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M embrane potential

M embrane potential. R esting potential A ction potential. M embrane potential. Membrane potential ( transmembrane potential or membrane voltage ) is the difference in electrical potential between the interior and the exterior of a biological cell.

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M embrane potential

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  1. Membrane potential Resting potential Action potential

  2. Membrane potential • Membrane potential ( transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell. • Typical values of membrane potential range from –40 mV to –100 mV.

  3. Excitable cells • Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells. • Action potentials in neurons are also known as "nerve impulses“. • It sends the messages from our muscles to our brains and back, as well as all the thought processes in our brain. • We could stimulate an excitable cell chemically, electrically, or mechanically.

  4. Voltage gated channels • Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane. • Two types of channels are present: • 1. Voltage gated Na+channels • 2. Voltage gated K+channels

  5. Resting membrane potential to threshold level (-70 to -50 mv) • 1. Opening of voltage gated Na+channels (electrical stimulus) • 2. Opening of mechanically gated Na+channels (mechanical stimulus) • 3. Opening of ligand gated Na+channels (chemical stimulus)

  6. Action potential

  7. All or none principle ►Action potential will either be generated or not…no gradations or intensities or possible ►Suprathreshold stimulus will elicit same action potential as elicited by threshold stimulus ►Subthreshold stimulus will not elicit action potential

  8. Stages of action potential • 1. Depolarization (-50 to +40 mv) ►Opening of voltage gated Na+channels ►About 5000 fold increase in Na+permeability ►Voltage rises and crosses zero (overshoot)

  9. Stages of action potential • 2. Repolarization (+40 to -70) • ►Opening of voltage gated K+channels • ►Closure of voltage gated Na+channels

  10. Stages of action potential • 3. Hyperpolarization • ►Some voltage gated K+channels remain open even after RMP (-70 mv) is restored • ►Potential decreased more than resting level • ►Na+ -K+pump restores RMP from hyperpolarization

  11. Re-establishment of ionic gradients • During action potential Na+& K+ionic gradients reverse. In this condition cells contain: ►Large amount of Na+(due to massive Na+influx) ►Too less amount of K+(due to massive K+ efflux) • Na+ -K+pump re-establishes ionic gradients (recharges the nerve fiber)

  12. Refractory period 1. Absolutely refractory period • Period during which a 2nd action potential can not be generated. This can be elicited: ►From start of depolarization to initial 1/3 of repolarization ►After closure, the inactivation gates do not reopen until RMP is restored • It is mostly of 0.4 ms in large myelinated nerve fibers. 2. Relative refractory period • Period during which 2nd action potential can be generated but with stronger than normally required stimulus. This can be elicited: ►From end of initial 1/3 of repolarization to start of after depolarization (middle 1/3rd) ►Some voltage gated Na+channels regain their resting configuration • During this period K+efflux continues.

  13. Refractory period • limits frequency of action potentials. ►Longer the refractory period, less will be the frequency ►Absolutely refractory period of large myelinated nerve fiber is 0.4 ms, therefore, frequency of action potential is 2500/second ►Determine direction of action potential ►Action potentials can not be summated

  14. Local anesthetics • Procaine, Tetracaine etc block voltage gated Na+channels, thus ►No action potential occurs ►No nerve signal from periphery to brain ►No sensation of pain

  15. Propagation (conduction) of action potential • Propagates along nerve fiber as nerve signal or nerve impulse ►Means of communication between neurons or nerves and muscles. ► Causes muscle contraction

  16. Conduction of nerve impulse ►Nerve impulse conduction is always unidirectional ►Chemical synapses are unidirectional ►Ensure one way transmission of nerve impulse

  17. Types of nerve fibers (based upon myelination) • Myelinated fibers ►Covered by myelin sheath ►Large diameter fibers (A fibers) carrying touch and pressure sensations to CNS ►Somatic motor fiber to skeletal muscle • Unmyelinated fibers ►Not covered by myelin sheath ►Small diameter fibers (C fibers) carrying dull pain sensation to CNS ►Postganglionic autonomic fibers

  18. Myelin sheath • ►Fatty material • ►Produced by Schwann cells • ►Wraps around the axon in multiple layers • ►Insulates the nerve fiber • ►Ionic exchange can not take place through myelin sheath

  19. Nodes of Ranvier • ►Parts of myelinated nerve fiber devoid of myelin sheath • ►Present after every1-3 mm of myelinated part of nerve fiber • ►Are in contact with ECF • ►Have abundance of voltage gated Na+channels • ►Sites of action potential generation

  20. Propagation • Theactionpotentialgeneratedattheaxonhillockpropagatesasawavealongtheaxon. Thecurrentsflowinginwardsatapointontheaxonduringanactionpotentialspreadoutalongtheaxon, anddepolarizetheadjacentsectionsofitsmembrane. Ifsufficientlystrong, thisdepolarizationprovokesasimilaractionpotentialattheneighboringmembranepatches.

  21. Types of conduction • Contiguous conduction • ►Occurs in unmyelinated fibers • ►Every part of nerve fiber undergoes depolarization • ►Slow speed of impulse conduction • ►More energy consumption • Saltatory conduction • ►In myelinated nerve fibers • ►Depolarization occurs only at nodes of ranvier • ►Myelinated parts do not depolarize • ►Activation ‘jumps’ from node to node

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