The action potential is a phenomenon of excitable cells like nerve and muscle and consists of rapid depolarization (upstroke) followed by repolarization of the membrane potential.

Action potentials are the basic mechanism for transmission of information in the nervous system and in all types of muscle.

Terminology

We will need to know some basic terminology for this chapter. Here goes:

• Depolarization
  • The process of making the membrane potential less negative
  • We should not ask questions like the following because the terms increased and decreased are ambiguous.
    • When the membrane potential depolarizes, has the membrane potential increased or decreased?
• Hyperpolarization
  • The process of making the membrane potential more negative
  • Again, we should not describe this with increasing or decreasing
• Inward Current
  • The flow of positive charge into the cell
    • Inward currents depolarize the membrane potential
      • Inward current is the flow of Na+ into the cell during the upstroke of the action potential
• Outward Current
  • The flow of positive charge out of the cell
    • Outward currents hyperpolarize the membrane potential
      • Outward current is the flow of K+ out of the cell during the repolarization phase of the action potential
• Threshold Potential
  • The membrane potential at which occurrence of the action potential is inevitable
    • This is caused when the net inward current becomes larger than net outward current and the resulting depolarization becomes self-sustaining
• Overshoot
  • The portion of the action potential where the membrane potential is positive
• Undershoot (hyperpolarizing afterpotential)
  • The portion of the action potential following repolarization where the membrane potential is actually more negative than it is at rest
• Refractory Period
  • The period during which another normal action potential cannot be elicited in an excitable cell
  • Absolute and Relative RP
    • In cardiac cells, there is also Effective RP

Characteristics of Action Potentials

Action potentials have three basic characteristics:

• Stereotypical size and shape

  • Each normal action potential for a given cell type looks identical, depolarizes to the same potential, and then repolarizes back to the same resting potential

• Propagation

  • An action potential at one size causes depolarization at adjacent sites bringing adjacent sites to threshold
  • Propagation from one site to the next is nondecremental
  • Nondecremental means the amplitude is constant

• All-or-none response

  • An action potential either occurs or does not occur
  • If an excitable cell is depolarized normally, then the action potential occurs
  • If an excitable cell is not depolarized, then the action potential cannot occur

  Ionic Basis of the Action Potential

  The action potential is a fast depolarization (upstroke) followed by repolarization back to the resting membrane potential.

  

  Sorry is the picture is hard to see. This illustrates the events of the action potential in nerve and skeletal muscle.

Let's go in steps.

1. Resting Membrane Potential

   • At rest, the MP is -70 mV and the K+ conductance is high
   • The K+ ion channels are fully open allowing K+ ions to diffuse out of the cell down the existing concentration gradient
   • The diffusion creates a K+ diffusion potential which drives the membrane potential toward the K+ EP
   • The conductance to Cl- is also high (not shown) and at rest is near ECE
   • At rest, Na+ conductance is low and the RMP is far from the Na+ EP and Na+ is farm from ECE

2. Upstroke of the Action Potential

   • An inward current usually caused by adjacent cells causes depolarization of the nerve cell membrane to threshold which is at -60 mV
   • The initial depolarization causes rapid opening of the activation gates of the Na+ channel and the Na+ conductance promptly increases and becomes even higher than the K+ conductance
   • The increase in Na+ conductance results in an inward Na+ current and the membrane potential is further depolarized toward the Na+ EP of +65 mV

   <aside> 💡 Tetrodotoxin comes from the Japanese Pufferfish and the local anesthetic lidocaine (the one your grandma uses) block these voltage-sensitive Na+ channels and prevent the occurrence of nerve action potentials.

   Doctors prescribe these lidocaine to people with pain and prevent the action potentials saying "there's pain here" from going to the brain

   </aside>