Organization of somatic thalamus in monkeys with and without section of dorsal spinal tracts.

In an attempt to understand the modifications which appears at the thalamic level when dorsal cord sections are performed, peripheral fields of thalamic units were studied in normal and dorsal-cord sectional monkeys, totally awake and implanted with glass micropipettes. Six normal Macaca cynomolgus and 7 having received spinal sections, were studied. Ventricular radiography was performed and all the coordinates were related to new stereotaxic coordinates using the posterior commissure as the origin. Cell-bodies and axon units were recognized on the basis of the shape of the spikes. In normal animals, 972 units were studied; 307 were thalamic units with peripheral fields and 177 were derived from cell bodies. Localization of these cells was studied as a function of their peripheral field and response characteristics. The majority of ventralis posterior (VP) cell bodies were only activated by contralateral stimulation, their peripheral field being frequently found on the extremities. Somatotopic organization consisted of concentric layers rather than zones in apposition. Dorsoventral segregation of afferent modality sensitivity (movement, pressure, light touch) was observed. Somatosensory convergence was found in VP inferior (VPi) as well as convergence of different types of afferents on a few VP cells. Units responding to bilateral or ipsilateral stimulations were found only in posterior VP and in surrounding nuclei. A particular somatotopy was shown to exist in n. reticularis. A total of 838 units were studied in animals having had dorsal columns and Morin's bundle served. Only a few cells (13) responded in VPl to contralateral hindlimb stimulation. Their characteristics recall those noted in layer V spinal cord cells. The other cells (55) still driven by a hindlimb were dispersed in the nuclei just adjacent to VPl. The majority of cells found in the VPl were activated from the forelimb. They were observed in their normal VPl localization as well as in areas where hindlimb representation was found in normal animals. This change of afferent input is attributed to a reinnervation of hindlimb cells (probably at the gracilis level) by sprouting from forelimb afferent.