THE SURVIVAL OF AN INDIVIDUAL requires tight control of body temperature, water balance, and blood pressure, together with sufficient food intake and appropriate regulation of sleep/wakefulness cycles. Survival of a species requires that individuals be fertile, mate, and nurture their offspring, and that aggression toward others be appropriate and adaptive. Neurons in the hypothalamus control all of these key survival activities.
As we shall learn in this chapter, the hypothalamus together with interconnected areas of the brain responds to bodily and emotional challenges by recruiting appropriate behavioral and physiological responses. Coordination of these activities ensures constancy of the internal environment, a process known as homeostasis. The hypothalamus acts on three major systems: the autonomic motor system, the neuroendocrine system, and neural pathways that mediate motivated behavior.
The autonomic motor system is distinct from the somatic motor system, which controls skeletal muscle. Whereas somatic motor neurons regulate contractions of striated muscles (Chapter 31), autonomic motor neurons regulate blood vessels, the heart, the skin, and visceral organs through synapses upon smooth and cardiac muscle cells, upon glands cells that serve endocrine and exocrine functions, and upon metabolic targets such as adipocytes. The neuroendocrine system works differently, by secreting several peptide hormones from the pituitary, the “master gland,” located just beneath the hypothalamus. These pituitary hormones control water retention by the kidney, parturition, lactation, somatic growth, gamete development, and also the release of nonpeptide hormones from three downstream glands—the gonads, adrenal cortex, and thyroid.
Although largely involuntary, autonomic and neuroendocrine responses are tightly integrated with voluntary behavior executed by the somatic motor system. Running, climbing, and lifting exemplify voluntary actions that have metabolic, cardiovascular, and thermoregulatory consequences. These needs are automatically met by the autonomic and neuroendocrine systems through changes in cardiorespiratory drive, cardiac output, regional blood flow, heat dissipation, and fuel mobilization. Such compensatory changes are implemented primarily by feedforward central commands, supplemented by reflexes activated by sensory feedback. Similarly, emotional states evoke autonomic and neuroendocrine responses. Feelings of fear, anger, happiness, and sadness have characteristic autonomic and hormonal manifestations.
In this chapter, we first explore the concept of homeostasis and the general means by which it is achieved. We then discuss the anatomical and functional organization of the hypothalamus and its two “involuntary” motor arms—the autonomic and neuroendocrine systems. After that, we focus in depth on three classic examples of hypothalamic homeostatic control—regulation of body temperature, of water balance and its related deficiency drive, thirst, and of energy balance and its drive, hunger. We conclude by examining sexually dimorphic regions of the hypothalamus and their role in regulating sexual behavior, aggression, and parenting. Additional discussion of sleep cycles and regulation of circadian rhythms can be found in Chapter 44.
Homeostasis Keeps Physiological Parameters Within a Narrow Range and Is Essential for Survival
In the mid-19th century, the French ...