Brain Research 899 (2001) 66–81
www.elsevier.com / locate / bres
Research report
Reciprocal and Renshaw (recurrent) inhibition are functional in man at
birth
S.M. Mc Donough, G.J. Clowry, S. Miller, J.A. Eyre*
Developmental Neuroscience, Department of Child Health, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 4 HH, UK
Accepted 2 January 2001
Abstract
The aims were (1) to determine when in human postnatal development Group Ia reciprocal and Renshaw inhibition can be
demonstrated; (2) to explore the relationship between the expression reciprocal inhibition and the disappearance of Group Ia excitatory
reflexes between agonist and antagonist muscles. Studies were performed on 99 subjects, aged 1 day to 31 years, of whom 53 were
neonates. A longitudinal study was also performed on 29 subjects recruited at birth and studied 3 monthly until 12 months of age.
Reciprocal inhibitory and excitatory reflexes were recorded in the surface EMG of contracting biceps brachii (Bi), evoked by taps applied
to the tendon of triceps brachii (Tri). Reciprocal excitatory reflexes were recorded in all but one neonate. Reciprocal inhibition was
observed in 25% of neonates; evidence is provided that it was likely to have been masked by low threshold reciprocal excitation in the
remaining neonates. Reciprocal inhibition was demonstrated in all subjects after 9 months of age. In four neonates there was depression of
inhibition of Bi during co-contraction of Bi and Tri implying that Group Ia interneurones may be under segmental and suprasegmental
control at birth. Renshaw cells, identified in human postmortem cervical spinal cord by their morphology, location and calbindin D28K
immunoreactivity, were present at 11 weeks post-conceptional age (PCA) and by 35 weeks PCA had mature morphological
characteristics. In four neonates reciprocal inhibitory responses in Bi disappeared when the tap to Tri evoked its own homonymous phasic
stretch reflex, providing neurophysiological evidence for Renshaw inhibition of Group Ia inhibitory interneurones. 2001 Elsevier
Science B.V. All rights reserved.
Theme: Development
Topic: Motor systems
Keywords: Reciprocal inhibition; Renshaw inhibition; Recurrent inhibition; Neonate; Development; Phasic stretch reflex; Human
1. Introduction
During spontaneous movement human neonates show a
predominant pattern of co-contraction of agonist / antagonist muscle pairs in the limbs, with distinct alternating
patterns of activation of agonist / antagonist pairs not being
achieved until several years later in childhood [17–19].
Abbreviations: Bi, biceps brachii; C, condition; CMCD, central motor
conduction delay; MVC, maximum voluntary contraction; PCA, post
conceptional age; PRBS, pseudo-random binary sequence; PMCD,
peripheral motor conduction delay; R, cross-correlation function; t, test;
T, threshold; Tri, triceps brachii; TMCD total conduction delay; V,
cross-covariance
*Corresponding author. Department of Child Health, The Royal
Victoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP,
UK. Tel.: 144-191-2023-013; fax: 144-191-2023-022.
E-mail address: j.a.eyre@ncl.ac.uk (J.A. Eyre).
The degree of co-contraction expressed by agonist / antagonist muscle pairs during limb movement depends upon
many interrelated factors, in particular the descending
commands to each motoneuronal pool and the balance
between reciprocal excitation [32,36,37] and reciprocal
inhibition [4,30]. The magnitude of reciprocal inhibition is
fine-tuned by recurrent inhibition [4]. Descending commands and segmental afferents modulate the excitability of
Group Ia inhibitory interneurones and Renshaw cells [26]
and there is also a pattern of mutual agonist / antagonist
inhibition such that the Group Ia inhibitory interneurones
inhibit the Group Ia inhibitory interneurones of the antagonist muscle [4,22] and Renshaw cells excited by agonists
inhibit those excited by antagonists and vice versa [40].
Thus a-motoneurones, Group Ia inhibitory interneurones
and Renshaw cells together with Group Ia primary spindle
afferent input form part of an ‘output stage’ spinal
0006-8993 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved.
PII: S0006-8993( 01 )02151-5
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
network, which interacts with descending pathways to
mediate the final relative output of agonist / antagonist
motoneurones [26].
The inappropriate degree of co-contraction observed
early in development is likely to reflect immaturity both of
this spinal neural network and of supraspinal descending
commands. Moreover, the rapid improvement of co-ordination, smoothness of limb movements and dexterity,
which occurs in the first 24 months after birth, is related to
the progressive development and refinement of both systems. The present study forms part of an investigation to
ascertain the time of expression of essential components of
the ‘output stage’ spinal network in man and which factors
are important in shaping the functional relationships of the
mature network. It addresses when in postnatal development Group Ia reciprocal inhibition and Renshaw inhibition can be demonstrated and the relationship between the
functional expression of these reflexes and the restriction
of heteronymous Group Ia excitatory projections between
agonist and antagonist muscles, which occurs in the first
four years after birth [36,37].
2. Methods
2.1. Subjects
A cross sectional study was performed on 99 subjects,
63 females, 36 males, aged from 1 day to 31 years; of
whom 53 were term neonates, aged 1–4 days. A longitudinal study was performed on a further 29 subjects, 20
females and nine males, recruited at birth and studied at 3
monthly intervals until 12 months of age. Informed,
written consent was obtained from the adult subjects and
the parent(s) of the babies and children. The study received
approval from the Newcastle Health Authority and University of Newcastle upon Tyne Joint Ethics Committee.
2.2. Electromyograms
EMGs in biceps brachii (Bi) and triceps brachii (Tri)
were recorded using Ag–AgCl skin-mounted, standard
EEG electrodes, 5 mm in diameter, with centres separated
by 15 mm for babies and children up to 9 years and 20 mm
for older children and adults. To promote integration of
motor unit activities in the surface EMG the electrodes
were orientated in alignment with the main direction of the
underlying muscle fibres. The electrodes were placed on
the muscle belly for Bi and over the belly of the lateral
head for Tri, in positions approximately midway between
the motor point and the tendon. The EMG was recorded
unrectified [31] and was fed to differential preamplifiers of
60 dB voltage gain, 100 dB at 50 Hz common mode
rejection ratio, 100 MV input impedance, 50 V output
impedance and 23 dB frequency bandpass of 10–1000
Hz. The impedance of the electrodes was maintained at ,5
67
kV. The preamplifiers were a.c. coupled, so that the
reflexes recorded do not show a positive d.c. shift corresponding to background muscle activity. The signals were
digitized using an intelligent interface (Cambridge Electronic Design, Cambridge, UK, type 1401 plus), sampled at
5 kHz, and stored on a computer for analysis.
2.3. Positioning of subjects and generation of
background muscle activity
Neonates and babies aged less than 6 months were
studied lying quietly in a cot. An evacuable plastic bag
filled with polystyrene beads was used to stabilize the head
and trunk in the mid-line anatomical position. The left arm
was adducted with the elbow held in 458 of flexion from
full extension. Subjects older than 6 months were seated
either on a parent’s knee or on a chair, with the head, trunk
and arm positioned as described above. Babies and young
children are not able to maintain a steady level of
contraction or perform a maximum voluntary contraction
(MVC) on request. Reflexes requiring muscle contraction
were therefore obtained in subjects aged less than 12
months during spontaneous periods of contraction. For
contraction of Bi, children over the age of 18 months held
a weight in the half supinated hand with the flexed arm
supported at the elbow; the weight varied between 100 and
200 g, as judged appropriate to age and stature. To contract
Tri children were encouraged to push the pronated arm
onto a horizontal support, while an examiner placed a hand
over the child’s upper forearm to resist extension at the
elbow.
Adults sat in a chair with the head and trunk in the
mid-line anatomical position and the left forearm resting
on a support. As with babies and younger children, the left
arm was adducted with the elbow held in 458 of flexion
from full extension. Contraction of Bi was obtained with
the arm half supinated and a strap was placed over the
wrist to resist elbow flexion. For contraction of Tri the
forearm was pronated and a strap placed across the upper
forearm to resist elbow extension. For each muscle the
EMG was also rectified and integrated (time constant 1 s)
and displayed to the subject as a horizontal line on an
oscilloscope. The MVC of each muscle was recorded as
the greatest level of rectified, integrated EMG, obtained by
the subject in three attempts separated by 1 min rest
intervals. During reflex testing the subject contracted the
target muscle to raise the EMG display to meet a target
line set at 10% MVC.
2.4. Electromechanical tapper
The reflexes were elicited in Bi or Tri with a handheld
electromechanical tapper (Ling Altec, Royston, UK, Model
101) delivering a sequence of taps (cross-correlation
method) or a single tap (spatial summation procedure), in
each case of 5 ms duration, comprising rise and decay
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S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
times of 2.5 ms. The stylus of the tapper, a Perspex round
disc (diameter 8 mm for babies, 10 mm for adults) was
applied to the skin overlying the tendon of the muscle. The
peak force (N) of the tap was measured with a transducer
placed in series with the stylus. The tapper was driven by a
constant gain power amplifier. The peak voltage of the
pulse feeding the amplifier correlated linearly with the
peak force delivered over a range of 0–15 N (r50.998). In
practice the reflexes were obtained with peak forces in the
range 0.5–10.0 N [31].
2.5. Cross-correlation method for recording spinal
reflexes
Spinal myotatic reflexes were elicited by a pseudorandom binary sequence (PRBS) of taps applied to the
muscle tendon and were recorded in the surface EMG (Fig.
1A). The characteristics of the reflexes were measured
from the functions obtained by cross-correlation of the
PRBS with the EMG. The cross-covariance function V
estimates the magnitude of the reflex and the cross-correla-
Fig. 1. Cross-correlation method. (A) The PRBS signal (upper trace) and corresponding sequence of taps (middle trace). The lower trace shows surface
EMG of Bi contracting at 10% MVC in response to taps applied over the tendon at an intensity of 1.2 T for the phasic stretch reflex. The time base is
indicated on the abscissa. (B–I) Excitatory and inhibitory responses. Each response represents the average of six consecutive sequence responses (defined
as a Trial). Each sequence response was defined as the cross-covariance function of the EMG and PRBS following the application of the 64 taps in a single
PRBS signal lasting 1.28 s. (B–E) Adult. (F–I) Neonate. (B & F) Tri stretch reflex, homonymous phasic stretch reflex in contracting Tri at 1.2 T. (C & G)
Tri→Bi heteronymous excitation, excitatory response in contracting Bi following tap to the tendon of relaxed Tri. (D & H) Bi stretch reflex, homonymous
phasic stretch reflex in contracting Bi at 1.2 T. (E & I) Tri→Bi inhibition; inhibitory response in contracting Bi following a tap to the tendon of relaxed Tri
at 0.8 T. Horizontal scale bar gives the time base for responses (B–I). Vertical scale bar: (B–E & I) 100 mV, (F & H) 200 mV, (G) 300 mV.
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
tion function R estimates the degree of correlation between
input PRBS and output EMG [31]. The authors showed
that the onset latencies of cross-correlation functions of the
reflexes are advanced in time by 10 ms, the clock period of
the PRBS, and 10 ms has therefore been added to all
measures of latency [31].
A sequence response was defined as the cross-covariance function of the EMG and PRBS following the
application of the 64 taps in a single PRBS signal lasting
1.28 s. A trial comprised the average of six sequence
responses obtained with consecutively delivered PRBS
signals [31].
69
both to soft tissue near to the tendon of triceps and also to
the lateral humeral epicondyle. If these taps evoked a
reflex response in Tri or Bi the subjects were excluded
from the study. Two neonates were excluded on this basis.
2.8. Control for volume conduction of the EMG
If in phase or out of phase deflections in the crosscovariance functions were observed at any stage in Bi and
Tri, all the data from these muscles were excluded from
the analysis. In fact no subject was excluded on this basis.
2.9. Reflex threshold
2.6. Reflex response
A response was defined as present if it was discernible
by eye in the trial average of six cross-covariance functions and occurred with a consistent onset latency (Fig.
1B–I). In each experimental session the polarity of the
initial phase of the homonymous phasic stretch reflex in
biceps was determined. If it was positive the polarity of the
recording electrodes was appropriate. If it was negative the
polarity of the differential electrodes was reversed so that
the homonymous phasic stretch reflex showed an initial
positive deflection. Heteronymous reflexes evoked in
biceps by a tap to triceps were then defined as excitatory if
the initial component was positive going (Fig. 1B–D and
Fig. 1F–H) and as inhibitory if the initial component was
negative going (Fig. 1E and I). This convention was
validated by corresponding changes in torque about the
elbow. Thus initially upward going responses in the EMG
of biceps, designated as excitatory, have been shown to
correspond to an increase in flexor contractile force, while
downward going responses in the EMG of biceps, designated as inhibitory, to correspond to a decrease in flexor
contractile force [31].
2.7. Controls for mechanical spread of the tap stimulus
In 10 neonates and 10 adults the onset of the mechanical
pulse induced by the tap to triceps was measured using an
accelerometer placed over the site of the recording electrodes on biceps (Table 1). In all subjects, controls were
made for reflex effects induced by the direct mechanical
spread of the tap to biceps [36,37]. Taps of 1.5 T for the
homonymous phasic stretch reflex in Tri, were applied
Table 1
Onset of mechanical impulse in Bi following taps to the tendon of Tri
Neonates
Adults
n
Mean6S.E.M.
(ms)
10
10
0.0560.08
1.6360.48
In determining reflex threshold (T), the tap magnitude
was first set suprathreshold and then reduced stepwise until
reflexes were identified in three out of the six sequences in
a trial (a 50% response rate).
2.10. Reflexes studied
2.10.1. Homonymous phasic stretch reflexes
Homonymous phasic stretch reflexes were elicited in
contracting Bi and Tri. The thresholds and onset latencies
of the trial averages were recorded (Fig. 1B, D, F and H).
2.10.2. Short latency heteronymous excitatory and
reciprocal inhibitory responses from Tri to Bi
Once the threshold for the homonymous phasic stretch
reflex in Tri had been determined (T), the force of the tap
was set at a level between 0.2 and 0.5 T and applied to
relaxed Tri. The responses were recorded in contracting Bi.
The force delivered was increased stepwise in units of 0.1
T up to a maximum of 1.5 T. At each of these stimulus
levels the trial average was inspected for evidence of
excitatory heteronymous responses (Fig. 1C and G) or
reciprocal inhibitory responses (Fig. 1E and I). The
thresholds, onset latencies and peak to peak amplitudes of
the responses were recorded.
2.11. Spatial summation experiment: corticospinal
projection to Group Ia inhibitory interneurones ( Fig. 4)
This component of the corticospinal projection was
investigated by spatial summation at presumed Tri Group
Ia inhibitory interneurones in two neonates and two adults.
Summation was tested between a Tri Group Ia afferent
volley, subthreshold for evoking a heteronymous excitatory response or a reciprocal inhibitory response in Bi, and
a corticospinal volley, subthreshold for evoking a response
in Bi or Tri.
2.11.1. Excitation of a corticospinal volley
Transcranial magnetic stimulation (TMS) using a Magstim 200 stimulator, (MagStim, Whitland, UK), with a 90
70
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
mm circular coil placed tangentially above the contralateral
motor cortex, excited corticospinal neurones [9,12–14].
The optimal position for exciting corticospinal axons and
the conduction delay from TMS to the arrival of the
corticospinal volley at Tri Group Ia interneurones was
assumed to be the same as for corticospinal axons projecting to Tri a-motoneurones. The optimal coil position for
evoking EMG responses in Tri was determined using
suprathreshold stimuli. The intensity of TMS was then
decreased until responses were evoked in contracting Tri in
50% of trials, defined as threshold (T). The shortest onset
latency of 20 responses to TMS at 1.2 T was obtained to
give the total motor conduction delay (TMCD) to Tri
muscle. Magnetic stimulation, with a 5 cm circular coil,
placed in the coronal plane overlying cervical spines 5–8,
was used to excite spinal motor roots at the point of
emergence from the intervertebral foramina [39]. The
peripheral motor conduction delay (PMCD) to the Tri
muscle was estimated from the longest onset latency
obtained from at least 20 responses [13]. The subtraction
of PMCD from TMCD was used to estimate the corticospinal conduction and trans-synaptic delay to Tri amotoneurones.
measured as the peak of the short latency inhibitory
response and expressed as a percentage of the RMS value
of the background EMG in Bi during each set of repeats.
2.12. Renshaw (recurrent) inhibition
2.11.2. The conduction and trans-synaptic delay from
Tri Group Ia afferents to Tri Group Ia inhibitory
interneurones
The conduction and trans-synaptic delay from Tri Group
Ia afferents to Tri Group Ia inhibitory interneurones was
assumed to be the same as that to Tri a-motoneurones. It
was estimated by subtracting the PMCD to Tri muscle
from the shortest onset latency of the homonymous phasic
stretch reflex in Tri obtained from responses to at least 20
single taps delivered at 1.2 T to the tendon of Tri.
2.12.1. Anatomical study ( Fig. 6)
The characteristic morphology and location and calbindin D28K (calbindin) immunoreactivity were used to
identify Renshaw cells in human postmortem cervical
spinal cord obtained from a surgically aborted fetus of 11
weeks post-conceptional age (PCA) and nine preterm
babies aged 26–35 weeks PCA, who died from nonneurological causes (Fig. 5). The spinal cords were
immersion fixed in buffered 4% paraformaldehyde solution
for 24–72 h and then sectioned into segments and immersed in buffered 30% sucrose solution for at least 24 h.
Frozen 50 mm sections were cut on a freezing microtome
and then immunostained free floating using standard
techniques. A well characterized monoclonal antibody to
calbindin (Sigma), was employed as a primary antibody at
a dilution of 0.05% in phosphate buffered saline containing
0.1% Triton X-100 (Sigma) and 3% normal horse serum
(Vector Labs.). The sections were incubated overnight with
gentle agitation. The immuno-localization of calbindin was
visualized by incubation with biotinylated horse antimouse antibodies (0.5%, Vector Labs.), followed by streptavidin–horseradish peroxidase (HRP) conjugate (0.5%,
Vector Labs.) in phosphate-buffered saline between and
after incubations. The sections were reacted with
diaminobenzidine (0.05%, Sigma) in the presence of
hydrogen peroxide (0.003%) in phosphate-buffered saline
and washed and mounted on slides.
2.11.3. Spatial summation trials
Subthreshold tap to Tri (Fig. 4C), TMS at 0.8 T (Fig.
4D) and the two stimuli together at the interstimulus
interval of expected convergence of both volleys at Group
Ia interneurones (Fig. 4E), were delivered in an interleaved
sequence. Five repeats of this sequence were performed
three times on each neonate and adult subject. To define
the time course of spatial summation in the two adults five
repeats of the sequence were used, conducted over a range
of interstimulus intervals such that the corticospinal EPSP
arrived at biceps a-motoneurones from 2 ms before to 10
ms after the estimated arrival of the Group Ia volley (Fig.
5). The condition to test interval (C–t), the interval of
expected convergence, was defined as C–t interval zero
and all other C–t intervals were defined relative to this.
Thus negative C–t intervals indicated the Group Ia volley
arriving before the corticospinal volley and positive C–t
intervals indicated the corticospinal volley arriving before
the Group Ia volley. The magnitude of the inhibition was
2.12.2. Neurophysiological study ( Fig. 7)
Group Ia inhibitory interneurones are inhibited by
Renshaw cells [4,20]. Evidence was therefore sought for
depression of reciprocal inhibition between Tri and Bi by
recurrent activity in Tri motoneurone axon collaterals. The
cross-correlation method for spinal reflexes was used. Four
neonates were chosen, who had clear inhibitory responses
in Bi following a tap to relaxed Tri with a low threshold,
i.e., ,0.7 T, and who also had high thresholds for
heteronymous excitatory responses in Bi, i.e., .1.3 T.
Threshold taps to Tri (1.0 T) were applied to evoke a
homonymous phasic stretch reflex in contracting Tri in
50% of sequence responses. These taps were suprathreshold for evoking reciprocal inhibition in Bi, but substantially below the threshold for evoking a heteronymous
excitatory response in Bi. Evidence was sought for consistent absence of reciprocal inhibition of Bi in those trials
when a homonymous phasic stretch reflex in Tri was
simultaneously evoked [30] (Fig. 7).
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
2.13. Relationship between level of contraction in Tri
and the probability and magnitude of the inhibition
evoked in contracting Bi during co-contraction of Bi and
Tri ( Fig. 8)
In the same four neonates studied above, taps at 0.8 T
were applied to the tendon of Tri, so that the tap would be
subthreshold for either a homonymous phasic stretch reflex
in Tri or a heteronymous excitatory response in Bi, but
suprathreshold for an inhibitory response in Bi. The
magnitude of the inhibitory response in Bi was determined
(1) during periods of isolated activity in Bi and (2) during
spontaneous co-contraction of Bi and Tri when the magnitude of the inhibitory response evoked in Bi was related
to the level of contraction in Tri.
For each trial the levels of contraction in Tri and Bi
were determined as the root mean square (RMS) of their
respective EMGs. The magnitude of the inhibition in each
trial was determined to be the difference between the
baseline to peak values of the inhibitory response in the
trial average expressed as a percentage of the RMS value
of the EMG in Bi during that trial. The amplitudes of
inhibitory responses had a skewed distribution and underwent logarithmic transformation.
It is not possible to measure MVC in neonates and
therefore the magnitude of the EMG in individual subjects
could not be normalized by reference to EMG values at
MVC. To remove between child variation in the magnitudes of EMG in Bi and Tri, the mean values across all
trials for the level of contraction of Tri and the magnitude
of the inhibition in Bi were determined for each neonate.
The magnitudes of contraction in Tri or inhibition in Bi in
each trial were then expressed in terms of their difference
from the mean value (called a residual value). Thus
negative values indicate magnitudes which were less than
the mean for that subject and positive values those which
were greater than the mean (Fig. 8).
3. Results
Unless otherwise
means6S.E.M.
stated
all
data
are
given
71
of the homonymous phasic stretch reflex in Bi (Table 2,
P,0.0001), with a mean difference of 1.8 ms60.1 ms
(Fig. 2B and C and Fig. 3A).
When reciprocal inhibitory responses from Tri to Bi
were observed in neonates, the onset latencies also
occurred significantly later than the homonymous phasic
stretch reflex in Bi (Table 2: cross-sectional study, P,
0.0001; longitudinal study, P,0.001), with a mean difference in onset latency of 1.860.3 ms for the cross-sectional
study and 1.060.4 ms for the longitudinal study (Fig. 2B
and C and Fig. 3B).
The difference in onset latency between reciprocal
inhibition from Tri to Bi and the homonymous phasic
stretch reflex in Bi, remained remarkably consistent with
age, the mean varying within the range 11.0 to 11.8 ms
for both the cross sectional and longitudinal study groups
(Fig. 2B and C).
3.2. Group Ia heteronymous excitatory responses from
Tri to Bi ( Figs. 2 and 3)
Heteronymous excitatory responses [36,37] were observed in 52 / 53 neonates (98%) in the cross-sectional
study and 29 / 29 neonates (100%) in the longitudinal study
(Fig. 2E). The probability of demonstrating a heteronymous excitatory response in Bi decreased slowly with age.
At 12 months of age 38 / 53 (72%) of cross-sectional
subjects and 20 / 29 (71%) of longitudinal subjects still
demonstrated heteronymous excitatory responses, but in
adult subjects these responses were only present in two
subjects (10%; Fig. 2E).
For neonates there were no significant differences
between the onsets of homonymous phasic stretch reflexes
in Bi and the heteronymous excitatory response from Tri to
Bi (Table 2, Fig. 2F and G and Fig. 3B). However, the
difference in the onset latencies of the heteronymous
excitatory responses and the homonymous phasic stretch
reflex in Bi increased progressively with age (Fig. 2F and
G), so that by adulthood the onsets of the heteronymous
excitatory responses were 11 ms and 12 ms longer than the
onset of the homonymous phasic stretch reflex.
as
3.3. Thresholds of heteronymous excitatory and
reciprocal inhibitory responses
3.1. Reciprocal inhibition from Tri to Bi ( Figs. 2 and 3)
Short latency reciprocal inhibition from Tri to Bi was
demonstrated in neonatal subjects in 15 / 53 (28%) of the
cross sectional and 6 / 29 (21%) of the longitudinal study.
The probability of demonstrating short latency inhibition
however increased rapidly postnatally so that after 9
months postnatal age all subjects in both the cross-sectional and longitudinal studies demonstrated short latency
reciprocal inhibition (Fig. 2A).
For the adult subjects the onset latency of reciprocal
inhibition from Tri to Bi was significantly longer than that
The thresholds for reciprocal inhibitory and heteronymous excitatory responses in Bi (Fig. 2D and H and Fig. 3
and D) were normalised by reference to the threshold (T)
of the homonymous phasic stretch reflex in contracting Tri.
In adult subjects the threshold of reciprocal inhibitory
responses in Bi were significantly lower than those of the
homonymous phasic stretch reflex in Tri (Table 2, P,
0.02, Fig. 3C) and of the heteronymous excitatory responses in the two adult subjects (Table 2, Fig. 3C).
Similarly, when reciprocal inhibitory responses in Bi
were demonstrated in neonates, the thresholds were sig-
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S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
Fig. 2. Reciprocal inhibitory (A–D) and heteronymous excitatory responses (E–F) in Bi following taps to Tri. The symbols represent the means and the
vertical lines the 95% confidence limits of the means. Filled circles: data from the cross-sectional study, numbers above the abscissa give subjects in each
age group. Open circles: data from 29 children studied longitudinally. (A and E) The percentage of subjects demonstrating a response in relation to age. (B
and F) Onset latencies of the trial average responses in relation to age. The onset latencies of homonymous phasic reflexes in Bi are also plotted as
triangles. Filled triangles: cross-sectional data; open triangles: longitudinal data. (C and G) Relative onset latencies of responses, defined as the onset
latency of responses minus the onset latency of the homonymous phasic stretch reflex in Bi. The dashed horizontal line through 0 represents the onset of
the homonymous phasic stretch reflex in Bi. (D and H) Threshold of responses. Thresholds are defined as percentages of the threshold of the homonymous
phasic stretch reflex in contracting Tri. The horizontal dashed line at 100% represents the threshold of the homonymous phasic stretch reflex of Tri. The
horizontal dotted line represents the mean threshold of the reciprocal inhibitory responses in adults.
nificantly lower than those for the homonymous phasic
stretch reflexes in Tri (Table 2: cross-sectional study,
P,0.02; longitudinal study, P,0.02, Fig. 3D) and lower
than the threshold of the heteronymous excitatory re-
sponses in Bi (Table 2: cross-sectional study, P.0.05,
longitudinal study, P,0.01; Fig. 3D).
In contrast, the neonates, who did not demonstrate
reciprocal inhibition, had heteronymous excitatory re-
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
73
Table 2
Onset and threshold of reflexes
Onset latency
Adults
Neonates:
Cross-sectional
Longitudinal
Bi homonymous phasic stretch reflex
Bi→Bi
Reciprocal inhibition
Tri→Bi
Heteronymous excitatory reflexes
Tri→Bi
n
Mean6S.E.M. (ms)
n
Mean6S.E.M. (ms)
n
20
18.560.4
20
20.360.5
2
53
29
13.960.2
14.060.3
15
6
15.760.5
15.060.5
52
28
Threshold
Adults
Neonates with inhibition:
Cross-sectional
Longitudinal
Neonates not demonstrating inhibition:
Cross-sectional
Longitudinal
sponses in Bi with thresholds that were significantly lower
than the threshold of their homonymous phasic stretch
reflex in Tri (Table 2: cross-sectional study, P,0.001;
longitudinal study, P,0.001; Fig. 3D) and were also
significantly lower than the thresholds of the heteronymous
excitatory responses observed in those neonates in whom
reciprocal inhibition was demonstrated (Table 2: crosssectional study, P,0.01; longitudinal study, P,0.001; Fig.
3D). The thresholds of the heteronymous excitatory responses in the neonates without reciprocal inhibition were
similar to the thresholds of the inhibitory responses
observed in the neonates with reciprocal inhibition (Fig.
3D).
The threshold for reciprocal inhibition relative to that of
the homonymous phasic stretch reflex in Tri remained
remarkably consistent with age with the mean lying
between 0.65 T and 0.8 T at all ages in both the
longitudinal and cross-sectional studies (Fig. 2D). In
contrast, the threshold for heteronymous excitatory responses increased progressively with age relative to that of
the homonymous phasic stretch reflex in Tri. By 9 months
the mean threshold of the heteronymous responses was
greater than that of reciprocal inhibition, and by 12 months
the mean threshold of the heteronymous responses was
greater than that of the homonymous phasic stretch reflex
in Tri (Fig. 2H).
3.4. Corticospinal projection to Group Ia inhibitory
interneurones ( Fig. 4)
Spatial summation of a Tri Group Ia afferent volley
subthreshold for evoking a heteronymous excitatory or
reciprocal inhibitory response in Bi and a corticospinal
volley subthreshold for evoking a response in either Bi or
Mean6S.E.M. (ms)
Mean6S.E.M. (T)
31, 33
13.760.3
13.960.3
Mean6S.E.M. (T)
20
0.860.10
2
1.35, 1.20
15
6
0.760.09
0.660.17
14
6
1.0060.09
1.0960.05
38
23
0.6060.06
0.5660.06
Tri occurred at the estimated coincidence of volleys at Tri
Group Ia inhibitory interneurones in the two neonates and
the two adults studied. In all subjects spatial summation
resulted in inhibition of Bi EMG with onset latencies 1–2
ms longer than EMG responses evoked in Bi by suprathreshold TMS (Fig. 4E). In the neonatal subjects the
EMG was depressed by a mean of 32.365.2% (P,0.01)
and 29.366.1% (P,0.01) and in the adults by 1563.2%
(P,0.01) and 3864.1% (P,0.01), respectively.
The time course of the spatial summation was defined in
the two adult subjects (Fig. 5). Spatial summation occurred
over C–t intervals of 0 to 14 ms and 0 to 17 ms, with
peak depressions of the EMG of 32.564% and 55.6613%
respectively (Fig. 5A).
3.5. Renshaw (recurrent) inhibition
3.5.1. Anatomical study ( Fig. 6)
In the human post-mortem material calbindin positive
cells were observed in the appropriate location for Renshaw cells at all the ages examined [2,3] (Fig. 6). At 11
weeks PCA most putative Renshaw cells had a strongly
immunoreactive cell nucleus with only weakly immunoreactive cytoplasm (Fig. 6A and B). Every cell exhibited
weak immunoreactivity in two or three dendritic processes
but only close to the cell body. In the motoneuronal pools
the occasional cell resembling a motoneurone was calbindin positive, but no calbindin positive axons were observed
either in the grey matter or in the white matter. By 23
weeks PCA (Fig. 6C and D) calbindin immunoreactivity
was much stronger in the cytoplasm of cells with the
characteristic morphology and location of Renshaw cells.
Each cell had two or three immuno-positive proximal
dendrites that were thin and gave rise to even thinner
74
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
Fig. 3. Onset latencies and thresholds of trial average responses. (A and C) Adults; (B and D) neonates. The subjects are divided into those in whom
reciprocal inhibition was not evoked (inhibition absent) and those in whom it was (inhibition present). The circles represent means and the vertical lines
95% confidence limits of the means. For neonates filled circles represent data from the cross-sectional study, open circles from the longitudinal study. (A
and B) Onset latencies. The horizontal dotted line defines the mean onset latency of the homonymous phasic stretch reflex in Bi in adults and neonates,
respectively. (C and D) Threshold. Thresholds are defined as percentages of the threshold of the homonymous phasic stretch reflex in contracting Tri. The
horizontal dotted line at 100% represents the threshold of the homonymous phasic stretch reflex of Tri. Bi, SR homonymous stretch reflex in Bi; Tri→Bi
EXCIT, heteronymous excitatory responses from Tri to Bi; Tri→Bi INHIB, inhibition from Tri to Bi; the numbers above the absicca indicate the number of
subjects; NA, not applicable.
branches. Many calbindin positive axons could be seen
towards the inner boundary of the white matter all round
the ventral horn. No axons were present in the grey matter
of the motoneuronal pools and no motoneurones were
calbindin positive at this age. Over the next 11 weeks of
development the appearance of presumed Renshaw cells
changed, so that by 35 weeks post-conceptional age (Fig.
6E, F and G) the proximal dendrites and their branches had
thickened considerably and could be traced for several
hundred microns. They often appeared to run in parallel
bundles along the medial edge of the motoneuronal pools
or along the grey / white matter boundaries. Many calbindin
immunoreactive varicose axons were present in the
motoneuronal pools and some appeared to make contact
with motoneurone cell bodies.
3.5.2. Neurophysiological study ( Fig. 7)
In all four neonates studied it was a consistent finding
that when a tap of 1.0 T evoked a homonymous phasic
stretch reflex in contracting Tri, inhibition was not observed in contracting Bi (Fig. 7D).
3.6. Relationship between level of co-contraction in Tri
and Bi and the probability and magnitude of the
inhibition evoked in contracting Bi ( Fig. 8)
One hundred and forty-five informative trial averages
were obtained in the four neonates studied: minimum
number of trial averages per subject530 (representing 180
sequences), maximum543 (representing 258 sequences).
The mean RMS values for EMG in Tri varied from 12.3 to
14.9 mV and the mean values for the amplitude of
inhibitory responses in Bi, expressed as the percentage
reduction in the background level of contraction in Bi,
varied from 38 to 64%, respectively.
Reciprocal inhibitory responses were observed in 98 /
145 (68%) trials. Reciprocal inhibition was most frequently observed and had its maximum amplitude when there
was no contraction in Tri (Fig. 8; frequency, Tri not active,
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
75
Fig. 4. Spatial summation of subthreshold corticospinal volley and
subthreshold Tri Group Ia volley in a neonatal subject. Open arrows
indicate onset of tap to Tri. Filled arrows indicate onset of TMS. (A and
B) Taps delivered to the tendon of Tri and responses recorded in Tri (A)
and Bi (B–E). The intensity of the tap was decreased until it was
subthreshold for evoking excitation or inhibition in Bi. (Note: heteronymous excitation of Bi with a threshold of 0.4 T was observed but reciprocal
inhibition was not demonstrated). (C) Subthreshold tap to Tri. (D) TMS
subthreshold for evoking a response in Bi or Tri (0.8 T). (E) The two
stimuli together at the interstimulus interval of estimated convergence at
Tri Group Ia interneurones resulting in inhibition of contracting Bi. T,
Threshold; TMS, transcranial magnetic stimulation.
2
12 / 13 trials, 93%; Tri active, 86 / 132 trials, 65%; x 598
P,0.001; amplitude of reciprocal inhibition; Tri not
active, n512, 2.0360.14 l n %; Tri active, n586,
0.1060.09 l n %; P,0.0001). When reciprocal inhibition in
Bi was evoked during co-contraction of Tri and Bi, there
was a significant negative correlation between the level of
contraction in Tri during the trial and the magnitude of the
inhibitory response in Bi (r 2 50.22, P,0.001; Fig. 8).
4. Discussion
Short latency inhibition and excitation of Bi EMG was
demonstrated in neonates and adults following a tap to the
tendon of relaxed Tri. Direct volume conduction of
reflexes evoked in Tri is excluded since the tap was
subthreshold for reflexes in relaxed Tri. It is possible that
these heteronymous responses could have arisen from
Fig. 5. Inhibition of biceps following spatial summation of a subthreshold
triceps Group Ia afferent volley and a subthreshold corticospinal volley at
triceps Group Ia inhibitory interneurones. (A) Time course and magnitude
of spatial summation, open symbols represent the mean for each subject
and vertical lines the S.E.M. of the peak of the depression of the EMG
expressed as a percentage of the background EMG. Time base C–t
interval 0: the interstimulus interval of expected spatial convergence of a
corticospinal and Group Ia volley at triceps Group Ia inhibitory interneurones. Negative C–t intervals: Group Ia before corticospinal volley,
positive C–t intervals: corticospinal before Group Ia volley. Stars indicate
statistically significant (P#0.05) differences between conditioned and test
reflexes. (B) EMG of biceps when the subthreshold MAG and subthreshold tap to Tri are delivered together at the defined interstimulus
(C–t) intervals for the adult subject illustrated by s in (A). Open arrows
indicate onset of tap to Tri. Filled arrows indicate onset of TMS.
mechanical transmission of the tap through the arm
exciting spindle afferents of Bi (Table 1). However, a tap
of up to twice the force delivered to Tri’s tendon failed to
evoke reflexes in Bi when applied to soft tissue near Tri’s
tendon or to the lateral epicondyle. This indicates that if
76
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
Fig. 6. Calbindin D28K immunoreactivity in the ventral horn of human fetal spinal cord at C 6 segmental level. At all ages a strongly stained group of
neurones is located where the ventral roots coalesce before traversing the ventral white matter. These are putative Renshaw cells (R). (A) Calbindin D28K
positive neurones are present as early as 11 weeks post-conceptional age which include putative Renshaw cells. (B) A higher power image at 11 weeks
post-conceptional age shows the nucleus (arrow) to be more intensely stained than the cytoplasm. Some staining of proximal dendrites can also be seen.
Most motoneurones (M) appear weakly immunoreactive with occasional motoneurone-like cell strongly positive (arrow in A). There is no axonal staining
in either the grey or white matter. (C and D) By 23 weeks post-conceptional age all motoneurones have lost calbindin D28K immunoreactivity, and there is
no immunoreactivity in the surrounding neuropil, although calbindin D28K positive axons are present in the white matter. The Renshaw cell group (R) is
again prominent. At higher magnification the cytoplasm is seen to be heavily immunostained and thin dendrites can be clearly seen. (E, F and G) By 35
weeks post-conceptional age axonal staining is strong both in the ventral white matter and in the neuropil around motoneurones (M in E and G). The
Renshaw cells have thick dendrites that can be arranged in bundles (arrow in F). Scale bars: A, C and E, 250 mm; D, F and G, 50 mm; B, 25 mm.
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
Fig. 7. Trial averages recorded in the EMGs of contracting Tri and Bi in
response to PRBS taps to the tendon of Tri in a neonate. The tap intensity
is expressed as multiples of the threshold (T) of the homonymous phasic
stretch reflex in contracting Tri. (A–C) Inhibitory responses are observed
in Bi in response to taps of Tri at 0.5, 0.9 and 1.0 T in the absence of a
homonymous phasic stretch reflex in Tri. D. Inhibition of Bi is not present
when a homonymous phasic stretch reflex is evoked in Tri by a tap of 1.0
T.
mechanical spread of the stimulus did activate spindles in
Bi, the activity was subthreshold for evoking reflex
responses. Since activation of Bi spindle afferents excites
Bi motoneurones, mechanical transmission of the tap is
likely to have reduced the probability of demonstrating
reciprocal inhibition and enhanced the probability of
demonstrating heteronymous excitation. Furthermore, to
demonstrate inhibition of Bi, using the paradigm of the
present study, background contraction was required. Voluntary contraction of one muscle in an agonist / antagonist
pair leads to descending excitation of its Group Ia inhibitory interneurones [10,21,35,44]. Thus, isolated contraction of Bi, would have led to inhibition of the antagonists
(Tri) Group Ia inhibitory projections to Bi [21]. Overall
therefore the paradigm used in the present study will have
reduced the probability of demonstrating reciprocal inhibition, but will have enhanced the ability to detect
heteronymous excitation.
4.1. Reciprocal inhibition
A major finding of this study was that short latency
reciprocal inhibition from Tri to its antagonist Bi was
77
Fig. 8. (A) The magnitude of the inhibitory responses in Bi in relation the
level of contraction in Tri in four neonates. To remove between subject
variability in the absolute value of the EMG in Bi and Tri, the mean value
for the level of contraction of Tri and the magnitude of the inhibition in
Bi, was determined for each neonate. The magnitudes of contraction in
Tri or inhibition in Bi were then expressed as a residual from their
respective mean. Thus negative values indicate magnitudes which were
less than the mean for that subject and positive values those which were
greater than the mean. The amplitudes of inhibitory responses had a
skewed distribution and underwent logarithmic transformation. NI, no
inhibition of Bi; NC, no contraction in Tri.
present in approximately 25% of neonates, with thresholds
and central synaptic delays similar to those of adults [30].
4.1.1. Group Ia afferents mediate short latency
reciprocal inhibition in adults and neonates and
throughout postnatal development
Group Ia inhibitory interneurones have been shown to
mediate short latency reciprocal inhibition between antagonist muscles in the cat [28]. The threshold for evoking
inhibitory responses in Bi lay significantly below that of
the homonymous phasic stretch reflex in Tri (Fig. 2D,
Table 2) implying that the inhibition was mediated by low
threshold afferents. A brief, low intensity tap to relaxed Tri
was chosen to evoke the reflexes since it excites predominantly Group Ia afferents, although a small number of
low threshold cutaneous afferents are also excited [6,38].
The inhibition mediated by cutaneous afferents is not
present in neonates and does not appear until 18 months of
age [25]. Furthermore, in adults cutaneous inhibition is
78
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
part of a triphasic reflex where short latency inhibition (I 1 )
is preceded and followed by early (E 1 ) and late (E 2 )
excitation, respectively. Thus the inhibition observed in the
present study is unlikely to have been mediated by
cutaneous afferents.
4.1.2. Short latency reciprocal inhibition has a
disynaptic linkage from birth
Group Ia afferents project monosynaptically to Group Ia
interneurones leading to a disynaptic inhibition of antagonist muscles [1,28]. The estimated additional synaptic
delay above that for a monosynaptic reflex, for the subjects
in the present study was 1.0–1.8 ms, consistent with a
disynaptic linkage. This additional synaptic delay is longer
than that previously estimated for reciprocal inhibition in
adults [10,30], when electrical stimulation of the afferent
fibres from the antagonist muscle was used to evoke
reciprocal inhibition. The rise time of motoneuronal EPSPs
following tendon tap is longer than that following electrical stimulation to the peripheral nerve [5] and together
with the time required for transduction of the tap at muscle
spindles, would be more than sufficient to account for the
longer central delays of the present study. Katz et al. [30]
also included a tendon tap to the antagonist muscle as part
of the protocol of their study, and demonstrated a 12 ms
discrepancy in the onset of the inhibitory responses in
comparison to electrical stimulation, which they also
attributed to the temporal dispersion of the volley evoked
by the tendon tap.
4.1.3. Direct corticospinal projections to Group Ia
inhibitory interneurones
In the monkey Group Ia interneurones receive a monosynaptic projection from Group Ia afferents and from
corticospinal axons [29]. In the present study we demonstrated inhibition of Bi EMG following spatial summation
of a subthreshold Tri Group Ia afferent volley and a
subthreshold corticospinal volley at their expected time of
convergence on Group Ia interneurones. This spatial
summation establishes a monosynaptic corticospinal projection to Group Ia inhibitory interneurones is also present
in human neonates and adults.
4.2. Heteronymous excitatory responses
Heteronymous excitatory responses evoked from Tri to
Bi were observed in all but one of the neonates (Fig. 1F).
Heteronymous excitatory reflexes to non-synergistic
motoneurones, including those of the antagonist, have been
reported previously in immature animals [11,41,43] and by
ourselves in babies [32,36,37]. We confirmed our previous
observations that the initial component of the heteronymous excitatory reflexes is monosynaptically linked in the
newborn, since heteronymous excitatory reflexes in Bi had
similar onsets to that of the homonymous phasic stretch
reflex [36,37]. The present study however was the first to
demonstrate that the threshold for evoking heteronymous
excitatory responses is significantly lower in neonates than
that for evoking homonymous phasic stretch reflexes (see
Fig. 4 and Figs. 2H and 3D). Heteronymous excitatory
responses occurred less frequently with age and their
thresholds and onset latencies increased. Thus with development heteronymous excitatory responses may require
more temporal summation and / or be mediated by afferents
with slower conduction velocities. The homonymous
phasic stretch reflex evoked by a tap produces oligosynaptic as well as monosynaptic excitation of the motoneurones
[5]. Such oligosynaptic projections are also likely also to
be involved in heteronymous excitation of Bi following a
tap to Tri. The later onset of heteronymous excitation of Bi
in adults may therefore reflect withdrawal of the monosynaptic projection during development leaving the onset
of the reflex dependent upon an oligosynaptic linkage.
Neonates and infants who demonstrated reciprocal inhibition, had significantly higher thresholds for their
heteronymous excitatory responses than those not demonstrating reciprocal inhibition (Fig. 3D). We suggest two
possible explanations for this observation. First, the restriction of heteronymous excitation between agonist and
antagonists may occur because of the establishment of
short latency reciprocal inhibition during development.
Thus the establishment of short latency reciprocal inhibition leads to higher thresholds for heteronymous reciprocal
excitation. If this is so then the establishment during
development of reciprocal inhibition is not the only, nor
the most significant process involved in the restriction of
heteronymous reciprocal excitation. From 9 months of age
all subjects demonstrate reciprocal inhibition but 70% still
had heteronymous reciprocal excitation (Fig. 2A, E, F and
G).
An alternative explanation may be that low threshold
and short onset heteronymous excitatory responses in
neonates and infants mask reciprocal inhibitory responses
which have at that stage both a higher threshold and a later
onset. Thus, as the threshold for heteronymous reciprocal
excitation increases with development to be greater than
that for reciprocal inhibition, reciprocal inhibition can be
demonstrated. The latter argument receives strong support
from an observation in the present study. The neonates in
whom the corticospinal projection to Group Ia inhibitory
interneurones was investigated had, by chance, very low
thresholds for heteronymous excitation between Tri and
Bi. Neither demonstrated short latency reciprocal inhibition from Tri to Bi when a tap was delivered to Tri (Fig.
4B). However, spatial summation of a Tri Group Ia
afferent volley (subthreshold for modulating the activity of
Bi motoneurones) with a subthreshold corticospinal volley,
revealed short latency inhibition of Bi in both subjects
(Fig. 4E). If, as is likely from this observation, that
reciprocal inhibition from Tri to Bi is masked by low
threshold heteronymous excitatory reflexes, it is possible
that reciprocal inhibition may be present in all neonates.
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
4.3. Renshaw (recurrent) inhibition
4.3.1. Anatomical study
Calbindin immunoreactive neurones found in the ventral
horn of both rat [2] and monkey [3] have the same
morphology and location as electrophysiologically identified and intracellularly labelled Renshaw cells in the cat
[27]. Furthermore, in both rat and cat, calbindin positive
neurones in this location express characteristic glycine
receptor complex clusters of identified Renshaw cells
[3,7,8]. Injury to sciatic motor axons in rat down-regulates
calbindin expression in putative Renshaw cells of the
lumbar cord, suggesting a functional link between sciatic
motoneurones and these cells [42]. In rat cervical cord,
using double immunofluorescence techniques, putative
calbindin positive Renshaw cells have been shown to be
richly innervated by motor axon collateral terminals,
labelled by injection of cholera toxin B into muscle
[15,16]. Taken together, these observations make a strong
case for using calbindin immunoreactivity as a marker for
Renshaw cells in human post-mortem tissue, particularly
as the immunostaining is reliable even in less than
optimally fixed tissue.
In human spinal cord putative Renshaw cells were
identified from as early as 11 weeks post-conceptional age
and by 35 weeks Renshaw cells were identified which
were morphologically mature. In addition, calbindin immunoreactive varicose axons were present in the
motoneuronal pools, some of which appeared to make
contact with motoneurone cell bodies. Without formally
identifying them as Renshaw cells, Ince et al. [24] described a population of calbindin positive neurones in a
ventromedial location in adult human spinal cord. They
also noted the presence of calbindin positive axonal
varicosities in the motoneuronal pools indicating that this
is a feature of mature spinal cord. This pattern of axonal
staining has also been observed in the lumbar cord of the
adult macaque monkey [3] where it was suggested that
most, if not all, calbindin positive varicose axons around
the motoneurones derive from Renshaw cell axons.
4.3.2. Neurophysiological study
In the four neonates investigated, reciprocal inhibition of
Bi was consistently absent when an homonymous phasic
stretch reflex was evoked in Tri (Fig. 7D). The intensity of
the tap to Tri was chosen to be at threshold so that a
homonymous phasic stretch reflex was evoked in 50% of
responses. Inhibition of Bi occurred however when Tri was
contracting, but a tap to the tendon of the same intensity
did not evoke the homonymous phasic stretch reflex (Fig.
7C). This observation excludes mechanical spread of the
stimulus to the Group Ia afferents of Bi and / or excitation
of heteronymous excitatory reflexes to Bi [36,37] counterbalancing the inhibitory reflex, since both these excitatory
inputs to Bi motoneurones would be the same irrespective
of whether a homonymous phasic stretch reflex in Tri was
79
evoked or not. Similarly descending inhibition of the
respective pools of reciprocal inhibitory interneurones
during spontaneous co-contraction of Tri and Bi [34] will
also remain the same, irrespective of whether a homonymous phasic stretch reflex in Tri was evoked or not. It is
concluded, therefore, that the suppression of reciprocal
inhibition in Bi arose from Renshaw inhibition evoked by
recurrent activity in Tri motoneurone axon collaterals, in a
similar manner to that described by Katz et al. [30] in adult
subjects. In that study a suprathreshold tap to Tri had to be
delivered at least 1.5 ms before the electrical conditioning
stimulus to abolish the inhibition in Bi. In the present
study a series of taps was delivered rapidly to Tri over a
period of 7.68 s. Thus activation of Tri motoneurones and
their coupled Renshaw cells could be evident in the crosscovariance functions and co-incidentally collected EMG
from Bi.
The combined anatomical and physiological data of the
present study provide the first evidence for Renshaw cells
and Renshaw inhibition in human neonates. On the basis
of intracellular recordings Naka [33] concluded that Renshaw inhibition was well developed in the kitten 1 week
prior to birth, indicating that Renshaw inhibition is likely
to be established early in development.
4.3.3. Central inhibition of Group Ia interneurones is
present in the newborn
During isolated contraction of Bi there was a higher
probability of evoking inhibition which had a greater
magnitude than when there was co-contraction of Bi and
Tri (Fig. 8). In addition a significant negative correlation
was found between increasing levels of contraction of Tri
and the magnitude of reciprocal inhibition, in Bi. Nielsen
and Kagamihara [34] have demonstrated central depression
of Group Ia inhibition during co-contraction of agonist and
antagonist muscles. As proposed by Nielsen and Kagamihara [34], it is possible that the depression of Group Ia
interneurones is also produced indirectly by facilitation of
recurrent inhibition, both by increasing levels of contraction in Tri and also by descending pathways. A further
mechanism which may underlie the depression of reciprocal inhibition in the present study is presynaptic
inhibition of Group Ia afferents, which has been shown to
be modulated during various voluntary movements in
adults [23]. The observation that the reciprocal inhibition
in the neonate is depressed during co-contraction between
Tri and Bi implies that Group Ia inhibitory interneurones
may be under segmental and suprasegmental control.
4.4. Development of inhibitory and excitatory spinal
reflexes
Around the time of birth in several animal species,
including man, spinal motor centres are undergoing extensive development and refinement of connections
[33,36,37,41,43]. The present study has demonstrated that
80
S.M. Mc Donough et al. / Brain Research 899 (2001) 66 – 81
reciprocal inhibition is present in approximately 25% of
babies at birth and is established in all subjects after 9
months of age, but co-contraction underlies limb movements until at least 18 months of age [18,17]. The
discrepancy between the times of appearance of reciprocal
inhibition and independent agonist / antagonist muscle control may lie in the gradual maturation and integration of
spinal interneuronal reflex systems and the maturation of
their differential control by supraspinal centres.
Acknowledgements
The support of Scope, which provided a research
training scholarship for S.M.M. and of the Wellcome Trust
is gratefully acknowledged. Technical support from Mr. S.
Kelly, Mr. J. Clarke and Mr. G. Arnott is warmly
acknowledged.
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