Page 293 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Hypothalamic–Pituitary System mothers, suckling stimulates nerve endings in
the nipples, triggering the secretion of oxy-
In the hypothalamus, (1) humoral signals from tocin (and prolactin, ! p. 303) via neuro-
the periphery (e.g., from circulating cortisol) humoral reflexes.
can be converted to efferent neuronal signals, Releasing hormones (RH) or liberins that
and (2) afferent neuronal signals can be con- stimulate hormone release from the adenohy-
verted to endocrine messengers (neurosecre- pophysis (Gn-RH, TRH, SRH, CRH; ! p. 270ff.)
tion). are secreted by hypothalamic neurons into a
kind of portal venous system and travel only a
The first case is possible because the hypothalamus is short distance to the anterior lobe (! A). Once
situated near circumventricular organs like the or-
Hormones and Reproduction no blood-brain barrier there, hydrophilic peptide temic circulation (! A). Some anterior pitui-
in its vascular network, they trigger the release
ganum vasculosum laminae terminalis (OVLT), sub-
of anterior pituitary hormones into the sys-
fornical organ, the median eminence of the hypo-
thalamus, and the neurohypophysis. Since there is
tary hormones are regulated by release-inhib-
hormones can also enter.
iting hormones (IH) or statins, such as SIH and
PIH = dopamine (! p. 270ff.). Peripheral hor-
The hypothalamus is closely connected to
mones, ADH (see above) and various neu-
other parts of the CNS (! p. 330). It controls
rotransmitters such as neuropeptide Y (NPY),
many autonomous regulatory functions and
norepinephrine (NE), dopamine, VIP and
tions. The hypothalamus is related to the
functions (! p. 272).
sleeping-waking rhythm (! p. 334) and to
The four glandotropic hormones (ACTH,
psychogenic factors. Stress, for example,
11 its neuropeptides influence higher brain func- opioids also help to regulate anterior pituitary
stimulates the release of cortisol (via CRH, TSH, FSH and LH) and the aglandotropic hor-
ACTH) and can lead to the cessation of hor- mones (prolactin and GH) are secreted from
mone-controlled menstruation (amenorrhea). the anterior pituitary (! A). The secretion of
Neurosecretion. Hypothalamic neurons growth hormone (GH = somatotropic hor-
synthesize hormones, incorporate them in mone, STH) is subject to control by GH-RH, SIH
granules that are transported to the ends of the and IGF-1. GH stimulates protein synthesis
axons (axoplasmic transport ! p. 42), and (anabolic action) and skeletal growth with the
secrete them into the bloodstream. In this way, aid of somatomedins (growth factors formed in
oxytocin and ADH are carried from magno- the liver), which play a role in sulfate uptake by
cellular hypothalamic nuclei to the neurohy- cartilage. Somatomedin C = insulin-like
pophysis, and RHs and IHs (and ADH) reach the growth factor-1 (IGF-1) inhibits the release of
median eminence of the hypothalamus (! A). GH by the anterior pituitary via negative feed-
The action potential-triggered exocytotic re- back control. GH has lipolytic and glycogeno-
lease of the hormones into the bloodstream re- lytic actions that are independent of soma-
sults in Ca 2+ influx into the nerve endings tomedin activity.
(! p. 50ff.). Proopiomelanocortin (POMC) is a peptide precur-
Oxytocin (Ocytocin) and antidiuretic hor- sor not only of ACTH, but (inside or outside the ante-
mone (ADH) are two posterior pituitary hor- rior pituitary) also of !-endorphin and α-melano-
mones that enter the systemic circulation cyte-stimulating hormone (α-MSH = α-melanocor-
directly. ADH induces water retention in the tin). !-endorphin has analgesic effects in the CNS
renal collecting ducts (V 2-rec.; ! p. 166) and and immunomodulatory effects, while α-MSH in the
hypothalamus helps to regulate the body weight
induces vasoconstriction (endothelial V 1 rec.) (! p. 230) and stimulates peripheral melanocytes.
by stimulating the secretion of endothelin-1
(! p. 212ff.). ADH-bearing neurons also
secrete ADH into the portal venous circulation
(see below). The ADH and CRH molecules
regulate the secretion of ACTH by the adenohy-
280 pophysis. Oxytocin promotes uterine contrac-
tions and milk ejection (! p. 304). In nursing
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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