The Glands and Hormones of the Endocrine System

The Endocrine System

• The endocrine system is the body's second messenger system, with the nervous system being the first.
• It has slower, longer-lasting effects compared to the nervous system.
• It uses chemical messengers called hormones that are released into the bloodstream.

Functions of the Endocrine System

• Reproduction
• Growth and development
• Mobilization of body defenses
• Maintenance of homeostasis
• Regulation of metabolism
• Regulation of stress response
• Regulation of blood sugar
• Regulation of water balance

Endocrinology

• Endocrinology is the medical specialty that studies disorders of the endocrine system.
• The endocrine system, in vertebrates, works in parallel with the nervous system to maintain homeostasis by releasing chemical hormones from various glands. It is composed of the hormone-producing glands and tissues of the body.
• A hormone is a chemical messenger sent to many parts of the body to produce a specific effect on a target cell or organ.

Location of Major Endocrine Organs

• Pineal gland
• Hypothalamus
• Pituitary gland
• Thyroid gland
• Parathyroid glands (on dorsal aspect of thyroid gland)
• Thymus gland
• Adrenal glands
• Pancreas
• Ovary (female)
• Testis (male)

Endocrine vs. Exocrine Glands

• Exocrine glands:
  • Transport substances to target tissues via ducts to the surface of the body or body cavities.
  • Examples: tear glands, salivary glands, mammary glands.
• Endocrine glands:
  • Ductless (no tubes).
  • Secrete hormones directly into the bloodstream.
  • ONLY endocrine glands can produce hormones.
  • Blood hormone concentration is very low.

Hormone Action on Target Cells

• Each target cell contains receptor proteins that bind to specific hormones.
• When the hormone binds, it triggers other reactions within the target cell.

Steroid and Water-Soluble Hormones

• Steroid Hormones – Lipid-based
  • Can easily diffuse through the lipid bilayer of cells and bind to receptors inside the cell.
  • Examples: Testosterone, estrogen, cortisol
• Water-Soluble Hormones – e.g., Amino-acid based
  • Bind to receptor protein on the surface of the target cell, leading to a cascade of reactions.
  • Examples:
    • Epinephrine in the liver triggers the breakdown of many glycogen molecules into glucose.
    • Other examples: Human growth hormone, thyroxine, insulin

Steroid Hormone Action

1. Hormone diffuses through plasma membrane because it is lipid soluble.
2. Hormone binds to receptor inside nucleus.
3. Hormone-receptor complex activates gene and synthesis of a specific mRNA molecule.
4. mRNA moves to ribosomes, and protein synthesis occurs.

Water-Soluble Hormone Action

1. Epinephrine binds to a receptor in the plasma membrane.
2. Binding leads to activation of an enzyme that changes ATP to cAMP.
3. cAMP activates an enzyme cascade.
4. Many molecules of glycogen are broken down to glucose, which enters the bloodstream.

• Epinephrine (first messenger) binds to a receptor protein.
• This activates an enzyme that converts ATP to cAMP (second messenger).
• cAMP activates an enzyme cascade.
• Glycogen is broken down into glucose.

Steroid Hormone Action (Detailed)

(a) Steroid hormone action

1. Steroid hormone enters cytoplasm.
2. Hormone binds to a receptor protein.
3. Hormone-receptor complex forms.
4. Hormone-receptor complex enters the nucleus.
5. Hormone-receptor complex binds to DNA and causes transcription producing mRNA.
6. mRNA is translated producing new proteins.

Non-Steroid Hormone Action (Detailed)

(b) Nonsteroid hormone action

1. Nonsteroid hormone (first messenger) binds to a receptor protein on the plasma membrane of the target cell.
2. This binding activates an enzyme.
3. The activated enzyme converts ATP to cAMP (second messenger).
4. cAMP causes an effect on cellular function, such as glycogen breakdown.

Stimuli of Endocrine Glands

1. Hormonal Stimuli
   • Endocrine glands are activated by hormones.
   • Tropic Hormones - hormone that targets endocrine glands and stimulates them to release other hormones.
2. Humoral Stimuli
   • Changing blood levels of certain ions or other substances stimulate hormone release.
3. Neural Stimuli
   • Nerve impulses stimulate hormone release.
   • Most are under control of the sympathetic nervous system.

Maintaining Homeostasis - Feedback Loops

1. Hypothalamus releases hormone 1 (releasing hormone).
2. Hormone 1 stimulates the anterior pituitary.
3. Anterior pituitary releases hormone 2 (stimulating hormone).
4. Hormone 2 stimulates the target gland.
5. Target gland releases hormone 3 (target gland hormone).
6. Hormone 3 inhibits the release of hormone 2 and Hormone 1 (negative feedback).