Endocrine System Overview

Endocrine System Signaling Pathways

• Endocrine pathway: Secreting cell (endocrine cell) secretes hormones into blood, targeting distant cells.
• Paracrine pathway: Tissue cell secretes paracrine chemicals into extracellular fluid, targeting nearby cells.
• Autocrine pathway: Specialized cells secrete autocrine chemicals into extracellular fluid, targeting the same cell.

Endocrine Glands

• Primary endocrine glands: Glands solely in the endocrine system.
• Secondary endocrine glands: Glands that contribute to the endocrine system, part of another organ system (e.g., kidney).

Hormone Types

• Amino acid-based hormones:
  • Free hormones: Travel freely in plasma.
  • Usually hydrophilic, bind to membrane receptors.
  • Small, amino acid-based, hydrophilic, transported in plasma.
  • Derived from essential fatty acids.
• Steroid hormones:
  • Bound hormones: Require binding proteins to travel in plasma.
  • Usually hydrophobic, bind to intracellular receptors.
  • Made from cholesterol.

Target Cells and Receptors

• Receptors are highly specific for their hormones, crucial for homeostasis.
• Upregulation: Target cells increase receptor number for increased sensitivity.
• Downregulation: Target cells decrease receptor number in response to prolonged exposure.

Hormone Effects

• Gene transcription and expression
• Enzyme activation/inhibition
• Mitosis and meiosis
• Membrane potential changes
• Secretion modulation (start/stop, increase/decrease)

Hormone Interactions

• Complementary action: Different hormones with same goal, operate independently.
• Synergistic hormones: Different hormones work together to achieve the same effect.
• Antagonistic hormones: Different hormones have opposing effects.

Hormone Half-Life

• Measures hormone production rate and breakdown rate.

Hormone Stimuli

• Hormonal, humoral, and neural stimuli trigger hormone release.
• Upregulation and downregulation adjust sensitivity.

Homeostasis (Negative and Positive Feedback)

• Homeostatic process: Stimulus→Sensor→Control Center→Effector.
• Negative feedback: The increase/decrease becomes the stimulus.
• Positive feedback: Example includes breastfeeding.

Hypothalamic Control

• Hypothalamus: Master control center affecting anterior pituitary activity.
• Regulatory hormones: Releasing and inhibiting hormones influence pituitary secretion.

Pituitary Gland

• Pea-shaped gland in the hypophyseal fossa (sphenoid bone).
• Infundibulum: Connects pituitary to hypothalamus, regulating hormone release.
• Anterior pituitary (adenohypophysis): Controlled by hypothalamic regulatory hormones—hormones released directly into target cells.
• Posterior pituitary (neurohypophysis): Hormones produced in hypothalamus, stored and released by posterior pituitary.

Anterior Pituitary Hormones

• FLAT PiG: Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Adrenocorticotropic hormone (ACTH), Thyroid-stimulating hormone (TSH), Prolactin, Growth hormone

• Growth hormone (GH): Affects all cells and tissues, elevated before epiphyseal plate closure causing gigantism, or after causing acromegaly.

Posterior Pituitary Hormones

• Antidiuretic hormone (ADH): Regulates water balance.
• Oxytocin: Involved in childbirth and breastfeeding.

Thyroid Gland

• Butterfly-shaped gland below the larynx, highly vascularized.
• Follicles and follicular cells secrete T3/T4 into the colloid.
• Parafollicular cells (C-cells) secrete calcitonin (decreases blood calcium).
• T3 (triiodothyronine) and T4 (thyroxine): Important for metabolism, and altering gene expression (increasing/decreasing), in nearly all cells of the body.

Thyroid Hormone Effects

• Regulates Basal Metabolic Rate (BMR) (energy used at rest).
• Increases ATP consumption.
• Influences gluconeogenesis.
• Affects protein breakdown in skeletal muscle and adipose tissue
• Increases anabolic and catabolic processes
• Thermoregulation
• Growth and development
• Neurodevelopment
• Sympathetic nervous system modulation

Hyperthyroidism and Hypothyroidism

• Hypothyroidism: Low T3/T4 levels.
  • Causes: Inflammation of thyroid (e.g., Hashimoto's thyroiditis), inability to respond, or communication issues between the hypothalamus and pituitary.
  • Symptoms: Lethargy, weight gain TSH high, with low T3/T4.
• Hyperthyroidism: High T3/T4 levels
  • Causes: Autoimmune disease (e.g., Graves' disease), tumors, excessive iodine.
  • Symptoms: Weight loss, intolerance to cold TSH low, with high T3/T4.

Parathyroid Glands

• Four small glands on the posterior thyroid.
• Chief cells: Secrete parathyroid hormone (PTH) for calcium homeostasis.
• PTH regulates calcium levels by affecting bone resorption, calcitriol synthesis, and calcium absorption.

Adrenal Glands

• Located superior to kidneys.
• Cortex (bark) and medulla (center).
• Zona glomerulosa, fasciculata, and reticularis produce various hormones

Adrenal Cortex Hormones

• Zona glomerulosa (outside): Mineralocorticoids (e.g., aldosterone) regulate salt balance, blood pressure, sodium retention.
• Zona fasciculata and reticularis (middle): Glucocorticoids (e.g., cortisol) regulate stress response, and catabolism.
• Zona reticularis (inside): Androgenic steroids regulate sex hormones.

Adrenal Medulla

• Chromaffin cells: Secrete epinephrine (adrenaline) for immediate stress response, sympathetic hormone.

Pancreas

• Endocrine and exocrine organ.
• Pancreatic islets (islets of Langerhans): Contain Alpha, Beta, and Delta cells.
• Alpha cells: Secrete glucagon (promotes glycogenolysis and gluconeogenesis).
• Beta cells: Secrete insulin (reduces blood glucose, promotes glucose uptake and storage)
• Delta cells: Secrete somatostatin (inhibits pancreatic hormone secretion).

Diabetes Mellitus

• Type 1 Diabetes: Beta cell destruction, insulin deficiency, high ketone bodies, significant symptoms and requires insulin.
• Type 2 Diabetes: Insulin resistance (downregulation), inadequate/ineffective insulin—symptoms often less pronounced.

Pineal Gland

• Small pine cone-shaped gland in epithalamus.
• Secretes melatonin, regulates circadian rhythm (sleep-wake cycle).

Thymus

• Located in mediastinum, associated with lymphatic system.
• Produces thymopoietin and thymosin for T-lymphocyte maturation

Gonads

• Testes: Produce testosterone for sperm production and development of secondary sex characteristics.
• Ovaries: Produce estrogen (secondary female sex characteristics, menstrual cycle) and progesterone (pregnancy development).

Adipose Tissue

• Produces leptin, signaling satiety in hypothalamus.

Heart

• Produces atrial natriuretic peptide (ANP) for vasodilation and natriuresis.

Kidneys

• Produce erythropoietin (EPO), increases red blood cell production.
• Produce renin for the RAAS (renin-angiotensin-aldosterone system).
• Produce calcitriol (Vitamin D), promotes calcium absorption.

Explore the fascinating signaling pathways of the endocrine system, including endocrine, paracrine, and autocrine mechanisms. Learn about the primary and secondary endocrine glands and the various types of hormones, such as amino acid-based and steroid hormones. This quiz will test your knowledge of how these components interact with the body.