Synaptic organization of frontal eye field and vestibular afferents to interstitial nucleus of Cajal in the cat

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Abstract
The interstitial nucleus of Cajal (INC) is presumed to play an important role in vertical and torsional eye and head movements by virtue of its efferent monosynaptic projections to ocular and spinal motoneurons. Sources of synaptic input to the INC were located and its synaptic organization was studied using electrophysiological techniques. Interstitiospinal tract neurons (IST) comprised 18% of the sample and were identified by their antidromic response to stimulation of the cervical spinal cord (latency < 1.0 ms). Vestibular inputs were found in 68% of the sample. Most of these cells were excited from the contralateral labyrinth (Vc) and inhibited from the ipsilateral labyrinth (Vi). Latency measurements from intracellular recordings of Vc-evoked EPSP (1.8 ms, n = 18) and Vi-evoked IPSP (2.1 ms, n = 27) suggested a disynaptic pathway from the labyrinths to the INC. Acute bilateral lesions of the medial longitudinal fasciculus rostral to the abducens nucleus abolished vestibular responses. Only 13% of the neurons receiving vestibular input were IST neurons. Cortical inputs from the frontal eye fields (FEF) were found in 56% of the sample. The latencies of some of these responses were as short as 1.0 ms, suggesting a monosynaptic connection. IST neurons were excited from the FEF at shorter latencies (mean latency 2.4 ms, extracellular spikes) than non-IST cells (mean latency 5.1 ms for non-IST cells receiving vestibular input). Bilateral stimulation of the optic tract or the superior colliculi produced no response in INC neurons. Extracellular recordings obtained during evoked vestibular nystagmus under ketamine anesthesia showed that many INC neurons that received disynaptic vestibular and polysynaptic FEF input also discharged in relation to vertical eye movements (type A neurons). In contrast, 83% of the IST neurons did not discharge in relation to eye movements (type B neurons).

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