To determine anatomically the boundaries and internal organization of the kinesthetic and cutaneous mechanosensory regions of the ventrobasal thalamus, alternate section series from electrophysiologically mapped tissues from 14 raccoons were stained for cytochrome oxidase, myelinated fibers, acetylcholinesterase, and Nissl substance. Microelectrode tracks, along with electrolytic lesions placed as tissue markers, reveal that the mechanoreceptor projection zones have higher cytochrome oxidase and lower acetylcholinesterase staining than some neighboring regions. Both these enzymatic stains reveal particularly sharp boundaries separating the mechanoresponsive region, from the lateral posterior nucleus dorsally and from the ventroposterior inferior nucleus ventrally. The kinesthetic projection zone is often separated from other mechanoreceptor projections by bundles as well as laminae of myelinated fibers, similar to those separating cutaneous projections from distinct body parts. These subdivisions are particularly well marked by the cytochrome oxidase stain. The combination, in neighboring sections, of the use of the several stains adds considerably to the visible delineation of these functionally distinct regions, beyond what can be seen in Nissl-stained sections.

To determine the presence and organization of kinesthetic, as compared with other mechanosensory projection zones in the thalamus of raccoons, unit-cluster responses to mechanical stimulation of the postcranial body were mapped electrophysiologically in the thalami of 14 raccoons anesthetized with Dial-urethane. A distinct zone of kinesthetic projections (from receptive fields in muscles, tendons, and joints) was found in the rostral and dorsal aspects of the mechanosensory projection zone. These projections are somatotopically organized: those from axial structures lie dorsalmost and those from successively more distal limb regions are successively more caudoventral. The kinesthetic forelimb representation is large and lies rostrodorsal to a large central core of cutaneous projections from the forepaw digits. A few scattered kinesthetic projections were found at the caudal edge of the sensory thalamic region. The large, spatially and somatotopically distinct kinesthetic projection zone in the thalamus parallels those seen in the cortex and medulla of raccoons. Similar findings in monkeys, and suggestions from data in cats and humans support the hypothesis of a distinct pathway to the cortex for kinesthetic information in all mammals.

To assess the locations and densities of cells in the dorsal medulla giving rise to ipsilateral versus contralateral projections to the mechanosensory regions of cerebellar cortex in the anterior lobe and paramedian lobule, these cortical regions were injected unilaterally with horseradish peroxidase in each of 5 raccoons. To show injection sites and retrogradely labelled cells in the medulla, sections through the medulla and the cerebellum, in at least two different planes for each, were reacted with tetramethylbenzidine; alternate sections were reacted with cobalt-enhanced diaminobenzidine. Labelled were 60–80% of cells in the ipsilateral and 15–25% of cells in the contralateral external cuneate nuclei, as well as 25–50% of cells in the ipsilateral and the contralateral cell groups f and x near the descending vestibular nucleus.Substantial contralateral, as well as ipsilateral cerebellar projections from external cuneate nuclei and cell group x may be related to development of forelimb dexterity in raccoons, since these nuclei mediate forelimb muscular sensibility. The numerical complementarity of ipsilaterally versus contralaterally projecting cells suggests that they represent two separate populations.