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Hormones are chemical substances made by endocrine glands. These glands pass the hormones directly into the bloodstream, which carries them around the body. In this way, a hormone can cause a change in the body at some distance from the gland where it was made. They are also known as Chemical Messengers.
Both endocrine and neurosecretory cells produces hormones, which is carried in the blood to the target organs.
Three general structures are known. • Steroid hormones are fat-soluble molecules made from cholesterol. Among these are the three major sex hormones groups: estrogens, androgens and progesterones. Males and females make all three, just in different amounts. Steroids pass into a cell's nucleus, bind to specific receptors and genes and trigger the cell to make proteins. • Amino acid derivatives, such as epinephrine, are water-soluble molecules derived from amino acids (the building blocks of protein). These hormones are stored in endocrine cells until needed. They act by binding to protein receptors on the outside surface of the cell. The binding alerts a second messenger molecule inside the cell that activates enzymes and other cellular proteins or influences gene expression. • Insulin, growth hormone, prolactin and other water-soluble polypeptide hormones consist of long chains of amino acids, from several to 200 amino acids long. They are stored in endocrine cells until needed to regulate such processes as metabolism, lactation, growth and reproduction.
The Hypothalamus is a region of the brain located just above the pituitary gland. In fact, it’s intimately related to the pituitary and connected via the hypophyseal stalk. The hypothalamus releases six hormones, which regulate function of the pituitary gland. 1. Thyrotropin-Releasing Hormone (TRH) stimulates the release of thyroid-stimulating hormone (TSH) and prolactin. 2. Gonadotropin-releasing hormone (GnRH) stimulates the release of LH and FSH from the anterior pituitary; thus indirectly increasing increased testosterone levels in males and increased estrogen and progesterone levels in females. 3. Growth hormone-releasing hormone (GHRH) stimulates the anterior pituitary to release GH. GHRH is secreted in pulses preceding the GH release pulse.
4. Corticotropin-releasing hormone (CRH). The hypothalamus secretes CRH into the hypophysial portal blood. CRH then stimulates the release of ACTH (adrenocorticotropin hormone) in the anterior pituitary. 5. Somatostatin .Somatostatin can also be called growth hormone-inhibiting hormone (GHIH) due to its effects on the release of GH. Somatostatin inhibits the release of both GH and TSH by the anterior pituitary. 6. Dopamine acts as an inhibiting hormone, much like somatostatin. But rather than inhibiting GH and TSH, dopamine inhibits prolactin production by the anterior pituitary.
The hormones secreted from the pituitary and the effects they have on the body. Two hormones (ADH and oxytocin) are secreted from the posterior pituitary and six (prolactin, ACTH, growth hormone, TSH, FSH, and LH) from the anterior pituitary.
The Pituitary gland is the major and controlling gland of the endocrine system.The anterior pituitary produces a number of hormones that control hormone production in the endocrine glands. The pituitary is located near the base of the brain. About the size of a pea, the pituitaryis connected to the hypothalamusby a slender pituitary stalk (hypophyseal stalk). In addition to anatomical connection via the hypophyseal-hypothalamic tract, the hypothalamus regulates pituitary function providing a bridge between the nervous system and the endocrine system through the hypophyseal portal system. The hypophyseal portal system is a localized collection of blood vessels that allow regulating agents secreted by the hypothalamus to travel directly to the pituitary. • Neural imput can be interpreted by the hypothalamus and translated into pituitary control and therefore affect the endocrine response. The connection between the hypothalamus and the pituitary establishes a clear chain of control: nervous signals in the hypothalamus cause changes in pituitary hormone secretion that, in turn, control production of hormones in other endocrine glands. • The anterior lobe secretes seven different hormones: • 1. Growth Hormone (GH), • 2. Thyroid-Stimulating Hormone (TSH), • 3. Adrenocorticotropic Hormone (ACTH), • 4. the Gonadotropins (LH and FSH), • 5. Prolactin (PRL), • 6. Melanocyte-Stimulating Hormone (MSH).
Growth Hormone or Somatotropin, acts directly on tissues to stimulate overall body growth. GH acts to stimulate growth by increasing protein synthesis and acts metabolically to shunt glucose from ATP synthesis pathways, while at the same time promoting fat usage. GH stimulates the secretion of somatomedin hormones. • Growth hormone is essential for muscle growth. Hundreds of studies have been conducted on GH and its relation to skeletal muscle growth. In one study, conducted by UCLA indicates weight training increases the release of GH.
Thyroid-Stimulating Hormone (TSH) stimulates thyroid growth and secretion of thyroid hormones, including Thyroxine and Triiodthyronine. • Adrenocorticotropic Hormone (ACTH) stimulates adrenocortical growth (growth of cortex of adrenal glands) and the subsequent secretion of corticosteroids. There is also a negative feedback mechanism involved in ACTH production. Cortisol is the principle corticosteroid produced by the adrenal glands. As levels of cortisol increase in the bloodstream, they act to inhibit further production of ACTH by the anterior pituitary.
The pituitary secretes gonadotropins, LH (lutinzing hormone) and FSH (follicle stimulating hormone). In males, LH is usually referred to as interstitial cell stimulating hormone (ICSH). In females, LH acts to stimulate ovulation and the formation of the corpus luteum on an ovarian follicle. In males, the action of LH acts to stimulate testosterone production. In females, FSH stimulates estrogen secretion and supports the growth and maturation of the ovarian follicle. In males, FSH stimulates spermatogenesis, the formation of sperm cells. • Pituitary stimulation of melanocytestimulating hormone (MSH) contributes to the regulation of pigmentation of the skin by stimulating melanocytes to produce melanin.
POSTERIOR PITUITARY GLAND • The posterior lobe, produces Oxytocin and Antidiuretic Hormone (ADH). • Oxytocin issecreted by the cells of the paraventricular nuclei. There are no known functions of oxytocin in males. In females, however, oxytocin plays a large role in mammary gland stimulation and contractions during birth. • Antidiuretic Hormone (ADH), also called vasopressin, is synthesized and secreted in the supraoptic nucleus. The primary function of ADH is regulation of body water and is secreted whenever the water levels in the blood are decreased. Decrease of water in the blood can be caused by osmotic diuresis (brought on by an increase in blood glucose levels, ketone bodies, or sodium loss). ADH is also secreted when mechanoreceptors (blood volume receptors) in the heart and pressure receptors in the vasculature are stimulated after blood loss. After a hemorrhage, ADH causes vasoconstriction, which leads to an increase of blood pressure.
