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Study Notes: The Endocrine System (Anatomy & Physiology, Chapter 9)

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The Endocrine System

Overview and Functions

The endocrine system is the body's second major controlling system, working alongside the nervous system to regulate physiological processes. Unlike the rapid electrical signaling of the nervous system, the endocrine system uses chemical messengers called hormones that are released into the blood and act on distant target organs.

  • Major processes controlled by hormones:

    • Reproduction

    • Growth and development

    • Mobilization of body defenses

    • Maintenance of homeostasis

    • Regulation of metabolism

  • Endocrinology: The scientific study of hormones and endocrine organs.

The Chemistry of Hormones

Classification of Hormones

Hormones are classified based on their chemical structure, which determines their mechanism of action and solubility.

  • Amino acid-based hormones:

    • Proteins

    • Peptides

    • Amines

  • Steroids: Derived from cholesterol.

  • Prostaglandins: Made from highly active lipids that act as local hormones.

Hormone Action

Target Cells and Receptors

Hormones affect only specific tissues or organs known as target cells or target organs. Target cells must possess specific protein receptors for the hormone to bind and exert its effect.

  • Hormone binding alters cellular activity.

Cellular Effects of Hormones

Hormones can arouse cells or alter their activity in several ways:

  1. Change plasma membrane permeability or membrane potential by opening or closing ion channels.

  2. Activate or inactivate enzymes.

  3. Stimulate or inhibit cell division.

  4. Promote or inhibit secretion of a product.

  5. Turn on or turn off transcription of certain genes.

Mechanisms of Hormone Action

  • Direct gene activation: Used by steroid hormones and thyroid hormone.

  • Second-messenger system: Used by protein and peptide hormones.

Direct Gene Activation

  1. Steroid hormone diffuses through the plasma membrane of target cells.

  2. Once inside, the hormone enters the nucleus.

  3. The hormone binds to a specific protein within the nucleus.

  4. The hormone-receptor complex binds to specific sites on the cell's DNA.

  5. Certain genes are activated to transcribe messenger RNA (mRNA).

  6. New proteins are synthesized.

Second-Messenger System

  1. Hormone (first messenger) binds to a receptor protein on the plasma membrane.

  2. Activated receptor sets off a series of reactions that activates an enzyme.

  3. Enzyme catalyzes a reaction that produces a second-messenger molecule (e.g., cyclic AMP, cAMP).

  4. Second messenger molecule oversees additional intracellular changes to promote a specific response in the target cell.

Stimuli for Control of Hormone Release

Negative Feedback Mechanisms

Hormone levels in the blood are regulated primarily by negative feedback mechanisms.

  • A stimulus or low hormone levels in the blood trigger the release of more hormone.

  • Hormone release stops once an appropriate level in the blood is reached.

Categories of Stimuli

Endocrine glands are activated by three major types of stimuli:

  1. Hormonal stimuli: Most common; endocrine organs are activated by other hormones. Example: Hormones of the hypothalamus stimulate the anterior pituitary to secrete its hormones.

  2. Humoral stimuli: Changing blood levels of certain ions and nutrients stimulate hormone release. Examples:

    • Parathyroid hormone and calcitonin are produced in response to changing levels of blood calcium.

    • Insulin is produced in response to changing levels of blood glucose.

  3. Neural stimuli: Nerve fibers stimulate hormone release. Example: Sympathetic stimulation of the adrenal medulla to release epinephrine and norepinephrine.

The Major Endocrine Organs

List of Major Endocrine Organs

  • Hypothalamus

  • Pituitary gland

  • Pineal gland

  • Thyroid gland

  • Parathyroid glands

  • Thymus

  • Adrenal glands

  • Pancreas

  • Gonads (testes and ovaries)

Types of Endocrine Glands

  • Purely endocrine: anterior pituitary, thyroid, adrenals, parathyroids

  • Mixed glands (endocrine and exocrine functions): pancreas, gonads

  • Endocrine glands are ductless; hormones are released directly into blood or lymph.

Pituitary Gland and Hypothalamus

Pituitary Gland

  • Pea-sized gland hanging by a stalk from the hypothalamus.

  • Protected by the sella turcica of the sphenoid bone.

  • Two functional lobes:

    • Anterior pituitary: glandular tissue

    • Posterior pituitary: nervous tissue

  • Called the "master endocrine gland" due to its control over many other endocrine glands.

Hypothalamus

  • Produces releasing and inhibiting hormones that control the anterior pituitary.

  • Hormones are released into portal circulation connecting hypothalamus to anterior pituitary.

  • Makes two hormones: oxytocin and antidiuretic hormone (ADH), which are stored in the posterior pituitary and released in response to nerve impulses.

Posterior Pituitary Hormones

  • Oxytocin:

    • Released during childbirth and nursing.

    • Stimulates uterine contractions and milk ejection (let-down reflex).

  • Antidiuretic hormone (ADH):

    • Inhibits urine production by promoting water reabsorption in kidneys.

    • Decreases urine volume, increases blood volume and pressure.

    • In large amounts, causes vasoconstriction (vasopressin).

    • Alcohol inhibits ADH secretion.

Anterior Pituitary Hormones

  • Six major hormones:

    1. Growth hormone (GH): General metabolic hormone; stimulates growth of skeletal muscles and long bones, protein synthesis, and fat breakdown.

    2. Prolactin (PRL): Stimulates and maintains milk production after childbirth; function in males is unknown.

    3. Follicle-stimulating hormone (FSH): Stimulates follicle development in ovaries and sperm development in testes.

    4. Luteinizing hormone (LH): Triggers ovulation in females; stimulates testosterone production in males.

    5. Thyroid-stimulating hormone (TSH): Influences growth and activity of the thyroid gland.

    6. Adrenocorticotropic hormone (ACTH): Regulates endocrine activity of the adrenal cortex.

  • Four are tropic hormones (stimulate other endocrine glands to release hormones).

Pineal Gland

  • Located in the roof of the third ventricle of the brain.

  • Secretes melatonin, which regulates sleep/wake cycles and may coordinate reproductive hormones.

Thyroid Gland

  • Located at the base of the throat, inferior to the Adam's apple.

  • Consists of two lobes and a connecting isthmus.

  • Follicles store colloidal material from which thyroid hormone is derived.

  • Produces two hormones:

    1. Thyroid hormone: Major metabolic hormone; controls oxidation of glucose for ATP and heat, needed for growth and development.

      • Composed of Thyroxine (T4) and Triiodothyronine (T3).

    2. Calcitonin: Decreases blood calcium by promoting calcium deposition in bone; antagonistic to parathyroid hormone.

Parathyroid Glands

  • Tiny masses on the posterior of the thyroid gland.

  • Secrete parathyroid hormone (PTH), the most important regulator of blood calcium (Ca2+) homeostasis.

  • PTH is a hypercalcemic hormone (increases blood calcium):

    • Stimulates osteoclasts to remove calcium from bone.

    • Stimulates kidneys and intestine to absorb more calcium.

Hormonal Control of Blood Calcium

Calcium homeostasis is maintained by the antagonistic actions of calcitonin and PTH.

Thymus

  • Located in the upper thorax, posterior to the sternum.

  • Largest in infants and children; decreases in size with age.

  • Produces thymosin, which matures certain white blood cells and is important for immune system development.

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