Electrical or chemical?
wisey at pipeline.com
Sun Nov 17 21:44:34 EST 1996
In article <328F7C91.12A9 at cadvision.com>, pdunn at cadvision.com wrote:
> I've always understood neural transmission to be of a chemical nature.
> Why then is epilepsy explained in terms of electrical discharges in the
> brain. I know that differences in electrical potential exist, but I
> thought it was as a result of chemical phenomena. Can anyone shed
> light? Also, this raises the question; at what speed do nuerons
> transmit? I suppose one could measure how long it takes one of the
> longer nerves to release transmitter substances in response to
> excitation. Am I overlooking something obvious?
1. Conduction and synaptic release of neurotransmitters result from
electrical depolarization of celllar membranes. The electrical potential
difference across membranes stem from ionic concentration differences
across cellular membranes.
2. The term "electrical discharges" for epilepsy is a misnomer. We
measure electrical manifestations of neuronal activity. The electrical
field potentials resulting from synchronous depolarization and
hyperpolarizations of membranes give rise to potential changes that can be
recorded with electrodes. Epilepsy is abnormally synchronized
hyperactivity of neurons in the brain. Don't confuse the activity itself
and the electrical manifestations of that activity that we use to monitor
the presence of epilepsy.
3. Axons conduct at rates varying from 0.1 to 150 m/sec, depending on the
size of the axon and wheter or not it is myelinated.
3. The time required for neurotransmitter release is short, typically less
than 0.1 msec. However, the time required for the neurotransmitter to
produce sufficient depolarization of the post-synaptic membrane can be
longer, on the order of 0.5-3 msec.
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