The Motor Output and Behavior Produced by Rhythmogenic
Sacrocaudal Networks in Spinal Cords of Neonatal Rats
I. DELVOLVÉ, H. GABBAY, AND A. LEV-TOV
Department of Anatomy and Cell Biology, The Hebrew University Medical School, Jerusalem 91120, Israel
Received 6 September 2000; accepted in final form 11 January 2001
Kiehn 1996; Kremer and Lev-Tov 1997; Tresch and Kiehn
1999) and that it decreases gradually in the rostrocaudal direction (Cowley and Schmidt 1997; Kjaerulff and Kiehn 1996;
Kremer and Lev-Tov 1997; Tresch and Kiehn 1999). In our
recent studies, we showed that the rhythmogenic capacity of
the mammalian spinal cord is not restricted to its caudalthoracic and the limb moving segments and that activation of
sacrocaudal afferents (SCA) produced alternating left-right
efferent bursts in lumbosacral efferents and rhythmic tail
movements. The rhythm persisted in the sacral cord after
surgical removal of the lumbo-thoracic and cervical segments
of the cord (Lev-Tov et al. 2000b). More recent studies showed
that the rhythmogenic capacity of the sacral cord could be
activated also by bath-applied N-methyl-D-aspartate (NMDA)
and serotonin (5HT) (Cazalets and Bertrand 2000; Lev-Tov
and Delvolvé 2001). This rhythm, however, is not as regular as
the SCA-induced rhythm, and additional or different neurochemicals are often required to sustain it (Lev-Tov and Delvolvé 2001). The present study was aimed at establishing the
optimal conditions required for inducing drug-free activation
of the sacrocaudal rhythm and at testing the dynamic range of
the rhythm as a function of the stimulation parameters. The
study was also aimed at understanding the activity pattern of
the principal motoneuron pools of the sacrocaudal region during the rhythm and its relevance to the organization of the
sacrocaudal circuitry. This aim was obtained by studying the
movements produced by SCA stimulation and the pattern of
muscular activity underlying these movements. Some of the
preliminary findings appeared in an abstract (Lev-Tov et al.
2000a) and in a mini-review (Lev-Tov and Delvolvé 2001).
METHODS
INTRODUCTION
The isolated spinal cord of the neonatal rat is a frequently
used model of mammalian central pattern generation. A regular
locomotor-like rhythm is produced in this preparation by bath
application of various neurochemicals (Cazalets et al. 1992;
Cowley and Schmidt 1994; Kjaerulff and Kiehn 1996; Kremer
and Lev-Tov 1997; Kudo and Yamada 1987; Smith et al.
1988). Studies of the neonatal rat spinal cord showed that the
rhythmogenic capacity associated with hindlimb locomotion is
maximal at the caudal-thoracic and rostral-lumbar segments
(Cazalets et al. 1995; Cowley and Schmidt 1997; Kjaerulff and
Address for reprint requests: A. Lev-Tov, Dept. of Anatomy and Cell
Biology, The Hebrew University Medical School, P.O. Box 12272, Jerusalem
91120, Israel (E-mail: [email protected]).
2100
Preparations
Spinal cord preparations were isolated from P4 –P6 ether-anesthetized rats with or without an intact tail (Lev-Tov et al. 2000b). The
cord was transferred to a recording chamber and superfused continuously with an oxygenated Krebs saline (e.g. Kremer and Lev-Tov
1997; Lev-Tov et al. 2000b).
Stimulation and recordings
Slow ventral root potentials (VRPs) were recorded by suction
electrodes from pairs of sacral ventral roots at 0.1 Hz to 10 kHz using
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0022-3077/01 $5.00 Copyright © 2001 The American Physiological Society
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Delvolvé, I., H. Gabbay, and A. Lev-Tov. The motor output and
behavior produced by rhythmogenic sacrocaudal networks in spinal
cords of neonatal rats. J Neurophysiol 85: 2100 –2110, 2001. The
characteristics of the rhythmic motor output and behavior produced by
intrinsic sacrocaudal networks were studied in isolated tail-spinal cord
preparations of neonatal rats. An alternating left-right rhythm could be
induced in the sacral cord by stimulus trains applied to sacrocaudal
afferents at various intensities. Strengthening the stimulation intensity
enhanced the rhythmic efferent firing and accelerated the rhythm by
ⱕ30%. High stimulation intensities induced tonic excitation or inhibition and thereby perturbed the rhythm. Increasing the stimulation
frequency from 1 to 10 Hz decreased the cycle time of the rhythm by
36%. The rhythm was blocked during prolonged afferent stimulation
but could be restored by stimulation of contralateral afferents. Sacrocaudal afferent activation produced ventroflexion accompanied by
either low- or high-amplitude rhythmic abduction of the tail. The
low-amplitude abductions were produced by alternating flexor bursts
during long stimulus trains. The activity of abductors and extensors
was substantially reduced during these trains, their recruitment lagged
after that of the flexors, and their activity bursts were much shorter. It
is suggested that tail extensor/abductor motoneurons were suppressed
during the stimulus train by inhibitory afferent projections. The highamplitude abductions appeared after cessation of stimulus trains.
Alternating left-right activation of the tail muscles, and coactivation of
the principal muscles on each side of the tail were observed during
these abductions. It is suggested that flexors and extensors assist the
abductors to produce the high-amplitude abductions. This suggestion
is supported by the finding that tail abduction could be produced by
direct unilateral stimulation of any of the principal tail muscles. The
relevance of the findings described in the preceding text to the use of
regional sacral circuits in generation of stereotypic motor behaviors
and to future studies of rhythmogenic sacrocaudal networks is
discussed.
AFFERENT-INDUCED RHYTHMS IN NEONATAL RAT SPINAL CORDS
a high-gain AC amplifier. Sharp electrode intracellular recordings
were obtained from S2–S3 motoneurons impaled from the ventral or
ventrolateral aspect of the cord and identified by the presence of
antidromic spikes. Microwire electromyographic (EMG) recordings
(100 Hz to 10 kHz) were obtained from the flexor caudae longus
(FCL), extensor caudae lateralis (ECL), and abductor caudae dorsalis
(ACD). Muscle stimulation was obtained by constant-current squarepulses applied to the EMG microwires. Rhythmic activity was obtained by repetitive stimulation of SCA. The threshold (T) was measured at the beginning of each experiment from short-latency
(monosynaptic) responses induced by single-pulse stimulation in the
homologous or one of the adjacent ventral roots.
Video recordings and analyses
Statistical analysis
Three characteristics of the rhythm were measured and tested: the
cycle time, burst duration, and phase (with respect to a desired
reference). The frequency of motoneuron firing during bursts was
measured in some of the intracellular studies. The cycle time, burst
duration, and firing frequency were analyzed by linear statistics while
the phase data were analyzed by circular statistics. Because the
duration of regular rhythmic activity induced by afferent stimulation
is limited, there was a need to repeat identical stimulus trains a
number of times during each experiment. The resultant data were
pooled only if one-way ANOVA (linear data) or Watson and Williams
test (circular data) revealed no significant differences between the data
samples. Similar approach was used to pool data samples from different experiments in a given series.
ANOVA followed by Tukey method for multiple comparisons or
by Tumhane method (when nonequal variance was detected by Bartlett’s test) was used to compare means of cycle time, burst duration,
and firing frequency of different stimulation frequencies or intensities.
The mean phase and the vector r that describes the concentration of
phase values around the mean were calculated from the raw phase
values. Rayleigh’s test was used to determine whether the phase
values are uniformly distributed around the circle. One-sample test for
the mean angle (Zar 1984) was applied to determine whether the onset
of activity of ipsilateral abductors and extensors during the rhythm is
different from that of the flexors (used as the 0-cycle reference).
Multi-sample testing was performed to compare the mean phase
values of any pair of tested factors (the Watson-Williams test) (Zar
1984).
RESULTS
Induction and maintenance of sacrocaudal rhythmicity by
afferent stimulation
Although the rhythmogenic sacrocaudal networks could be
activated by drug-applied neurochemicals (Cazalets and Bertrand 2000; Lev-Tov and Delvolvé 2001), controlled drug-free
activation of these networks is obtained by mechanical or
electrical stimulation of SCA (Lev-Tov et al. 2000b). Figure
1A shows rhythmic activity recorded from S2 ventral roots
during a 50-pulse 1-Hz stimulus train applied to the right S4
dorsal root. The cycle time of the rhythm was fairly constant,
while the burst duration increased with time and then reached
a plateau (Fig. 1A, bottom). Analysis of the phase difference
between the left and right efferent bursts revealed a mean phase
shift of 0.51 ⫾ 0.06 (mean ⫾ circular SD), n ⫽ 22 cycles. The
phase concentration vector (r) was 0.94.
In some preparations, the cycle time of the rhythm was
slowed down during the train and the firing became less intense
toward the end of the train (Fig. 1B). The phase relation
remained relatively stable under these conditions (0.5 ⫾ 0.08
cycle, r-vector ⫽ 0.87, n ⫽ 33 cycles in 2 identical stimulus
trains). To determine whether the changes in cycle time and
burst duration reflect the beginning of a possible breakdown of
the rhythm, we prolonged the stimulus train and monitored the
changes in the sacral efferent activity. Figure 2A shows that
stimulation of the right S4/Ca1 dorsal roots at 1 Hz and 1.9 T
produced a regular left-right alternating rhythm (left). Stimulation of the left S4/Ca1 dorsal roots at 1 Hz and 2T produced
a similar rhythm (right). When the stimulus train applied to
right S4/Ca1 was prolonged (Fig. 2B), the rhythm was replaced
by bilateral tonic activity, then long-duration intermittent
rhythmic bursts could be detected (B1), and finally the efferent
activity was virtually blocked (B2). Stimulation of the left
S4/Ca1 dorsal roots at this stage (Fig. 2B3, 3) restored the
rhythm. Thus the suppression of the rhythm during the train
does not reflect an impairment of the rhythmogenic circuitry
but rather a failure of the stimulated afferents to sustain a
steady rhythmic drive.
Modulation of sacrocaudal rhythmicity by the intensity of
afferent stimulation
Our previous intracellular studies of sacral motoneurons
showed that stimulation of sacral afferents induced slow
phasic membrane potential oscillations superimposed on a
progressive depolarizing drive (Lev-Tov et al. 2000b). Figure 3, A and B, shows intracellular recordings from a sacral
motoneuron and VRP recordings from the same segment
during rhythmic activity induced by 50-pulse, 4-Hz trains at
different stimulation intensities. Stimulation of the afferents
at 1 T failed to produce the rhythm (not shown). Stimulation
at 1.2 T (Fig. 3A) produced eight activity cycles superimposed on a 15-mV depolarization. Bursts of low-frequency
spikes (mean firing frequency per burst: 11.6 –16.8 Hz) were
elicited at the peaks of the last six cycles. The concurrently
recorded VRPs exhibited rhythmic cycles with marginal
spiking activity. Changing the stimulation intensity from 1.2
to 6 T (Fig. 3B) increased the depolarizing drive to 30 mV,
elevated the firing rate during the oscillatory peaks to means
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Tail movements in the mediolateral direction were monitored by a
frontally positioned video camera. Movements in the dorsoventral
plane were reflected by a mirror and monitored concurrently. EMGs
produced in two different pairs of tail muscles (left and right flexors
and abductors or flexors and extensors) during the video-monitored
movements were recorded using a pulse code modulated (PCM)
recorder. The activity of the left flexor was recorded also on the audio
track of the video camera and used as a timing mark to match the
video-recorded movements and the PCM-recorded EMGs. Video clips
of tail movements were stored on a computer hard disk at 25 fps using
a frame grabber, and the movements were analyzed from the respective video frames. Consecutive stick diagrams (see Figs. 5 and 6)
constructed from four digitized reference points along the tail (base,
proximal, distal, and tip) describe movements in the mediolateral and
dorsoventral planes [after correcting the distortions (angle, direction)
of the reflected images]. To allow better understanding of the temporal
relation between the activity of the tail musculature and corresponding
movements in the mediolateral plane, the data digitized from the video
frames were displayed as a function of time and expressed as displacements from the midline.
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I. DELVOLVÉ, H. GABBAY, AND A. LEV-TOV
of 28 –56 Hz per burst, and significantly decreased (t-test,
P ⬍ 0.01) the cycle duration from 1.58 ⫾ 0.26, n ⫽ 8, to
1.08 ⫾ 0.22, n ⫽ 11. The rhythmic efferent firing recorded
from the ventral root was augmented substantially under
these conditions.
In this and other experiments, a limited range of stimulation
intensities could induce regular rhythmicity. The minimal stimulus intensity required to produce the rhythm at any given
stimulation frequency was found to be higher than the measured threshold (T, see METHODS). This intensity, the “rhythmic
threshold” (RT), was inversely related to the stimulation fre-
quency. Above a given intensity level, dorsal root stimulation
induced tonic excitation or inhibition and thereby perturbed the
rhythmicity (not shown).
Quantitative analyses of the effects of stimulation intensity on the rhythm were performed on data from eight
different experiments. The stimulation-intensity range for
evoking rhythmic activity during 50-pulse, 5-Hz stimulus
trains was determined for each experiment. These intensities
were expressed with respect to the 5-Hz RT. Figure 3C
shows that the cycle duration was inversely related to the
stimulation intensity. The decrease in the cycle time was
FIG. 2. Prolonged activation of sacrocaudal
networks. A: rhythmic activity elicited by 50pulse 1-Hz stimulus trains of the right S4–Ca1
dorsal roots at 1.9 T (Right SCA, left) and the
left S4–Ca1 at 2 T (Left SCA, right) was recorded from the left and right (L and R) S2
ventral roots at 0.1 Hz to 10 kHz. B: sample
recordings from the same preparation taken during a prolonged repetitive stimulus train of the
right S4–Ca1 dorsal root at 1 Hz, 113, 211, and
297 s after the beginning of the train (1, 2, and
3, respectively). 3 in 3, the time at which the
stimulation of the ipsilateral dorsal root was
stopped and stimulation of the contralateral S4–
Ca1 dorsal root (left SCA) started.
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FIG. 1. The regularity of the sacrocaudal
rhythm. Recordings (50 Hz to 10 kHz) of
rhythmic activity from the left and right ventral roots of S2 in 2 different preparations (A
and B) during a 50-pulse, 1-Hz stimulus train
applied to the right S4 dorsal root at 1.6 T (A)
and at 2 T (B). Raw data of the cycle time
and burst duration during this train are plotted as a function of time below (A and B,
bottom left). The circular distribution of the
raw phase values measured during the train
(single train in A and 2 identical trains in B)
are shown superimposed with the r vector
describing the concentration of the phase
values around the mean (A and B, bottom
right).
AFFERENT-INDUCED RHYTHMS IN NEONATAL RAT SPINAL CORDS
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statistically significant: 1.5 RT ⬍ 1–1.25 RT ⬎ 2–2.5 RT
(ANOVA followed by Tamhane method for multiple comparisons, P ⬍ 0.01).
Modulation of sacrocaudal rhythmicity by the frequency of
afferent stimulation
Rhythmic activity could be produced by single stimuli
and by stimulus trains applied at various frequencies. Because the rhythm produced by stimulation frequencies ⬎10
Hz was often perturbed by tonic excitation or inhibition, the
effects of frequency on the rhythm were tested over a
frequency range of 1–10 Hz, and the stimulus intensity was
adjusted to produce regular rhythmic activity over this
range. Figure 4A shows intracellular recordings from an S2
motoneuron and VRP recordings from the same segment
during and following 30-pulse stimulus trains applied to
S4–Ca1 dorsal roots at 1 and 5 Hz. Regular rhythmic activity
developed gradually during the 1-Hz train. The rhythm was
superimposed on a slow rising (mean slope ⫽ 1.01 mV/s)
depolarization, reaching 19 mV at the end of the train. The
mean firing frequency per burst varied from 9 to 24 Hz.
During the 5-Hz train, the depolarizing drive developed
much faster (4.2 mV/s), reaching 14.1 mV at the end of the
train, and the regular rhythmic activity was obtained from
the beginning of the train. The mean firing frequency per
burst was 11–21 Hz. The mean cycle time was 1.51 ⫾ 0.3,
n ⫽ 15 and 1.31 ⫾ 0.21, n ⫽ 5 at 1 and 5 Hz, respectively.
The VRPs recorded from the S2 ventral roots during 50pulse stimulus trains applied at 1, 5, and 10 Hz to the right S4
dorsal root in another experiment are shown in Fig. 4B. The
cycle duration (superimposed rectangles) decreased as the
stimulus frequency was increased from 1 to 5 or 10 Hz. Figure
4C shows the results of quantitative analyses of five experiments performed in this series. The mean normalized cycle
periods decreased significantly with the increased frequency of
stimulation (10 Hz ⬍ 1–2 Hz ⬎ 4 –5 Hz; 4 –5 Hz ⬎ 10 Hz,
ANOVA followed by Tukey method for multiple comparisons,
P ⬍ 0.01).
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FIG. 3. Effects of the intensity of afferent
stimulation on the sacrocaudal rhythm. Intracellular recordings from a motoneuron in the right
S2 segment (R-S2 MN) are superimposed with
extracellular recordings of ventral root potentials
(VRPs, 0.1 Hz to 10 kHz) from the left and right
(L, R) S2 ventral roots. Rhythmic activity was
induced by 50-pulse, 4-Hz stimulus trains at 1.2
and 6 T in A and B, respectively. The resting
membrane potential before the train ( 䡠 䡠 䡠 ) was
⫺77 mV. Effects of stimulation intensity on the
cycle time of the rhythm are shown in C. The
rhythmic threshold of the afferents (RT) was
determined for each preparation during 50-pulse
5-Hz trains, and the tested stimulation intensities
were normalized by these values. Data obtained
on stimulation intensities of 1–1.25, 1.5, and
2–2.5 RT in 8 different experiments were pooled
(METHODS), and the respective means of cycle
duration ⫾ SD were plotted as a function of
intensity. The mean cycle duration was: 1.57 ⫾
0.45, 1.42 ⫾ 0.34, and 1.16 ⫾ 0.4 s, for 1–1.25,
1.5, and 2–2.5 RT, respectively.
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The same was true for the duration of the efferent bursts
(Fig. 4 legend). Significant shortening of the normalized
burst duration was observed with increasing stimulation
frequency: 10 ⬍ 1–2 ⬎ 4 –5 Hz; 4 –5 ⬎ 10 Hz (1-way
ANOVA followed by Tukey method for multiple comparisons, P ⬍ 0.01). The phase lag between the left and right
efferent bursts was found to be invariable on the different
intensities and frequencies of afferent stimulation tested in
our experiments.
Activity of the tail musculature and the movements produced
following stimulation of SCA
Figure 5A shows stick diagrams describing the mediolateral
tail movements during one of the activity cycles produced by
mechanical stimulation of the mid-tail region. The tail (ventral
side up) moved first from midline to left (I), then all the way to
right (II) and then back to midline (III). Figure 5B shows the
mediolateral rhythmic movement of the tip of the tail as a
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FIG. 4. Effects of the frequency of afferent
stimulation on the sacrocaudal rhythm. A: intracellular recordings from a right S2 motoneuron
(R-S2 MN) and VRPs recorded from the left and
right S2 ventral roots are shown during and following 30-pulse, 2.4-T stimulus trains applied to
the right S4–Ca1 dorsal root at 1 and 5 Hz. The
membrane potential before the train ( 䡠 䡠 䡠 ) was
⫺62 mV in both cases. Note initial time locked
bursts (at 1 and 5 Hz) followed by slow appearance of the rhythm at 1 Hz and a much faster
development of cyclic activity at 5 Hz. Also note
the progressive reduction in spike amplitude with
the increasing depolarizing drive and the gradual
recovery with the posttetanic decay of the drive.
B: samples of efferent bursts recorded at 50 Hz to
10 kHz, from the left and right (L and R) S2
ventral roots during 50-pulse trains applied to the
left and right Ca2–Ca3 dorsal root at 1, 5, and 10
Hz at 1.2 T. The superimposed frames denote a
single activity cycle. C: the mean normalized cycle duration obtained in 5 different experiments is
plotted as a function of the stimulation frequency.
The cycle times obtained during 50-pulse stimulus
trains applied at 1, 2, 4, 5, and 10 Hz in each
experiments were normalized by the mean cycle
time obtained at 1 Hz (that way, the effect of
variations in stimulation intensity in different experiments on the frequency response, was factored
out). Data were compared by 1-way ANOVA and
consequently pooled into 3 groups: 1–2, 4 –5, and
10 Hz. Cases in which the cycle time was slowed
down substantially during the train were omitted.
The mean cycle time was: 1 ⫾ 0.21, 0.81 ⫾ 0.25,
and 0.64 ⫾ 0.53 for 1- to 2-, 4- to 5-, and 10-Hz
trains, respectively. The mean normalized burst
duration in these experiments was: 1.03 ⫾ 0.36,
n ⫽ 155; 0.88 ⫾ 0.27, n ⫽ 117; and 0.7 ⫾ 0.26,
n ⫽ 64 cycles for 1–2, 4 –5, and 10 Hz, respectively.
AFFERENT-INDUCED RHYTHMS IN NEONATAL RAT SPINAL CORDS
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FIG. 5. Tail movements and the activity of the tail musculature following sacrocaudal afferents (SCA) activation. A: stick
diagrams describing 1 of 5 activity cycles of mediolateral movements produced by tail pinching (see METHODS). The stick
diagrams are divided into 3 consecutive parts (I, II, and III) for clarity. The changes in the x-y coordinates are expressed as
percentage of tail length (26.5 mm). B: stick diagrams describing the tail movements in the dorsoventral plane following the
tail pinch described in A are superimposed with the time domain display of the concurrently monitored lateral movements of
the tip of the tail. A bar and 2 arrows denote the time of occurrence of the movements described by the 1st 9 consecutive
stick diagrams. The other stick diagrams represent the dorsoventral position of the tail at different poststimulus times
(arrows). The heavy dotted segment of the time domain display denotes the cycle described in A. The peaks and troughs in
the time domain display of the abductions correspond to maximal right and left abduction, respectively. C: electromyographic
(EMG) recordings of right (R) and left (L) tail flexors and abductors are superimposed with 2 identical traces describing the
resultant abductions of the proximal tail. Based on simultaneous video and EMG recordings. The dotted reference lines in
B and C denote the midline (⌬x ⫽ 0). D: bar diagrams of the activity of the principal tail muscles following 10-pulse 10-Hz
stimulus trains of the S4 dorsal roots. The bars denote the mean burst duration (from the mean onset to offset of the EMG
bursts) of each muscle normalized by the cycle period of the flexor caudae longus (FCL). The circular standard deviations
of the onset and offset of the EMG bursts are superimposed. Data were pooled from 6 different experiments. Ipsi- and
contralateral refer to the side of stimulated afferents (either left or right S4 dorsal roots in this case). The r vector and the
number of cycles were ipsilateral-onset: flexors (n ⫽ 134), extensors (r ⫽ 0.89, n ⫽ 24) and abductors (r ⫽ 0.94, n ⫽ 84);
ipsilateral offset: flexors (r ⫽ 0.84, n ⫽ 131), extensors (r ⫽ 0.84, n ⫽ 18), abductors (r ⫽ 0.88, n ⫽ 82); contralateral onset:
flexors (r ⫽ 0.88, n ⫽ 131), extensors (r ⫽ 0.85, n ⫽ 16), and abductors (r ⫽ 0.85, n ⫽ 85); contralateral offset: flexors (r ⫽
0.8, n ⫽ 126); extensors (r ⫽ 0.63, n ⫽ 13) and abductors (r ⫽ 0.84, n ⫽ 85).
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function of time. Five constant-amplitude cycles are clearly
detectable. The stimulus also produced a prominent ventroflexion of the tail. The concurrent positions of the tail in the
dorsoventral plane are displayed as stick diagrams on top (Fig.
5B, arrows denote the time of occurrence). After the stimulus
(the 1st 360 ms), the tail was elevated gradually, and curled
ventrally to reach a steady ventroflexed position (1st sequence
of stick diagrams). This ventroflexion was sustained nearly
throughout the rhythmic episodes, and the tail was lowered
back to its prestimulus position just before cessation of the
rhythm (right-most stick diagram).
Movements induced after short stimulus trains (5–10 pulse,
10 Hz) were similar to those described in the preceding text
(ventroflexion accompanied by alternating abductions). The
relation between the activity of the tail musculature and the
movements following mechanical stimulation of the tail is
shown in Fig. 5C. Coactivation of the left flexors and abductors
was observed during left abduction, and coactivation of the
right flexors and abductors was evident as the tail moved all the
way to the right.
The temporal relation between the activities of tail flexors
extensors and abductors following short stimulus trains (data
were pooled from 6 different experiments) is demonstrated in
Fig. 5D. The bars represent the average burst duration normalized by the cycle time of the tail flexors. This diagram shows
that in addition to the longer duration of the flexor bursts, the
flexors were recruited significantly (P ⬍ 0.01) before the
abductors, and nonsignificantly (P ⬎ 0.05) after the extensors
(Rayleigh’s 1-sample test for the mean angle).
Activity of the tail musculature and the movements produced
during SCA stimulation
Figure 6A shows that during the stimulus train (rectangle),
the central part of the tail moved laterally to the left and right
while the tip and base of the tail remained virtually immobile
(stick diagrams on the left). These low-amplitude excursions
were gradually attenuated toward the end of the stimulus train
(see the time domain display). High-amplitude abductions appeared immediately after the stimulus train, (stick diagrams on
the right, and the superimposed time domain display). These
latter movements resembled those produced following mechanical and short-train activation of SCA (e.g., Fig. 5). The
rhythmic movements during and following the long stimulus
train were superimposed on a sustained ventroflexion (not
shown). This ventroflexion developed gradually during the
train, reaching a maximal level toward the end of the train and
declining in amplitude after the train as the high-amplitude
abductions began. The tail returned to its initial horizontal
position on termination of the rhythmic movements (not
shown).
Simultaneous EMG and video recordings showed leftright activation of flexors and a suppressed activity of
abductors during the train, followed by a strong coactivation
of flexors and abductors at a given side of the tail, after the
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FIG. 6. Tail movements produced during
long stimulus trains of SCA and the muscle
activities associated with them. A: lateral
movements of the 4 digitized regions of the
tail are shown as a function of time during
(frame) and following 50-pulse, 10-Hz train
applied to the right S4 dorsal root of a tail
spinal cord preparation. Two sets of stick
diagrams corresponding to activity cycles
sampled during and after the train (heavy
dots) are superimposed above. Each of these
sets is also shown subdivided into 3 consecutive groups for clarity. Peaks and troughs in
the time domain display are maximal right
and left abduction, respectively. B: EMG
recordings of the left (L) and right (R) tail
flexors and abductors are shown superimposed with 2 identical traces describing the
concurrently monitored abductions of the
“proximal” tail. The superimposed frame denotes the stimulus train.
AFFERENT-INDUCED RHYTHMS IN NEONATAL RAT SPINAL CORDS
2107
stimulus train (Fig. 6B). Thus the low-amplitude abductions
during the train were produced by alternating left-right
activation of flexors, while the high-amplitude abductions
after the train involved coactivation of flexors and abductors
on a given side of the tail, and alternating left-right activation of these muscles (Fig. 6B). Interestingly, the suppression of the activity of abductors/extensors during the train
was enhanced with the intensity of afferent stimulation. The
left-right alternating flexor activity persisted and was re-
placed by bilateral tonic bursts of flexors only at very high
stimulation intensities (not shown).
Analyses of EMGs recorded in six experiments during the
stimulus trains (Fig. 7A) indeed showed that the majority of the
flexor bursts during the stimulus trains (58%) was not accompanied by abductor bursts. Extensor bursts were totally abolished in 43% of the cases where flexor bursts were clearly
detected. The duration of abductor and extensor bursts during
the train was significantly shorter than that of the flexor bursts
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FIG. 7. The activity patterns of the tail muscles during SCA stimulation. A: frequency histograms of the duration of flexor,
abductor, and extensor bursts produced during 50-pulse 10-Hz stimulus trains in 6 different experiments. F, the number of failures
to induce EMG bursts (burst duration ⫽ 0). The mean duration of the activity bursts was 0.51 ⫾ 0.26, 0.25 ⫾ 0.15, and 0.23 ⫾
0.09 s for flexor, abductor, and extensor, respectively. B: bar diagrams of the activity of the principal tail muscles during 50-pulse
10-Hz stimulus trains of the S4 dorsal roots. Means shown with circular SD were pooled from 6 different experiments (METHODS).
The phase values were normalized by the cycle of the flexor caudae longus. Ipsi- and contralateral refer to the side of stimulated
afferents (either left or right S4 dorsal roots in this case). The r vector and the number of cycles were ipsilateral-onset: flexors (n ⫽
155), extensors (r ⫽ 0.75, n ⫽ 29), and abductors (r ⫽ 0.81, n ⫽ 44); ipsilateral-offset: flexors (r ⫽ 0.66, n ⫽ 150), extensors
(r ⫽ 0.76, n ⫽ 26), and abductors (r ⫽ 0.88, n ⫽ 44); contralateral-onset: flexors (r ⫽ 0.81, n ⫽ 113), extensors (r ⫽ 0.78, n ⫽
16), and abductors (r ⫽ 0.71, n ⫽ 41); contralateral-offset: flexors (r ⫽ 0.73, n ⫽ 109), extensors (r ⫽ 0.79, n ⫽ 12), and abductors
(r ⫽ 0.84, n ⫽ 41). C: contribution of the principal tail muscles to the mediolateral tail movements. Abductions of the tip of the
tail produced by alternating stimulus trains (10 pulse, 10 Hz) applied to the left (light bars) and right (heavy bars) abductor caudae
dorsalis (abductors), extensor caudae lateralis (extensors), and flexor caudae brevis (flexors). Data were digitized from video frames
sampled at 12.5 fps. The stimulus train to the right muscle (r-stim. bar) was applied as the stimulus train to the left muscle (l-stim.
bar) was ended. The repetition rate was 0.5 Hz. The 2 sets of bars superimposed on the abductor data demonstrate the stimulation
paradigm (L-stim. and R-stim. denote stimulation of the left and right muscle respectively). The activity of the unstimulated
muscles was monitored during the experiments and the stimulation intensity was adjusted to exclude nonspecific muscle activation
by current spread from the stimulation electrodes.
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I. DELVOLVÉ, H. GABBAY, AND A. LEV-TOV
(ANOVA/Tukey, P ⬍ 0.001). The pronounced differences
between the duration of the flexor bursts and that of the
abductor and extensor are also evident in the bar diagrams in
Fig. 7B. The bar diagrams also show that the onset of flexor
activity was followed by a delayed onset of extensor and
abductor activity. These delays were statistically significant
(Rayleigh’s 1-sample test for the mean angle, P ⬍ 0.01). These
findings suggest that flexors were recruited before the extensors and abductors during the stimulus trains and that some of
the stimulated afferents are capable of diminishing the abductor and extensor activity under these conditions.
Direct unilateral stimulation of the tail muscles produces
abduction of the tail
DISCUSSION
In the first part of this work, we studied the dynamic range
of SCA-induced rhythms. We found that an alternating leftright rhythm could be induced in sacral efferents by activation
of SCA and that increasing the stimulation intensity or frequency accelerated the rhythm by 30 – 40%. High stimulation
intensities and rates perturbed the rhythm. The rhythm was
blocked during prolonged (usually ⬎50 s) stimulus trains of
SCA but could be restored by stimulation of contralateral
afferents.
In the second part of the study, we studied the tail movements produced by SCA and the underlying activity pattern of
the tail muscles. Our main findings were that SCA activation
produced ventroflexion followed by rhythmic abductions of the
tail. Low-amplitude abductions appeared during stimulus trains
and high-amplitude abductions appeared after the trains. During stimulus trains the activity of abductors/extensors was
suppressed and the activity of flexors was not changed. After
stimulus trains, we observed an alternating left-right activation
of the tail muscles, and coactivation of the principal muscles on
each side of the tail.
The findings described in the preceding text and their relevance to neural control of automatic movements are discussed
in the following text.
Drug-free rhythmicity in the mammalian spinal cord,
boundary conditions, and dynamic range
Sensory-induced rhythmogenesis has been reported in a number of preparations including lampreys (McClellan 1984), amphibian tadpoles (Soffe 1991), cats (Grillner and Zangger 1979),
and neonatal rats (Smith et al. 1988). Moreover, stimulation of the
skin of the perineal region (including the base of the tail) has also
reported to initiate treadmill locomotion in spinal cats (Pearson
Movements induced by SCA stimulation and the muscles
involved in their generation
What are the movements produced by SCA stimulation?
How are these movements produced? The sacrocaudal rat
spinal cord controls the movements executed by the tail muscles (Brink and Pfaff 1980; Grossman et al. 1982; Masson et al.
1991). Our studies revealed that SCA stimulation produced
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Because flexor activity was sufficient of producing rhythmic
abduction of the tail, we assessed the relative contribution of
each pair of the principal tail muscles to the abduction of the
tail by application of alternating sequences of stimulus trains
through the EMG microwires to the muscles. We first stimulated the left and right abductors, then the extensors, and finally
the left and right flexors. The lateral movements of the mid-tail
during these stimulus trains (see METHODS) are plotted as a
function of time in Fig. 7C. These data show that both flexors
and extensors are capable of producing significant abductions
of the tail.
and Rossignol 1991). The SCA-induced rhythm described in the
present study has several advantages. The induction of the rhythm
is obtained in drug-free media, the duration of the rhythmic
activity can be set by the experimenter (between 1–2 and 30 – 60
s), the stimulus trains can be repeated many times in each experiment, the resultant rhythm has a substantial drive, and it is rather
reproducible. The rhythm induced by dorsal root stimulation is
different from the rhythm induced by bath-applied drugs. First,
dorsal root stimulation activates afferent projections to central
pattern generator (CPG) neurons (reviewed in Rossignol 1996) as
well as excitatory and inhibitory afferent projections to motoneurons (e.g., Pinco and Lev-Tov 1993a for the neonatal rat; review
by Baldissera et al. 1981). Therefore the rhythm is superimposed
on tonic excitation and/or inhibition of motoneurons that is capable of perturbing the rhythm during high-intensity or -frequency
stimulus trains. Second, the rhythmic drive produced by dorsal
root stimulation is not constant. It increases at the beginning of the
stimulation, reaching a plateau-like level at a later stage, and
decreasing gradually after the trains. Therefore the cycle time and
burst duration of the rhythm varies during parts of the stimulus
trains and following these trains. Third, long stimulus train of
dorsal root afferents is capable of producing regular rhythmic
activity for tens of seconds (30 – 60 s). During longer stimulus
trains, the rhythmic drive decreases, the cycle time increases, and
the rhythm is finally blocked. Because normal rhythmicity could
be restored at that stage by stimulation of contralateral afferents,
it is suggested that the decline in the rhythmic drive and its block
reflect an inability of the stimulated afferents to sustain a steady
rhythmic drive. This inability may result from a prolonged synaptic depression similar to the depression described for neonatal
afferent pathways during high- and low-frequency stimulus trains
(Lev-Tov and Pinco 1992; Pinco and Lev-Tov 1993a,b; Seebach
and Mendell 1996). Failure of action potentials to invade afferent
terminals, branch point blockade of action potentials, and depletion of neurotransmitter stores or inactivation of presynaptic release sites (reviewed in Lev-Tov 1995, 1998) are only few of the
possible mechanisms that should be considered in this regard.
Finally, it is possible to modulate the rhythm within given
ranges of intensity, frequency and duration of sacral dorsal root
stimulation. In this way, a 2.5-fold increase in intensity and a
10-fold increase in frequency of afferent stimulation were capable
of speeding the rhythm by ⱕ30 – 40% and of changing the firing
rate during each cycle. This dynamic range is rather narrow
comparing to that observed for the neurochemically induced locomotor rhythm at different NMDA concentrations (Cazalets et
al. 1992; Kudo and Yamada 1987; Smith et al. 1988).
In summary, stimulation of sacral dorsal roots is an effective
and reliable mean to induce drug-free rhythmicity in the sacrocaudal cord of the neonatal rat. The SCA-induced rhythm
can be modulated by controlled variations in the stimulation
parameters, and its dynamic range is narrower than that of the
neurochemically induced locomotor rhythm.
AFFERENT-INDUCED RHYTHMS IN NEONATAL RAT SPINAL CORDS
Rhythmogenesis and motor output in the mammalian spinal
cord
What can we learn about the sacrocaudal circuitry from the
activity pattern of the tail muscles? Locomotion (reviewed by
Rossignol 1996); FRA-induced rhythmicity (Jankowska et al.
1967a,b; Lundberg 1979, also see: Baldissera et al. 1981;
Hultborn et al. 1998) paw shaking (Pearson and Rossignol
1991), and fictive scratching in the cat (reviewed in Gelfand et
al. 1988), and turtle (reviewed in Stein et al. 1998) are characterized by flexor-extensor alternations within a limb during
the rhythm. In contrast, as mentioned in the preceding text, the
rhythmic pattern of the sacrocaudal network exhibits mainly an
alternating left-right activation of the tail muscles and coactivation of the three principal muscles on each side of the tail
during each cycle. This pattern might imply that there is
crossed inhibition between the two sides of the sacral cord
during the rhythm and that there is no reciprocal inhibition
between ipsilateral flexors and extensor/abductor motoneurons
during the rhythm.
The notion of crossed inhibition between the sacral hemicords during the rhythm is supported by our intracellular studies of sacral motoneurons during the rhythm (Lev-Tov et al.
2000b). The issue of reciprocal inhibition between CPGs at a
given side of the sacrocaudal cord is more complicated. Inhib-
itory postsynaptic potentials (IPSPs) could be produced in
sacral motoneurons by either ipsi- or contralateral stimulation
of SCA (Lev-Tov et al. 2000b). In the present study, stimulus
trains of the ipsi or contralateral SCA produced partial or
complete suppression of the rhythmic bursts of extensors and
abductors. The rhythmic activity of flexors was not perturbed
under these conditions. The degree of extensor/abductor suppression increased with the intensity of afferent stimulation.
Therefore it is suggested that stimulation of SCA, activated
strong reciprocal and crossed inhibitory projections to tail
extensor/abductor motoneurons and thereby produced tonic
inhibition of these motoneurons. This tonic inhibition is alleviated after the stimulus trains. Further clarification of the
extent of reciprocal inhibitory projections from SCA to flexor
and extensor/abductor motoneuron pools, their tonic and possible phasic activation during the rhythm, must await additional intracellular recordings from identified flexor and extensor/abductor motoneurons. Functional identification of
motoneurons in theses recordings can be now based on the
differential suppression of abductor/extensor motoneurons during stimulus trains (e.g., Figs. 6 and 7).
In summary, the present work provided basic understanding
of the organization of the recently described sacrocaudal rhythmogenic network and the behavior produced by it. The simplicity of this network and its accessibility makes it a potential
model for studies of neural control of automatic movements in
the isolated spinal cord.
The authors thank Dr. M. J. O’Donovan for helpful comments on the
manuscript.
This work was supported by Grants 724/97 and 497/00 from the Israel
Academy for Sciences and Humanities, Jerusalem, Israel, to A. Lev-Tov.
REFERENCES
BALDISSERA F, HULTBORN H, AND ILLERT M. Integration in spinal neuronal
systems. In: Handbook of Physiology. The Nervous System. Motor Control.
Bethesda, MD: Am. Physiol. Soc., 1981, sect. 1, vol. II, part 1, p. 509 –595.
BENNETT DJ, GORASSINI M, FOUAD K, SANELLI L, HAN Y, AND CHENG J.
Spasticity in rats with sacral spinal cord injury. J Neurotrauma 16: 69 – 84,
1999.
BRINK EE AND PFAFF DW. Vertebral muscles of the back and tail of the albino
rat (Rattus norvegicus albinus). Brain Behav Evol 17: 1– 47, 1980.
CAZALETS JR AND BERTRAND S. Coupling between lumbar and sacral motor
networks in the neonatal rat spinal cord. Eur J Neurosci 12: 2993–3002,
2000.
CAZALETS JR, BORDE M, AND CLARAC F. Localization and organization of the
central pattern generator for hindlimb locomotion in newborn rat. J Neurosci
15: 4943– 4951, 1995.
CAZALETS JR, SQALLI HOUSSAINI Y, AND CLARAC F. Activation of the central
pattern generators for locomotion by serotonin and excitatory amino acids in
neonatal rat. J Physiol (Lond) 455: 187–204, 1992.
COWLEY KC AND SCHMIDT BJ. A comparison of motor patterns induced by
N-methyl-D-aspartate, acetylcholine and serotonin in the in vitro neonatal
rat spinal cord. Neurosci Lett 171: 147–150, 1994.
COWLEY KC AND SCHMIDT BJ. Regional distribution of the locomotor patterngenerating network in the neonatal rat spinal cord. J Neurophysiol 77:
247–259, 1997.
GELFAND IM, ORLOVSKY GM, AND SHIK ML. Locomotion and scratching in
tetrapodes. In: Neural Control of Rhythmic Movement in Vertebrates, edited
by Cohen AH, Rossignol S, and Grillner S. New York: Wiley, 1988, p.
167–199.
GRILLNER S AND ZANGGER P. On the central generation of locomotion in the
low spinal cat. Exp Brain Res 34: 241–261, 1979.
GROSSMAN ML, BASBAUM AI, AND FIELDS HL. Afferent and efferent connections of the rat tail flick reflex (a model used to analyze pain control
mechanisms). J Comp Neurol 206: 9 –16, 1982.
Downloaded from http://jn.physiology.org/ by 10.220.33.1 on June 17, 2017
ventroflexion of the tail accompanied by low-amplitude rhythmic abduction during stimulus trains and by high-amplitude
rhythmic abduction after the trains.
We showed that the ventroflexion of the tail during the
stimulus trains was produced by left-right alternating activation
of the flexors. The action of extensors and the abductors was
suppressed under these conditions by a presumed tonic inhibition (see following text). The ventroflexion, however, was
maintained with a slightly reduced angle after the train when
the extensors and abductors were coactivated with the flexors
at a given side of the tail. This latter finding indicates a strong
bias of the tail flexors on SCA stimulation. This flexor bias
may be ascribed to the absence of supraspinal control in the
isolated spinal cord preparation. Interestingly, flexor spasms
and prolonged tonic activity of tail flexors that could be augmented by cutaneous stimulation have also been described at
different stages following spinalization of rats at the S2 level
(Bennett et al. 1999).
What is the basis for the rhythmic abduction induced by
SCA stimulation? The high-amplitude abductions are accounted for mainly by alternating left-right activation of the
abductors. Our studies revealed, however, that the coactivated
flexors and extensors could also contribute to the rhythmic
abduction (Fig. 7C) and that during the trains when the abductors and extensors are suppressed, the alternating activity of the
flexors was sufficient to produce low-amplitude rhythmic abduction of the mid-tail (Fig. 6).
These findings imply that the alternating left-right efferent
bursts observed during high-intensity stimulus train are generated mainly by activation of flexors, while those produced
during low-intensity trains or after stimulus trains involve
coactivation of flexor, extensor, and abductor motoneurons
within a given hemicord, and alternating activation of the left
and right pools of these motoneurons. The relevance of these
patterns is discussed in the following text.
2109
2110
I. DELVOLVÉ, H. GABBAY, AND A. LEV-TOV
LEV-TOV A, DELVOLVE I, AND KREMER E. Sacrocaudal afferents induce rhythmic efferent bursting in isolated spinal cords of neonatal rats. J Neurophysiol
83: 888 – 894, 2000b.
LEV-TOV A AND PINCO M. In vitro studies of prolonged synaptic depression in
the neonatal rat spinal cord. J Physiol (Lond) 447: 149 –169, 1992.
LUNDBERG A. Multisensory control of spinal reflex pathways. Prog Brain Res
50: 11–28, 1979.
MASSON RL JR, SPARKES ML, AND RITZ LA. Descending projections to the rat
sacrocaudal spinal cord. J Comp Neurol 307: 120 –130, 1991.
MCCLELLAN AD. Descending control and sensory gating of “fictive” swimming and turning responses elicited in an in vitro preparation of the lamprey
brainstem/spinal cord. Brain Res 302: 151–162, 1984.
PEARSON KG AND ROSSIGNOL S. Fictive motor patterns in chronic spinal cats.
J Neurophysiol 66: 1874 –1887, 1991.
PINCO M AND LEV-TOV A. Synaptic excitation of alpha-motoneurons by dorsal
root afferents in the neonatal rat spinal cord. J Neurophysiol 70: 406 – 417,
1993a.
PINCO M AND LEV-TOV A. Modulation of monosynaptic excitation in the
neonatal rat spinal cord. J Neurophysiol 70: 1151–1158, 1993b.
ROSSIGNOL S. Neural control of stereotypic limb movements. In: Handbook of
Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, p. 173–216.
SEEBACH BS AND MENDELL LM. Maturation in properties of motoneurons and
their segmental input in the neonatal rat. J Neurophysiol 76: 3875–3885,
1996.
SMITH JC, FELDMAN JL, AND SCHMIDT BJ. Neural mechanisms generating
locomotion studied in mammalian brain stem-spinal cord in vitro. FASEB J
2: 2283–2288, 1988.
SOFFE SR. Triggering and gating of motor responses by sensory stimulation:
behavioural selection in Xenopus embryos. Proc R Soc Lond B Biol Sci 246:
197–203, 1991.
STEIN PS, MCCULLOUGH ML, AND CURRIE SN. Spinal motor patterns in the
turtle. Ann NY Acad Sci 860: 142–154, 1998.
TRESCH MC AND KIEHN O. Coding of locomotor phase in populations of
neurons in rostral and caudal segments of the neonatal rat lumbar spinal
cord. J Neurophysiol 82: 3563–3574, 1999.
ZAR JH. Biostatistical Analysis. Englewood Cliffs, NJ: Prentice Hall, 1984.
Downloaded from http://jn.physiology.org/ by 10.220.33.1 on June 17, 2017
HULTBORN H, CONWAY BA, GOSSARD JP, BROWNSTONE R, FEDIRCHUK B,
SCHOMBURG ED, ENRIQUEZ-DENTON M, AND PERREAULT MC. How do we
approach the locomotor network in the mammalian spinal cord? Ann NY
Acad Sci 860: 70 – 82, 1998.
JANKOWSKA E, JUKES MG, LUND S, AND LUNDBERG A. The effect of DOPA on
the spinal cord. V. Reciprocal organization of pathways transmitting excitatory action to alpha motoneurones of flexors and extensors. Acta Physiol
Scand 70: 369 –388, 1967a.
JANKOWSKA E, JUKES MG, LUND S, AND LUNDBERG A. The effect of DOPA on
the spinal cord. VI. Half-centre organization of interneurones transmitting
effects from the flexor reflex afferents. Acta Physiol Scand 70: 389 – 402,
1967b.
KJAERULFF O AND KIEHN O. Distribution of networks generating and coordinating locomotor activity in the neonatal rat spinal cord in vitro: a lesion
study. J Neuroscience 16: 5777–5794, 1996.
KJAERULFF O AND KIEHN O. Crossed rhythmic synaptic input to motoneurons
during selective activation of the contralateral spinal locomotor network.
J Neurosci 17: 9433–9447, 1997.
KREMER E AND LEV-TOV A. Localization of the spinal network associated with
generation of hindlimb locomotion in the neonatal rat and organization of its
transverse coupling system. J Neurophysiol 77: 1155–1170, 1997.
KUDO N AND YAMADA T. N-methyl-D, L-aspartate-induced locomotor activity
in a spinal cord-hindlimb muscles preparation of the newborn rat studied in
vitro. Neurosci Lett 75: 43– 48, 1987.
LEV-TOV A. Afferent, propriospinal and descending control of lumbar motoneurons in the neonatal rat. In: Alpha and Gamma Motor Systems, edited
by Taylor A. New York: Plenum, 1995, p. 23–28.
LEV-TOV A. Dynamic aspects of the presynaptic function in the developing
mammalian spinal cord. In: Presynaptic Inhibition and Motor Control,
edited by Rudomin P, Romo R, and Mendell LM. Oxford, UK: Oxford Univ.
Press, 1998, p. 271–281.
LEV-TOV A AND DELVOLVE I. Pattern generation in non limb-moving segments
of the mammalian spinal cord. In: Neural Development of Motor Behavior,
a special issue of Brain Res Bull 53: 671– 675, 2001.
LEV-TOV A, DELVOLVE I, AND GABBAY H. The motor outputs produced by sacral
pattern generators in neonatal rats. Soc Neurosci Abstr 26: 255, 2000a.