Introduction to the Endocrine System

Body Communication

• The body is made up of various parts: organ systems, organs, tissues, and cells.
• Approximately 100 trillion cells in the body.
• Communication occurs not just through the nervous system, but also through the endocrine system.

Endocrine System Basics

• Composed of glands that secrete hormones into the bloodstream.
• Hormones travel through the body to initiate effects.
• Major endocrine organs are referred to as endocrine glands.

Key Endocrine Glands and Hormones

Hypothalamus

• Located in the forebrain.
• Receives nerve signals and funnels them to the pituitary gland.
• Produces hormones: ADH (regulates fluid volume) and Oxytocin (stimulates uterine contractions).
• Acts as the control center of the endocrine system.

Pituitary Gland

• Positioned below the hypothalamus, size of a pea.
• Known as the "master gland."
• Directs functions of other endocrine glands.

Thyroid Gland

• Located in the neck around the trachea.
• Regulates metabolism via T3 (triiodothyronine) and T4 (thyroxine).

Parathyroid Glands

• Positioned behind the thyroid gland.
• Regulate blood calcium levels through parathyroid hormone (PTH).

Adrenal Glands

• Located on top of kidneys.
• Composed of cortex (produces corticosteroids like cortisol and aldosterone) and medulla (produces catecholamines like epinephrine and norepinephrine).

Gonads

• Ovaries in females, testes in males.
• Produce sex hormones: testosterone (males), estrogen and progesterone (females).

Pancreas

• Located in the upper abdomen.
• Regulates blood sugar via insulin and glucagon.
• Less directly controlled by the pituitary gland.

Hormone Function and Classification

• Hormones act like radio waves; require specific receptors to function.

Hormone Classes

1. Autocrine Hormones: Function within the cell that makes them (e.g., T-cell interleukins).
2. Paracrine Hormones: Function regionally (e.g., hypothalamus to pituitary gland communications).
3. Endocrine Hormones: Function at a distance (e.g., pituitary stimulating gonads).

Conclusion

• Hormones are crucial for the communication among the 100 trillion cells in the body.
• Understanding hormones gives insight into bodily functions and endocrine communication.

Endocrine Control Lecture Notes

Key Glands

• Hypothalamus

  • Located in the forebrain.
  • Receives neural signals from the brain and peripheral nervous system.
  • Acts as a bridge between the nervous system and endocrine system.
  • Controls the pituitary gland and thus regulates the body’s hormonal response.

• Pituitary Gland

  • Situated below the hypothalamus.
  • Divided into two parts:
    • Anterior Pituitary Gland
    • Posterior Pituitary Gland

Interaction with Pituitary Gland

Anterior Pituitary Gland

• Communicates through the hypophyseal portal system (a capillary network).
• Secretes hormones that signal the pituitary.
• Hormones from the hypothalamus that affect the anterior pituitary:
  • Gonadotropin-releasing hormone (GnRH): Stimulates release of FSH and LH → stimulates gonads to release hormones.
  • Corticotropin-releasing hormone (CRH): Stimulates release of ACTH → stimulates adrenal glands.
  • Thyroid-releasing hormone (TRH): Stimulates release of TSH → stimulates thyroid gland.
  • Growth-hormone-releasing hormone: Stimulates release of growth hormone → stimulates growth in bones and muscles.
  • Prolactin inhibitory factor (PIF): When stopped, signals release of prolactin → involved in milk production.

Hormone Types

• Tropic Hormones (FLAT): Stimulate other endocrine glands.

  • F: FSH
  • L: LH
  • A: ACTH
  • T: TSH

• Direct Hormones (PEG): Directly affect parts of the body.

  • P: Prolactin
  • E: Endorphins
  • G: Growth Hormone

Posterior Pituitary Gland

• Communicates through nerve stimulation.
• Hormones made in the hypothalamus but stored and released by the posterior pituitary:
  • ADH (Antidiuretic Hormone): Retains water in kidneys.
  • Oxytocin: Involved in uterine contractions.

Summary

• The pituitary gland controls other endocrine glands and is itself regulated by the hypothalamus.
• Hormones act as chemical messengers to stimulate different parts of the body, integrating neural and endocrine responses.

Endocrine System and Hormone Regulation

Overview

• Endocrine organs and glands release hormones throughout the body.
• It may seem random, but hormone release is controlled and precise.
• Hormone effects (e.g., adrenal glands' fight or flight response) are sensitive and need regulation.

Hormone Concentration Control

1. Metabolism and Excretion

   • Hormone levels in the blood are tightly regulated.
   • Liver: Metabolizes excess hormones into bile, excreted via the digestive system.
   • Kidneys: Filter blood, remove waste products through urine.
   • Some hormones break down directly in the blood, then processed by the liver or kidneys.
   • Hormones can also be excreted through sweat.

2. Feedback Loops

   • Majority are negative feedback loops.
   • Negative Feedback Loops: Conditions resulting from hormone action suppress further hormone release.
   • Example:
     • Hypothalamus releases thyroid-releasing hormone (TRH).
     • Pituitary Gland receives TRH, releases thyroid-stimulating hormone (TSH).
     • Thyroid Gland receives TSH, releases T3 (triiodothyronine) and T4 (thyroxine).
     • Thyroid hormones travel through the body, increase metabolism.
     • Some receptors are on the pituitary gland and hypothalamus.
     • Thyroid hormones signal to stop production when levels are adequate.

Importance of Feedback Control

• Redundancy in receptors (e.g., both hypothalamus and pituitary gland) reflects the importance of feedback control.
• Ensures precise hormone concentration in the body.

Conclusion

• Hormone levels are not random; they are well-regulated.
• Proper hormone concentration is crucial for bodily functions.

Lecture on Hormone Classification by Structure

Introduction

• Hormones can be classified by their function and structure.
• Structure determines how a hormone works.

Major Types of Hormones

1. Proteins and Polypeptides

• Composed of amino acids linked by peptide bonds.
• Form most of the body's hormones.
• Range from small (3 amino acids) to large (hundreds of amino acids).
• Made in the rough endoplasmic reticulum (RER) and processed in the Golgi apparatus.
• Characteristics:
  • Typically charged, water-soluble.
  • Difficulty crossing cell membranes.
  • Receptors located on cell surfaces.
  • Initiate a response inside cells through a cascade of secondary messengers.
• Example: Insulin.

2. Steroids

• Derived from lipids, primarily cholesterol.
• Characteristic structure: four-ring backbone (3 cyclohexane rings and 1 cyclopentane ring).
• Characteristics:
  • Lipid-based, easily pass through cell membranes.
  • Receptors located inside the cell (cytoplasm or nucleus).
  • Function as primary messengers affecting transcription and translation.
• Examples:
  • Adrenal cortex hormones: Cortisol, Aldosterone.
  • Sex hormones: Testosterone, Estrogen, Progesterone.

3. Tyrosine Derivatives

• Derived from the amino acid tyrosine.
• Unique as they can act like either proteins/polypeptides or steroids.
• Characteristics:
  • Tyrosine-based hormones act like steroids (e.g., thyroid hormones - T3, T4).
  • Catecholamines act like proteins/polypeptides (e.g., Epinephrine, Norepinephrine).
• Examples:
  • Thyroid hormones (act like steroids).
  • Catecholamines (act like proteins and polypeptides).

Conclusion

• Hormones' structure is crucial as it determines their function and ability to affect body responses.
• Hormones signal everything from emotions to bodily functions such as fear, hunger, and childbirth.

Hormone Interaction with Receptors

Overview

• Hormones travel through blood vessels and interact with specific receptors on target cells.
• Hormone interaction occurs in two main ways: via secondary messengers or as primary messengers.

Secondary Messenger Mechanism

• Initial Interaction: Hormone binds to a receptor on the cell surface, initiating a chain reaction.
• Receptor and G Protein:
  • Receptors (depicted in pink) are located in the cell membrane's phospholipid bilayer.
  • A G protein (depicted in green) binds with guanine nucleotides.
  • Initially, G protein is bound to guanine diphosphate (GDP).
• Activation Process:
  • Hormone binding changes receptor shape, allowing interaction with G protein.
  • GDP is exchanged for GTP, enabling G protein to interact with adenylate cyclase.
  • Adenylate cyclase converts ATP to cyclic adenosine monophosphate (CAMP).
• Signal Amplification:
  • CAMP activates target proteins inside the cell.
  • A single hormone can trigger substantial CAMP production, amplifying the signal.
• Variability:
  • Mechanisms differ across cells; several secondary messengers exist aside from CAMP.
• Analogy: Similar to using a phone service to communicate over distances.
  • Peptide hormones and catecholamines, which cannot cross the cell membrane, utilize secondary messengers.

Primary Messenger Mechanism

• Direct Interaction:
  • Steroid and thyroid hormones can cross the cell membrane directly.
  • Hormones bind to receptors in either the cytosol or nucleus.
• Process Efficiency:
  • Binding directly affects transcription in the nucleus or translation in the cytoplasm.
  • Fewer steps are involved compared to the secondary messenger system.
• Characteristics:
  • Steroid and thyroid hormones are lipid-based and can cross cell membranes without extra mechanisms.

Key Takeaway

• Hormones influence target cells primarily through secondary messengers or as primary messengers.
• Understanding these processes is vital for grasping hormonal regulation in the body.

Lecture Notes: Steroids and Terpenes

Introduction

• Steroids are chemical messengers in the body, classified as hormones.
• This lecture focuses on the origin and synthesis of steroids.

Terpenes

• Definition: Terpenes are a class of lipid molecules composed of isoprene units.
• Isoprene Structure:
  • Isoprene is a 5-carbon unit with a distinct structure: four carbons in a chain and a fifth carbon branching off.
• Terpene Classification by Isoprene Units:
  • Monoterpene: 10 carbons (2 isoprene units). Example: Menthol.
  • Sequiterpene: 15 carbons (3 isoprene units). Example: Ginger.
  • Diterpene: 20 carbons (4 isoprene units).
  • Sesterterpene: 25 carbons (5 isoprene units).
  • Triterpene: 30 carbons (6 isoprene units).
  • Tetraterpene: 40 carbons (8 isoprene units).
• Applications: Terpenes are present in plant oils and vitamins.

Biosynthesis of Steroids

• Starting Material: Isoprene bound to pyrophosphate.
• Pyrophosphate:
  • Weak base, good leaving group in organic chemistry.
  • Essential for the reaction to build isoprene blocks.
• Key Molecules in Biosynthesis:
  • Dimethyl Allyl Pyrophosphate
  • Isopentyl Pyrophosphate
  • Combination of these forms geranyl pyrophosphate (10 carbons).
  • Further reactions yield farnesyl pyrophosphate (15 carbons).
• Formation of Triterpene:
  • Resulting in a 30-carbon molecule called squalene.

Importance of Squalene

• Squalene is the precursor to all steroid hormones in the body.

Lecture on Cholesterol and Steroid Hormones

Cholesterol Formation

• Starting Material: Squalene
• Process:
  • Series of ring-closing or cyclization reactions.
  • Converts squalene into cholesterol.
• Location: Liver
• Transportation: Travels through the blood, present inside and on the surface of cells.

Function of Cholesterol

• In endocrine organs, cholesterol is altered to form the steroid backbone.
• Steroid Backbone: Composed of four characteristic rings.

Steroid Hormones

• Classes of Steroid Hormones:
  • Sex Hormones
  • Adrenal Cortex Steroids

Sex Hormones

1. Estrogens

   • Examples: Estradiol, Estrone
   • Produced In: Ovaries
   • Function: Regulate secondary sex characteristics in females.

2. Progesterone

   • Function: Prepares the uterus; maintains pregnancy.

3. Androgens

   • Examples: Testosterone, Androsterone
   • Produced In: Testes
   • Function: Regulate secondary sex characteristics in males.

Adrenal Cortex Steroids

1. Cortisone and Cortisol

   • Function: Stress hormones; anti-inflammatory; increase carbohydrate metabolism.

2. Aldosterone

   • Function: Regulates blood pressure and fluid volume.

Conclusion

• Understanding Steroids: Recognizing the steroid backbone helps in understanding their significance.
• Everyday Examples: Menthol cough drops and ginger snap cookies contain terpenes, related compounds.

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This lecture highlights cholesterol's role in steroid hormone production and their diverse functions within the body.