THE ACTION POTENTIAL

ACTION POTENTIAL

Conveys information over distance in the nervous system Rapid reversal of the membrane potential at rest

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

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

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).

THE ACTION POTENTIAL IN THEORY

If only K+ channel are open then the membrane would reach EK+

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

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

THE ACTION POTENTIAL IN REALITY

The Voltage-Gated Sodium Channel

1) sensitivity to change in membrane potential

2) selectivity for Na

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)

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

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

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

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

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

THE ACTION POTENTIAL CONDUCTION

Saltatory conduction

0.2 - 2 mm

THE ACTION POTENTIAL INITIATION