Anatomy Lecture 4: The Endocrine System

Endocrine and Nervous Systems

• The endocrine and nervous systems work in tandem to regulate body functions.
• The nervous system operates through nerve impulses and neurotransmitters, providing fast responses with brief effects on specific targets.
• The endocrine system utilizes hormones, which are released in one area but influence cells elsewhere, resulting in slower but longer-lasting effects.

Endocrine Glands

• Glands are categorized into exocrine (ducted) and endocrine (ductless).
• Endocrine glands secrete hormones directly into the interstitial fluid for diffusion into the bloodstream.
• Major endocrine glands include the pituitary, thyroid, parathyroid, adrenal, and pineal glands, alongside other organs like the hypothalamus, pancreas, and ovaries, among others.

Hormone Activity

• Hormones target specific tissues through specialized receptors, which are actively synthesized and degraded.
• Receptor regulation includes down-regulation, up-regulation, and the blocking of hormone receptors, exemplified by the drug RU486 (Mifepristone).

Types of Hormones

• Circulating hormones: Travel throughout the body via the bloodstream.
• Local hormones: Act in the vicinity of secretion, classified as:
  • Paracrine hormones: Influence neighboring cells (e.g., IL-2).
  • Autocrine hormones: Affect the secreting cell itself (e.g., NO).

Chemical Classes of Hormones

• Lipid-soluble hormones: Require transport proteins and include:
  • Steroids derived primarily from cholesterol.
  • Thyroid hormones (T3 and T4) linked to iodine.
  • Nitric oxide (NO), functioning as both neurotransmitter and hormone.
• Water-soluble hormones: Function in free form and encompass:
  • Amines.
  • Peptides/proteins.
  • Eicosanoids.

Mechanisms of Hormone Action

• Hormonal responses vary based on the hormone and receptor type.
• Lipid-soluble hormones interact with intracellular receptors, while water-soluble hormones bind to plasma membrane receptors, activating secondary messenger systems.
• Target cell responsiveness is influenced by hormone concentration, receptor abundance, and other hormonal interactions (permissive, synergistic, or antagonistic).

Hormonal Regulation and Feedback

• Hormone secretion is governed by:
  • Nervous system signals.
  • Blood chemical changes.
  • Other hormones.
• Most hormone regulation occurs through negative feedback, with few exceptions for positive feedback.

Hypothalamus and Pituitary Gland

• The hypothalamus connects the endocrine and nervous systems.
• The pituitary gland, linked to the hypothalamus via infundibulum, consists of anterior and posterior sections.
• The anterior pituitary releases hormones regulated by hypothalamic releasing and inhibiting hormones, often classified as tropic hormones that influence other endocrine glands.

Anterior Pituitary Hormones

• Human Growth Hormone (hGH): Stimulates insulin-like growth factors (IGFs) for growth and protein synthesis.
• Thyroid-Stimulating Hormone (TSH): Promotes thyroid hormone production.
• Follicle-Stimulating Hormone (FSH): Supports ovarian and testicular functions.
• Luteinizing Hormone (LH): Triggers ovulation and testosterone production.
• Prolactin (PRL): Promotes milk secretion from mammary glands.
• Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex glucocorticoid secretion.
• Melanocyte-Stimulating Hormone (MSH): Role in humans remains unclear.

Pancreatic Islets

• The pancreas functions as both an exocrine and endocrine gland, with 99% of its cells producing digestive enzymes.
• Pancreatic islets (Langerhans) contain:
  • Alpha cells: Secrete glucagon to raise blood sugar levels.
  • Beta cells: Release insulin to lower blood sugar levels.
  • Delta cells: Produce somatostatin to inhibit glucagon and insulin.
  • F cells: Secrete pancreatic polypeptide to inhibit several digestive functions.

Ovaries and Testes

• Gonads produce gametes and hormones.
• Ovaries generate estrogen and progesterone, crucial for reproductive functions, and inhibin to regulate FSH.
• Testes produce testosterone, regulating sperm production and male characteristics, also inhibiting FSH.

Pineal Gland

• Located at the midline of the brain’s third ventricle, it contains pinealocytes that secrete melatonin.
• Melatonin plays a role in regulating circadian rhythms, with production increasing in darkness.

Thymus and Endocrine Functions

• The thymus, behind the sternum, is vital for T cell maturation.
• It produces thymosin, thymic humoral factor (THF), thymic factor (TF), and thymopoietin.

Stress Response

• Eustress is beneficial, while distress is harmful.
• The body activates a general adaptation syndrome (GAS) in response to stress, comprising:
  • Initial fight-or-flight reaction.
  • Slower resistance phase.
  • Potential exhaustion from prolonged cortisol exposure, affecting muscle, immune, GI, and pancreatic functions.

Explore the functions of the endocrine system in this lecture. Understand how it interacts with the nervous system to coordinate body functions through hormones and nerve impulses. This quiz focuses on the mechanisms of hormonal regulation and the contrasting speed of responses between these two systems.