Neuroendocrinology
Neuroendocrinology is the branch of
The
Neuroendocrine system
Hypothalamus
The hypothalamus is commonly known as the relay center of the brain because of its role in integrating inputs from all areas of the brain and producing a specific response. In the neuroendocrine system, the hypothalamus receives electrical signals from different parts of the brain and translates those electrical signals into chemical signals in the form of hormones or releasing factors. These chemicals are then transported to the pituitary gland and from there to the systemic circulation.[2]
Pituitary gland
The pituitary gland is divided into three lobes: the anterior pituitary, the intermediate pituitary lobe, and the posterior pituitary. The hypothalamus controls the anterior pituitary's hormone secretion by sending releasing factors, called tropic hormones, down the hypothalamo-hypophysial portal system.[3] For example, thyrotropin-releasing hormone released by the hypothalamus in to the portal system stimulates the secretion of thyroid-stimulating hormone by the anterior pituitary.[citation needed]
The posterior pituitary is directly innervated by the hypothalamus; the hormones
Major neuroendocrine axes
By contrast, the hormones of the
For example, the secretion of growth hormone is controlled by two neuroendocrine systems: the
Functions
The neuroendocrine systems control reproduction
The neurons of the neuroendocrine system are large; they are mini factories for producing secretory products; their nerve terminals are large and organised in coherent terminal fields; their output can often be measured easily in the blood; and what these neurons do and what stimuli they respond to are readily open to hypothesis and experiment. Hence, neuroendocrine neurons are good "model systems" for studying general questions, like "how does a neuron regulate the synthesis, packaging, and secretion of its product?" and "how is information encoded in electrical activity?"[citation needed][It appears that this is a primary source observation.]
History
Pioneers
Walter Lee Gaines noted the activity of the pituitary in the lactation of cows in 1915.[14] He also noted that anaesthesia could block lactation and response to the suckling reflex.[15]
Ernst and
The first of these factors to be identified are thyrotropin-releasing hormone (TRH) and gonadotropin-releasing hormone (GnRH). TRH is a small peptide that stimulates the secretion of thyroid-stimulating hormone; GnRH (also called luteinizing hormone-releasing hormone) stimulates the secretion of luteinizing hormone and follicle-stimulating hormone.
In 1952,
Modern scope
Today, neuroendocrinology embraces a wide range of topics that arose directly or indirectly from the core concept of neuroendocrine neurons. Neuroendocrine neurons control the
Neuroendocrine systems have been important to our understanding of many basic principles in neuroscience and physiology, for instance, our understanding of stimulus-secretion coupling.[21] The origins and significance of patterning in neuroendocrine secretion are still dominant themes in neuroendocrinology today.
Neuroendocrinology is also used as an integral part of understanding and treating
Experimental techniques
Since the original experiments by Geoffrey Harris investigating the communication of the hypothalamus with the pituitary gland, much has been learned about the mechanistic details of this interaction. Various experimental techniques have been employed. Early experiments relied heavily on the electrophysiology techniques used by Hodgkin and Huxley. Recent approaches have incorporated various mathematical models to understand previously identified mechanisms and predict systemic response and adaptation under various circumstances.[citation needed]
Electrophysiology
Electrophysiology experiments were used in the early days of neuroendocrinology to identify the physiological happenings in the hypothalamus and the posterior pituitary especially. In 1950, Geoffrey Harris and Barry Cross outlined the oxytocin pathway by studying oxytocin release in response to electrical stimulation.[24] In 1974, Walters and Hatton investigated the effect of water dehydration by electrically stimulating the supraoptic nucleus—the hypothalamic center responsible for the release of vasopressin.[24] Glenn Hatton dedicated his career to studying the physiology of the Neurohypophyseal system, which involved studying the electrical properties of hypothalamic neurons.[24] Doing so enabled investigation into the behavior of these neurons and the resulting physiological effects. Studying the electrical activity of neuroendocrine cells enabled the eventual distinction between central nervous neurons, neuroendocrine neurons, and endocrine cells.[25]
Mathematical Models
Hodgkin-Huxley Model
The Hodgkin–Huxley model translates data about the current of a system at a specific voltage into time-dependent data describing the membrane potential. Experiments using this model typically rely on the same format and assumptions, but vary the differential equations to answer their particular questions. Much has been learned about vasopressin, GnRH, somatotrophs, corticotrophs, and lactotrophic hormones by employing this method.[8]
Integrate-and-Fire Model
The integrate-and-fire model aims for mathematic simplicity in describing biological systems by focusing on, and only on the threshold activity of a neuron. By doing so, the model successfully reduces the complexity of a complicated system; however it ignores the actual mechanisms of action and replaces them with functions that define how the output of a system depends on its input.[8] This model has been used to describe the release of hormones to the posterior pituitary gland, specifically oxytocin and vasopressin.[9]
Functional or Mean Fields Model
The functional or mean fields model relies on the premise "simpler is better".[8] It strives to reduce the complexity of modelling multi-faceted systems by using a single variable to describe an entire population of cells. The alternative would be to use a different set of variables for each population. When attempting to model a system where multiple populations of cells interact, using several sets quickly becomes overcomplicated. This model has been used to describe several systems, especially involving the reproductive cycle (menstrual cycles, luteinizing hormone, prolactin surges).[9] Functional models also exist to represent cortisol secretion, and growth hormone secretion.[9]
See also
References
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