Abstract
The left hypoglossal nucleus was examined by electron microscopy in normal rats, and rats from 7–84 days after axotomy of the left hypoglossal nerve. Boutons of different types were counted on micrograph samples. Boutons with clear spherical vesicles decreased in number from 7 to 35 days after axotomy, if somatic, or from 21 to 35 days if in the neuropil. Restitution of boutons followed nerve regeneration. Boutons with flat or mixed vesicles were unaffected by axotomy. Synaptic thickenings were lost from neuropil boutons prior to the decrease in bouton number, and the thickenings were not regained until after the bouton numbers began recovery. During the retractive phase, there was a significant increase in the number of boutons and dendrites with unusual inclusions suggesting surface membrane activity. The results were interpreted as indicating loss (possibly by involution) and restoration of excitatory boutons without alteration of inhibitory terminals.
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