Anatomical changes in cat dorsal horn cells after transection of a single dorsal root

EXPERIMENTAL

NEUROLOGY

64,

453-468 (1979)

Anatomical Changes in Cat Dorsal Horn Cells After Transection of a Single Dorsal Root PAUL B. BROWN, Department

R. BUSCH, AND JEFFREY WHITTINGTON’

GINA

of Physiology Morgantown,

Received

October

and Biophysics, West Virginia West Virginia 26504

I I, 1978; revision

received

January

University,

9, 1979

Adult cats were subjected to unilateral L6, L7, or Sl rhizotomy. After survival times of 3 to 224 days each cat’s spinal cord segments Ll to S2 were serially sectioned and stained with either Golgi or Nissl stains. The Nissl-stained material was used to determine whether or not significant cell death occurred as a result of transneuronal degeneration. The Golgi material was used to determine if changes in dendritic structure occurred. Laminae IV to VI were used for these analyses. There were no statistically significant changes in cell counts when the operated side was compared with the normal side, suggesting that cell death was rare or absent. However, dendritic complexity (mean number of branches of different orders per cell) and total dendritic length decreased with time after L7 rhizotomy in segments L3 to S2. There were no significant changes in segments Ll and L2. Atrophy of dendrites was most rapid and severe in the L7 segment, decreasing as a function of distance from L7. Two hypotheses are advanced, on the basis of these data: (a) The rapidity and degree of atrophy of a dendrite is a function of severity of deafferentation of that dendrite; and (b) rapid dendritic atrophy without cell death, as is seen in L6 to Sl after L7 rhizotomy, is possible because some dendrites are totally or largely deprived of their synaptic complement, but enough of the neuron’s total synaptic input is left intact to sustain the cell and prevent cell death.

INTRODUCTION It has been known since the work of Liu and Chambers (31) that intact dorsal root afferent fibers grow collateral sprouts in the spinal cord when other dorsal roots are cut [see also (20, 34)]. This was observed in other i This research was supported by a grant from the West Virginia University Senate, and U.S. Public Health Service grant NS 12061. Drs. M. and J. Bernstein, M. Wells, and L. and B. F’ubols provided many useful suggestions and criticisms. Dr. C. Pinkstaffprovided access to a phase microscope. 4.53 0014-4886/79/060453-16$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

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central nervous system regions as well (47, 48, 51, 57). It is also possible that other anatomical changes may occur after partial deafferentation, such as sprouting of axons of local intemeurons, of collaterals of ascending spinal tract cells, and of axonal arbors of descending axons (20, 21); atrophy or proliferation of dendritic processes of denervated neurons (2,3); and even death of denervated neurons [e.g. (16, 17, 25, SS)]. Bernstein and Bernstein (2) found that after hemisection of rat spinal cord, neurons near the site of lesion developed dendritic varicosities with progressive loss of fine dendritic terminal processes and concomitant reduction of dendritic field. Such hemisections may cause dendritic atrophy as a consequence of interference with blood supply, retrograde reaction to severing axons of the affected cells, or transneuronal atrophy due to deafferentation. Bernstein and Bernstein favor the last of these alternatives. Many dorsal horn cells receive direct afferent connections [e.g., (7, 27, 46)], so rhizotomy constitutes at least a partial denervation of these cells. Taub et al. (58) reported dorsal horn cell death in cats with L6 to S 1 dorsal roots cut. Only a single dorsal root, L6, L7, or Sl, was cut in this study, but because the major input to some dorsal horn cells is from one dorsal root [see (4-6) for a discussion of afferent convergence upon hindlimb dorsal horn neurons], there is still a possibility of cell death which must be investigated. No studies to date have quantitatively examined the changes of cell numbers or dendritic morphology which may occur with rhizotomy. Such quantitative analyses are useful for determination of the extent of such effects as a function of survival time, as a function of the number and locations of severed dorsal roots, and as a function of segmental relationships between the cells under study and the severed root(s). Only through the use of quantitative methods will it be possible to develop hypotheses linking morph.ological changes to physiological changes. In the study reported here, two anatomic variables were examined: (a) changes in cell numbers in laminae IV to VI after cutting L6, L7, or Sl dorsal root, and (b) changes in dendritic morphology of cells in laminae IV to VI after cutting the L7 dorsal root. METHODS

AND MATERIALS

Surgery in Chronic Preparations. Sterile surgery was carried out on adult cats (2 to 5 kg, exact ages unknown) anesthetized with intramuscular ketamine hydrochloride, 35 mg/kg, supplemented as needed with 25mg doses. In those cats where L7 was cut, the lateral portion of the lamina of the L7 vertebra and some of the L6 lamina were removed to expose the left L7 ganglion. The left L7 dorsal root was then cut immediately proximal to

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the ganglion. Ventral roots were not cut, and ganglia were not severed from their distal processes. This means that ventral root afferents (8) were left intact. The distal processes were left attached to the ganglia to minimize which has been demonstrated in possible peripheral reorganization, amphibians (12). In some animals, similar procedures were used to cut L6 and Sl dorsal roots. After surgery, the wound was closed in layers; after 3 to 224 days the animals were anesthetized for perfusion for the Golgi studies, or single-unit recording and perfusion for the Nissl studies. At perfusion, the identity of the sectioned root was verified by dissection. This was further confirmed, in the Nissl-stained material, by the location of the glial scar which was clearly visible in the dorsal columns in animals with survival times of 20 days or more. Clear-cut demyelination and atrophy of the severed dorsal root was visible with the naked eye in some of the animals with longer survival times. Anatomical Methods-Cell Counts. For cell count studies, three animals were used: One had the L6 dorsal root cut (survival time, 28 days); one had L7 cut (204 days); and one had Sl cut (224 days). As described below, each animal served as its own control, by comparing operated and unoperated sides. Before histological processing, the rhizotomized animals used for cell counts were studied in acute microelectrode recording experiments. Cells in laminae I to VI were sampled in one electrode track per quarter segment (Brown et al., in progress), using stainless-steel microelectrodes (-125~pm-diameter shaft). Recording sites were marked by depositing Fe3+ ions. The animals were perfused at the end of the recording experiments with physiological saline followed by 10% formalin containing 10% potassium ferrocyanide, to visualize recording sites by the method of Green (23). Segments L4 to S2 were embedded in paraffin, and serial cross sections were cut at 25 to 28 pm. All sections were mounted and stained with cresyl violet. The serial sections were used for reconstruction of recording sites [see (4) for details of recording site reconstructions], and then for the cell counts. The quality of these histological preparations was undistinguishable from animals prepared without physiological recording. Fifteen sections per segment were used for cell counts. Segments LS to L7 were examined in the cat with the L6 rhizotomy; segments L6 and L7 in the cat with the L7 rhizotomy; and L7 to S2 in the cat with the Sl rhizotomy. The 15 sections used for cell counts in each segment were evenly spaced throughout the segment; if a section selected using such an algorithm contained damage to the dorsal horn due to recording, the nearest undamaged section was used. In the present work, cells in laminae IV to VI of these cats were counted. In each segment examined, sections were drawn using a camera lucida, showing the outline of the gray matter and locations of all neuronal somata

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which contained nucleoli. As a control, similar drawings were made of cells on the unoperated side of the same animals and on the left side of unoperated animals. Cells were counted in each section, to provide a cell count Cti, for segment i, sectionj. The distributions of the Cj within a given segment i were compared between normal and operated cats and between the operated and unoperated side in operated cats using the one-tailed Mann-Whitney U test, to determine whether or not they decreased significantly in the operated cats. The one-tailed test was selected because an increase in number of cells is not possible for cells in the central nervous system. No correction was made for nucleoli overlapping more than one section, because equal errors should occur in histological material from control and experimental dorsal horns, making statistical comparisons of raw cell counts uncomplicated. There were significant but random interanimal variations between operated and unoperated animals, as well as among the unoperated cats. There were no significant differences between left and right dorsal horns in unoperated cats, so the operated and unoperated sides were compared in the operated animals, using counts from the unoperated (right) side as control and counts from the operated (left) side as experimental data. Because there are only minor crossed projections of roots L6 to Sl in the cat (4,l l), deafferentation effects on the unoperated side should be negligible, making it a legitimate control. Analysis ofDendritic Morphology. The Nissl-stained material was used only for cell counts. A separate group of animals with L7 rhizotomies was used for Golgi studies. They were perfused with physiological saline followed by 10% formalin, and segments Ll to S2 were cut in blocks, each block the full length of a segment. After a minimum of two months in formalin, they were stained with the zinc chromate modification (15) of the Golgi stain. The tissue blocks were immersed 2 days in 6% zinc chromate-formic acid and then transferred to 0.75% AgN03 for 3 days. Then the tissue blocks were cut in half, one half was sectioned in the transverse plane and mounted, and the other half was stored in fresh AgN03 for 1 more day. Then the second half of each block was sectioned. All serial sections were cut at 250 pm in the transverse plane on a freezing microtome, dehydrated, cleared, and mounted. All cells in laminae IV to VI which had unobscured dendritic trees were used in this study. In this initial study, only cross-sectioned material was used, which permits reasonable visualization of the roughly cone-shape dendritic arbors of lamina IV and the transverse disk-shape dendritic arbors of laminae V and VI. This plane is not optimal for laminae I to III. Of course, there are a number of different cell types, with different projections to them, in laminae IV to VI, but the purpose here was to look for changes in the population as a whole. For this reason, no selection procedure was used

DENDRITIC

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except to exclude cells with dendritic trees which were partially obscured by other stained elements. Later studies will attempt to determine which cell types are affected, in which laminae, and perhaps even which dendrites are affected. In each animal, a number of cells was examined using phase microscopy, to determine whether or not dendrites were completely impregnated. Incompletely impregnated dendrites would show a continuation of the dendrite’s external membrane beyond the end of the impregnation. The dendrites of all cells were completely impregnated by this criterion. Figure 1 illustrates the procedure used to quantify dendritic structural properties. Typical cells encountered in normal animals and 34 and 122 days after rhizotomy in the L7 segment are illustrated in Fig. 1A. Figures IB and 1C illustrate how neurons were represented as stick-figure drawings. In Fig. lC, some dendritic branches are numbered to illustrate the classification of branches. First-order dendrites are defined as going from the cell body to the first branch point (or end of dendrite, or truncation at the edge of the section); second-order segments extend from first branch point to second branch point (or end or truncation), etc. This nomenclature avoids the necessity of making subjective decisions about which element at a branch point is a continuation of the parent dendrite and which is the offshoot. The few trichotomous branch points encountered were broken down into pairs of dichotomous branch points for the purpose of keeping the classification scheme simple. The two-tailed Mann-Whitney U test was used to test for statistically significant differences in the distribution of numbers of branches of each order per cell, in operated and unoperated animals. In this application and in other applications described here, differences are said to be statistically significant if P < 0.01. Total dendritic length for each cell was measured using a program written in BASIC on a PDP-12 computer. Dendrites were traced on a Numonics graphic digitizer, measuring and cumulating the lengths of all the line segments in the stick figure representation of each cell. For each segment at each survival time, the distribution of total dendritic lengths was compared between cells in operated (left) dorsal horns and cells in the left dorsal horns of unoperated animals. The two-tailed Mann-Whitney U test was used to determine the significance of differences in distributions of total dendritic lengths for cells in corresponding segments in unoperated animals and in operated animals at various survival times. RESULTS Cell Counts. The Nissl-stained material did not reveal any chromatolytic changes at any survival time, nor was there any sign of shrinkage of the dorsal horn. Cook et al. (10) reported scattered chromatolytic cells in

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DENDRITIC

459

ATROPHY

dorsal horn of cats with L6 to Sl severed, but these were present on both sides in cats with rhizotomies and even in sham-operated cats, and they concluded that there were no obvious differences in partially deafferented dorsal horn. Table 1 presents the cell counts in operated animals, comparing the operated side (“ipsi”) with the unoperated side (“contra”) in the same animal. The rhizotomies were done in the L6, L7, and S 1 segments. A total of 18,161 cells was counted. Differences in mean cell counts per section (laminae IV to VI combined) for operated and unoperated sides were generally less than 10%. None of the eight distributions of cell counts per section from the operated side differed significantly from the unoperated side. Therefore, there was no significant cell loss in laminae IV to VI in such animals. This means that dendritic changes observed in the Golgi studies (such as those of Fig. 1A) were not due to selective cell death. TABLE

1

Comparison of Cell Counts on Operated and Unoperated Sides in Cats with L6, L7, or Sl Dorsal Root Transected” Cut root (survival) Segment counted Cut L6 (28 days) ipsi LS contra LS ipsi L6 contra L6 ipsi L7 contra L7

Mean no. cells per section ?SEM

48.9 49.5 61.8 63.8 77.7 82.7

2 2 2 k f f

2.2 2.9 1.7 2.3 2.2 2.3

58.8 53.7 83.0 92.3

f 2.2 f 2.1 -c 4.3 k 3.8

90.6 89.2 81.8 87.4 89.9 99.8

2 4.6 + 3.0 2 4.0 2 5.2 +- 3.8 f 5.1

l!J

P

112

>o.os

99

>0.05

79

>0.05

77

>0.05

72

0.05

107

>0.05

72

l-o.05

79

>0.05

Cut L7 (204 days) ipsi L6 contra L6 ipsi L7 contra L7 Cut Sl (224 days) ipsi L7 contra L7 ipsi Sl contra Sl ipsi S2 contra S2

a Fifteen sections were counted for each row of the table; hence, N, = NZ = 15.

460

BROWN, BUSCH, AND WHI’ITINGTON TABLE

2

Sample Sizes (Number of Stained and Analyzed Cells) for Go&Stained in Different Segments at Different Survival Times

Normal 3 days 17 days 34 da)is

55 days 122 days Totals

Cells

Ll

L2

L3

L4

L5

L6

L7

Sl

s2

Totals

21 0 0

19 0 0

20 11

24 13

33 14

33

38

0

0

8 0

7 2

7 8

8 8 15

9 12 5

0 16

0 16

0 16

0 19

0 22

0 28

0 23

37

35

55

65

84

92

87

16 12 6 8 13 36 91

8 0 0 13 2 14 37

212 67 48 51 15 190 583

Dendritic Morphology. Table 2 summarizes the segments for which Golgi material was available, for each of the survival times. One animal was used for each survival time except 34 days (two animals) and normal (two animals). Bernstein and Bernstein (2) reported frequent varicosities in neurons undergoing rapid dendritic atrophy in hemisected rat spinal cords. However, no varicosities were seen in our material. In addition, no growth cones or filopodia, or other signs of dendritic regeneration or sprouting were observed, although Bernstein and Bernstein (3) reported those signs of dendritic regeneration in hemisected rat spinal cord. However, dendritic stumps, similar to those described by Machado-Salas et al. (32) in the aging mouse spinal cord, were common (Fig. 1). Figure 2 illustrates the changes in dendritic complexity which occurred in segments Ll through S2 at different survival times after cutting dorsal root L7. Each point on each graph is a plot of the average number of dendritic branches of a given order, the average being computed for all cells in that segment at that survival time. Each graph depicts results for a different segment. Order of dendrite is the independent variable. Each curve connects points from a single survival time (or from normal cats, zero days), as indicated by the label next to the curve. Using the Mann-Whitney U test to compare numbers of branches of a given order in normal and operated animals, it was found that statistically significant differences in numbers of high-order dendrites (of order 6 or greater) were common at all levels, at all survival times. It was therefore concluded that statistically significant interanimal variation in numbers of distal branches occurred normally. Changes in dendritic complexity are described as significant only if statistically significant changes were detected in the five lowest-order dendrites. Even as far away as L3, there were changes in dendritic morphology which were statistically significant by this criterion. There were no statistically significant changes in Ll or L2.

DENDRITIC

Order

ATROPHY

of

461

Dendrite

FIG. 2. Analysis of dendritic complexity. Each graph plots the mean number of dendritic segments per cell (y axis), for different orders of dendritic segment (X axis), for each different segment. Each curve within a graph connects the means for each survival time after L7 rhizotomy, or for normals, as indicated.

Segments which were closer to the operated segment showed a more rapid and more pronounced dendritic atrophy than did segments which were farther from the operated segment. The operated segment (L7) had the most pronounced total changes, and the loss of dendritic detail occurred most rapidly in the operated segment. No segments showed a net restoration of dendritic compiexity with time, indicating that if regeneration occurred it was not significant in terms of this average measure of dendritic complexity. Figure 3 depicts the corresponding changes in mean total dendritic length per cell, in the same cells. Again, statistically significant effects occurred as

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Survival

AND

WHITTINGTON

Time (days)

FIG. 3. Analysis of total dendritic length. Each graph plots total dendritic length per cell as a function of survival time. Zero survival time represents data from a normal cat.

far away as L3, and the changes occurred faster and with larger magnitude in those segments nearest the rhizotomy. There was no significant effect of L7 rhizotomy in segments Ll and L2. In Figs. 2 and 3, net atrophy was complete by 34 to 122 days; the atrophy was complete earliest in segments closest to the rhizotomy.

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463

DISCUSSION Previous investigations (9, 18,28,33,45,59,61) reported a tendency for deafferented cells to resist Golgi impregnation, which would bias our results in favor of staining normal cells, and hence fail to reflect any change. It seems probable, however, that the phenomena observed reflect real changes in the morphology of lamina IV to VI cells, rather than changes in staining selectivity in cats with rhizotomies, because there were no systematic changes in overall number of cells stained per animal as a function of survival time. In some preparations spines or even large portions of dendrites may have failed to stain even though their parent processes or cell bodies were normally stained (9,18,28,59,61). However, unstained portions of dendrites should have shown up as extensions of dendrites in our phase microscopic examinations of Golgi-stained cells. No such unstained portions of dendrites were seen, so this type of artifact did not occur. Largely irreversible dendritic atrophy occurred in cells of laminae IV to VI of segments L3 through S2 (and possibly more caudal segments). Any regeneration of dendrites was undetected in terms of restoration of dendritic length or complexity, or presence of specialized structures such as growth cones or filopodia. However, such regeneration may have occurred, and be undetectable using our relatively gross light microscopic and statistical methods. In a different preparation (hemisected rat spinal cord) Bernstein and Berstein (3) reported signs of dendritic sprouting. The rostrocaudal extent of our effect (L3 to S2) was interesting because the anatomic distribution of primary afferents from the L7 dorsal root is largely limited (excluding some more rostra1 projections to Clarke’s column) to segments LS through S2 (43,55,56), and only dorsal horn cells in segments L5 to S 1 respond to inputs from the L7 dorsal root dermatome (4-6). Merrill and Wall (35) and Wall (60) reported the presence of normally ineffective synapses in cat lumbosacral dorsal horn; perhaps these connections exerted trophic effects. Bernstein and Bernstein (2) observed qualitatively similar degeneration in dendrites of cells in hemisected spinal cords. However, dendritic varicosities were not observed during the degeneration process in our studies. Bernstein and Bernstein illustrated pronounced varicosities. Of course, the two preparations were different. In a hemisection, neurons near the level of the lesion probably have a large fraction of their inputs removed, because all descending projections (proptiospinal, descending afferent collaterals, descending fibers from higher central nervous system levels) are lost for cells below the hemisection, and ascending projections (propriospinal, ascending afferent collaterals) are lost for cells immediately above the hemisection. A single-root rhizotomy thus probably constitutes a

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lower level of denervation for most lamina IV to VI cells. In addition, hemisection may constitute a graver insult to the blood supply of nearby neurons than unilateral L7 rhizotomy. Finally, hemisection undoubtedly severs axons of many cells of origin of propriospinal or other ascending axons: The dendrites of such cells could conceivably undergo retrograde degenerative changes. The rate and extent of degeneration, especially at the operated segment, were impressive but not entirely novel. Gobel and Binck (19) report transsynaptic degenerative changes in cell bodies and dendrites of layer I of nucleus caudalis by 14 days, and by 60 days in layers II and III after extirpation of tooth pulp in adult cats. This transsynaptic effect also constituted a transganglionic (22) effect, because the peripheral portions of sensory fibers were cut, rather than the central branches. One example of very rapid transneuronal degeneration comparable to that reported here is provided by Heimer (24), who reported strongly argyrophilic neurons in olfactory cortex of adult rats as early as 12 h after removal of the ipsilateral olfactory bulb. Electron microscopy revealed degenerative changes as early as 13 h (29). Heimer and Kalil(25) presented convincing arguments that this is a transsynaptic rather than a retrograde effect due to axotomy. Benes et al. (1) reported dramatic changes in the ventral dendrites of cells in the laminar nucleus [laminaris] of 5- to 7-day-old chicks after selective deafferentation of these dendrites by cutting the crossed dorsal cochlear tract. Although their electron microscopic study concentrated on changes evident 96 h postlesion, they reported that dendritic atrophy is well advanced as early as 48 h after lesion. It is not clear from their paper, however, whether cell death occurred. The greater severity and rate of change in segments around L7 (particularly L6, L7, and Sl) suggest that the rate and severity of atrophy is a function of the fraction of the cell’s dendritic surface which is denervated. The L7 dorsal root dermatome (6, 14,30) has its most marked physiological representation in segments L6 through Sl (4), particularly L7, and degeneration studies (43, 55, 56) showed that the L7 dorsal root projects predominantly to these segments. Rapid dendritic atrophy is often associated with cell death [e.g., (19,25)]. Taub et al. (58) reported cell death in the dorsal horn of cats with larger numbers of dorsal roots cut. Failure to detect cell loss in this study may have been a consequence of a less total denervation of these neurons. Rapid dendritic atrophy in the absence of cell death appears, from these data, to be possible in the dorsal horn. This could have been due to the fact that primary afferent terminals are concentrated on different parts of the cells’ synaptic surface than are other inputs, such as descending fibers from higher levels of the neuraxis (13, 26, 36-44, 49, 50, 52-56).

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The fact that, even with severe and rapid atrophy of dendrites, there was no statistically significant cell death after cutting one dorsal root, suggests that all neurons of laminae IV to VI have a remaining complement of connections from other sources (other dorsal roots, or central nervous inputs including local interneuronal connections, or descending tract connections), and/or that a fraction of the lost synaptic connections was replaced by new ones (sprouting). In conclusion, therefore, we advance two hypotheses: (a) The relative rate and degree of atrophy of a dendrite is a function of the relative fraction of that dendrite’s surface which is denervated. This would account for the greater rate and extent of dendritic atrophy in those segments nearest the rhizotomy. (b) The conjunction of rapid dendritic atrophy and lack of cell death is made possible by virtue of some dendritic surfaces being severely denervated, with sufficient innervation remaining intact to sustain the cell. REFERENCES 1. BENES, F. M., T. N. PARKS, AND E. W. RUEIEL. 1977. Rapid dendritic atrophy following deafferentation: an EM morphometric analysis. Brain Res. 122: 1- 13. 2. BERNSTEIN, J. J., AND M. E. BERNSTEIN. 1971. Axonal regeneration and formation of synapses proximal to the site of lesion following hemisection of the rat spinal cord. Exp. Neural. 30: 336-351. 3. BERNSTEIN, M. E., AND J. J. BERNSTEIN. 1977. Dendritic growth cone and filopodia formation as a mechanism of spinal cord regeneration. Exp. Neural. 57: 419-425. 4. BROWN, P. B., AND J. L. FUCHS. 1975. Somatotopic representation ofhindlimb skin incat dorsal horn. J. Neurophysiol. 39: l-9. 5. BROWN, P. B., AND H. R. KOERBER. 1977. Cat hindlimb dermatomes with single-unit recording. Neurosci. Abstr. 3: 477. 6. BROWN, P. B., AND H. R. KOERBER. 1978. Cat hindlimb tactile dermatomes determined with single unit recordings. J. Neurophysiol. 41: 260-267. 7. BROWN, P. B., H. MORAFF, AND D. N. TAPPER. 1973. Functionalorganizationofthe cat’s dorsal horn: spontaneous activity and central cell response to single impulses in single type I fibers. J. Neurophysiol. 36: 827-839. 8. CLIFTON, G. L., W. H. VANCE, M. L. APPLEBAUM, R. E. COGGESHALL, AND W. D. WILLIS. 1974. Responses of unmyelinated afferents in the mammalian ventral root. Brain Res. 82: 163- 167. 9. COLEMAN, P. D., AND H. A. RIESEN. 1968. Environmental effects on cortical dendritic fields. I. Rearing in the dark. J. Anat. 102: 363-374. 10. COOK, W. H., J. H. WALKER, AND M. L. BARR. 1951. A cytological study of transneuronal atrophy in the cat and rabbit. J. Comp. Neural. 94: 267-291. 11. CULBERSON, J. L., D. L. KIMMEL, AND P. B. BROWN. 1976. Laterality of primary afferent distribution in the mammalian spinal cord. Anat. Rec. 184: 385. 12. DIAMOND, J., E. COOPER, C. TURNER, AND L. MACINTRYE. 1976. Trophic regulation of nerve sprouting. Science 193: 371-377. 13. DYACHKOVA, L. N., P. G. KOSTYUK, AND N. CH. POGORELAYA. 1971. An electron microscopic analysis of pyramidal tract terminations in the spinal cord of the cat. Exp. Brain Res. 12: 105- 119. 14. EKHOLM, J. 1967. Postnatal changes in cutaneous reflexes and in the discharge patterns of cutaneous and articular sense organs. Acta Physiol. Stand. 71 (Suppl. 297): l-30.

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15. Fox, C. A., M. UBEDA-PURKISS, H. K. IHRIG, AND D. GIAGIOLI. 1951. Zinc chromate modification of the Golgi technic. Slain Techof. 26: 109- 114. 16. GHETTI, B., D. S. HOROUPIAN, AND H. M. WISNIEWSKI. 1972. Transynaptic response of the lateral geniculate nucleus and the pattern of degeneration of nerve terminals in the rhesus monkey after eye enucleation. Brain Res. 45: 31-48. 17. GHETTI, B., D. S. HOROUPIAN, AND H. M. WISNIEWSKI. 1975. Acute and long term transneural response of dendrites of lateral geniculate neurons following transection of the primary visual afferent pathways. Pages 401-429 in G. W. KREUTZBERG, Ed., Physiology and Pathology of Dendrites. Raven Press, New York. 18. GLOBUS, A., AND A. B. SCHEIBEL. 1966. Loss of dendritic spines as an index of presynaptic terminal patterns. Nature (London) 212: 463-465. 19. GOBEL, S., AND J. M. BINCK. 1977. Degenerative changes in primary trigeminal axons and neurons in nucleus caudalis following tooth pulp extirpations in the cat. Brain Res. 132: 347-354. 20.

21. 22. 23.

24. 25.

26. 27.

28. 29. 30. 31. 32. 33. 34. 35. 36.

GOLDBERGER, M. E. 1973. Restitution of function and collateral sproutingin the cat spinal cord: the deafferented animal. Anat. Rec. 175: 329. GOLDBERGER, M. E., AND M. MURRAY. 1974. Restitution of function and collateral sprouting in the cat spinal cord: the deafferented animal. J. Camp. Neurol. 158: 37-54. GRANT, G., AND J. ARVIDSSON. 1975. Transganglionic degeneration in trigeminal primary sensory neurons. Brain Res. 95: 265-279. GREEN, J. D. 1958. A simple microelectrode for recording from the central nervous system. Nature (London) 182: 962. HEIMER, L. 1968. Synaptic distribution of centripetal and centrifugal nerve fibers in the olfactory system of the rat. An experimental anatomical study. J. Anat. 103: 413-432. HEIMER, L., AND R. KALIL. 1978. Rapid transneuronal degeneration and death of cortical neurons following removal of the olfactory bulb in adult rats. J. Comp. Neural. 178: 559-610. HEIMER, L., AND P. D. WALL. 1968. The dorsal root distribution to the substantia gelatinosa of the rat with a note on the distribution in the cat. Exp. Brain Res. 6: 89-99. HONGO, T., E. JANKOWSKA, AND A. LUNDBERG. 1%8. Post-synaptic excitation and inhibition from primary aRerents in neurons of the spinocervical tract. J. Physiol. (London) 199: 569. JONES,W. H., AND D. B. THOMAS. 1%2. Changes in the dendritic organization of neurons in the cerebral cortex following deafferentation. J. Anal. 96: 375-381. KALIL, R., AND L. HEIMER. 1974. Rapid cell death in the prepiriform cortex after lesion of the olfactory bulb. Anat. Rec. 178: 385-386. KUHN, R. A. 1953. Organization of tactile dermatomes in cat and monkey. J. Neurophysiol. 16: 169- 182. LIU, C. N., AND W. W. CHAMBERS. 1958. Intraspinal sprouting of dorsal root axons. Arch. Neurol. Psychiat. 79: 46-61. MACHADO-SALAS, J., M. E. SCHEIBEL, AND A. B. SCHEIBEL. 1977. Neuronal changes in the aging mouse: spinal cord and lower brain stem. Exp. Neurol. 54: 504-512. MATTHEWS, M. R., AND T. P. S. POWELL. 1972. Some observations on transneuronal celI degeneration in the olfactory bulb of the rabbit. J. Anal. 96: 89-102. MCCOUCH, 0. P., G. M. AUSTIN, C. N. LIU, AND C. Y. LIU. 1958. Sprouting as a cause of spasticity. J. Neurophysiol. 21: 205-216. MERRILL, E. G., AND P. D. WALL. 1972. Factorsformingthe edgeofareceptive field. The presence of relatively ineffective afferents. J. Physiol. (London) 226: 825-846. NYBERG-HANSEN, R. 1964. Origin and termination of fibers from the vestibular nuclei descending in the medial longitudinal fasciculus. An experimental study with silver impregnation methods in the cat. J. Comp. Neurol. 122: 355-367.

DENDRITIC

ATROPHY

467

R. 1%5. Sites and mode of termination of reticulospinal fibers in the cat. An experimental study with silver impregnation methods. J. Camp. Neurol. 124: 71-99. 38. NYBERG-HANSEN, R. 1969. Further studies on the origin of cortico-spinal fibres in the cat. An experimental study with the Nauta method. Brain Res. 16: 39-54. 39. NYBERGHANSEN, R., AND A. BRODAL. 1963. Sites of termination of corticospinal fibers in the cat. An experimental study with silver impregnation methods. J. Comp. Neurol. 120: 369-391. 40. NYBERG-HANSEN, R., AND A. BRODAL. 1964. Sites and mode of termination of rubro-spinal fibres in the cat. An experimental study with silver impregnation methods. 37. NYBERGHANSEN,

J. Anat. 98: 235-253. 41. NYBERG-HANSEN, R., AND

42.

43. 44. 45. 46. 47.

T. A. MASCHITTI. 1964. Sites and modes of termination of fibers of the vestibulo-spinal tract in the cat. An experimental study with silver impregnation methods. J. Comp. Neural. 122: 369-387. PETIUS, J. M. 1965. Afferent peripheral nerve fibers to the spinal cord and dorsal column nuclei in the cat. An analysis and comparison with the distribution of terminal efferent brain fibers to the spinal cord. Anat. Rec. 151: 399-400. PETRAS, J. M. 1%6. Afferent fibers to the spinal cord. The terminal distribution of dorsal root and encephalospinal axons. Med. Sci. J. 22: 668-694. PETRAS, J. M. 1968. The substantia gelatinosa of Rolando. Experientia 24: 1045-1047. POWELL, T. P. S. 1%7. Transneuronrd cell degeneration in the olfactory bulb shown by the Golgi method. Nature (London) 215: 425-426. PRICE, D. D., C. D. HULL, AND N. A. BUCHWALD. 1971. Intracellularresponsesofdorsal horn cells to cutaneous and sural nerve A and C fiber stimuli. Exp. Neural. 33: 291-307. RAISMAN, G. 1%9. Neuronal plasticity in the septal nuclei of the adult rat. Bruin Res. 14:

25-48. 48. RAISMAN,

G., AND P. FIELD. 1973. A quantitative investigation of the development of collateral reinnervation after partial deafferentation of the septal nuclei. Brain Res. 50:

241-264. 49. RALSTON, H. J., cord. J. Comp. 50. RETHELYI, M.

51. 52. 53.

54. 55.

56.

57.

58.

III. 1968. Dorsal root projections to dorsal horn neurons in the cat spinal Neural. 132: 303-330. 1977. Preterminal and terminal axon arborization in the substantia gelatinosa of cat’s spinal cord. J. Camp. Neural. 172: 511-528. ROSE, J. E., L. I. MALIS, L. KRUGER, AND C. P. BAKER. l%O. Effects of heavy, ionizing monoenergetic particles on the cerebral cortex. J. Comp. Neurol. 115: 243-296. SCHEIBEL, M. E., AND A. B. SCHEIBEL. 1966. Terminal axonal patterns incat spinal cord. I. The lateral corticospinal tract. Bruin Res. 2: 333-350. SCHEIBEL, M. E., AND A. B. SCHEIBEL. 1968. Terminal axonal patterns in cat spinal cord. II. The dorsal horn. Brain Res. 9: 32-58. SCHEIBEL, M. E., AND A. B. SCHEIBEL. 1969. Terminal axonal patterns in cat spinal cord. III. Primary afferent collaterals. Bruin Res. 13: 417-443. SPRAGUE, J. M. 1958. The distribution of dorsal root fibers on motor cells in the lumbosacral spinal cord of the cat and the site of excitatory and inhibitory terminals in monosynaptic pathways. Proc. R. Sot. Lond. (Biol.) 149: 534-556. SPRAGUE, J. M., AND H. HA. 1964. The terminal fields of dorsal root fibers in the lumbosacral spinal cord of the cat and the dendritic organization of the motor nuclei. Progr. Brain Res. 11: 120-154. STEWARD, O., X. W. COTMAN, AND G. S. LYNCH. 1974. Growth of a new fiber projection in the brain of adult rats: reinnervation of the dentate gyrus by the contralateral entorhinal cortex following ipsilateral entorhinal lesions. Exp. Brain Res. 20: 45-66. TAUB, A., L. M. KITAKATA, AND T. YAMASHITA. 1976. The effect of chronic dorsal

468

BROWN,

BUSCH,

rhizotomy upon intemeuronal

AND

WHITTINGTON

activity in the feline dorsal horn. Neurosci.

Absfr.

2:

967.

VALVERDE, F. 1967. Apical dendritic spines of the visual cortex and light deprivation in the mouse. Exp. Brain Res. 3: 337-352. 60. WALL, P. D. 1977. The presence of ineffective synapses and the circumstances which unmask them. Phil. Trans. R. Sot. Lond. (Biol.) 278: 361-372. 61. WHITE, L. E., JR., AND L. E. WESTRUM. 1964. Dendritic spine changes in the prepyriform cortex following olfactory bulb lesions-rat, Golgi method. Anat. Rec. 148: 410-411. 59.