Figure 10.

Effects of a midline section between the horizontal IBN regions on SC-evoked PSPs in an OPN. A, Experimental setup. Thick midline bar between the bilateral horizontal IBN regions indicates the rostrocaudal extent of the midline section shown in (F). B, Partial antidromic spikes were evoked at the initial segment by stimulation of the right FFH at 290 μA. C,D, Synaptic inputs from the left (C) and right SC (D) to an OPN. Short-latency PSPs could not be evoked from the caudal SC sites (C3,4 and D7,8), but disynaptic IPSPs with preceding monosynaptic EPSPs were evoked from the rostral SC sites (C1 and D5). The most rostral SC stimulation sites activated “fixation neurons” exciting this OPN (Gandhi and Keller, 1997, 1999) and vertical saccade neurons simultaneously, so that PSPs in OPNs became monosynaptic excitation followed by disynaptic inhibition. This result confirmed that vertical saccade driving signals from the rostral SC inhibit OPNs via INC IBNs. Calibration in D also applies to B and C. E, Lateral view of the brainstem showing the distribution of horizontal IBNs in the PMRF of the brainstem that were labeled transneuronally after injection of wheat germ agglutinin–horseradish peroxidase into the contralateral abducens nerve in the cat. Labeled neurons are projected onto a parasagittal plane 1.2 mm from the midline. Broken line indicates the dorsal surface of the fourth ventricle in the midline [modified from Fig. 1B in Sugiuchi et al. (2005)]. F, Lateral view of a midline section between the horizontal IBN regions reconstructed from 100-µm-thick coronal sections of the brainstem stained with thionine. This lesion covered the distribution area of the labeled IBNs in E, and sectioned their main crossing axons, because the main axons of horizontal IBNs cross the midline at almost the same level as their cell bodies (Takahashi et al., 2022a).