Figure 22-15 (a) Location of the hypothalamus and pituitary gland. (b) Details of the hypothalamus-pituitary system. Signals arriving from connecting neurons stimulate the hypothalamus to secrete hormones destined for the anterior pituitary into a special blood vessel, which carries the hormones directly to a capillary network in the anterior pituitary. In response to each hypothalamic hormone, the anterior pituitary releases its appropriate hormone into the general circulation. Posterior pituitary hormones are made in neurons arising in the hypothalamus, transported in axons to nerve endings in the posterior pituitary, and stored there until released into the blood in response to a neuronal signal.
The hypothalamic hormones pass directly to the nearby pituitary gland through special blood vessels and neurons that connect the two glands (Fig. 22-15b). The pituitary gland has two functionally distinct parts. The anterior pituitary responds to hypothalamic hormones carried in the blood, by producing six tropic hormones or tropins (from the Greek tropos, meaning "turn"), relatively long polypeptides that activate the next rank of endocrine glands (Fig. 22-14). Adrenocorticotropic hormone (ACTH, also called corticotropin; Mr 4,500) stimulates the adrenal cortex; thyroid-stimulating hormone (TSH, also called thyrotropin; Mr 28,000) acts on the thyroid gland; follicle-stimulating hormone (FSH; Mr 34,000) and luteinizing hormone (LH; Mr 20,500) act on the gonads; and growth hormone (GH, also called somatotropin; Mr 21,500) stimulates the liver to produce several growth factors.
Lehninger-Nelson-Cox: Principles of Biochemistry, 752.o.
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