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Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. ... Almost all plasma membranes have an electrical potential across them, with the inside usually negative with respect to the outside.
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definition • It is the difference in electrical potential between the two sides of the membrane surface under resting conditions. • The inside is negative relative to the outside of the membrane (polarizedstate).
Excitable Tissue • Nerve cell and Muscle are excitable tissue, they produce rapid, transient changes, in their membrane potential when stimulated • These changes in potential cause Electrical signals.
Resting membrane potential (RMP) • When cells are at rest (not stimulated) – there is Resting Membrane Potential. • myelinated nerve& skeletal muscle is -90 mV. • The RMP in Medium-sized neuron is -70 mV. • The RMP of the cardiac muscle is -60 Mv.
Causes of RMP Diffusion potential • Diffusion Potential caused by an ion concentration difference on the two sides of the membrane. • The Na+ - K+ PUMP transports THREE Na+ out, for TWO K+ it transports inside the cell. • As both are positive ions, unequal transport causes more positive charge outside negative charge inside. Na-K ATP pump Selective permeability for: Na +K
Selective permeability Inside of the cell: K+. is the main cation &proteins are the main anions. The cell membrane is : • It is more permeable to K+, less permeable to Na+,freely permeable to Cl- & impermeable to proteins.
Cont” selective permeability 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
Cont” selective permeability 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.Na+ -K pump which pump Na outside the cell.
Na-K ATP pump • Pumps 3 Na+ outside the cell for every 2 K+ • Electrogenic pump
Action potential A short-term change in the electrical potential on the surface of a cell (e.g. a nerve cell or muscle cell) in response to stimulation, and spread rapidly along the nerve fiber membrane.
The Nobel Prize in Physiology or Medicine (1963) Alan Lloyd Hodgkin Andrew Fielding Huxley “for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane”
Basic Electrophysiological Terms • Stimulus: a sudden change of the (internal or external) environmental condition of the cell. • includes physical and chemical stimulus. • Threshold(intensity): the lowest or minimal intensity of stimulus to elicit an action potential
Basic Electrophysiological Terms : • Polarization: a state in which membrane is polarized at rest, negative inside and positive outside. • Depolarization: the membrane potential becomes less negative than the resting potential (close to zero). • Hyperpolarization: the membrane potential is more negative than the resting level. • Repolarization: restoration of normal polarization state of membrane.
Phases of action potential 1.resting stage 2 depolarization stage 3.repolarization stage
Resting stage • The membrane is polarized. • all Na+ and K+ gated channels are closed.
depolarization stage Depolarization phase • the voltage gatedfast Na+ channels open partially & when it reaches -55mV :Na+ channels are fully open and the Na+ moves inside the cell.
Activation gate Inactivation gate
The Nobel Prize in Physiology or Medicine (1991) Erwin Neher Bert Sakmann "for their discoveries concerning the function of single ion channels in cells"
Repolarization stage • Repolarization • hyperpolarization
Properties of the Action Potential • “All or none” phenomenon A threshold or suprathreshold stimulus applied to a single nerve fiber always initiate the same action potential with constant amplitude, time course and propagation velocity. • Propagation • Transmitted in both direction in a nerve fiber
Inhibition of action potential Local anesthetics: • Proacaine and Tetracaine. • Work on gate of Na channels: difficult to open.
Refractory periods • relative • Absolute.
