adult turtles, we demonstrate that irregular firing and high-conductance states are present not only during scratching but also during swimming. Our findings suggest that irregular firing and high-conductance states could be a general feature for motor behaviors.