jonesmat at ohsu.edu
Wed Apr 29 11:19:22 EST 1998
In article <354525CF.3A3E7B5E at spectra.net> eric Wolf, ewolf at spectra.net
>Hello, looking for information on neural oscillations. I was wondering
>if a neuron is capable of oscillitory firing behaivor without recieving
>a driving stimulation. The source of these oscilations intruiges me.
Yes, this is possible, it is observed in some types of neurons naturally,
and it is predicted by the Hodgkin-Huxley equations (an example of
mathematical equations that describe action potential generation) for
certain values of the equation parameters.
The basis of spontaneous firing is that there are two opposing "forces"
that shape the membrane potential: a depolarizing force (usually mediated
by sodium or calcium channels) and a hyperpolarizing force (usually
mediated by potassium channels). The key is that both of these
potential-altering forces themselves depend on potential (or some
downstream result of changing potential, like calcium entry).
The oscillations work like this: A small depolarization (provided by an
external stimulus, or by an intrinsic depolarizing "leak") moves the
membrane to threshold (about -40 mV). This turns on voltage-gated sodium
or calcium channels which cause a further depolarization which turns on
more channels which causes a further depolarization, etc. The result is
an abrupt depolarization to high positive potentials. This strong
depolarization also turns on potassium channels, but those open later
than the sodium and calcium channels. They have a built-in delay (some of
them are called delayed rectifiers) either because of their voltage
sensitivity or because they depend on calcium entry. These potassium
channels act to bring the potential back into the hyperpolarized range.
This not only terminates the action potential, but also has the very
important effect of "removing inactivation" of the sodium channels
(inactivation means that the channels tend to close and stay closed
during prolonged depolarization). Once their inactivation has been
removed, sodium channels are ready to go again, and the entire action
potential will be repeated if the depolarizing leak is still active.
Whether you get a single action potential or short bursts or continuous
oscillation depends on the relative densities of the different types of
channels, and on their specific voltage dependences and kinetics.
For more info, try "Ionic Channels of Excitable Membranes" by Hille, an
excellent and very popular electrophysiology textbook.
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