3.3.1.1. The Complex Hypothalamys, Pituitary gland and Thyroid.

The Hypothalamus

hypothalamus1318026807983The hypothalamus, a collection of specialized cells that is located in the lower central part of the brain, is the primary link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It’s often called the “master gland” because it makes hormones that control several other endocrine glands.

The production and secretion of pituitary hormones can be influenced by factors such as emotions and seasonal changes. To accomplish this, the hypothalamus relays information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The pituitary gland

Pituitary_Hormones_largeThe tiny pituitary gland is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. Among the hormones it produces are:

  • Growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body’s handling of nutrients and minerals
  • Prolactin, which activates milk production in women who are breastfeeding
  • Thyrotropin, which stimulates the thyroid gland to produce thyroid hormones
  • Corticotropin, which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain. In addition, the pituitary secretes hormones that signal the ovaries and testes to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic hormone, which helps control body water balance through its effect on the kidneys and urine output; and oxytocin, which triggers the contractions of the uterus that occur during labor.

The Thyroid and the Parathyroid

thyroid-gland-left-and-right-lateral-lobes-isthmus-and-aorta-and-trachea-anterior-view Thyroid_systemThe thyroid, located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine and triiodothyronine. These hormones control the rate at which cells burn fuels from food to produce energy. As the level of thyroid hormones increases in the bloodstream, so does the speed at which chemical reactions occur in the body.

Thyroid hormones also play a key role in bone growth and the development of the brain and nervous system in children. The production and release of thyroid hormones is controlled by thyrotropin, which is secreted by the pituitary gland.

Attached to the thyroid are four tiny glands that function together called the parathyroids. They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin, which is produced in the thyroid.

Hypothalamus

Secreted hormone

Abbreviation Produced by Effect
Thyrotropin-releasing hormone TRH Parvocellular neurosecretory neurons Stimulate thyroid-stimulating hormone (TSH) release from anterior pituitary (primarily)
Dopamine(Prolactin-inhibiting hormone) DA or PIH Dopamine neurons of the arcuate nucleus Inhibit prolactin released from anterior pituitary
Growth hormone-releasing hormone GHRH Neuroendocrine neurons of the Arcuate nucleus Stimulate Growth hormone (GH) release from anterior pituitary
Somatostatin(growth hormone-inhibiting hormone) SS, GHIH, or SRIF Neuroendocrine cells of thePeriventricular nucleus Inhibit Growth hormone (GH) release from anterior pituitaryInhibit thyroid-stimulating hormone (TSH) release from anterior pituitary
Gonadotropin-releasing hormone GnRH or LHRH Neuroendocrine cells of the Preoptic area Stimulate follicle-stimulating hormone (FSH) release from anterior pituitaryStimulate luteinizing hormone (LH) release from anterior pituitary
Corticotropin-releasing hormone CRH or CRF Parvocellular neurosecretory neuronsof the Paraventricular Nucleus Stimulate adrenocorticotropic hormone (ACTH) release from anterior pituitary
Oxytocin OT or OXT Magnocellular neurosecretory neuronsof the Supraoptic nucleus andParaventricular nucleus Uterine contractionLactation (letdown reflex)
Vasopressin(antidiuretic hormone) ADH or AVP or VP Parvocellular neurosecretory neurons,Magnocellular neurosecretory neuronsof the Paraventricular nucleus andSupraoptic nucleus Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume

Pineal body (epiphysis)

Secreted hormone From cells Effect
Melatonin Pinealocytes AntioxidantMonitors the circadian rhythm including inducement of drowsiness and lowering of the core body temperature

Pituitary gland (hypophysis)

Anterior pituitary lobe (adenohypophysis)

Secreted hormone Abbreviation From cells Effect
Growth hormone(somatotropin) GH Somatotrophs Stimulates growth and cell reproductionStimulates Insulin-like growth factor 1 release from liver
Thyroid-stimulating hormone(thyrotropin) TSH Thyrotrophs Stimulates thyroxine (T4) and triiodothyronine (T3) synthesis and release from thyroid glandStimulates iodine absorption by thyroid gland
Adrenocorticotropic hormone(corticotropin) ACTH Corticotrophs Stimulates corticosteroid (glucocorticoid and mineralcorticoid) and androgen synthesis and release fromadrenocortical cells
Beta-endorphin Corticotrophs Inhibits perception of pain
Follicle-stimulating hormone FSH Gonadotrophs In females: Stimulates maturation of ovarian follicles in ovaryIn males: Stimulates maturation of seminiferous tubulesIn males: Stimulates spermatogenesis

In males: Stimulates production of androgen-binding protein from Sertoli cells of the testes

Luteinizing hormone LH Gonadotrophs In females: Stimulates ovulationIn females: Stimulates formation of corpus luteumIn males: Stimulates testosterone synthesis from Leydig cells (interstitial cells)
Prolactin PRL Lactotrophs Stimulates milk synthesis and release from mammary glandsMediates sexual gratification
Melanocyte-stimulating hormone MSH Melanotropes in the Pars intermedia of the Anterior Pituitary Stimulates melanin synthesis and release from skin/hair melanocytes

Posterior pituitary lobe (neurohypophysis)

Secreted hormone Abbreviation From cells Effect
Oxytocin Magnocellular neurosecretory cells Uterine contractionLactation (letdown reflex)
Vasopressin(antidiuretic hormone) ADH or AVP Parvocellular neurosecretory neurons Increases water permeability in the distal convoluted tubule and collecting duct of nephrons, thus promoting water reabsorption and increasing blood volume

Thyroid

Secreted hormone Abbreviation From cells Effect
Triiodothyronine T3 Thyroid epithelial cell (More potent form of thyroid hormone)Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rateStimulates RNA polymerase I and II, thereby promoting protein synthesis
Thyroxine(tetraiodothyronine) T4 Thyroid epithelial cells (Less active form of thyroid hormone)(Acts as a prohormone to triiodothyronine)Stimulates body oxygen and energy consumption, thereby increasing the basal metabolic rate

Stimulates RNA polymerase I and II, thereby promoting protein synthesis

Calcitonin Parafollicular cells Stimulates osteoblasts and thus bone constructionInhibits Ca2+ release from bone, thereby reducing blood Ca2+
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