Electrophysiological study of projections from mesencephalic central gray matter to forebrain in the rabbit

An electrophysiological study has been made of pathways from mesencephalic central gray to forebrain in sixteen rabbits. Attention was focussed on those potentials evoked by electrical stimulation of central gray which, on the basis of latency (less than 7 msec), stability to high-frequency stimulation, and resistance to barbiturate anesthesia or anoxemia, were considered to be mediated by direct fibers. Such responses had a relatively limited distribution, being mainly confined to the hypothalamus, although they were also obtained from caudate nucleus and thalamus. Responses of a different type were recorded in the tuberal region of the hypothalamus, consisting of an initial small component of short latency (sometimes biphasic), followed by a broad wave. Whereas the initial components were depressed by barbiturate, the broad component increased in amplitude and duration. The present study suggests that the dorsal longitudinal fasciculus and periventricular-diencephalic system of fibers may be an important afferent pathway to the hypothalamus, including the rostral portion of the latter and the hypothalamohypophyseal system.