Endocrine System Overview

Endocrine System Overview

• The endocrine system is responsible for producing and regulating hormones.
• Hormones are chemical messengers that travel through the blood to target cells.
• Hormones trigger responses in target cells leading to a physiological response.

Signaling Molecules

• Hormones are signaling molecules.
• The word hormone comes from the Greek word "horman" meaning "to excite" or "to activate".

Types of Signaling Molecules

• Some signaling molecules act within the body, while others can act outside the body (e.g. pheromones).

Classes of Hormones

• Hormones are classified into three major chemical classes:
  • Polypeptides (e.g., insulin): water-soluble
  • Amines (e.g., thyroxine, epinephrine): water-soluble
  • Steroids (e.g., cortisol): lipid-soluble

Hormone Action

• Water-soluble hormones cannot diffuse through plasma membranes. They bind to cell-surface receptors, influencing cytoplasmic molecules and sometimes altering gene transcription.
• Lipid-soluble hormones diffuse across the membranes. They bind to transport proteins, enter target cells, and bind to receptors within the cytoplasm or nucleus, triggering changes in gene transcription.

Hormone Effects

• Some hormones can produce more than one effect on different target cells.
  • For example, epinephrine affects liver cells, blood vessel smooth muscle cells in skeletal muscle, and blood vessel smooth muscle cells in the intestines in different ways.

Hormone Production and Control:

• The endocrine system contains different structures like glands and other organs that produce hormones.
• Hormone secretion is regulated by feedback mechanisms:
  • Some hormones are released in response to internal or external stimuli.
  • A stimulus could be a low pH level in the duodenum.

Negative Feedback

• The response to the hormone opposes to the initial stimulus.
• Example: in secretin signaling, low pH in duodenum stimulates the release of secretin which causes a response of bicarbonate release.

Positive Feedback

• The hormone's response amplifies the initial stimulus.
• Example: suckling stimulates oxytocin release, oxytocin stimulates milk ejection, and milk ejection reinforces suckling.

Neuroendocrine

• The endocrine system and nervous systems work together to maintain homeostasis.
• The hypothalamus is a primary link between two systems using chemical messengers to stimulate or inhibit hormone secretion from the pituitary gland.

Hormone Secretion Stimulus

• Hormones are secreted in response to:
  • Blood concentrations of non-hormone chemicals.
  • Another hormone.
  • Neural stimuli.

Evolution of Hormone Function

• Same hormones can have different functions in different organisms.
  • For example, prolactin regulates milk production in mammals but regulates fat metabolism and reproduction in birds.
  • Hormones produce different responses in amphibians, and freshwater fish.