ACTION POTENTIALS AND GRADED POTENTIALS
I. Introduction
A. All cells possess a membrane potential related to the non-uniform distribution of, and differential
permeability of, Na+, K+ and A-.
B. Nerve and muscle are excitable tissue that use this potential by undergoing controlled, transient,
rapid changes in membrane potential. Such fluctuations in membrane potential serve as
electrical signals.
C. Two kinds of such electrical signals
1. Graded potentials are short distance signals
2. Action potentials are long distance signals
II. Action Potentials
A. Terms (Fig 7.9)
1. Polarization: separation of charges
2. Depolarization: reduction in potential
3. Hyperpolarization: increase in potential
4. Repolarization: return to resting potential
B. Events in an Action Potential (Fig 7.13)
1. Occurrence of a triggering event
2. Slow depolarization from -70 mV to -55 mV
3. -55 mV is threshold potential; as soon as it is reached, a very rapid depolarization to +30
mV occurs
4. repolarization then occurs
5. frequently overshoots to -80 mV before repolarizing at -70 mV; this is hyperpolarization
6. in a nerve, this whole process takes 0.001 seconds
C. Mechanisms underlying events in Action Potential (Fig 7.13, 7.14)
1. Membrane contains Na+ channels and K+ channels. Both of these are voltage regulated
channels, meaning that changes in voltage (i.e., membrane potential) open or close them.
2. At rest (-70 mV) some K+ channels are open, fewer Na+ channels open; the “leaking” of
these two ions across membrane balanced by Na-K-ATPase pump (see Topic 3-VI-C-4)
3. As depolarization begins in response to triggering event, the change in voltage causes
additional Na+ channels to open
4. Na+ concentration and electrical gradients are INTO cell, so Na+ begins to move into cell
5. This causes more depolarization, which opens more voltage-gated Na+ channels
6. Positive feedback loop established
7. When threshold reached at -55 mV membrane becomes 600 times more permeable to
Na+ than K+ because all voltage-gated Na+ channels opened at -55 mV
8. Na+ rushes in, and inside of cell becomes positive (30 mV); note that this is close to the
Na+ equilibrium potential
9. Na+ voltage-gated channels close at 30 mV
10. When membrane voltage is 30 mV, K+ voltage gated channels open wide and K+ rushes
out of cell down both its concentration and electrical gradient.
11. Positive charges leaving with K+ repolarizes the membrane back to resting potential of
-70 mV
12. Relative to the total number of Na+ ions outside neuron very few Na+ ions move into cell
during depolarization; likewise, relative to the total number of K+ ions inside neuron,
very few K+ ions move out of the cell during repolarization.
Biology 251 Fall 2007
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D. Restoration of Ion Gradients
1. At completion, the resting membrane potential of -70 mV is restored, but concentrations
of Na+ and K+ are not.
2. However, very few K+ and Na+ actually cross the membrane during an action potential
compared to total amount of these ions available (only about 1 out of 100,000 ions
crosses the membrane) so their overall concentration is not changed much.
3. Eventually the original concentrations restored by Na+-K+ ATPase pump. Without this
pump, repeated action potentials would eventually erode concentrations of Na+ and K+

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