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Resting membrane potential. Resting membrane potential (RMP). Unbalanced charges distributed across the plasma membrane that are responsible for membrane potential. It is the difference in electrical potential between the inside &the outside membrane surface under resting conditions.
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Resting membrane potential (RMP) Unbalanced charges distributed across the plasma membrane that are responsible for membrane potential It is the difference in electrical potential between the inside &the outside membrane surface under resting conditions. The inside is negative relative to the outside of the membrane (polarized state). Extracellular fluid Closed Na+ channel Intracellular fluid Entire membrane at resting potential myelinated nerve& skeletal muscle is -90mV. The RMP in Medium-sized neuron is -70mV. The RMP of the cardiac muscle is -60Mv. excitable cell Portion of an
How to measurecell electrical potentials • RMP can be recorded when two microelectrodes ,one is placed outside the nerve fiber &the other is placed inside nerve fiber are connected to a cathode ray oscilloscope (CRO).
Causes of resting membrane potential The unequal distribution of ions on both sides of membrane is due to: 1-Selective permeability of the membrane for (Na+ &K+). 2- Na+ &K+ pump.
1-Selective permeability of the membrane for (Na+ &K+). Outside the cell : Na+ is the main cation & Cl- is the main anion. Inside of the cell: K+. is the main cation &proteins are the main anions. The cell membrane Is semipermeable membrane. It is more permeable to K+, less permeable to Na+ ,freely permeable to Cl- & impermeable to proteins.
1-Selective permeability of the membrane for (Na+ &K+) K+ ions: Although K+is more concentrated inside the cell , it tends to diffuse outside the cell according to 1-The concentration gradient 2- the high permeability of the cell membrane. However the K+ outflux is limited by : 1-Attraction between K+ &intracellular proteins. 2-Repulsion between K+& Na+ outside cell membrane. 3- K+-Na+ pump which derive K+inside the cell. Na+ions: Although Na+ is more concentrated outside the cell , it tends to diffuse inside the cell according to: 1-The concentration gradient. 2-Electrical gradient. However the Na+ influx is limited by : 1-the low permeability of the cell membrane to Na+. 2- K+-Na+ pump which pumpsNa+ outside the cell.
2- Na+ &K+ pump It is an electrogenic pump because it pumps3 Na+ outside the cell for every2 K+ inside the cell .This will add more positive charges outside the cell membrane producing the resting membrane potential. Na+ &K+ pump acts against the concentration gradient .So, it needs energy derived from hydrolysis of ATP
Basis and important of RMP The basis of RMP: 1- the existence of the Na+ &K+ pump. 2-The permeability of the membrane to the K + to the outside is a hundred times greater than that to the of Na+. 3-The large amount of protein anions within the cell. The cell membrane acts as a charged capacitor. At rest the outer surface carries positive charged with respect to inner surface.( polarized)
Action potential Action potential : Is the change in the membrane potential after application of an effective stimulus (threshold).
Phases of action potential 1-Stimulus artifact: A short irregular deflection of the base line at the time of stimulus application. 2-Latent period (isoelectric period) It is isoelectric period & indicates the Time taken by the current to travel from the stimulating to the recording electrode. It depend on: 1-The distance between site of application of the stimulus & the recording electrode. (direct proportional). 2-The velocity of nerve impulse. )indirect proportional).
Depolarization phase Depolarization phase Sudden shoots up (upstroke) from a potential -70mV to +35mV& it represents the reversal of polarity of the cell membranes .It is divided into:
Repolarization phase Downstroke ,the membrane potential return to the original polarity ,it is divided into: The peak value of the AP is known as Spike potential
component of AP Caused by Na+ influx Rpolarization is followed by After hyperpolarization: Prolonged period of mild hyperpolarization. Finally the RMP is restored. Caused by K+ efflux Rising phase Falling phase Threshold potential Resting potential
Ionic basis of action potential At resting potential all Na+ and K+ gated channels are closed At resting potential
As membrane potential decreases from -70mV,the voltage gated fast Na+ channels open partially & when it reaches -55mV Na+channels are fullyopen and the Na+ movement inside the cell The membrane potential Decreases to +35mV • As membrane potential decreases • from -70mV,the voltage gated • fast Na+ channels open partially • & when it reaches -55mV • Na+channels are fullyopen • and the Na+ movement • inside the cell • The membrane potential • Decreases to +35mV Explosive Na influx by both concentration and electrical gradient
Inward movement of Na+ makes the outside progressively less positive Explosive depolarization; potential reaches 0 mV Action potential begins Na influx produces Membrane depolarization until 0 mV is reached
Continued inward movement of Na+ reverses the potential with the inside becoming Positive and the outside becoming negative as the action potential Peaks Reversal of polarity or Overshoot K+ gate opens Na+ inactivation gate begins to close Peak of action potential; potential reversed
At the peak of action potential, the Na+gates close and the K+ gates open. Entry of Na+ ceases and K+ starts to leave the cell. Repolarization begins Outward movement of K+ (both by concentration and electrical gradient) makes the insideprogressively less positive and the outside less negative
Na+ inactivation gate opens; Na+ activation gate closes Continued outward movement of K+ (K EFFLUX) restores the resting membrane potential Action potential complete; after hyperpolarization begins Further outward movement of K+ through the still-open K+ gates transiently hyperpolarizes the membrane
then the K+ gates close, and the membrane returns to resting potential After hyperpolarization is complete; return to resting potential