Restructuring of the somatotopic map and appearance of abnormal neuronal activity in the gracile nucleus after partial deafferentation

Results imply that “latent” connections of some sort exist between afferents and cells in the gracile nucleus, and that these latent connections are unmasked by partial deafferentation, which is initially poor, but increases greatly with time.

Abstract

Tungsten microelectrodes were used for single and multiunit recordings in the gracile nucleus of cats. The nuclei were “mapped” by making a set of penetrations 100 μm apart in one transverse plane through the nucleus, sampling the receptive fields of units at 100 μm-depth intervals in each track. In the first set of experiments, the gracile nucleus on one side was partially deafferented by transection of all ipsilateral dorsal roots from L4 caudally except S1 or L7. In these animals, units with no peripheral receptive fields and unusual spontaneous activity patterns were found. However the area of the nucleus occupied by such cells was less than expected from the degree of deafferentation. The representation of the remaining intact dorsal roots had increased in the nucleus, and cells with abnormal receptive fields were found in areas previously occupied by the cut roots. In a second set of experiments, similar partial deafferentation operations were carried out on cats which were then allowed to survive at least 8 months. Maps through these chronically deafferented nuclei showed that the number of nonresponding cells in the nucleus had decreased relative to the acute preparations, and that a further increase in the representation of the remaining intact roots had occurred. These results imply that “latent” connections of some sort exist between afferents and cells in the gracile nucleus, and that these latent connections are unmasked by partial deafferentation. The effectiveness of such connections is initially poor, but increases greatly with time.