The pituitary has been called the ‘master endocrine gland’. It lies immediately below the brain, tucked into the bony portion of the skull that serves as the floor that supports the brain, and in one sense is almost an extension of the brain itself. Direct connections exist between it and the bottom structure of the DIENCEPHALON, the HYPOTHALAMUS. These connections are of two types. The first is direct neural projections, in which neurons that lie in the hypothalamus(in particular within the PARAVENTRICULAR NUCLEUS OF THE HYPOTHALAMUS and the SUPRAOPTIC NUCLEUS OF THE HYPTHALAMUS) send their axons (projection fibres) directly through the stalk (INFUNDIBULUM) that connects the brain to the pituitary and into the anterior portion of the pituitary gland, where they may stimulate endocrine cells to secrete HORMONES into the general BLOOD circulation. The second type of connection involves a special set of blood vessels that travel from the hypothalamus to the pituitary (HYPOTHALAMO-PITUITARY PORTAL SYSTEM) which acts as a porter to allow chemical neurosecretory signals secreted by neurons in the hypothalamus (see NEUROSECRETION) to be passed directly to cells in the anterior portion of the pituitary gland. These chemicals stimulate cells in this portion of the pituitary to secrete their own hormones into the general blood supply. In this way, groups of neurons in the hypothalamus can activate the pituitary as fingers playing upon a keyboard: causing a particular combination of pituitary hormones to be released, and any combination can be produced depending only upon the combination of hypothalamic command neurons that are activated.
Thus the pituitary acts as an interface between the brain and the rest of the body, broadcasting signals via chemical hormones to receptors that may lie anywhere else. Pituitary hormones are particularly important for controlling physiological events relevant to set of basic motivational states such as SEXUAL BEHAVIOUR, MATERNAL BEHAVIOUR, STRESS, THIRST, and SALT APPETITE. For example, the secretion of SEX HORMONES by the OVARIES of females or the TESTES of males is regulated by groups of cells in the anterior pituitary, which secrete LUTEINIZING HORMONE when the hypothalamus commands. In females, luteinizing hormone secretion occurs in cyclic pulses, and leads to the conditions required for conception. In males, the hormone has a different name but is released continuously to stimulate the testes. The difference between luteinizing hormone secretion in males and females is not caused by differences in their pituitary glands themselves, but instead by differences in their hypothalamic signals that control the pituitary. Other endocrine cells in the anterior pituitary secrete the stress hormone, adrenocorticotropin (ACTH) when stimulated by the hypothalamic-pituitary portal system. Conversely, in the posterior pituitary some cells secrete VASOPRESSIN (also known as antidiuretic hormone) which can stimulate thirst and cause the body to retain fluid, when neurally stimulated by the hypothalamus. Others secrete OXYTOCIN, which promotes lactation, maternal behaviour, and may also be activated in sexual behaviour.
KENT C.BERRIDGE
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