Perhaps the major advance in our understanding of the autonomic nervous system occurred with the elaboration of the autonomic regulating functions of the hypothalamus. Small, insignificant-appearing nuclei in the walls of the third ventricle and in buried parts of the limbic cortex have rich bidirectional connections with autonomic centers in various parts of the nervous system. As indicated in Chap. 24, the hypothalamus serves as the integrating mechanism of the autonomic nervous system and limbic system. The regulatory activity of the hypothalamus is accomplished in two ways, through direct pathways that descend to particular groups of cells in the brainstem and spinal cord, and through the pituitary and thence to other endocrine glands. The supranuclear regulatory apparatus of the hypothalamus includes three main cerebral structures: the frontal lobe cortex, the insular cortex, and the amygdaloid and adjacent nuclei.
The ventromedial prefrontal and cingulate cortices function as the highest levels of autonomic integration. Stimulation of one frontal lobe may evoke changes in temperature and sweating in the contralateral arm and leg; massive lesions here, which usually cause a hemiplegia, may modify the autonomic functions in the direction of either inhibition or facilitation. Lesions involving the posterior part of the superior frontal and anterior part of the cingulate gyri (usually bilateral, occasionally unilateral) result in loss of voluntary control of the bladder and bowel. Most likely a large contingent of these fibers terminates in the hypothalamus, which, in turn, sends fibers to the brainstem and spinal cord. The descending spinal pathways from the hypothalamus are believed to lie ventromedial to the corticospinal fibers.
The insular cortex receives projections from the NTS, the parabrachial nucleus of the pons, and the lateral hypothalamic nuclei. Direct stimulation of the insula produces cardiac arrhythmias and a number of other alterations in visceral function. The cingulate and hippocampal gyri and their associated subcortical structures (substantia innominata and the amygdaloid, septal, piriform, habenular, and midbrain tegmental nuclei) have been identified as important cerebral autonomic regulatory centers. Together they have been called the visceral brain (see Chap. 24). Of particular importance in autonomic regulation is the amygdala, the central nucleus of which is a major site of origin of projections to the hypothalamus and brainstem. The anatomy and the effects of stimulation and ablation of the amygdala have been discussed in Chap. 24, in relation to the neurology of emotion.
In addition to the aforementioned central relationships, it should be noted that important interactions between the autonomic nervous system and the endocrine glands occur at a peripheral level. The best-known example is in the adrenal medulla. A similar relationship pertains to the pineal gland, in which norepinephrine (NE) released from postganglionic fibers that end on pineal cells stimulates several enzymes involved in the biosynthesis of melatonin. Similarly, the juxtaglomerular apparatus of the kidney and the islets of Langerhans of the pancreas may function as neuroendocrine transducers insofar as they convert a neural stimulus (in these cases adrenergic) to an endocrine secretion (renin, glucagon, and insulin, respectively). The numerous autonomic–endocrine interactions are elaborated in the next chapter.