Projecting to the Contralateral Trochlear and Oculomotor Nucleus

22 Neurons in the interstitial nucleus of Cajal (INC) that are known to be involved in eye 23 and head movements are excitatory. We investigated the input-output organization of 24 inhibitory INC neurons involved in controlling vertical saccades. Intracellular recordings 25 were made in INC neurons activated antidromically by stimulation of the contralateral 26 trochlear or oculomotor nucleus, and their synaptic input properties from the superior colliculi 27 (SCs) and the contralateral INC were analyzed in anesthetized cats. Many INC neurons 28 projected to the contralateral trochlear nucleus, Forel’s field H (FFH), INC and oculomotor 29 nucleus, and mainly received monosynaptic excitation followed by disynaptic inhibition from 30 the ipsiand contralateral SCs. After sectioning the commissural connections between the 31 SCs, these neurons received monosynaptic excitation from the ipsilateral medial SC and 32 disynaptic inhibition via the INC from the contralateral lateral SC. Another group of INC 33 neurons were antidromically activated from the contralateral oculomotor nucleus, INC and 34 FFH, but not from the trochlear nucleus, and received monosynaptic excitation from the 35 ipsilateral lateral SC and disynaptic inhibition from the contralateral medial SC. The former 36 group was considered to inhibit contralateral trochlear and inferior rectus motoneurons in 37 upward saccades, whereas the latter was considered to inhibit contralateral superior rectus and 38 inferior oblique motoneurons in downward saccades. The mutual inhibition existed between 39 these two groups of INC neurons for upward saccades on one side and downward saccades on 40 the other. This pattern of input-output organization of inhibitory INC neurons suggests that 41 the basic neural circuits for horizontal and vertical saccades are similar. 42