BIO222_Ch_18_Endocrine_System_OSp2024_copy

Chapter 18: Endocrine System

Cell Communication

• Four ways cells communicate:

  • Gap Junctions: Pores in cell membrane allow signaling chemicals to move between cells.

  • Neurotransmitters: Released from neurons to travel across a gap to a second cell.

  • Paracrine Hormones: Chemicals secreted into tissue fluids (interstitial fluid) affecting nearby cells.

  • Hormones: Chemical messengers that travel in the bloodstream.

Nervous vs. Endocrine Systems

• Nervous System:

  • Uses both electrical and chemical communication.

  • Fast response (1 - 10 ms), specific effects on fewer cells.

• Endocrine System:

  • Uses only chemical communication.

  • Slow response (hormone release takes seconds to days), widespread effects on the whole body.

Chemical Overlap

• Hormones as neurotransmitters:

  • Examples: Norepinephrine (NE), cholecystokinin, thyrotropin-releasing hormone, dopamine, and ADH.

• Neuroendocrine Cells: Neurons that act like glands, releasing hormones such as oxytocin, ADH, and catecholamines.

• Overlapping effects: Both systems can signal to each other, e.g., NE and glucagon in liver glycogen hydrolysis.

Types of Glands

• Exocrine Glands: Secrete products into a duct leading to epithelial surfaces (skin or digestive tract).

• Endocrine Glands: Secrete hormones into surrounding fluid that diffuse into capillary blood.

• Endocrine Glands Include: Pituitary, thyroid, parathyroid, adrenal, pineal glands, among others.

Hormone Targeting

• Receptors: Hormones affect specific target tissues that possess specific receptors.

• Receptors Dynamics: Constantly synthesized and broken down.

  • Down-regulation: Example: insulin resistance.

  • Up-regulation: Increased receptor availability.

Hormone Classification

• Circulating Hormones: Travel throughout the body in blood.

• Local Hormones: Act locally as paracrine (neighboring cells) or autocrine (same cell).

Hormonal Mechanisms

• Lipid-Soluble Hormones: Bind with receptors inside target cells.

  • Examples: Steroids (cortisol, estrogens), thyroid hormones (T3 and T4).

• Water-Soluble Hormones: Bind to receptors on plasma membranes, activating G proteins activating adenylate cyclase, producing cAMP, which activates kinases and enzymes.

Hormone Interactions

• Permissive Effect: One hormone enhances the action of another.

• Synergistic Effect: Two hormones work together for a greater effect.

• Antagonistic Effect: Two hormones oppose each other.

Control of Hormone Secretion

• Hormonal regulation involves feedback mechanisms to maintain homeostasis:

  • Negative Feedback: Most common regulatory mechanism.

  • Positive Feedback: Example: oxytocin and prolactin during childbirth.

Hypothalamus & Pituitary Gland

• Role of Hypothalamus: Major link between nervous and endocrine systems.

  • Produces hormones that are stored and released by the posterior pituitary (e.g., oxytocin, ADH).

Anterior Pituitary Hormones

• Releasing Hormones from Hypothalamus:

  • TRH (Thyrotropin-Releasing Hormone) → TSH (Thyroid-Stimulating Hormone)

  • GnRH (Gonadotropin-Releasing Hormone) → FSH and LH (Follicle- and Luteinizing Hormones)

• Inhibitory Hormones:

  • GHIH (Somatostatin) → inhibits hGH, gland and hormone regulation.

Thyroid Gland

• Functions of Thyroid Hormones:

  • Increase basal metabolic rate (BMR), stimulate protein synthesis, and regulate development.

  • Calcitonin (from C cells) helps lower blood calcium levels.

• Disorders:

  • Hypothyroidism/managing symptoms and disorders like goiter due to iodine deficiency.

Pancreatic Hormones

• Insulin: Lowers blood sugar by triggering cells to absorb glucose, enhances fat synthesis.

• Glucagon: Raises blood sugar by converting glycogen to glucose; acts on liver cells.

Hormonal Dysregulation

• Diabetes Mellitus Types:

  • Type I: Insulin-dependent, autoimmune destruction of beta cells.

  • Type II: Insulin resistance; risk factors include genetics, age, and obesity.

Adrenal Gland Functions

• Adrenal Cortex: Produces corticosteroids:

  • Mineralocorticoids: Aldosterone increases Na+ retention.

  • Glucocorticoids: Cortisol manages stress responses.

• Adrenal Medulla: Produces epinephrine and norepinephrine, influencing fight-or-flight responses.

Stress Response

• Eustress vs. Distress: Positive versus harmful stress.

• Stress Mechanisms: Counteract stress through increased secretion of hormones like cortisol, hGH; can lead to muscle wasting and immune suppression upon prolonged exposure.