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

Action Potentials

An action potential is where things really become interesting and exciting—no pun intended. The action potential represents a rapid change in the membrane potential, followed by a rapid return to the resting membrane potential (see figure below). In other words, a rapid depolarization followed by a rapid repolarization. The action potential is the basis for transmitting signals in nerve cells, inducing muscle contraction, and perception of all our senses. The action potential is the result of the activation of the voltage-gated ion channels, most often the Na+ voltage-gated ion channel.

Most commonly, we talk about action potentials as they relate to nerve cells. In nerve cells, at rest, the movement of Na+ through the membrane is extremely low (very few Na+ leak channels). However, if the surface (cell membrane) of the neuron receives a graded potential that is sufficient to exceed the set threshold value, the voltage sensitive proteins will respond by changing conformation (note: all voltage sensitive proteins respond to a threshold, but with varying levels of sensitivity. The Na+ channel is the most sensitive and opens very rapidly). Because the driving force for Na+ is extremely high (≈-147mV), the opening of Na+ channels will cause a rapid influx of Na+, therefore disrupting the negative membrane potential and resulting in depolarization. The membrane potential will increase rapidly in response to the increased positive charge until the Na+ voltage-gate inactivates and closes. It is important to note that depolarization occurs with minimal changes in the overall concentration of Na+ or K+ (Only one out of every 100,000 Na+ ions need to enter the cell to produce a 100mV change in potential).

Once activated, the protein channel is quick to reestablish a new conformation, but during the interim (about 0.5 msec), the protein allows sodium to pass through the membrane. In the case of the Na+ channel, there are two gates, an activation gate and an inactivation gate. The activation gate is very sensitive to voltage changes and is the basis of threshold. The inactivation gate is slightly delayed compared to the activation gate, which allows for the channel to be permeable for a brief moment. After a slight delay, voltage-gated K+ channels open, resulting in an efflux of potassium out of the cell. This efflux is in addition to the efflux resulting from K+ leak channels that are always open. The additional efflux of potassium, in combination with the termination of Na+ (because the inactivation gate closes) influx, reverses the initial depolarization, and the membrane potential moves back towards the resting potential (repolarization) and even beyond (hyperpolarization, ie. Moving towards the Nernst for K+). After which, the K+ voltage channels close, and the resting membrane is re-established (the potassium channels have only one gate which is activated by depolarization and inactivated by repolarization). Hyperpolarization occurs because of the additional movement of K+ through the voltage-gated K+ channel. Once the voltage-gated K+ channel closes, the membrane will return to the resting potential established initially by the K+ leak channels. The small depletions that occur in K+ and Na+ concentrations following each action potential are then reestablished by the Na-K ATPase pump, but this is not necessary for another action potential. In fact, it has been demonstrated that the ion gradients in a neuron are sufficient to be able to generate 10,000 action potentials without replenishment from the Na-K ATPase pump. It is important to note that in order for activation of the Na+ channels to occur, there needs to be a sufficient stimulus of current that exceeds the threshold value. For example, the threshold value for a typical neuron is near -55 mV, while the resting membrane potential is near -70 mV. If a graded potential is not sufficient to bring the membrane up to the threshold value (-55 mV), then an action potential cannot be initiated. This kind of stimulus is referred to as a sub–threshold stimulus. If the threshold value is exceeded by a given stimulus, the action potential will always occur. This phenomenon is referred to as the all-or-nothing principle. In addition, unlike graded potentials, the action potential cannot be summed or added upon, but once an action potential starts it becomes self-propagating.

Action Potential
Image by BYU-I Kaylynn Loyd 2013

When current stimulus is sufficient to reach the threshold value, an action potential is triggered. Notice that the first stimulus came close but did not exceed threshold. This stimulus failed to initiate an action potential. However, the second stimulus must have exceeded threshold because a relatively large and rapid depolarization occurred, followed by a rapid repolarization.

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