An anatomical reinvestigation of the termination of the spinothalamic tract in the monkey

Abstract The projections of the spinothalamic tract in the macaque monkey have been reinvestigated using the Wiitanen modification of the Fink‐Heimer technique. In agreement with previous studies in the monkey (Mehler, Bowsher, Kerr) it was found that the spinothalamic tract ascends outside the medial lemniscus, enters the thalamus just dorsal to this structure, and terminates in the posterior, intralaminar and ventral regions, as well as in the zona incerta. The posteromedial nucleus (POm) receives a dense spinothalamic projection medially and ventromedially; elsewhere in the POm the projection is more scattered. The fibers to the intralaminar region terminate in the nucleus centralis laterials (CL) with a distinct pattern of the distribution. The nucleus centralis medialis (CeM) has a minute projection. There was no evidence for somatotopic organization in the projections to the POm or to the intralaminar region. The distribution of the terminal degeneration in the ventral nuclear region was more complex. Although present in the whole nucleus ventralis posterolateralis (VPL), the degeneration was unevenly distributed and also extended beyond the VPL. So‐called clusters of dense degeneration lay in the outskirts of the forelimb and hindlimb representation areas, namely at its ventral, ventrolateral, dorsolateral, and medial borders. Centrally the degeneration was scattered. Thus, most of the VPL receives only a sparse spinothalamic projection, but a small portion contains dense networks of terminal spinal fibers. A somatotopic pattern was evident, for after low thoracic lesions most of the medial VPL lacked degeneration. Spinothalamic fibers pass beyond the VPL to terminate in a zone of transition (nucleus ventralis intermedius or V.im of Hassler, '59; Mehler, '71) between the rostral pole of the VPL and the nucleus ventralis lateralis (VL). This zone also reportedly receives cerebellar and vestibular afferent fibers. Observations suggesting that the evolution of the spinothalamic tract and the spino‐cervico‐thalamic pathway in carnivores and primates may be linked are discussed. The spinothalamic clusters in the monkey's VPL appear to be homologous to much of the cervicothalamic tract projection to the VPL in the cat.