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The ra- ther large tear in this section obscures Gr organization and makes it difficult to completely determine the organization of Cu. However, the large volume devoted to volar hand and the general trend toward an inverted figure, except in the lateral aspect of the Cu, is still present. The deviation from the inverted figure is seen to occur only in the ear, neck, and arm representation. Since the ear and neck are continuous with the Sp Trg and since the Sp Trg is somewhat dorsal as well as lateral to the Cu at this level, the loca- tion of the neck and ear in the dorsal Cu can be understood. The ear seems to lie along the dorsal part of the Cu on its lateral edge and the neck and arm successively more ventral to this. The dorsal hand is found ventral to these body parts and is also generally ventral to the volar hand through- out the remainder of the nuclei. This deviation may also re- sult from the slight tilt of the electrode penetrations which caused the deeper responses in each puncture to also be more medial. Figure 8b. Photomicrograph of section 47 (hematoxylin stained, Heidenhain method). 11.11. :1 . 8 SJ /» e m: d1. _ HA 33 Figure 9b Figure 9. Response distribution in Plane 4-6. Figure 9a. This plane, about 4 mm caudal to the obex, is again consistent with the general somatotopic organization and illustrates the large volar hand representation still present at this level. A decrease in the density (number) of responses per puncture is also beginning to become appar- ent as it will at the very rostral end of the nuclei. The lower thoracic region is not represented in the caudal Gr as opposed to the absence of caudal body representation in the rostral Gr. The volar hand is the only body area in obvious representation in the caudal Cu with the other upper body ar- ea representations decreased significantly. (Three other planes from this level, not illustrated in detail here, are completely consistent with this observation as is the data from this level in one additional animal in which the elec- trode tracks have not been identified histologically.) Figure 9b. Photomicrograph of section 259 (hematoxylin stained, Well method). Portions of all five punctures are evident. 34 Figure 9a 35 At the extreme rostral end of Cu-Gr, the same general organization exists, but the degree to which the various bo- dy parts are represented has changed. As mentioned previ- ously, the tail and perianal region are not represented above the obex, and indeed, at the very rostral portion of Cu-Gr only the lower thoracic regions are found to be repre- sented in Gr, the representation of lower body regions hav- ing stopped in an orderly manner at more caudal levels. Cu has maintained the same apparent organization with the volar hand representation still exaggerated relative to other body parts. No change is apparent in the size of receptive fields, but a decrease in the density (number) of responses per punc- ture is obvious. These trends, along with the continuing continuity between Cu and Gr and between Cu and Sp Trg are evidenced in all three of the planes of data from this level that are presented in Figures 10 through 12. 36 Figure 10b Figure 10. Response distribution in Plane 1-3. Figure 10a. As mentioned in the text, the number or responses per puncture has decreased at this level -- about 2 mm rostral to the obex. The fiber response in P28 suggests that the foot may still be represented at this level, but, as mentioned previously, caudal body regions have a reduced re- presentation. Abbreviation: E.-Cu .......... external cuneate nucleus Figure 10b. Photomicrograph of section 193 (hematoxylin stained, Heidenhain method). 37 Figure 10a 38 Figure 11b Figure 11. Response distribution in Plane 1-2. Figure 11a. At the rostral tip of Gr, about 2.5 mm rostral to the obex, caudal body representations are again noticably absent from Gr, which is greatly reduced in size. The volar hand is still prominent in Cu. Abbreviation: E.-Cu ..... external cuneate nucleus Figure 11b. Photomicrograph of section 291 (hematoxylin stained, Wail method). 39 Figure 11a Figure 12b Figure 12. Response distribution in Plane 1-1. Figure 12a. Again about 2.5 mm rostral to the obex, at the rostral tip of Gr, the same trend is evident. Volar hand representations are present in Cu but more proximal bo- dy regions are diminished in the degree of representation. In Gr, only representation from the lower thoracic region is observed. Abbreviation: E.-Cu .......... external cuneate nucleus Figure 12b. Photomicrograph of section 157 (hematoxylin stained, Heidenhain method). Portions of P16 through P18 are visible. Figure 12a DISCUSSION Somatotopic Organization The general somatotopic organization of Cu-Gr found in the placental mammals and the one reptile previously studied has been determined to exist in the opossum. That is, neighboring peripheral body regions are represented in adja- cent locations in the nuclear mass. Also, the representa- tion can be visualized as an animal lying on its back with its extremities dorsal and tail dorso-medial. The represen- tations of the various body parts are distorted in such a manner that the peripheral regions having the largest nuclear representation are those which, judging from their usefulness to the animal in mechanoreception, have the greatest behav- ioral significance. All representations, with the exception of some tail responses, were ipsilateral. Many of the tail responses were asymmetrically bilateral, which is to be ex- pected because the relatively long effective recording radi- us of the electrodes spans the small tail representation of the central Gr. In the opossum Cu-Gr the velar hand has the largest representation, but the detail of representation of the vi- brissae and rhinerium in Sp Trg is even more elaborate (Wel- ler,1971). The detail of representation that exists in the 42 43 medulla is not consistent with the extremely large receptive fields reported for the ventrobasal thalamus by Erickson 23 El: (1964). But the detail reported here and by Weller (1971) as well as the relative importance of the representa- tion of facial parts over that of the lower body is consis- tent with the representations found in later microelectrode studies in the ventrobasal thalamus (Pubols and Pubols,1966; Oswaldo-Cruz and Rocha-Miranda,1971) and in an earlier evoked potential study in the somatosensory cortex (Lende, 1963). .Although no attempt was made to determine the somato- topic organization of any but the mechanoreceptor projec- tions, it was noted that the deep pressure and propriocep- tive responses were generally found to be more ventral in Cu-Gr than, and except for location in the appropriate nu- cleus (either Cu or Gr) did not follow the same somatotopic organization as, the mechanoreceptor projections. One of the major differences between the Opossum Cu-Gr and that of most placental mammals studied thus far is the relatively reduced detail in the organization of mechanore- ceptor projections in the opossum. Most of the receptive fields covered a relatively large body area. Gr responses often included the entire ipsilateral hindquarter. Even the receptive fields of the relatively more detailed volar hand representation usually included at least several digits and/ or pads. The use of predominantly CU and MU responses un- doubtedly increased the average receptive field size (of. 44 Johnson pp gl.,1968, p 28); for those SU responses encoun- tered innervated smaller receptive fields: but even these were generally large compared with SU responses in other ani- mals (Welker pp gl.,1968: McComas,1962; Kruger pp al.,1961; Perl gp'gl.,1962) and in the Sp Trg of opossums (Weller, 1971). The representation of the volar hand also did not show any indications of the fine grain resolution present in the raccoon (Johnson 21 §;.,1968). Indeed, although there was a tendency for the digit 1 side of the hand to be represented laterally in Cu and the digit 5 side medially, and a ten- dency for the digits to be represented more dorsally than the palm, all parts of the forepaw were represented to some extent in all parts of Cu devoted to hand representation. Thus, a general somatotopic map can be constructed for ra- ther broad peripheral body areas, but a detailed mapping of body topography to the nuclear space is impossible. Such a generalized map is shown in Figure 13 for a Cu-Gr level slightly caudal to the obex. Variations in this map at dif- ferent nuclear levels are observed to occur only in the ex- tent to which the various body parts are represented (if re- presented at all at some levels) and not in the relative or- ganization of anatomical representation (somatotopy). Woudenberg (1970) described the location of the pinna in the sheep to be confined to a bump of cells located be- tween Cu and Sp Trg. The neck was found to be located medi- al to this and the throat and cheek laterally. The same 45 Figure 13. Schematic map of the somatotopic organization of Cu-Gr mechanoreceptor projections. Top: The figurine at the upper left represents the opossum body and has been divided into several areas, each of which is designated by a letter. The drawing represents a coronal section of Cu-Gr about 1 mm caudal to the obex. The nuclear area has been divided into regions that correspond to the various body parts labeled in the opossum figurine. The diagram is partially a summary of the data collected and partially an estimation of the actual somatotopic organiza- ion. Bottom: This illustration is a very schematized figurine of an opossum body that has been drawn into the nuclear area in a manner that approximates the location of corresponding body part representations. 45 0.5 mm Figure 13 47 situation exists in the opossum except that no bump of cells exists between Cu and Sp Trg and the pinna representation is instead found in the dorsolateral Cu, the doreomedial Sp Trg, and in the connection between the two nuclei. This has been previously reported by Weller (1971) and is consistent with the pattern found in the rat (Nord,1967), monkey (Kerr, Kru- ger, Schwassmann, and Stern,1968), and raccoon (Johnson 23 al.,1968). However, the pinna representation in the opossum extends to other regions of the Sp Trg as well, and thus is more widely distributed than has been reported for other animals (Weller,1971). In agreement with Weller (1971), the somatotopic re- presentation is continuous between Sp Trg and Cu. The addi- tional demonstration that the representation is also contin- uous between Cu and Gr shows that the entire body and head have a continuous representation at the level or the medulla. This conflicts with the assertions of Kerr 23.2; (1968, p. 133) that such continuity does not exist at brain stem lev- els. Longitudinal Variations The continuity of representations along the longitudi- nal axis of Cu-Gr has also been subject to controversy. Several investigators (Gordon and Jukes,1961; Gordon and Seed,1960; McComas,1962,1963) reported a rostral-caudal dif- ferentiation of the gracile nucleus on the basis of recep- tive field size. The smallest receptive field sizes were reported in the middle third of the nucleus and largest 48 fields in the rostral third. Intermediate size receptive fields were reported in the caudal third of the nucleus. But other investigations (Kruger 23,3l.,1961; Nord,1967; Perl pp gl.,1962; Winter,1965; Woudenberg,1968) find no such differentiation and maintain that the receptive field size is solely a function of the position on the body surface which is being represented. This study on the opossum Cu-Gr agrees completely with the latter assertions; for no differ- ences in receptive field size were evident along the rostral- caudal axis of Cu-Gr. Furthermore, those body areas having the smallest receptive fields occupied the largest nuclear volume. Similarly, no rostral-caudal differentiation was noted in the general somatotopic organization of Cu-Gr. But the extent to which the various body parts were represented at different nuclear levels did not remain constant. No tail representation was found rostral to the obex and at the ex- treme rostral end of the Gr, only the lower thoracic regions (the most anterior regions represented in the Gr) are found to be represented. Likewise, the representations of the pinna, neck, and shoulder (the most anterior regions repre- sented in the Cu) are either absent or greatly reduced at the most caudal levels of the Cu. This is consistent with findings in other animals (cf. Norton,1970, p 24). W “Studies Since Cu-Gr was mapped using mainly CU responses, a more detailed study, employing mainly SU responses would be 49 beneficial for determining the limit of resolution of periph-- eral representation. It may also help clarify the organiza- tion of the volar forepaw representation in the Cu. Parti- cular attention should also be given to deep pressure and proprioceptive responses so that the projections of these modalities of sensory receptors can be compared to the cu- taneous mechanoreceptor somatotopy. A 30 study should also be capable of clarifying the exact location of tail repre- sentations and of investigating more thoroughly the possi- bility of rostral caudal nuclear differences in receptive field sizes. Also, as suggested by Weller (1971), additional exper- iments should be performed in which electrode penetrations deviate from the verticle by as much as 45°. These experi- ments will give a much better insight into the medial-later- al organization of the nuclei than is possible from the ver- tical punctures of the present study, and may also help to determine the role of the central portion of the ventral Cu. BIBLIOGRAPHY BIBLIOGRAPHY Baldwin, H.A., S. Frenk, and J.I. Lettvin. 1965. Glass coated tungsten microelectrodes. Science 148: 1462- 1463. Chang, H.T. and T.C. Ruch. 1947. Organization of the dorsal columns of the spinal cord and their nuclei in the spider monkey. g, Anat. 81: 140-149. Erickson, R.P., J.A. Jane, R. Weite, and I.T. Diamond. 1964. Single neuron investigation of sensory thalamus of the Opossum. g. Neurophysiol. 21: 1026-1047. Gordon, G. and M.G.M. Jukes. 1962. 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Neurgphysiol. 24: 333- 349. Kuhn, R.A. 1949. Tepographical pattern of cutaneous sensi- bility in the dorsal column nuclei of the cat. Trans. Amer. NOWOIO A330 E: 22l-230. 50 51 Lende, R.A. 1963a. Sensory representation in the cerebral cortex of the opossum (D d h ziggxnigng). g, 92.192. Neurpl. El: 395- 3. 1963b. Motor representation in the cerebral cor- tex of the opossum (Ma 21.351.21.292)- .J.- 9.222- aami. 1.2.: 405-415- McComas, A.J. 1962. Longitudinal organization of the gra- cile nucleus. g. Physiol. 161: 21P-22P. 1963. Responses of the rat dorsal column system to mechanical stimulation of the hind paw. g, Ppysiol. 1964. Innervation of cells in rat gracile nucle- us. is Pm810 0 Hi: 46?. Nord, S.G. 1967. Somatotopic organization in the spinal tri- geminal nucleus, the dorsal column nuclei and related structures in the rat. ,1. Comp. Neurol. 12: 343-355. Norton, Allen C. (ed.). Updated through 12 Dec 1970. 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General References Keefe, James F. 1967. The World prthe Oppssum. J.B. Lippon- cott Company, New York. Oswaldo-Cruz, E. and C.E. Rocha-Miranda. 1968. The Brain pfl the Opossum (Didelphis marsupialis): £,Cytoarchi?ec- tonic At as in Stereotaxic Coordinates. Instituto de BIETIS ca, UiIversIHaEe Federa o 0 de Janeiro. Voris, H.C. and N.L. Hoerr. 1932. The hindbrain of the opos- sum Didelphis virginiana. J, Comp. Neurol. 54: 277- 355. "1117111111111 ([1111! 1113111 111111111 11'”