The ANS, like the somatic nervous system, is organized in segmental levels. Neurons in the cerebral cortex, basal forebrain, hypothalamus, midbrain, pons, and medulla participate in autonomic control. Autonomic neurons are reciprocally interconnected by neurochemically complex pathways and constitute a functional unit referred to as the central autonomic network (CAN).y , [4 The CAN integrates visceral, humoral, and environmental information to produce coordinated autonomic, neuroendocrine, and behavioral responses to external or internal stimuli. Within the CAN, there is segmental and regional specialization for the control of autonomic functions. A coordinated response is generated through interconnections among the amygdala and the neocortex, basal forebrain, ventral striatum, hypothalamus, and autonomic and somatic motor nuclei of the brain stem.y [4 [5
Figure 21-1 (Figure Not Available) Organization of visceral and somatic afferents in the spinal corReproduced from Westmoreland BF, et al: Medical Neurosciences: An Approach to Anatomy, Pathology, and Physiology by Systems and Levels, 3rd ed. Boston, Little, Brown and Company, 1994. By permission of Mayo Foundation.)
The insular and medial prefrontal cortices (paralimbic areas) and nuclei of the amygdala are the higher centers involved in the processing of visceral information and the initiation of integrated autonomic responses. The insular cortex is the primary viscerosensory cortex and receives viscerotopically organized sensory information from visceral and arterial chemoreceptors and baroreceptors.  The ventromedial prefrontal and anterior cingulate cortices initiate autonomic responses associated with affective behavior. The amygdala receives exteroceptive and interoceptive information and provides it with emotional significance. M
The hypothalamus integrates the autonomic and endocrine responses that are critical for homeostasis. It is functionally divided into three longitudinal zones: a periventricular zone involved in circadian rhythms and endocrine responses, a medial zone involved in the control of sexual function, osmoregulation, and thermoregulation, and a lateral zone involved in behavioral arousal [8 (Fig. 21-2 (Figure Not Available) ). All three zones project to the autonomic nuclei of the brain stem and spinal cord.y
The paraventricular hypothalamic nucleus is the site of integrated autonomic and neuroendocrine responses to stress. It contains different subpopulations of "effector" neurons including magnocellular neurons that secrete vasopressin, parvicellular neurons that produce corticotrophic-releasing hormone and other regulatory hormones that control pituitary function, and neurons that innervate all brain stem and spinal autonomic nuclei. [d The medial preoptic nucleus contains thermosensitive neurons that initiate autonomic, endocrine, and motor responses. Warm-sensitive neurons, excited by an increase in core temperature, initiate responses such as skin vasodilatation, sweating, and slow-wave sleep, which lead to heat loss. Cold-sensitive neurons initiate vasoconstriction and shivering. In humans, these thermoregulatory sudomotor and vasomotor pathways descend in the equatorial plane of the lateral column of the spinal cord.
The ventromedial nucleus is involved in energy metabolism and reproduction. In animals, ventromedial hypothalamic lesions result in obesity, which is associated with a decrease in sympathetic and an increase in parasympathetic nervous system activity. Finally, the lateral hypothalamic area participates in arousal, motivated motor behavior, and autonomic control and has widespread projections. The hypothalamic autonomic pathways descend predominantly ipsilaterally in the dorsomedial and ventrolateral tegmentum of the brain stem.
The periaqueductal gray matter (PAG) of the midbrain is the site of integrated autonomic, behavioral, and antinociceptive stress responses. It is organized into separate columns that control specific patterns of response to stress. y The lateral PAG mediates sympathoexcitation, opioid-independent analgesia, and motor responses consistent with the fight-or-flight reaction. The ventrolateral PAG produces sympathoinhibition, opioid-dependent analgesia, and motor inhibition. The PAG constitutes a critical relay
Figure 21-2 (Figure Not Available) Subdivisions of the hypothalamu (Reproduced from Westmoreland BF, et al: Medical Neurosciences: An Approach to Anatomy, Pathology, and Physiology by Systems and Levels, 3rd ed. Boston, Little, Brown and Company, 1994. By permission of Mayo Foundation.)
station for the micturition reflex (see later discussion on Bladder). BRAIN STEM
Neurons in the medulla are critical for the control of cardiovascular, respiratory, and gastrointestinal functions. The medullary nucleus of the solitary tract (NTS) is the first relay station for the arterial baroreceptors and chemoreceptors, as well as cardiopulmonary and gastrointestinal afferents. y The NTS integrates multiple autonomic reflexes and projects to other central autonomic regions. Neurons in the reticular formation of the ventrolateral medulla are arranged in distinct groups that maintain cardiomotor, vasomotor, and respiratory functions. Neurons in the rostral ventrolateral medulla project directly to sympathetic preganglionic neurons and maintain basal arterial blood pressure, y while neurons in the ventral medulla are critical for respiratory rhythmogenesis. The nucleus ambiguous provides parasympathetic innervation to the heart. y
These ventromedullary vasomotor, cardiomotor, and respiratory neurons form a network that is coordinated via interneurons of the medullary reticular formation. Together with the NTS, this network is involved not only in the generation of respiratory and cardiovascular rhythms but also in complex motor patterns such as vomiting, swallowing, sneezing, and coughing. y The descending reticulospinal pathways for the automatic control of breathing and blood pressure, as well as those for sweating and micturition, run in the ventral half of the white matter of the lateral columns of the spinal cord.
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