Hypothalamic pathways involved in metabolic regulatory functions, as identified by track-tracing methods.

The present review of the fiber connections of the hypothalamus has been concerned basically with recent data obtained by the aid of the autoradiographic and HRP tracer techniques. Evidence presented has shown that, besides confirming many of the older data, recent studies have resulted in the introduction of several conceptual modifications into the classic picture of hypothalamic hodological relationships. Among these conceptual modifications, the following can be mentioned: (1) the medially placed nuclei of the hypothalamus have a great number of long efferent and afferent connections with many extrahypothalamic structures; (2) many hypothalamic nuclei send direct projections to cell territories in the brainstem and spinal cord that contain preganglionic autonomic motor neurons; (3) several neural districts that lie caudal to the mesencephalon send direct projections to the hypothalamus; (4) in addition to the olfactory channel, other sensory pathways (including interoceptive and gustatory conduction lines) have a relatively direct access to hypothalamic mechanisms; (5) the hypothalamus sends fibers to several brainstem territories that give rise to widespread monoaminergic projections; and (6) there are anatomical pathways that establish reciprocal connections between the hypothalamus and the basal ganglia. Some of the possible physiological correlates of these anatomical findings in the context of metabolic regulatory functions have been briefly indicated.