During the past year, we have studied the effects of adult infraorbital nerve transection upon response properties of cells in trigeminal brainstem subnucleus interpolaris (SpVi). The infraorbital nerve was transected in adult rats more than 60 days old. At least 60 days later, the animals were prepared for electrophysiological recording. Class microelectrodes were used to sample the response properties of cells in the region of SpVi. Recording sites were histologically verified. Identical recording procedures were used in SpVi of normal adult rats. A total of 255 cells (198 local circuit, 38 thalamic projecting, 19 cerebellar projecting) were characterized from SpVi of nerve damaged rats and compared with 95 cells (74 local circuit, 11 thalamic projecting, 9 cerebellar projecting, 1 cerebellar + thalamic projecting) from normal animals. The representation of several response characteristics examined were significantly different in the two groups. Four types of cells were observed in nerve cut rats that were nerve seen in SpVi of normal adult rats. One such cell type exhibited discontinuous or split receptive fields. Another was responsive to stimulation of both trigeminal and cervical regions of the body. A third type could only be activated by a sharp poke or deep pressure to the site of the nerve transection and a fourth type could not be activated by any of the somatic stimuli used. Thus, the chronic effects of adult infraorbital nerve transection include changes in: a) numbers of SpVi cells activated from the infraorbital region of the face, b) receptor modalities driving SpVi cells, c) intra- and d) inter-receptor modality convergence, e) inter-divisional convergence, f) cervical convergence and g) receptive field continuity (e.g. split fields). The respective roles of primary and central afferents to SpVi, in the functional alterations noted above, await further clarification.
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