Endocrine System

Lecture 20: Endocrine System

Learning Objectives

  1. Compare and contrast the endocrine and nervous systems of control

    • The nervous system:

      • Composed of nervous tissue.

      • Utilizes electrical signals and chemical messages (neurotransmitters).

      • Provides fast, direct communication to target tissues.

    • The endocrine system:

      • Composed of glandular tissue that secretes hormones.

      • Utilizes chemical messages (hormones).

      • Provides slower, indirect communication to target tissues (target cells must have receptors).

  2. Differentiate between steroid and protein hormones in terms of their structure and location of receptors

    • Steroid Hormones:

      • Derived from cholesterol (lipids).

      • Examples: Testosterone and estrogen.

      • Receptors are typically located in the cytoplasm or nucleus of target cells.

    • Protein Hormones:

      • Made from amino acids, often polar and/or negatively charged.

      • Example: Insulin.

      • Receptors are typically located in the plasma membrane of target cells.

  3. Outline a hormone axis (use hGH as an example)

    • The hypothalamic-pituitary axis involves several key hormones:

      • Growth Hormone-Releasing Hormone (GHRH) from the hypothalamus stimulates the release of human growth hormone (hGH) from the anterior pituitary.

      • hGH then triggers the liver to produce Insulin-like Growth Factor 1 (IGF-1).

      • IGF-1 promotes growth and development in various tissues.

      • The feedback mechanism includes somatostatin which inhibits hGH release.

  4. Describe the relationship between GHRH, hGH, IGF-1, and somatostatin in the body and their function related to growth

    • GHRH regulates the release of hGH from the anterior pituitary.

    • hGH acts mainly by promoting the production of IGF-1 in the liver.

    • IGF-1 promotes differentiation and proliferation of chondrocytes and osteoblasts, facilitating growth.

    • Somatostatin counters this effect by inhibiting hGH production, thus maintaining balance in growth regulation.

  5. Analyze the effects of disturbances to the hGH axis

    • Disturbances often lead to abnormal growth conditions:

      • Gigantism: Overproduction of hGH in children results in excessive height.

      • Acromegaly: Overproduction of hGH in adults leads to bone thickening.

  6. Explain the normal hormonal factors that control and eventually end growth, and analyze the effects of disturbances to this process

    • Growth ceases when the epiphyseal plates close.

    • This process is regulated by the interplay of growth hormone, sex hormones (especially estrogen), and factors like body composition.

    • Disturbances, such as hormone imbalances, can lead to conditions such as dwarfism or excessive growth.

  7. Describe the function of parathyroid hormone in calcium regulation

    • Parathyroid hormone (PTH) is crucial for maintaining calcium homeostasis.

      • Secreted by parathyroid glands in response to hypocalcemia (low calcium levels).

      • It increases calcium levels by stimulating bone resorption, enhancing intestinal absorption, and promoting renal reabsorption of calcium.

Vocabulary List

  • Endocrine system

  • Steroid hormone

  • Protein hormone

  • Hypothalamus

  • Anterior pituitary gland

  • Posterior pituitary gland

  • Hypothalamic-pituitary axis

  • Human growth hormone (hGH, GH)

  • Growth hormone releasing hormone (GHRH)

  • Insulin-like growth factors (IGF-1)

  • Somatostatin

  • Gigantism

  • Acromegaly

  • Estrogen

  • Osteoporosis

  • Parathyroid hormone

  • Parathyroid glands

  • Hypocalcemia

  • Hypercalcemia

Homeostasis and Hormonal Regulation

  • Homeostasis requires sensory input and integration between various organs.

  • To attain homeostasis:

    • Nervous System: Uses electrical and chemical signals for rapid communication.

    • Endocrine System: Utilizes hormones for slower communication requiring specific receptors in target cells.

Hormones and Their Mechanism of Action

  • Hormones modulate target cell activity, mostly through protein synthesis.

  • General process includes:

    1. Hormone travels through blood from the source gland to target tissue.

    2. Interaction with target tissue is determined by the presence of receptors:

      • Protein hormones bind to receptors on the membrane.

      • Steroid hormones diffuse into cells and bind to cytoplasmic/nuclear receptors.

    3. Hormones act as switches to alter gene expression, leading to protein synthesis and other molecular changes.

    4. Ultimately, the hormone/receptor complex must be terminated once its action is accomplished.

Endocrine Glands

  • Digestive Glands:

    • Pancreas, liver

  • Reproductive Glands:

    • Gonads (testes and ovaries)

  • Nervous Glands:

    • Hypothalamus, pituitary gland, pineal gland

  • Unique Endocrine Organs**:

    • Thyroid, parathyroid, adrenal glands

  • Some glands are dependent on the CNS, while others function independently.

Hypothalamus and Pituitary Interaction

  • Hypothalamus: Part of the diencephalon, collaborates with the pituitary gland to manage hormone release.

  • Pituitary Gland: Divided into two parts:

    • Anterior Pituitary (Adenohypophysis): Synthesizes and secretes hormones, classifying it as a true endocrine gland.

    • Posterior Pituitary (Neurohypophysis): Stores and secretes hormones and is considered an extension of neural tissue.

  • The pituitary regulates various endocrine organs via hormonal axes.

Hypothalamic-Pituitary Axis

  • Neurons in the hypothalamus produce releasing hormones (RH).

  • These releasing hormones trigger the release of additional hormones from the pituitary gland.

  • The sequence is as follows:

    1. Hypothalamus releases GHRH.

    2. GHRH stimulates GH release from the anterior pituitary.

    3. GH triggers the release of IGF-1 from the liver, impacting growth processes in target tissues.

    4. Somatostatin production is stimulated to inhibit further GH release, establishing a negative feedback loop.

Growth Hormone (GH) Effects

  • GH influences metabolism and growth, including:

    • Stimulating the liver to produce IGFs, which promote:

      • Chondrocyte division and osteoblast activity.

      • Increased protein synthesis and amino acid uptake in muscle and bone.

  • GH levels fluctuate, typically peaking during childhood and adolescence.

    • Average secretory rates: approximately 700 µg/day in teens and around 400 µg/day in adults, decreasing with age.

    • Growth declines as GH levels drop after puberty.

Growth Cessation

  • Growth ends due to:

    • Decreased GH levels post-puberty.

    • Peak sex hormones during puberty, particularly estrogen, which promote osteoblast activity and inhibit osteoclasts.

    • Closure of epiphyseal plates:

      • High GH stimulates chondrocyte division alongside rising estrogen, leading to bone growth until the closure of growth plates occurs through osteoblast competition.

Growth Disorders

  • Achondroplastic Dwarfism:

    • Genetic alteration limiting chondrocyte division in the epiphyseal plate, notably affecting limb growth.

  • Osteoporosis:

    • A condition where bone resorption outpaces bone deposition, primarily affecting spongy bone, particularly in older women post-menopause due to decreased estrogen levels.

    • It is characterized by brittle bones and increased fracture risk.

Parathyroid Hormone and Calcium Regulation

  • PTH is essential for regulating levels of free calcium in the body.

    • Secreted by parathyroid glands, it responds to hypocalcemia and is inhibited by hypercalcemia.

    • PTH targets bone to increase calcium levels through resorption during low calcium states, with feedback mechanisms to stop secretion upon normalization of calcium levels.

Anatomy of Long Bones and Calcium Storage

  • Bone Mineral Composition:

    • Hydroxyapatite ($Ca{10}(PO4)6(OH)2$) is the mineralized constituent of bone tissue, acting as a calcium reservoir.

    • Calcium ions play vital roles in neuronal and muscular functions.

Summary of PTH Action

  • Released PTH elevates calcium concentrations in the body fluids during induced hypocalcemia, and once levels stabilize, its release ceases, exemplifying negative feedback regulation in endocrine function.