The study of the motion of intracellular organelles which are far smaller than the limit of resolution of light microscopy has become possible only since the advent of video microscopy, that is the coupling of fast image processors to state-of-the-art light microscopes. The techniques required to make such objects visible are summarized and procedures to extract and analyse positional data of moving organelles are explained. Examples of our work on organelle motion along free, native microtubules from squid giant axons and on movements in human nerve fibers are presented.

Such studies have revealed that in animal cells microtubules serve as tracks for organelle movement. Thus, the questions studied at present deal with the analysis of motion along tracks, and especially with the possible existence of regular, high frequency components (steps), or low frequency velocity fluctuations in such movements. The question of whether this movement is stochastic or deterministic is also treated.

The present status of our analysis points to the existence of a molecular motor producing constant motion with only little variations of stochastic rather than deterministic nature. An additional stochastic component is especially pronounced in intact cells but almost absent from the cell-free motile system studied. It is therefore ascribed to cytoskeletal impediment and not to the motor itself.