Action potential

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  • Insulation – almost all flow of ions is prevented
  • Action potential

    1. 1. ACTION POTENTIAL
    2. 2. OVERVIEW OF A NERVE IMPULSE 1.Resting potential – neuron is not stimulated at threshold level 2.Action potential – neuron responds to stimulus, sends “message” along axon a. Depolarization b. Repolarization ** Potential = difference in charge (measured in volts)
    3. 3. RESTING POTENTIAL  Inactive neuron  Inside cell membrane – more K+ ions  Outside cell membrane – more Na+ ions  Polarized cell membrane  Inside cell membrane – negative charge  Outside cell membrane – positive charge  Cell membrane relatively impermeable to both ions
    4. 4. WHY IS MEMBRANE POLARIZED AT REST? 1. Na+ /K+ pump maintains ion distribution - transports 3 Na+ out and 2 K+ in 2. K+ diffuses out faster – membrane is more permeable 3. Na + is attracted to cell because of its concentration gradient 4. Membrane is impermeable to large negatively charged ions
    5. 5. RESTING POTENTIAL
    6. 6. ACTION POTENTIAL = NERVE IMPULSE  Occurs in excitable membranes – neurons and muscle fibers  Critical level must be reached (“threshold”) before impulse is sent  Positive feedback mechanism  All-or-none response  Lasts a few milliseconds  2 steps:  Depolarization  Repolarization
    7. 7. DEPOLARIZATION  Stimulus causes adjacent Na+ ion channels to open along axon  Na+ ions rush into cell (high  low concentration)  Movement of Na+ ions reverses the charge of the membrane (depolarization):  Inside cell membrane – positive charge  Outside cell membrane – negative charge  If threshold is reached, adjacent Na+ ion channels to open along axon
    8. 8. DEPOLARIZATION
    9. 9. REPOLARIZATION  K+ ion channels open  K+ ions rush out of cell (high  low concentration)  Restores polarization of cell membrane  Inside cell membrane – negative charge  Outside cell membrane – positive charge  Ion distribution is different than at resting potential – can’t send another impulse yet  Inside cell – low K+ , high Na+  Outside cell – high K+ , low Na+
    10. 10. REPOLARIZATION
    11. 11. AFTER REPOLARIZATION  Na+ /K+ pump corrects ion distribution  Inside cell – high K+ , low Na+  Outside cell – low K+ , high Na+  Another impulse can now occur
    12. 12. ROLE OF MYELIN  Myelin insulates the axon  Action potential must “jump” from node to node  Saltatory conduction  Action potential is MUCH faster than along an unmyelinated axon
    13. 13. FACTORS AFFECTING NERVE IMPULSE  Local anesthetics – block Na+ channels – no Na+ movement = no AP  Continuous cold and/or pressure – blocks circulation (flow of nutrients and oxygen), so there is no energy for Na+ /K+ Pump = no AP

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