Brain Research 899 (2001) 66–81www.elsevier.com/ locate /bres

Research report

Reciprocal and Renshaw (recurrent) inhibition are functional in man atbirth

*S.M. Mc Donough, G.J. Clowry, S. Miller, J.A. EyreDevelopmental Neuroscience, Department of Child Health, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 4HH, UK

Accepted 2 January 2001

Abstract

The aims were (1) to determine when in human postnatal development Group Ia reciprocal and Renshaw inhibition can bedemonstrated; (2) to explore the relationship between the expression reciprocal inhibition and the disappearance of Group Ia excitatoryreflexes between agonist and antagonist muscles. Studies were performed on 99 subjects, aged 1 day to 31 years, of whom 53 wereneonates. 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 appliedto the tendon of triceps brachii (Tri). Reciprocal excitatory reflexes were recorded in all but one neonate. Reciprocal inhibition wasobserved in 25% of neonates; evidence is provided that it was likely to have been masked by low threshold reciprocal excitation in theremaining neonates. Reciprocal inhibition was demonstrated in all subjects after 9 months of age. In four neonates there was depression ofinhibition of Bi during co-contraction of Bi and Tri implying that Group Ia interneurones may be under segmental and suprasegmentalcontrol at birth. Renshaw cells, identified in human postmortem cervical spinal cord by their morphology, location and calbindin D28Kimmunoreactivity, were present at 11 weeks post-conceptional age (PCA) and by 35 weeks PCA had mature morphologicalcharacteristics. In four neonates reciprocal inhibitory responses in Bi disappeared when the tap to Tri evoked its own homonymous phasicstretch reflex, providing neurophysiological evidence for Renshaw inhibition of Group Ia inhibitory interneurones. 2001 ElsevierScience 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 The degree of co-contraction expressed by agonist /antago-nist muscle pairs during limb movement depends upon

During spontaneous movement human neonates show a many interrelated factors, in particular the descendingpredominant pattern of co-contraction of agonist /antago- commands to each motoneuronal pool and the balancenist muscle pairs in the limbs, with distinct alternating between reciprocal excitation [32,36,37] and reciprocalpatterns of activation of agonist /antagonist pairs not being inhibition [4,30]. The magnitude of reciprocal inhibition isachieved until several years later in childhood [17–19]. fine-tuned by recurrent inhibition [4]. Descending com-

mands and segmental afferents modulate the excitability ofGroup Ia inhibitory interneurones and Renshaw cells [26]

Abbreviations: Bi, biceps brachii; C, condition; CMCD, central motorand there is also a pattern of mutual agonist /antagonistconduction delay; MVC, maximum voluntary contraction; PCA, postinhibition such that the Group Ia inhibitory interneuronesconceptional age; PRBS, pseudo-random binary sequence; PMCD,

peripheral motor conduction delay; R, cross-correlation function; t, test; inhibit the Group Ia inhibitory interneurones of the antago-T, threshold; Tri, triceps brachii; TMCD total conduction delay; V, nist muscle [4,22] and Renshaw cells excited by agonistscross-covariance inhibit those excited by antagonists and vice versa [40].

*Corresponding author. Department of Child Health, The RoyalThus a-motoneurones, Group Ia inhibitory interneuronesVictoria Infirmary, Queen Victoria Road, Newcastle upon Tyne NE1 4LP,and Renshaw cells together with Group Ia primary spindleUK. Tel.: 144-191-2023-013; fax: 144-191-2023-022.

E-mail address: j.a.eyre@ncl.ac.uk (J.A. Eyre). afferent input form part of an ‘output stage’ spinal

0006-8993/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0006-8993( 01 )02151-5

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network, which interacts with descending pathways to kV. The preamplifiers were a.c. coupled, so that themediate the final relative output of agonist /antagonist reflexes recorded do not show a positive d.c. shift corre-motoneurones [26]. sponding to background muscle activity. The signals were

The inappropriate degree of co-contraction observed digitized using an intelligent interface (Cambridge Elec-early in development is likely to reflect immaturity both of tronic Design, Cambridge, UK, type 1401 plus), sampled atthis spinal neural network and of supraspinal descending 5 kHz, and stored on a computer for analysis.commands. Moreover, the rapid improvement of co-ordi-nation, smoothness of limb movements and dexterity, 2.3. Positioning of subjects and generation ofwhich occurs in the first 24 months after birth, is related to background muscle activitythe progressive development and refinement of both sys-tems. The present study forms part of an investigation to Neonates and babies aged less than 6 months wereascertain the time of expression of essential components of studied lying quietly in a cot. An evacuable plastic bagthe ‘output stage’ spinal network in man and which factors filled with polystyrene beads was used to stabilize the headare important in shaping the functional relationships of the and trunk in the mid-line anatomical position. The left armmature network. It addresses when in postnatal develop- was adducted with the elbow held in 458 of flexion fromment Group Ia reciprocal inhibition and Renshaw inhibi- full extension. Subjects older than 6 months were seatedtion can be demonstrated and the relationship between the either on a parent’s knee or on a chair, with the head, trunkfunctional expression of these reflexes and the restriction and arm positioned as described above. Babies and youngof heteronymous Group Ia excitatory projections between children are not able to maintain a steady level ofagonist and antagonist muscles, which occurs in the first contraction or perform a maximum voluntary contractionfour years after birth [36,37]. (MVC) on request. Reflexes requiring muscle contraction

were therefore obtained in subjects aged less than 12months during spontaneous periods of contraction. For

2. Methods contraction of Bi, children over the age of 18 months helda weight in the half supinated hand with the flexed arm

2.1. Subjects supported at the elbow; the weight varied between 100 and200 g, as judged appropriate to age and stature. To contract

A cross sectional study was performed on 99 subjects, Tri children were encouraged to push the pronated arm63 females, 36 males, aged from 1 day to 31 years; of onto a horizontal support, while an examiner placed a handwhom 53 were term neonates, aged 1–4 days. A longi- over the child’s upper forearm to resist extension at thetudinal study was performed on a further 29 subjects, 20 elbow.females and nine males, recruited at birth and studied at 3 Adults sat in a chair with the head and trunk in themonthly intervals until 12 months of age. Informed, mid-line anatomical position and the left forearm restingwritten consent was obtained from the adult subjects and on a support. As with babies and younger children, the leftthe parent(s) of the babies and children. The study received arm was adducted with the elbow held in 458 of flexionapproval from the Newcastle Health Authority and Uni- from full extension. Contraction of Bi was obtained withversity of Newcastle upon Tyne Joint Ethics Committee. the arm half supinated and a strap was placed over the

wrist to resist elbow flexion. For contraction of Tri the2.2. Electromyograms forearm was pronated and a strap placed across the upper

forearm to resist elbow extension. For each muscle theEMGs in biceps brachii (Bi) and triceps brachii (Tri) EMG was also rectified and integrated (time constant 1 s)

were recorded using Ag–AgCl skin-mounted, standard and displayed to the subject as a horizontal line on anEEG electrodes, 5 mm in diameter, with centres separated oscilloscope. The MVC of each muscle was recorded asby 15 mm for babies and children up to 9 years and 20 mm the greatest level of rectified, integrated EMG, obtained byfor older children and adults. To promote integration of the subject in three attempts separated by 1 min restmotor unit activities in the surface EMG the electrodes intervals. During reflex testing the subject contracted thewere orientated in alignment with the main direction of the target muscle to raise the EMG display to meet a targetunderlying muscle fibres. The electrodes were placed on line set at 10% MVC.the muscle belly for Bi and over the belly of the lateralhead for Tri, in positions approximately midway between 2.4. Electromechanical tapperthe motor point and the tendon. The EMG was recordedunrectified [31] and was fed to differential preamplifiers of The reflexes were elicited in Bi or Tri with a handheld60 dB voltage gain, 100 dB at 50 Hz common mode electromechanical tapper (Ling Altec, Royston, UK, Modelrejection ratio, 100 MV input impedance, 50 V output 101) delivering a sequence of taps (cross-correlationimpedance and 23 dB frequency bandpass of 10–1000 method) or a single tap (spatial summation procedure), inHz. The impedance of the electrodes was maintained at ,5 each case of 5 ms duration, comprising rise and decay

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times of 2.5 ms. The stylus of the tapper, a Perspex round 2.5. Cross-correlation method for recording spinaldisc (diameter 8 mm for babies, 10 mm for adults) was reflexesapplied to the skin overlying the tendon of the muscle. Thepeak force (N) of the tap was measured with a transducer Spinal myotatic reflexes were elicited by a pseudo-placed in series with the stylus. The tapper was driven by a random binary sequence (PRBS) of taps applied to theconstant gain power amplifier. The peak voltage of the muscle tendon and were recorded in the surface EMG (Fig.pulse feeding the amplifier correlated linearly with the 1A). The characteristics of the reflexes were measuredpeak force delivered over a range of 0–15 N (r50.998). In from the functions obtained by cross-correlation of thepractice the reflexes were obtained with peak forces in the PRBS with the EMG. The cross-covariance function Vrange 0.5–10.0 N [31]. 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 surfaceEMG 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 isindicated on the abscissa. (B–I) Excitatory and inhibitory responses. Each response represents the average of six consecutive sequence responses (definedas 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 singlePRBS 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, homonymousphasic 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 Triat 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.

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tion function R estimates the degree of correlation between both to soft tissue near to the tendon of triceps and also toinput PRBS and output EMG [31]. The authors showed the lateral humeral epicondyle. If these taps evoked athat the onset latencies of cross-correlation functions of the reflex response in Tri or Bi the subjects were excludedreflexes are advanced in time by 10 ms, the clock period of from the study. Two neonates were excluded on this basis.the PRBS, and 10 ms has therefore been added to allmeasures of latency [31]. 2.8. Control for volume conduction of the EMG

A sequence response was defined as the cross-covar-iance function of the EMG and PRBS following the If in phase or out of phase deflections in the cross-application of the 64 taps in a single PRBS signal lasting covariance functions were observed at any stage in Bi and1.28 s. A trial comprised the average of six sequence Tri, all the data from these muscles were excluded fromresponses obtained with consecutively delivered PRBS the analysis. In fact no subject was excluded on this basis.signals [31].

2.9. Reflex threshold2.6. Reflex response

In determining reflex threshold (T), the tap magnitudeA response was defined as present if it was discernible was first set suprathreshold and then reduced stepwise until

by eye in the trial average of six cross-covariance func- reflexes were identified in three out of the six sequences intions and occurred with a consistent onset latency (Fig. a trial (a 50% response rate).1B–I). In each experimental session the polarity of theinitial phase of the homonymous phasic stretch reflex in 2.10. Reflexes studiedbiceps was determined. If it was positive the polarity of therecording electrodes was appropriate. If it was negative the 2.10.1. Homonymous phasic stretch reflexespolarity of the differential electrodes was reversed so that Homonymous phasic stretch reflexes were elicited inthe homonymous phasic stretch reflex showed an initial contracting Bi and Tri. The thresholds and onset latenciespositive deflection. Heteronymous reflexes evoked in of the trial averages were recorded (Fig. 1B, D, F and H).biceps by a tap to triceps were then defined as excitatory ifthe initial component was positive going (Fig. 1B–D and 2.10.2. Short latency heteronymous excitatory andFig. 1F–H) and as inhibitory if the initial component was reciprocal inhibitory responses from Tri to Binegative going (Fig. 1E and I). This convention was Once the threshold for the homonymous phasic stretchvalidated by corresponding changes in torque about the reflex in Tri had been determined (T), the force of the tapelbow. Thus initially upward going responses in the EMG was set at a level between 0.2 and 0.5 T and applied toof biceps, designated as excitatory, have been shown to relaxed Tri. The responses were recorded in contracting Bi.correspond to an increase in flexor contractile force, while The force delivered was increased stepwise in units of 0.1downward going responses in the EMG of biceps, desig- T up to a maximum of 1.5 T. At each of these stimulusnated as inhibitory, to correspond to a decrease in flexor levels the trial average was inspected for evidence ofcontractile force [31]. excitatory heteronymous responses (Fig. 1C and G) or

reciprocal inhibitory responses (Fig. 1E and I). The2.7. Controls for mechanical spread of the tap stimulus thresholds, onset latencies and peak to peak amplitudes of

the responses were recorded.In 10 neonates and 10 adults the onset of the mechanical

pulse induced by the tap to triceps was measured using an 2.11. Spatial summation experiment: corticospinalaccelerometer placed over the site of the recording elec- projection to Group Ia inhibitory interneurones (Fig. 4)trodes on biceps (Table 1). In all subjects, controls weremade for reflex effects induced by the direct mechanical This component of the corticospinal projection wasspread of the tap to biceps [36,37]. Taps of 1.5 T for the investigated by spatial summation at presumed Tri Grouphomonymous phasic stretch reflex in Tri, were applied Ia inhibitory interneurones in two neonates and two adults.

Summation was tested between a Tri Group Ia afferentvolley, subthreshold for evoking a heteronymous excitat-ory response or a reciprocal inhibitory response in Bi, and

Table 1 a corticospinal volley, subthreshold for evoking a responseOnset of mechanical impulse in Bi following taps to the tendon of Tri

in Bi or Tri.n Mean6S.E.M.

(ms)2.11.1. Excitation of a corticospinal volley

Neonates 10 0.0560.08 Transcranial magnetic stimulation (TMS) using a Mag-Adults 10 1.6360.48 stim 200 stimulator, (MagStim, Whitland, UK), with a 90

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mm circular coil placed tangentially above the contralateral measured as the peak of the short latency inhibitorymotor cortex, excited corticospinal neurones [9,12–14]. response and expressed as a percentage of the RMS valueThe optimal position for exciting corticospinal axons and of the background EMG in Bi during each set of repeats.the conduction delay from TMS to the arrival of thecorticospinal volley at Tri Group Ia interneurones wasassumed to be the same as for corticospinal axons project- 2.12. Renshaw (recurrent) inhibitioning to Tri a-motoneurones. The optimal coil position forevoking EMG responses in Tri was determined usingsuprathreshold stimuli. The intensity of TMS was then 2.12.1. Anatomical study (Fig. 6)decreased until responses were evoked in contracting Tri in The characteristic morphology and location and calbin-50% of trials, defined as threshold (T). The shortest onset din D28K (calbindin) immunoreactivity were used tolatency of 20 responses to TMS at 1.2 T was obtained to identify Renshaw cells in human postmortem cervicalgive the total motor conduction delay (TMCD) to Tri spinal cord obtained from a surgically aborted fetus of 11muscle. Magnetic stimulation, with a 5 cm circular coil, weeks post-conceptional age (PCA) and nine pretermplaced in the coronal plane overlying cervical spines 5–8, babies aged 26–35 weeks PCA, who died from non-was used to excite spinal motor roots at the point of neurological causes (Fig. 5). The spinal cords wereemergence from the intervertebral foramina [39]. The immersion fixed in buffered 4% paraformaldehyde solutionperipheral motor conduction delay (PMCD) to the Tri for 24–72 h and then sectioned into segments and im-muscle was estimated from the longest onset latency mersed in buffered 30% sucrose solution for at least 24 h.obtained from at least 20 responses [13]. The subtraction Frozen 50 mm sections were cut on a freezing microtomeof PMCD from TMCD was used to estimate the corticospi- and then immunostained free floating using standardnal conduction and trans-synaptic delay to Tri a- techniques. A well characterized monoclonal antibody tomotoneurones. calbindin (Sigma), was employed as a primary antibody at

a dilution of 0.05% in phosphate buffered saline containing0.1% Triton X-100 (Sigma) and 3% normal horse serum

2.11.2. The conduction and trans-synaptic delay from (Vector Labs.). The sections were incubated overnight withTri Group Ia afferents to Tri Group Ia inhibitory gentle agitation. The immuno-localization of calbindin wasinterneurones visualized by incubation with biotinylated horse anti-

The conduction and trans-synaptic delay from Tri Group mouse antibodies (0.5%, Vector Labs.), followed by strep-Ia afferents to Tri Group Ia inhibitory interneurones was tavidin–horseradish peroxidase (HRP) conjugate (0.5%,assumed to be the same as that to Tri a-motoneurones. It Vector Labs.) in phosphate-buffered saline between andwas estimated by subtracting the PMCD to Tri muscle after incubations. The sections were reacted withfrom the shortest onset latency of the homonymous phasic diaminobenzidine (0.05%, Sigma) in the presence ofstretch reflex in Tri obtained from responses to at least 20 hydrogen peroxide (0.003%) in phosphate-buffered salinesingle taps delivered at 1.2 T to the tendon of Tri. and washed and mounted on slides.

2.11.3. Spatial summation trials 2.12.2. Neurophysiological study (Fig. 7)Subthreshold tap to Tri (Fig. 4C), TMS at 0.8 T (Fig. Group Ia inhibitory interneurones are inhibited by

4D) and the two stimuli together at the interstimulus Renshaw cells [4,20]. Evidence was therefore sought forinterval of expected convergence of both volleys at Group depression of reciprocal inhibition between Tri and Bi byIa interneurones (Fig. 4E), were delivered in an interleaved recurrent activity in Tri motoneurone axon collaterals. Thesequence. Five repeats of this sequence were performed cross-correlation method for spinal reflexes was used. Fourthree times on each neonate and adult subject. To define neonates were chosen, who had clear inhibitory responsesthe time course of spatial summation in the two adults five in Bi following a tap to relaxed Tri with a low threshold,repeats of the sequence were used, conducted over a range i.e., ,0.7 T, and who also had high thresholds forof interstimulus intervals such that the corticospinal EPSP heteronymous excitatory responses in Bi, i.e., .1.3 T.arrived at biceps a-motoneurones from 2 ms before to 10 Threshold taps to Tri (1.0 T) were applied to evoke ams after the estimated arrival of the Group Ia volley (Fig. homonymous phasic stretch reflex in contracting Tri in5). The condition to test interval (C–t), the interval of 50% of sequence responses. These taps were suprathres-expected convergence, was defined as C–t interval zero hold for evoking reciprocal inhibition in Bi, but substan-and all other C–t intervals were defined relative to this. tially below the threshold for evoking a heteronymousThus negative C–t intervals indicated the Group Ia volley excitatory response in Bi. Evidence was sought for con-arriving before the corticospinal volley and positive C–t sistent absence of reciprocal inhibition of Bi in those trialsintervals indicated the corticospinal volley arriving before when a homonymous phasic stretch reflex in Tri wasthe Group Ia volley. The magnitude of the inhibition was simultaneously evoked [30] (Fig. 7).

S.M. Mc Donough et al. / Brain Research 899 (2001) 66 –81 71

2.13. Relationship between level of contraction in Tri of the homonymous phasic stretch reflex in Bi (Table 2,and the probability and magnitude of the inhibition P,0.0001), with a mean difference of 1.8 ms60.1 msevoked in contracting Bi during co-contraction of Bi and (Fig. 2B and C and Fig. 3A).Tri (Fig. 8) When reciprocal inhibitory responses from Tri to Bi

were observed in neonates, the onset latencies alsoIn the same four neonates studied above, taps at 0.8 T occurred significantly later than the homonymous phasic

were applied to the tendon of Tri, so that the tap would be stretch reflex in Bi (Table 2: cross-sectional study, P,

subthreshold for either a homonymous phasic stretch reflex 0.0001; longitudinal study, P,0.001), with a mean differ-in Tri or a heteronymous excitatory response in Bi, but ence in onset latency of 1.860.3 ms for the cross-sectionalsuprathreshold for an inhibitory response in Bi. The study and 1.060.4 ms for the longitudinal study (Fig. 2Bmagnitude of the inhibitory response in Bi was determined and C and Fig. 3B).(1) during periods of isolated activity in Bi and (2) during The difference in onset latency between reciprocalspontaneous co-contraction of Bi and Tri when the mag- inhibition from Tri to Bi and the homonymous phasicnitude of the inhibitory response evoked in Bi was related stretch reflex in Bi, remained remarkably consistent withto the level of contraction in Tri. age, the mean varying within the range 11.0 to 11.8 ms

For each trial the levels of contraction in Tri and Bi for both the cross sectional and longitudinal study groupswere determined as the root mean square (RMS) of their (Fig. 2B and C).respective EMGs. The magnitude of the inhibition in eachtrial was determined to be the difference between the 3.2. Group Ia heteronymous excitatory responses frombaseline to peak values of the inhibitory response in the Tri to Bi (Figs. 2 and 3)trial average expressed as a percentage of the RMS valueof the EMG in Bi during that trial. The amplitudes of Heteronymous excitatory responses [36,37] were ob-inhibitory responses had a skewed distribution and under- served in 52/53 neonates (98%) in the cross-sectionalwent logarithmic transformation. study and 29/29 neonates (100%) in the longitudinal study

It is not possible to measure MVC in neonates and (Fig. 2E). The probability of demonstrating a heteronym-therefore the magnitude of the EMG in individual subjects ous excitatory response in Bi decreased slowly with age.could not be normalized by reference to EMG values at At 12 months of age 38/53 (72%) of cross-sectionalMVC. To remove between child variation in the mag- subjects and 20/29 (71%) of longitudinal subjects stillnitudes of EMG in Bi and Tri, the mean values across all demonstrated heteronymous excitatory responses, but intrials for the level of contraction of Tri and the magnitude adult subjects these responses were only present in twoof the inhibition in Bi were determined for each neonate. subjects (10%; Fig. 2E).The magnitudes of contraction in Tri or inhibition in Bi in For neonates there were no significant differenceseach trial were then expressed in terms of their difference between the onsets of homonymous phasic stretch reflexesfrom the mean value (called a residual value). Thus in Bi and the heteronymous excitatory response from Tri tonegative values indicate magnitudes which were less than Bi (Table 2, Fig. 2F and G and Fig. 3B). However, thethe mean for that subject and positive values those which difference in the onset latencies of the heteronymouswere greater than the mean (Fig. 8). excitatory responses and the homonymous phasic stretch

reflex in Bi increased progressively with age (Fig. 2F andG), so that by adulthood the onsets of the heteronymous

3. Results excitatory responses were 11 ms and 12 ms longer than theonset of the homonymous phasic stretch reflex.

Unless otherwise stated all data are given asmeans6S.E.M. 3.3. Thresholds of heteronymous excitatory and

reciprocal inhibitory responses3.1. Reciprocal inhibition from Tri to Bi (Figs. 2 and 3)

The thresholds for reciprocal inhibitory and heteronym-Short latency reciprocal inhibition from Tri to Bi was ous excitatory responses in Bi (Fig. 2D and H and Fig. 3

demonstrated in neonatal subjects in 15/53 (28%) of the and D) were normalised by reference to the threshold (T)cross sectional and 6/29 (21%) of the longitudinal study. of the homonymous phasic stretch reflex in contracting Tri.The probability of demonstrating short latency inhibition In adult subjects the threshold of reciprocal inhibitoryhowever increased rapidly postnatally so that after 9 responses in Bi were significantly lower than those of themonths postnatal age all subjects in both the cross-section- homonymous phasic stretch reflex in Tri (Table 2, P,

al and longitudinal studies demonstrated short latency 0.02, Fig. 3C) and of the heteronymous excitatory re-reciprocal inhibition (Fig. 2A). sponses in the two adult subjects (Table 2, Fig. 3C).

For the adult subjects the onset latency of reciprocal Similarly, when reciprocal inhibitory responses in Biinhibition from Tri to Bi was significantly longer than that were demonstrated in neonates, the thresholds were sig-

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Fig. 2. Reciprocal inhibitory (A–D) and heteronymous excitatory responses (E–F) in Bi following taps to Tri. The symbols represent the means and thevertical lines the 95% confidence limits of the means. Filled circles: data from the cross-sectional study, numbers above the abscissa give subjects in eachage group. Open circles: data from 29 children studied longitudinally. (A and E) The percentage of subjects demonstrating a response in relation to age. (Band F) Onset latencies of the trial average responses in relation to age. The onset latencies of homonymous phasic reflexes in Bi are also plotted astriangles. Filled triangles: cross-sectional data; open triangles: longitudinal data. (C and G) Relative onset latencies of responses, defined as the onsetlatency of responses minus the onset latency of the homonymous phasic stretch reflex in Bi. The dashed horizontal line through 0 represents the onset ofthe homonymous phasic stretch reflex in Bi. (D and H) Threshold of responses. Thresholds are defined as percentages of the threshold of the homonymousphasic stretch reflex in contracting Tri. The horizontal dashed line at 100% represents the threshold of the homonymous phasic stretch reflex of Tri. Thehorizontal dotted line represents the mean threshold of the reciprocal inhibitory responses in adults.

nificantly lower than those for the homonymous phasic sponses in Bi (Table 2: cross-sectional study, P.0.05,stretch reflexes in Tri (Table 2: cross-sectional study, longitudinal study, P,0.01; Fig. 3D).P,0.02; longitudinal study, P,0.02, Fig. 3D) and lower In contrast, the neonates, who did not demonstratethan the threshold of the heteronymous excitatory re- reciprocal inhibition, had heteronymous excitatory re-

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Table 2Onset and threshold of reflexes

Bi homonymous phasic stretch reflex Reciprocal inhibition Heteronymous excitatory reflexesBi→Bi Tri→Bi Tri→Bi

n Mean6S.E.M. (ms) n Mean6S.E.M. (ms) n Mean6S.E.M. (ms)

Onset latencyAdults 20 18.560.4 20 20.360.5 2 31, 33

Neonates: 53 13.960.2 15 15.760.5 52 13.760.3Cross-sectional 29 14.060.3 6 15.060.5 28 13.960.3Longitudinal

Threshold Mean6S.E.M. (T) Mean6S.E.M. (T)

Adults 20 0.860.10 2 1.35, 1.20

Neonates with inhibition:Cross-sectionalLongitudinal 15 0.760.09 14 1.0060.09

6 0.660.17 6 1.0960.05

Neonates not demonstrating inhibition:Cross-sectional 38 0.6060.06Longitudinal 23 0.5660.06

sponses in Bi with thresholds that were significantly lower Tri occurred at the estimated coincidence of volleys at Trithan the threshold of their homonymous phasic stretch Group Ia inhibitory interneurones in the two neonates andreflex in Tri (Table 2: cross-sectional study, P,0.001; the two adults studied. In all subjects spatial summationlongitudinal study, P,0.001; Fig. 3D) and were also resulted in inhibition of Bi EMG with onset latencies 1–2significantly lower than the thresholds of the heteronymous ms longer than EMG responses evoked in Bi by supra-excitatory responses observed in those neonates in whom threshold TMS (Fig. 4E). In the neonatal subjects thereciprocal inhibition was demonstrated (Table 2: cross- EMG was depressed by a mean of 32.365.2% (P,0.01)sectional study, P,0.01; longitudinal study, P,0.001; Fig. and 29.366.1% (P,0.01) and in the adults by 1563.2%3D). The thresholds of the heteronymous excitatory re- (P,0.01) and 3864.1% (P,0.01), respectively.sponses in the neonates without reciprocal inhibition were The time course of the spatial summation was defined insimilar to the thresholds of the inhibitory responses the two adult subjects (Fig. 5). Spatial summation occurredobserved in the neonates with reciprocal inhibition (Fig. over C–t intervals of 0 to 14 ms and 0 to 17 ms, with3D). peak depressions of the EMG of 32.564% and 55.6613%

The threshold for reciprocal inhibition relative to that of respectively (Fig. 5A).the homonymous phasic stretch reflex in Tri remainedremarkably consistent with age with the mean lying 3.5. Renshaw (recurrent) inhibitionbetween 0.65 T and 0.8 T at all ages in both thelongitudinal and cross-sectional studies (Fig. 2D). In 3.5.1. Anatomical study (Fig. 6)contrast, the threshold for heteronymous excitatory re- In the human post-mortem material calbindin positivesponses increased progressively with age relative to that of cells were observed in the appropriate location for Re-the homonymous phasic stretch reflex in Tri. By 9 months nshaw cells at all the ages examined [2,3] (Fig. 6). At 11the mean threshold of the heteronymous responses was weeks PCA most putative Renshaw cells had a stronglygreater than that of reciprocal inhibition, and by 12 months immunoreactive cell nucleus with only weakly immuno-the mean threshold of the heteronymous responses was reactive cytoplasm (Fig. 6A and B). Every cell exhibitedgreater than that of the homonymous phasic stretch reflex weak immunoreactivity in two or three dendritic processesin Tri (Fig. 2H). but only close to the cell body. In the motoneuronal pools

the occasional cell resembling a motoneurone was calbin-3.4. Corticospinal projection to Group Ia inhibitory din positive, but no calbindin positive axons were observedinterneurones (Fig. 4) either in the grey matter or in the white matter. By 23

weeks PCA (Fig. 6C and D) calbindin immunoreactivitySpatial summation of a Tri Group Ia afferent volley was much stronger in the cytoplasm of cells with the

subthreshold for evoking a heteronymous excitatory or characteristic morphology and location of Renshaw cells.reciprocal inhibitory response in Bi and a corticospinal Each cell had two or three immuno-positive proximalvolley subthreshold for evoking a response in either Bi or 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 whomreciprocal inhibition was not evoked (inhibition absent) and those in whom it was (inhibition present). The circles represent means and the vertical lines95% confidence limits of the means. For neonates filled circles represent data from the cross-sectional study, open circles from the longitudinal study. (Aand 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. Thehorizontal dotted line at 100% represents the threshold of the homonymous phasic stretch reflex of Tri. Bi, SR homonymous stretch reflex in Bi; Tri→BiEXCIT, heteronymous excitatory responses from Tri to Bi; Tri→Bi INHIB, inhibition from Tri to Bi; the numbers above the absicca indicate the number ofsubjects; NA, not applicable.

branches. Many calbindin positive axons could be seen stretch reflex in contracting Tri, inhibition was not ob-towards the inner boundary of the white matter all round served in contracting Bi (Fig. 7D).the ventral horn. No axons were present in the grey matterof the motoneuronal pools and no motoneurones were 3.6. Relationship between level of co-contraction in Tricalbindin positive at this age. Over the next 11 weeks of and Bi and the probability and magnitude of thedevelopment the appearance of presumed Renshaw cells inhibition evoked in contracting Bi (Fig. 8)changed, so that by 35 weeks post-conceptional age (Fig.6E, F and G) the proximal dendrites and their branches had One hundred and forty-five informative trial averagesthickened considerably and could be traced for several were obtained in the four neonates studied: minimumhundred microns. They often appeared to run in parallel number of trial averages per subject530 (representing 180bundles along the medial edge of the motoneuronal pools sequences), maximum543 (representing 258 sequences).or along the grey /white matter boundaries. Many calbindin The mean RMS values for EMG in Tri varied from 12.3 toimmunoreactive varicose axons were present in the 14.9 mV and the mean values for the amplitude ofmotoneuronal pools and some appeared to make contact inhibitory responses in Bi, expressed as the percentagewith motoneurone cell bodies. reduction in the background level of contraction in Bi,

varied from 38 to 64%, respectively.Reciprocal inhibitory responses were observed in 98/

3.5.2. Neurophysiological study (Fig. 7) 145 (68%) trials. Reciprocal inhibition was most frequent-In all four neonates studied it was a consistent finding ly observed and had its maximum amplitude when there

that when a tap of 1.0 T evoked a homonymous phasic 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 andsubthreshold Tri Group Ia volley in a neonatal subject. Open arrowsindicate onset of tap to Tri. Filled arrows indicate onset of TMS. (A andB) 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 wassubthreshold for evoking excitation or inhibition in Bi. (Note: heteronym-ous excitation of Bi with a threshold of 0.4 T was observed but reciprocalinhibition was not demonstrated). (C) Subthreshold tap to Tri. (D) TMSsubthreshold for evoking a response in Bi or Tri (0.8 T). (E) The twostimuli together at the interstimulus interval of estimated convergence atTri Group Ia interneurones resulting in inhibition of contracting Bi. T,Threshold; TMS, transcranial magnetic stimulation.

Fig. 5. Inhibition of biceps following spatial summation of a subthreshold2 triceps Group Ia afferent volley and a subthreshold corticospinal volley at12 /13 trials, 93%; Tri active, 86 /132 trials, 65%; x 598

triceps Group Ia inhibitory interneurones. (A) Time course and magnitudeP,0.001; amplitude of reciprocal inhibition; Tri not of spatial summation, open symbols represent the mean for each subjectactive, n512, 2.0360.14 l %; Tri active, n586, and vertical lines the S.E.M. of the peak of the depression of the EMGn

expressed as a percentage of the background EMG. Time base C–t0.1060.09 l %; P,0.0001). When reciprocal inhibition inninterval 0: the interstimulus interval of expected spatial convergence of aBi was evoked during co-contraction of Tri and Bi, therecorticospinal and Group Ia volley at triceps Group Ia inhibitory inter-was a significant negative correlation between the level ofneurones. Negative C–t intervals: Group Ia before corticospinal volley,

contraction in Tri during the trial and the magnitude of the positive C–t intervals: corticospinal before Group Ia volley. Stars indicate2inhibitory response in Bi (r 50.22, P,0.001; Fig. 8). statistically significant (P#0.05) differences between conditioned and test

reflexes. (B) EMG of biceps when the subthreshold MAG and sub-threshold tap to Tri are delivered together at the defined interstimulus(C–t) intervals for the adult subject illustrated by s in (A). Open arrows4. Discussionindicate onset of tap to Tri. Filled arrows indicate onset of TMS.

Short latency inhibition and excitation of Bi EMG wasdemonstrated in neonates and adults following a tap to the mechanical transmission of the tap through the armtendon of relaxed Tri. Direct volume conduction of exciting spindle afferents of Bi (Table 1). However, a tapreflexes evoked in Tri is excluded since the tap was of up to twice the force delivered to Tri’s tendon failed tosubthreshold for reflexes in relaxed Tri. It is possible that evoke reflexes in Bi when applied to soft tissue near Tri’sthese heteronymous responses could have arisen from 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 segmental level. At all ages a strongly stained group of6

neurones is located where the ventral roots coalesce before traversing the ventral white matter. These are putative Renshaw cells (R). (A) Calbindin D28Kpositive neurones are present as early as 11 weeks post-conceptional age which include putative Renshaw cells. (B) A higher power image at 11 weekspost-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 stainingin either the grey or white matter. (C and D) By 23 weeks post-conceptional age all motoneurones have lost calbindin D28K immunoreactivity, and there isno immunoreactivity in the surrounding neuropil, although calbindin D28K positive axons are present in the white matter. The Renshaw cell group (R) isagain prominent. At higher magnification the cytoplasm is seen to be heavily immunostained and thin dendrites can be clearly seen. (E, F and G) By 35weeks post-conceptional age axonal staining is strong both in the ventral white matter and in the neuropil around motoneurones (M in E and G). TheRenshaw 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 77

Fig. 7. Trial averages recorded in the EMGs of contracting Tri and Bi inresponse to PRBS taps to the tendon of Tri in a neonate. The tap intensityis expressed as multiples of the threshold (T) of the homonymous phasicstretch reflex in contracting Tri. (A–C) Inhibitory responses are observed Fig. 8. (A) The magnitude of the inhibitory responses in Bi in relation thein Bi in response to taps of Tri at 0.5, 0.9 and 1.0 T in the absence of a level of contraction in Tri in four neonates. To remove between subjecthomonymous phasic stretch reflex in Tri. D. Inhibition of Bi is not present variability in the absolute value of the EMG in Bi and Tri, the mean valuewhen a homonymous phasic stretch reflex is evoked in Tri by a tap of 1.0 for the level of contraction of Tri and the magnitude of the inhibition inT. Bi, was determined for each neonate. The magnitudes of contraction in

Tri or inhibition in Bi were then expressed as a residual from theirrespective mean. Thus negative values indicate magnitudes which wereless than the mean for that subject and positive values those which were

mechanical spread of the stimulus did activate spindles in greater than the mean. The amplitudes of inhibitory responses had askewed distribution and underwent logarithmic transformation. NI, noBi, the activity was subthreshold for evoking reflexinhibition of Bi; NC, no contraction in Tri.responses. Since activation of Bi spindle afferents excites

Bi motoneurones, mechanical transmission of the tap islikely to have reduced the probability of demonstratingreciprocal inhibition and enhanced the probability of present in approximately 25% of neonates, with thresholdsdemonstrating heteronymous excitation. Furthermore, to and central synaptic delays similar to those of adults [30].demonstrate inhibition of Bi, using the paradigm of thepresent study, background contraction was required. Vol- 4.1.1. Group Ia afferents mediate short latencyuntary contraction of one muscle in an agonist /antagonist reciprocal inhibition in adults and neonates andpair leads to descending excitation of its Group Ia inhib- throughout postnatal developmentitory interneurones [10,21,35,44]. Thus, isolated contrac- Group Ia inhibitory interneurones have been shown totion of Bi, would have led to inhibition of the antagonists mediate short latency reciprocal inhibition between antago-(Tri) Group Ia inhibitory projections to Bi [21]. Overall nist muscles in the cat [28]. The threshold for evokingtherefore the paradigm used in the present study will have inhibitory responses in Bi lay significantly below that ofreduced the probability of demonstrating reciprocal inhibi- the homonymous phasic stretch reflex in Tri (Fig. 2D,tion, but will have enhanced the ability to detect Table 2) implying that the inhibition was mediated by lowheteronymous excitation. threshold afferents. A brief, low intensity tap to relaxed Tri

was chosen to evoke the reflexes since it excites pre-dominantly Group Ia afferents, although a small number of

4.1. Reciprocal inhibition low threshold cutaneous afferents are also excited [6,38].The inhibition mediated by cutaneous afferents is not

A major finding of this study was that short latency present in neonates and does not appear until 18 months ofreciprocal inhibition from Tri to its antagonist Bi was 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 ) demonstrate that the threshold for evoking heteronymous1

is preceded and followed by early (E ) and late (E ) excitatory responses is significantly lower in neonates than1 2

excitation, respectively. Thus the inhibition observed in the that for evoking homonymous phasic stretch reflexes (seepresent study is unlikely to have been mediated by Fig. 4 and Figs. 2H and 3D). Heteronymous excitatorycutaneous afferents. responses occurred less frequently with age and their

thresholds and onset latencies increased. Thus with de-4.1.2. Short latency reciprocal inhibition has a velopment heteronymous excitatory responses may requiredisynaptic linkage from birth more temporal summation and/or be mediated by afferents

Group Ia afferents project monosynaptically to Group Ia with slower conduction velocities. The homonymousinterneurones leading to a disynaptic inhibition of antago- phasic stretch reflex evoked by a tap produces oligosynap-nist muscles [1,28]. The estimated additional synaptic tic as well as monosynaptic excitation of the motoneuronesdelay above that for a monosynaptic reflex, for the subjects [5]. Such oligosynaptic projections are also likely also toin the present study was 1.0–1.8 ms, consistent with a be involved in heteronymous excitation of Bi following adisynaptic linkage. This additional synaptic delay is longer tap to Tri. The later onset of heteronymous excitation of Bithan that previously estimated for reciprocal inhibition in in adults may therefore reflect withdrawal of the mono-adults [10,30], when electrical stimulation of the afferent synaptic projection during development leaving the onsetfibres from the antagonist muscle was used to evoke of the reflex dependent upon an oligosynaptic linkage.reciprocal inhibition. The rise time of motoneuronal EPSPs Neonates and infants who demonstrated reciprocal inhi-following tendon tap is longer than that following electri- bition, had significantly higher thresholds for theircal stimulation to the peripheral nerve [5] and together heteronymous excitatory responses than those not demon-with the time required for transduction of the tap at muscle strating reciprocal inhibition (Fig. 3D). We suggest twospindles, would be more than sufficient to account for the possible explanations for this observation. First, the restric-longer central delays of the present study. Katz et al. [30] tion of heteronymous excitation between agonist andalso included a tendon tap to the antagonist muscle as part antagonists may occur because of the establishment ofof the protocol of their study, and demonstrated a 12 ms short latency reciprocal inhibition during development.discrepancy in the onset of the inhibitory responses in Thus the establishment of short latency reciprocal inhibi-comparison to electrical stimulation, which they also tion leads to higher thresholds for heteronymous reciprocalattributed to the temporal dispersion of the volley evoked excitation. If this is so then the establishment duringby the tendon tap. development of reciprocal inhibition is not the only, nor

the most significant process involved in the restriction of4.1.3. Direct corticospinal projections to Group Ia heteronymous reciprocal excitation. From 9 months of ageinhibitory interneurones all subjects demonstrate reciprocal inhibition but 70% still

In the monkey Group Ia interneurones receive a mono- had heteronymous reciprocal excitation (Fig. 2A, E, F andsynaptic projection from Group Ia afferents and from G).corticospinal axons [29]. In the present study we demon- An alternative explanation may be that low thresholdstrated inhibition of Bi EMG following spatial summation and short onset heteronymous excitatory responses inof a subthreshold Tri Group Ia afferent volley and a neonates and infants mask reciprocal inhibitory responsessubthreshold corticospinal volley at their expected time of which have at that stage both a higher threshold and a laterconvergence on Group Ia interneurones. This spatial onset. Thus, as the threshold for heteronymous reciprocalsummation establishes a monosynaptic corticospinal pro- excitation increases with development to be greater thanjection to Group Ia inhibitory interneurones is also present that for reciprocal inhibition, reciprocal inhibition can bein human neonates and adults. demonstrated. The latter argument receives strong support

from an observation in the present study. The neonates in4.2. Heteronymous excitatory responses whom the corticospinal projection to Group Ia inhibitory

interneurones was investigated had, by chance, very lowHeteronymous excitatory responses evoked from Tri to thresholds for heteronymous excitation between Tri and

Bi were observed in all but one of the neonates (Fig. 1F). Bi. Neither demonstrated short latency reciprocal inhibi-Heteronymous excitatory reflexes to non-synergistic tion from Tri to Bi when a tap was delivered to Tri (Fig.motoneurones, including those of the antagonist, have been 4B). However, spatial summation of a Tri Group Iareported previously in immature animals [11,41,43] and by afferent volley (subthreshold for modulating the activity ofourselves in babies [32,36,37]. We confirmed our previous Bi motoneurones) with a subthreshold corticospinal volley,observations that the initial component of the heteronym- revealed short latency inhibition of Bi in both subjectsous excitatory reflexes is monosynaptically linked in the (Fig. 4E). If, as is likely from this observation, thatnewborn, since heteronymous excitatory reflexes in Bi had reciprocal inhibition from Tri to Bi is masked by lowsimilar onsets to that of the homonymous phasic stretch threshold heteronymous excitatory reflexes, it is possiblereflex [36,37]. The present study however was the first to that reciprocal inhibition may be present in all neonates.

S.M. Mc Donough et al. / Brain Research 899 (2001) 66 –81 79

4.3. Renshaw (recurrent) inhibition evoked or not. Similarly descending inhibition of therespective pools of reciprocal inhibitory interneurones

4.3.1. Anatomical study during spontaneous co-contraction of Tri and Bi [34] willCalbindin immunoreactive neurones found in the ventral also remain the same, irrespective of whether a homonym-

horn of both rat [2] and monkey [3] have the same ous phasic stretch reflex in Tri was evoked or not. It ismorphology and location as electrophysiologically iden- concluded, therefore, that the suppression of reciprocaltified and intracellularly labelled Renshaw cells in the cat inhibition in Bi arose from Renshaw inhibition evoked by[27]. Furthermore, in both rat and cat, calbindin positive recurrent activity in Tri motoneurone axon collaterals, in aneurones in this location express characteristic glycine similar manner to that described by Katz et al. [30] in adultreceptor complex clusters of identified Renshaw cells subjects. In that study a suprathreshold tap to Tri had to be[3,7,8]. Injury to sciatic motor axons in rat down-regulates delivered at least 1.5 ms before the electrical conditioningcalbindin expression in putative Renshaw cells of the stimulus to abolish the inhibition in Bi. In the presentlumbar cord, suggesting a functional link between sciatic study a series of taps was delivered rapidly to Tri over amotoneurones and these cells [42]. In rat cervical cord, period of 7.68 s. Thus activation of Tri motoneurones andusing double immunofluorescence techniques, putative their coupled Renshaw cells could be evident in the cross-calbindin positive Renshaw cells have been shown to be covariance functions and co-incidentally collected EMGrichly innervated by motor axon collateral terminals, from Bi.labelled by injection of cholera toxin B into muscle The combined anatomical and physiological data of the[15,16]. Taken together, these observations make a strong present study provide the first evidence for Renshaw cellscase for using calbindin immunoreactivity as a marker for and Renshaw inhibition in human neonates. On the basisRenshaw cells in human post-mortem tissue, particularly of intracellular recordings Naka [33] concluded that Re-as the immunostaining is reliable even in less than nshaw inhibition was well developed in the kitten 1 weekoptimally fixed tissue. prior to birth, indicating that Renshaw inhibition is likely

In human spinal cord putative Renshaw cells were to be established early in development.identified from as early as 11 weeks post-conceptional ageand by 35 weeks Renshaw cells were identified which 4.3.3. Central inhibition of Group Ia interneurones iswere morphologically mature. In addition, calbindin im- present in the newbornmunoreactive varicose axons were present in the During isolated contraction of Bi there was a highermotoneuronal pools, some of which appeared to make probability of evoking inhibition which had a greatercontact with motoneurone cell bodies. Without formally magnitude than when there was co-contraction of Bi andidentifying them as Renshaw cells, Ince et al. [24] de- Tri (Fig. 8). In addition a significant negative correlationscribed a population of calbindin positive neurones in a was found between increasing levels of contraction of Triventromedial location in adult human spinal cord. They and the magnitude of reciprocal inhibition, in Bi. Nielsenalso noted the presence of calbindin positive axonal and Kagamihara [34] have demonstrated central depressionvaricosities in the motoneuronal pools indicating that this of Group Ia inhibition during co-contraction of agonist andis a feature of mature spinal cord. This pattern of axonal antagonist muscles. As proposed by Nielsen and Kagami-staining has also been observed in the lumbar cord of the hara [34], it is possible that the depression of Group Iaadult macaque monkey [3] where it was suggested that interneurones is also produced indirectly by facilitation ofmost, if not all, calbindin positive varicose axons around recurrent inhibition, both by increasing levels of contrac-the motoneurones derive from Renshaw cell axons. tion in Tri and also by descending pathways. A further

mechanism which may underlie the depression of re-4.3.2. Neurophysiological study ciprocal inhibition in the present study is presynaptic

In the four neonates investigated, reciprocal inhibition of inhibition of Group Ia afferents, which has been shown toBi was consistently absent when an homonymous phasic be modulated during various voluntary movements instretch reflex was evoked in Tri (Fig. 7D). The intensity of adults [23]. The observation that the reciprocal inhibitionthe tap to Tri was chosen to be at threshold so that a in the neonate is depressed during co-contraction betweenhomonymous phasic stretch reflex was evoked in 50% of Tri and Bi implies that Group Ia inhibitory interneuronesresponses. Inhibition of Bi occurred however when Tri was may be under segmental and suprasegmental control.contracting, but a tap to the tendon of the same intensitydid not evoke the homonymous phasic stretch reflex (Fig. 4.4. Development of inhibitory and excitatory spinal7C). This observation excludes mechanical spread of the reflexesstimulus to the Group Ia afferents of Bi and/or excitationof heteronymous excitatory reflexes to Bi [36,37] counter- Around the time of birth in several animal species,balancing the inhibitory reflex, since both these excitatory including man, spinal motor centres are undergoing exten-inputs to Bi motoneurones would be the same irrespective sive development and refinement of connectionsof whether a homonymous phasic stretch reflex in Tri was [33,36,37,41,43]. The present study has demonstrated that

80 S.M. Mc Donough et al. / Brain Research 899 (2001) 66 –81

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