EXPERIMENTAL NERVE THERMAL-INJURY

Thermal injury to rat sciatic nerve was produced by local heating (47 and 58 degrees C) and studied physiologically and morphologically. Unm yelinated nerve fibres showed a greater direct vulnerability to hypert hermia; first manifest as a reversible conduction block of C fibre act ion potentials and at higher temperatures by immediate and selective a xonal degeneration. By contrast, lower grade nerve thermal injury resu lted in delayed, selective loss of myelinated fibres Evidence from thi s study suggests that this is secondary to a heat-induced angiopathy, immediately and diffusely manifest in the vasa nervorum and giving ris e to a progressive and ultimately severe reduction in nerve blood flow . The relative sparing of unmyelinated fibres is likely to be a result of their greater resistance to ischaemia. The pathological vulnerabil ity of unmyelinated fibres to thermal injury, coupled with the suscept ibility of large myelinated nerve fibres to secondary ischaemia, large ly resolves previous contradictions in the literature.