The Hypothalamus And Pituitary Gland
The pituitary gland, which is under the direct control of the brain from the hypothalamus, provides endocrine control of many major physiological functions. The hypothalamus is composed of a number of nuclei (collections of cell bodies) and vaguely defined ‘areas’, and surrounds the third ventricle at the base of the medial forebrain. The most important hypothalamic areas for endocrine function are the paraventricular, periventricular, supraoptic and arcuate nuclei, and the ventromedial hypothalamus. Some of the hypothalamic neurones can secrete hormones (a process called neurosecretion), releasing chemicals in exactly the same way as other nerve cells (Chapter 7), albeit that their signals are liberated into the bloodstream rather than into synapses (Fig. 44a) The pituitary is located immediately beneath the hypothalamus and comprises three divisions: the anterior pituitary, the intermediate lobe (almost vestigial in humans) and the posterior pituitary (Fig. 44a,b). The anterior pituitary develops from tissues originating in the roof of the mouth, is non-neural and is sometimes known as the adenohypophysis. The posterior gland is really an extension from the hypothalamus itself, consists of neural tissue and is referred to as the neurohypophysis. All pituitary hormones are either peptides or proteins. As befits their developmental origins, the adeno- and neuro-hypophyses are controlled in different ways.
The anterior pituitary and intermediate lobe
The adenohypophyseal hormones and their actions are listed in Figure 44b. They are released under the control of chemical signals (hypothalamic releasing or inhibiting hormones) originating from small (parvocellular) neurones with their cell bodies in the hypothalamus (Fig. 44a–c). These hormones are peptides or proteins released into the blood at the median eminence (Fig. 44a) when the appropriate parvocellular neurones are electrically active. The hypothalamic hormones are transported directly to the anterior pituitary via the hypophyseal portal vessels (Fig. 44a). The portal vessels carry hypophysiotropic signals directly to the anterior pituitary to stimulate or inhibit the release of pituitary hormones by the activation of receptors on specific groups of pituitary cells (Fig. 44b). It should be noted that some hypothalamic hormones control more than one pituitary hormone. Figure 44c illustrates the basic principles that underlie the control of anterior pituitary hormones; this is a form of chemical cascade that allows for the precise control of pituitary output with two stages of signal amplification: first, at the pituitary itself, where tiny amounts of hypothalamic hormones control the release of larger quantities of pituitary hormone; and then at the final target gland, where the pituitary signals stimulate the release of still larger quantities of hormones such as steroids. The cascade allows for feedback control of hormone release at several points. The final hormone (and often some of the intermediate signals) inhibits further activity in the axis to provide the fine regulation of hormone release (Fig. 44c). This is a characteristic feature of anterior pituitary control systems.
The posterior pituitary
The posterior gland secretes two peptide hormones: oxytocin and antidiuretic hormone (ADH; also known as vasopressin). The hormones are manufactured in the cell bodies of large (magnocellular) neurones in the supraoptic and paraventricular nuclei of the hypothalamus, and are transported down the axons of these cells to their terminals on capillaries originating from the inferior hypophyseal artery within the posterior pituitary gland (Fig. 44a). When magnocellular neurones are activated (see Chapters 35, 52 and 53), they release oxytocin or ADH into the general circulation, from whence they can reach the relevant target tissues to produce the required effect. The signals that drive the release of posterior gland hormones are entirely neural, so that the hormones are said to be involved in neuroendocrine reflexes. These hormones operate over shorter time courses (minutes) than most endocrine events (hours to days). The release of ADH is controlled by conventional negative feedback mechanisms based on plasma osmolality and blood volume (Chapter 35). Oxytocin, however, is involved in positive feedback mechanisms (Chapters 52 and 53).
Pulsatile release of pituitary hormones Hormones released from the hypothalamus tend to appear in the blood in discrete pulses, rather than as continuous secretions. This is achieved by the synchronous activation of hormone-releasing neurones of the hypothalamus. As will be seen in later chapters, episodic release has profound implications for the operation of the endocrine system. It also raises a number of interesting and as yet unanswered questions as to how many separate and more or less widely scattered neurones can be activated simultaneously to give rise to pulsatile release.