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5 Neuroendocrinology 41

Vascular Supply external environment such as temperature, light, pain,
VetBooks.ir The vascular organization of the neuroendocrine system and other sensory stimuli.
In the hypothalamus, cell bodies responsible for hor-
mone synthesis are anatomically intermingled, and there
is responsible for the effectiveness of feedback regula-
tion and the hormone cascade. The hypothalamus is often redundancy in production of a single hormone
receives arterial blood from the circle of Willis, while the by separate hypothalamic nuclei. A single hypothalamic
pituitary gland is supplied by the superior and inferior hormone may exert an effect on multiple pituitary hor-
hypophyseal arteries (branches of the internal carotid mones, and multiple hypothalamic hormones may influ-
artery). The hypophyseal portal circulation connects the ence regulation of a single hypophyseal hormone. This
hypothalamus with the anterior pituitary, thereby allow- overlap and redundancy throughout the neuroendocrine
ing direct transport of hypophysiotropic hormones to system allows for a high degree of regulation.
their corresponding hypophyseal receptors. Venous
blood from the pituitary drains into the cavernous sinus Pituitary Gland
where it enters the systemic circulation to reach target
glands and tissues. Short portal veins connect the Pituitary hormone regulation is not only under hormonal
posterior and anterior lobes of the pituitary. A small feedback but is also influenced by central and peripheral
portion of pituitary outflow is thought to ascend in ret- neural input, an example of which is illustrated by lacta-
rograde to the hypothalamus where it can directly tion and nursing. Suckling activates sensory pathways
participate in regulation of hypophyseal hormone pro- leading to the hypothalamus that trigger release of PRLH
duction. It is important to note that the blood–brain and TRH, and inhibit release of dopamine (prolactin‐
barrier is incomplete surrounding the portal system, inhibiting hormone), and result in PL secretion by the
which ensures exposure to circulating hormones ema- anterior pituitary. Concurrently, suckling stimulates
nating from peripheral tissues. release of OT by the posterior pituitary, and together PRL
and OT stimulate milk production and initiate milk ejec-
tion in mammary glands, respectively. Central factors
Regulation of Hormone Secretion also influence pituitary hormone release, as is seen when
on occasion lactating mothers expel a small amount of
milk upon the thought of their baby. The images stimulate
Hypothalamus
the cerebral cortex, which results in release of OT and
The hypothalamus is responsible for integrating sensory contraction of myoepithelium in mammary gland ducts.
stimuli from the external environment with input from The majority of pituitary hormones are released in a
the internal environment to maintain homeostasis. pulsatile manner, therefore regulation of target organs
Therefore, neuroendocrine transduction is regulated not and tissues relies not only on absolute hormone concen-
only by hormonal feedback but also by neural input from trations but also on the frequency and amplitude of hor-
higher brain centers that convey information about the mone pulses.