Most synaptic excitatory connections are made on dendritic spines. But how the voltage in spines is modulated by its geometry remains unclear. In this talk I shall focus on possible impacts of the synapse geometry on its electrical properties that are surprisingly not well understood on a basic level. I will use as an example the dendritic spine which has a peculiar shape composed of a bulby head connected to a thin neck. I developed models and mathematical tools to deal with electro-diffusion in such non-trivial shapes. The first part of the talk will be about the ionic steady-state distribution and the electrical potential distribution for various geometries (spherical, cusp-like) for a system with a non-zero net charge. Then I will present how based on voltage imaging data and electro-diffusion modeling, it is possible to extract spines electrical properties that are not yet accessible experimentally. Results presented in this talk are from collaboration with D. Holcman (ENS, Fr.), R. Yuste (Columbia, NY), T. Kwon (Columbia, NY) and Z. Schuss (Tel Aviv Univ.)