3. ACTION POTENTIAL
The Generation of an Action Potential is caused by depolarization of the
membrane beyond threshold
“All-or-none” event
Chain reaction
e.g., Puncture foot, stretch membrane of nerve fibers
Opens Na+-permeable channels Na+ influx depolarized
Membrane reaches threshold action potential
4. ACTION POTENTIAL
A way to study the properties of AP is the Generation of Multiple Action Potentials
Artificially - inject current into a neuron using a microelectrode
5. ACTION POTENTIAL
Firing frequency reflects the magnitude of the depolarizing current
The maximum firing frequency is 1000 Hz. This means that after an AP, is not
possible to initiate another one for at least 1 msec (absolute refractory period).
Also the initiation of another AP after few msec requires more current
(relative refractory period).
6. THE ACTION POTENTIAL IN THEORY
If only K+ channel are open then the membrane would reach EK+
7. THE ACTION POTENTIAL IN THEORY
But if the membrane is also permeable to Na+ , the EP will go towards ENa+
Rising phase (depolarization):
Inward sodium current
Falling phase (repolarization):
Outward potassium current
8. THE ACTION POTENTIAL IN REALITY
First described by Hodgkin and Huxley, with the use of a voltage Clamp: “Clamp”
membrane potential at any chosen value
Rising phase transient increase in gNa, influx of Na+ ions
Falling phase increase in gK, efflux of K+ ions
Existence of sodium “gates” in the axonal membrane sensitive to change
in membrane potential and selective for Na
9. THE ACTION POTENTIAL IN REALITY
The Voltage-Gated Sodium Channel
1) sensitivity to change in membrane potential
2) selectivity for Na
10. THE ACTION POTENTIAL IN REALITY
The Voltage-Gated Sodium Channel
Open with little delay
Stay open for about 1msec
Cannot be open again by
depolarization (Absolute
refractory period: Channels
are inactivated)
11. THE ACTION POTENTIAL IN REALITY
The Voltage-Gated Potassium Channels
Open in response to depolarization but later than sodium gates
Potassium conductance serves to rectify or reset membrane potential
(Delayed rectifier)
Structure: Four separate polypeptide subunits join to form a pore
12. THE ACTION POTENTIAL IN REALITY
To summarize- Key Properties of the Action
Potential are
•Threshold
•Rising phase
•Overshoot
•Falling phase
•Undershoot
•Absolute refractory period
•Relative refractory period
13. THE ACTION POTENTIAL CONDUCTION
Down axon to the axon terminal
Orthodromic: Action potential travels in one direction
Antidromic (experimental): Backward propagation
Typical conduction velocity: 10 m/sec and length of action potential: 2 msec
14. THE ACTION POTENTIAL CONDUCTION
Factors Influencing Conduction Velocity:
1) Spread of action potential along membrane follows the path of less
resistance. It depends upon axon structure and direction of positive
charge
2) Path of the positive charge
Inside of the axon (faster)
Across the axonal membrane (slower)
3) Axonal excitability
Axonal diameter (bigger = faster)
Number of voltage-gated channels opens
15. THE ACTION POTENTIAL CONDUCTION
Layers of myelin sheath facilitates current flow (saltatory conduction)
Myelinating cells
1) Schwann cells in the PNS
2) Oligodendroglia in CNS
