T H E HUMAN MOTOR TRIGEMINAL NUCLEUS A QUANTITATIVE STUDY J. TOMASCH AND A. J. MALPASS' Department of Anatomy, Queen's University, Eingston, On.tario, Canada FIVE FIGURES The morphological study of the human brain stem up to date has been centered chiefly on the description of fiber tracts and nuclei. As their location and extent has become sufficiently clear, further research is required on these structures to evaluate them quantitatively. Not until complete informa t'ion of that nature is collected, can we hope to obtain a perfect concept of the functioning of the Central Nervous System. I n the study presentecj here, we undertook to examine at least one cell group quantitatively. At our disposal was a complete series of transverse sections of the brain stem of a 42 year old male. The subject from whom we procured this specimen was reported free of neurological disorders. The brain stem was fixed in formalin, embedded in paraffin and sectioned at 2011. Using toluidine blue, the sections were stained by Nissl's method. Because it forms a well defined cell group, we selected for our study the motor trigeminal nucleus. The nuclei of the cranial nerves, other than those for the eye muscles, have, to our knowledge, not been considered from a quantitative point of view. Our idea of a quantitative study is to assess as many measureable features of a structure and of its component elements, as possible. Such information, however, is only a first step towards providing normal standard values for future consideration of individual variations and differences. By aid of grants from the National Research Council of Canada. One of US, A. J. M. received a Lederle Student Research Grant enabling him t o carry out this work. 91 92 J. TOMASCH AND A. J. MALPASS In each section the cell group was located and photographed. By using the combination of lenses giving the lowest magnification it was just possible to include the entire nucleus in one 35 mm photomicrograph. From these photographs 5” x 8” enlargements were made. Serial photographs were hereby &tained f o r both left and right nuclei throughout their extent. While viewing a section under the high power lenses of the microscope, these photomicrographs were used to mark off the cells under examination. For the counting of the total number of cells, only those in which the nucleolus appeared were considered. Overall measurements of cell sizes, and also the dimensions of the nuclei and nucleoli within the cells, were taken, using a graduated eyepiece. A magnification of 1350 was used. The caudal pole of the nucleus begins 2 mm above the oral pole of the facial nucleus and from this point it extends rostrally, terminating at the level at which the heavily pigmented cells of the locus coeruleus first appear. Ventrally and mcdially lie the cells of the nucleus pontis centralis caudalis. Latcrally the root fibers of I t e fifth cranial nerve separate the motor from the main sensory trigeminal nucleus. Posteriorly the transversely coursing fibers of the facial nerve intervene between the motor trigeminal and superior vestibular nuclei. Caudally and somewhat medially the retrotrigeminal nucleus is located. Its functional relationship to the motor trigeminal nucleus itself is undecided. Although our study was extended to include this cell group, its data were separately listed. I n sagittal sections the nucleus is ovoid, its long axis directed upwards and forwards. I n horizontal sections, as in our series, its outline is also ovoid. Although the left and right nuclei were similar in shape, differences in size were noted. While the left nucleus extended through 146 sections, the right appeared in only 139. As the thickness of each section was 2011, the overall lengths of the cell groups were therefore 2.92 mm and 2.78 mm respeckively. Sections through the widest parts of each nucleus were compared. It was found that the left nucleus was also of a slightly greater cross sec- H U M A N MOTOR TRIGEMINAL NUCLEUS 93 tional area. No systematic grouping of cells could be observed t within either nucleus. With regard to our attempt to enumerate the total number of cells present within each nucleus, our method was as follows. Each cell was examined under high power magnification. As a cell containing a nucleolus was encountered, its counterpart in the photograph was encircled. By this procedure, we have avoided counting any cell more than once, as most cells and in some cases their nuclei, are of such magnitude as to extend through more than one section, while nucleoli appear in one section only. A total cell count was then made by obtaining the sum of the circles described on all the photos. I n accordance with the differences in size between the left and the right nucleus, as mentioned before, there also exists a difference in the total number of cells. The number of cells in the left nucleus was counted as 5443, and in the right 4960. As these figures are from actual counts and not estimates, this left-right difference is obviously of significance. This difference could be attributed to the sidedness of the individual from whom the brainstem was obtained, but we have no relevant information on this point. I n a study of the trigeminal root by 0. Sjoqvist ('38), he estimated the number of motor fibers in it. His figure is 8100 fibers. This comparatively large difference of 2898 fibers between our eel1 count and his fiber estimate may be due to an actual difference in the two individuals compared, or perhaps to the less dependable result obtained by an estimate. Consideration may next be given to the retrotrigeminal nucIeus. It is located immediately caudal, and somewhat posteromedial, to the motor trigeminal nucleus. A slight overlap of these nuclei causes the cells of the oral pole of the retrotrigeminal nucleus to appear in those sections containing the most caudal cells of the motor trigeminal nucleus. Related dorsally and laterally are the fibers of the seventh nerve as they proceed towards their genu interum. Medially and ventrally it borders against the dispersed cells of the nucleus parvocellularis. The ceIls of the retrotrigeminal nucleus are of the large 94 J. TOMASCH AND A. J. MALPASS multipolar type, similar to those of the main nucleus in size and shape. They differed, however, in that their processes showed a greater affinity for the stain. The shape of the total cell group resembles that of the main nucleus, the long axis of the ovoid, however, is directed dorsoventrally. Its widest horizontal dimension is slightly less than one mm. The right nucleus was found slightly larger than the left, the former measuring 1.04 mm in height, the latter only 0.88 mm. On counting the number of cclls, we found the right t o contain 494 cells, the left 415. No individual cell measurements were made of this nucleus. I n order to further comprehend the quantitative problems connected with the motor trigcminal nucleus, we have attempted t o find means of expressing the characteristics of the cells represented. A Wliipple disc was inserted in the eyepiece of the microscope and carefully calibrated to a micrometer scale. Since the smallest division in t h e disc’s grid was 3.36p, and the average diameter of a cell body mas 33.0,u,a fair degree of accuracy in our measurements was assured. Thus measurements of the length and tvidth of cell, length of nucleus and diameter of nucleolus were taken and recorded. Each cell measured was assigned a number on the photomicrograph for purposes of re-identification. We spent some thought on the choice of a method of expressing the size distribution of the cells. Cellular volume would render the best assessment of a cell’s magnitude. The rather bizarre shapes of the multipolar neurones, however, do not lend themselves to be dealt with by a general volumetric formula. On cross sections these cells were for the most part diamond shaped, therefore, a long and a short diameter could easily be measured. As to at hird dimension, however, the extension of most cells into more than one section made depth measurements impossible. The best alternative we found was to calculate the cross sectional area of each cell, since the area undoubtedly is a more direct function of the volume than either length or width alone. The following method was adopted for the determination of the cross sectional area of a cell. As HUMAN MOTOR TRIGEMINAL NUCLEUS 95 mentioned, most cells appear diamond shaped in section, and half the product of the long and short diameters expresses the area of a cell of such an outline. According to the high values these calculations rendered, we chose a semilogarithmic curve for their representation (figs. 1 and 2). These curves show virtually normal distribution with well emphasized differences between left and right. I n comparing the curves it is interesting to note that although the number of cells on the left is 9.7% greater, the cell sizes are generally smaller on this side, suggesting the idea that the amount of nerve cell material might so be kept equal on the left and right side. After the consideration of the cell size we turned our attention to the cell elements. Measurements on about 20% of the total cell population of both left and right motor trigeminal nuclei were carried out with respect to their nuclear and nucleolar diameters. Figure 3 contains the essence of these measurements. Contrary to our findings of side differences in over all cell sizes, these graphs indicate that the ranges of these two cell components are quite similar on both sides. The nucleolar diameters range from 3 . 3 ~to 6 . 6 ~; the nuclear diameters from 1 0 . 5 ~to 2 4 . 5 ~ . Recent studies of cell activities have indicated that the nucleolar size as well as the nuclear size are indicative of the functional state of the cell. Bucher, 0. and Gattiker, R. ('54) in tissue cultures showed the existence of a linear correlat'ion between these cell elements, using a quotient of nuclear diameter divided by nucleolar diameter. Applying this means of representation to our own material we obtained the graph depicted in figure 4. Although we covered no functional aspects, we feel the inclusion of this graph is justified, f o r it provides yet another criterion for comparison of the properties of this cell group. The next problem we felt 01value to consider was the volume of the nerve cell nuclei. Karyometry, as a biological method of investigation, is proving to be of increasing usefulness in many fields. Ludi, P. ( '51) studied the nuclear volumes of many cell groups in the human brain stern. His findings on the nuclear 3 9 +. r A P2 100.0 - 109.0 119.0 - 0 oulcn ~ 00O $ & - - - - _ ( ? , 4 c ? o p O ~ 4 O W r n 4 O N cn 0 01 0 rn 0 m o rn ~ y 08 u ) 0 108-9 118.9 129.6 R 733.2 - 799.9 s - 616.7 ' 0 566.0 1233-6- 1345.5 96 SSVdTVN 'f 'V (INV H D S V N O L 'f * 100.0- 108.9 119.0- 1296 672.8 - 733.1 0 872-0- 951.9 - 1466.4 15999 1600 - % - P Fig. 3 Size distribution of the cell nuclei and cell iiucleoli of the cells of the left and right motor trigeminal nucleus. The long diameter of each was measured. Measurements were made on 1100 cells of each side. 25- 30 - 35 H U M A N MOTOR TRIQEMINAL NUCLEUS 99 I . d s. 0 P 0 m ~1 cn 2 8 c p - 583 584 - 693 694 - 825 826 - 981 982 -1167 1168 -1388 8. 1389 -1651 1652 -1964 0 1965 - 2335 - 2777 i o % 5 2336 ti' 2778 - 3303 a$ 9 3304 - rt s 0 0 rt. SSVdTVN 'f 'V CINV HDSVNOJ, 'f OOT HUMAN MOTOR TRIGEMINAL NUCLEUS 101 volumes in the motor nucleus of the trigeminal could be compared to our own results. We measured the short and long diameters of the nuclei of 100 nerve cells chosen at random. F o r calculating the volume the formula V = %nab2was used, considering each nucleus as a rotation ellipsoid. These results are contained in our figure 5. A considerable degree of asymmetry characterizes the distribution of the nuclear volumes depicted in this graph. Comparing it with Ludi's tabulated results, the ranges of volumes almost coincide. As his and our materials are from a different population, a noteworthy consistency in these measurements of this nucleus is shown. This consistency is even more surprising, as Ludi found considerable differences in nuclear volume distribution among the various cell groups of the brain stem. During the examination of our slides we paid attention to the possibility of cell congregations within the motor trigeminal nucleus suggestive of any functional significance. However in accordance with the statements by Olszewsky, A., Baxter ('54) no such grouping could be distinguished. They drew attention to the fact that in man there is no proof for any somatotopic representation within this nucleus. SxentagothaiScliimert ('49), however, could in the cat find experimental evidence of such correlations. He found, as in the facial nucleus, a clear dorso ventral representation of muscles, the upper muscles being represented in the ventral portion and the lower ones in the dorsal portion of the nucleus. He raises the interesting point that the posterior belly of the digastric muscle has its cell group within the facial nucleus so closely related to the cell group within the motor trigeminal nucleus for the anterior belly, that the two cell groups form a single column of cells, forming part of both nuclei. SUMMARY A count was carried out of all cells within the human motor trigeminal nucleus. The number of cells in the left nucleus was found at 5443, in the right nucleus at 4960 cells. Cslculating the cross sectional areas of these cells, their size and 102 J. T O M A S C H AND A. J. M A L P A S S distribution was plotted. While the left nucleus was spread over a larger volume and contained 9.7% more cells, the right nucleus was made up of larger cells. Graphs were made to show the distribution of nuclear and nucleolar diameters of the cells. The nuclear volume was also considered and plotted in a graph. ACKNOWLEDGMENT The authors wish to express their thanks to the head of the Department, Professor D. C. Matheson, f o r helpful assistance in the writing of this paper. LITERATURE CITED FWCHER,O., AND R. GATTIRER 1954 Karpiiietriscl~eUntersuchungen :in Gewcl)ekulturen. Acta Anatomica 23: 312-326. LUDI,P. 1951 Kernvolumetrische Untersuchungen a n Ganglienzellen im Rautenhirn und Mittelhirn des Menschen. Zeit. f. Zellfors. 36: 476-502. OLSZEWSKY, J., AND D. BAXTER1954 Cytoarchiterture of the Human Brain Stem. J. B. Lippincott Company, Philadelphia. SJOQVIST, 0. 1938 Studies on pain conduction in the trigeniinal nerve. Acta Psychiat. 1.91: suppl. 2. SZENTAGOTH-4I-SCHIMERT, J. 1949 Functional representation in the motor trigeminal nucleus. J. Comp. Neur., 90: 111-120.