Excitable Tissues & Action Potentials

• Local Potentials (LP) to Action Potentials (AP)

  1. If strong enough, local potentials (LPs) can spread (Na+ diffusion) and reach the trigger zone at the axon hillock

  2. If LP can depolarize the hillock to a threshold voltage it causes an action potential A(P)

  3. Threshold = minimum voltage required to trigger AP (i.e., voltage that causes voltage-gated channels in axon to open)
• Action Potential


  • Changing the resting potential temporarily at one point
  • Response of excitable cells to a stimulus
  • Mostly due to movement of Na+ & K+

  
• Voltage-gated Na+ Channel

  • Has Two Gates: Activation gate and Inactivation gate
• Action Potential


  1. Local potential to threshold

  2. Depolarization

  3. Repolarization & Hyperpolarization
• Voltage-Gated Na+ Channel

  • Activation gate closed; inactivation gate open (at rest)
  • Activation gates first open at threshold; -55 mV
  • Both gates stay open during depolarization; -55 mV to +35 mV
  • Inactivation gate closes at peak voltage (+35 mV) and stays closed during most (or all) of repolarization
  • Activation gate closes & inactivation gate opens ("resets" to resting state) sometime between threshold and resting potential
• Depolarization


  • Voltage-gated Na+ channels are open
  • Lots of Na+ diffuses into cell
• Repolarization


  • At peak (+35 mV) inactivation gates on Na+ channels close (no more Na+ can move into cell)
  • K+ still diffuses out of cell
• Hyperpolarization


  • Membrane potential becomes more negative than resting (i.e., below -70 mV)
  • K+ diffuses out of cell (past RMP)
  • New AP could be triggered but would take a larger stimulus
  • Prevents backward movement of APs
• Action Potentials

  • All-or-none: reaching threshold will always depolarize to around +35 mV
  • Non-decremental: each AP has the same magnitude from the hillock to the end of axon
  • Irreversible: cannot be stopped once it reaches threshold
• Action Potential vs. Nerve Signal

  • AP: reversal of membrane potential at one point
  • Nerve signal: traveling wave of APs, each one stimulating (propagating) the next

  
• Action Potentials Self-Propagate

  1. Once one action potential happens, the depolarization will trigger voltage-gated channels in adjacent segments of the axon causing more action potentials

  2. This process is called conduction (series of action potentials along the length of the axon)

  3. Positive feedback mechanism

  4. Local anesthetics (e.g., lidocaine) bind to voltage-gated Na+ channels, preventing Na+ influx

  
• Controlling Conduction

  1. We need nerve impulses to move in ONE direction along a neuron

  2. Refractory periods prevent "backwards" movement of APs

  3. After depolarization of a certain segment of axon, there is a delay to the start of another AP
• Refractory Periods

  • Absolute refractory period: Na+ channels are either already open (depolarization) or blocked by inactivation gate (during repolarization), impossible to generate AP
  • Relative refractory period: Na+ channels reset, inactivation gate reopens, activation gate closed (but can be opened if threshold reached), hyperpolarization (below RMP), would need a stronger than normal stimulus to get a new AP