1. An action potential, or nerve impulse, occurs when a neuron reaches its threshold level and is stimulated to send a message along its axon. 2. The action potential involves two steps - depolarization, where sodium ions rush into the neuron, reversing the charge; and repolarization, where potassium ions rush out, restoring the charge. 3. After repolarization, the sodium-potassium pump restores the ion concentrations to prepare the neuron to fire again if stimulated. Myelin insulation allows the impulse to jump from node to node, speeding up conduction.

1. ACTION
POTENTIAL

2. OVERVIEW OF A NERVE IMPULSE
1.Resting potential – neuron is not stimulated at threshold level
2.Action potential – neuron responds to stimulus, sends “message” along axon
a. Depolarization
b. Repolarization
** Potential = difference in charge (measured in volts)

3. RESTING POTENTIAL
 Inactive neuron
 Inside cell membrane – more K+
ions
 Outside cell membrane – more Na+
ions
 Polarized cell membrane
 Inside cell membrane – negative charge
 Outside cell membrane – positive charge
 Cell membrane relatively impermeable to both
ions

4. WHY IS MEMBRANE POLARIZED AT REST?
1. Na+
/K+
pump maintains ion distribution - transports 3 Na+
out and 2 K+
in
2. K+
diffuses out faster – membrane is more permeable
3. Na +
is attracted to cell because of its concentration gradient
4. Membrane is impermeable to large negatively charged ions

5. RESTING POTENTIAL

6. ACTION POTENTIAL = NERVE IMPULSE
 Occurs in excitable membranes – neurons and muscle
fibers
 Critical level must be reached (“threshold”) before
impulse is sent
 Positive feedback mechanism
 All-or-none response
 Lasts a few milliseconds
 2 steps:
 Depolarization
 Repolarization

7. DEPOLARIZATION
 Stimulus causes adjacent Na+
ion channels to open along axon
 Na+
ions rush into cell (high  low concentration)
 Movement of Na+
ions reverses the charge of the membrane
(depolarization):
 Inside cell membrane – positive charge
 Outside cell membrane – negative charge
 If threshold is reached, adjacent Na+
ion channels to open along axon

8. DEPOLARIZATION

9. REPOLARIZATION
 K+
ion channels open
 K+
ions rush out of cell (high  low concentration)
 Restores polarization of cell membrane
 Inside cell membrane – negative charge
 Outside cell membrane – positive charge
 Ion distribution is different than at resting potential – can’t send another
impulse yet
 Inside cell – low K+
, high Na+
 Outside cell – high K+
, low Na+

10. REPOLARIZATION

11. AFTER REPOLARIZATION
 Na+
/K+
pump corrects ion distribution
 Inside cell – high K+
, low Na+
 Outside cell – low K+
, high Na+
 Another impulse can now occur

12. ROLE OF MYELIN
 Myelin insulates the axon
 Action potential must “jump” from node to node
 Saltatory conduction
 Action potential is MUCH faster than along an
unmyelinated axon

13. FACTORS AFFECTING NERVE IMPULSE
 Local anesthetics – block Na+
channels – no Na+
movement = no AP
 Continuous cold and/or pressure – blocks circulation (flow of nutrients
and oxygen), so there is no energy for Na+
/K+
Pump = no AP