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Fiber composition of the rat sciatic nerve.

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T H E ANATOMICAL RECORD 2 1 5 7 - 8 1 0986)
Fiber Composition of the Rat Sciatic Nerve
Institute
of
H. SCHMALBRUCH
Neurophysiology, University of Copenhagen, The Panum Institute, DK 2200
Copenhagen N, Denmark
ABSTRACT
The rat sciatic nerve originates from the spinal segments L4-L6. It
is unifascicular at the trochanter; 5-7 mm distally, the nerve splits into two and
then into four fascicles. The tibial portion gives rise to the tibial and the sural
nerves, and the peroneal portion gives rise to the peroneal nerve and a cutaneous
branch that perforates the lateral hamstring muscles to innervate the proximolateral face of the calf.
The number and type of the axons in these branches were determined in light and
electron micrographs of normal nerves, and after de-efferentiation or sympathectomy. Deafferentiation was technically not feasible because spinal ganglia and
ventral roots were supplied by the same vascular plexus.
The tibial nerve contained 1,000 motor and 3,500 myelinated afferent axons, 3,700
sympathetic axons, and 5,400 unmyelinated afferent axons. The peronal nerve
contained 600 motor and 1,300 myelinated afferent axons, 1,100 sympathetic axons
and 3,000 unmyelinated afferent axons. The sural nerve contained 1,100 myelinated
and 2,800 unmyelinated afferent axons; in addition, there were 1,500 unmyelinated
sympathetic axons. The cutaneous branch consisted of 400 myelinated and 1,800
unmyelinated afferent axons. Thus, the entire sciatic nerve a t midthigh is composed
of about 27,000 axons; 6% are myelinated motor axons, 23% and 48% are myelinated
and unmyelinated sensory axons, respectively, and 23% are unmyelinated sympathetic axons. The techniques used did not demonstrate sympathetic axons in the
cutaneous branch and did not reveal the few motor axons contained in the sural
nerve.
The sciatic nerve branches of cat and rat are frequently used in experimental studies. Boyd and Davey
(1966, 1968), in a now classical study, counted the myelinated axons in a large number of nerves of cat and,
using surgical deafferentiation or de-efferentiation, determined the percentages of motor and sensory fibers.
Comparable data for rat nerves do not exist. Experimental studies of rat nerves may contain counts of myelinated axons in the normal sural and tibial nerves (for
references, see Peyronnard and Charron, 1982 and Mayhew and Sharma, 1984a), but usually the nerves were
taken from the calf region, possibly after branching had
commenced. Recently, Jenq and Coggeshall(1984,1985)
reported counts of myelinated and unmyelinated fibers
in the sural nerve, and in the entire sciatic nerve just
after its exit from the pelvis. The results will be discussed below. Separate counts of sympathetic and unmyelinated sensory fibers to my knowledge do not exist.
The present paper is a n anatomical study of the fiber
composition of the four principal branches that arise
from the rat sciatic nerve in the thigh region. The number of motor fibers was determined by de-efferentiation,
and the contribution of sympathetic fibers to the pool of
unmyelinated fibers was determined by sympathectomy.
MATERIALS AND METHODS
Forty male Wistar rats were used, each weighing about
300 gm. The fiber counts reported in this paper were
(c)
1986 ALAN R. LISS, INC.
derived from 18 animals. Nerve fibers of different origin
were identified by cle-efferentiation or sympathectomy.
For de-efferentiation, 4-week-old rats were used because
in younger rats laminectomies are technically easier.
All operations were performed in fentanyl-apozepam
anesthesia; for the terminal perfusion fixation, the rats
were anesthetized with Halotha+. Surgery was performed under clean but not sterile conditions; infections
did not occur.
De-efferenriation
Three to four lumbar laminectomies were performed
on the right side including the spinal processes. The
spinal ganglia were identified; the crest of the ilium
that is closest to L5 served a s landmark. The ventral
roots L4-L6 were cut close to the ganglia and bent
upward to prevent regeneration. The spinal cord was
covered with a hemostatic sponge, and muscles and skin
were sutured separately. The rats were sacrificed after
4 weeks. The deficit of myelinated fibers in the sciatic
nerve compared to the contralateral side reflected the
number of efferent motor fibers.
Deafferentiation, i.e., excision of the spinal ganglia
L4-L6, always resulted in paresis of leg muscles. The
operation caused profuse bleeding from the site where
Received August 6, 1985; accepted December 18, 1985.
72
H. SCHMALBRUCH
the ventral root came close to the ganglion, and it was
difficult to identify both structures. To elucidate the
cause of this difficulty, spaced serial sections of the ganglion-ventral root complex were investigated by light
microscopy.
Syrnpathectorny
After laparotomy the abdominal aorta and vena cava
were approached from the left side and mobilized, but
not separated. Both sympathetic chains were found dorsal to the renal blood vessels and were excised from here
to the aortic bifurcation. This corresponds to L2 to S2
(Suh et al., 1984). To ensure completeness, the ventral
surfaces of the spinal column and of the prevertebral
muscles were cleaned, and all connections to the connective tissue around the large vessels were severed. Care
was taken not to injure the left ureter and the dorsal
vascular branches supplying the spinal cord. It did not
appear to be possible to completely excise one chain
alone without injuring the contralateral one. The abdominal wall was sutured in layers. The rats were sacrificed after 10-12 days, which suffices for the
unmyelinated axons to degenerate (Coggeshall et al.,
1980; Suh et al., 1984).
Specimen Preparation
The nerves were fixed in situ by perfusion of the hind
limbs via the abdominal aorta. The vascular bed was
rinsed for 2 min with Ringer’s solution containing heparin (5,000 unitditer) and procaine (0.1 m i t e r ) ; perfusion was continued for 10-12 min with 2.5%
glutaraldehyde in Ringer’s solution (1,000 ml, pH 7.4,
20°, 350 mOsM). The nerves remained in the glutaraldehyde solution a t 4°C overnight; the individual
branches were postfixed in 1%Os04 in phosphate buffer,
and embedded in Embed 812.
For light microscopy, cross sections 1- to 3-pm-thick
were stained with p-phenylenediamine. For electron microscopy cross sections of a n entire branch were collected
on 150-mesh grids without support film, and were
stained with methanolic uranyl acetate (Landon, personal communication) and aqueous lead citrate.
The samples used to study the topographical relation
between spinal ganglia and ventral roots (see above)
were taken from normal rats that had been fixed by
vascular perfusion through the left cardiac ventricle.
Counts and Measurements
The myelinated nerve fibers were counted on light
micrographs (objective Zeiss Planapo 40 oil; A: 1.0)
printed to 1,100 x or 1,800 x . In small nerves all fibers
were counted; in nerves with more than 1,000 fibers 3070% of the fibers were counted and the total number
was computed from the sample area and the total area
of the nerve cross section. The micrographs to be assessed were taken nonoverlappingly according to a
scheme that covered the entire cross section. On the
same prints, the outer perimeters of the myelin sheaths
were measured and divided by 3.14 to obtain “idealized
diameters” (see Sources of Error, below).
For electron microscopy, one complete cross section of
a nerve branch was selected, and a series of micrographs
was taken at 3,600 x according to a scheme that covered
the entire cross section. The total area photographed
corresponded to a t least 3% of the total cross-sectional
area of the nerve; in small nerves up to 70% of the area
was photographed. Areas with artifacts, contamination,
or blood vessels were not excluded. Myelin sheaths and
unmyelinated axons were counted on prints at 10,800
x ; structures bisected by two of the four margins were
included. The total number of unmyelinated axons was
calculated from their relative frequency compared to
myelin sheaths, and the total number of myelinated
fibers, which already was known from light microscopy
(see Sources of Error). The “idealized diameters” of the
unmyelinated axons were obtained from their perimeters measured at 26,500 X.
The magnifications of light and electron micrograph
prints were calibrated against gratings (100 lines per 1
mm, and 2,160 lines per 1mm, respectively), which were
photographed and printed together with each series of
micrographs. All counts and measurements were done
with the aid of a digitizer tablet (Kontron MOP 2) connected to a computer (Digital PDP 11/34). The effect of
the hysteresis of the lenses of the electron microscope on
its magnification was eliminated by exciting all lenses
maximally before focusing; the focus setting was not
readjusted during a series of micrographs.
Sources of Error
“Idealized diameters” of the nerve fibers were calculated from their perimeters because the circumference
in contrast to the area is generally independent of the
irregular configurations of the cross section (Friede and
Samorajski, 1967). Nevertheless, this precaution, in
hindsight, turned out to be unnecessary. The mean diameters of myelinated fibers were found to differ by at
most 0.1 pm when they were calculated from the perimeter and from the cross-sectional area. This indicates
that the effect of distortion was negligible.
The micrographs used for the light microscope fiber
counts covered a t least 30% of the cross-sectional area of
the nerve, and sampling bias was avoided by spreading
the micrographs systematically over the entire cross
section. Mayhew and Sharma (1984a,b), who tested different sampling methods for the rat tibia1 nerve, found
that the gain in accuracy is small when more than 6%
of the fibers are assessed, and that the sampling procedure has little effect. Sampling was critical, however,
when relatively large nerve branches were studied by
electron microscopy. The unmyelinated axons were clustered and a micrograph covering a 350-pm2 cross-sectional area exposed 2-11 (mostly 5-7) myelin sheaths
and 0-80 unmyelinated axons. To ensure randomness,
the photographs were taken according to a predetermined scheme without inspection of the site to be photographed, and a n equal number of micrographs was
placed upon each grid square. The areas hidden by the
grid bars were assumed to be randomly selected.
Initially the numbers of both unmyelinated and myelinated axons were calculated from counts in electron
micrographs, the area assessed, and the total cross-sectional area of the nerve branch measured by light microscopy. The result for myelinated fibers was then
compared to counts in light micrographs; this procedure
should ascertain that the areas studied by electron microscopy were representative. Unexpectedly, in each
nerve the number of myelinated fibers was 10-20%
higher when it was determined by electron microscopy
than when determined by light microscopy alone. Re-
LH!L
peated photographing and counting, or counting axons,
or counting centers of axons rather than myelin sheaths,
gave results identical within f5%. The difference in the
number of myelinated fibers could not be attributed to
the presence of very thin fibers that might have been
overlooked in light micrographs. This indicated that the
density of niyelinated fibers (number per unit area) was
indeed higher in electron than in light micrographs.
Section compression might represent part of the explanation, but it is most likely that the sections were not
stretched across the grid holes but that they sagged or
scallopped. Folds often disappeared when the sections
were exposed to the electron beam, and a t high magnification differences of the section position in relation to
the focal plane were seen when different sites were
inspected. To account for a 20% error in area, a 150-pm
by 150-pm square of the grid would have to expose a
164-pm by 164-pm square of the section; this does not
appear unlikely. To circumvent the error in area, the
total number of unmyelinated fibers was calculated from
their frequency in relation to myelinated fibers, and the
counts of myelinated fibers in light micrographs.
RESULTS
Branches of the Sciatic Nerve
The anatomy of the rat has been described by Greene
(1963).Nevertheless, the branching pattern of the sciatic
nerve of the rats of this study differed in several aspects
from that found by Greene (1963).
The segmental nerves L4, L5, and L6 contributed to
the sciatic nerve; no contribution of L3 and S1 was
found. None of the nerves L4-L6 were exclusive to the
sciatic nerve. After the sciatic nerve had left the pelvis
and had given rise to several branches to the glutei and
hamstring muscles, it curved around the greater trochanter. At this site it was always unifascicular. At a
point 3-5 mm distal to the greater trochanter a septum
dividing the future main branches of the nerve occurred,
and a t about the same level a thin branch originated
from the peroneal portion of the nerve. This nerve
branch entered the lateral hamstring muscle group and
without branching, perforated the dense connective tissue between the anterior and posterior heads of the
biceps femoris muscle and innervated the skin of the
proximolateral face of the calf. In the following this
nerve is referred to as “cutaneous branch.” At a point
distal to the point where the tibial and peroneal portion
of the sciatic nerve separated, the sural nerve originated
from the dorsal face of the tibial nerve. Cross sections at
the middle of the thigh might show four nerve branches:
the large tibial nerve, the smaller peroneal nerve, the
still smaller sural nerve, and the very small cutaneous
branch (Fig. 1).The peroneal nerve on its way through
the thigh gave off two to four very thin nerve branches,
each consisting of 10-20 myelinated and several hundred
unmyelinated nerve fibers. In respect to fiber composition these nerve branches did not resemble muscular
branches and probably innervated the femur and the
knee joint. These nerve branches were seen in serial
cross sections but could not be identified by dissection.
Within the fat tissue filling the popliteal fossa the three
remaining branches of the sciatic nerve divided. The
sural nerve ran superficially through the middle of the
popliteal fossa and, together with prominent blood vessels, continued on the dorsal face of the gastrocnemius
KXI b L l A l l L N h K V h
/Y
muscle; the tibial nerve ran through the depth of the
popliteal fossa and disappeared between the heads of the
gastrocnemius muscle to innervate the flexor muscles
and eventually, by means of the plantar nerves, the
palm of the foot; the peroneal nerve took a superficial
and lateral course and entered the anterior muscle
group.
Myelinated Fibers
The normal number of myelinated fibers in the sciatic
nerve was determined in 18 rats. Four of these were
control rats, in nine sympathectomy had been performed, and in five the contralateral sciatic nerve had
been de-efferentiated. Sympathectomy or contralateral
de-efferentiation did not affect the myelinated fibers,
and the results were pooled (Table 1).The entire sciatic
nerve contained 7,800 fibers, of which 4,500 formed the
tibial nerve and 1,900 the peroneal nerve. In these
branches the variation with respect to fiber numbers
was small and the interindividual standard deviations
were less than 10% of the mean values. The sural nerve
and the cutaneous branch contained 1,050 and 350 fibers, respectively. The large interindividual variation of
these nerves probably reflected the variability of the
sensory innervation of the skin.
Four weeks after de-efferentiation, the number of fibers in the right tibial nerve was reduced by about
1,000, and that of the peroneal nerve was reduced by
about 600 (Table 2). At that time, almost no degenerating myelin sheaths were left, and no regenerating fibers
were found. The loss of myelinated axons after de-efferentiation reflected the numbers of motor fibers in these
two nerves. No fiber deficit occurred in the cutaneous
branch or in the sural nerve, although the latter nerve
in rat contains about 80 motor fibers (Peyronnard and
Charron, 1982).
The diameters of the myelin sheaths ranged from 1.5
pm to 12.5 pm. The distribution was unimodal in the
tibial and peroneal nerves, whereas the sural nerve and
the cutaneous branch showed discontinuous distributions. The sural nerve contained a larger proportion of
thick fibers than the cutaneous branch (Fig. 2).
The shape of the diameter histograms changed considerably during the process of sampling; in places distinct
bimodal distributions were found. These areas might
have contained groups of fibers determined to form motor branches; the bimodal shapes of the histograms always disappeared with increasing sample size.
The diameter distribution of the motor fibers was obtained by constructing “difference histograms.” For each
size class the number of fibers in the de-efferentiated
nerve was subtracted from the number of fibers in the
normal contralateral nerve. In two of three rats the
difference histograms of the tibial nerves were bimodal;
in one it was skewed (Fig. 3).
Unmyelinated Fibers
The total number of unmyelinated axons was about
19,000. In all, five nerves were assessed; three were from
normal rats and two were control nerves from rats in
which the contralateral side had been de-efferentiated.
The interindividual variation in all nerve branches was
larger than that of myelinated fibers; the standard deviations were about 20% of the mean values. In the
tibial and peroneal nerves there were about twice as
74
H. SCHMALBRUCH
Fig. 1. The branching pattern of the rat sciatic nerve. A. Lateral
view of the right thigh enlarged 1.8 times. The gluteal and lateral
hamstring muscles and the fat tissue filling the popliteal fossa have
been removed. The position of the trochanter and the knee joint are
marked by pushpins labeled t and k, respectively. The cutaneous
branch (c) has been cut. Below left the gastrocnemius muscle is seen.
1,2,3) The arrows mark the sites of the cross sections shown in panels
B,C,D. B. Cross section of the sciatic nerve at site 1. C. Cross section of
the sciatic nerve at site 2. The nerve starts to divide into its tibial and
peroneal portions. D. Cross section at site 3. The tibial portion of the
nerve has divided into the tibial (t) and the sural (s) nerves, and the
peroneal portion has divided into the peroneal nerve (p) and the cutaneous branch (c).
FIBER COMPOSITION O F THE RAT SCIATIC NERVE
75
TABLE 1. Number of myelinated axons in sciatic nerve branches of normal rats or in rats after
experimental procedures not affecting the myelinated nerve fibers
Tibial n.
Rat No.
Control animals
1
2
3
4
(SD)
Cutaneous
branch
Total
1,754
1,806
1,865
1,611
1,058
979
1,099
884
272
438
328
352
7,309
7,085
8,369
7,114
4,651
4,945
4,647
5,472
4,781
4,266
4,594
4,208
4,985
1,882
2.123
1:964
2,139
1,994
1,592
1,975
1,815
1,973
1,091
1,017
1.417
lj212
833
834
1,049
957
979
466
469
349
427
281
346
397
309
341
8,090
8,554
8,377
9,250
7,889
7,038
8,015
7,289
8.278
4,474
3,999
4,391
4,870
4,124
4,547
(422)
1,797
1,760
1,912
1,952
1,765
1,871
(150)
1,066
1,070
1,123
1,081
1,217
1,054
(142)
348
407
287
375
183
354
(74)
7,685
7,236
7,713
8,278
7,289
7,825
(622)
Contralaterally
de-efferentiated
14
15
16
17
18
Mean
Sural n.
4,225
3,862
5,077
4.267
Sympathectomized
5
6
7
8
9
10
11
12
13
Peroneal n.
TABLE 2. Loss of myelinated nerve fibers after de-
efferentiation’
Cutaneous
Tibial n.
Peroneal n.
Sural n.
Rat 14
1,027
605
149
branch
Lost
Rat 15
Rat 16
Rat 17
Rat 18
Mean
(SD)
948
921
985
1,189
1,014
(106)
549
485
700
601
588
(79)
31
( - 6912
( - 136)
387
25
(-52)
17
(-128)
-
-
‘Most data from Schmalbruch, 1984.
2Note: (-69) means that the de-efferentiated nerve contained 69 more
nerve fibers than the contralateral nerve.
many unmyelinated as myelinated fibers; in the sural
nerve and the cutaneous branch the ratios were 4.1:l
and 5.3:1,respectively (Fig. 4).
Sympathectomy reduced the number of unmyelinated
fibers to roughly 13,000,i.e., the branches of the sciatic
nerve together contained 6,000 efferent (sympathetic)
and 13,000afferent (sensory) unmyelinated axons. The
absolute loss of unmyelinatd fibers was largest in the
tibia1 nerve; 3,700,or 40%, of the unmyelinated axons
were sympathetic. The deficit in the peroneal nerve was
1,100,which corresponded to 27% of all unmyelinated
axons; in the sural nerve it was 1,500,corresponding to
35% of the unmyelinated axons (Table 3). Sympathectomy did not produce a significant loss of unmyelinated
fibers in the cutaneous branch.
The diameter distribution of the unmyelinated axons
was unimodal in all nerves. The peak of the histograms
was at 0.7-0.8pm; the standard deviation of the axon
diameters was 20-25% of the mean. After sympathectomy, the mean diameters tended to be reduced. “Difference histograms” (see above) revealed that not only large
but also small axons had been lost. There was no change
in diameter in the cutaneous branch after sympathectomy, suggesting that this nerve contained no or only
few autonomic fibers
4).
Topography of the Ventral Root-Spinal Ganglion Complex
Cross sections of plastic-embedded samples for light
microscopy showed that the ventral roots were practically devoid of a connective tissue sheath. The ventral
root L5,which was the thickest one of the three roots to
the sciatic nerve (Schmalbruch, 1984),had a diameter of
about 400 pm. When it approached the ganglion both
became enclosed by a common connective tissue sheath
about 10 pm thick. Cross sections of samples fixed by
vascular perfusion demonstrated a vascular plexus that
supplied the ganglion and the ventral root; there were
also vessels in the 20-pm-wide gap between the two
structures. Before the distal pole of the ganglion, peripheral afferent and efferent ventral root fibers became so
close that even in cross sections it was impossible to
delineate afferent and efferent structures without the
aid of proximal serial sections (Fig. 5). The situation at
the ganglia L4 and L6 was the same as at L5; the
ventral root L6 was much thinner than the L4 or L5
roots.
DISCUSSION
The rat sciatic nerve in the thigh, distal of the greater
trochanter, consists of 7,800 myelinated and 19,400unmyelinated axons. Of these, 1,600myelinated and 6,400
unmyelinated axons are efferent, and are motor axons
and sympathetic axons, respectively (Tables 1-3,5). The
ratio unmyelinated afferent axons to myelinated afferent axons is 2 2 1 , and, hence, smaller than the corresponding 3:l ratio in thoracic dorsal roots (Hulsebosch
and Coggeshall, 1983). Whether this reflects different
cell populations in lumbar as compared to thoracic dor-
H. SCHMALBRUCH
76
rat 2
rat 1
tibial n
n 1219(=32%)
tibial n.
n 688(=16%)
10
h
peroneal n.
n 548(=30%)
sura n .
n 58 (=55%)
sural n.
n 452(=46%)
iI
15
0
peroneal n .
cut. branch
1
100%)
cut. branch
n 438(=100%)
unmyelinated fibers. Their data for myelinated fibers
are in harmony with those reported here (Table 1).It is
interesting that the number of myelinated fibers does
not increase when the nerve splits up into its branches;
i.e., that there is no branching within a distance of at
least 10 mm. Their counts of unmyelinated fibers in both
nerves, however, are almost 20% less than in the present
study (Table 3). This might suggest axonal branching in
the sciatic nerve. Nevertheless, this assumption does not
explain the difference in the sural nerve; in addition,
our own unpublished experiments indicated that, within
a 20-mm-long segment of the peroneal nerve, the unmyelinated axons do not branch. Jenq and Coggeshall(1984,
1985)counted nerve fibers in photomontages of electronmicrographs of the entire nerve. The final magnification
is not given, but it seems reasonable to assume that it
was less than 10,000 x . In my experience, it is difficult
to identify the smallest axons below a magnification of
10,000.
The sciatic nerve gives rise to four nerve branches
(Fig. 1;Table 5). Its two largest branches, the tibial and
peroneal nerves, supply the dorsal and ventral compartments of the lower leg, and contain 1,000 and 600 motor
axons, respectively. The motor percentage of these
nerves (22% and 31%) is much smaller than that reported for cat nerves by Boyd and Davey (1968) (4070%). Nevertheless, tibial and peroneal nerves at their
origin from the sciatic nerve still contain many skin
afferents, whereas Boyd and Davey (1968)preferentially
studied muscular branches. It is unlikely that the number of motor axons in the present study was underestimated. By using horseradish peroxidase (HRP)as a label,
Brushart and Mesulam (1980)found 866 tibial motoneurons (one rat) and 368-434 peroneal motoneurons (six
rats).
The 80 motor axons of the sural nerve, shown by
Peyronnard and Charron (1982)by HRP labeling and by
deafferentiation (see below), did not cause a consistent
fiber loss after de-efferentiation in this study (Table 2).
Nevertheless, it is unquestioned that the sural nerve of
rat contains motor axons tht eventually enter the lateral
plantar nerve (see, for example, Betz et al., 1979). The
“cutaneous branch” apparently is purely afferent, but
the large interindividual variation of the size of this
nerve might have obscured the loss of efferent unmyelinated fibers after sympathectomy.
It is well established that the ventral roots of mammals may contain myelinated and unmyelinated sensory fibers (for references, see Coggeshall et al., 1974).
These sensory fibers were severed during de-efferentiation as well. Nevertheless, this did not influence the
fiber counts in the peripheral nerves. Sectioning of the
sensory ventral root fibers causes degeneration of the
central segments only, whereas the distal segments and
hence also the ganglion cells survive (Coggeshall et al.,
1974).
The diameter of myelinated axons in rat is smaller
than in cat. Boyd and Davey (1968) measured fiber diameters of up to 18 pm, whereas in rat only a few fibers
are 12 pm thick. This species difference seems real,
despite differences in the histological technique: Boyd
and Davey (1968) studied paraffin sections 7 pm thick,
whereas in this study plastic sections 1-3 pm thick were
used. The histograms of fiber diameters in all intact and
de-efferentiated tibial and peroneal nerves are unimodal
’L
I‘LL
7
0
2
4
6
8
1012
tJm
0
2
4
6
8
1 0 1 2
Prn
Fig. 2. Diameter distribution of the myelinated fibers in the four
sciatic nerve branches of rats 1 and 2. n, number of axons measured
(= 410 of all axons of the nerve branch). The diameters range from 2 pm
to 12 pm. The distributions are unimodal with peaks at about 8 pm in
the tibial and peroneal nerves, but appear discontinuous in the sural
nerve and the cutaneous branch (see also Table 1).
sal roots, or is due to branching of myelinated afferents,
is unknown.
Jenq and Coggeshall (1984, 1985) found in the rat
sural nerve 1,062 myelinated and 3,557 unmyelinated
fibers; the unifascicular most proximal segment of the
sciatic nerve contained 8,120 myelinated and 15,542
R A T 16
R A T 17
C O N T R O L
u
%IBi
m
20
R A T 18
1
12
n= 1 7 6 5
24
i
A ' F F E R E N
k?!+
n= 1 2 5 2
200
r
" E1F F E R E N 1T "
n
150
100
50
0
u
-
0
2
l
4
l
6
l
8
1
P
l
0
l
0
I
I
1
l
2
4
Fig. 3. Diameter distribution of the myelinated fibers in the normal
(top) and the de-efferentiated (middle) tibia1 nerves of rats 16, 17, and
18. The bottom-row histograms were constructed by subtracting the
upper two histobvams from each other to obtain the diameter distri-
6
J
8
1
0
0
P
I
2
1
4
1
6
I
8
I
1
0
P
bution of the eliminated efferent fibers. Note that the class frequency
in the two upper rows is given in percentage of all axons; the scale for
the bottom row indicates the computed number of axons in each class.
n, total number of myelinated axons. (See also Tables 1 and 2).
TABLE 3. Number of unmyelinated axons in sciatic nerve branches of normal rats and after
sympathectomy
Rat No.
Normal
1
2
3
14
15
Mean
(SD)
Sympathectomized
5
7
9
10
11
12
13
Mean
(SD)
Peroneal n.
Sural n.
Cutaneous
branch
8,640
6,742
9,918
11,049
9,037
9,077
(1,600)
4,155
3,351
4,795
3,967
4,588
4,171
(565)
4,209
3,773
3,654
lost
5,590
4,307
(888)
1,752
2,198
2,100
2,176
1,176
1,880
(433)
18,756
16,064
20,467
20,391
18,9201
(2,061)
7,821
5,913
3,075
4,173
5,642
7,579
3,624
5,404
(1.871)
2,963
2,771
3,631
2,548
2,583
3,938
2,885
3,046
(544)
2,073
3,812
2,911
2,594
2,811
2,668
2,660
2,790
(523)
2,523
1,344
1,625
2,541
2,070
1,205
1,357
1,809
(568)
15,380
13,840
11,242
11,856
13,106
15,390
10,526
13,048
(1.940)
Tibia1 n.
Total
'The mean total number of unmyelinated axons calculated from the mean numbers of axons in each nerve branch
is 19,435;this explains the discrepancy between this figure and the data given in the text.
78
H. SCHMALBRUCH
Fig. 4. Typical electron micrographs of cross sections of the normal peroneal nerve (A) and
the cutaneous branch (B).The peroneal and also the tibia1 nerves consist of rather large axons
with interspersed groups of unmyelinated axons. In the cutaneous branch and also in the sural
nerve large areas of the cross section are occupied by unmyelinated axons.
(Fig. 2). In the rat L4 dorsal root the fiber distribution is
unimodal; it is bimodal in the L4 ventral root (Rao and
Krinke, 1983). The shape of the histogram in the intact
nerve is probably dominated by the large number of
afferent fibers, whereas the “difference histograms,” reflecting the diameter distribution of the efferent fibers,
tend to be bimodal (Fig. 3). Muscular branches in rat
always show a bimodal diameter distribution (Zelena
and Hnik, 1963; Eisen et al., 1974).The diameter distribution of the unmyelinated nerve fibers in all nerves is
unimodal and remains so after sympathectomy; preferentially but not exclusively, large fibers are lost. This
Fig. 5. The anatomical relation between the L5 ventral root and the
L5 dorsal root ganglion shown in stepped serial cross sections. A. The
ventral root proximal to the ganglion. The circular defects represent
blood vessels ballooned by perfusion fixation. B. The ventral root and
the ganglion are enclosed by a common connective tissue sheath about
10 pm thick. C . The gap between ganglion and ventral root is about 20
pm wide and contains blood vessels. Note the large number of afferent
fibers within the ganglion, close to the ventral root. D. Towards the
distal pole of the ganglion fascicles of peripheral afferent axons and
efferent ventral root fibers are difficult to delineate without the aid of
proximal sections. There were numerous ganglion cells outside the
area depicted. There seems to exist a discrepancy between the anatomical situation in the rat shown in these micrographs, and the reports
of complete surgical deafferentiation, assuming that this operation
inflicted no or only little damage on the ventral roots (see text).
H. SCHMALBRUCH
80
TABLE 4. Number and diameter of the unmyelinated axons in branches of the sciatic nerve of a normal rat as compared to
a rat in which the autonomic fibers have been eliminated’
P
Sural n.
Mean
diam.
(pm)
Cutaneous br.
Mean
diam.
SD
N
(pm)
(pm)
(pm)
N
8,640
0.80
0.19
4,155
0.78
0.18
4,209
0.77
0.16
1,752
0.73
0.14
5,642
0.75
0.20
2,583
0.74
0.18
2,811
0.70
0.16
2,070
0.72
0.14
N
Rat 1,
normal
Rat 11,
sympathectomized
Peroneal n.
Mean
SD
diam.
(pm)
(pm)
Tibial n.
Mean
diam.
(pm)
SD
< 0.1
N
< 0.05
SD
(pm)
< 0.5
<0.001
‘Note that there is no change in the cutaneous branch. SD, standard deviation; P, significance limits of the diameter difference according to
Student’s t test.
TABLE 5. Fiber composition of the rat sciatic nerve branches: Summary of results
presented in Tables 1-3’
Tibial n.
Efferent
Myelinated
motor axons
Unmyelinated
sympathetic
axons
Afferent
Myelinated
axons
Unmyelinated
axons
Peroneal n.
Sural n.
Cutaneous branch
-
-
1,000
600
3,700
1,100
1,500
(ioo?
3,500
1,300
1,100
400
5,400
3,000
2,800
1,800
‘Data are mean values to nearest hundred.
2Not significant.
suggests that the sympathetic fibers on the average are
thicker than unmyelinated afferent fibers (Table 4).
It would have been desirable to deafferentiate the
nerves by excision of the appropriate ganglia in order to
assess the number and size of the motor axons in a direct
approach. This turned out to be impossible (Fig. 5). Muscle paresis regularly occurred after attempts to excise
the ganglia; it was probably due to direct trauma during
the operation and also to ischemia following the destruction of the vascular plexus supplying ventral root and
ganglion. Connective tissue that would allow separation of efferent and afferent structures in a blunt fashion
is scarce. Wray (1969), who determined the number of
motor units in the much larger baboon, encountered
similar difficulties and often was forced to use de-efferentiation rather than deafferentiation. Nevertheless,
Zelena and Hnik (1963), Gutmann and Hanzlikova
(19661, and Peyronnard and Charron (1982, 1983) determined in rats the number of motor axons in deafferentiated nerves of the soleus and anterior tibial muscles,
and in the deafferentiated sural nerve. Zelena and Hnik
(1963) and Peyronnard and Charron (1982, 1983) do not
report technical difficulties. Gutmann and Hanzlikova
(1966) cut only the dorsal roots, but did not discuss
whether this caused the desired degeneration of afferent
fibers in the peripheral nerve.
The branching pattern of the sciatic neve in several
aspects differed from that found by Greene (19631, although she used Wistar rats as well. According to Greene
(19631, the tibial and peroneal portions of the nerve are
separatable up to their origin from the plexus; in the
rats used for the present study, and also in those investigated by Jenq and Coggeshall(1985), the sciatic nerve
was unifascicular at the trochanter. Greene (1963) reports that the sural nerve originates from the peroneal
nerve; in the present rats it unequivocally and constantly originated from the tibial nerve. Greene (1963)
described and depicted a prominent articular branch to
the knee originating from the peroneal nerve a t the
trochanter and running along the inner surface of the
biceps femoris muscle; this nerve was not found in the
present study. The “cutaneous branch” is not described
by Greene (1963).
ACKNOWLEDGMENTS
This work was supported by grants from the Danish
Medical Research Council and the Foundation of Experimental Research in Neurology.
I wish to thank Mrs. M. Bjaerg for unfailing assistance, and Dr. E. Richter for showing me his technique
of sympathectomy.
LITERATURE CITED
Betz, W.J., J.H. Caldwell, and R.R. Ribchester (1979) The size of motor
units during post-natal development of rat lumbrical muscle. J.
Physiol. Gond.), 297:463-478.
Boyd, LA., and M.R. Davey (1966)The composition of peripheral nerves.
In: Control and Innervation of Skeletal Muscle. B.L. Andrew, ed.
University of St. Andrews, Dundee, pp. 35-52.
Boyd, I.A., and M.R. Davey (1968) Composition of peripheral nerves.
E. and S. Livingstone, Edinburgh and London.
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.
Coggeshall, R.E., J.D. Coulter, and W.D. Willis, Jr. (1974) Unmvelinated axons in the ventral roots of the cat lumbosacral enlargement.
J. Comp. Neurol., 153:39-58.
FIBER COMPOSITION OF THE RAT SCIATIC NERVE
Coggeshall, R.E., C.W. Maynard, and L.A. Langford (1980) Unmyelinated sensory and preganglionic fibers in rat L6 and S1 ventral
spinal roots. J. Comp. Neurol., 193:41-47.
Eisen, A., G. Karpati, S. Carpenter, and J. Danon (1974) The motor
unit profile of the rat soleus in experimental myopathy and reinnervation. Neurology (Minneap.),24:878-884.
Friede, R.L., and T.Samorajski (1967) Relation between the number of
myelin lamellae and axon circumference in fibers of vagus and
sciatic nerves of mice. J. Comp. Neurol., 130:223-232.
Greene, E.C. (1963) Anatomy of the Rat. Hafner, New York and London.
Gutmann, E., and V. Hanzlikova (1966)Motor unit in old age. Nature,
209:921-922.
Hulsebosch, C.E., and R.E. Coggeshall (1983) A comparison of axonal
numbers in dorsal roots following spinal cord hemisection in neonate and adult rats. Brain Res., 265:187-197.
Jenq, C.-B., and R.E. Coggeshall (1984) Effects of sciatic nerve regeneration on axonal populations in tributary nerves. Brain Res.,
29591-100.
Jenq, C.-B., and R.E. Coggeshall(1985)Numbers of regenerating axons
in parent and tributary peripheral nerves in the rat. Brain Res.,
326:27-40.
Mayhew, T.M., and A.K. Sharma (1984a) Sampling schemes for estimating nerve fibre size. I. Methods for nerve trunks of mixed
81
fascicularity. J. Anat., 139:45-58.
Mayhew, T.M., and A.K. Sharma (1984b) Sampling schemes for estimating nerve fibre size. 11. Methods for unifascicular nerve trunks.
J. Anat., 139:59-66.
Peyronnard, J.-M., and L. Charron (1982) Motor and sensory neurons
of the rat sural nerve: A horseradish peroxidase study. Muscle
Nerve, 5:654-660.
Peyronnard, J.M., and L. Charron (1983) Motoneuronal and motor
axonal innervation in the rat hindlimb: A comparative study using
horseradish peroxidase. Exp. Brain Res., 50:125-132.
Rao, R.S., and G. Krinke (1983) Changes with age in the number and
size of myelinated axons in the rat L, dorsal spinal root. Acta
Anat., 117r187-192.
Schmalbruch, H. (1984) Motoneuron death after sciatic nerve section
in newborn rats. J. Comp. Neurol., 224.252-258.
Suh, Y.S., K. Chung, and R.E. Coggeshall (1984) A study of axonal
diameters and areas in lumbosacral roots and nerves in the rat. J.
Comp. Neurol., 222473-481.
Wray, S.H. (1969) Innervation ratios for large and small limb muscles
in the baboon. J. Comp. Neurol., 137:227-250.
Zelena, J., and P. Hnik (1963) Motor and receptor units in the soleus
muscle after regeneration in very young rats. 12:277-290.
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