I ll? ' El‘ “ E m. l \JCD E I $3133 {3F EELHA EURE CEPTE‘E‘E EMU-E - EGRACELE AND EAEEEEAL NEECLEAR REGE’JNS EN SHEEP & m—v' SEE?EAL ERG Thesés for the Degree of M. A AEECE EruAAI STATE UE‘EE‘EERSITY RGGER A. WGUDENBERG 1958 LIBRARY IVA EC ‘Etc '11 S til EC THES‘S University E, BINDl-NG BY me & sons' annx 3mm mc. _ I E”(ARV BINDERS ABSTRACT PROJECTIONS OF MECHANORECEPTIVE FIELDS IN CUNEATE-GRACILE AND SPINAL TRIGEMINAL NUCLEAR REGIONS IN SHEEP by Roger A. Woudenberg Neural unit discharges in response to mechanical stimu- lation oF peripheral body tissues were mapped using tungsten microelectrodes in medullas oF sheep anesthetized with Dial- urethane. It was determined that the First order somatic sensory aFFerents From the sheep's body terminate in the cuneate-gra- cile and spinal trigeminal nuclear regions oF the medulla in a.somatotopic pattern that is similar to that oF the receptors at the periphery but distorted in size and shape. The pattern oF somatotopic organization was largely reiterative at succes- sive rostro-caudal levels when changes in the shape and size oF the nuclei were considered. In the relatively small cuneate-gracile nuclear complex, projections From the rear halF oF the body were conFined to a small region near the midline. Responses From the Front halF oF the body were Found over a larger, more lateral region. In this complex distal body regions were represented dorsally, and proximal body parts, ventrally. The relatively large spi- nal trigeminal nucleus received projections From the Face with the dorsal Face regions represented ventrally in the nucleus, and the ventral Face parts, dorsally. Projections From the neck and ear were located between the cuneate-gracile and spinal trigeminal cellular regions. Roger A. woudenberg The chieF diFFerences between the projection patterns in sheep and those in other animals studied were related to the disproportionally larger volumes oF nuclear mass that receive projections From body areas with greater innervation densities. In contrast to the sheep which has a small representation oF the limbs in the cuneate-gracile nuclear complex, in the alligator a large part oF the volume oF this complex cotains projections From limb apices. Similarly in the cat the Forelimb apices have a large nuclear representation, and in the raccoon the representation oF the volar hand makes up almost halF oF the volume oF the cuneate-gracile nuclear complex. The rat like the sheep has a large representation oF the Face in the Spinal trigeminal nucleus. However in the rat the proportionally great representation is 0F the nose and vibrissae, while in the sheep it is oF lips and surrounding area. In sheep the small representation oF the limbs and the relatively large volume oF tissue receiving projections From the lips in these medulla nuclei is analagous with the corres- ponding pr0portions oF tissue receiving these cutaneous pro- jections at thalamic and cerebral neocortical levels. PROJECTIONS OF MECHANORECEPTIVE FIELDS IN CUNEATE-ORACILE AND SPINAL TRIGEMINAL NUCLEAR REGIONS IN SHEEP By “R \., Roger A. Woudenberg A THESIS Submitted to Michigan State University in partial fulfillment oF the requirements For the degree of MASTER OF ARTS Department of Psychology 1968 ACKNOWLEDGEMENTS The author wishes to express his gratitude to the chair- man of this thesis committee, Dr. J. I. Johnson, whose guid- ance made this research possible. Special thanks are also giuen to Dr. G. I. Hatton and Dr. J. P. McKinney for serving on the thesis committee. The assistance of Dr. R. L. Raisler in the completion of the final draft of this thesis is also gratefully acknowledged. To my wife and parents, who have helped me to reach this stage of my education, I dedicate this thesis. This research was supported by NIH predoctoral fellowship NH 38130 and research grant NB 05982. 11 TABLE OF CONTENTS ACKNowLEDGEMENTS 0.00.00.00.00...0.000.000.0000...OOI. LIST OF TABLES OOOOOOOOOOOOOOOODOOOOO.000...0.0.0....7 LIST OF FIGURES OOOOOOOOOOOOOOOOOOOOOC000.000.0000..O INTRODUCTION OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO METHOD 0....0.0000000COOOOOOOOOOOOOOOOIOOCO000......O RESULTS 0......0.0.0.0000...OOOOOOOOOOOOOOO0.0.000... DISCUSSION 0.00.00.00.00.0000000000000000000000...... REFERENCES OOOOOOOOOOOCIOOOOOOOOOOOOOOOOOOOIOOOOOOOOO 111 page ii iv v 1 4 13 35 43 LIST OF TABLES Table Page 1. ReSponse criteria used to differentiate unit spike activity of nuclear areas from activity 0? fiber traCtsoooooooooo0000000000000...cocoon 9 2. Abbreviations used in Figures 5-16............. 16 iv 6. 10. 11. LIST OF FIGURES Page Sheep SUSpension apparatus to allow access to the receptive fields on the body surface........ 5 summary diagram 0F apparatUS.....o.........o.... 7 Neural unit spike discharge cluster used as response criteria in medUllaooooooo0000000000000 10 Photograph of entire sheep brain showing location of relevant medulla region............. 14 Summary diagram of the pattern of nuclear loci reSponding to mechanical stimulation of periph- eral body receptive fields at two coronal lev- els in the caudal portion of the rostro-caudal BXtent 0F the CU-Groooooo00000000000000.oooooooo 17 Figurines and diagram showing actual receptive fields and the nuclear location of reaponding loci and photomicrographs of cell and fiber stained coronal sections. (Level of Figure 5).. 18 Figurines and diagram showing actual receptive fields and the nuclear location of responding loci and photomicrographs of hematoxylin éweil and Heidenhain) stained coronal sections. Laval 0? Figure 5)oooooooooooooooooo00000oooooo 19 Figurines and diagram showing actual receptive fields and the nuclear location of reSponding loci and photomicrographs of cell and fiber stained coronal sections. (Level of Figure 5).. 20 Summary diagram of the pattern of nuclear loci reSponding to mechanical stimulation of periph- eral body receptive fields at two coronal levels in the central portion of the rostro- caUdal BXtent 0f the CU-Groooooooooooooooooooooo 22 Figurines and diagram showing actual receptive fields and the nuclear location of reSponding loci and photomicrographs of cell and fiber stained coronal sections. (Level of Figure 9).. 23 Figurines and diagram showing actual receptive fields and the nuclear location of responding loci and photomicrographs of cell and fiber stained coronal sections. (Level of Figure 9).. 24 Figure 12. 13. 14. 15. 16. LIST OF FIGURES (continued) Page Summary diagram of the pattern of nuclear loci reSponding to mechanical stimulation of periph- eral body receptive fields at two coronal levels in the rostral portion of the rostro- CaUdal eXtent 0f the CU-crooooooooooooooooococo 27 Figurines and diagram showing actual receptive fields and the nuclear location of reSponding loci and photomicrographs of cell and fiber stained coronal sections. (Level of Figure 12)....00000000000000000OOOOOOOOOOOOOOOOOOOOOOO 2B Figurines and diagram showing receptive fields and responsive nuclear loci and photomicrographs of cell and fiber stained coronal sections. (Level 0? Figure 12)....000000000000ooooooooooo 29 Figurines and diagram shbwing receptive fields and reaponsive nuclear loci and photomicrographs of cell and fiber stained coronal sections. (LBVBl 0? Figure 12)....0000000000000000000.00. 30 Figurines and diagram showing receptive fields and reSponsive nuclear loci and photomicrographs of cell and fiber stained coronal sections. (Rostral to level of Figure 12)................ 31 vi Introduction A number of recent electrophysiological studies have de- scribed the somatotopic organization of cutaneous input to the cuneate-gracile nuclear complex (Cu-Gr) and the spinal trigeminal nucleus (Sp Trg) of various animals including the cat (Darian-Smith, Proctor, & Ryan, 1963: Gordon, Landgren, & Seed, 1961: Gordon & Paine, 1960; Kruger & Michel, 1962; Kruger, Siminoff, & witkovsky, 1961; Wall & Taub, 1962; Winter, 1965), the raccoon (J. I. Johnson, melker, & Pubols, 1968; welker, Johnson, & Pubols, 1964), the rat (McComas, 1963: Nord, 1967), and a reptile, Alligator mississippiensis (Kruger & witkovsky, 1961). In general this research substantiates and expands the earlier physiological studies with cats (F. H. Johnson, 1952; Kuhn, 1949) and the early anatomical studies with monkeys ( Chang & Rush, 1947; Ferraro & Barrera, 1935a; Ferraro & Barrera, 1935b; Walker & Weaver, 1942). These studies have shown that the first order somatic sensOry afferents from the body terminate on the cells of Cu- Gr and Sp Trg in an orderly fashion along the longitudinal extent of these nuclei. The projections are somatotopically organized in a pattern similar to that of the receptors at the periphery although the total representation is distorted lfin size and shape. Generally in a cross section of the medul— la, the more distal body regions are represented dorsally and the proximal regions, ventrally. In the media-lateral dimen- sion the more caudal parts of the body are represented near the midline and the rostral body parts, more laterally. This 1 2 is conceptualized more simply by considering the representation of the animal in the nuclei as an inverted figure with the tail dorsomedial, the extremities dorsal and the face lateral. This organization exists between the nuclei as well as within them so that they may be considered together as a functional unit to which the entire body projects cutaneous input (Kruger et al., 1961). The chief differences between the projection patterns of the different animals are related to the disproportionally larger volumes of nuclear mass that receive projections from body areas with greater innervation densities (Rose & Mount- castle, 1959). Thus half of the volume of Cu-Gr in the alli- gator contains projections from the apices of the limbs (Kruger & Witkovsky, 1961). Similarly in the cat the nuclear represen- tation of the limb apices is larger than the representation of any other body part. However in the cat the representation of the forelimb and sepecially its digits is larger than that of the alligator and larger than the representation of the hindlimb in the cat itself (Kruger et al., 1961). In the raccoon the nuclear representation of the volar hand makes up at least 44 percent of the volume of Cu-Gr and occupies the cuneate portion of the nucleus throughout its dorso-ventral extent, whereas in the alligator and cat it is restricted to the dorsal portion of the nucleus (J. 1. Johnson et al., 1968; Walker et al., 1964). In the raccoon the projections from each digit were contained in a discrete columnar subnucleus separated by fibrbus laminae from the other digit projections. In the 3 rat which has a relatively small Cu-Gr the central represen- tation of the vibrissae and nose fields in Sp Trg was propor- tionally larger than the representation of any other body part (Nord, 1967). In sheep previous research on this afferent system in our laboratory at the cerebral neocortical level (Hatton & Rubel, 1967) and concurrent research at the thalamic level (Cabral & Johnson, 1967) has shown that a relatively large mass of these areas receives cutaneous projections from the upper and lower lips and very little input from the trunk or limbs. The purpose of the present study was to determine if an anal- agous somatotopic organization exists in the second-order cells of the medulla of sheep. Since the sheep does not belong to the same family as any of the other animals studied and because the relative size of its medullary tactile nuclei is different from these animals, it provides a useful comparative neurolog- ical study. Method Subjects Seven adult sheep (Qvig arias) were used for microelec- trode studies and one for histological study of the normal sheep brain. Subject Preparation The subjects were anesthetized by intraperitoneal injection of a dial urethane mixture. The initial dose was 1cc per pound ($2.2 cc/kg) of body weight. Additional injections of approx- imately & the original dosage were given when needed (every 8-10 hours). The body hair was clipped and the animals were su3pended on two large bars to allow access to the receptive fields on the entire body surface. This is shown in Figure 1. The skull was immobilized by means of metal rods attached to it and a head holder device. The skull, atlas, and axis were held to- gether by dental acrylic poured around screws inserted into them. The trachea was cannulated and a mechanical respiratOr was used when necessary. Body temperature was measured by a thermometer inserted into the muscle surrounding the scapula and was kept near 36°C by means of infra-red lamps. In the surgical BXposure of the medulla, the caudal por- tion of the cerebellum was aspirated and the dura and epi-pial tissue covering the dorsal surface of the medulla were removed. The eXposed area was surrounded by an acrylic dam filled with mineral oil. Recording Equipment Fig. 1. Sheep swepension apparatus to allow access to the receptive fields on the body surface. The skull was immobilized by means of metal rods attached to it and a head holder device. During the exceriments the head was tilted further down than shown in the drawing to allow maximum exeosure of the medulla region. 6 Glass insulated tungsten microelectrodes were used (Bald- win, Frenk, & Lettvin, 1965; Hubel, 1957). These electrodes had shaft diameters of 40-70 microns and were insulated to with- in 30-80 microns of the tip. The electrodes were oriented perpendicular to the dorsal surface of the medulla and inserted in that direction. Mapping was carried out on the right side of the medulla except for several punctures on the left side near the midline. Unit reSponses were led through a preamplifier and passed through low (80 Hz) and high (10 kHz) cut off filters. The output was visually diaplayed on an oscilloscope and audibly through an 8-inch Speaker. Evoked activity and verbal descrip- tion were recorded on two channels of a magnetic tape. The neural unit activity could later be reproduced using a Honey- well Visicorder. This part of the procedure is summarized in Figure 2. Recording Procedure The medullary regions were mapped moving the electrode in a systematic fashion while mechanically stimulating the body surface. This stimulation was done using the hands, 1/8 inch diameter wood applicators, tongue depressors, and pieces of fine polyethylene tubing. The microelectrodes were advanced vertically by a micromanipulator for lengths of approximately 200 microns and held at these points while the sheep's body was stimulated. When this mechanical stimulation evoked a neural spike or spike cluster electrical reaponse (see below) each time it was applied, the area that activated that response CRO VIS Fig. 2. Summary diagram of apparatus. Voltages were recor- 'ded with glass insulated tungsten microelectrodes with rBSpect to a reference electrode attached to the scalp of the sheep. Unit reSponses were led through an amplifier (AND), visually displayed on an oscillosc0pe screen (CRO), and audibly through anB-inch Speaker (AUD). The output was recorded on magnetic tape (TAPE) and later reproduced when necessary with a Honey- well Uisicorder (VIS). 8 was carefully delineated and drawn on photographs of the sheep's body. A written record describing these reSponsive body areas was also made. Finally it was also noted whether gentle strok- ing of a skin receptor or sharsh probing of deep tissue recep- tors evoked the neural reaponse. The first punctures in each experiment were in the most rostral position possible to minimize interruptlon of ascen- ding dorsal column fibers. Punctures were epacedeither .966 or .483 mm apart in medio-lateral rows that were 1 mm apart in the rostro-caudal dimension except where the vascular pat- tern made this impossible. Each placement of the electrode was measured using the coordinate system of the electrode man- ipulator and was recorded on a grid and on photographs of the exposed medullary region. This procedure left neat rows of punctures which could be identified histologically and related to the written, taped, and photographic records. ReSponse Criteria The criteria used to differentiate the unit Spike activ- ity of Cu-Cr, Sp Trg, and other nuclear areas from the activity of neighboring fiber tracts were those of J. 1. Johnson et a1. (1968) as shown in Table 1. Fiber responses consisted of one or more isolated Spikes that could be easily distinguished from any background activity and which were associated with a low noise level. These responses were easily lost with slight movements of the microelectrode and usually could not be re- trieved. A nuclear neural unit Spike discharge cluster, typ- ical of those obtained in this experiment, is shown in Figure 3. Table 1 ReSponse criteria used to differentiate unit Spike activity of nuclear areas from activity of fiber tracts (Abbreviated from Table 1 in J. I. Johnson et al., 1968) Fiber tract Spikes well isolated, sharply differentiated from back- ground; low noise level. ReSponse suddenly disappears with electrode movement, and is rarely retrieved. Nuclear region Spikes of variable amplitude, occurring in clusters with smaller spikes merging into the background; background relatively noisy. ReSponses persist with considerable electrode movement, gradually changing in amplitude. When lost, the responses can often be retrieved by reversing the direction of elec- trode movement. 10 100 1 UV. - 1’ 3‘: - *- -.—~ -- W.- - .- A Fig. 3. Neural unit Spike discharge cluster used as response criteria in medulla. This particular reSponse was evoked by stimulation of the ipsilateral face just rostral to the eye. 11 These responses consisted of a cluster of Spikes of variable amplitude associated with relatively noisy background activity with which the smaller spikes merged. These reSponses endured with considerable electrode movement so that the electrode could be manipulated in order to obtain a maximum response. Only those electrical reSponses which met the criteria of the 11uclear neural unit spike discharge cluster were in- cluded in the data except for two fiber reSponses from the solitary tract (see below). Histological procedures At the completion of each experiment, the brain was fixed by intracardial perfusion of the anesthetized animals with. 0.9% NaCl solution followed by a mixture of 10% formalin and 0.9% NaCl. The part of the medulla containing the electrode tracks was cut from the brain and photographed. In order to insure that the medulla was sliced from the brain in the cor- onal plane of the electrodes, it was out along the line of reference electrodes placed in the tissue with the micromanip- ulator before perfusion. The medullas were dehydrated, embedded in celloidin, and sectioned approximately every 25 microns. Alternate sections were stained to show cell bodies with thionin (Nissl method) or myelinated fibers with hematoxylin (Well and Heidenhain stains). These sections were compared with similar sections of a sheep medulla that did not undergo microelectrode recording. All sections were studied by projection through a light mag- nifying device. 12 The final localization of recording sites was determined from the following data: Dorso-ventral dimension: 1. Recorded micromanipulator readings. 2. Microelectrode tracks identified and drawn from brain sections. Data from electrode tracks not histologically identified were not included in the reSults. 3. Identification of microlesions created dur- ing the course of the experiments by sending current through the electrode tip ( 50 HA, 5 sec., Anodal). Rostro-caudal and medic-lateral dimensions: 1. Correlation of identified electrode tracks with the marks made on photographs of the exposed medulla and on coordinate grids. 2. Comparison between and within experiments of electrode tracks on the basis of three- dimensional plexiglass reconstructions of tracks and nuclear regions. I All data regarding the activating receptive fields and activated neural sites were combined and organized to show the somatotopic organization within and between Cu-Cr and Sp Trg. Results The data with regard to the somatotOpic organization with- in Cu-Cr and Sp Trg of projections of the peripheral mechano- receptive fields reveal that these nuclei exhibit a relatively high degree of somatotopic organization within themselves and with reference to each other. In general the sheep is repre- sented in a cross section of these nuclei as an inverted figure with the tail and rump ventromedial, the extremities dorsal and the face lateral. In the main the somatotopic organiza- tion is reiterated at successive rostro-caudal levels of these nuclei. However there are some changes in the organization associated with variations in the position and shape of the nuclei along the rostro-caudal axis. This will be discussed in more detail below. A total of 251 electrode punctures were made. Approxi- mately 40% (102) of these punctures yielded no data. In the other 60%, 229 responsive nuclear loci were identified. 'These reSponsive loci and the peripheral mechanoreceptive fields projecting to them are summarized diagramatically in six cor- onal sections along the rostro-caudal extent of Cu-Cr in Fig- ures 5, 9, and 12. Figures 6-8, 10-11, and 13-16 supplement these with diagrams of actual reSponses obtained and photo- micrographs of cell and fiber stained coronal sections showing the tracks of the electrode when these responses were obtained. Table 2 lists the abbreviations used in these figures. The relevant medulla region and its location within the sheep brain are shown in Figure 4. Figures 5 through 16 show 13 14 Fig. 4. Photograph of entire sheep brain (lateral view) showing the location of the relevant medulla region. The correSponding drawing of the medulla (dorso-lateral view) is the same as that Shown in Figures 5, 9, and 12. 25 caudal portion. The external cuneate nucleus is now directly lateral to Cu-Cr in the area occupied more caudally by Sp Trg» The boundary between Cu-Gr and Sp Trg is less definite than more caudally and this is complicated by the fact that all three nuclei meet each other in the same region. 1 Consistent with what has been said above about the organ- ization mere caudally, Cu-Cr receives projections from the body up to and partially including the neck. The neck and pinna are represented in the region where the three nuclear groups are contiguous (Figure 9). However, projections from the pinna are located more directly between Cu-Cr and Sp Trg as they were more caudally. Figures 9, 10, and 11 show that the area dir- ectly between Cu-Cr and Sp Trg receives projections from the neck, throat, and cheek. Projections from the rear part of the body are again loca- ted near the midline with projections from the front part of the body ventrolateral to them as Shown in Figures 9, 10, and 11. This change from lateral to ventrolateral organization is con- sistent with the change in position of the nuclei. The prox- imal body portions were always represented ventral to the dis- tal body part (Figure 9) reiterating the more caudal organi- zation. The somatotOpic organization of Sp Trg at this central level of the rostro-caudal extent of Cu-Cr also reiterates the organization shown more caudally, but the data at this level provide mde detail. A large portion of Sp Trg receives projec- tions from the upper and lower lips and the immediate 26 surrounding area including the buccal cavity (Figures 9 and 10). The ventral aspects of the face (lower lip, chin) are represented dorsally and the dorsal aSpects (upper lip, snout, and ophthalmic region) are represented ventrally. This is illustrated clearly in the two series of face reSponses shown in Figure 10 where the receptive fields of cells lying from dorsal to ventral in Sp Trg Show the following progression: chin, lower lip, lower incisors, upper palate, upper lip, nose, snout. The buccal cavity reSponses that are shown in Figure 10 were the most laterally located reSponses that were obtained from that peripheral area in Sp Trg. Figure 11 Shows that some exceptions were found in the boundary region between Cu-Cr' and Sp Trg where projections from the upper lip were sometimes dorsal to projections from the lower lip. However it is also possible that these electrical discharges were recorded from the nucleus of the solitary tract rather than from Sp Trg. Rostral Region In the rostral portion of the rostro-caudal extent of Cu-Cr the Sp Trg lies ventrolateral to Cu-Cr, and in the ex- treem rostral portions of Cu-Cr the Sp Trg is almost directly ventral to it (Figures 12-16). In general the more caudal body portions were represented dorsal to the rostral body parts in Cu-Gr at this level. This is shown in Figure 14 where when moving from dorsal to ventral in Cu-Cr, the successive projections are from the rump and back, then only from the back above the middle of the body and finally from the shoulder. The decision that these and similarly located 22 ,Cu-GkiM \ .. O I O. C O C. O O . . .H 3.353;. S; . g ,:,:.:.:.:.:.:.. om”é 5. o o o o o 7 5 C . ' .0. E - N 2;:;I;I;I;I;I;I;I;.: R. éfm g -P- ° . / P °. ' C” i4¢3§¢hfi¢v. f:mw oooumo in g. :;.......°.°.°. MeduIIa (mm. Fig. 9. Summary diagram of the pattern of nuclear loci reSponding to mechanical stimulation of peripheral body recep- tive fields at two coronal levels in the central portion of the rostro-caudal extent of the Cu-Cr. The bottom section is most caudal and both are located by the darker lines surrounding the shaded area on the bottom diagram of a section of medulla. Dots in the unshaded nuclear areas represent actual reSponding loci identified in Similar histological coronal sections. The letters designate areas of the body within which the receptive field was located or which the receptive field included as its main portion. Sheep 67|42 Sections l64. |65 hum. I-—: Fig. 6. Figurines and diagram showing actual receptive fields (shaded black) and the nuclear location of reSponding loci (dots and stars) and photomicrographs of cell and fiber stained cor- onal section: Showing parts of the electrode tracks. This section is at the level of Figure 5: Caudal medulla, and shows the most caudal responses obtained from the Cu—Cr and 5p Trg. The elec- trode tracks that are visible in the photomicrographs are drawn on the diagram as single dashed lines. Slack dots represent the location of the responsive nuclear loci. Jhite stars represent micrnlesions made at reSponsive loci but which are not visible in the photomicrographs. 24 . 3 .. . A . .Q A. . . v o r V... e . o'er—J, _ aziL-W - : "ad—J C‘aa’ 1...; “n‘- " . . . .._ Q U. . . . . . “If: 3.6.» .. ... : L4 ... I . sumac. hum»... twwlmne r \ \aI (.- .. .- 00 p.- :f d“ 1 Add 1 0 AD rfiw ‘rd [7.9 F“! r... 32 reSponses were recorded from Cu-Cr and not from the nearby external cuneate nucleus is based on the assumption that these nuclei in the sheep are similar to those in the raccoon in cer- tain anatomical and physiological ways. More Specifically it is assumed that the external cuneate nucleus can be distin- guished from Cu-Cr by its relatively larger cells and by the fact that harsher tactile stimulation is required to activate the deep muscle receptors that project to it (J. I. Johnson et al., 1968). In the caudal portions of Cu-Cr the rostral body projection was lateral to the caudal body projection, and in the central Cu-Cr the rostral projections were ventrolateral to the caudal body projections. At this rostral portion of Cu-Cr the rostral body projections are almost directly ventral to the caudal body projections. This is consistent with the changes in shape and position of the nuclei. However Figure 13 shows a reSponse that was obtained in the dorsomedial corner of Cu-Cr following stimulation of the forelimb, a rostral body part; and which was marked by a microlesion. This is inconsistent with the rostro-caudal body projection pattern as just described. This was also the only distal body area whose receptive field pro- jected to the rostral portion of Cu-Cr. Therefore any differ- ences in representation of distal and proximal body areas at this level of the rostro-caudal extent of Cu-Cr have not been determined. Projections from the pinna, throat, and cheek lie in the boundary region between the Cu-Gr, Sp Trg, and external cuneate 33 nucleus (Figures 12 and 15). The somatotopic organization within Sp Trg at this level of the Cu-Cr reiterates that shown at more caudal regions of the nucleus. The ventral parts of the face are represented dorsally in Sp Trg and the dorsal parts, ventrally (Figures 14 and 15). Figure 16 shows a series of responses that were obtained in a single electrode puncture through Sp Trg at a level where Cu-Cr is no longer present. Moving from dorsal to ventral in this puncture, the successive projections begin from the chin, then move from the lower lip around to the more dorsal upper lip. All but two of the electrical discharges obtained in Cu-Cr and Sp Trg occured in reSponse to mechanical stimulation of the ipsilateral body. The two exceptions which are Shown in Figure 10 occured in the same electrode puncture into Sp Trg and followed stimulation of ipsilateral and contralateral upper and lower lips. In general the peripheral receptive fields activating cells in Sp Trg were larger than those activating CuéCr cells. A typical receptive field that activated Sp Trg cells would cover an area of a few square centimeters and in some cases, involving the lips, only a few hairs. The receptive fields activating Cu-Gr cells typically included large body areas (eg. entire shoulder or rump). The Smallest Cu-Gr receptive fields were located on the limbs and always included several square inches or more. In terms of the type of stimulation required to evoke a 34 neural reSponse (gentle stroking of a skin receptor vs. harsh probing of deep tissue receptors), light tactile stimulation of hairs and skin produced unit discharges in both Cu-Cr and Sp Trg, although occasionally deep pressure was required to produce electrical discharges in Cu-Cr. In general units in Sp Trg responded to more delicate stimulation than that required to activate reSponses in Cu-Cr. Evoked electrical reSponses were also obtained from the external cuneate nucleus and from the solitary tract and its surrounding nucleus following tactile stimulation of certain body areas. The data with regard to the external cuneate nu- cleus, Shown in Figures 9 and 12, although generally consistent with studies of other animals, are too scanty to be conclusive. Only two reSponses were identified within the solitary tract (Figure 11) and one within its surrounding nucleus (Figure 13). In general every part of the sheep's body was represented within Cu-Cr and Sp Trg. These projections of peripheral body receptive fields exhibited a high degree of somatotopic organ- ization which was largely reiterative along the rostro-caudal axis when changes in the shape and position of the nuclei were considered. Discussion The somatotopic organization that exists in the Cu-Cr and Sp Trg of the sheep is consistent with the gross organi- zation described for other animals. In general the sheep may be considered as being represented in a cross section of these nuclei as an inverted figure with the hindlimb dorsomedial in the gracile portion of the nucleus and forelimb represented dorsally in the cuneate portion of the nucleus. The proximal body parts are located ventrally in Cu-Cr. The representation of the neck, cheek, and pinna are located between Cu-Cr and Sp Trg which receives projections from the face. Thus the projections to the boundary area between the nuclei are from peripherally adjacent body areas demonstrating that the somato- topic organization exists between the nuclei as well as within them. 9.9.132 In an early anatomical study Chang and Ruch (1947) found a separate mass (pars dorsomedialis of nucleus gracilis) in the medulla for the relay of tactile impulses from the tail of the spider monkey. In a more recent physiological study with raccoons (J. I. Johnson et al., 1968) the tail represen- tation was found always to lie in the dorsomedial aSpect of Cu-Cr, and at more caudal levels this representation was lo- cated in a cell column that straddled the midline which in some animals became completely seperated from the rest of Cu-Cr. The authors identified this cell column as Bischoff's (1899) ”median accessory nucleus," although Chang and Ruch claimed 35 36 that they had not found Bischoff's "tail nucleus." In the rec- coon the ventral portion of this cell column contained the rep- resentation of the hindlimb. In the sheep the dorsomedial por- tion of the gracile nucleus contained the representation of the hindlimb. The tail on the other hand sent projections to the cells in the ventromedial portion of the gracile nucleus and was usually part of a larger receptive field including the rump. This is probably related to the fact that relative to the other animals mentioned, the Sheep has a very small tail. Although the representation of the tail and rump is less conspicuous in the medic-lateral and dorso-ventral dimensions than in other animals, it seems to be represented throughout a greater rostro-caudal extent than that reported for other animals. ReSponses from the rostral portions of the longitudinal extent of Cu-Cr have been obtained in other animals only from stimulation of the forelimb and forequarterssince the gracile portion of the Cu-Cr is no longer present at this level (J. I. Johnson et al., 1968). In the rat Nord (1967) found that the tail was observed only in the caudal 60% of the hindquarter representation. It would seem from this that the gracile por- tion of the Cu-Gr complex in sheep is present throughout the extent of the complex, and that the tail representation is us- ually included within a larger receptive field including at least part of the hindquarters so that it is represented as far rostrally as the hindquarters. Sp Trg 37 Within the Sp Trg the ventral face areas were represented dorsally in the nucleus, and the dorsal face areas, ventrally. A great deal of the nuclear volume of Sp Trg, which is large relative to Cu-Cr and relative to Sp Trg of most other animals studied, receives projections from the upper and lower lips and the immediately surrounding area including the buccal cav- ity. This is consistent with the results obtained at the cere- bral neocortical (Hatton & Rubel, 1967) and thalamic levels (Cabral & Johnson, 1967) of the sheep. The relatively small Size of Cu-Cr and the concomitant small representation of trunk and limb parts is also consistent with the results at the other levels of this afferent system. The representation of the buccal cavity of Sheep is loca- ted in approximately the medial two-thirds of Sp Trg between the projections from the upper and lower lips. The fact that this representation extends more laterally than in other animals reported (Kruger & michel, 1962; Nord, 1967) is probably related to the relatively extensive representation of the lips and surrounding areas in sheep. This might also eXplain the fact that the buccal cavity was represented in Sp Trg at least through— out the length of Cu-Cr in sheep, although Nord reports that in the rat the representation does not occur caudal to the obex. KrUger and MiChel (1962) report that in the cat the buccal cavity representation extends to the caudal extreem of Sp Trg which seems meme congruent with the sheep data. Relative Slag gfl Nuclear Formations The general somatotopic organization and relative size 38 of Cu-Cr and Sp Trg of the sheep is most similar to that de- scribed for the rat (Nord, 1967). In both animals Sp Trg is large relative to Cu-Cr and to Sp Trg of other animals studied, and both animals have a proportionally large central represen- tation of a Specific face area. However in the sheep the rel- atively large representation is from the lips and immediate surrounding area and in the rat it is from the vibrissae and nose. The alligator, cat, and raccoon, which have relatively and successively larger Cu-Cr, have proportionally greater cen- tral representations of Specific limb regions (J. I. Johnson et al., 1968: Kruger et al., 1961; Kruger & Witkovsky,1961). These larger central representations correSpond to a greater density of peripheral receptors in the area represented. It is usually inferred that these denselyinnervated peripheral areas with large central representations are the areas most discrim- inatively used by the animal in activities requiring tactile sensation. There has been some controversy with regard to the presence or absence of rostro-caudal variations in general somatotopic organization and peripheral field sizes. The general somato- topic organization that exists in medio-lateral and dorso-ventral dimensions in the sheep was seen to be reiterative along that part of the rostro-caudal axis that was sampled in this study when changes in the position and shape of the nuclei were con- sidered. That is, stimulation of any particular part of the body led to electrical discharges in similarly located cells throughout the length of the nuclei so that most of the body 39 projected to every rostro-caudal level. It has been suggested on the basis of similar results with other animals that the cells receiving projections from particular body regions extend as columns along the rostro-caudal extent of the nuclei (Gordon et al., 1961: J. I. Johnson et al., 1968: Kruger & Michel, 1962; Kruger et al., 1961: Kruger & Witkovsky, 1961). Receptive Fields Although no exact quantitative records were kept in this study with regard to receptive field size, there seemed to be no systematic variance in field size along the rostro-caudal axis of either Cu-Cr or Sp Trg in Sheep. However the contrary has been suggested by some authors with regard to other animals (Darian-Smith et al., 1963: Gordon & Paine, 1960; McComas, 1963: Wall & Taub, 1962). The only factor that seemed to be related to peripheral receptive field size in sheep was the relative proportion of central nuclear volume containing the representation of a given body part. Thus the smallest recep- tive fields were found on the upper and lower lip of sheep which were the peripheral body areas that had the greatest represen- tation in the large Sp Trg. The trunk and limbs on the other hand which were represented centrally with a prOportionally smaller nuclear volume had large receptive fields. Similar results can be found for other animals(J. I. Johnson et al., 1968; Kruger et al., 1961: Nord, 1967). Other studies have Shown that receptive fields of a given size and of a specific body area project uniformly throughout the rostro-caudal extent of the nuclei (J. I. Johnson et al., 40 1968; Kruger & Michel, 1962; Kruger et al., 1961; Nord, 1967; Winter, 1965). Most of these authors conclude that the major determinant of receptive field size is the peripheral location of the fields. Here as in the controversy with regard to rostro-caudal variation in somatotOpic organization, it seems important to take into account the changes in shape and position of the nuclei along the rostro-caudal axis in order to be sure that homologous portions of the somatotopic pattern are being sampled at the different rostro-caudal levels. J. I. Johnson et a1. (1968)_have made a similar point with regard to differ- ences in size, shape, and length of the columns of subnuclei and emphasize the importance of knowing the exact location of the electrode tip within their three dimensional organization. Possible Inhibitory Interactions There is one other factor which seems to be pertinent to both controversies-~3patial inhibition. It has been report- ed that tactile stimulation outside of a receptive field can inhibit the firing of the cells within the receptive field (Gordon et al., 1961; Cordon & Paine, 1960). Since all animals must be SUSpended by some means during the experiments to allow access to most body parts, there is always some form of tactile stimulation in similar areas across eXperiments. This means that some receptive fields might be inhibited in the experiments which would lead to different conclusions about the somatotooic organization and possibly about receptive field size than those that might have been arrived at if this unwanted stimulation were not present. The fact that different animals were 41 SUSpended by different means complicates this even further. All of the electrical discharges obtained in Cu-Gr were in reSponse to stimulation of the ipsilateral body. In Sp Trg only two reSponses included part of the contralateral face within their receptive fields. The two exceptions which occured in the same electrode puncture involved the upper and lower lips, the body areas with the largest central representa- tion. Similarly, Kruger et al. (1961) reported a few contra- lateral peri-oral reSponses in the cat. In general this is consistent with all other studies of these nuclei. Further Studies Needed There are a number of areas within Cu-Cr and Sp Trg where the relationship between nuclear morphology and somatotopic organization has not been completely described by this study. The somatotOpic organization of the lateral portion of Sp Trg has not been thoroughly determined. Thus the somato- topic pattern in this nucleus along the medic-lateral dimension including the full extent of the buccal cavity representation is not clear. There is also some question with regard to the organization of projections from the eye, cheek, and nostril. The somatotopic pattern within Cu-Cr in its rostral por- tion requires more extensive mapping and confirmation of pres- ent results. Accepting the results thus far, there still re- mains the question as to how the proximal vs. distal body parts project to Cu-Cr at this level. The essential problem at this level of the medulla, however, is the means by which to differ- entiate Cu-Cr and external cuneate nucleus responses. 42 Ultimately this will require electrOphysiological mapping ex- periments of the external cuneate nucleus as well as a more extensive study of this portion of Cu-Cr. Similarly microelec— trode mapping in the region of the solitary tract and its nucleus would help clarify the somatotOpic organization and boundary of the medial portion of Sp Trg. References Baldwin, H. A., Frenk, 5., & Lettwin, J. Y. Class-coated tung- sten microelectrodes. Science, 1965, 148, 1462-1464. Bischoff, E. Zur anatomie der Hinter-strangkerne bei saug- etheiren. gghrbucher fur Psychiat. Neurol., 1889, 18, 371-384. - Cabral, R. J. & Johnson, J. I. Somatic sensory projections in thalamus of sheep. Amer. 2001., 7: in press (Abstract) Chang, H. -T. & Ruch, T. C. Organization of the dorsal columns of the spinal cord and their nuclei in the Spider monkey. J. 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Microelectrode studies on the cuneate and gracile nuclei of the cat. Am, J. Physiol., 1952, 171, 737. Johnson, J. I. Jr., Welker, W. I. & Pubols, B. H. Jr. Somato- topic organization of raccoon dorsal column nuclei. 3, Kruger, L. & Michel, F. A single neuron analysis of buccal cavity representation of the Sensory trigeminal complex of the cat. Arch. oral Biol., 1962, 7, 491-503. 43 44 Kruger, L., Liminoff, R., & Witkovsky, P. Single neuron anal- ysis of dorsal column nuclei and Spinal nucleus of trig- eminal in cat. 3. Neurophysiol., 1961, 24, 333-349. Kruger, L. & Witkovsky, P. A functional analysis of neurons in the dorsal column nuclei and Spinal nucleus of the trigeminal in the reptile (Alligator mississippiensis). Comp. Neur., 1961, 117, 97-106. Kuhn, R. A. T0pographical pattern of cutaneous sensibility in the dorsal column nuclei of the cat. Trans. Amer. Neurol. 800., 1949, 74, 227-230. McComas, A. J. 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Functional organiza- tion and corticofugal effects. 3. Neurophysiol., 1965, 28’ 48-70. Y\:\ b8 to; [I] " IllE