Chp 19 Mechanisms of Hormonal Regulation-RL

CHAPTER 19 Mechanisms of Hormonal Regulation

19.1 The Endocrine System

  • Functions of the endocrine system include:

    • Differentiation of the reproductive system and central nervous system (CNS) during fetal development.

    • Stimulation of growth and development throughout life.

    • Coordination of male and female reproductive systems.

    • Maintenance of internal environmental homeostasis.

    • Adaptation to emergency demands placed on the body.

19.2 Major Endocrine Glands

  • Pineal gland

  • Pituitary gland

  • Parathyroid glands (on posterior surface of thyroid gland)

  • Thymus

  • Adrenal gland

  • Pancreas

  • Ovary

  • Testis

19.3 Hormones: General Characteristics

  • Hormones have specific rates and rhythms of secretion:

    • Diurnal, pulsatile, and cyclic patterns based on circulating substances.

  • Operate within feedback systems to maintain homeostasis.

  • Affect only target cells with the appropriate receptors.

  • Are excreted by kidneys or deactivated through liver mechanisms or cellular processes.

19.4 Regulation of Hormone Release

  • Hormones can be released in response to:

    • Alterations in the cellular environment.

    • The need to maintain regulated levels of specific substances or other hormones.

  • Regulated by:

    • Chemical factors

    • Endocrine interactions

    • Neural signals

  • Released into the circulatory system by endocrine glands for distribution throughout the body.

19.4.1 Feedback Mechanisms

  • Positive feedback:

    • Response to a stimulus increases hormone synthesis and secretion (e.g., thyrotropin-releasing hormone).

  • Negative feedback:

    • Response to a stimulus decreases hormone synthesis and secretion (e.g., thyroxine and triiodothyronine).

19.5 Feedback Loops

  • Involves interactions between:

    • Central nervous input

    • Hypothalamus and pituitary gland

    • Releasing hormones and factors (inhibiting and releasing)

    • Anterior pituitary and target organs (e.g., TSH influencing thyroid hormone production).

19.6 Hormone Receptors

  • Target cells contain:

    • Plasma membrane or intracellular hormone receptors that recognize and bind with specific hormones.

    • Initiates signals to intracellular effectors.

  • Up-regulation: occurs with low hormone concentrations, increasing number or affinity of receptors.

  • Down-regulation: occurs with high hormone concentrations, decreasing number or affinity of receptors.

19.7 Lipid-Soluble Hormones

  • Include steroid hormones:

    • Circulate primarily bound to carriers.

    • Small, allow for rapid and long-lasting responses.

    • Diffuse across plasma and nuclear membranes, binding with cytosolic or nuclear receptors.

19.8 Water-Soluble Hormones

  • Includes protein hormones and catecholamines:

    • Circulate in free, unbound forms.

    • High molecular weight and produce short-term responses.

    • Cannot diffuse across plasma membranes, bind to surface receptors to initiate signaling pathways.

  • First messenger: The hormone that initiates the signal upon binding.

  • Second messenger: Small molecule produced within the cell as a result of first messenger action (e.g., cAMP, cGMP).

19.9 The Hypothalamic-Pituitary System (HPA)

  • Integrates neurologic and endocrine systems through the neuroendocrine system.

  • Comprises:

    • Hypothalamus

    • Pituitary gland (anterior and posterior)

19.10 Pituitary Gland Functions

  • Anterior Pituitary:

    • Secretes hormones such as TSH, ACTH, LH, FSH, GH, Prolactin.

  • Posterior Pituitary:

    • Secretes oxytocin and vasopressin.

  • Involved in regulating numerous body functions via hormonal signals.

19.11 Specific Hormones and Their Functions

19.11.1 Thyroid Gland

  • Composed of two lobes on either side of the trachea.

  • Secretes Thyroid Hormone (TH):

    • Regulated by the TSH.

    • 90% T4 and 10% T3 with various physiological actions including metabolism and growth regulation.

19.11.2 Parathyroid Glands

  • Produce parathyroid hormone (PTH):

    • Increases serum calcium.

    • Promotes calcium absorption and bone mineralization in conjunction with Vitamin D.

19.11.3 Endocrine Pancreas

  • Contains islets of Langerhans:

    • Alpha cells produce glucagon.

    • Beta cells produce insulin and amylin.

    • Delta cells produce somatostatin, which regulates alpha and beta-cell activities.

19.11.4 Adrenal Glands

  • Comprised of:

    • Adrenal Cortex:

      • Site of steroid hormone synthesis (glucocorticoids and mineralocorticoids).

    • Adrenal Medulla:

      • Secrets catecholamines (epinephrine and norepinephrine), regulating fight or flight response.

19.12 Case Studies

Case Study 1: Breastfeeding Challenges

  • Discussion on the use of oxytocin for milk ejection in lactating women, as well as the roles of prolactin and other hormones in lactation.

Case Study 2: Fight or Flight Response

  • Discussion on catecholamines’ role in the body’s quick response to perceived threats, emphasizing the adrenal medulla's involvement.