The neural connections of the reticular formation (RF) with the vestibular nuclei (8V) and the ascending medial longitudinal fasciculus (MLF) were studied, because many neurons in these structures carry eye-movement and head-movement (vestibular) signals and are only one or two synaptic connections removed from eye motorneurons. We used stimulating electrodes placed in specific brainstem structures and a single-neuron recording microelectrode in anesthetized or decerebrate cats. Connections were determined when the neurons were excited either forwards (orthodromically) or backwards (antidromically) by a shock. Four classes of neurons were studied. One neuronal class in the pontine RF projects axons into the ascending ipsilateral MLF; these axons terminate in the midbrain. Some of these cells receive excitation from both vestibular nerves and are probably involved in the vestibulo-ocular reflex. Another class of RF neurons projects to either the ipsilateral or the contralateral 8V. A third class located amidst lateral-rectus motorneurons in the VIth nucleus projects into the contralateral ascending MLF and excites medial-rectus motorneurons for the contralateral eye so that the two eyes move horizontally in the same direction. A fourth class located in and just beneath the 8V receives monosynaptic input from the vestibular nerve and projects into the contralateral MLF. The possible roles for these neurons in controlling eye movements are discussed. Abbreviations: MLF: medial longitudinal fasciculus; RF: reticular formation; 3: oculomotor (IIIrd cranial nerve) nucleus; 4: trochlear (IVth cranial nerve) nucleus; 6: abducens (VIth cranial nerve) nucleus; 8V: vestibular nuclear complex of the VIIth cranial nerve.

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