Learning objectives - The Endocrine System
Indicate important differences between hormonal and neural controls of body functioning
The means of control and the speed of the endocrine system are very different from those of the nervous system.
• The nervous system regulates the activity of muscles and glands via electrochemical impulses delivered by neurons, and those organs respond within milliseconds.
• The endocrine system influences metabolic activity by means of hormones, which are chemical messengers released into the blood to be transported throughout the body. Binding of a hormone to cellular receptors initiates responses that typically occur after a lag period of seconds or even days. But, once initiated, those responses tend to be much more prolonged than those induced by the nervous system.
o Growth and Development
o Maintenance of Electrolyte, water, and nutrient balance of the blood
o Regulation of cellular metabolism and energy balance
o Mobilization of body defenses
List the major endocrine organs and major hormones in each. Describe their body locations and how their major hormones function.
Pineal Gland: Tiny, pine cone-shaped, hangs from the roof of the third ventricle in the diencephalon in the brain, endocrine function is still a mystery, but releases melatonin(sleep cycle).
Hypothalamus/Pituitary Gland: Above the pituitary gland. Produces oxytocin and ADH which exits out the posterior pituitary(just stores the hormones, doesn't produce any itself, part of NS). Also secretes releasing and inhibiting hormones which cause the anterior pituitary to produce hormones such as TSH, FSH, LH, ACTH, GH, and PRL.
Thyroid Gland: Located in the anterior neck on the trachea just inferior to the larynx. Largest pure endocrine gland in the body, produces Thyroid Hormone (affects virtually every cell in the body) and Calcitonin (bone-sparing effect).
Parathyroid Glands: On posterior side of thyroid gland, usually 4 of them. Secretes Parathyroid hormone(single most important hormone controlling the calcium balance of the blood).
Thymus Gland: Deep to the sternum in the thorax, loses size in adulthood, doesn't really do much.
Adrenal Glands: Pyramid-shaped organs on top of the kidneys, inner is adrenal medulla which releases EPI and NE, outer is adrenal cortex and synthesizes corticosteriods. All adrenal hormones help us cope with stressful situations.
Pancreas: Partially behind the stomach in the abdomen, produces glucagon and insulin.
Gonads: Testes and Ovaries, produce steroid sex hormones identical to those produced by adrenal cortical cells. Estrogen, Progesterone, Testosterone
Distinguish between hormones, paracrines, and autocrines.
• Hormones are long-distance chemical signals that travel in blood or lymph throughout the body.
• Autocrines are short-distance chemical signals that exert their effect on the same cells that secret them. (ex. Prostaglandins released by smooth muscle causes that smooth muscle to contract.)
• Paracrines are short-distance chemical signals that exert their effect on cell types other than those secreting the paracrine chemicals.
Describe how hormones are classified chemically.
•Amino acid based: Most hormones are amino acid based. Molecule size varies widely in this group.
o Simple amino acid derivatives – including thyroxine constructed of amino acids tyrosine and amines
o Peptides – short chains of amino acids
o Proteins – long polymers of amino acids
• Steroids: Steroids hormones are synthesized from cholesterol.
o Gonadal hormones
o Adrenocortical hormones
Describe the two major mechanisms by which hormones bring about their effects on their target tissues.
• Water-soluble hormones (all amino acid-based hormones except thyroid hormones) act on receptors in the plasma membrane. These receptors are usually coupled via regulatory molecules called G proteins to one or more intracellular second messengers which mediates the target cell response.
• Lipid-soluble hormones (steroid and thyroid hormones) act on receptors inside the cell, which directly activates genes and stimulating synthesis of specific proteins
Explain how hormone release is regulated.
The synthesis and release of most hormones are regulated by some type of negative feedback mechanism. Some internal or external stimulus triggers hormone secretion. As levels of hormones rise, it causes target organ effects, which then feedback to inhibit further hormone release. As a result, blood levels of many hormones vary only within a narrow range
List three kinds of interaction of different hormones acting on the same target cell.
• Permissiveness – one hormone cannot exert its full effects without another hormone being present (ex. Reproductive system hormones regulate the development of the reproductive system. However thyroid hormone is also necessary for normal timely development of reproductive structures. Lack of thyroid hormone delays reproductive development.
• Synergism – occurs when more than one hormone produces the same effect at the target cell and their combined effects are amplified. (ex. both glucagon (pancreas) and epinephrine causes the liver to release glucose into the blood. When they act together, the amount of glucose released is about 150% of what is released when each hormone acts alone
• Antagonism – occurs when one hormones opposes the action of another hormone. (ex. insulin which lowers blood glucose levels, is antagonized by glucagon, which raises blood glucose levels.
compete for the same receptor
Act through different metabolic pathways
Cause down-regulation of the receptors for the antagonistic hormone.
Describe structural and functional relationships between the hypothalamus and the pituitary gland.
The pituitary gland, also known as the hypophysis, is a roundish organ that lies immediately beneath the hypothalamus, resting in a depression of the base of the skull called the sella turcica.
The posterior pituitary or neurohypophysis is not a separate organ, but an extension of the hypothalamus. It is composed largely of the axons of hypothalamic neurons which extend downward as a large bundle behind the anterior pituitary. It also forms the so-called pituitary stalk, which appears to suspend the anterior gland from the hypothalamus.
A key to understanding the endocrine relationship between hypothalamus and anterior pituitary is to appreciate the vascular connections between these organs. As will be emphasized in later sections, secretion of hormones from the anterior pituitary is under strict control by hypothalamic hormones. These hypothalamic hormones reach the anterior pituitary through the following route:
• A branch of the hypophyseal artery ramifies into a capillary bed in the lower hypothalamus, and hypothalmic hormones destined for the anterior pituitary are secreted into that capillary blood.
• Blood from those capillaries drains into hypothalamic-hypophyseal portal veins. Portal veins are defined as veins between two capillary beds; the hypothalamic-hypophyseal portal veins branch again into another series of capillaries within the anterior pituitary.
• Capillaries within the anterior pituitary, which carry hormones secreted by that gland, coalesce into veins that drain into the systemic venous blood. Those veins also collect capillary blood from the posterior pituitary gland.
The utility of this unconventional vascular system is that minute quantities of hypothalamic hormones are carried in a concentrated form directly to their target cells in the anterior pituitary, and are not diluted out in the systemic circulation.
Discuss the structure of the posterior pituitary, and describe the effects of the two hormones it releases
The posterior pituitary consists largely of axon terminals of hypothalamic neurons. These neurons synthesize one of two neurohormones. Axon terminals in the posterior pituitary release these hormones “on demand” in response to action potentials that travel down the axons of these same hypothalamic neurons
• Oxytocin – A strong stimulant of uterine contractions, released in higher amounts during childbirth and nursing women. The number to oxytocin receptors in the uterus peak near the end of pregnancy. Oxytocin also acts as a neurotransmitter in the brain. There is is involved in sexual and affectionate behavior (as the cuddle hormone) and promotes nurturing, couple bonding and trust.
• Antidiuretic hormone (ADH) – Diuresis is urine production, so antidiuretic hormone (ADH) is a substance that inhibits or prevents urine formation. ADH prevents wide swings in water balance, helping the body avoid dehydration and water overload.
List and describe the chief effects of anterior pituitary hormones.
• Growth Hormone (GH) – Anabolic hormone, stimulates somatic growth, mobilizes fats, spares glucose – target organs liver, muscle, bone, cartilage and other tissue
• Thyroid stimulating hormone (TSH)– stimulates thyroid gland to release thyroid hormone.
• Adrenocorticotropic hormone (ACTH) – promotes the release of glucocorticoids and androgens (mineralocorticoids to a lesser extent) target Adrenal cortex
• Follicle-stimulating hormone (FSH) – In females stimulates ovarian follicle maturation and estrogen production, in males stimulates sperm production. Targets ovaries and testes
• Lueinizing hormone (LH) - in females triggers ovulation and stimulates ovarian production of estrogen and progesterone, in males promotes testosterone production. Targets ovaries and testes
• Prolactin (PRL) – promotes lactation. Target breast secretory tissue.
Describe important effects of the two groups of hormones produced by the thyroid gland.
• Increasing basal metabolic rate and body heat production, by turning on transcription of genes concerned with glucose oxidation. This is the hormone’s calorigenic effect = heat producing
• Regulating tissue growth and development, TH is critical for normal skeletal and nervous system development and maturation and for reproductive capabilities.
• Maintaining blood pressure by increasing the number of adrenergic receptors in blood vessels.
Follow the process of thyroxine formation and release.
• Thyroglobulin is synthesized and discharged into the follicle lumen
• Iodide is trapped
• Iodide is oxidized to iodine
• Iodine is attached to tyrosine
• Iodinated tyrosines are linked together to form T3 and T4
• Thyroglobulin colloid is endocytosed
• Lysosomal enzymes cleave T4 and T3 from thyroglobulin and the hormones difuse from the follicular cell into the bloodstream.
Indicate general functions of parathyroid hormone
Controls calcium balance in the blood.
List major hormones produced by the adrenal gland, and cite their physiological effects.
• Aldosterone - mineralcorticoid steroid hormone that regulates the excretion of salt, potassium, and water.
• Cortisol - glucocorticoid steroid hormone that affects fat, carbohydrate, and protein levels in the blood as well as regulating the immune response.
• Androgens – Gonadocorticoid steroid that converts to testosterone of estrogen after release. Contributes to female libido, produces pubic and axillary hair, also a source of estrogen after menopause
• Epinephrine and norepinephrine – catecholamine steroids increase heart rate and metabolic rate, increase blood pressure
Briefly describe the importance of melatonin.
An amine hormone derived from serotonin. Melatonin concentrates in the blood rise and fall in a daily cycle. Peak levels occur during the night and make us drowsy.
Compare and contrast the effects of the two major pancreatic hormones.
• Glucagon – 29 amino acid polypeptide is a very potent hyperglycemic agent. The major target of glucagon is the liver
o Breakdown of glycogen to glucose (glycogenolysis)
o Synthesis of glucose from lactic acid and from noncarbohydrate molecules (gluconeogenesis)
o Release of glucose to the blood by liver cells, causing blood glucose levels to rise.
• Insulin – is a small 51 amino acid protein consisting of two amino acid chains linked by disulfide bonds. It synthesizes as part of a larger polypeptide chain called proinsulin. Enzymes then excise the middle portion of this chain releasing functional insulin. Effects are most obvious when we have just eaten.
o Enhances membrane transport of glucose into most body cells (muscle and fat)
o Inhibits the breakdown of glycogen to glucose.
o Inhibits the conversion of amino acids or fats to glucose. These inhibiting effects counter any metabolic activity that would increase plasm levels of glucose
Describe the functional roles of hormones of the testes, ovaries, and placenta.
• Estrogen is responsible for maturation of the reproductive organs and the appearance of the secondary sex characteristics of the female at puberty. Acting with progesterone promotes breast development and cyclic changes in the uterine mucosa
• Testosterone initiates the maturation of the male reproductive organs and the appearance of secondary sex characteristics and sex drive. Also responsible for normal sperm production and maintains the reproductive organs in the mature functional state.
• The placenta is a temporary endocrine organ that sustains the fetus during pregnancy. It also secrets several steroids and protein hormones that influence the course of pregnancy. These hormones incluse estrogen, progesterone and human chorionic gonadotropin (hCG)
Name a hormone produced by the heart.
The heart produces atrial natriuretic peptide (ANP). ANP decreases the amount of sodium in the extracellular fluid thereby reducing blood volume and blood pressure.
State the location of enteroendocrine cells
Enteroendocrine cells are hormone-secreting cells sprinkled in the mucosa of the gastrointestinal tract (GI) These hormones help regulate a wide variety of digestive function.
Briefly explain the hormonal function of the kidneys, skin, adipose tissue, bone and thymus.
• Kidneys – secrets erythropoietin is a glycoprotein hormone that signals bone marrow to increase production of red blood cells.
• Skin – produces cholecalciferol and inactive form of vitamin D then becomes fully activated within the kidneys. The active form calcitriol is a regulator of the carrior system that intestinal cells use to absorb Ca2+ from food.
• Adipose tissue – release leptin which serves to tell your body how much stores energy (as fat) you have.
• Bone – osteoblast secrete osteocalcin prods pancreatic beta cells to divide and secrete more insulin.
• Thymus – secrets several hormones including thymulin, thymopoietins and thymosins these hormones are involved in normal development of T lymphocytes and the immune response.
Describe the effect of aging on endocrine system functioning.
Most endocrine organs operate smoothly until old age. Aging may alter the rate of hormone secretion, breakdown and excretion, or the sensitivity of target cell receptor. It can also be affected by the chronic illnesses common in old age.