The nerve impulse

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  • THE NERVE IMPULSE
    • RESTING POTENTIAL
      • Movement of ions (Na+ & k+) , across axon membrane - controlled in + ways:
        • 1} Phospholipid bilayer of axon plasma membrane prevents Na+ & K+ diffusing across
          • = Inside axon  -vely charged compared to outside - RESTING POTENTIAL
            • RP- 50-90mv
              • 65mv- humans
            • Axon is POLARIZED
            • POTENTIAL DIFF (diff in charges between inside & outside of axon) DUE TO:
              • 1] Na+ actively transported OUT axon by SP PUMP
              • 2] K+ actively transported INTO axon by SP PUMP
              • 6] Most gates in channel that allow K+ OPEN But most for Na+  CLOSED
              • 5] Na+ naturally diffuse back in axon & K+ out axon
              • 3] Active transport of Na+ greater than K+ (3 Na+ out for ever 2 K+ in)
              • 4] = + Na+ in tissue fluid around axon than in        cytoplasm               &           + K+ in cytoplasm than tissue fluid
                • = ELECTROCHEMICAL GRADIENT
        • 3} Channel proteins on bilayer-> have channel ions
          • Some have gates -> open & close -> ions can move through facilitated diffusion
            • Some always stay open -> ions freely move (facilitated diffusion)
          • = Inside axon  -vely charged compared to outside - RESTING POTENTIAL
            • RP- 50-90mv
              • 65mv- humans
            • Axon is POLARIZED
            • POTENTIAL DIFF (diff in charges between inside & outside of axon) DUE TO:
              • 1] Na+ actively transported OUT axon by SP PUMP
              • 2] K+ actively transported INTO axon by SP PUMP
              • 6] Most gates in channel that allow K+ OPEN But most for Na+  CLOSED
              • 5] Na+ naturally diffuse back in axon & K+ out axon
              • 3] Active transport of Na+ greater than K+ (3 Na+ out for ever 2 K+ in)
              • 4] = + Na+ in tissue fluid around axon than in        cytoplasm               &           + K+ in cytoplasm than tissue fluid
                • = ELECTROCHEMICAL GRADIENT
        • 2} Sodium potassium pump
          • A carrier protein -> actively transports K+ in axon & Na+ out
    • ACTION POTENTIAL
      • When stimulus of sufficient size detected by receptor, its energy = TEMPORARY REVERSAL of charges either side of axon membrane
        • If stimulus great enough,  -ve charge inside membrane (-65mv) turns +ve ( ar. +40mv) - ACTION POTENTIAL
          • Axon is DEPOLARISED
            • Depolaristion -occurs d.t change of channel's shape so open/close, depending on voltage across membrane
              • Therefore called voltage-gated channels
      • 1. At RP some K v-g channels open but Na v-g channels closed
        • 2. Energy of stimulus = Na v-g channels to open -> Na+ diffuse into axon through these channels along ELECTROCHEMICAL GRADIENT.
          • 3. As Na+ diffuse into axon + Na channels open = greater influx of Na+ via diffusion
            • 4. Once action potential at ar +40mv -> v-g on Na+ channel close & v-g on K+ channel open
              • 5. -> electrical gradient which prevented further outward movment of K+ now reversed = opening of + K+ channels
                • 6. Outward diffusion of K+ = temporary overshoot of the electrical gradient, with inside A + -ve  (relative to outside) than usual (=HYPERPOLARISATION)
                  • Closable gates on K+ channel now close & Na k pump again pump out Na+ & K+ in
                    • RP (-65mv) re-established
                      • Axon -> REPOLARIZED
                • Means + K+  diffuse out-> REPOLARISATION of axon
          • Trigger reversal in PD across membrane d.t being +ve
    • Self-propagating wave of elec activity -> travels along axon membrane
    • Temporary reversal of the elec Potential Difference across axon membrane
      • Reversal is between 2 states: RP & AP

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