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Endocrine System 2: Hypothalamus and Pituitary Gland

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Endocrine System 2: Hypothalamus and Pituitary Gland Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Keywords Hypothalamus/Pituitary gland/Endocrine system/Hormones Systems of life This article has been Endocrine system double-blind peer reviewed In this article... ● Endocrine functions of the hypothalamus ● Hormones of the anterior and posterior pituitary gland ● Role of pituitary gland hormones in mediating the actions of the hypothalamus Endocrine system 2: hypothalamus and pituitary gland Key points Authors Zubeyde Bayram-Weston is senior lecturer in biomedical science; Maria The hypothalamus Andrade is honorary lecturer in biomedical science; John Knight is associate and pituitary gland professor in biomedical science; all at College of Human and Health Sciences, both lie in the cranial Swansea University. cavity of the skull Abstract The endocrine system consists of glands and tissues that produce and Two major secrete hormones to regulate and coordinate vital bodily functions. This article, the hormones released second in an eight-part series on the endocrine system, explores the anatomy and by the posterior physiology of the hypothalamus and pituitary gland, and how they work together to pituitary gland are regulate and coordinate vital physiological processes in the body through hormonal antidiuretic action. It shows how many of the actions initiated by the hypothalamus are mediated hormone and through hormonal secretions produced by the pituitary gland beneath it. oxytocin Citation Bayram-Weston Z et al (2021) Endocrine system 2: hypothalamus and The anterior pituitary gland. Nursing Times [online]; 117: 6, 49-53. pituitary gland produces several stimulating he first article in this eight-part system is well developed in all higher ver- hormones that series on the endocrine system tebrates and plays a key role in emotional regulate the activity gave an overview of the nature of responses, long-term memory, sense of of other endocrine Tendocrine glands and high- smell (olfaction) and the acquisition of glands lighted the role of hormones as chemical new skills, as well as contributing to a signals that help maintain the homeostatic range of behavioural responses (Wróbel, Although the balance essential to health; the remaining 2018). The hypothalamus is the location of pituitary gland is articles will each explore different major the thermoregulatory centre, which regu- often referred to as endocrine glands and tissues. This article lates body temperature (VanPutte et al, the master gland, examines the anatomy and physiology of 2017); it plays an essential role in water bal- many of its actions the hypothalamus and pituitary gland, ance, regulation of blood pressure and the are directed by the which lie in the cranial cavity of the skull. sensations of thirst and hunger. hypothalamus This article focuses on the endocrine The hypothalamus functions of the hypothalamus and its role Clusters of neurons The hypothalamus is located at the base of as the key link between the nervous and in the hypothalamus the brain just below the thalamus. A small endocrine systems. The hypothalamus is synthesise releasing but vital region of the brain, it is roughly connected directly to the pituitary gland and inhibiting the size of an almond and weighs around via a thin stalk, called the infundibulum hormones that 4g (Saper and Lowell, 2014). It accounts for (Fig 1). Many actions initiated by the hypo- regulate the activity <1% of the total brain mass but performs a thalamus are mediated through secretions of the anterior multitude of functions that are essential to produced by the pituitary gland beneath it. pituitary survival and the enjoyment of life. The hypothalamus is part of the limbic The pituitary gland system, a region of the brain that also The pituitary gland is a pea-sized gland includes the thalamus, amygdala, hip- that is typically around 0.8-1.0cm in diam- pocampus and cingulate gyrus. The limbic eter and weighs around 500mg. It resides Nursing Times [online] June 2021 / Vol 117 Issue 6 49 www.nursingtimes.net Copyright EMAP Publishing 2021 This article is not for distribution except for journal club use Clinical Practice Systems of life Fig 1. Anatomy of the hypothalamus and pituitary gland by ADH acting as a powerful vasopressor (which promotes the constriction of blood vessels). ADH-induced vasoconstriction, Hypothalamus particularly in the peripheral arterioles Mammillary body (small arteries), further increases and nor- malises blood pressure (Kanbay et al, 2019). As a result, ADH is also known as vaso- pressin, particularly in the United States. Reduced secretion of ADH can lead to Optic Median eminence chiasm diabetes insipidus (DI). Patients with DI Infundibulum cannot concentrate their urine, resulting in polyuria. Large volumes of urine (3-20L/ Anterior pituitary Pars tuberalis Posterior pituitary day) are usually produced; if not treated, Pars intermedia Infundibular stalk this can lead to severe dehydration. Pars distalis Pars nervosa DI is rare, affecting around 1 in 25,000 people; two major types are recognised: ● Neurogenic or central DI is caused by the undersecretion (hyposecretion) of ADH by the posterior pituitary. This is most often due to trauma (commonly in the sella turcica (Turkish saddle), a pro- ● Antidiuretic hormone (ADH); head injuries), tumours affecting the tective pocket in the sphenoid bone of the ● Oxytocin. hypothalamus or pituitary or, more skull (Fig 1). Increased pituitary size is These hormones are synthesised in the rarely, infections; often indicative of endocrine pathologies, cell bodies of neurons in the hypothalamus ● Nephrogenic DI is a rarer form, in particularly tumours of the pituitary (De and transported down the axons of the which patients usually have normal Sousa et al, 2015). The pituitary gland com- neurons running through the infundib- ADH synthesis and secretion, but their prises two regions (Fig 1): ulum. ADH and oxytocin are concentrated kidneys are insensitive to the effects of ● Posterior pituitary (neurohypophysis) and stored in the pars nervosa (Fig 3), ADH – most commonly due to kidney – neural tissue extends from the before being released into the blood when disease or drug-induced kidney hypothalamus through the required. Both are peptide hormones and, damage (Kalra et al, 2016). infundibulum into a larger, bulbous as they are produced by neurons, they are DI requires careful management. Ini- region called the pars nervosa; this often called neuropeptides. tially patients may be severely dehydrated, forms the bulk of the posterior pituitary; feel nauseous and shivery, and experience ● Anterior pituitary (adenohypophysis) Antidiuretic hormone headache; careful monitoring of water – derived from the epithelial tissue of ADH plays a vital role in regulating fluid intake and urine output, with ongoing the embryonic oral cavity. balance and blood pressure. Specialised assessment of urine and blood concentra- During embryonic development, the osmoreceptors located in the hypothal- tion, is essential. Neurogenic DI is usually roof of the mouth bulges upwards (invagi- amus continually monitor the solute con- treated with desmopressin, a synthetic nates) to form a tiny, bubble-like structure centration of the blood. When the body analogue of ADH that acts on the kidneys known as Rathke’s pouch, which then loses water (for example, through sweating in the same way to concentrate the urine fuses with the posterior portion of the during exercise or following vomiting and and increase blood volume. Treatment of pituitary gland (Fig 2). Failure of this pro- diarrhoea) dehydration may occur and the nephrogenic DI is more complex and cess to occur normally may lead to an plasma solute concentration rises. This is depends on the underlying cause of the abnormal pituitary structure or the forma- detected by the hypothalamic osmorecep- disease (The Pituitary Foundation, 2016). tion of cysts and clefts (Babu et al, 2013). tors, which initiate the release of ADH The anterior pituitary accounts for from the posterior pituitary. Oxytocin approximately 70-80% of the total mass of ADH primarily acts on the kidneys, Oxytocin is released into the blood at high the gland and includes two major parts: increasing the volume of fluid absorbed concentration towards the end of the ● Pars distalis – larger, bulbous portion; from the renal filtrate back into the blood. ● Pars tuberalis – highly vascular sheath This reduces the volume of urine produced Fig 2. Pituitary gland origins wrapped around the infundibular stalk. (hence the name antidiuretic hormone), Base of brain A third (intermediate) region of the pitu- resulting in the urine being darker and (diencephalon) itary gland is often recognisable; this is more highly concentrated. By increasing Rathke’s known as the pars intermedia and is usually fluid reabsorption back into the blood, pouch present as a thin band of tissue that marks ADH helps normalise the solute concen- the point where the anterior and posterior tration of the blood (VanPutte et al, 2017). Roof of mouth pituitaries fuse (Ilahi and Ilahi, 2020). ADH is also released after a drop in (stomadeum) blood volume or pressure. By promoting Stalk Hormones of the posterior water reabsorption in the kidney, ADH Anterior Posterior pituitary increases blood volume, which then starts gland gland Two major hormones are released from the to increase blood pressure. This normalisa- Cleft JENNIFER N. R. SMITH posterior pituitary: tion of blood pressure is further
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