1. When an axon is depolarized past its threshold, voltage-gated sodium channels open, allowing sodium ions to enter and further depolarize the membrane in a positive feedback loop. 2. As the membrane depolarizes further, voltage-gated potassium channels open, allowing potassium ions to exit and repolarize the membrane. 3. After an action potential occurs, sodium-potassium pumps actively transport ions to restore the resting membrane potential, before the axon can initiate another action potential.

1. Action Potential

2. • The channel is closed at the resting
membrane potential but opens in response
to a threshold level of depolarization. This
permits the diffusion of ions required for
action potentials. After a brief period of
time, the channel is inactivated by the "ball
and chain" portion of a polypeptide chain.

3. A model
of a
voltage-
gated
ion
channel

4. The ion channels

5. Action potential
• is a property of excitable cells (i.e., nerve,
muscle) that consists of a rapid
depolarization, or upstroke, followed by
repolarization of the membrane potential.
Action potentials have stereotypical size
and shape, are propagating, and are all-
or-none.

6. Threshold
• is the membrane potential at which the
action potential is inevitable. Inward
current depolarizes the membrane. If the
inward current is not sufficient to
depolarize the membrane to threshold, it
does not produce an action potential. If the
inward current depolarizes the membrane
to threshold, it produces an action
potential.

7. Action
Potential

8. I. When the axon membrane has been
depolarized to a threshold level the
Na+ gates open and the membrane
becomes permeable to Na+. This
permits Na+ to enter the axon by
diffusion, which further depolarizes
the membrane (makes the inside less
negative, or more positive).

9. • Since the gates for the Na+ channels
of the axon membrane are voltage
regulated, this additional
depolarization opens more Na+
channels and makes the membrane
even more permeable to Na+. As a
result, more Na+ can enter the cell
and induce a depolarization that
opens even more voltage-regulated
Na+ gates.

10. • The overshoot is the brief portion at
the peak of the action potential
when the membrane potential is
positive.
• II. The explosive increase in Na+
permeability results in a rapid
reversal of the membrane potential
in that region from -70 mV to +30
mV.

11. At that point in time, the channels for
Na+ close (they actually become
inactivated), causing a rapid decrease
in Na+ permeability. Also at this time,
as a result of a time-delayed effect of
the depolarization, voltage-gated K+
channels open and K+ diffuses
rapidly out of the cell.

12. • Since K+ is positively charged, the
diffusion of K+ out of the cell makes the
inside of the cell less positive, or more
negative, and acts to restore the original
resting membrane potential of -70 mV.
This process is called repolarization.
• These changes in Na+ and K+ diffusion
and the resulting changes in the
membrane potential they produce
constitute an event called the action
potential, or nerve impulse.

13. Undershoot
(hyperpolarizing afterpotential)
• The K+ conductance remains high for
some time after closure of the
Na+ channels. During this period, the
membrane potential is driven
very close to the K+ equilibrium potential

14. Depolarization of an axon affects Na+
and K+ diffusion in sequence
• Na+ gates open and Na+ diffuses into the cell.
• After a brief period, K+ gates open and K+
diffuses out of the cell. An inward diffusion of
Na+ causes further depolarization, which in turn
causes further opening of Na+ gates in a
positive feedback (+) fashion.
• The opening of K+ gates and outward diffusion
of K+ makes the inside of the cell more negative,
and thus has a negative feedback effect (-) on
the initial depolarization.

15. Membrane potential changes ion
movements during an action potential

16. III. Once an action potential has been
completed, the Na+/K+ pumps will extrude
the extra Na+ that has entered the axon
and recover the K+ that has diffused out of
the axon. This active transport of ions
occurs very quickly because this events
occurs across only a very small area of
membrane.

17. Absolute and relative refractory
periods

18. Refractory periods
Absolute refractory period
• is the period during which another action
potential cannot be elicited, no matter how large
the stimulus.
• It occurs almost the entire duration of the action
potential.
The inactivation gates of the Na+ channel
are closed when the membrane potential is
depolarized. They remain
closed until repolarization occurs. No action
potential can occur until the inactivation gates
open.

19. Relative refractory period
• begins at the end of the absolute refractory
period and continues until the membrane
potential returns to the resting level.
• An action potential can be elicited during this
period only if a larger than usual inward current
is provided.
The K+ conductance is higher than at rest,
the membrane potential is closer to the K+
equilibrium potential and, therefore, farther from
threshold; more inward current is required to
bring the membrane to threshold.

20. Local anesthetics block the conduction
of action potentials in sensory axons
• They do this by reversibly binding to
specific sites within the voltage-gated Na+
channels, reducing the ability of
membrane depolarization to produce
action potentials.

21. The gap-junction. Connexon