A&P: Endocrine Lecture 1

• The endocrine system releases hormones into the bloodstream.
• The nervous and endocrine systems release chemical messengers that bind to target cells.
• Both systems coordinate body functions and maintain homeostasis.
• Despite sharing similar functions, there are major differences between the two systems.

Similarities Between the Nervous and Endocrine Systems:

• Both release chemicals that bind to target cell receptors.
• Both systems share chemical messengers called neurotransmitters in the nervous system and hormones in the endocrine system.
• Both systems are primarily regulated by negative feedback mechanisms.
• Both systems share a common goal to preserve homeostasis by coordinating and regulating cells, tissues, and organs.

Differences Between the Nervous and Endocrine Systems:

• The nervous system uses electrical impulses and neurotransmitters for signaling
  • Signals have fast responses and brief effects
  • Affecting specific targets
• The endocrine system uses hormones for signaling
  • Signals have slow responses and longer lasting effects
  • Hormones can act on many targets
  • Hormones have very powerful effects, even in low concentrations (signal amplification)

Functions of Hormones include regulating:

• The chemical composition and volume of the internal environment (interstitial fluid).
• Metabolism and energy balance.
• Contraction of smooth and cardiac muscle fibers.
• Glandular secretions.
• Certain immune system activities.
• Hormones control growth and development.
• Hormones regulate the operation of reproductive systems.
• Hormones help establish circadian rhythms.

Glands

• There are two types, exocrine and endocrine

Exocrine Glands:

• Secrete products into ducts that carry secretions into a body cavity, lumen, or the body's outer surface.

Endocrine Glands:

• Secrete products into the interstitial fluid
• Diffuses into the blood and travels to target cells.

Endocrine System Components:

• The endocrine system includes endocrine glands whose main function is hormone production:
  • Hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pineal gland, and pancreatic islets (pancreas).
• Organs with secondary endocrine functions:
  • Heart, thymus, digestive tract, kidneys, and gonads (testes, ovaries).

Hormones Defined:

• Hormones are mediator molecules.
• They are released in one part of the body but regulate activity in another.
• Hormones typically travel in the bloodstream and are present in low concentrations.

Exocrine Glands Defined:

• Glands secrete products into ducts that carry secretions into body activities, lumens, or surfaces of the body.

Endocrine Glands Defined:

• They secrete their products into the interstitial fluid surrounding the gland.
• The product then diffuses into the blood and travels to target cells.

Hormone Activity:

• Hormones influence their target cells by chemically binding to specific protein receptors.
• Only target cells possess receptors that bind and recognize specific hormones.
• Hormones do not affect every cell, and this specificity is due to the presence of receptors.

Hormone Activity: Receptor Regulation

• Receptors are constantly synthesized and broken down
• Down-regulation: occurs when excess hormone is present.
  • High hormone levels decrease number of receptors in the cell.
  • Decreasing the cell's sensitivity to the hormone.
• Up-regulation: occurs when the hormone level is deficient.
  • The cell responds by increasing the number of receptors.
  • Making the target tissue more sensitive to the hormone.

Circulating Hormones:

• Circulate in the blood throughout the body and have longer duration.

Local Hormones:

• Act locally and are quickly inactivated.
• Types:
  • Paracrine: act on neighboring cells
  • Autocrine: act on the same cell that secreted them

Chemical Classes of Hormones:

• There are various types of hormones that exert their effects via different mechanisms.
• Two main chemical classes:
  • Lipid-soluble hormones.
  • Water-soluble hormones.

Lipid-Soluble Hormones include:

• Steroid hormones
  • Derived from cholesterol
  • Examples: testosterone and estrogen
• Thyroid hormones
  • The tyrosine ring makes T3 and T4 hormones very lipid soluble
• Nitric Oxide (NO)
  • Acts as both a neurotransmitter and a hormone

Functions of transport proteins that attach to lipid-soluble hormones:

• They temporarily make lipid-soluble hormones water-soluble.
• They slow the passage of small hormones through the kidney's filtration system.
• They provide a ready reserve of hormones in the bloodstream.

Water-Soluble Hormones include:

• Amine Hormones
  • Amino acids as their base
  • Examples include epinephrine and norepinephrine
• Peptide and Protein Hormones
  • Amino acid polymers classified as small or large hormones
• Eicosanoid Hormones
  • Derived from arachidonic acid, a 20-carbon fatty acid
  • Major types are prostaglandins and leukotrienes

Down-Regulation Defined:

• High hormone levels decrease the number of receptors, making the target cell less sensitive to the hormone.

Up-Regulation Defined:

• When a hormone is deficient, the number of receptors will increase, making the target tissue more sensitive to the hormone.

Paracrine Hormone Defined:

• A local hormone that acts on neighboring cells.

Autocrine Hormone Defined:

• Hormones that act on the same cell that secretes them.

Mechanisms of Hormone Action:

• The particular response depends on both hormone and the target cell.
• Responsiveness of target cell depends on:
  • Hormone amount (concentration)
  • Target cell receptor abundance
  • Influence of other hormones (permissive, synergistic, and antagonistic effects)

Permissive Effect;

• Hormones require simultaneous or recent exposure to another hormone to fully affect target cell.
• An example is how epinephrine alone weakly stimulates breakdown of triglycerides but with T3 or T4 present, the epinephrine stimulates breakdown much more powerfully.

Synergistic Effect:

• The effect of two hormones acting together is greater or more extensive than the effect of each hormone acting alone.
• Both hormones are needed for full hormone activity, such as the need for both FSH and estrogens for normal oocyte development.

Antagonistic Effect:

• When one hormone opposes the action of another hormone.
• Examples include insulin and glucagon, and PTH and calcitonin.

Hormone Binding

• Lipid-soluble hormones bind to receptors inside target cells.
  • They may bind in the cytoplasm or nucleus.
• Water-soluble hormones bind to receptors on the plasma membrane.
  • Binding activates a second messenger system, amplifying the original small signal.

The Difference in Binding Location Relates To Hormone Permeability

• Lipid-soluble hormones molecule binds to and activates receptors within the cytosol or nucleus.
• Water-soluble hormones cannot diffuse into cells, so they bind to the cell's surface.

Hormone Regulation

• Hormone secretion is regulated by signals from the nervous system, chemical changes in the blood and other hormones.
• Hormonal regulation is mostly through negative feedback.
• Few examples of positive feedback exist; an example is how oxytocin stimulates uterine contractions which leads to more oxytocin released.