NYMC Faculty Publications

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Dendritic spines are key elements underlying synaptic integration and cellular plasticity, but many features of these important structures are not known or are controversial. We examined these properties with newly developed simultaneous sodium and calcium imaging that had single spine resolution in pyramidal neurons in rat hippocampal slices from either sex. Indicators for both ions were loaded through the somatic patch pipette, which also recorded electrical responses. Fluorescence changes were detected with a high speed, low noise CCD camera. Following subthreshold electrical stimulation postsynaptic sodium entry is almost entirely through AMPA receptors with little contribution from entry through NMDA receptors or voltage gated sodium channels. Sodium removal from the spine head is through rapid diffusion out to the dendrite through the spine neck with a half removal time of about 16 ms, which suggests the neck has low resistance. Peak [Na+]i changes during single EPSPs are about 5 mM. Stronger electrical stimulation evoked small plateau potentials that had significant longer lasting localized [Na+]i increases mediated through NMDA receptors.SIGNIFICANCE STATEMENTDendritic spines are small structures that are difficult to investigate, but are important elements in the fundamental processes of synaptic integration and plasticity. Although calcium imaging has been the main tool for examining these structures there are limits to the kinds of information it reveals. We used newly developed, high speed, simultaneous sodium and calcium imaging to examine ion dynamics in spines in hippocampal pyramidal neurons. We found that following single subthreshold synaptic activation most sodium entry was through AMPA receptors and not through NMDA receptors or voltage gated sodium channels and that the spine neck is not a significant resistance barrier. Most spine mechanisms are linear. However, regenerative NMDA conductances can be activated with stronger stimulation.