Isometric Retraction and Invisible Nerve Cell Processes

Many physiological studies in recent years on stress and hibernation have noted an unusual morphological phenomenon consisting of the rapid disappearance and restoration of the apical dendrites of pyramidal neurons in the hippocampus, prefrontal cortex, and other parts of the brain. The present article seeks to explain this on the basis of morphological analysis of the natural elastic properties of the neuroplasm and the kinetics of partially preserved processed of living isolated neurons. Movement of neuroplasm in processes has been demonstrated with a bidirectional flow. A new physiological phenomenon is described – the isometric retraction of nerve cell processes, in which flows of neuroplasm are in opposite directions, leading to marked thinning of the middle parts and thickening of both ends. It is suggested that extreme thinning of processes can shrink them to submicroscopic dimensions, such that they become invisible under the light microscope. Repeated reversible “disappearance” and “reappearance” of processes has been demonstrated in neuron cultures and C-1300 neuroblastoma cells. Decreases in process diameter to the limits of visibility have been demonstrated using natural stretching of processes as an example. This same effect is seen in areas between reversible varicosities along processes. These areas become extremely thin and then invisible. By becoming thinner, the processes are able to become much longer. This review of the existing literature and our own data leads to the suggestion that the phenomenon of disappearing apical dendrites can be explained in terms of their isometric retraction, leading to the development of “invisible processes.”