What's All the Fuss About Hormones?

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An article discussing the mechanisms of endocrine secretion, neurohormones and the importance of these invisible agents in the regulation of our body's most vital physiological processes.

Submitted: December 29, 2011

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Submitted: December 29, 2011



What’s All the Fuss about Hormones?




Hormones are the invisible agents behind the most vital biological processes and reactions. So how exactly do they work?

Hormones are generally defined as organic substances synthesised in the body and secreted into the bloodstream in order to regulate important bodily functions, such as growth and cell division, and maintain homeostasis. They are detected by target cells, specialised cells which are adapted to respond to certain quantities of the hormone after they have been secreted by glands into the bloodstream. This form of hormone regulation is called endocrine regulation, as it involves the synthesis and secretion of hormones by the endocrine gland directly into the bloodstream. Certain less developed organisms which lack vascular systems transmit hormones by the process of simple diffusion across cell membranes.

So what is the main distinction between neural and endocrine regulation. One can differentiate between the two processes by examining the duration of the effect, the rate at which the hormone acts in the body, and extent to which the hormone is distributed. Endocrine regulation is characterised by slow action, prolonged effect and a wide distribution of the hormone; conversely, neural regulation provokes rapid responses, lasts for a brief period of time and has localized effects.

The pituitary gland or hypophysis is the predominant control centre for endocrine secretion in vertebrates, consisting of the neurohypophysis (posterior pituitary lobe) and the adenohypophysis (anterior pituitary lobe). The cells of the anterior lobe comprise of five different types, each giving rise to the discharge of a disparate endocrine hormone. Abnormalities in levels of secretion of these hormones may be traced back to pathological changes or changes in the numbers of these secretory cells (in hyperplasia for example), which regulate the hormone secretion.  These hormones are classed as proteins consisting of long polypeptide chains, although certain hormones, such as gonadotropins and thyroid stimulating hormone are glycoproteins. The structure of the glycoproteins differs in that it consists of protein joined to carbohydrate to form polysaccharides. The hormones secreted by the adenohypophysis regulate a range of different physiological processes, including sexual maturation, growth of the epiphysial plates (end plates of large bones), and the carbohydrate metabolism.

The anterior lobe or adenohypophysis secretes most of the pituitary hormones, many of which stimulate the release of hormones by other endocrine glands. These glands include the adrenal cortex, thyroid and gonads (testes and ovaries) which are classed as target organs of the hypothalamus and pituitary. This is an elegant model of hormone regulation: an interconnected chain comprising the hypothalamus stimulating the pituitary and the pituitary stimulating the target organ through a series of “negative feedback loops” or circuits. The “stimulants” behind this finely-tuned, complex mechanism are the neurohormones and the pituitary hormones respectively. The action of this mechanism is best illustrated through the negative feedback relationship between corticotropin, a pituitary hormone, and the adrenal cortex, a target gland. Corticotropin maintains normal levels of steroid hormones, which have been synthesized in the outer region or cortex of the adrenal glands, namely glucocorticoids such as cortisol and corticosterone. A shortage of such steroid hormones in the blood triggers an increased secretion of corticotropin, which in turn stimulates its target gland (the adrenal cortex) to release the hormones which are deficient in the blood.

Other pituitary hormones include prolactin, somatotropin (the growth hormone), thyrotropin (thyroid-stimulating hormone; TSH), and gonadotropins.  Prolactin is associated with the secretion of milk in female, mammalian mammary glands; it has also been found to have a similar molecular structure as somatotropin.  The thyroid-stimulating hormone works according to the same negative-feedback mechanism as corticotropin; it stimulates the thyroid gland to increase hormone secretion, and if overactive, can cause an enlargement in the thyroid organ and number of cells (hyperplasia). Gonadotropins are chemicals which stimulate the secretion of steroid hormones such as oestrogen and progesterone by the gonads; they regulate the development of the ovaries and testes, as well as governing reproductive processes such as ovulation.  Examples include follicle-stimulating hormone and interstitial-cell-stimulating hormone or luteinizing hormone. The human growth hormone (somatotropin) achieves growth of the organism through increased protein synthesis, while a surplus of the chemical can cause pancreatic strain due to heightened need for insulin to reduce blood-glucose levels.

Neural regulation occurs when neurons, nerve cells, produce chemical transmitters called neurohumors which are released at nerve endings, triggering a response in the adjoining neuron. This allows for the transmission of electrical impulses from neuron to neuron throughout the nervous system. Neural signals may be converted into chemicals called neurohormones by specialised nerve cells called neurosecretory cells. These neurohormones, consisting of polypeptides, travel along axons, the extensions of nerve cells, before being released into the bloodstream at neurohemal organs. At these regions, the axon is in close proximity to the blood capillaries, where the neurohormones may be transmitted to the target cells. In certain simpler organisms which lack vascular systems, the axon of the neurosecretory cell is closer to the target tissue, without requiring the involvement of blood capillaries.




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