Endocrine System Overview

The Endocrine System

• How organs and tissues communicate with each other through the bloodstream.
• Allows organisms to respond to changes in internal and external stimuli to maintain homeostasis
• Relies on hormonal communication
• Influenced by and influences the nervous system.

The endocrine system works in what we call axes which are systems of glands and organs that communicate through hormones, which are chemical messengers.

Hormone categorization

Hormones are categorized through:

• Their structure and mechanism
  • Peptide Hormones
    • Made of polar polypeptides
      • Act extracellularly through G-Protein Coupled Receptors (GPCRs)
    • Quick, short-lived responses
  • Steroid Hormones
    • Made of nonpolar cholesterol
      • Bind to receptors inside the cell
    • Slow, transient responses
  • Amino-Acid Derived Hormones
• Their intended action
  • Tropic Hormones
  • Direct Hormones

Many of the common axes (that we will go over) start with the hypothalamus.

Hypothalamic connections

The hypothalamus connects to the pituitary gland. The pituitary, however, is split into two different lobes: the anterior pituitary and the posterior pituitary.

Anterior Pituitary

The anterior pituitary is made of glandular tissue, which means it can both produce and secrete hormones.

The hypothalamus, thus, secretes tropic hormones into a structure of blood vessels called the hypophyseal portal system.

The hypothalamus secretes these hormones to the anterior pituitary:

• Gonadotropin Releasing Hormone (GnRH)
  • AP releases Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH)
• Corticotropin Releasing Hormone (CRH)
  • AP releases Adrenocorticotropic Hormone (ACTH)
  • AP releases Endorphins
• Thyrotropin Releasing Hormone (TRH)
  • AP releases Thyroid Stimulating Hormone (TSH)
• Growth Hormone Releasing Hormone (GHRH)
  • AP releases Growth Hormone (GH)

Prolactin

Prolactin is produced by the anterior pituitary, but it is negatively regulated by the hypothalamus through DOPAMINE!

• In this axis, dopamine is known as the Prolactin Inhibiting Factor (PIF)

Hypothalamus	Anterior Pituitary	Destination	Hormone released or resulting action
GnRH	FSH	Ovary (granulosa cells) Testis (Sertoli cells)	Follicular development Spermatogenesis
GnRH	LH	Ovary Testes (Leydig cells)	Ovulation Sperm production
CRH	ACTH	Adrenal Cortex	Glucocorticoids
CRH	Endorphins		Inhibit pain, relieve stress
TRH	TSH	Thyroid gland	T3 + T4
PIF (Dopamine) - NEGATIVE	Prolactin	Mammary glands	Lactation
GHRH	Growth Hormone (GH)	Most tissues	Stimulate growth, cell reproduction, etc.

Posterior Pituitary

The Posterior Pituitary is made of neural tissues, so it cannot produce hormones. It only stores hormones that the hypothalamus produces.

The hypothalamus sends axons down into the posterior pituitary. When depolarized, the posterior pituitary releases the hormones (known as neurohormones) from the axon terminals.

The two major hormones that the posterior pituitary releases are Oxytocin and Anti-diuretic Hormone (ADH).

• Oxytocin promotes uterine contractions during labor, thus pushing the baby out

  • It’s a canonical example of a PostitiveFeedbackLoop

• ADH (also known as Vasopressin) opens up aquaporins in the collecting duct, allowing for water to be reabsorbed by the body

  • Increases blood pressure
  • Released when dehydrated