A focal zone of thalamic plasticity.

In this study, sensory maps in the thalamus were investigated by examining their volume and shape. We determined the forelimb representation in adult rats after the removal of hindlimb input by nucleus gracilis lesions. Three-dimensional reconstructions of thalamic sensory maps were obtained from a grid of electrode penetrations. We found that the volume of the shoulder sensory map contracted >50% at an acute time interval (n = 6), followed by a robust volumetric sensory map expansion of 25% at 1 week (n = 8) and 1 month (n = 8) after lesion relative to controls (n = 8). The topology of the volumetric increase was scrutinized by slicing functional maps in the coronal, sagittal, and horizontal planes. The equivalence of such slices from each animal was established by virtue of their distance from either a functional or neuroanatomical landmark. Surprisingly, all of the volumetric increase unequivocally occurred in a circumscribed coronal slice 300 micron thick. This focal zone was located toward the rostral pole of the thalamic tactile relay, the ventroposterolateral nucleus. Analysis in the sagittal plane revealed that, unexpectedly, the shoulder map volume expanded by superimposing its representation on that of the forepaw, via an advancement of the shoulder representation by 0.6 mm medially. We propose a "hot spot" hypothesis in which focal zones of plasticity may not be specific to the thalamus but may have manifestations elsewhere in the nervous system, such as the cerebral cortex or dorsal column nuclei.