Endocrine System and Hormones

Regulatory Systems of the Body

• The endocrine and nervous system regulate the body together.
• The nervous system uses action potentials and neurotransmitters, with effects in milliseconds and high specificity.
• The endocrine system uses the circulatory system and hormones which can have varying response times, and affect broad cells where there are receptors for that particular hormone
• Most hormones operate through negative feedback mechanisms.

The Endocrine System

• Ductless glands secrete hormones into the circulatory system.
• A hormone is defined as something "set into motion".
• Hormones are secreted in small amounts and stimulate cells for specific responses.
• Target cells have receptors for specific hormones and respond when the hormones bind to the receptors.

Hormone Overview

• Hormones are produced by specialized cells and secreted into extracellular fluids.
• Blood transfers hormones to target sites.
• These hormones regulate the activity of other cells.
• Endocrinology is the scientific study of hormones and endocrine organs.

Chemistry of Hormones

• Hormones are classified chemically as either proteins or steroids/lipids.
• Protein hormones include those made of amino acids, peptides, or amines.
• Steroid hormones are derived from cholesterol and include sex hormones.
• Prostaglandins are made from highly active lipids and act as local hormones.
• Lipid hormones are also referred to as fatty acid hormones.

Hormone Action on Target Cells

• Hormones act on target cells which have specific receptors for the hormone to interact with the cell.

Hormone Receptors and Cell Location

• Protein hormones cannot pass through the plasma membrane because they are membrane receptors.
• Lipid hormones pass directly through the membrane to act on receptors inside the cell and are intracellular receptors.

Classes of Receptors

• Nuclear receptors bind to: lipid hormones/cholesterol/steroids.
• Membrane receptors include membrane-spanning subgroups:
  • Ionotropic receptors open channels.
  • Metabotropic receptors activate G-proteins or enzymes inside the cell when a ligand binds.
• Protein hormones uses membrane receptors.

Hormone Action on Cellular Activity

• Hormones alter cellular activity by:
  • Changing plasma membrane permeability by opening or closing ion channels.
  • Activating or inactivating enzymes.
  • Stimulating or inhibiting cell division.
  • Promoting or inhibiting secretion of a product.
  • Turning on or off transcription of certain genes.

Direct Gene Activation

• Steroid hormones diffuse through the plasma membrane into target cells.
• Steroid hormones enter the nucleus and bind to a specific protein within the nucleus.
• The hormone-receptor complex then binds to specific sites on the cell's DNA.
• Certain genes are activated, resulting in the synthesis of new proteins.

Second-Messenger System

• A hormone (first messenger) binds to a membrane receptor.
• This triggers protein/nonsteroid hormone action that is metabotropic.
• The activated receptor initiates a series of reactions activating an enzyme.
• The enzyme catalyzes a reaction that produces a second-messenger molecule, such as cyclic AMP (cAMP).
• This second messenger oversees additional intracellular changes to promote a specific response in the target cell.

Release of Hormones & Gland Stimulation

• Hormone release is controlled by:
  • Other hormones, where an endocrine gland is stimulated by a hormone to release its own hormone.
  • Humoral or blood levels of chemicals.
  • The nervous system.

Hormonal Stimulus

• Involves the hypothalamus secreting hormones that stimulate the anterior pituitary gland to secrete hormones that stimulate other endocrine glands.

Humoral Stimulus

• Occurs when capillary blood contains a low concentration of Calcium which stimulates secretion of parathyroid hormone (PTH) by parathyroid glands.

Neural Stimulus

• A preganglionic sympathetic fiber stimulates adrenal medulla cells.
• This causes secretion of catecholamines (epinephrine and norepinephrine).

Regulation of Receptors

• Up-regulation involves an increase in the number of receptors on a target cell, increasing sensitivity to a hormone.
  • Example: More oxytocin receptors during the last trimester of pregnancy to stimulate uterine smooth muscle contraction.
• Down-regulation involves a decrease in the number of receptors on a target cell, decreasing sensitivity.
  • Example: This can be caused by certain medications or type 2 diabetes.

Major Endocrine Organs

• The major endocrine organs include the pineal gland, hypothalamus, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands, pancreas, ovaries (female), and testes (male).

Pituitary Gland and Hypothalamus

• The pituitary gland is the size of a pea and hangs from the hypothalamus in the brain.
• It is protected by the sphenoid bone and has two functional lobes: the anterior (glandular tissue) and posterior (nervous tissue).
• The pituitary gland is often called the "master endocrine gland."

Posterior Pituitary

• The posterior pituitary (lobe) releases hormones that are produced in the hypothalamus and stored in the posterior pituitary:
  • Antidiuretic hormone (ADH).
  • Oxytocin (uterine contractions, milk letdown).

Anterior Pituitary Hormones

• Synthesizes and releases hormones.
• Many are tropic hormones that regulate the secretion of hormones from other endocrine glands.
• Several of these hormones have the word "stimulating" in their name.
• The anterior pituitary produces 6 hormones which have a direct effect on tissues, and 4 stimulate other endocrine glands:
  • Growth Hormone.
  • Prolactin.
  • Thyroid Stimulating Hormone/thyrotropic hormone.
  • Follicle Stimulating Hormone.
  • Luteinizing Hormone.
  • Adrenocorticotropic Hormone.

Growth Hormone (GH)

• The hormone does not go to another gland, but to tissues.
• Major effects are directed towards growth of skeletal muscles and long bones.
• Pituitary dwarfism results from hyposecretion of GH during childhood.
• Gigantism results from hypersecretion of GH during childhood.
• Acromegaly results from hypersecretion of GH during adulthood.

Thyroid Gland

• Responsible for regulating metabolism and found in the anterior neck, inferior to the thyroid cartilage (Adam's apple) and superior to the trachea.
• It produces thyroid hormone (T3, T4) and calcitonin.
• Thyroid hormone is the major metabolic hormone that controls the rate of oxidation of glucose for body heat and chemical energy.
• Iodine is necessary for the synthesis of thyroid hormone.
• Calcitonin hormone is produced in the thyroid gland, and sends blood calcium to bone if blood calcium is too high.

Thyroid Hormone Disorders

• Goiters: The thyroid gland enlarges due to lack of iodine; salt is iodized to prevent goiters.
• Hypothyroidism:
  • Myxedema is hypothyroidism in adults, resulting in mental and physical sluggishness.
  • Cretinism is severe hypothyroidism in children, causing dwarfism, intellectual impairment, scanty hair, dry skin, and a thick protruding tongue.
• Graves' disease/Hyperthyroidism: This results in increased metabolism, heat intolerance, rapid heartbeat, weight loss, and exophthalmos.

Adrenal/Suprarenal Glands

• Found superior to the kidneys, consists of two parts.
  • Inner Medulla: neural tissue layer
  • Outer Cortex: synthesizes corticosteroids from cholesterol.
• Adrenocorticotropic hormone released from the anterior pituitary stimulates the cortex.
• The nervous system (sympathetic) stimulates the medulla.

Adrenal Cortex

• Outer Layer (mineralocorticoids): This mainly produces aldosterone which regulates mineral (salt) content in blood, water, and electrolyte balance. Target organ is the kidney.
• Middle Layer (glucocorticoids: cortisone, cortisol): Helps resist long-term stressors by increasing blood glucose levels and provides anti-inflammatory properties.
• Inner Layer: Produces sex hormones.

Adrenal Medulla

• The adrenal medulla produces two similar hormones (catecholamines):
  • Epinephrine (adrenaline)
  • Norepinephrine (noradrenaline)
• These hormones prepare the body to deal with short-term stress ("fight or flight") by:
  • Increasing heart rate.
  • Increasing blood pressure.
  • Increasing alertness.
  • Increasing blood glucose levels.
  • Dilating small passageways of lungs.

Cushing’s Syndrome

• High levels of cortisol over a prolonged period of time can lead to Hypercortisolism/Cushing syndrome.
• Can be caused by prescribed medications or due to hypersecretion of cortisol.
• Symptoms include fat pads (“buffalo hump”) and “moon face”.

Gonads

• FSH from the anterior pituitary stimulates follicle development in the ovaries and sperm development in the testes.
• Gonads are the male and female reproductive organs that produce gamates, the spermatozoa and ova that fuse to start new life.
• Testes and ovaries secrete hormones to regulate the development of the reproductive system and secondary sex characteristics.
  • Testes: secrete testosterone.
  • Ovaries: secrete estrogen and progesterone.

Parathyroid Glands

• Four small oval masses on the posterior side of the thyroid gland.
• Produces parathyroid hormone, the most important hormone in controlling blood calcium.
• Stimulates osteoclasts to dissolve bone, releasing calcium into the blood.
• Increases calcium absorption at the intestine and decreases excretion at the kidney.
• If blood calcium drops too low (hypocalcemia), this can causes spasms, tetany, heart arrhythmia, seizures, etc.

Pancreas

• Known as "double gland".
• Exocrine: Acinar cells produce digestive pancreatic enzymes.
• Endocrine: Islet cells produce insulin (B cells) and glucagon (a cells).
• Endocrine portion regulates blood sugar.

Blood Glucose

• When glucose levels in the blood rise, beta islet cells release insulin hormone.
• Insulin stimulates glycogen formation, enhances glucose and fatty acid transport into cells, and storage of energy.
• When glucose levels fall, alpha islet cells release glucagon.
• Glucagon stimulates glycogen breakdown and the release of glucose by the liver.
• Diabetes:
  • Type 1: Islets do not make insulin.
  • Type 2: Receptors are faulty.

Homeostatic Imbalance of Glucose

• Diabetes occurs in the absence of insulin or the inability to use insulin.
• Blood sugar levels increase dramatically, and blood glucose is lost in the urine, drawing water along with it.
• The three cardinal signs of diabetes are:
  • Polyuria.
  • Polydipsia.
  • Polyphagia.

Thymus Gland

• Located posterior to the sternum, it is largest in infants and children.
• It produces thymosin, which supports the normal development of T-lymphocytes and is important for the white blood cell immune system.

Pineal Gland

• The pineal gland secretes melatonin in response to light detected by the eyes.
• Melatonin levels normally rise and fall in a diurnal (daily) cycle, peaking at night and is important for sleep.
• It is affected by reproduction and associated with circadian rhythms, which are 24-hour cycle in living entities.
• Melatonin affects jet lag.

Other Hormone Producing Structures

• Other hormone-producing cells occur in various organs.
• The heart produces atrial natriuretic peptide to reduce blood volume.
• The GI tract has hormone-secreting cells sprinkled in the mucosa.
• Skin produces cholecalciferol, a precursor to Vitamin D.
• Kidneys create erythropoietin for production of blood cells and renin (enzyme) for blood pressure.
• Adipose tissue produces leptin and cytokines.
• The placenta is a temporary gland producing HCG, the hormone tested for in pregnancy tests.