Effects of sciatic nerve crush on the L7 spinal roots and dorsal root ganglia in kittens

EXPERIMENTAL

NEUROLOGY

79, 176-187 (1983)

Effects of Sciatic Nerve Crush on the L7 Spinal Roots and Dorsal Root Ganglia in Kittens M. RISLING, H. ALDSKOGIUS, AND C. HILDEBRAND’ Department of Anatomy, Karolinska Institutet, S-104 01 Stockholm, Sweden Received July 13, 1982 The number and size distribution of axons and neurons were examined in the L7 spinal roots and ganglia of kittens 14 to 220 days after early postnatal sciatic nerve crush. The results show that motoraxons in the ventral root as well as axons and perikarya of sensory neurons in the dorsal root remained growth-retarded throughout the examined period. This was most evident in the dorsal root. Both ventral and dorsal roots showed some loss of myelinated axons, but this was only half that previously observed after sciatic nerve resection. Whereas in the dorsal roots and dorsal root ganglia the loss seemed to be nonselective with respect to size, axons in the gamma range were primarily affected in the ventral roots. In the dorsal roots the proportion of unmyelinated axons was comparable with controls but in the ventral roots it was somewhat elevated. In most cases the loss of dorsal root ganglion neurons was relatively greater than the decrease of dorsal root axons.

INTRODUCTION In a previous study a substantial loss of myelinated axons was found in the spinal roots L7 of kittens after early postnatal sciatic nerve resection (25). The loss in the dorsal roots exceeded that in the ventral roots. In addition, the number of dorsal root ganglion neurons decreased relatively more than the number of axon profiles in the dorsal root (2). Due to a persistent growth retardation of surviving ventral and dorsal root axons and dorsal root ganglion neurons, it could not be decisively determined if the Abbreviations: EDR, CDR-experimental, control dorsal roots; EG, CG-experimental, control ganglia; EVR, CVR-experimental, control ventral roots. ’ This work was supported by the Swedish Medical Research Council (projects 3761 and 5420) and by the Karolinska Institutet. Excellent technical assistance was given by Anna Dahlberg, Inga Lam, Pippi Lindqvist, Maria Meier, and Bobbe Wire. Please send correspondence to M. Risling, Department of Anatomy, Karolinska Institutet, P.O. Box 60 400, S- 104 0 1 Stockholm, Sweden. 176 0014-4886/83/010176-12$03.00/O Copyright 0 1983 by Academic Press. Inc. All rights of reproduction in any form reserved.

CHANGES

IN SPINAL ROG’I-S AND GANGLIA

177

losspreferentially affected a particular sizerange. Axonal atrophy or growth retardation proximal to a nerve lesion is known to be linked with lack of peripheral reinnervation [( 13, 14), cf. (12)]. In the dorsal roots the proportion of unmyelinated axons remained essentialIyunaltered, but it increased markedly in the ventral roots (25). This increase,which is directly related to lack of successfulfunctional regeneration from the proximal stump to the periphery (Risling, Hildebrand, Cullheim, and Aldskogius, in preparation), obscured the true loss of axons. It cannot be excluded that the dorsal root had been similarly invaded by unmyelinated axons. Nerve crush allows a much more efficient axonal regeneration than resection (23, 28). In the present study we tried to minimize the axotomy-induced growth retardation and sprouting in the spinal roots by crushing instead of resectingthe sciatic nerve. The aim was to seek answers to the following questions: (i) Does a neonatal crush lesion of this nerve result in a persistent axonal growth retardation in contributing spinal roots? (ii) Is there a significant loss of dorsal and/or ventral root axons? (iii) If so, does this loss preferentially affect a particular sizerange of axons and/or dorsal root ganglion neurons? (iv) How does the number of dorsal root axons compare with the number of dorsal root ganglion neurons after a crush lesion? MATERIAL AND METHODS Nine 8-day-old kittens were anesthetizedwith halothane. The left sciatic nerve was exposed and crushed (three times, 20 s each) with watchmaker’s forceps, just distal to the hamstring branch. Fascia and skin were sutured in layers. Postoperatively (p.o.), all animals at first presented symptoms like those after sciaticnerve division. Function gradually returned in the affected hind limb and animals with long survival times showed a grossly normal walking behavior 8 to 12 weeks p.o. After 14,48, or 220 days of survival the animals were anesthetizedwith Mebumal(40 mg/kg, i.p.) and perfused through the descending thoracic aorta with Tyrode’s solution followed by 5% glutaraldehyde in phosphate buffer (3). The ventral and dorsal roots L7 and, in some cases(Table 4), the corresponding dorsal root ganglia, were removed bilaterally and posffixed 4 h in phosphate-buffered glutaraldehyde. The root specimens were osmicated, acetone-dehydrated, and embedded in Vestopal W (3). Complete transverse semithin sections were cut from each root at midroot level and stained with toluidine blue. Total crosssectional areas of the roots, as well as number and size distribution of myelinated fibers were determined on light micrographs [see (291. Thin sectionsfrom the central part of each crosscut root were collected on onehole copper grids and contrasted with uranyl acetate and lead citrate. The relative proportion of unmyelinated axons wasdetermined by sample countings in a Philips EM 300 electron microscope (8, 25-27). The L7 ganglia

I

Survival time (days)

14 14 14

48 48 48

220 220 220

Animal

1 2 3

4 5 6

7 8 9

7.38 6.22 5.70

3.46 3.82 3.64

1.86 2.02 1.53

CVR

6.21 5.51 5.00

3.12 3.38 3.12

1.39 1.58 1.39

EVR

0.84 0.89 0.88

0.90 0.89 0.86

0.75 0.78 0.91

E:C

Cross-sectional area (X10’ pm’)

5568 4936 5276

5416 6068 5816

5238 5681 4836

CVR

5143 444 1 4730

5431 5619 4965

4344 4905 4286

EVR

0.92 0.90 0.90

1.oo 0.93 0.85

0.83 0.86 0.89

E:C

Calculated total number of myelinated axons

28.9 24.3 19.5

11.2 12.5 14.3

7.9 14.8 9.2

CVR

36.8 28.7 25.7

13.0 14.7 15.1

18.3 17.3 11.6

EVR

Percentage unmyelinated axon profiles

7831 6521 6554

6099 6935 6787

5687 6668 5326

CVR

8138 6629 6366

6243 6587 5848

5317 5931 4848

EVR

1.04 1.02 0.97

1.02 0.95 0.86

0.94 0.89 0.91

E:C

Calculated total number of axons

Ventral Roots: Cross-Sectional Areas, Number of Myelinated Axons, Proportion of Unmyelinated Axons, and Calculated Total Number of Axons in Ventral Roots L7 from Experimental (EVR) and Control (CVR) Sides of Operated Kittens

TABLE

E $ 5

z z F

k E 8 “$

5

irr t; c

CHANGES

i

IN SPINAL

ROOTS AND GANGLIA

CVR

179

EVR

FIG. 1. Histograms illustrating caliber spectra of myelinated fibers (including myelin sheaths) in the ventral roots L7 of control (CVR) and experimental (EVR) sides at various survival times after early postnatal sciatic nerve crush (kittens 1, 6, and 8).

were dehydrated in ethanol and embedded in paraffm. Serial IO-pm longitudinal sections, including the whole ganglion, were stained with cresyl violet. To assess the total number of primary sensory neurons, the number of neuronal nucleoli was determined by counts in every 10th section at a magnification of 250X (2). For each ganglion, nucleolar diameters were measured with an ocular scale (X 1250) in about 400 neurons. The average and the smallest nucleolar diameters were used to calculate a correction factor for split nucleoli ( 19). In animals 7 through 9 the size distribution of neuronal perikarya was assessed on light micrographs (X200), considering about 400 neurons with clearly discernible nucleoli in each ganglion. Perikaryal size was approximated as the mean of the smallest and the largest diameters (2). RESULTS ventral Roots. The ventral roots on the operated side (experimental ventral roots = EVR) were thinner than those on the control side (CVR) and

2

14

14

14

48

48

48

220

220

220

1

2

3

4

5

6

7

8

9

7.8 1

9.51 8.25

5.38

5.83

5.74

3.36

3.95

3.90

CDR

5.58

8.18 6.04

3.45

4.15

3.60

2.60

2.45

2.76

EDR

E:C

0.71

0.86 0.73

0.64

0.71

0.63

0.77

0.62

0.71

Cross-sectional area (XIOs pm2)

9,935

11,082

12,341

9,807

10,518

11,442

10,902

11,012

10,931

CDR

9,328

10,984 9,635

7,912

10,088

9,614

8,636

8,490

9,140

EDR

Calculated total of myelinated

number axons

0.94

0.89 0.87

0.81

0.96

0.84

0.79

0.77

0.84

E:C

59.2

60.9 60.2

66.7

57.8

59.8

69.2

66.2

72.6

CDR

56.3

59.2 67.7

59.5

66.8

59.3

69.2

67.5

63.0

EDR

Percentage unmyelinated axon profiles

24,35 1

31,563 27,844

29,45 1

24,924

28,463

35,396

32,580

39,894

CDR

Number

21,346

26,922 29,830

19,536

30,386

23,622

28,039

26,123

24,703

EDR

total number of axons

Total

Calculated

Cross-Sectional Areas, Numbers of Myelinated Axons, Proportion of Unmyelinated Axons, and Calculated Axons in Dorsal Roots L7 from Experimental (EDR) and Control (CDR) Sides of Operated Kittens

Animal

Roots:

Survival time WY4

Dorsal

TABLE

0.88

0.85 1.07

0.66

1.22

0.83

0.79

0.80

0.62

EC

of

ii

B

5

8 $

t R

“8

E F

%% Li CHANGES

IN SPINAL

14d

FO.

EDR

CDR

20 10

0

5

201

0 flm

5

w

,c;,o%L

5

181

ROOTS AND GANGLIA

m

15

,um

5

x)

,

0

15

,iJm

FIG. 2. Histograms illustrating caliber spectra of myelinated fibers (including myelin sheaths) in the dorsal roots L7 on the control (CDR) and experimental (EDR) sides at various survival times after sciatic nerve crush (kittens 1, 6, and 8).

the EVR content of myelinated axons varied around 90% of the control values (Table 1). At 14 days survival the size spectra of myelinated EVR axons appeared slightly shifted to the left relative to the controls, at 48 days p.o. the proportion of medium size axons was increased, and at 220 days p.o. EVR and CVR spectra were comparable with respect to range and general configuration (Fig. 1). A closer analysis of the calculated total numbers of axons in different size groups indicated that the loss of myelinated EVR fibers fairly selectively a&ted the gamma size range (Fig. 1; Table 3). Within the alpha size range the calculated number of large (> 10 rm) axons was decreased, whereas the number of axons measuring 5 to 10 pm was increased (Table 3). Although the proportion of unmyelinated axon

182

RISLING,

ALDSKGGIUS,

AND HILDEBRAND

TABLE 3 Numbers of Myelinated Axons and Neurons in Different Size Groups in Experimental Ventral and Dorsal Roots and Dorsal Root Ganglia L7,220 Days after Early Postnatal Sciatic Nerve Crush” Animal

<5 pm

>5 flrn

>5 < 10 jbm

>lO pm

7 EVRb 8 EVR 9 EVR

86 72 78

96 98 96

117 111 139

87 88 59

7 EDR 8 EDR 9 EDR

95 103 116

86 78 81

91 128 96

69 21 37

7EG 8 EG 9 EG

<40 pm

40-80 pm

380 pm

90 76 74

100 81 81

77 55 40

’ Numbers are expressed as percentages of corresponding control values. b EVR, EDR-experimental ventral, dorsal root; EG-experimental ganglia.

profiles was within the normal range in all EVR, it was consistently higher than in the CVR (Table 1). This tended to compensate for the loss of myelinated axons, so that the total number of axons was largely unchanged. Dorsal Roots. With respect to all examined parameters the experimental dorsal roots (EDR) were more affected than the corresponding ventral roots. The average EDR cross-sectional area was reduced to 7 1% of the control area (range 62 to 86%; Table 2) and the number of myelinated EDR axons varied around 86% of the CDR (control dorsal roots) values (Table 2). At all survival times the EDR showed a marked reduction both in the proportion (Fig. 2) and in the calculated number (Table 3) of larger myelinated fibers. Both in EDR and CDR the proportion of unmyelinated axons varied irregularly around a mean of about 63% (Table 2). Thus, on the average, the total number of EDR axons was reduced to the same extent as the number of myelinated axons (Table 2). Dorsal Root Ganglia. The experimental ganglia (EG) contained various numbers of chromatolytic neurons. In some of these the nucleus occupied an eccentric position. At 220 days, however, such neurons were very rare. In all animals the EG showed a reduced neuronal number compared with the control ganglia (CC). The average corrected and uncorrected EG values varied around 8 1% of the controls (Table 4). In comparison with controls, the proportion of medium size neurons was increased in the EG (Fig. 3).

CHANGES

183

IN SPINAL ROOTS AND GANGLIA TABLE 4

Calculated Number of Neuronal Perikarya (Uncorrected and Corrected Figures) in the L7 Spinal Ganglia from Experimental (EC) and Control (CC) sides Number of neuronal perikarya Uncorrected

EGCG

Animal

Survival time (days)

Corrected

CC

EC

CC

EC

Uncorrected

corrected

4 5 6

48 48 48

24,710 24,720 19,980

17,400 22,390 17,700

18,533 18,293 14,785

13,224 16,569 13,098

0.70 0.9 1 0.89

0.71 0.91 0.89

7 8 9

220 220 220

28,200 22,180 21,130

25,880 16,130 14,860

20,304 15,970 15,214

18,634 11,614 10,699

0.92 0.73 0.70

0.92 0.73 0.70

Analysis of the loss in terms of calculated neuronal numbers in different size ranges at 220 days survival showed that the upper size range (>80 pm) suffered the greatest deficit (mean 57.7% of CG) followed by the lower size range (~40 pm; mean 80% of controls). The average number of neurons with intermediate sizes was also reduced (mean 87.3% of CG; Table 3). On both sides the total number of dorsal root axons surpassed the number of neurons in the corresponding ganglia (Table 5). In most animals this difference was larger on the operated side than on the control side, particularly at 220 days survival (Table 5). DISCUSSION For the present study the reliability of the unoperated side as a control is of crucial importance. In the L7 spinal roots and ganglia of normal kittens and adult cats, side differences are very limited with regard to axonal and neuronal numbers (2,25,26). In kittens with unilateral sciatic nerve lesions the L7 spinal roots and ganglia contralateral to the operated side are quite similar to those in normal animals (2, 25). Thus, the left-right differences

FlG. 3. Histograms illustrating size spectra of neuronal perikarya in the L7 dorsal root ganglia of control (CC) and experimental (EC) sides in kitten 8 (220 days survival).

184

RISLING,

ALDSKOGIUS,

AND HILDEBRAND

TABLE 5 Calculated Total Numbers of Neuronal Perikarya (Corrected Figures) in L7 Dorsal Root Ganglia and L7 Dorsal Root Axons from Experimental and Control Sides

Animal

suNival time (days)

Axons:Neurons Control side

Experimental side

4 5 6

48 48 48

1.54 1.36 1.99

1.79 1.83 1.49

7 8 9

220 220 220

1.56 I .74 1.60

I .45 2.57 2.00

observed in the present study should, to a large extent, reflect changes induced by the crush lesion. Size measurements of dorsal root ganglion neurons were made in the nucleolar plane. In normal primary sensory neurons, nuclei and nucleoli occupy a central position but some neurons in the examined ganglia from the operated side showed eccentric nuclei. At 220 days survival the frequency of neurons with eccentric nuclei, however, was very low, and the influence of this source of error, which would shift the spectrum to the left, should be negligible. The results show that after sciatic nerve crush the loss of axons in the L7 ventral and dorsal roots as well as of spinal ganglion neurons is only about half that seen after sciatic nerve resection (2, 25). This supports the view that the type of lesion is an important determinant for the outcome of the axon reaction [for discussion see (21)]. Both by a crush lesion and by nerve resection the soma is deprived of part of the axon. After nerve crush, however, the basal laminae remain intact and regenerating sprouts have immediate access to hyperplastic Schwann cell columns (28). In contrast, regeneration after nerve resection involves migration of axon sprouts and sheath cells from the proximal stump through scar tissue, some regenerating axons fail to reach the distal stump and a neuroma develops (28). The crush-resection difference with respect to neuronal survival is apparent early, probably before peripheral contacts have been reestablished [cf. (15, 28,3 l)]. It therefore seems possible that rapid contact with the environment in the distal stump enhances the survival of axotomized neurons [cf. (4 231. Considering the ventral roots, the present data suggest that predominantly gamma axons were lost. After nerve resection the loss likewise seemed to be greater among gamma axons in the ventral root (25). These findings

CHANGES

IN SPINAL

ROOTS AND GANGLIA

185

indicate that immature gamma neurons are more susceptible to axotomy than alpha neurons. Along the same line it has been claimed that alpha neurons have a greater ability to regenerate their axons than gamma neurons (5,29,30). Neurons in which the proximal part of the axon possesses intact branches appear to survive axotomy better than neurons lacking such branches (9; 23, p. 604). Conceivably, the presence of recurrent collaterals from the central nervous system segment of alpha neuronal axons but usually not from gamma neuronal axons (10, 11) might be a critical factor for postaxotomy survival. In the dorsal root no clearcut preferential degeneration of a particular size range of axons or neurons could be detected, as after sciatic nerve resection (2, 25). On the basis of studies in the rat it has been claimed that small dorsal root ganglion neurons are preferentially lost subsequent to peripheral nerve transection (7, 24). This is not supported by the present findings in the kitten. The picture, however, is obscured by the short-term chromatolytic swelling of neuronal perikarya (24) and the long-term growth retardation of axons and neurons after nerve crush. The loss of dorsal root ganglion neurons was relatively larger than the decrease in dorsal root axon profiles, as afler sciatic nerve resection (2). This could be due to a preferential sparing of neurons with more than one branch in the dorsal root (20), or compensatory local sprouting of surviving dorsal root axons (16). Recurrent growth of axons from the site of lesion to the dorsal root (23), however, appears to be the most likely explanation. Although no neuroma is formed, a nerve crush causes some local endoneurial disorganization (28). Conceivably, this could induce some axon sprouts to grow in a reverse direction from the lesion site (23, Fig. 109). This would eventually add new axons to the dorsal roots on the lesion side. In the ventral roots the number of unmyelinated axon profiles increased with development during the examined period, as in normal kittens (26, 27). In all animals this increase was slightly higher on the experimental than on the control side. Ongoing studies show that sciatic nerve resection in kittens is followed by growth of unmyelinated axon sprouts into the ventral root L7 (Risling, Hildebrand, Cullheim, and Aldskogius, in preparation). Similarly, the postcrush side difference observed in the present study might reflect a growth of sprouts from, e.g., the lesion site into the ventral roots. As seen from the results, both the axon loss and the distortion of the size distribution of surviving myelinated axons was more marked in dorsal than in ventral roots. Similar differences have been observed after amputations or nerve transections in kittens (17,25) and adult cats (6, 14, 18,22). These differences might be related to the fact that the lesion is closer to the dorsal root ganglion than to the ventral horn (about 30 mm and 45 mm, respectively, at operation) (21). Regeneration of the axon and reestablishment of

RISLING,

186

ALDSKQGIUS,

AND HILDEBRAND

peripheral contacts seems to be one prerequisite for restoration of the fiber size distribution proximal to a peripheral nerve lesion [( 13), cf. (12)]. The different effects of nerve lesions on fiber size spectra in ventral and dorsal roots might then reflect different abilities of motor and sensory neurons to reestablish peripheral contacts (30, 32). Other possible explanations were discussed by Hoffer et al. (14). In spite of favorable conditions for regeneration in the present material, many large ventral and dorsal root axons remained growth-retarded 220 days p.o. and this was most pronounced in the dorsal root. The atrophy or growth retardation following axotomy is accompanied by a decrease in conduction velocity, which can be relatively larger than the reduction of fiber diameter (22). In kittens, but not in adult cats, axonal conduction velocities proximal to a muscle nerve transection remain subnormal for at least 1 year postoperatively (Cullheim, Berglund, Linda, and Risling, in preparation). It may be that division of a prospective large immature axon can interfere irreversibly with some critical developmental events in the surviving proximal segment, such as, e.g., the elimination of supernumerary internodes or nodalization (4). This disturbed timing might then cause a permanent structural maldevelopment with accompanying functional deficits. REFERENCES 1. AGUAYO, A. J., S. DAVID, AND G. M. BRAY. 1981. Influences of the glial environment on the elongation of axons after injury: transplantation studies in adult rodents. J. Exp. Biol. 95: 231-240. 2. ALDSKOGIUS, H., AND M. RISLING. I98 1. Effect of sciatic neurotomy on neuronal number and size distribution in the L7 ganglion of kittens. Exp. Neural. 74: 597-604. 3. BERTHOLD, C.-H. 1968. A study on the fixation of mature feline myelinated ventral lumbar spinal-root fibres. Acta Sot. Med. Upsal. 73 (suppl. 9): l-36. 4. BERTHOLD, C.-H. 1978. Morphology of normal peripheral axons. Pages 3-63 in S. G. WAXMAN, Ed., Physiology and Pathobiology qfdxons. Raven Press, New York. 5. BRUSHART, T. M., AND M.-M. MESULAM. 1980. Alteration in connections between muscle and anterior horn motoneurons after peripheral nerve repair. Science 208: 603-605. 6. CARLSON, J., A. C. LAIS, AND P. J. DYCK. 1979. Axonal atrophy from permanent peripheral axotomy in adult cat. J. Neuropathol. Exp. Neural. 38: 579-585. 7. CAVANAUGH, M. W. 195 1. Quantitative effects of the peripheral innervation area on nerves and spinal ganglion cells. J. Comp. Neural. 94: 18 l-2 19. 8. CCCCESHALL, R. E., J. D. COULTER, AND W. D. WILLIS, JR. 1974. Unmyelinated axons in the ventral roots of the cat lumbosacral enlargement. J. Comp. Neural. 153: 39-58. 9. CRAGG, B. G. 1970. What is the signal for chromatolysis? Brain Rex 23: l-21. 10. CULLHEIM, S., AND J.-O. KELLERTH. 1978. A morphological study of the axons and recurrent collaterals of the cat sciatic cr-motoneurons after intracellular staining with horseradish peroxidase. J. Comp. Neural. 178: 537-558. Il. CULLHEIM, S., AND B. ULFHAKE. 1979. Observations on the morphology of intracellularly stained y-motoneurons in relation to their axon conduction velocity. Neurosci. Lett. 13: 47-50.

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12. CULLHEIM, S., AND M. RISLING. 1982. Observations on the morphology and the axon conduction velocity of axotomixed and regenerating sciatic motoneurons in the kitten. Exp. Brain Res. 45: 428-432. 13. GORDON, T., AND R. B. STEIN. 1982. Time course and extent of recovery in reinnervated motor units of cat triceps surae muscles. J. Physiol. (bndon) 323: 307-323. 14. HOFFER, J. A., R. B. STEEN,AND T. GORDON. 1979. Differential atrophy of sensory and motor fibrea following section of cat peripheral nerves. Brain Res. 178: 347-361. 15. HORCH, K. 1978. Central responses of cutaneous sensory neurons to peripheral nerve crush in the cat. Brain Res. 151: 581-586. 16. HULSEEIOSCH,C. E., AND R. E. COGGESHALL. 198 1. Sprouting of dorsal root axons. Brain Res. 224: 170-174. 17. JORGENSEN, D., AND P. J. DYCK. 1979. Axonal underdevelopment from axotomy in kittens. J. Neuropathol. Exp. Neural. 38: 571-578. 18. KIRALY, J. K., AND K. KRNJEVI:. 1959. Some retrograde changes in function of nerves atIer peripheral nerve section. Q. J. Exp. Physiol. 44: 244-257. 19. KONIGSMARK, B. W. 1970. Methods for the counting of neurons. Pages 315-340 in W. J. H. NALITA AND S. 0. E. EBBESSON, Eds., Contemporary Research Methods in Neuroanatomy. Springer-Verlag, Berlin/Heidelberg/New York. 20. LANGFORD, L. A., AND R. E. COGGESHALL. 1979. Branching of sensory axons in the dorsal root and evidence for the absence of dorsal root efferent fibers. J. Comp. Neural. 184 193-204. 21. LIEBERMAN, A. R. 1974. Some factors affecting retrograde neuronal responses to axonal lesions. Pages 71-105 in R. BELLAIRS AND E. G. GRAY, Eds., Essays on the Nervous System. Clamndon, Oxford. 22. MILNER, T. E., AND R. B. STEIN. 1981. The effects of axotomy on the conduction of action potentials in peripheral sensory and motor nerve fibres. J. Neurol. Neurosurg. Psychiat. 44: 485-496. 23. RAM6N Y. CAJAL, S. 1928. Degeneration and Regeneration of the Nervous System. (R. M. MAY, trans. and Ed.), reprinted. Hafner, New York, 1959. 24. RANSON, S. W. 1909. Alterations in the spinal ganglion cells following neurotomy. J. Comp. Neural. 19: 125-153. 25. RISLING, M., S. REMAHL, C. HILDEBRAND, AND H. ALDSKOGIUS. 1980. Structuralchanges in kittens’ ventral and dorsal roots L7 after early postnatal sciatic nerve transection. Exp. Neural. 67: 265-279. 26. RISLING, M., C. HILDEBRAND, AND H. ALDSKOGIUS. 1981. Postnatal increase of unmyelinated axon profiles in the feline ventral root L7. J. Comp. Neural. 201: 343-35 1. 27. RISLING, M., AND C. HILDEBRAND. 1982. Occurrence of unmyelinated axon profiles at distal, middle and proximal levels in the ventral root L7 of cats and kittens. J. Neural. Sci., in press. 28. SUNDERLAND, S. 1978. Nerves and Nerve Injuries. Churchill Livingstone, Edinburgh/ London/New York. 29. TAKANO, K. 1976. Absence ofthe gamma-spindle loop in the reinnervated hind leg muscles of the cat: “alpha-muscle.” Exp. Brain Res. 26: 343-354. 30. THULIN, C.-A. 1960. Electrophysiological studies of peripheral nerve regeneration with special reference to the small diameter (gamma) fibers. Exp. Neural. 2: 598-612. 3 1. WALL, P. D., AND M. DEVOR. 198 I. The effect of peripheral nerve injury on dorsal root potentials and on transmission of afferent signals into the spinal cord. Brain Rex 209: 95-111. 32. ZELEN~, J., AND P. Hti~. 1963. Motor and receptor units in the soleus muscle after nerve regeneration in very young rats. Physiol. Bohemoslov. 12: 277-289.