Endocrine System Review

Overview of the Endocrine System

  • The endocrine system coordinates with the nervous system to maintain homeostasis using hormones as signaling molecules.

  • Unlike the rapid, localized communication of neurons, the endocrine system releases hormones into the bloodstream to reach distant target cells with specific receptors.

Major Endocrine Glands

  • Primary glands include the pituitary, pineal, thyroid, parathyroid, and adrenal glands.

Types of Hormone Stimulation

  1. Hormonal Stimulation: A hormone triggers the release of another (e.g., TSH stimulating thyroid hormone).

  2. Humoral Stimulation: Changes in blood nutrient or ion levels trigger release (e.g., high glucose stimulating insulin).

  3. Nervous System Stimulation: Nerve signals trigger release (e.g., sympathetic signals stimulating the adrenal medulla).

Chemical Classes of Hormones

  • Steroids: Lipid-soluble; derived from cholesterol (e.g., estrogen, cortisol).

  • Biogenic Amines: Usually water-soluble (except thyroid hormone); derived from amino acids (e.g., melatonin, epinephrine).

  • Proteins: Water-soluble amino acid chains (e.g., insulin, growth hormone).

Mechanisms of Action

  • Lipid-Soluble Hormones: Diffuse across plasma membranes to bind intracellular receptors, forming a hormone-receptor complex (HRC) that initiates mRNA and protein synthesis via DNA binding.

  • Water-Soluble Hormones: Bind to plasma membrane receptors to activate G-proteins and intracellular second messengers (e.g., cAMP).

Receptor Regulation and Interactions

  • Regulation: Cells perform up-regulation (increasing receptors) when hormone levels are low and down-regulation (decreasing receptors) when levels are high.

  • Interactions:

    • Synergistic: Hormones work together for a greater effect.

    • Permissive: One hormone allows another to act.

    • Antagonistic: One hormone opposes the action of another.

The Hypothalamic-Pituitary Axis

  • The hypothalamus regulates the pituitary gland. It produces hormones stored in the posterior pituitary (oxytocin, ADH) and releasing/inhibiting hormones that control the anterior pituitary (GH, TSH, ACTH, etc.).

Key Hormone Functions

  • Growth Hormone (GH): Stimulated by GHRH; promotes tissue growth and nutrient release.

  • Thyroid Hormone (TH): Regulated by TRH and TSH; increases metabolic rate.

  • Cortisol: Regulated by CRH and ACTH; manages stress and increases blood nutrient levels.

  • Insulin & Glucagon: Pancreatic hormones that decrease and increase blood glucose levels, respectively.