Functional Anatomy of the Hypothalamus and Pituitary

In mammals and man, historical investigation suggests that early recognition for a role of the hypothalamus as a site for integration of endocrine, autonomic and behavioral responses can be dated to the 2 -18 centuries A.D. Although the hypothalamus comprises only 2% of the total brain volume, it is a key regulator of pituitary function and homeostatic balance. In this chapter, we provide an overview of the historical landmarks, embryologic, gross, microscopic and functional anatomy of the mammalian and human hypothalamus and pituitary, and how the hypothalamus relates to the rest of the brain and responds to peripheral signals. In particular, we show that its rostral, nuclear portion exerts prominent regulation of homeostatic behaviors related to energy balance and reproduction. The two caudal portions are primarily involved in ensuring adequate metabolic resources for defensive and exploratory behaviors and responses to sudden changes in endogenous and exogenous stimuli. In addition, we discuss how its network of neurons is made of cells with different functions (neurosecretory, autonomic, motor), how they interact, and how these neural circuitries are woven into a complex architecture of conduits for the movement of intercellular fluids (vasculature, glymphatic channels, meningeal lymphatic vessels). Finally, we focus on the hypothalamic mechanisms involved in the regulation of anterior and posterior pituitary secretion (hypothalamic tuberoinfundibular and neurohypophysial systems), as well those involved in food and fluid intake, lactation, thermoregulation, circadian rhythmicity and the sleep-wake cycle.