Hypothalamic Control Of Autonomic Outflow

As noted earlier, the hypothalamus plays an essential role in the maintenance of homeostasis. Neuroendocrine regulation of pituitary function constitutes one of the primary ways through which this is achieved. However, the autonomic nervous system (ANS) is also intimately involved in homeostatic regulation. A substantial body of work clearly indicates that the hypothalamus plays a major role in controlling the activity of the ANS. Extensive descending connections to brain stem and spinal preganglionic neurons of the ANS arise from hypothalamic nuclei, and these hypothalamic nuclei are the target of feedback regulation that is both synaptic and humoral in nature. However, it is important to emphasize that these hypothalamic nuclei function within a larger set of forebrain areas that influence autonomic outflow either indirectly by virtue of their connections with hypothalamus or directly via descending projections to autonomic nuclei in the brain stem and spinal cord. These areas include the visceral cortices (prefrontal and insular cortex) and components of the limbic system such as the amygdala.

The major hypothalamic efferents to autonomic nuclei arise in the paraventricular nucleus and the lateral hypothalamic area. Neurons in both of these regions give rise to descending projections to pregan-glionic neurons in both the sympathetic and para-sympathetic subdivisions of the spinal cord and brain stem. The paraventricular nucleus (PVN) is a particularly important integrative center for both neuroendocrine and autonomic regulation of homeostatic function. As noted earlier, parvicellular neurons in the PVN are major contributors to the neuroendocrine regulation of anterior pituitary secretion by virtue of their projections to the portal plexus in the median eminence. Additionally, magnocellular vasopressiner-gic and oxytocinergic neurons in the PVN and supraoptic nuclei project through the median eminence to terminate in the posterior lobe of the pituitary gland. These peptidergic systems are important for the regulation of fluid homeostasis and lactation and are distinct from the large numbers of PVN parvicellular neurons that give rise to descending projections to autonomic nuclei. The latter projections arise from phenotypically distinct PVN neurons sequestered within subfields of the nucleus that are devoted to autonomic function (dorsal parvicellular subdivision) or to both neuroendocrine and autonomic regulation (medial parvicellular subdivision). This juxtaposition of neurons in the PVN that contribute to homeostatic regulation via neuroendocrine or autonomic pathways makes efficient use of the sensory feedback signals that are relevant to both modes of regulation. Defining the means through which this sensory information is integrated in the PVN to influence endocrine and autonomic function remains an active and important area of research. The complexity of the integrative capacities of the PVN is illustrated in the following section on the neural control of feeding.

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