Content Summary:
The endocrine system is one of two systems within the human body that maintains homeostasis and coordinates its functions. That coordination is provided through its ductless glands and tissues which secrete hormones that act as chemical messengers to cells. Specific hormones target specific cells that have specific receptor proteins which affect specific tissues and organs. All of this specificity is the reason homeostasis can be maintained. Some endocrine glands are also organs which produce hormones along with secreting them.

Summary #1
There are four chemical classifications of hormones:
  • Amines-they are derived from tyrosine and typtophan amino acids and are secreted by the adrenal medulla, thyroid and pineal glands. Examples are epinephrine, norepinephrine, melatonin and thyroxine.
  • Polypeptides and proteins-both are similar in structure, but differ in size. Proteins are large polypeptide molecules. ADH is a polypeptide and GH is considered a protein.
  • Glycoproteins-proteins bound to one or more carbohydrate groups, such as FSH and LH.
  • Steroids-lipids derived from cholesterol such as testosterone, estradiol, progesterone and cortisol.

Summary #2
Hormones interact in ways that either complement or inhibit. A complementary interaction is synergistic when two or more hormones work together to produce a result that is larger than if working singularly. Another type of complementary action, permissive, occurs when one hormone enhances responsiveness of the 2nd hormone's target organ. Inhibited interaction, antagonistic, occurs when one hormone limits the effect of another hormone.

Summary #3
Regulatory molecules within organs that produce them act in many ways. The two that were discussed in this chapter were paracrine and autocrine regulators. Paracrine regulators are produced within an organ tissue but regulate a different tissue of that same organ. Autocrine regulators are produced within an organ tissue and regulate cells within that same tissue. Some regulatory molecules are cytokines found in the immune system, growth factors that promote growth and cell division, and neutrophins which are regulators within the nervous system. Prostaglandins are another kind which are produced in almost every organ within the body and have varying functions such as promoting inflammation, smooth muscle contractions, inhibit gastric secretion and even a role in ovulation.

Application:
How does endocrinology affect my career as an OTA? My first thought is that I will be working with patients who may have diseases involved with the endocrine system such as SAD (Seasonal Affective Disorder), Diabetes Mellitus, Asthma, Hyperthyroidism, Gigantism and Hypothyroidism to name a few. It is important for me to understand these illnesses, how they may happen and how they are affecting my patients. This will help me to be a better OTA.

Essential Questions:
The hypothalamus regulates the action of the posterior pituitary because it produces the two hormones, ADH and oxytocin, that are stored and secreted by the posterior pituitary. The hormones are transported from the hypothalamus, along the hypothalamo-hypophyseal to the posterior pituitary. Once stored there, the neuroendocrine reflexes within the hypothalamus control the release of ADH and oxytocin. Oxytocin in females stimulates contractions of the uterus during labor and stimulates contractions of the mammary gland alveoli and ducts in lactating women to eject milk for their baby. In men, oxytocin may be secreted during an ejaculation, but that is still being determined.

The hypothalamus regulates the action of the anterior pituitary through its own secretion of hypothalamic hormones which control secretion of hormones in the anterior pituitary. The hypothalamo-hypophyseal portal system is the vascular link between the hypothalamus and the anterior pituitary. One hormone the anterior pituitary produces is prolactin which in females, stimulates milk production of the mammary glands after the birth of a baby. In males, prolactin plays a role in the regulation of the male reproductive system with FSH and LH. It also acts on the kidneys in regulation of water and electrolyte balance.

Trophic hormones are the hormones of the anterior pituitary. They are called trophic because high concentrations of them within an organ can cause hypertrophy, meaning the organ will grow. Low concentrations of these hormones will cause an organ to atrophy, meaning the organ will shrink. They are controlled by releasing and inhibiting factors within the hypothalamus and by the target organs they regulate. Secretion of ACTH, TSH, FSH and LH are controlled by the negative feedback inhibition of target gland hormones. Inhibition can take place by: 1) the target gland hormones can influence the hypothalamus to inhibit secretion of releasing hormones, and 2) the target gland hormones can influence the anterior pituitary to inhibit its response to releasing hormones. There is one instance of positive feedback effect during a woman's menstrual cycle when estradol secretion from the ovaries stimulates the anterior pituitary to secrete a large amount of LH that results in ovulation. Higher levels of estradol later in the menstrual cycle will have the opposite affect and inhibit LH secretion.

References: Human Physiology by Stuart Ira Fox, Anatomy and Physiology by Stanley E Gunstream
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