Somatotopic organization of raccoon dorsal column nuclei

Abstract

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...

Opossum Trigeminal Afferents Associated with Vibrissa and Rhinarial Mechanoreceptors

“…3). Of the 220 ganglion units, all but 13 also displayed ini tially positive spike discharges (amplitudes, base to positive peak, be tween 20 and 125 /¿V), suggesting that, in the overwhelming majority of cases, fiber, rather than cell body, activity was being recorded [Johnson et al, 1968]. Amplitudes of the negative discharges ranged between 40 and 120 /< V, base to negative peak ( fig.…”

Organization of Projections from the Gracile, Medial Cuneate and Lateral Cuneate Nuclei in the North American Opossum

“…Reports on the somatotopic organization of the CuL in placental mammals indicate that dorsomedial parts of the nucleus con tain representation for muscle afferents from the forepaw [Johnson et al, 1968;Campbell et al, 1974;Liu, 1956], A projec tion from the CuL to lateral portions of the anterior lobe has also been document ed in the cat using several techniques, in cluding anterograde degeneration [Grant, 1962], electrophysiology [Cooke et al, 1971] and the retrograde transport of HRP [Rinvik and Walberg, 1975;Cheek et al, 1975]. Topographical details of the CuL projections to the cerebellum have been provided in the cat [Cooke et al, 1971;Ekorot and Larson, 1972;Korlin and Lar son, 1970;Rinvik and Walberg, 1975].…”

Functional and Anatomical Features of the Dorsal Column Nuclei in Mammals and Lower Animals

“…The afferent fibers carrying vibration, hair stimulation and foot pad stimulation are somatotopically arranged and terminated mainly by synapsing with the neurons of the middle region -cell nest region‖ [30,31,36,53]. The neurons of the cell nest region have restricted receptive fields, small cortical input and, most of them, project to the medial lemniscus [30,31].…”

“…The DCN can be divided clearly into Cu, which receives afferents from the forelimb (upper limb) [29] and Gr, which receives afferents the hindlimb (lower limb) [30]. A third nucleus known as Bi, is present between the gracile nuclei of both sides in some tailed-mammals such as raccoon [31] and opossum [32]. The Bi in tailed-mammals resembles those described in some tailed-reptiles and receives afferents from the tail [2,6].…”

“…The neurons of the cell nest region have restricted receptive fields, small cortical input and, most of them, project to the medial lemniscus [30,31]. There are also neurons which are responsive to hair stimulation, light pressure and touch located at the rostral region -reticular region‖ of the Gr [30,31,53]. The neurons of this region are loosely organized homogeneous triangular or fusiform cells [36,37,39,54].…”