Sympathetic-sensory axon to axon contacts in the dental pulp

The activity in stimulated intrapulpal sensory nerve fibres can be increased by simultaneous stimulation of the sympathetic system. The anatomical basis for this interaction is unknown. Unmyelinated nerve fibres in the dental pulp are characterized by incomplete isolation and ensheathment. Individual pockets of Schwann cells often contain several axons in close contact within a confined space. If a pocket contained both sensory and adrenergic axons it is possible that activity in one could affect the properties of the other. Four young adult ferrets were, under general anesthesia injected intraperitoneally with the false neurotransmitter 6-hydroxydopamine which accumulates as dark vesicles within adrenergic axons. The animals were, still under general anesthesia, perfused four hours later with a fixative mixture. The canine teeth were removed, decalcified and prepared for electron microscopy. Ultrathin sections of the dental pulp were examined in the electron microscope. Convincingly labelled axons were rare. Labelled sympathetic axons were found in common Schwann cell pockets with unlabelled, presumably sensory, axons. This confirms studies in which degeneration has been observed following either surgical removal of the superior cervical ganglion or chemical sympathectomy. While this "axonal mingling" probably occurs in other sites it may be more extensive in the dental pulp.