Homeostasis and autonomic control

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The nervous system plays the directing role in achieving homeostasis, or the maintenance of a constant internal environment. The hypothalamus and limbic systems, which are closely linked in this function, act to regulate endocrine secretion and the autonomic nervous system and to influence behavior through emotions and drives. The hypothalamus plays a central role in synthesizing information from various sources and in directing a response. It is the 'final pathway' for emotional expression and serves as the most important center for homeostasis, including autonomic and endocrine control ( CD Figure 3). Its roles include the regulation of intravascular volume, maintenance of temperature, motivation for drinking and eating, and activities involved in pleasure or aversion. In addition, it serves as a pacemaker for cyclical activities, including the wake-sleep and menstrual cycles.

CD Figure 3. Location and structure of the hypothalamus: (a) medial view of the hemisphere showing the hypothalamus in relation to the pituitary and thalamus; (b) medial view showing some of the nuclei within the hypothalamus, lying above the pituitary. (Adapted with permission from Kandel et al. (1991).

Endocrine function is regulated by the hypothalamus through its control of pituitary hormones by releasing and inhibiting hormones or factors ( Table 1). For example, thyrotropin-releasing hormone produced in the hypothalamus flows through the portal system to the anterior pituitary gland where it stimulates the release of thyrotropin, which in turn stimulates the production and release of thyroid hormone from the thyroid gland.

Table 1 Hypothalamic and pituitary factors

The autonomic nervous system is under the control of the hypothalamus and the nucleus of the solitary tract in the medulla. There are important influences from the cerebral cortex and limbic system as well as from ascending sensory pathways. The autonomic nervous system, with its three major divisions (sympathetic, parasympathetic, and enteric) (CD F.igure.,4), is largely an involuntary system. It allows the body to adjust to rapidly changing external events (the 'flight or fight'

response of the sympathetic division) and to regulate internal activities (blood pressure, temperature, airway and breathing, urinary function, and digestion by the parasympathetic and enteric divisions). The autonomic nervous system also regulates pupil size and plays a major role in reproductive activity, at least in males.

CD Figure 4. The components of the autonomic nervous system including the sympathetic (right) and parasympathetic (left) divisions. Preganglionic neurons of the sympathetic nervous system extend from the first thoracic spinal segment down to lower lumbar segments and synapse in the paravertebral sympathetic chain or in various ganglia. Postganglionic neurons travel to the target organs. Parasympathetic nervous system preganglionic neurons are located within the brainstem and in spinal cord segments S2 to S4, and travel to the target organ where they synapse with the postganglionic neurons. This diagram demonstrates the dual innervation of most organs by both divisions of the autonomic nervous system. (Adapted with permission from Kandel et al. (1991).

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