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.
