Endocrine System

The endocrine system is a system of ductless glands that secrete hormones into the bloodstream to regulate body functions

It is different to the exocrine system, which releases products from ducted glands into cavities (e.g. digestive tract) or into the external environment 

While the endocrine system describes chemical messengers that act on distant sites, hormones may also act locally via autocrine or paracrine signalling

Types of Cell Signalling

Types of Hormones

There are three main classes of hormones:

Steroid Hormones

  • Steroid hormones are lipophilic - meaning they can freely diffuse through the plasma membrane
  • They bind to receptors either in the cytoplasm or in the nucleus of the target cell, to from an active receptor-hormone complex
  • This activated complex will move to the nucleus and bind the DNA directly, acting as a transcription factor for the regulation of specific genes

Mechanism of Action for a Steroid Hormone

Protein Hormones

  • Protein hormones (peptides and amines) are lipophobic - meaning they cannot freely pass through the plasma membrane
  • They bind to receptors on the surface of the plasma membrane, which generates a chemical signal within the cell
  • The receptor is typically coupled to a G-protein (or adenylate cyclase), which activates an intracellular molecule called a second messenger
  • This process is called signal transduction, because the initial chemical signal (the hormone) is transduced via a G protein to an intermediary molecule (second messenger) within the cell
  • Examples of second messengers include: ¬†cyclic AMP (cAMP), protein kinases, phosphatases, calcium ions (Ca2+), nitric oxide (NO)

Mechanism of Action for a Protein Hormone

Hormone Action

  • Hormones may induce cell activity in a number of ways, including:
    • Changing plasma membrane polarity or permeability by opening or closing protein channels - e.g. regulating glucose uptake into adipose tissue
    • Regulating expression of functional proteins (via gene activation or suppression) - e.g. producing cytoskeletal proteins to alter cell morphology
    • Moderating enzyme activity (via activation or deactivation) - e.g. altered cell metabolism
    • Inducing or supressing release of secretory products - e.g. secretion of ovarian hormones in response to stimulation from pituitary hormones
    • Stimulating mitosis and cell division
  • A small stimulus can cause a large response if it involves many intermediaries, as each step can activate multiple molecules at the next step (amplification)

The Endocrine System

  • The endocrine system releases hormones into the bloodstream to act on distant target cells within the body
  • The hormone will only be able to activate cells that possess the appropriate receptor (the receptor and hormone share specificity)
  • The endocrine system is slow to initiate, but may have a much more prolonged and sustained response when compared to the nervous system

General Overview of the Endocrine System