HAP_11e_Lecture_Ch 16A

Overview of the Endocrine System

• The endocrine system works with the nervous system to coordinate and integrate body activities.

• Hormones are the chemical messengers of the endocrine system, transported via blood.

• Endocrine responses are slower but have longer-lasting effects compared to the nervous system.

• Endocrinology: The study of hormones and the glands that produce them.

Functions of the Endocrine System

• The endocrine system controls and integrates:

  • Reproduction

  • Growth and development

  • Maintenance of electrolyte, water, and nutrient balance of blood

  • Regulation of cellular metabolism and energy balance

  • Mobilization of body defenses

Glands of the Endocrine System

• Endocrine Glands (secrete hormones directly into the bloodstream):

  • Pituitary gland

  • Thyroid gland

  • Adrenal gland

  • Pineal gland

  • Parathyroid gland

• Exocrine Glands (produce non-hormonal substances and have ducts):

  • Examples include sweat and salivary glands.

• Neuroendocrine Organs: The hypothalamus acts as a neuroendocrine organ that controls various bodily functions.

Types of Cell Signaling in Endocrine Systems

• Hormones: Long-distance signals affecting target cells in different locations.

• Autocrines: Chemicals that exert effects on the same cells that secrete them.

• Paracrines: Local signaling chemicals that affect nearby cells.

Classification of Hormones

1. Chemical Nature:

   • Amino Acid-Based Hormones: Include amino acid derivatives, peptides, and proteins.

   • Steroid Hormones: Derived from cholesterol; includes gonadal and adrenocortical hormones.

2. Mechanism of Action:

   • Water-Soluble Hormones (Amino acid-based): Act on plasma membrane receptors and often use second-messenger systems.

   • Lipid-Soluble Hormones (Steroid and thyroid hormones): Pass through cell membranes and act on intracellular receptors, directly influencing gene expression.

Mechanism of Action of Water-Soluble Hormones

• Second-Messenger Systems: Include cAMP and PIP2-calcium mechanisms.

  • cAMP signaling: hormone binds to receptor → activates G protein → activates adenylate cyclase → converts ATP to cAMP (second messenger) → cascades cause target cell response.

  • PIP2-Calcium signaling: hormone binds → activates phospholipase C → splits PIP2 into DAG (activates protein kinases) and IP3 (causes Ca2+ release).

Hormone Release Regulation

• Hormone levels are maintained within narrow ranges by negative feedback:

  • Increased hormone action on target organs inhibits further hormone release.

• Release can be triggered by various stimuli:

  • Humoral Stimuli: Changes in blood levels of ions/nutrients (e.g., declining blood Ca2+ triggers PTH release).

  • Neural Stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic nervous system stimulates adrenal medulla).

  • Hormonal Stimuli: Hormones from one gland stimulate hormones from another (hypothalamus triggers anterior pituitary).

Target Cell Specificity

• Target cells must have specific receptors for hormones to exert effects, determined by:

  • Blood levels of hormone

  • Number of receptors

  • Affinity of binding between hormone and receptor

• Receptor Regulation:

  • Up-regulation: Increased receptor number in response to low hormone levels.

  • Down-regulation: Decreased receptor number in response to high hormone levels.

Half-Life, Onset, and Duration of Hormone Activity

• Hormones may circulate free or bound to plasma proteins.

• Half-life: Time for the hormone level to decrease by half; varies by hormone type.

• Onset: Varies from immediate responses to several hours/days (especially for steroid hormones).

• Effects typically last from seconds to several hours depending on concentration and hormone type.

Hormonal Interactions

• Permissiveness: One hormone requires presence of another to exert effects.

• Synergism: Multiple hormones produce mutual amplifying effects.

• Antagonism: One hormone opposes the action of another (e.g., insulin vs. glucagon).

The Hypothalamus and Pituitary Gland

• Structure: Separation of the anterior and posterior pituitary:

  • Posterior Pituitary: Neural tissue, stores and releases neurohormones (oxytocin, ADH).

  • Anterior Pituitary: Glandular tissue, regulated by hypothalamus through releasing/inhibiting hormones.

• The hypothalamus plays a crucial role in regulating the secretion of hormones from the pituitary gland, using a two-part system: neural connection for posterior pituitary and portal system for anterior pituitary.