Action Potential

Define: Threshold, Depolarization, Hyperpolarization, Repolarization, Overshoot, Undershooot

• Threshold: level to which the membrane must depolarize to trigger and action potential

• Depolarization: When the membrane voltage becomes more positive

• Repolarization: When membrane voltage is becoming more negative after having reached a “peak”

• Hyperpolarization: Transient “dip” below normal Vm

  • Due to potassium “leak” channels

    • slow to open and close

• Overshoot: When membrane potential is positive

• Undershoot: When membrane potential is more negative than at rest

What is the relationship between Stimulus and distance?

Delay between stimulus and response (action potential) increases w/ distance

What is Spatial Summation? Temporal Summation?

• Spatial summation

  • Two or more presynaptic inputs arrive at a post synaptic cell at the same time

  • EPSP-IPSP Cancelation can occur this way

• Temporal summation

  • Two presynaptic inputs arrive at the post- synaptic cell in rapid succession

Describe the stages of Action Potential

• Resting stage

  • Before AP

  • Membrane is “polarized” (-70 mV)

  • Potassium current out of the cell dominates

• Depolarization stage

  • At threshold, sodium and potassium currents are equal and opposite, moving us up to threshold

  • Once threshold is met, many more voltage-gated sodium channels open causing rapid depolarization

    • Na+ conductance increase 5000x

  • Cell now depolarizes (+35 mV)

• Repolarization stage

  • Na channesl close; K+ channesl open

    • K+ leaves through BOTH VOLTAGE GATED AND LEAKAGE CHANNELS

    • K+ channels = “delayed rectifier” channels because they are slow to open and close → undershoot

• Return to rest:

  • Na-K pump pumps Na out, K in

Describe the three steps of Sodium Gate Activation

Step 1:

• Closed, but available

• When an AP arrives, the attendant voltage change
  opens the sodium channel (voltage-gated)
  

Step 2:

• Open (some open as we move towards threshold)

• Sodium conductance increases rapidly as we
  reach threshold and depolarization occurs rapidly
  

Step 3:

• Inactivated (no more sodium current into the cell)

• This plays a role in the “refractory period”

Differentiate between Absolute and Relative Refractory period. Describe the mechanism for both.

• Absolute refractory period

  • sodium channel inactivation gate is closed → impossible to open them

  • Also, during this time, the membrane potential is already positive and action potential propagation can not be simulated

• Relative refractory period

  • Occurs when potassium conductance out of cell increases

  • Membrane potential → more negative → Na+ channels changes status from Inactivated to Closed 

    • now possible for AP propagation but requires a greater than normal stimulus

(T/F) In action potential, Na+ channels are stimulated first, then K+ channels are stimulated to open; Explain

False; They are both stimulated at same time. The reason why they open at separate times is because K+ voltage gated channels are slower to act then Na channels.

Where is myelin usually found? Describe why myelin increases AP propagation speed

• Myelin is found in larger nerves, like the alpha-motor neurons

• Increases speed due to two factors:

  • Provides insulation 

    • provides resistance to electrical flow leaving the neuron → electrical flow Is always down the neuron

  • Saltatory conduction

    • the sodium channels are only at Nodes of Ranvier

      • Thus, the AP leaps from node to node unlike in unmyelonated cells where it is limited by the speed with which the impulse can spread from channel to channel