Electrical properties of dendritic spines

Dendritic spines are small protrusions that mediate most of the excitatory synaptic transmission in the brain. Initially, the anatomical structure of spines has suggested that they serve as isolated biochemical and electrical compartments. Indeed, following ample experimental evidence, it is now widely accepted that a significant physiological role of spines is to provide biochemical compartmentalization in signal integration and plasticity in the nervous system. In contrast to the clear biochemical role of spines, their electrical role is uncertain and is currently being debated. This is mainly because spines are small and not accessible to conventional experimental methods of electrophysiology. Here, I focus on reviewing the literature on the electrical properties of spines, including the initial morphological and theoretical modeling studies, indirect experimental approaches based on measurements of diffusional resistance of the spine neck, indirect experimental methods using two-photon uncaging of glutamate on spine synapses, optical imaging of intracellular calcium concentration changes, and voltage imaging with organic and genetically encoded voltage-sensitive probes. The interpretation of evidence from different preparations obtained with different methods has yet to reach a consensus, with some analyses rejecting and others supporting an electrical role of spines in regulating synaptic signaling. Thus, there is a need for a critical comparison of the advantages and limitations of different methodological approaches. The only experi-mental study on electrical signaling monitored optically with adequate sensitivity and spatiotemporal resolution using voltage-sensitive dyes concluded that mushroom spines on basal dendrites of cortical pyramidal neurons in brain slices have no electrical role.