Somatotopic organization was demonstrated by means of microelectrode mapping studies of three somatic sensory nuclear regions in the raccoon medulla.Projections of peripheral receptive fields in the cuneate-gracile nuclear complex occur iteratively in rostro-caudal columns. These projections are organized into a detailed 3-dimensional pattern, in contrast to the more diffuse intermingling of such projections in the spinal dorsal roots and columns. 'This indicates that afferent fibers are re-sorted before synaptic termination. In the extensive representation of the volar hand, there are distinct subnuclei, delineated by intervening fiber laminae, each containing the projections from one of the forepaw digits.Units in the external-cuneate nucleus responded only to stimulation of deeperlying tissues in anterior body regions. These projections were also somatotopically organized. Retrograde degeneration studies indicated that some cells, in that region where cuneate-gracile adjoins external-cuneate, may project through both the ipsilatera1 cerebellar peduncles and the contralateral midbrain.( 3 ) The spinal trigeminal nucleus adjoining the cuneate-gracile complex displayed many characteristics similar to those of the latter.Neighboring, although anatomically distinct, regions contained projections from neighboring peripheral receptive fields.( 1) ( )The raccoon's propensity to palpate and and manipulate objects and surfaces with its hands has prompted a series of investigations of the somatic sensory system in these animals. A pronounced differential enlargement of cellular populations which are activated by light mechanical stimulation of each of the hand digits was found at both the cerebral neocortical and thalamic levels of this afferent system (Welker and Seidenstein, '59; Welker and Johnson, ' 6 5 ) . The relatively large development of these digital representations was found to be associated at both cortex and thalamus with certain morphological specializations. Thus, the individuated digital representations were projected: at the cortex into adjacent gyri separated by sulci, and within the thalamus into adjacent subnuclei separated by fibrous laminae.The purpose of the present study was to determine whether a similar degree of differentiation, and of correlation between somatotopic organization and nuclear morphology, also exists in the dorsal column nuclei which contain synapses of the first order somatic sensory afferents from the body. By means of the combined use of J. COMP. NEUR., 132: 1-44. microelectrode mapping and neuroanatomical methods it was found that such a correlation does exist. Our results also emphasize the importance of determining the exact peripheral locus of mechanoreceptors that activate each of the many subnuclei which constitute the three-dimensional cellular mosaic of the cuneategracile nuclear complex. SubjectsEighteen adult raccoons (Procyon Zotor) were used in electrophysiological recording, four in retrograde degeneration, and three in normal histological studies. El...
Organization of opossum somatic sensory cortex has been investigated utilizing closely spaced microelectrode penetrations (0.25-0.5 mm apart) and delicate mechanical stimulation of body surfaces including the facial vibrissae. Results may be summarized as follows: (1) the general organization of somatic sensory cortex, as originally defined by Lende ('63a) has been confirmed; (2) a double representation of the contralateral mystacial vibrissae and rhinarium, implicit in Lende's original data, was revealed in detail, the two representations being orderly, adjacent, mirror-images of each other; (3) units at a given cortical locus responded to deflection of between one and five mystacial vibrissae, about half responding to movement of a single vibrissa only; (4) about 40% of mystacial vibrissa units showed a directional specificity to the extent that they responded to deflections in only one or two cardinal directions; (5) units located in the medial vibrissa area showed a greater directional specificity than did units located in the lateral vibrissa area; (6) the surface area of rhinarial receptive fields was about ten times the area of first-order rhinarial unit receptive fields (B. Pubols et al., '73); (7) representation of the contralateral forelimb, especially the ventral surface of the forepaw, is extensive, orderly, and precise; (8) representation of the contralateral hindlimb, foot, and tail is minimal, and is confined to the midline convexity; (9) the presence of a small region of bilateral representation, lateral to the regions of contralateral representation, was confirmed. It is suggested that the region of contralateral postcranial representation plus the medial rhinarium and mystacial vibrissa areas are the homologue of SmI in placental mammals, and the region of bilateral representation is homologous to SmII of placental mammals, but that the lateral vibrissa and rhinarium areas are a specialization of somatic sensory cortex unique to the Virginia opossum.
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