Figure 9.

Paired recordings show that alx neurons make monosynaptic connections onto motoneurons. The pictures (Aa, Ba, Ca, Da) show images of the recorded cells. Motoneurons (moto) were identified by their torn axons at the lateral edge of the ventral spinal cord. Fictive locomotion was elicited by applying a brief electrical stimulation near the ear at the time point indicated by the arrowheads (Ab, Ac, Bb, Bc, Cb, Db). In the experiments shown in A and B, the stimulation was applied on the contralateral side, whereas in the experiments shown in C and D, it was applied on the ipsilateral side. Aa–Ae, A paired recording between a strong-movement-type alx neuron and a motoneuron. Ab, Ac, Activity of the alx neuron and the motoneuron during fictive locomotion. Ac shows a close-up view near the onset of locomotion. Ad, The current-evoked spikes (the first and the fourth spikes) in the alx neuron lead to EPSPs in the motoneuron. Synaptic failures occurred for the second and third spikes. Ae, Five superimposed traces of EPSPs in the motoneuron. The traces show short, constant latency EPSPs. Only one spike in the alx neuron is shown for simplicity. Traces of EPSPs in the motoneuron are aligned with the peak of the spikes of the alx neuron. Ba–Be, A paired recording between a strong-movement-type alx neuron and a motoneuron. Bb, Bc, Activity of the alx neuron and the motoneuron during fictive locomotion. Bc shows a close-up view near the onset of locomotion. Bd, Four current-evoked spikes in the alx neuron lead to EPSPs in the motoneuron. All four EPSPs show two components as indicated by the arrowhead and arrow in the first EPSP. Be, A close-up view of the dual-component EPSP. The first component EPSP (electrical; arrowhead) occurred earlier than the peak of the presynaptic spike, whereas the second component EPSP (chemical; arrow) occurred with a very short latency. Ca–Cc, A paired recording between a strong-movement-type alx neuron and a primary motoneuron. Cb, Activity of the alx neuron and the motoneuron during fictive locomotion. The spiking activity in the motoneuron is only observed during the initial phase of the locomotor episode. Cc, Current-evoked spikes in the alx neuron lead to EPSPs in the motoneuron. Two traces are shown. Both traces show dual-component EPSPs (arrowhead and arrow). Da–Dd, A paired recording between a weak-movement-type alx neuron and a motoneuron. Db, Activity of the alx neuron and the motoneuron during fictive locomotion. Dc, A close-up view in the middle of swimming. Three cycles of swimming are shown. Dd, Current-evoked spikes in the alx neuron lead to EPSPs in the motoneuron. Four traces are shown. All of the traces show dual component EPSPs (arrowhead and arrow).