The Hypothalamo-Pituitary Axis The

The pituitary gland was considered for a long time as the maestro
gland because of its dynamic role in regulating the body needs and its
control of important glands. The hormones secreted by the pituitary gland
regulate either other major glands or affect most, if not all, cells of the body
i.e. they have a general effect.
With the progress in science, the question of what regulates the
pituitary gland itself was answered. It was found that a part of the brain, the
hypothalamus, controls the pituitary gland. The hypothalamus is part of the
brain that regulates autonomic nervous reflexes and temperature control. The
hypothalamus is now also considered as an endocrine gland.
The hypothalamus receives signals from the higher centers of the brain,
translates them and sends the appropriate messengers (neurohormones) to the
pituitary gland, which in turn secretes the necessary hormone.
As mentioned previously neurohormones are so called because they
are synthesized and secreted by nerve cells into the blood circulation to
affect its target cell by means of a chemical mechanism, and not in a synapse
to affect another nerve cell or muscle by means of a nerve impulse.
The pituitary gland lies at the base of the brain, below the
hypothalamus. It weighs 0.5- 0.8 grams in the human and is slightly larger in
females. Another name for the pituitary is the hypophysis - so named
because it seems to be growing on a stalk (neck) underneath the
hypothalamus. It is connected to the hypothalamus by this stalk containing
nerve fibers and blood vessels. The pituitary gland is composed of two
lobes- anterior pituitary (adenohypophysis) and posterior pituitary
(neurohypophysis).
The posterior pituitary is an outgrowth of the hypothalamus and is
composed of nervous tissue. That is why the posterior pituitary is referred to
as neurohypophysis. The two hormones, vasopressin (antidiuretic hormone,
ADH) and oxytocin, are synthesized in the hypothalamus, but are stored and
secreted by the posterior pituitary.
The anterior pituitary is ectodermal in origin and secretes six
hormones. The regulation of these hormones is controlled by five releasing
and two inhibiting hypothalamic neurohormones. They are so called because
they are synthesized in nerve cells in the hypothalamus. A system of small
blood vessels carries these hormones from the hypothalaums to the anterior
pituitary.
Regulation of Hormone Secretion:
The hypothalamic neurohormones, with the exception of one case,
control the anterior pituitary hormones in a three hormone sequence. The
sequence begins with (1) a hypothalamic hormone, which controls the
secretion of (2) an anterior pituitary hormone, which controls the secretion
of (3) a hormone from some other endocrine gland. This system is controlled
mostly by a negative feedback mechanism exerted upon the hypothalamus
and/or pituitary gland by one or more of the hormones in the sequence.
Precise regulation of the three hormone sequence system keeps the
hormones at levels that maintain homeostasis.
Table 3: Hypothalamic hormones and the pituitary hormones they
control