Bill,
your statement may be a bit misleading. The 'voltage dependence' of the
NMDA receptor is different from that of the group of voltage-dependent
(or voltage-activated) ion channels. The NMDA receptor is blocked from
outside by magnesium ions at the resting potential. When the membrane
depolarizes, magnesium is released and the channel can open when
activated by glutamate. Thus, it is not a real voltage dependence. The
channel can also be opened by glutamate at the resting membrane
potential when magnesium is removed from the external solution.
Calcium can enter the spines also through the receptor channel of the
AMPA-type of glutamate receptors (Eilers & Konnerth, Nature 1995), and
can be released from intracellular stores in the spines by activating
metabotropic glutamate receptors (Finch & Augustine, Nature 1998;
Takechi et al, Nature 1998). Check reviews for detailes:
Yuste et al, Nature Neuroscience 3, 2000
Berridge, Neuron 21, 1998
Cheers
Chris
>> The basic signal is a massive calcium influx into the spine, through a
> special kind of voltage-dependent membrane channel called the NMDA
> receptor, which is a receptor for the neurotransmitter glutamate. The
> NMDA receptor only permits calcium influx if two things happen
> simultaneously: 1) glutamate is bound to the receptor; 2) the membrane
> of the spine is electrically depolarized. The necks of dendritic
> spines are narrow enough to restrict calcium flow, so the
> concentration reached inside the spine is considerably higher than the
> concentration in the neighboring dendrite.
>> The influx of calcium triggers a cascade of biochemical processes
> whose workings are still only poorly understood, though quite a bit of
> research has been done on it. Ultimately, the changes in spine shape
> may result from changes in actin, which is known to be present in
> spines in substantial quantities.
>> -- Bill
--
Christian Lohr, Ph.D.
ARL Division of Neurobiology
University of Arizona
PO Box 210077
Tucson, AZ 85721-0077
Phone: (520) 621-6671
FAX: (520) 621-8282
clohr at neurobio.arizona.edu
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