Human Physiology - Action Potential Part 1 PDF

HUMAN PHYSIOLOGY Membrane Properties of excitable cells (Action Potential) part 1 Physiology Department Faculty of Medicine Helwan University  Lecture objectives By the end of discussing “Membrane properties of excitable cells (neurons & m...

HUMAN PHYSIOLOGY Membrane Properties of excitable cells (Action Potential) part 1 Physiology Department Faculty of Medicine Helwan University  Lecture objectives By the end of discussing “Membrane properties of excitable cells (neurons & muscle cells)”, student will be able to: 1. Describe Electrical activity (action potential), development, and, properties. 2. Interpret the impact of changing ionic conductance on AP. 3. Describe Excitability changes that occurs during the electrical activity of the membrane. 4. Define and list mechanisms of cellular propagation of electrical signals. 2  Changes accompanying the propagation of a nerve impulse: 1- Electrical change l RMP AP 3 Resting Membrane Potential (RMP) Potential difference across the cell membrane in millivolts (mV). It is established by diffusion potentials that result from concentration differences of permeant ions. 1 4  Resting membrane potential(RMP) In the nerve - 70mv  How to ensure the negativity of the membrane ? K+ Leak channels ( Selective permeability)plays essential role in the development of RMP. Presence of proteins (-ve charged)inside the cells ensure that the membrane tends to be negative. if any disturbance occurs the presence of Na+,K+ pump ,quickly corrects the change Then comes the stimulus and….  Action Potential 9  Action Potential Definition: The action potential is the rapid changes in membrane potential produced by application of an effective (threshold or suprathreshold) stimulus to the nerve. These changes transmitted along the nerve as a self propagated disturbances known as the nerve impulse. 1 1 Thershold stimulus  Recording of action potential: Action potential can be recorded by cathode ray oscilloscope through 2 microelectrodes, one is inserted inside the nerve and the other is placed on its outer surface. 1 3  Phases (stages) of the action potential  Stimulating the nerve by electric stimulator is marked by a stimulus artifact, which is due to current leakage from stimulating to recording electrodes.  The stimulus artifact is followed by latent period.  The latent period is isopotential interval, representing the time taken by the impulse to reach recording electrode. 1 4 After the latent period, the successive stages of the action potential are recorded, including: Spike potential: 1. Depolarization stage. 2.Repolarization stage. After potential 3. The negative after-potential 4. The positive after-potential 10  Action potential 11 12 13  The action potential consists of: Spike potential: It is large wave of a short duration (0.5-1 m. sec) in myelinated nerve fibers). It consists of: 1) Ascending limb: which represents the process of depolarization. It is due to Na+ influx which occurs in two stages: slow until the threshold potential: gradual Na+ influx changes the membrane potential form the resting potential (-70 m.v.) to the threshold potential (-55 m.v.). 14  Rapid : at the threshold potential or the firing level, the voltage sensitive Na+ channels open suddenly leading to rapid Na+ influx and rapid depolarization of the membrane With continuous Na+ influx, the membrane potential becomes positive (+ 35 m.v.). This is known as reversal of polarity or overshoot. 15  Descending limb: Which represents the major part of the process of repolarization (about 70%). Repolarization is caused by K+ efflux which occurs at first rapid due to the sudden opening of the voltage sensitive K+ channels (open immediately after closure of Na+ channels). So, the descending limb is caused by rapid K+ efflux. 16  Ionic basis (mechanism) of Depolarization A. The stimulus increases the permeability of the cell membrane (several hundred folds) to Na+ ions, which diffuse inside causing gradual change in the membrane potential from the resting potential (-70m.v) to the threshold potential or the firing level (-55m.v). B. At that level, the gates of the voltage activated sodium channels open and Na+ ions flow into the cell (Na influx ) C. These inactivating gates close after a certain time. As a result of sudden Na+ influx, the membrane potential quickly reaches zero potential and then overshoots to about + 35 m.v, so that there is a momentary reversal in polarity. 17  N.B.: The Na+ channels: Each Na+ channel has two gates:  One at the outer surface of the membrane called an activation gate. The other at the inner surface of the cell membrane called an inactivation gate. Na + channel is voltage –gated because: Its opening and closure is controlled by the present potential. 18 Redistribution of ions inside and outside: Redistribution of Na+ and K+ ions to the normal resting condition is established by the Na–K pump which actively transports sodium out and potassium into the cell. 19  B) After potentials: These are small waves with relatively longer durations and consist of: a) Negative after potential (or after depolarization): When the membrane becomes 70% repolarized, the rate of repolarization decreases and the resting membrane potential is reached slowly. 20 b) Positive after potential (after hyperpolarization): Prolonged opening of the K+ channels cause a continued efflux of k efflux ( slow k channels) 21  Question time 27  References BRS Physiology 6th edition. Chapter 1 pages 9- 12 28 Its Not the end