Biol2302_Chapter16

Chapter 16: The Endocrine System

16.1: Cell-Cell Communication

• Function of the Endocrine System: Maintain homeostasis through communication between cells via signaling.

• Key Players:

  • Sending Cell: Releases signals (ligands) to communicate.

  • Target Cell: Receives signals and responds.

Types of Signaling

• Signaling Methods by Distance:

  • Autocrine Signaling: Communication in which a cell targets itself.

  • Paracrine Signaling: Signals are released to nearby cells.

  • Endocrine Signaling: Signals travel through the circulation to distant target cells.

Autocrine Signaling

• Mechanism: Cells release ligands that act on their own surface receptors.

Paracrine Signaling

• Local Signaling: Chemical messengers released by nearby cells affect local target cells.

• Example: Neurotransmitters released from a neuron diffuse across a synapse.

Endocrine Signaling

• Mechanism: Signals produced by glands enter the bloodstream to reach distant targets.

Types of Glands

1. Exocrine Glands:

   • Produce non-hormonal substances (e.g., sweat, saliva).

   • Secrete substances via ducts.

2. Endocrine Glands:

   • Ductless, secreting hormones directly into the extracellular fluids.

   • Example Glands: Pituitary, Thyroid, Parathyroid, Adrenal, Pineal.

Long-Distance Communication

• Neuronal Signaling: Involves rapid communication through electric signals in the nervous system.

• Endocrine Signaling: Slower than neuronal signaling; involves hormones in the bloodstream.

Differences Between Systems

• Nervous System: Rapid responses, specific locations, short durations, signal strength via action potentials.

• Endocrine System: Slower responses, diffuse locations, long durations, signal strength via hormone concentration.

Overview of the Endocrine System

• Composed of various glands and tissues producing hormones that regulate metabolic activities.

• Major glands include: Pineal, Thymus, Thyroid, Adrenal, Pancreas, and reproductive organs.

Functions of the Endocrine System

• Major functions include:

  • Reproduction

  • Growth and Development

  • Electrolyte, water, and nutrient balance

  • Metabolic regulation

  • Mobilization of body defenses

Hormones Overview

• Definition: Chemical messengers released into extracellular spaces that travel through the blood.

• Response Times: Vary from seconds (e.g., adrenaline) to days (e.g., reproductive hormones).

16.2: Types of Hormones

• Two Major Groups:

  • Amino Acid Derivatives: Include amines, peptides, and proteins.

  • Lipid Derivatives: Includes steroid hormones.

Hormone Structure and Function

• Chemical Structure & Solubility: Determines degradation speed, transport method in blood, receptor binding, function, and distribution.

• Water-Soluble Hormones: Cannot cross plasma membrane; bind to surface receptors.

• Lipid-Soluble Hormones: Can cross the plasma membrane, bind to intracellular receptors, directly activating gene expression.

Pathways of Hormone Action

1. Hormone Binding: Hormones bind to receptors on target cells.

2. Alteration of Activity: Increases/decreases rates of cellular processes.

Specificity of Target Cells

• Only cells with specific receptors bind hormones, producing various responses based on the hormone and receptor type.

Hormonal Response Mechanism

• Hormones can stimulate enzyme synthesis, activate/deactivate enzymes, change membrane permeability, and affect cell growth.

Water-Soluble vs Lipid-Soluble Hormones

• Water-Soluble: Act through second-messenger systems (e.g., cAMP, PIP2-Ca).

• Lipid-Soluble: Act on intracellular receptors and initiate gene expression directly.

Hormone Regulation and Feedback Mechanisms

• Feedback Systems:

  • Negative Feedback: Inhibits hormone secretion to maintain homeostasis.

  • Positive Feedback: Accelerates hormone secretion in response to stimuli.

16.4: Regulation of Hormone Secretion

• Endocrine glands respond to:

  • Humoral stimuli (ion/nutrient levels)

  • Neural stimuli (nerve fiber activity)

  • Hormonal stimuli (hormones from other glands).

Pineal Gland

• Produces melatonin, with increasing levels during the night promoting sleepiness.

Pancreas

• Contains both exocrine (digestive enzymes) and endocrine (insulin, glucagon) functions.

Clinical Aspects of Hormonal Imbalance

1. Diabetes Mellitus:

   • Type 1: Lack of insulin production.

   • Type 2: Insulin resistance.

2. Thyroid Disorders: Include hyperthyroidism and hypothyroidism, affecting metabolism and growth.

3. Adrenal Gland Disorders: Include Cushing’s syndrome (hypersecretion) and Addison’s disease (hyposecretion).

Summary

• The endocrine system is essential for regulating various body functions through hormones, relying on complex signaling pathways, feedback loops, and interactions between different types of glands.