Physiological activity of newly differentiated olfactory receptor neurons correlated with morphological recovery from olfactory nerve section in the salamander.

1. Extracellular unitary recordings were made from the olfactory epithelium of the salamander, Ambystoma tigrinum, at numerous time points following olfactory nerve section. Unitary response properties were correlated with histological examination of the same tissues. 2. At 10 days following nerve section, unitary activity was rarely recorded in all regions of the epithelium. Histological examination indicated that virtually the entire mature olfactory receptor cell population had undergone retrograde degeneration. Transneuronal degeneration was not observed in the olfactory bulb, although the olfactory nerve and glomerular layers were substantially reduced in size. 3. At subsequent times, unitary impulse activity gradually returned, consisting of both spontaneous activity and odor-evoked discharges. Anatomical recovery of the olfactory epithelium preceded that of the olfactory bulb. A positive correlation was found between neuronal differentiation in the olfactory epithelium and the recovery of receptor cell function. 4. Patterns of spontaneous activity, odor specificities, intensity-response functions, and adaptive properties studied in newly differentiated olfactory receptor neurons were indistinguishable from those observed in control units. This indicated that these properties were intrinsic to the receptor neurons. 5. Spontaneously active and responsive units were encountered prior to olfactory nerve connection with the bulb. It is concluded that receptor neurons pass through two phases of functional maturity: the first independent of bulbar contact and the second dependent on presumed synaptic contact with bulbar neurons.