A thorough understanding of how neurons work is one of the greatest scientific goals in the field of experimental neuroscience. However, four fundamental technical limitations complicate any attempt to study neuronal function with sub-cellular resolution: First, neurons and neuronal processes are small, second, in realistic experimental situations they can be located deep within optically scattering tissue, third, the chemical and electrical signaling that characterizes neuronal behavior happens quickly, and fourth, neurons and neuronal processes have very three dimensional (3D) shapes. Here we develop a tool that overcomes all four listed limitations by combining the technique of multi-photon microscopy with a unique method for 3D laser beam steering. The result is an instrument capable of monitoring physiological signals at multiple locations in the volume of space occupied by a neuron, a task that is unachievable with any other available instrument.