On 8 Oct 2004 06:35:46 -0700, "Jake Gold" <jake.cog at gmail.com> wrote:
>I was simply wondering if there is ever a circumstance in the brain
>where a neuron may enter a state whereopun it continuously fires
>through its axons, even though it is not recieving sufficient input
>from its dendrites to overcome the output threshold. I would suppose
>this may happen if at one time the dendrite input exceeds the output
>threshold at an exceptionally high level.
>>I was wondering because this may be one of the few ways that a brain
>could form new connections based on passed experiences, IE - to bridge
>the gap of time between when input happens and when the neurons finally
>grow and make the new connections...
Neurons can definitely show "spontaneous" activity, firing action
potentials without any apparent input. All that is necessary is to
have the "resting" potential more depolarized than threshold. Neurons
can also produce patterned or bursting type spontaneous activity
without specific input.
There are a variety of circumstances involving ionic concentrations
and metabolic state that can produce this situation both by lowering
the resting potential and by reducing the threshold. Hormones, nerve
transmitters, and the previous history of activity can induce such a
state that can then long outlast these very non-specific "stimuli".
The classic case of a cell (although not a neuron) producing action
potentials without a stimulus is represented by the SA node pacemaker
cells in the heart. You ordinarily expect these to continue firing
for many many decades!
You mentioning of formation of new connections based on past
experience really doesn't require spontaneous activity. There are
many long-lasting changes that can be produced by nerve activity --
activation of cell signaling systems, protein phosphorylation, gene
activation -- that can span the gap between initiation of the change
and a resulting "permanent" change in synaptic structure.