Medial forebrain bundle and “feeding centers” of the hypothalamus

Phyletic, architectonic, and cytotopographic studies, in conjunction with physiological, psychological, and pharmacological investigations, attest that the limbic system represents a primitive neural evolvement concerned with the integration of affectively determined behavior patterns. These studies suggest that several integrants of the limbic system, including the amygdaloid complex of nuclei, are intimately concerned with self-preservation, especially as i t pertains to the organization of feeding behavior and to the survival mechanisms involved in obtaining, incorporating, and assimilating food (MacLean, ’ 5 t h ) . This primogenial system, in which the processes of motivation may have their physiological correlates (MacKay, ’59), is strongly related to the hypothalamus principally by components of the medial forebrain bundle which courses posteriorly’ with the lateral hypothalamic area serving as its bed nucleus. Nauta (’58) has defined the medial forebrain bundle as the cardinal connection of the septum and preoptic regions with the midbrain, considering it a common trajectory for mingled ascending and descending projections of various origins. By means of the several components of this bundle the basal telencephalic centers maintain strong reciprocating connections via the hypothalamus with the central gray and paramedian reticulum of the midbrain. This system of fibers is hence extremely heterogeneous anatomically as well as, no doubt, functionally. It is a difficult complex to analyze and fractionate anatomically due to the great diversity of fiber origin and trajectory and its tendency to relay at all levels. Contained in the system are long axons and numerous synaptic chains of shorter connections and microscopic examination of the entire lateral hypothalamic area reveals long dendrites and axons weaving through the fibers of the medial forebrain bundle with many axons passing into the medial hypothalamus (including the ventromedial nucleus) and periventricular system. Of the many fibers in this bundle which enter the midbrain, some have their origin in the lateral preoptic region and septum, but a far greater number appear to arise progressively as the bundle proceeds more caudally through the lateral hypothalamic area (Guillery, ’57). The entire lateral preoptic and lateral hypcthalamic regions accordingly compose a continuous, somewhat diffuse, longitudinal cell group intercalary to the components of the medial forebrain bundle, the lateral hypothalamic “n~cleus”~ itself including nerve cells in the interstices of this bundle with which it is practically co-extensive. The lateral hypothalamic area which has been defined in several animal species, including rats and cats (Anand and Brobeck, ’sl), monkeys (Anand, Dua and Shoenberg, ’55), and chickens (Feldman, Larsson, Dimick and Lepkovsky, ’57), as a ‘feeding center” at the level of the coronal plane of the median eminence and ventromedial nuclei of the hypothalamus, is thus apparently inextricably associated with Iongitudinal fibers of passage comprising the medial forebrain bundle. Krieg (personal communication) feels that this area must act both as an interstitial and relaying “nucleus” for this system of fibers. Anand and Brobeck‘s data suggest, however, that