L1.1.4.1: Nerve Impulse (Irritability)

• NERVE IMPULSE (irritability)

  1. Resting membrane is polarized
  2. Stimulus initiates local depolarization
  3. Depolarization and generation of an action potential
  4. Propagation of the action potential
  5. Repolarization
  6. Initial ionic conditions are restored
• (1) Resting membrane is polarized

  ➔ Resting state

  ➔ External face of membrane is slightly positive

  ➔ Internal face is slightly negative

  ➔ Chief extracellular (outside) ion: Sodium (Na+)

  ➔ Chief intracellular (inside) ion: Potassium (K+)

  ➔ TIP TO REMEMBER: “PISO” = Potassium Inside, Sodium Outside

  ➔ Polarized membrane is more permeable to K+ ions

  ➔ As long as the inside of the membrane is more negative (fewer positive ions) than the outside, the cell remains inactive
• (2) Stimulus initiates local depolarization

  ➔ Stimulus changes the permeability of a local patch of the membrane

  ➔ Na+ diffuse rapidly into the cell

  ➔ Depolarization - the inward rush of sodium ions changes the polarity at that site
• (3) Depolarization and generation of an action potential

  ➔ Graded potential (localized depolarization) - inside of the membrane is more positive, outside is less positive

  ➔ TIP TO REMEMBER: “IPON” = Inside: Positive, Outside: Negative

  ➔ If the stimulus is strong enough and sodium influx is great enough, - Local depolarization activates the neuron to conduct an action potential
• (3) Depolarization and generation of an action potential

  ➔ If the stimulus is NOT strong enough, - Cell returns to a resting membrane state - No action potential occurs

  
  

  ➔ NO SODIUM ENTRY = NO ACTION POTENTIAL

  
  

  ➔ All-or-nothing response - nerve impulse is either propagated, or it doesn’t happen
• (4) Propagation of the action potential

  ➔ Depolarization of the first membrane patch causes permeability changes in the adjacent membrane

  ➔ Step 2 repeats

  ➔ Action potential propagates rapidly along the entire length of the membrane falling one after another (like domino or wave)
• (5) Repolarization

  ➔ Potassium ions diffuse out of the cell

  ➔ Membrane permeability changes again; negative charge is restored inside, positive charge outside

  ➔ Repolarization occurs in the same direction as depolarization
• (6) Initial ionic conditions are restored

  ➔ The sodium-potassium pump restores the ionic conditions of the resting state

  ➔ Uses ATP to restore original configuration

  ➔ 3 Na + are ejected every 2 K + ions carried back (3s:2p)

  ➔ Repolarization needs to be COMPLETED; otherwise, a neuron cannot conduct another nerve impulse
• Saltatory conduction

  ● Faster type of electrical impulse propagation

  ● Nerve impulses jumps/leaps from node to node, along the length of the fiber

  ● “Saltare” = to dance or leap

  ● Occurs in myelinated fibers
• Factors that impair the conduction of impulses

  ● Alteration of membrane permeability to sodium ions

  ○ Sedatives

  ○ Anesthetics

  ● Interruption of blood circulation

  ○ Cold

  ○ Continuous pressure