Exp Brain Res (1992) 91:311 319

E nResearch �9 Springer-Verlag 1992

Pattern of projections of group I afferents from elbow muscles to motoneurones supplying wrist muscles in man

P. Cavallari*, R. Katz, and A. Penicaud

Neurophysiologie Clinique, D~partement de R~ducation, H6pital de la Salp~tri~re, 47 Boulevard de l'H6pital, F-75651 Paris Cedex 13, France

Received July 18, 1991 / Accepted May 29, 1992

Summary. The pat tern of projections of low threshold afferents from triceps and biceps brachii muscles onto motoneurones innervating muscles acting at the wrist was assessed by a reflex and a postst imulus time his togram (psth) technique. Activat ion of low-threshold afferents originating from elbow flexors or extensors resulted in an early, short-lasting inhibition of wrist flexor motoneu- rones (flexor carpi radialis, flexor carpi ulnaris). An inhibi- tion was also found in the extensor carpi radialis (ECR) motoneurones after st imulation of low-threshold afferents from triceps. Evidence is presented that Ia fibres contrib- ute to these effects. The inhibitory effects were found in all subjects, but they were constant in only 57% of the reflex experimental sessions and in 25% of the explored mo to r units. Stimulation of biceps low-threshold afferents was always ineffective on E C R motoneurones . N o early facili- tat ion was ever seen in m o t o r nuclei innervating wrist muscles following st imulation of low threshold afferents from biceps and triceps. The pat tern of transjoint projec- tions of group I afferents f rom proximal to distal muscles and from distal to proximal ones (Cavallari and Katz 1989) is discussed in relation to that described in the cat forelimb.

Key words: Ia projections - Monosynap t ic reflex - Post- stimulus time his togram Upper limb - H u m a n

Introduction

Projections of group I fibres originating from muscles acting at the wrist onto mo to r nuclei innervating muscles acting at the elbow have been recently described in man by Cavallari and Katz (1989). The most striking result was that Ia afferents originating from wrist antagonistic mus-

* Present address. Istituto di Fisiologia Umana II, Universitfi degli Studi di Milano, I-Milano, Italy Correspondence to: R. Katz

cles have similar projections onto a mo to r nucleus in- nervating a given elbow muscle, e.g. st imulation of flexor carpi radialis (FCR) or extensor carpi radialis (ECR) Ia afferents produced a facilitation in biceps motoneurones .

Results presented here deal with the projections of low- threshold afferents originating from elbow flexors and extensors onto m o t o r nuclei supplying muscles acting at the wrist. The role of the Ia fibres in determining the effects evoked by the condit ioning stimulations was established by using the experimental methods described in the pre- vious paper (Cavallari and Katz 1989).

The pat tern of distribution of group I afferents a m o n g mo to r nuclei innervating the human upper arm is dis- cussed with respect to the data obtained from the cat's forelimb (Fritz et al. 1989).

Materials and methods

Group I afferent fibres originating from the biceps and the triceps muscles were activated with electrical and mechanical stimuli. The effects of their activation on FCR, ECR and flexor carpi ulnaris (FCU) motor nuclei were tested both on motoneuronal populations (reflex technique) and on single motor units (poststimulus time histogram technique, psth).

The experiments were performed in 12 healthy volunteers from the staff of the department (age 26 45), all of whom gave their informed consent to the procedure, which was approved by the local Ethics Committee. The subjects were comfortably seated in an armchair, with their examined arm resting on an armrest. The surface electromyogram (emg) was recorded by two nonpolarizable disk electrodes (0.9 cm diameter).

Condit ionin9 stimuli

Electric pulses of 0.5 or 1 ms duration were delivered percutaneously to the musculocutaneous nerve and to the radial branches innerva- ting the triceps muscle (triceps nerve). The stimulating electrodes were placed a few centimetres below the shoulder, and we carefully checked that, even when using the maximal intensity delivered by the stimulators, there was no encroachment on other nerves (i.e. no motor response or paresthesia in areas supplied by ulnar, median or

312

radial nerves). Furthermore, in each case, the efficacy of this condi- tioning stimulus in exciting Ia fibres was verified by its ability to induce an increase in firing probability of a homonymous voluntarily activated motor unit (see below and Fig. 1).

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Latency (ms) Fig. 1A-F. Changes in firing probability evoked in a voluntarily activated biceps (upper row) and flexor carpi radialis (FCR; middle and lower rows) motor unit, by musculocutaneous (upper and lower rows) and median (middle row) nerve stimulations. A, C, E Open bars represent the histograms obtained in control conditions (i.e. without stimulation); shaded bars represent the histograms obtained in response to nerve stimulation. B, D, F Black bars represent the difference between these two histograms: in each 1-ms bin the control value (open bars) was subtracted from that obtained after stimulation (shaded bars). Statistical analysis were performed to determine the onset of the increase in firing probability after homonymous nerve stimulation and to determine the duration of the decrease in firing probability after heteronymous nerve stimulation. In the time inter- val 26-33 ms for the biceps motor unit and 21-28 ms for the FCR motor unit, a Z z test was performed: for the psths represented in A and B no significant differences between control situation and after stimulation were seen in the 26- and 27-ms bins (Z 2 =0.2), whereas the increases in 28 and 29 bins were highly significant (Z2=35; P<0.001). For the psths represented in C and D, no significant difference between control situation and after stimulation was seen in 21-, 22- and 23-ms bins (Z 2 = 0.2), whereas the increases in 24- and 25- ms bins were significant (9 < )~2 < 12; 0.01 < P < 0.001). Similar calcu- lations were made for psths represented in E and F: no significant differences between were seen in the 21-, 22- and 23-ms bins (g 2 =0.1) whereas the decrease in the 24- to 27-ms window was significant (Z2=7.6; P<0.01). Ordinate, number of counts expressed as a percentage of the number of triggers. Different scales were used in each row; abscissa, latency after stimulation (1-ms bin). These time histograms were obtained the same day in the same subject. Number of triggers, 600. The same conditioning stimulation was used in A (and therefore in B) and in E (and therefore in F)

The current delivered by the stimulators was measured by a current probe (Tektronix 6021), and the stimulus intensity was expressed in multiples of the threshold for the direct motor wave ( x MT).

In some experiments the electrical conditioning stimulus was delivered to the nerve after a long-lasting mechanical vibration (166 Hz for 25 rain) of the homonymous muscle, a manoeuvre that selectively increases the electrical threshold of the Ia afferents (Coppin et al. 1970), while the mechanical threshold to muscle stretch remains unimpaired (Fetz et al. 1979). Moreover, it has been shown in man that after such a prolonged tendon vibration, electrical stimulation of the posterior tibial nerve is markedly less effective in eliciting monosynaptic H-reflexes in the soleus muscle (Heckman et al. 1984). The efficacy of the vibration was verified by its ability to induce an illusive joint movement (Roll and Vedel 1982).

Since the conditioning stimuli also activated cutaneous afferents producing a local pricking sensation under the stimulating elec- trodes, a similar cutaneous sensation was evoked without stimu- lating the nerve trunk. The local pricking was reproduced by displacing the electrodes laterally on the arm, 2-3 cm away from the nerve trajectory, so that the conditioning stimulus was applied only to the skin.

In some cases Ia afferents were activated by a slight mechanical stimulation of the distal tendon of the triceps or biceps, applied by an electromagnetic hammer (Briiel and Kjaar, model 4809), which produced a very quick transient stretch (8 mm in 5 ms). The intensity of the percussion was graded using a power amplifier and expressed in multiples of the threshold for the tendon-jerk reflex (x TT). Special care was taken to ensure that the hammer struck the tendon at the same position throughout the experiments.

Test of the excitability of the motoneuronal population at rest (H and tendon reflex)

Electrical percutaneous stimulation of the median nerve through bipolar electrodes placed just above the elbow (rectangular shocks of 1 ms duration, every 4 s) regularly evoked an H-reflex in the FCR. The recording electrodes were placed 1.5 cm apart over the belly of the FCR. When the H-reflex increased markedly during a wrist flexion, and not during a pronation or during a flexion of the fingers, it was considered to originate from FCR. After amplification, the responses were computer-analysed on line and the results stored for further analysis. The reflex responses were measured as the peak-to- peak amplitude of muscle action potentials and expressed as a percentage of the maximal direct motor response (Mmax).

The electrical stimulation of the radial nerve through bipolar electrodes placed a few centimetres above the elbow resulted in a stable ECR H-reflex only in three subjects. Thus, in five subjects an ECR tendon reflex was used as test. The tap was applied to the distal tendon (at the wrist) by the electromagnetic hammer described in the preceding section. The recording electrodes were placed 1.5 cm apart over the belly of the ECR. It was regularly verified that both H and tendon ECR reflex amplitudes increased markedly during an exten- sion of the wrist.

Since it was not possible to evoke a stable H-reflex of sizeable amount in the FCU following the electrical stimulation of the ulnar nerve, no reflex experiments were performed in the FCU. In all cases the test reflexes were chosen to" be large enough so that inhibition as well as facilitation could be clearly seen.

By convention, the timing of the pulse triggering the test stimulus was always referred to the timing of the conditioning electric shock. Since the conditioning electrodes were located closer to the spinal cord that the test ones, the simultaneous arrival of the conditioning and test volleys in the spinal cord was obtained when the condi- tioning stimulus was delivered after the test. In such a case the conditioning-test interval was said to be negative. Two kinds of

313

stimulation were used: (A) test stimulation alone and (B) condi- tioning stimulus plus test stimulus. In each sequence, 20 of each group of stimuli were presented randomly. Variance analysis was used for testing the significance of the changes of the test reflex amplitudes.

Test of the excitability of single motor units (psth)

The effects of a conditioning stimulus on a voluntarily activated motor unit can be determined by constructing a time histogram of the occurrence of motor unit spikes following repeated presentation of the stimulus. The psth extracts from the naturally occurring spike train only those changes in firing probability that are time-locked to the stimulus (Stephens et al. 1976). The validity of the method to detect postsynaptic potentials in motoneurones has also been estab- lished in animal experiments (Fetz and Gustafsson 1983; Gustafsson and McCrea 1984).

A detailed description of the procedure used in this section has been already published (Fournier et al. 1986) and will only be summarized here. The emg from single motor units of the biceps, triceps, FCR, ECR and FCU was recorded by conventional surface electrodes. While the subject performed a very weak but steady contraction, the electrodes were moved on the skin (previously rubbed with abrasive paste) until it was possible to isolate a single motor unit, either because it was the only one active or because it was significantly larger than the others. After several training sessions this was achieved rather easily using auditory and visual feedback of the emg potential. In all the subjects the contraction strength was below 5% of maximal voluntary power. The motor units studied were therefore all in the low-threshold range. The emg potentials of single motor units were converted into standard pulses by a dis- criminator with variable trigger levels and were used to trigger a computer (Apple II) and the stimulators. The motor unit potential and the trigger pulse were continuously monitored on an oscillo- scope. Each motor unit was recognizable by its shape and amplitude, and the experimenter could verify which motor unit was triggering the discriminator. In case of false triggers, due to other active units, the experimental sequence was immediately stopped. Only ex- periments in which the motor unit shape and trigger positions remained constant within and between sequences were retained for further analysis.

When the nerve stimulation was delivered at a fixed interval after a motor unit discharge, it was possible to choose a delay at which the probability for a new discharge was high (provided that there was a relatively regular firing frequency during voluntary contraction). This implied that the number of triggers necessary to reveal obvious peaks in the psth was much lower than if nerve stimulations were given without reference to the motor unit discharge. The psths of the voluntary motor unit discharge were constructed for the period 10-40 ms following the stimulation using 1-ms bins. This method also implies that the probability of discharge depends not only on the postsynaptic potentials evoked by the stimulation but also on the motoneurone membrane trajectory during the interspike interval. To take account of the latter, a histogram of firing probability was also constructed in a control situation without stimulation (open columns in Fig. 1A, C, E). The control and the different conditioned situations were randomly alternated (same number of triggers) within a se- quence. The control histogram represented the background firing probability to which the results following stimulation were com- pared. To show the differences between the results obtained in the two situations, the control value in each bin was subtracted from that obtained after stimulation. Within different time interval windows a .~2 test was used to determine to what extent the distribution of firing probability after stimulation differed from that obtained in the control situation. Such an analysis was only performed after having checked (using a )f2 test) that in the control situation the firing probability within the window of analysis did not differ from the mean probability of discharge of this unit.

R e s u l t s

Responses of the motoneurone population

Flexor motoneurones. F i g u r e 2A shows an e x a m p l e of the t ime cou r se o f the i n h i b i t o r y effects e v o k e d by s t i m u l a t i o n of the m u s c u l o c u t a n e o u s n e r v e on the F C R H-re f lex (at an in tens i ty ju s t s u b t h r e s h o l d for the d i rec t m o t o r response) . I n this subject , the i n h i b i t i o n s t a r t ed for a c o n d i t i o n i n g - test i n t e rva l o f - 1 ms, p e a k e d at - 0 . 5 ms a n d las ted for a b o u t 3 ms. W h e n the i n h i b i t i o n was m a x i m a l , the test reflex a m p l i t u d e was 5 5 % of its u n c o n d i t i o n e d va lue (i.e. an i n h i b i t i o n o f 45%). N e g a t i v e c o n d i t i o n i n g - t e s t va lues (i.e. the test s t imu lus p r e c e d e d the c o n d i t i o n i n g one) were

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Fig. 2A-E. Changes in the amplitude of the FCR (A, B) and of the extensor carpi radialis (ECR) (C) H-reflex following a conditioning electrical stimulation of the musculocutaneous (A) and the triceps (B, C) nerve. Intensity of the conditioning stimulations, 1 x MT, where MT is the threshold for the direct motor wave. Ordinate, amplitude of the test reflex expressed as a percentage of the unconditioned value; abscissa, time interval between test and condi- tioning stimuli. By convention, the timing of the test stimulation is referred to that of the conditioning one. Negative time intervals correspond to the cases in which the test stimulus was given first. Each symbol represents the mean of 20 measurements. Vertical bars, 1 SEM. Results in A, B, C were obtained from different subjects

314

due to the fact that in these experimental conditions the stimulation site of the musculocutaneous nerve was 15 cm more proximal (upper arm) than the test site (elbow).

The 21 experiments were performed in five subjects. In each case the intensity of the conditioning stimulus was adjusted to be just subthreshold for the direct motor response. The efficacy of the conditioning stimulus to activate Ia fibres was always verified (see Materials and methods).

An inhibition similar to that in Fig. 2A was found in all the subjects. In three of them, the experiments were repeated at least five times: in subject 1 the inhibition was present in eight out of the eight experimental sessions, and its maximal value was 25% +_ 6.5; in subject 2 the inhibi- tion was disclosed in four out of the five experimental sessions, with a mean value of 37.25% _+ 7.9; and in subject 3 the inhibition was found in three out of the six experi- mental sessions, with a maximal value of 22.6%-t-7.6. A slight facilitation, as in Fig. 2A, followed the inhibition in only four cases.

Figure 2B shows the time course of the changes in the FCR test reflex when preceded by a stimulation of the triceps nerve (intensity just subthreshold for the direct motor response): a short-lasting inhibition (3 ms) started at a conditioning-test interval of - 1 ms, with an abrupt onset. Its maximum was observed at a conditioning-test interval of 0 ms. At this time interval the test reflex amplitude reached 56% of its unconditioned value (i.e. an inhibition of 44%), 33 experiments were performed in seven subjects and this inhibition was found in all of them. In four subjects the experiments were repeated at least five times. In subject 1 the inhibition was found in seven out of ten trials, with a mean value of 32% _+ 11.4. In subject 2 the inhibition was present in three out of six experimental sessions, with a mean value of 36% _+ 17.7. In subject 3 the inhibition was found in four out of seven experimental sessions, with a mean value of 37% ___ 13.3. In subject 4 the inhibition was found in four out of five experimental sessions, with a mean value of 54.4% • 15.6. Only in four cases was the inhibition followed by a slight facilitation.

Extensor motoneurones. Figure 2C shows the time course of the changes in an ECR H-reflex test when preceded by a triceps nerve stimulation (intensity just subthreshold for the direct motor response). The inhibition of the test reflex started abruptly, was maximal at a conditioning test interval of - 0 . 5 ms and ended at a. conditioning-test interval of 1 ms. When the inhibition was maximal, the test reflex size was lowered to 52% of its unconditioned value (i.e. an inhibition of 48%). Seven experiments were performed in two subjects with a conditioning stimulus intensity just below motor threshold. In subject 1 the inhibition was found in three out of five experimental sessions, with a mean value of 40% _+ 6.9; in subject 2, in two experimental sessions, with a mean value of 27% _ 2.8. In five subjects the same conditioning stimulus was tested on a monosynaptic tendon reflex evoked by percussion of the distal tendon of the ECR muscle. A slight inhibition was found only in 3 out of the 11 experiments.

The effects of the stimulation of the musculocutaneous nerve onto the ECR monosynaptic reflex were studied in

seven subjects: 11 experiments were performed with a tendon test reflex and 5 with an H-reflex; no significant changes in the test reflex amplitude were observed.

Responses of individual motor units

As already stated in the Materials and methods section, it was not possible to perform reflex experiments in the FCU muscle. To study the effects of the stimulation of group I fibres originating from biceps and triceps muscles in FCU motoneurones, psth experiments were performed in FCU motor units. In psth experiments, the changes in moto- neurone excitability were indicated by the modifications in the probability of discharge of a single activated motor unit, while in reflex experiments they were indicated by the modifications in amplitude of a reflex discharge. Condi- tions in psth and reflex experiments were therefore too different (see Discussion) to allow a valid comparison of the effects of a given conditioning stimulus between the motor nuclei as changes in excitability would have been indicated differently. Thus, to compare the effects of the activation of group I fibres originating from biceps and triceps muscles on FCU, FCR and ECR motoneurones, psth experiments were also performed with FCR and ECR motor units.

Effects of the stimulation of the musculocutaneous nerve. The modifications of the firing probability of FCU, FCR and ECR voluntarily activated motor units were studied following electrical stimulation of the musculocutaneous nerve. The intensity of the conditioning stimulus was adjusted to be just subthreshold for the direct motor response.

In seven subjects, 38 FCU motor units were recorded. In each subject, but only in 11 motor units (i.e. 29% of the sample), a decrease in firing probability was observed. An example of the results obtained in these 11 FCU motor units is represented in Fig. 3B. The stimulation of the musculocutaneous nerve resulted in an early, short-lasting (3 ms) and significant (P<0.01) decrease in firing prob- ability. Similar results were obtained in all 11 FCU motor units, and the mean duration of the inhibition was 2.9 ms.

In nine subjects, 50 FCR motor units were recorded. In each subject, but only in 12 motor units (24% of the sample), a decrease in firing probability was observed after musculocutaneous nerve stimulation. An example of the results is represented in Fig. 1E, F and in Fig. 3A. The stimulation of the musculocutaneous nerve evoked an early, short (4 ms) and significant (P < 0.01) decrease in the firing probability of the FCR motor unit. Similar results were obtained in all 12 FCR motor units, and the mean duration of the inhibition was 3.6 ms.

In seven subjects, 18 satisfactory recordings of ECR motor units were obtained. No change in firing probability was ever observed, despite the fact that the efficacy of the conditioning stimulus was systematically verified and that its intensity was increased above motor threshold.

Effects of the stimulation of the triceps nerve. A protocol similar to that described in the preceding section was used

315

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Fig. 3A-C. Changes in firing probability evoked by musculocuta- neous (upper part) and triceps (lower part) nerve stimulations in a flexor carpi radialis (FCR) (A, C), a flexor carpi ulnaris (FCU) (B, D) and an extensor carpi radialis (ECR) (E) motor unit. Time histograms were obtained in control situation and after stimulation, and each black bar represents the difference between the two histograms (like the right-hand columns in Fig. 1). Statistical analysis were performed to determine the duration of the decrease in firing probability after heteronymous nerve stimulation. For psth represented in A: no significant differences between control situation and after stimula- tion were seen in the 21-, 22- and 23-ms bins (;(2 =0.1), whereas the decrease in the 24- to 27-ms window was significant (Z2=7.6; P < 0.01). For psth represented in B: no significant difference between control situation and after stimulation was seen in 26-, 27-, 28- and 29-ms bins (Z 2 = 3), whereas the decrease in 30- to 32-ms window was significant ()~2= 8.8; P < 0.01). For psth represented in C: no signifi- cant difference between control situation and after stimulation was seen in 23-, 24- and 25-ms bins (X 2 = 1), whereas the decrease in the 26- to 31-ms window was significant (Z2=9; P<0.01). For psth represented in D: no significant difference between control situation and after stimulation was seen in 23-, 24- and 25-ms bins (ff =0.2), whereas the decrease in the 26- to 30-ms window was significant (g 2= 6.5; P <0.05). For psth represented in E: no significant differ- ence between control situation and after stimulation was seen in 25-, 26- and 27-ms bins (Z 2 =0.7), whereas the decrease in the 28- to 34- ms window was highly significant (g2= 18; P<0.001). The arrows indicate the latency of the onset of the homonymous monosynaptic peak. Number of triggers, 600. Abscissa and ordinate as in Fig. 1. The same scale was used in each row

to study the effects of the triceps nerve st imulation on FCU, FCR and ECR moto r units.

In six subjects, 26 F C U moto r units were recorded. In each subject, but only in 8 m o t o r units (31% of the sample), a decrease in firing probabil i ty was observed, as exemplified in Fig. 3D. An early, short (5 ms) and signifi- cant (P < 0.05) decrease in firing probabil i ty was observed following triceps nerve stimulation. The mean dura t ion of the inhibition was 6.7 ms.

In six subjects, 29 FCR m o t o r units were recorded. In each subject, but only in 6 mo to r units (21% of the sample), a decrease in firing probabil i ty was observed following triceps nerve stimulation. An example of the results obtained is represented in Fig. 3C. Stimulation of the triceps nerve resulted in an early, short-lasting (6 ms) and significant (P < 0.01) decrease in the firing probability. In all 6 mo to r units a similar decrease was observed, and the mean dura t ion of the inhibition was 4.3 ms.

In seven subjects, 30 ECR moto r units were recorded. In each subject, but only in 9 mo to r units (i.e. 30% of the sample), a decrease in firing probabil i ty was observed following triceps nerve stimulation. In all 9 ECR moto r units a decrease similar to that represented in Fig. 3E was observed, and the mean durat ion of the inhibition was 4.2 ms.

Afferent fibres responsible for the inhibitory effects

Inhibi tory effects induced by the electrical st imulation of the musculocutaneous or the triceps nerve appeared at a condit ioning stimulus intensity below m o t o r threshold. At such intensities only the largest diameter fibres are activa- ted. Figure 4B (filled circles) shows an example of the modifications of an F C R test reflex ampli tude in response to the st imulation of the triceps nerve when varying its intensity. Inhibi t ion started at 0.8 x MT, then grew pro- gressively to 1.2 x M T and finally reached a plateau at 1.2-1.5 x MT. Similar results were found in all 11 ex- periments performed at a fixed condi t ioning-tes t delay while varying the intensity of the condit ioning stimulus.

To characterize further the afferent fibres responsible for the inhibitory effects, we used an artifice which is known to increase the electrical threshold of Ia afferents (see Materials and methods). In four subjects the threshold of the inhibition was measured before and after a long- lasting vibration applied to the distal tendon of the muscle from which the afferent fibres originate (biceps or triceps). In six experiments the effects of a condit ioning stimulus applied to the triceps nerve were studied in an FCR test reflex, and in three, in an ECR test reflex. In two ex- periments the effect of a condit ioning stimulus applied to the musculocutaneous nerve was studied in an FCR test reflex. In Fig. 4B an example of the results obtained before and after vibrating the triceps tendon is represented. The intensity of the condit ioning stimulus applied to the triceps nerve was increased by steps of 0.1 MT, and the inhibition was tested at a condi t ioning- tes t interval of - 0 . 5 ms (arrow in Fig. 4A). Before vibration of the triceps (black circles) the progressive decrease of the ampli tude of the F C R test reflex started at an intensity of 0.8 MT. After

316

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Fig. 4A-C. Changes in the amplitude of a FCR H-reflex following various conditioning stimuli. A Time course of the inhibition of the FCR H-reflex following a conditioning electrical stimulus applied to the triceps nerve (intensity 1.0 x MT). Same coordinates as in Fig. 2. B Changes in the amplitude of a FCR H-reflex following changes in the intensity of the conditioning stimulus applied to the triceps nerve at a fixed conditioning test interval (-0.5 ms; arrow in A). Filled circles represent the variations obtained in control conditions; open circles, the effect after a prolonged vibration of the triceps tendon. C Time course of the inhibition of a FCR H-reflex induced by a mechanical tap applied to the distal triceps tendon. Same co- ordinates as in A. Each symbol represents the mean of 20 measure- ments. The vertical bars, 1 SEM. All these results were obtained in the same subject

vibration (open circles) the threshold of the inhibition had increased to 1.2 MT. In each case it was verified that 20 min after the end of the long-lasting vibration the threshold of the inhibition had returned to its control value. The threshold for the inhibition was enhanced more than 30% of MT in five cases, between 10 and 20% in four cases and unchanged in two cases, thus suggesting that Ia fibres play a role in these inhibitory effects.

The effects induced in the test reflex by an electrical conditioning stimulus were also compared to those ob- tained with a slight mechanical tap as the conditioning stimulus (0.5 x TT) since the latter is known to activate, at rest, predominantly pr imary spindle endings (Lundberg and Winsbury 1960). Six experiments were performed in three subjects. In Fig. 4C the changes in the FCR test reflex amplitude following a mechanical tap applied to the distal tendon of the triceps muscle are illustrated. The inhibition of the FCR test reflex was apparent for a conditioning-test interval of 2 ms. It peaked at 5 ms and vanished a few milliseconds thereafter. When the conditioning stimulus was an electric shock (Fig. 4A) the inhibition started

earlier. This was due: (1) to the fact that the site of the electrical conditioning stimulus was closer to the spinal cord than that of the mechanical tap; (2) to the delay introduced by the electromagnetic coupling of the ham- mer; and (3) to the delay associated with spindle activation. Comparable results were obtained when the conditioning stimuli involved biceps Ia fibres. The difficulty in obtaining an ECR test reflex did not allow us to perform such experiments with this latter test.

Control psth experiments were also performed with a conditioning intensity below motor threshold and the emg of the muscles involved by the conditioning stimulus was recorded and averaged during a whole sequence in order to be sure that during this sequence the conditioning stimulus had never resulted in any motor or reflex re- sponse. It was thus possible to exclude the activation of motor axons, which would have resulted in Renshaw cell activation.

These experiments suggest that the activation of Ia fibres by electrical conditioning stimuli applied to the musculocutaneous and/or the triceps nerves led to an inhibition of the FCR and ECR motoneurones. Moreover, the possibility that stimulation of cutaneous afferents could have been responsible for these effects was ruled out, using a purely cutaneous conditioning stimulus mimicking the sensation evoked by the musculocutaneous and/or triceps nerve stimulation (see Materials and methods). No modification of the amplitude of the test reflexes (FCR and ECR) and of the firing probability (FCR, FCU and ECR motor units) was found in this case. Since it was not possible to evoke any stable H-reflex in the FCU, only psth experiments were performed. Taking into account that the decrease in firing probability following the same stimulation of the musculocutaneous (or triceps) nerve had the same features in FCU, FCR and ECR motor units, we favour the hypothesis that the activation of Ia fibres also plays a role in F C U inhibition.

Synaptic linkage for the inhibitory effects

Hultborn et al. (1987) have set up a method to estimate the central delay of a reflex pathway. This method is based on the comparison, in a given motor unit, between the latency of the monosynaptic facilitation (evoked in each moto- neurone pool by the activation of the Ia homonymous afferents) and the latency of the heteronymous effect. Since homonymous and heteronymous effects are tested in the same motor unit the afferent conduction time is obviously the same. Taking into account the calculation of the afferent peripheral conduction time in each case and the comparison between the latencies of the homonymous and heteronymous effects, it is thus possible to estimate the central delay of the heteronymous effect. However, the precision of the estimation of the central delay relies on the accuracy of latency measurement and our experimental technique (bins of 1 ms) did not allow the exact number of synapses in the reflex pathway to be calculated. Neverthe- less in five FCR motor units and in three FCU motor units the latency of the onset of the inhibitory effect induced by the activation of group I biceps fibres was systematically

317

compared to that of the onset of the monosynaptic homonymous effect (cf. Fig. 1). The central delay never exceeded the monosynaptic one by more than 2 ms, which strongly suggested that the reflex pathway underlying the inhibitory effects is oligosynaptic.

Discussion

predominantly activates muscle spindle primary endings and that the long-lasting vibration results in a selective increase in the electrical threshold of Ia afferents (Coppin et al. 1970; Heckman et al. 1984). Therefore Ia fibres contained in the musculocutaneous and the triceps nerve very likely play a role in the inhibition of FCR, ECR and FCU motoneurones. Our results, however, do not exclude the possibility that biceps and triceps Ib fibres could also participate in this inhibition.

A short-latency inhibition was revealed in FCR, ECR and FCU motoneurones following triceps nerve stimulation and in FCR and FCU motoneurones following musculo- cutaneous nerve stimulation. Such early inhibition was found in all subjects in 57% of the reflex experimental sessions and in 25% of the tested motor units. The discrepancy between reflex and psth experiments may have the following explanations: (1) In reflex experiments inhibitory effects are tested on a motoneurone population, while in psth experiments they are tested on a single motoneurone; (2) In reflex experiments the motoneurone population is tested at rest, while in psth experiments the motor unit is voluntary activated.

During psth experiments the contraction strength was always below 5% of the maximum voluntary force, and thus the motor units studied were all in the low-threshold range. It is therefore likely that the test reflexes involved motoneurones whose threshold was higher than that of the motor units studied in the psth experiments. According to the results obtained by Clamann et al. (1974), these higher threshold motoneurones are more sensitive to inhibitory effects than low-threshold ones. This phenomenon may play a role in the difference between results obtained in psth and in reflex experiments.

Several authors (for references see Crone et al. 1987) have shown, using reflex experiments, that during a voluntary contraction of the muscle involved by the test reflex reciprocal inhibition is decreased. Similar changes during contraction involving the test muscle could not be excluded for transjoint inhibition and would thus also play a role in the reduced inhibition seen in psth experiments.

Inhibitory effects were observed at rest (i.e. in reflex experiments) in all subjects but only in 57% of the experimental sessions. In each experimental session (see Materials and methods) it was verified that the condi- tioning stimulus excited Ia fibres and no special circum- stances (size of the unconditioned test reflex, intensity of the conditioning stimulus, etc.) were associated with the trials in which inhibition was not detected, thus suggesting that such variability may be attributable to fluctuations in the excitability of the interneuronal pathway.

Group I fbres were responsible for the inhibitory effect

Inhibitory effects were obtained with an electrical stimulus whose intensity was below motor threshold and with a slight mechanical conditioning tendon tap. Furthermore, in most cases their electrical threshold was enhanced after a long-lasting vibration. It has been shown (Lundberg and Winsbury 1960) that at rest a weak mechanical stimulus

Central pathway

The central delay of the inhibition induced by the activa- tion of group I afferents contained in the musculo- cutaneous nerve onto FCR and FCU motor units was estimated in psth experiments. This central delay was never more than 2ms longer than that of the mono- synaptic one. To interpret this result it should be re- membered that psth experiments were performed with 1- ms bins, which allowed a precision of _+ 1 ms. This central delay is therefore compatible with an oligo- (di- or tri-)- synaptic linkage. The central delay of the inhibition ev- oked by the activation of triceps Ia afferents onto FCR, FCU and ECR motoneurones was not estimated, since it was not possible to determine the conduction velocity of these Ia afferent fibres. However, in all these cases, the differences in latency between the onset of the mono- synaptic increase and that of the heteronymous decrease in firing probability of a given motor unit were similar to that observed following musculocutaneous nerve stimulation. This suggests that the inhibition induced by the activation of triceps group I afferents is also mediated through an oligosynaptic pathway.

Distribution of la projections in the human upper limb: functional significance

The results presented in this paper extend the description of the pattern of distribution of group I afferents among motor nuclei innervating muscles of the human upper limb. Indeed, a previous paper (Cavallari and Katz 1989) was devoted to the projections of group I afferents of muscles acting at the wrist onto motoneurones of muscles acting at the elbow. The data obtained in the two experi- mental series are pooled together in Table 1. The same table also summarizes the results concerning the distribu- tion of Ia excitatory connections in the cat forelimb (Fritz et al. 1989). This facilitates the comparison between the neural organization of group I pathways in the two species. However, it should be underlined that the com- parison is possible only when analysing excitatory effects, since the distribution of the inhibitory connections in the cat forelimb is still unknown.

In the cat, as in man, upper limb motor nuclei receive Ia homonymous facilitation, a connection that is witnes- sed (a) by the presence of H-reflexes or tendon reflexes in each of the analysed muscles and (b) by the early increase in firing probability, which was always evoked in motor units belonging to the explored motor nuclei (ECR, FCR,

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Table 1. Comparison between the projections of group I afferents in the human upper limb and the cat forelimb

Motor nuclei tested Biceps Triceps FCR ECR

Nerve Man Cat Man Cat Man Cat Man Cat stimulated

Biceps + + - No - No 0 + facilitation facilitation

Triceps - ND + + - + -- No facilitation

Median + + - + + -t- - ND (Pronator teres)

Radial + + - No - ND + + facilitation

Ulnar 0 No 0 ND 0 ND ND ND facilitation

+, presence of monosynaptie excitatory connections; - , presence of inhibitory connections; 0, absence of early effects; ND, not determined.

FCU, triceps and biceps). Reciprocal inhibitory connec- tions have also been demonstrated between two couples of antagonistic muscles: FCR-ECR, which act at the wrist (Baldissera et al. 1983; Day et al. 1984), and biceps-triceps, which act at the elbow (Katz et al. 1991).

By contrast, the pattern of transjoint projections ex- hibits clear-cut differences between cat and man: in man the upper limb group I connections between proximal and distal muscles are strikingly asymmetrical (Table 1). The most striking feature of the transjoint projections from distal to proximal was that Ia afferents originating from antagonistic muscles have similar projections onto a given motor nucleus: activation of group Ia afferents contained in the median nerve resulted in a strong, short-latency facilitation of biceps motoneurones. A similar effect was evoked in biceps motoneurones after stimulation of the radial nerve. The central delay of both facilitations was similar to that of the homonymous effect and therefore compatible with a monosynaptic linkage. Activation of group Ia afferents contained in the two nerves resulted also in a strong inhibition of triceps motoneurones with a central latency compatible with a disynaptic linkage. Thus, a motor pool innervating a muscle acting at the elbow receives Ia actions of the same kind (excitation or inhibi- tion) from antagonist forearm muscles (Cavallari and Katz 1989).

The most striking feature of the transjoint projections from proximal to distal was that no facilitatory effect was ever shown: activation of biceps Ia fibres evoked an inhibition of FCR motoneurones, while the same condi- tioning stimulus had no effect on the ECR motor pool. The first result does not contrast with animal data, since Fritz et al. (1989) showed very small monosynaptic EPSPs in only 1 out of 42 FCR recorded motoneurones. In the case of the ECR, however, we were not able to confirm, either at rest (H-reflex technique) or during single unit contraction (psth), the clear excitatory effect shown in the cat. Stimula- tion of triceps Ia afferents produced inhibition of both ECR and FCR motoneurones, a result that only partially matches the animal data. In the cat, FCR motoneurones are facilitated by volleys in the triceps nerve in 93% of the cases, while ECR motoneurones do not receive Ia excitation.

In comparing the results obtained in man and in the cat, three main aspects should be outlined (Table 1): 1. In the cat, Ia transjoint projections from biceps to ECR and from triceps to FCR are excitatory and bidirectional, whereas in man bidirectional excitatory projections are lacking. It is tempting to analyse this result in relation to the loss of the deambulatory function of the human upper limb and the acquisition of the manipulatory capacity of the hand. In the cat, the presence of bidirectional Ia facilitatory connections between proximal and distal mus- cles has been interpreted in respect of the requirements of gait and posture (Fritz et al. 1989). In man, the absence of those projections would protect distal muscles from the excitatory influences that arise from reflex actions evoked by the stretch in the proximal muscles. This would guaran- tee that muscles involved in the manipulatory movement are not submitted to proximodistal synergies. 2. In man, the two antagonist wrist muscles (ECR and FCR) are linked to biceps motoneurones by unidirectional Ia facilitatory pathways. This connection seems to favour a flexed position of the elbow, independently of the position of the hand. This attitude is common during manipulatory movements and is present also in patients with upper motoneurone disease, where the reflex excit- ability is enhanced. 3. In man, inhibitory projections from biceps to FCR and from triceps to ECR are present. A possible explanation of these pathways may originate from the anatomy of the arm. The proximal FCR and ECR tendons are inserted on the humerus and cross the elbow joint. Thus, if the biceps is contracted and the elbow is flexed while the wrist is kept fixed in supine position, as when putting food up to the mouth, the ECR will be elongated and give rise to a reflex Ia excitation. Meanwhile, however, the Ia discharge ori- ginating from the triceps stretched by the biceps contrac- tion will inhibit ECR motoneurones, thus preventing the stretch reflex from occurring. This would help to maintain wrist position whatever the changes in the more proximal joint position.

Acknowledgements. The authors wish to express their gratitude to Professor Emmanuel Pierrot-Deseilligny for reading and comment- ing upon the manuscript. Thanks are also due to Annie Rigaudie and

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Mich+le Dodo for excellent technical assistance and to David MacGregor for scrutinizing the English. This work was supported by grants from the Pierre and Marie Curie University (Paris VI), from INSERM (89 60 12), ETP (Tw 86/57), and IRME.

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