Hypothalamus

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© Springer Nature Switzerland AG 2020
I. R. IlieIntroduction to Endocrinologydoi.org/10.1007/978-3-030-27382-8_2



2. Endocrine Hypothalamus



Ioana R. Ilie1 


(1)
Department of Endocrinology, University of Medicine and Pharmacy Iuliu Hațieganu, Cluj-Napoca, Romania

 


The hypothalamus and pituitary gland form a unit that exerts control over the function of several endocrine glands (thyroid, adrenals, and gonads), as well as a wide range of physiologic activities. The anatomic relationships between the pituitary and the main nuclei of the hypothalamus are shown in Fig. 2.1. The hypothalamus is anatomically and functionally related to the pituitary gland (hypophysis) by the infundibulum (or pituitary stalk), which consists of the hypophyseal portal system and also carries the axons of neurons whose cell bodies reside in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus.

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Fig. 2.1

The human hypothalamus, the portal hypophysial vessels and the pituitary gland


The relationship between the adenohypophysis (the anterior pituitary) and the main nuclei of the HPT allows control of the anterior pituitary function by the hypothalamic hypophysiotropic hormones secreted into the portal hypophyseal vessels.


The endocrine hypothalamus is organized into the:



  • Magnocellular system → Neurohormones



  • Parvocellular and Arcuate system → Hypophysiotropic hormones


Besides its endocrine role, the hypothalamus is involved in many nonendocrine functions such as regulation of body temperature, thirst, and food intake and is connected with many other parts of the nervous system.


Hence, the hypothalamic hormones can be divided into those secreted by the neurohypophysis directly into the general circulation (neurohormones) and those secreted into hypophysial portal blood vessels (hypophysiotropic hormones).


The former are synthesized in the magnocellular neurons in the SON and PVN and includes vasopressin or antidiuretic hormone (ADH) and oxytocin (both are nonapeptides). The neurosecretory granules with ADH and oxytocin travel down the long axons through the stalk to the posterior hypophysis where the granules are stored and then released into the general circulation. The posterior lobe of the pituitary (neurohypophysis) is made up of the endings of neurons whose cell bodies reside in the supraoptic and paraventricular nuclei of the hypothalamus.


The hypophysiotropic hormones that regulate the secretion of anterior pituitary hormones include growth hormone-releasing hormone (GHRH), somatostatin, dopamine, thyrotropin-releasing hormone (TRH), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (GnRH) (see Table 2.1).


Table 2.1

Hypophysiotropic hormones and their main functions





































Hormone and structure


Abbreviation


Function


Thyrotropin-releasing hormone (tripeptide hormone)


TRH


Stimulates release of TSH


Corticotropin-releasing hormone (a 41-amino-acid peptide)


CRH


Stimulates release of ACTH and other products of its precursor molecule, POMC


Gonadotropin-releasing hormone (a 9-amino-acid peptide)


GnRH, LHRH


Stimulates release of FSH and LH


Growth hormone-releasing hormone (the major isoform of GHRH is 44 amino acids in length)


GHRH


Stimulates release of GH


Growth hormone inhibiting hormone somatostatin, tetradecapeptide somatostatin 14 (found mostly in the hypothalamus) and, somatostatin 28 (found in the gut)


SST


Inhibits release of GH


Prolactin inhibiting hormone (dopamine)


PIH (DA)


Inhibits release of prolactin


Most of the anterior pituitary hormones are controlled by stimulatory hormones, but GH and especially prolactin (PRL) are also regulated by inhibitory hormones. Some hypophysiotropic hormones are multifunctional. The hormones of the hypothalamus are secreted episodically and not continuously, and in some cases, there is an underlying circadian rhythm.


2.1 Physiologic Puberty


Puberty is best considered as one stage in the continuing process of growth and development that begins during gestation and continues until the end of reproductive life.


After an interval of childhood quiescence—the juvenile pause—the hypothalamic pulse generator increases activity in the peripubertal period, just before the physical changes of puberty commence. This leads to increased secretion of pituitary gonadotropins and, subsequently, gonadal sex steroids that bring about secondary sexual development, the pubertal growth spurt, and fertility.


The normal age at onset of puberty for girls: 8–13 years and for boys: 9–14 years.


It is not clear what triggers the onset of puberty, but stimuli from the cerebral cortex and limbic system via various neurotransmitters, for example, leptin, kisspeptin, endorphins, catecholamines, GABA, and dopamine are invoked in the process.

Aug 8, 2021 | Posted by in ONCOLOGY | Comments Off on Hypothalamus
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