As would be expected of a part of the brain which integrates many different physiological functions, the various components of the hypothalamus have extensive, complex, and usually reciprocal connections with many regions of the forebrain, brainstem, and spinal cord. There are few large myelinated fibre tracts and only relatively recently have a variety of tracing techniques elucidated the more subtle connections.
Information about the light– dark cycle is passed to the hypothalamus via retino- hypothalamic fibres that originate from retinal ganglion cells and project to the suprachiasmatic nuclei. Olfactory inputs reach the hypothalamus from the pyriform cortex, amygdala, and orbitofrontal cortex; gustatory inputs from the brainstem and orbitofrontal cortex; both are involved in arousal and feeding be haviour. Visual and auditory inputs affect the hypothalamus after their significance has been assessed by the limbic system; nociceptive inputs influence hypothalamic function via brainstem connections and the medial forebrain bundle (MFB). The hypothalamus (especially the paraventricular nucleus) receives information from the viscera via ascending projections from the solitary nucleus of the medulla, which itself receives sensory input from the facial, glossopharyngeal, and vagus nerves. The MFB is actually a series of fibre bundles that run through the lateral part of the hypothalamus, interconnecting basal forebrain olfactory cortex, septum, and brainstem. The mesolimbic pathway of dopamine fibres from the ventral tegmental area to the nucleus accumbens is a component of the MFB and of the reward system. Together with mesocortical dopamine fibres it forms one of the ‘fountains’ that spread diffusely throughout the forebrain from the locus coeruleus (noradrenaline), raphe nuclei (serotonin), and basal nucleus of Meynert (acetyl choline). Brainstem reticular afferents from the central grey of the midbrain reach the hypothalamus via the dorsal longitudinal fasciculus.
T he fornix is a large bundle that connects the hippocampal formation with the septal cortex, anterior thalamus, hypothalamus, and mamillary bodies. Most of its fibres originate from pyramidal hippocampal neurons. Commissural fibres link the two hippocampi. Anteriorly, precommissural fibres pass rostral to the anterior commissure to the septal nuclei, the preoptic and dorsal hypothalamic areas; postcommissural fibres pass caudally through the lateral part of the hypothalamus to the medial mammillary nucleus. The fornix also carries cholinergic axons from the basal nucleus to the hippocampus; degeneration of these is a feature of Alzheimer dis ease. The nuclei of the amygdala are connected to many parts of the hypothalamus by two distinct pathways: the stria terminalis (which parallels the course of the fornix) ends in the bed nucleus of the stria terminalis and medial hypothalamus; the ventral amygdalofugal pathway links the basolateral amygdala to the lateral hypothalamus and MFB. These convey olfactory and visceral information to the hypothalamus and are important in mediating autonomic and endocrine responses particularly to aversive stimuli (‘fear reaction’).
Apart from its neuroendocrine efferents, most neural efferent connections of the hypothalamus are reciprocal to its afferents. Fibre tracts link hypothalamic autonomic centres to brainstem and spinal cord preganglionic autonomic neurons; descending fibres controlling sympathetic activity pass through the lateral medulla and lateral medullary (Wallenberg) lesions cause a Horner syn drome. Neurons of the posterior part of the PVN project oxytocin fibres to the sex dimorphic nuclei of the lumbosacral cord.
