Describe the different types of communication between the hypothalamus and pituitary gland to control hormone regulation?
The human body’s internal environment maintained by homeostasis. Homeostasis attained by the regulatory mechanisms which is performed by many organs of the body. There are mainly two systems which help in regulation of body organ, and they are nervous system and endocrine system. The circulatory system together with the nervous system and endocrine system helps in homeostatic maintenance of the body. The circulatory system gives the path to hormones to reach their target tissues whereas nervous system synthesized the hormones such as oxytocin and released them to regulate the function of many endocrine glands (Biocourse, 2016).
Hypothalamus is the interface between the central nervous system and endocrine system. The hypothalamus controls the functions of the pituitary gland and other endocrine glands such as thyroid and adrenal glands (Mitrovic, 2016).
The hypothalamus contains neurosecretory neurons which synthesize peptides and catecholamine hormones and released in systemic circulation to the targeted tissues. Some of these hormones act on the anterior pituitary to inhibit and stimulate the secretion of anterior pituitary hormones (Mitrovic, 2016). The actions of hypothalamus hormones is given below in the table no. 1.
Table no. Showing hypothalamus hormones and their actions. Available at biochemistry2.ucsf.edu/programs/ptf/…/HPA%20Axis%20Physio.pdf.
The pituitary gland is found at the base of the hypothalamus of the brain. It is pea shaped and attached to the hypothalamus with help of nerve fibers called as stalk. It is divided into the anterior pituitary and posterior pituitary (Endocrinesurgeon, 2016). Both section of the pituitary gland have the unlike origin, thus secrete different hormones and thus regulated by the different controls system (Biocourse, 2016).
Figure no.1. Showing structure of pituitary gland and hypothalamus. Available at https://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_56.pdf.
The anterior lobe of pituitary gland is glandular. The anterior pituitary or adenohypophysis develop from ectodermal pouch called as rathke’s pouch. The origin of the anterior pituitary is from the epithelial roof of the oral cavity (Mitrovic, 2016). The anterior pituitary lost its link with the oral cavity during pituitary development. The hormones secrets from it stimulate the growth and development of other endocrine glands (Barron, 2010). The hormones released from the anterior pituitary is called as trophic hormones or tropins (Biocourse, 2016).
The posterior lobe of pituitary gland is fibrous because it is composed of axon that originates from the nervous tissues of the hypothalamus and develops as an extension of the hypothalamus along the stalk as the tract of fibers (Boundless, 2015). The origin of the neurohypophysis or posterior pituitary is the neural ectoderm at the base of the third ventricle. The hypothalamus is developed from the base of the third ventricle and some part of these nervous tissues extended downwards to forms the posterior pituitary (Boundless, 2016).
Figure no. 2 Magnetic resonance image of the hypothalamus, neurohypophyseal stalk, and pituitary gland. Available at biochemistry2.ucsf.edu/programs/ptf/…/HPA%20Axis%20Physio.pdf.
There are two types of communication which occurred between hypothalamus and pituitary gland, and these are neural control which occurs between the hypothalamus and posterior pituitary gland and second is negative feedback which occurs between hypothalamus and anterior pituitary gland (Mitrovic, 2016).
The signals transferred from the hypothalamus and posterior pituitary occurs through the neurosecretory cell. The hormones produced inside the cell bodies of the neurosecretory cells and these cells packed in the form of vesicles. The hormones then transferred through the axon, which stored in the axon terminals that lie in the posterior pituitary. The stimulation of the neurosecretory cells generate the action potential which activates the secretion of hormones from the axon terminals and travels through the capillaries in the posterior pituitary (Cliffs Notes, 2016). This process described below with the help of an example.
The neural stimuli activates the secretion of pituitary hormones which originates from the central nervous system. The posterior pituitary secretes oxytocin. Oxytocin stimulates milk ejection reflex. As baby sucks the nipple, the sensory receptors in the nipples and send impulse by the afferent pathway to the hypothalamus which activates the oxytocin and thus milk ejects. Oxytocin is necessary to stimulate uterine contraction in the women at the time of child birth (Biocourse, 2016).
The hypothalamus produces oxytocin and ADH hormones, which transported along the stalk (a connection link between the hypothalamus to the posterior pituitary) and gets stored in the posterior pituitary gland. When the stimulus occurs like decreased blood pressure or volume and suckling by baby occurs, ADH and oxytocin hormones released from the posterior pituitary gland into the blood and reaches their target organ (Mitrovic, 2016).
The secretion of ADH and oxytocin from the posterior pituitary gland is a reflex action. Therefore, it is controlled by the nervous system and endocrine system. Thus, the release of posterior pituitary hormones is neuroendocrine reflex (Biocourse, 2016).
The hypothalamus communicates with the anterior pituitary through hormones. The hypothalamus secretes the releasing hormone and the inhibiting hormone. The specialized cells of the hypothalamus produce the releasing and inhibiting hormones (Cliffs Notes, 2016). These hormones are then transferred through the primary plexus or capillary network and then transported through the hypothalamohypophyseal portal system. From the portal system, it travels through the second plexus and diffuse into the anterior pituitary gland cells and stimulate the release of a specific anterior pituitary hormone (Mitrovic, 2016).
The anterior pituitary gland does not originates from the brain. It also not activated by the axon tract of the hypothalamus therefore, the hypothalamus cannot controls the function of the anterior pituitary hormones. The hormones secreted by the neurons of the hypothalamus transferred through blood into the blood capillaries at the bottom of the hypothalamus. The blood capillaries divided into small veins inside the stalk of pituitary and then further divides into the second capillaries in the anterior pituitary. This system is called hypothalamohypophyseal portal system (Biocourse, 2016).
The hormones of hypothalamus transferred from the hypothalamohypophyseal portal system controls the function of other specific anterior pituitary hormones, like thyrotropin releasing hormone stimulates thyroid stimulating hormone (TSH), gonadotropin releasing hormone stimulate follicle stimulating hormone or luteinizing hormone, and corticotropin releasing hormone stimulates adrenocorticotropic stimulating hormone (Body Guide, 2001). The inhibiting hormones secreted by the hypothalamus inhibit the release of anterior pituitary hormones such as somatostatin inhibits release of growth hormone or melanotropin inhibiting hormone inhibits melanin stimulating hormone (Sargis, 2010).
The hypothalamus secretes hormones for the whole body because it controls the function of the anterior pituitary and anterior pituitary controls the other endocrine glands secretion. The homeostatic condition maintained in the body, when the negative feedback comes from the target gland which inhibit the secretion of the hypothalamus and anterior pituitary gland. This is called negative feedback inhibition (Ivyroses, 2016). A flowchart shown in the figure which describes how hypothalamus controls the secretion of anterior pituitary and how the anterior pituitary stimulates the release of hormones in the target gland. It also shows how negative feedback from target gland inhibit the release of hormones from the hypothalamus and anterior pituitary when hormone level exceeds in the target gland (CK12, 2016).
Figure No. 3 Feedback control of hormone secretion (Boulpaep, 2003).
Figure no. 4 Negative feedback inhibition. Available at https://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_56.pdf.
The thyrotropin releasing hormone secreted by the hypothalamus released into the hypothalamohypophyseal portal system which activates the anterior pituitary gland to release thyroid stimulating hormone which further stimulates the thyroid gland to release thyroxin hormone to produce iodine which needed for the growth of body (Biocourse, 2016). This is explained with the help of flowchart which is given below.
Figure No. 5 Feedback mechanism of thyroxine. Available at https://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_56.pdf.
The communication between the hypothalamus and pituitary gland discussed above which shows that communication occurs through neural control and hormonal control. The hypothalamus communicates anterior pituitary through hormones and with posterior pituitary through neurons.
References
Barron, J., 2010. The Endocrine System: Hypothalamus, Pituitary, & Pineal Glands. [Online]
Available at: https://jonbarron.org/article/endocrine-system-hypothalamus-pituitary-pineal-glands
[Accessed 2016].
Biocourse, 2016. the endocrine system. [Online]
Available at: https://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_56.pdf.
Body Guide, 2001. Endocrine system. [Online]
Available at: https://www.pennmedicine.org/health_info/body_guide/reftext/html/endo_sys_fin.html
Boulpaep, B. &., 2003. Medical Physiology. 1st ed. s.l.:s.n.
Boundless, 2015. Control of the Pituitary Gland by the Hypothalamus. [Online]
Available at: https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/the-endocrine-system-16/the-pituitary-gland-155/control-of-the-pituitary-gland-by-the-hypothalamus-785-4531/
Boundless, 2016. Hypothalamic-Pituitary Axis. [Online]
Available at: https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-endocrine-system-37/endocrine-glands-214/hypothalamic-pituitary-axis-804-12042/
CK12, 2016. Hormone Regulation. [Online]
Available at: https://www.ck12.org/book/CK-12-Biology-Concepts/section/13.25/
Cliffs Notes, 2016. The Hypothalamus and Pituitary Glands. [Online]
Available at: https://www.cliffsnotes.com/study-guides/anatomy-and-physiology/the-endocrine-system/the-hypothalamus-and-pituitary-
Courses washington, 2016. Negative Feedback Regulation of Hormone Release in the Hypothalamic-Pituitary Axis. [Online]
Available at: https://courses.washington.edu/conj/bess/feedback/newfeedback.html
Endocrinesurgeon, 2016. The Connection between the Pituitary and Hypothalamus. [Online]
Available at: https://www.endocrinesurgeon.co.uk/index.php/the-connection-between-the-pituitary-and-hypothalamus
Ivyroses, 2016. Hormone Regulation Feedback Mechanisms. [Online]
Available at: https://www.ivyroses.com/HumanBody/Endocrine/hormone-regulation-feedback-mechanisms.php
Mitrovic, I., 2016. Introduction to the hypothalamo pituitary adrenal axis. [Online]
Available at: ucsf.edu/programs/ptf/…/HPA%20Axis%20Physio.pdf
Sargis, R., 2010. An Overview of the Pituitary Gland. [Online]
Available at: https://www.endocrineweb.com/endocrinology/overview-pituitary-gland
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