Neurotubules have been the subject of intense research [8]. They usually are arranged haphazardly throughout the perikaryon of neurons but are aligned longitudinally in axons and dendrites. Each neurotubule consists of a dense-walled structure enclosing a clear lumen, in the middle of which may be found an electron-dense dot. Axonal neurotubules display 5-nm filamentous interconnecting side-arms known to be involved in axoplasmic transport in association with the proteins dynein and kinesin (see Chap. 28). The diameter of neurotubules varies between 22 and 24 nm. High-resolution studies indicate that each neurotubule wall consists of 13 filamentous subunits arranged helically around a lumen (see also Chaps. 8 and 28).