Residual cerebellothalamic terminal fields following hemispherectomy in the cat

Tritiated amino acids were injected into the nucleus interpositus, both the anterior (INA) and posterior (INP) divisions, and the projections to the ventrolateral nucleus (VL) of the thalamus mapped autoradiographically in 7 intact and 8 hemispherectomized adult cats. Five hemispherectomized cats receiving injections into the antero-medial one-third of INP, exhibited terminal fields in lateral portions of the contralateral VL. These terminals were patchy and appeared qualitatively identical to those seen in intact animals. In 3 hemispherectomized cats sparse terminal fields were also observed in both normal and gliotic regions of the parafascicular, central lateral, anterior pretectal and posterior thalamic nuclei. The presence of these terminal fields was specific to the injections into the INP. The density of the terminal labeling was greater in the hemispherectomized than in the intact cats. The VL areas receiving these terminals were conspicuously gliotic and devoid of large thalamocortical relay neurons. These results indicate that the INP projections to the VL persist despite degeneration of the thalamocortical neurons, suggesting that the site of the residual terminals is not directly upon such neurons. Attempts at reorganization are discussed.