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ACTION
POTENTIAL
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)
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
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
RESTING POTENTIAL
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
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
DEPOLARIZATION
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+
REPOLARIZATION
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
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
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

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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
  • 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
  • 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+
  • 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

Editor's Notes

  1. Insulation – almost all flow of ions is prevented