The extent of axonal exposure and axo-axonal apposition in the non-myelinated nerve fibres of peripheral nerve trunks and their dependence on buffer molarity.

Nineteen rats were perfused intracardially with a 2% glutaraldehyde solution in cacodylate buffers adjusted in molarity from 0 to 0 . 4 m. Ultrathin sections of the inferior alveolar nerve were photographed in the electron microscopy. From prints of unmyelinated nerve fibres, the following measurements were made: axon circumference, the proportion of axons which were incompletely covered by the Schwann cell and the extent of this 'exposure', the proportion of axons which were apparently in contact with each other and the extent of this contact and the axon-Schwann cell gap. Axonal size and the spacing between the axon and Schwann cell were not related to buffer molarity. Very few (0-3 . 4%) of the axons were in contact and the degree of contact was unrelated to buffer concentration. Of the axons, 10-34% were partially unsheathed, the proportion being linearly related to the molarity of the buffer vehicle. It is concluded that axo-axonal apposition is an insignificant feature in this nerve trunk, but that axonal exposure is a constant finding although its extent is dependent on the conditions of fixation.