Endocrine System Overview

Endocrine System

• Endocrine and nervous systems communicate in the body
• Nervous system is faster than the endocrine system
• Endocrine response is not always slower; examples include norepinephrine and insulin
• Endocrine system functions include homeostasis, growth regulation, reproduction regulation, and maturation regulation

Hormone Classification

• Steroid Hormone: Derived from cholesterol, insoluble in blood, requiring protein transporters
• Protein Hormone: Protein in nature, soluble in blood, does not need transporters, hydrophobic-lipophilic/hydrophilic-lipophilic, act on intracellular or extracellular receptors.

Types of Hormones

• Thyroid Hormone: Crosses cell membranes, stored in follicles, transported in blood via proteins, act on intracellular receptors, long half-life

• Catecholamines: Do not cross cell membranes, stored in granules, transported in blood freely or loosely bound to proteins, act on extracellular receptors, short half-life

• Amines: Hormones derived from tyrosine; act on intracellular (nuclear) or extracellular receptors

Hormone Receptors

• Function as glycoprotein structures that bind with hormones (lock-and-key theory)
• Physiological conditions, other hormones, and hormones acting on receptors affect hormone quality, quantity, and sensitivity

Hormone Regulation

• Example of Up-regulation: During parturition, oxytocin hormone leads to an increase in oxytocin receptors as estrogen hormone also increases receptor sensitivity.
• Example of Down-regulation: Insulin regulation decreases receptor synthesis and decreases receptor numbers when hormone levels are high

Hormone Action

• Autocrine action: Hormone acts within the organ of synthesis (e.g., testosterone influencing testes)
• Paracrine action: Hormone acts on nearby cells (e.g., somatostatin affecting nearby cells)
• Endocrine action: Hormone is produced by ductless endocrine glands and released into the blood (e.g., insulin and glucagon secretion)

Mechanism of Action of Protein Hormones

• Hormone is the first messenger, binding with receptors to start biochemical reactions
• Second messengers form, e.g., cAMP (adenyl cyclase), phosphatidyl inositol system, calcium calmodulin system, and tyrosine kinase system
• Examples of hormones using this pathway include epinephrine, norepinephrine, glucagon, and calcitonin

Mechanism of Action of Steroid and Thyroid Hormones

• Hormone binds with intracellular receptors, forming a hormone-receptor complex (HRC) to cause DNA sequence changes
• Receptors have ligand-binding, DNA-binding, and N-terminal domains
• This affects the transcription of specific genes, producing a specific protein.

Phosphatidyl Inositol System

• Hormone binding triggers conformational changes in HAC
• This activates phospholipase C, hydrolyzing PIP₂ into IP₃ and DAG
• IP₃ mobilizes Ca²⁺ from intracellular stores
• DAG stimulates protein kinase C (PKC)
• PKC phosphorylates enzymes, impacting transcription and causing changes

Calcium Calmodulin System

• In muscle and nerve cells, calcium ions (Ca²⁺) act as second messengers
• Ca²⁺ is released from the ER, and activation of Ca-calmodulin-dependent protein kinase
• Kinase phosphorylates target enzymes.

Explore the intricacies of the endocrine system and its comparison with the nervous system. Understand the classification of hormones and their roles in the body. This quiz covers types of hormones, their functions, and important characteristics, providing a comprehensive overview for students of biology.