CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings

Hormone Receptors

Hormones act by binding with specific receptors on the target cells. Receptors associated with cells are located in three areas:

The majority of plasma membrane receptors are G-protein-coupled receptors. Other types include receptor tyrosine kinases and receptor serine/threonine kinases. Activation of these receptors leads to an intracellular cascade that produce second messengers like cAMP, cGMP, or inositol triphosphate (IP3). These second messengers activate other enzymes such as those that phosphorylate or dephosphorylate proteins. Adding a phosphate to an enzyme is like flipping a light switch, sometimes that switch turns off the enzyme (inactive) and sometimes the switch turns on the enzyme (active). Additionally, the second messenger may increase intracellular calcium. Calcium can also act as a messenger that turns on events in the cell. Remember the role of calcium in muscle contraction.

Water-Soluble (Hydrophilic or Amino Acid Derived) Hormone Receptors: G-protein-coupled receptors. Synthesized by cell receptors on cell surface. Examples include Insulin, Growth Hormone, and Epinephrine
Author:  OpenStax College; License:  [CC BY 3.0 (], via Wikimedia Commons; Link:

Intracellular and nuclear receptors interact with DNA and affect mRNA synthesis. Nuclear receptors, or receptors on the nucleus of the cell, when activated stimulate transcription of various genes, resulting in the production of new proteins. The new proteins result in a change in the cell. Having different kinds of receptors such as membrane receptors and intracellular receptors accommodates both hydrophilic and hydrophobic hormones. Hydrophilic hormones easily dissolve in water, but do not easily enter the cell due to the phospholipid bilayer of the cell membrane and therefore act via membrane bound receptors, while nonpolar or hydrophobic hormones like testosterone that can easily cross the plasma membrane bind to cytosolic and nuclear receptors.  In addition, membrane receptors typically induce short term and rapid responses, while intracellular receptors tend to produce slower but prolonged responses. 

Lipid-Soluble (Hydrophobic or Cholesterol-derived) Steroid Hormone Receptors: Cytosolic and Nuclear Receptors.  Synthesized inside the nucleus of the cell.  Examples are Testosterone and Estrogen
Author: By OpenStax College; License: [CC BY 3.0 (], via Wikimedia Commons;

It is truly the receptor, not the hormone that ultimately determines the cellular response. Most receptors are highly selective to their particular hormone, that is, even similar hormones don't bind to the receptor with the same affinity. The receptor recognizes subtle differences in hormone structure which allows the receptor to distinguish between hormones. This concept has allowed pharmaceutical companies to design drugs (hormone analogs) that can interact with specific receptors to provide medicinal intervention. Drugs that bind to and stimulate a receptor are called agonists, while those that bind to a receptor and block its effects are called antagonists.

Agonist and Antagonist Ligands. Agonists fit hormone receptors and activate them.  Antagonists occupy hormone receptors and do not activate them but block them from activation.
Author: By Dolleyj (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons Link:

Approximately 60% of prescription drugs act by either activating or blocking specific receptors. The term ligand is a general term used to describe any substance that interacts with a receptor. The specificity of receptors allows hormones to be released at any site in the body and circulate throughout, but only affect the tissue that contains the particular receptor for the hormone.

Receptor numbers are constantly being changed within a given cell, thus, depending on the number of receptors present, a cell may become less or more responsive to a given hormone. These receptor numbers can be regulated by hormones in a positive (up-regulation, more) or negative (down-regulation, less) manner.

For example, an increased blood level of the hormone prolactin induces an up-regulation of prolactin receptors in the cells of the liver.  

In contrast, prolonged exposure of cells to the hormone insulin results in a down-regulation of the insulin receptor. In some instances, a hormone binding to its receptor may enhance the actions of a separate hormone on a different receptor of the same cell, a process called synergism.

End-of-Chapter Survey

: How would you rate the overall quality of this chapter?
  1. Very Low Quality
  2. Low Quality
  3. Moderate Quality
  4. High Quality
  5. Very High Quality
Comments will be automatically submitted when you navigate away from the page.
Like this? Endorse it!