Figure 9.

Morphological properties of unmyelinated axons may place them at elevated risk to traumatic brain injury (TBI). (A) Modeling axons as uniform cylinders of arbitrary length, a decrease in diameter corresponds to an increase in surface-to-volume ratio (upper panel). This increased ratio of axolemma area to axoplasm volume probably constitutes a risk to membrane-targeting pathogenetic mechanisms. Mean diameters for unmyelinated and myelinated axons measured in the present study are plotted on the curve-relating diameter to the surface-to-volume ratio (lower panel). The mean diameter of unmyelinated axons is about 60% smaller than the mean myelinated diameter, but this corresponds to an approximate 160% increase in the surface-to-volume ratio. (B) Diagram of unmyelinated axons and a myelinated axon in relation to non-neuronal elements (blood vessels, astrocytes). Unmyelinated axolemma is more exposed than myelinated axolemma to destructive extracellular influences (shading) arising locally (e.g. ionic imbalances and activated proteases) or from blood-borne factors penetrating through a compromised blood-brain barrier (arrows). These structural differences likely contribute to a greater vulnerability of unmyelinated axons in some injury conditions.