Ch. 18 Endocrine System Notes

Comparison of Control by Nervous and Endocrine Systems

• The nervous and endocrine systems coordinate body systems.
• The nervous system releases neurotransmitters locally.
• The endocrine system releases hormones that circulate in the blood and bind to target cells.

Nervous System vs. Endocrine System

• Nervous System:
  • Molecules: Neurotransmitters released locally.
  • Site of action: Close to the release site at a synapse.
  • Types of target cells: Muscle fibers, gland cells, other neurons.
  • Time to onset of action: Milliseconds.
  • Duration of action: Brief (milliseconds).
• Endocrine System:
  • Molecules: Hormones delivered throughout the body by blood.
  • Site of action: Far from the release site.
  • Types of target cells: Cells throughout the body.
  • Time to onset of action: Seconds to hours or days.
  • Duration of action: Longer (seconds to days).

Endocrine Glands

• Exocrine glands: Secrete products into ducts (e.g., sweat, oil, mucous, digestive glands).
• Endocrine glands: Secrete hormones directly into the interstitial fluid, then diffuse into the bloodstream.
  • Examples: Pituitary, thyroid, parathyroid, adrenal, and pineal glands.
• Certain organs and tissues (e.g., hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, adipose tissue, and placenta) also secrete hormones.

Endocrine Gland Locations

• Pineal gland
• Hypothalamus
• Pituitary gland
• Thyroid gland
• Parathyroid glands
• Thymus
• Heart
• Stomach
• Suprarenal glands
• Pancreas
• Small intestine
• Kidney
• Ovaries
• Testes

Hormone Activity

• Hormones affect target cells with specific protein receptors.
• Receptors are synthesized and broken down continuously.
• Downregulation: High hormone concentrations decrease receptor numbers.
• Upregulation: Low hormone concentrations increase receptor numbers.
• Circulating hormones: Travel throughout the body.
• Local hormones (paracrines): Act on neighboring cells.
• Autocrines: Act on the same cell that secretes them.
• Hormone Types
  • Lipid-soluble hormones: circulate bound to transport proteins
  • Water-Soluble hormones: circulate freely in the plasma

Chemical Classes

• Lipid-Soluble Hormones:

  • Steroid hormones
  • Thyroid hormones
  • Nitric oxide

• Water-Soluble Hormones:

  • Amine hormones
  • Peptide and protein hormones
  • Eicosanoid hormones

Mechanisms of Hormone Action

• Responses vary depending on the hormone and target cell.
• Responses include:
  • Synthesis of new molecules.
  • Changing cell membrane permeability.
  • Stimulating substance transport into or out of the cell.
  • Altering metabolic action rates.
  • Causing muscle contraction.
• Lipid-soluble hormones: Bind to receptors within target cells.
• Water-soluble hormones: Bind to receptors on the exterior surface of the target cell.

Factors Influencing Target Cell Response

• Hormone concentration in the blood.
• Number of hormone receptors on the target cell.
• Influences of other hormones (synergistic or antagonistic effects).
  • Synergistic effect: Hormones work more effectively together.
  • Antagonistic effect: Hormones oppose each other.

Control of Hormone Secretion

• Hormones are secreted in short bursts when needed.

• Regulated by:

  • Signals from the nervous system.
  • Chemical changes in the blood.
  • Other hormones.

• Negative feedback: Most hormone regulation is achieved via negative feedback.

• Positive feedback: Only a few hormones operate via positive feedback.

Hypothalamus and Pituitary Gland

• The hypothalamus and pituitary gland work together to control other endocrine glands via the infundibulum.

Pituitary Gland Lobes

• Anterior lobe (adenohypophysis): Makes up 75% of the pituitary gland's weight and secretes 7 hormones.
• Posterior lobe (neurohypophysis): Made of neural tissue and releases two hormones made by the hypothalamus.
• Hypothalamus secretes releasing and inhibiting hormones that control the release of hormones by the pituitary gland via the hypophyseal portal system.

Anterior Pituitary Hormones

• Growth hormone (GH, somatotropin):
  • Secreted by somatotrophic cells.
  • Stimulated by growth hormone-releasing hormone (GHRH, somatocrinin).
  • Suppressed by growth hormone-inhibiting hormone (GHIH, somatostatin).
• Thyroid-stimulating hormone (TSH, thyrotropin):
  • Secreted by thyrotrophic cells.
  • Stimulated by thyrotropin-releasing hormone (TRH).
  • Suppressed by growth hormone-inhibiting hormone (GHIH).
• Follicle-stimulating hormone (FSH):
  • Secreted by gonadotrophic cells.
  • Stimulated by gonadotropin-releasing hormone (GnRH).
• Luteinizing hormone (LH):
  • Secreted by gonadotrophic cells.
  • Stimulated by gonadotropin-releasing hormone (GnRH).
• Prolactin (PRL):
  • Secreted by prolactin cells.
  • Stimulated by prolactin-releasing hormone (PRH).
  • Suppressed by prolactin-inhibiting hormone (PIH, dopamine).
• Adrenocorticotropic hormone (ACTH, corticotropin):
  • Secreted by corticotrophic cells.
  • Stimulated by corticotropin-releasing hormone (CRH).
• Melanocyte-stimulating hormone (MSH):
  • Secreted by corticotrophic cells.
  • Stimulated by corticotropin-releasing hormone (CRH).
  • Suppressed by dopamine.

Human Growth Hormone

• Human growth hormone (hGH) is the most plentiful anterior pituitary hormone.
• It is released in bursts every few hours by somatotrophs.
• Two hypothalamic hormones control activity: growth hormone-releasing hormone (GHRH) and growth hormone-inhibiting hormone (GHIH).
• Factors that stimulate GHRH release:
  • Hypoglycemia
  • Decreased blood levels of fatty acids
  • Increased blood levels of amino acids
  • Sympathetic activity
  • Deep sleep
  • Testosterone, estrogens, thyroid hormones, and ghrelin
• Factors that stimulate GHIH release:
  • Hyperglycemia
  • Increased blood levels of fatty acids
  • Decreased blood levels of amino acids
  • Obesity
  • Aging
  • High blood levels of GH and IGFs

Anterior Pituitary Hormones Summary

• Human growth hormone (hGH)
• Thyroid-stimulating hormone (TSH)
• Follicle-stimulating hormone (FSH)
• Luteinizing hormone (LH)
• Prolactin (PRL)
• Adrenocorticotropic hormone (ACTH)
• Melanocyte-stimulating hormone (MSH)

Posterior Pituitary Gland

• The posterior pituitary gland does not synthesize any hormones but stores and releases two hormones produced by the neurosecretory cells of the hypothalamus:
  • Oxytocin (OT)
  • Antidiuretic hormone (ADH)
• Axons from the neurosecretory cells form the hypothalamohypophyseal tract.

Oxytocin

• Oxytocin is released in response to stretch placed on the cervix during childbirth.
• Oxytocin affects the:
  • Mother’s uterus – enhances contractions.
  • Mother’s breasts – stimulates milk ejection by the mammary glands in response to suckling.

Antidiuretic Hormone

• The amount of antidiuretic hormone (ADH) secreted varies with blood osmotic pressure.
  • ADH decreases urine output as part of a negative feedback loop where osmoreceptors in the hypothalamus monitor blood osmotic pressure.
• An increase in blood volume causes a decrease in ADH secretion.
• A decrease in blood volume causes an increase in ADH secretion.

Thyroid Gland

• The thyroid gland is a butterfly-shaped gland located inferior to the larynx and anterior to the trachea.
  • It has right and left lateral lobes connected by an isthmus.
  • Some glands also have a pyramidal lobe projecting from the isthmus.

Cells of the Thyroid

• Follicular cells are stimulated by TSH to produce thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3) known as thyroid hormones.
• Parafollicular cells produce the hormone calcitonin to help regulate calcium homeostasis.

Thyroid Hormones

• T3 and T4 are synthesized and secreted in an 8-step process.
• Thyroid hormones:
  • Increase basal metabolic rate (BMR).
  • Help maintain normal body temperature.
  • Stimulate protein synthesis.
  • Increase the use of glucose and fatty acids for ATP production.
  • Upregulate beta (\beta) receptors that attach to catecholamines.
  • Work with hGH and insulin to accelerate body growth.
• Thyrotropin-releasing hormone (TRH) from the hypothalamus and thyroid-stimulating hormone (TSH) from the anterior pituitary stimulate synthesis and release of thyroid hormones in a 5-step process.

Parathyroid Glands

• Located on the posterior aspect of each lobe of the thyroid gland are 2 parathyroid glands (one inferior and one superior).

Parathyroid Cells

• Parathyroid glands contain 2 types of cells:
  • Chief cells (principal cells) that produce parathyroid hormone (PTH, parathormone).
  • Oxyphil cells whose function is not known in normal parathyroid glands, but which secrete excess PTH in cases of parathyroid cancer.
• Calcitonin produced by the thyroid gland works in conjunction with PTH and calcitriol to regulate calcium homeostasis.

Suprarenal Glands

• The suprarenal glands are located on top of each kidney.
• The glands are covered by a connective tissue capsule.
• The glands are divided into two regions:
  • The outer cortex.
  • The inner medulla.

Suprarenal Cortex Layers

• The cortex is divided histologically into 3 regions:
  1. The zona glomerulosa
  2. The zona fasciculata
  3. The zona reticularis

Hormones of the Cortex

• The zona glomerulosa secretes hormones called mineralocorticoids used to regulate mineral homeostasis
• The zona fasciculata secretes hormones called glucocorticoids that affect glucose homeostasis.
• The zona reticularis secretes weak androgens (hormones with masculinizing effects).

Aldosterone

• Aldosterone is the major mineralocorticoid secreted by the adrenal gland. It helps regulate sodium and potassium homeostasis.
• The renin-angiotensin-aldosterone (RAA) pathway controls secretion of aldosterone.

Glucocorticoids

• Secretion of glucocorticoids (cortisol [hydrocortisone]-the most produced, cortisone and corticosterone) is regulated by negative feedback.
• They help control:
  • Protein breakdown.
  • Glucose formation.
  • Lipolysis
  • Resistance to stress.
  • Inflammation.
  • Immune responses

Androgens

• The major androgen secreted by the suprarenal cortex is dehydroepiandrosterone (DHEA).
  • In males, after puberty, the hormone testosterone is secreted in much larger quantities so DHEA has virtually no effect.
  • In females, DHEA and other adrenal androgens play a major role in promoting libido and are converted to estrogens.
  • In menopausal women, all female estrogens come from adrenal androgens.

Adrenal Medulla

• The adrenal medulla is stimulated by sympathetic preganglionic neurons of the autonomic nervous system (ANS).
• Chromaffin cells secrete epinephrine (adrenaline) and norepinephrine (noradrenaline) both of which are involved in the fight-or-flight response.

Pancreatic Islets

• The pancreas is located in the curve of the duodenum.
  • It is both an endocrine and exocrine gland.
• Almost all of the exocrine cells of the pancreas are arranged in clusters called acini.
  • Acini produce digestive enzymes that are delivered to the gastrointestinal tract through ducts.
• Scattered among the acini are clusters of endocrine tissue called pancreatic islets (islets of Langerhans).
  • The islets contain secreting cells.

Cells of the Pancreatic Islets

• The islets include 4 types of cells that secrete different hormones:
  • Alpha (A) cells - glucagon
  • Beta (B) cells - insulin
  • Delta (D) cells - somatostatin
  • F cells - pancreatic polypeptide

Insulin and glucagon regulation

• Secretion of insulin and glucagon are controlled by negative feedback.

Ovaries and Testes

• Gonads (ovaries and testes) produce gametes (oocytes and sperm respectively).
  • Ovaries produce two estrogens (estradiol and estrone), progesterone, relaxin, and inhibin.
  • Testes produce testosterone.

Pineal Gland and Thymus

• The pineal gland is attached to the roof of the third ventricle of the brain and secretes melatonin.
  • Melatonin helps to regulate the body’s biological clock.
• The thymus is located behind the sternum between the lungs and produces thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin.
  • These hormones promote maturation of the immune system’s T cells.

Other Endocrine Tissues and Organs

• Several tissues and organs which are not part of the endocrine system are able to produce hormones.
• Skin: Cholecalciferol plays a role in the synthesis of calcitriol, the active form of vitamin D.
• Gastrointestinal tract: Gastrin, Glucose-dependent insulinotropic peptide (GIP), Secretin, Cholecystokinin (CCK)
• Placenta: Human chorionic gonadotropin (hCG), Estrogens and progesterone, Human chorionic somatomammotropin (hCS)
• Kidney: Renin, Erythropoietin (EPO), Calcitriol
• Heart: Atrial natriuretic peptide (ANP)
• Adipose Tissue: Leptin

Eicosanoids

• Eicosanoids are locally-acting hormones derived from the 20-carbon fatty acid arachidonic acid.
  • Certain hormones stimulate cell growth and division.
  • Several newly discovered hormones called growth factors are involved in tissue development, growth, and repair.

Growth Factors

• Epidermal growth factor (EGF)
• Platelet-derived growth factor (PDGF)
• Fibroblast growth factor (FGF)
• Nerve growth factor (NGF)
• Tumor angiogenesis factors (TAFs)
• Transforming growth factors (TGFs)

The Stress Response

• Eustress is helpful, everyday stress that prepares us to meet challenges.
• Distress is any type of harmful stress that may be damaging.
  • The fight-or-flight response (first stage of the stress response) stimulates the body’s resources to prepare for immediate activity.
  • The resistance reaction is the second stage in the stress response and lasts longer than the fight-or-flight response.
  • If this lasts too long, exhaustion will result.

Development of the Endocrine System

• Glands of the endocrine system develop from all 3 primary germ layers.

Aging and the Endocrine System

• Aging brings about changes in the levels of most hormones.
  • Some hormone levels increase while some decrease.
  • Levels of other hormones, like epinephrine and norepinephrine, remain the same.
• Histologically, most endocrine glands reduce in size and contain increasingly more fibrous connective tissue with age.

Pituitary Gland Disorders

• Pituitary gigantism and acromegaly are caused by excess secretion of growth hormone.
• Goiter is caused by a reduction in the production of thyroid hormone.
• Graves disease (with associated exophthalmos) develops due to excess thyroid hormone.

Suprarenal Gland Disorders

• Cushing’s Syndrome is characterized by hypersection of the suprarenal cortex.
• Addison’s Disease is caused by hyposecretion of glucocorticoids and aldosterone.
• Pheochromocytomas are benign tumors causing hypersecretion of epinephrine and norepinerphrine.

Pancreatic Islet Disorders

• Diabetes Mellitus is the most common endocrine disorder, caused by the inability to produce insulin.
  • Type I Diabetes – autoimmune disease where beta cells are destroyed.
  • Type II Diabetes – more common type of diabetes often linked to lifestyle factors such as obesity and inactivity.