The Action Potential

I.            Phases*

            A. Initiation

1.            due to summation of PSPs (IPSPs and EPSPs) at axon hillock

2.            action potentials are stereotyped and follow an “all or nothing principle (unlike graded potentials that vary in relation to the size of the stimulus)

3..            a depolarizing change that brings the potential to threshold will activate electrically gated NA+ channels on hillock

B.            Rising Phase

1.            as Na+ channels open, the polarity of the membrane reverses and becomes very positive (there is approximately a 100 mV change in polarity—the neuron goes from –65 to approximately +35)—this is NOT relative to the size of the stimulus, it is because the neuron has reached its threshold

2.            Na+ will rush into the cell (diffusion and electrostatic pressure are both driving the Na+ ions inward)—this represents the rising phase of the AP

            3.         As Na+ enters, the Na+ channels will become inactivated

C.            Overshoot

1.            As the membrane potential becomes increasingly positive and the concentration of Na+ inside increases, less sodium will rush in, and the AP will reach its peak—this is known as overshoot

2.            At the peak of the AP, Na+ will no longer be electromagnetically attracted to the interior and since the concentration of Na+ inside the cell is now relatively great, the curve begins it descent

D.            Falling Phase

1.            Shortly (about 1 msec) after Na+ channels are activated, K+ channels will open and K+ will begin to rush out (because of diffusion)—this creates a reduction in polarity as the cell membrane potential goes back toward negative—known as the falling phase

2.            Because Na+ is no longer entering but K+ is still leaving the cell, there is a brief period of hyperpolarization known as undershoot

3.            Na+-K+ pumps restore the membrane to its original threshold value and ionic differential

II.            Refratoriness

            A.            Absolute Refractory Period

1.            After the Na+ channels are activated, they enter a period of inactivation—this inactivation makes the neuron unable to fire again

2.            this absolute refractory period is the rate-limiting factor for the maximum firing rate of the cell

            B.            Relative Refractory Period

1.            During the falling phase (repolarization), the Na+ channels become reactivated

2.            depending on how many are reactivated, a sufficiently large stimulus may trigger another AP

III.            Miscellaneous

            A.            Spontaneous firing

                        1.            is regular and necessary

                        2.            allows for a baseline of firing to compare excitation and inhibition

            B.            Conduction velocity

                        1.            depends on several things

                                    a.            resistance of the membrane

                                                i.            determined by density of ion channels

                                                ii.            determined by size of membrane

                                    b.            fiber diameter

                                                i.            larger fibers = less resistance at core

ii.            ratio of membrane area to cytosol (smaller axon, smaller membrane, ergo more membrane per segment than larger axon)

c.            myelin adds resistance, minimizing outward spread of charge, therefore increasing velocity

d.            myelin increases efficiency (less energy required to activate pumps)

e.            regions between myelin are nodes, regions under myelin are internodes

C.        Neurons may be categorized based on conduction velocity, diameter, function, etc.

*You should be able to depict the voltage changes of the action potential graphically as well as explain them verbally.