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The hypothalamo-pituitary axis is the
unit formed by the hypothalamus and pituitary
gland, which exerts control over many parts of
the endocrine system. This unit functions by means of
interaction of the nervous and endocrine systems whereby
the nervous system regulates the endocrine system and
endocrine activity modulates the activity of the CNS.  The Hypothalamus
The hypothalamus has many functions and
is one of the major regulators of homeostasis.
- It controls the autonomic nervous
system, acts with the limbic system to regulate
emotional and behavioural patterns, regulates
eating and drinking, controls body temperature
and regulates diurnal rhythms. It also controls
pituitary gland secretions.
- The hypothalamus receives input
from the external and internal environment as
well as having its own receptors. It receives
stimuli from the somatic and visceral sense
organs. These inputs travel via the medulla
oblongata and reach the hypothalamus through
innervation by fibres producing dopamine,
adrenaline, noradrenaline, serotonin and
acetylcholine as well as fibres releasing
neuropeptides such as enkephalins, NPY,
neurotensin, dynorphins and endorphins.
- The release of hormones from the
pituitary is therefore subject to many different
stimuli from 'higher centres' acting on the
hypothalamus.
In response to stimuli such as stress, pain and
emotions, the hypothalamus can exert effects on
the anterior and posterior pituitary gland in
order to respond rapidly to environmental change
as well as to feedback from internal systems.
The hypothalamus is the least well
understood area of the hypothalamo-pituitary axis and
research into certain aspects of its function is ongoing.
An example is current work involving the recently
discovered peptide hormone, Leptin, which has shed
light on the existance of an adipose tissue- brain
endocrine axis.
The hypothalmus exerts its effects on the pituitary gland
in two different ways:
Posterior
pituitary
The hypothalamic paraventricular and supraoptic nuclei
produce secretory droplets, which are the first stage in
formation of the neuropeptides vasopressin and oxytocin.
The paraventricular nuclei produce mostly oxytocin and
the supraoptic mostly vasopressin although both hormones
may also be produced by the other type of nuclei. These
hormones are packaged with the protein neurophysin into
granules which then move down the axon and are stored in
the posterior pituitary. Following stimulation of the
hypothalamus these hormones are then released into the
bloodstream. Small amounts of the hormones also enter the
portal blood supply.
Anterior
pituitary
Control of the anterior pituitary is not via a nervous
link. It was discovered in the 1930s that the
hypothalamus is linked to the anterior pituitary by a
network of microcapillaries - the hypophyseal portal
vessels. Control is maintained by release of hypothalamic
hormones, some of which stimulate release and others
inhibit release
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