Effects on Peroneal Motoneurons of Cutaneous Afferents Activated by Mechanical or Electrical Stimulations

Jean-François Perrier, Boris Lamotte D'Incamps, Nezha Kouchtir-Devanne, Léna Jami, and Daniel Zytnicki

Centre National de la Recherche Scientifique EP 1848, Université René Descartes, Unite de Formation et de Recherche Biomédicale, 75270 Paris 06, France


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Perrier, Jean-François, Boris Lamotte D'Incamps, Nezha Kouchtir-Devanne, Léna Jami, and Daniel Zytnicki. Effects on Peroneal Motoneurons of Cutaneous Afferents Activated by Mechanical or Electrical Stimulations. J. Neurophysiol. 83: 3209-3216, 2000. The postsynaptic potentials elicited in peroneal motoneurons by either mechanical stimulation of cutaneous areas innervated by the superficial peroneal nerve (SP) or repetitive electrical stimulation of SP were compared in anesthetized cats. After denervation of the foot sparing only the territory of SP terminal branches, reproducible mechanical stimulations were applied by pressure on the plantar surface of the toes via a plastic disk attached to a servo-length device, causing a mild compression of toes. This stimulus evoked small but consistent postsynaptic potentials in every peroneal motoneuron. Weak stimuli elicited only excitatory postsynaptic potentials (EPSPs), whereas increase in stimulation strength allowed distinction of three patterns of response. In about one half of the sample, mechanical stimulation or trains of 20/s electric pulses at strengths up to six times the threshold of the most excitable fibers in the nerve evoked only EPSPs. Responses to electrical stimulation appeared with 3-7 ms central latencies, suggesting oligosynaptic pathways. In another, smaller fraction of the sample, inhibitory postsynaptic potentials (IPSPs) appeared with an increase of stimulation strength, and the last fraction showed a mixed pattern of excitation and inhibition. In 24 of 32 motoneurons where electrical and mechanical effects could be compared, the responses were similar, and in 6 others, they changed from pure excitation on mechanical stimulation to mixed on electrical stimulation. With both kinds of stimulation, stronger stimulations were required to evoke inhibitory postsynaptic potentials (IPSPs), which appeared at longer central latencies than EPSPs, indicating longer interneuronal pathways. The similarity of responses to mechanical and electrical stimulation in a majority of peroneal motoneurons suggests that the effects of commonly used electrical stimulation are good predictors of the responses of peroneal motoneurons to natural skin stimulation. The different types of responses to cutaneous afferents from SP territory reflect a complex connectivity allowing modulations of cutaneous reflex responses in various postures and gaits.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In the peroneal group of cat leg muscles, peroneus brevis (PB) and peroneus tertius (PT) receive a rich supply of skeletofusimotor (beta ) axons: 30% of their motor units are beta  innervated (Emonet-Dénand et al. 1992; Jami et al. 1982) [the incidence of beta -innervation in peroneus longus (PL), is not known]. Activation of beta  motor units elicits simultaneous contraction and spindle discharges, i.e., group Ia (and II) afferent inputs to the spinal cord, producing an excitatory feedback in peroneal motoneurons (Kouchtir et al. 1995). Attempts to estimate the possible modulation of this feedback by other sensory afferents led us to examine the synaptic connections of peroneal motoneurons: in addition to their mutual links of monosynaptic Ia excitation, the three populations of peroneal motoneurons share excitatory influences from low-threshold cutaneous afferents in sural or superficial peroneal (SP) nerves (Kouchtir et al. 1997). However, mechanical stimulation of SP-innervated skin areas did not uniformly evoke excitation of peroneal motoneurons and has been observed to either enhance or reduce the positive feedback of PB contraction through convergence at either motoneuronal or interneuronal level. The various combinations of effects observed when mechanical skin stimulation was superimposed on PB contraction suggest that interaction of muscle and cutaneous inputs could occur via four different pathways (see the companion paper, Perrier et al. 2000). Many pathways are available for transmission of cutaneous inputs in the lumbosacral spinal cord, and variability of motoneuron responses to electrical stimulation of cutaneous nerves is a common observation (see the reviews, Jankowska 1992; Schomburg 1990). Furthermore, phasic modulations of cutaneous effects occur in single motoneurons during fictive locomotion (see e.g., Degtyarenko et al. 1996; Forssberg et al. 1977; Schmidt et al. 1988).

Data from a few studies using "natural" activation of cutaneous afferents again emphasized the variability of effects exerted on motor units and motoneurons (Hongo et al. 1990; Kanda et al. 1977; Schomburg and Steffens 1986). However, it is not known whether natural activation of cutaneous mechanoreceptors and electrical stimulation of the SP nerve with strengths in the Aalpha beta range elicit volleys in the same afferent fibers, and the matter is not easy to investigate experimentally. What could be investigated was whether both kinds of stimulation would produce similar effects on peroneal motoneurons. This was one of the questions addressed in the present analysis: the postsynaptic events evoked in peroneal motoneurons by trains of electric pulses in SP nerve were systematically compared with the events evoked by mild compression of SP-innervated skin areas. A second question was whether graded stimulation strengths would evoke similar evolutions of effects with both kinds of stimulations. The cutaneous region innervated by the terminal branches of SP, that is, the skin on lateral and medial aspects of toes, was chosen for this study because effects of inputs from these areas were examined in a previous study of a combination of contraction- and compression-induced effects (see Perrier et al. 2000). Repetitive electrical stimulation was applied in an attempt to mimic the discharges of cutaneous receptors occurring on mechanical stimulation (Burgess et al. 1968; Hunt and McIntyre 1960; see also Edin 1992 for human data). Electrical stimulation intensities were restricted to a one- to six-times threshold range, mostly activating Aalpha beta fibers (and very few, if any, Adelta fibers) of SP nerve (see Leahy and Durkovic 1991), and the strengths of toe compression remained in a range that did not activate nociceptive receptors.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Experiments were carried out on 10 adult cats (2.8-3.5 kg) anesthetized with pentobarbitone sodium (Sagatal, May and Baker, initial dose of 45 mg/kg ip, supplemented whenever necessary by additional intravenous doses of 4 mg/kg). All the procedures used in the present study were similar to those described in the companion paper (Perrier et al. 2000), except for the peripheral preparation, which was as follows. The right hindlimb was extensively denervated, except for the SP terminal branch kept in continuity with its innervated skin area and dissected over 2 cm to be mounted on stimulation electrodes in the distal portion of the leg, after the trunk has given off the nerves to peroneus tertius (PT) and peroneus brevis (PB) muscles. The nerve to peroneus longus (PL), and those to PB and PT, were cut and dissected over 2 cm and mounted on stimulation electrodes. As the nerve to PT is short and difficult to isolate over a length sufficient to rule out spread of stimulation to PB nerve, it was associated with the PB nerve in 9 of 10 experiments. Results obtained from separately identified PB and PT motoneurons did not display significant differences from those obtained from motoneurons identified by stimulating both nerves together and were therefore pooled in a single sample. When present, the PB nerve branch containing afferents from extensor digitorum brevis and extramuscular foot regions (Lundberg et al. 1975) was cut.

Devices for leg fixation and stimulation of SP-innervated cutaneous areas were as described in Perrier et al. (2000). Ramp-and-hold movements of a disk attached to a servo-length actuator (at 50 mm/s, with 2-5 mm amplitudes and 20-120 ms plateau durations), producing a mild compression of the toes between the disk and a vertical post, were used as mechanical stimulus for receptors in SP territory. In each sequence, this stimulation was repeated twice at 250-ms intervals (see e.g., Fig. 1), and five successive sequences at 1-s intervals were averaged on-line and recorded as explained in Perrier et al. (2000).

Trains of electric pulses (50 µs width, 20 Hz) were applied to the SP nerve. The 20-Hz frequency was within the range of the discharge rates of slow adapting cutaneous receptors (Burgess et al. 1968; Jänig et al. 1968) (see also DISCUSSION). Duration of trains was in the 500- to 800-ms range to match the duration of mechanical stimulation sequences. Systematical use of trains rather than single pulses allowed display of the quick decrease in amplitude of potentials in responses to repetitive stimulation (see Figs. 1B, 3C, and 5B) and of response components that were not readily visible in the response to the first pulse (see Fig. 6A). Stimulation strength was graded in multiples of a threshold (T) defined as the strength eliciting a just detectable afferent volley. The maximal stimulation strength used in the present experiments was about 6 T.

The responses of peroneal motoneurons were recorded intracellularly first, and the microelectrode was then withdrawn from the neuron by a few micrometers allowing extracellular recording of responses to the same stimulation. Subtractions of extracellular from intracellular averaged records were performed systematically (see Perrier et al. 2000). All the records in Figs. 1-3 and 5-6 show the results of these subtractions.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

This report is based on data from 63 peroneal motoneurons of which 32 (26 PB-PT, 4 PL, and 2 motoneurons that could not be ascribed to a specific pool) were tested for effects of both mechanical and electrical stimulations, 11 others (7 PB-PT, 3 PL, and 1 unspecified) for effects of the former and 20 (14 PB-PT, 5 PL, and 1 unspecified) for effects of the latter. Total samples of 43 and 52 motoneurons were thus tested for the effects of skin compression in SP-innervated areas or electric pulses on SP nerve. Data were collected from motoneurons with resting membrane potentials (rmp) between -50 and -85 mV (more than half of this sample had rmp larger than -60 mV). In addition, four motoneurons with rmp between -40 and -45 mV were included in the sample because their rmp was stable and their responses showed consistency. Each motoneuron received excitatory influences from low-threshold SP afferents whether mechanically or electrically activated. Stronger stimulations produced one of three patterns of responses: pure excitation (Fig. 1), predominant inhibition (Fig. 3), or mixed potentials (Fig. 5). A majority (24 of 32) of the motoneurons in which the effects of mechanical and electrical stimulations could be compared gave parallel responses to both kinds of stimuli (see Table 1).



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Fig. 1. Excitatory responses to mechanical (A) and electrical (B) stimulations of superficial peroneal nerve (SP) afferents in a peroneus brevis-peroneus tertius (PB-PT) motoneuron. A, in each panel, from top to bottom: cord dorsum potential, motoneuron membrane potential, and disk course. Responses to brief toe compressions produced by disk movements of increasing amplitudes as indicated on top of each panel. Arrows in the record of response to 4-mm disk course point to small excitatory postsynaptic potentials (EPSPs) caused by unwanted vibrations of the disk bearing device. B: responses of the same motoneuron to electrical stimulation of SP nerve at strengths indicated in multiples of threshold (T). In each panel, from top to bottom, cord dorsum potential, motoneuron membrane potential and stimulation at 20/s indicated by dots. All records show the result of subtraction of 5 averaged extracellular traces from 5 averaged intracellular traces.


                              
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Table 1. Distribution of the three types of responses among motoneurons that were tested for both modes of stimulation

Excitatory responses

An excitatory responses to both mechanical and electrical stimulation is illustrated by Fig. 1. In this PB-PT motoneuron, a brief toe compression produced by a 3-mm forward movement of the disk-bearing actuator followed by a 20-ms hold phase, evoked polyphasic excitatory postsynaptic potentials (EPSPs) of about 1.8 mV amplitude. Extension of the disk course to 4 and then 5 mm (increasing not only the pressure, that is, the strength of stimulation, but also the duration of compression because motion velocity remained constant) elicited longer lasting EPSPs and, occasionally, higher peak amplitudes. In the example of Fig. 1A, lengthening of disk course clearly resulted in extension of the area circumscribed by the EPSP. The polyphasic appearance of the responses most probably reflected the temporal dispersion of afferent discharges generated by asynchronously activated cutaneous receptors in the compressed skin area. It is difficult, however, to decide whether prolongation of disk course activated more receptors or simply produced longer lasting discharges from the same receptors. In the response to 5-mm disk course of Fig. 1A, a late peak was apparent, whose timing suggested that it could have been evoked by discharges from receptors activated (or re-activated) (see Hunt and McIntyre 1960) on release of toe compression. This interpretation was supported by the results of the test illustrated in Fig. 2 showing that, with the same disk course and the same velocity, an increase from 20 to 120 ms in the duration of the hold phase of compression produced an increase in the interval between early and late EPSPs so that the distinction between the two components of the responses became evident (Fig. 2, *).



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Fig. 2. Excitatory effects elicited in a PB-PT motoneuron by toe compressions of different durations. Same arrangement as in Fig. 1A. Plateau phase of disk movement lasted 20 ms in A and 120 ms in B. *, note that the interval between early and (*) late components of motoneuron responses increased with the duration of the plateau phase. Small EPSPs appearing, in A, after return of disk to initial position, were similar to those pointed at by arrows in Fig. 1. All records show the result of subtraction of 5 averaged extracellular traces from 5 averaged intracellular traces.

EPSP amplitudes, rarely exceeding 2.5 mV, were in the same range as those of the EPSPs evoked by PB muscle contractions in peroneal motoneurons (Kouchtir et al. 1995). Central latencies, as measured from the onset of the earliest components of afferent volleys to the onset of responses (after subtraction of extracellular from intracellular recordings) (see Pinter et al. 1982), were in a range of 7-19 ms (see Fig. 4) with a mean latency of 15.2 ± 3.5 (SD) ms. Excitation was the most frequently observed effect of mechanical stimulation, in 22 of 43 (51%) peroneal motoneurons (i.e., 17 of 33 in the PB-PT sample, 3 of 7 in the PL sample, and 2 of 3 unspecified motoneurons).

Nearly all the motoneurons excited by toe compression were also excited by trains of electric pulses applied on the SP nerve (Table 1). As shown in Fig. 1B, trains of 1.4 T pulses elicited small, quickly declining EPSPs, which disappeared before the end of the train. Amplitudes equivalent to those of compression-induced EPSPs required 2-4 T pulses, and 4-6 T pulses evoked 4 to 5 mV polyphasic EPSPs with a much briefer duration than in the responses to skin compression. In each series of responses and whatever the stimulus strength, the initial postsynaptic event was larger than the subsequent ones (Fig. 1B), and this was the case not only for excitatory but also for inhibitory responses (see Fig. 3C). Comparison of EPSP amplitudes elicited by mechanical and electrical stimulations (Fig. 1, A and B) suggested that light toe compression activated but a limited fraction of the cutaneous receptors innervated by afferent fibers traveling in SP nerve (see also Fig. 1 in Perrier et al. 2000). Central latencies of excitatory responses to electrical stimulation of SP nerve, in a range of 3-7 ms (mean 5.2 ± 1.2 ms) were shorter than the latencies of compression-induced EPSPs (Fig. 4), probably because the afferent discharges elicited by electric pulses were more synchronous than those elicited by toe compression. Altogether, electrical stimulation elicited excitatory responses in 20 of 52 (38.5%) peroneal motoneurons (i.e., 11 of 40 PB-PT, 7 of 9 PL, and 2 of the 3 unspecified motoneurons).



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Fig. 3. Inhibitory responses to mechanical and electrical stimulations of SP afferents in PB-PT motoneurons. A: from top to bottom, cord dorsum potential, motoneuron membrane potential without hyperpolarizing current, same during hyperpolarization by a 17-nA current, course of the disk. B: inhibitory postsynaptic potentials (IPSPs) elicited by toe compression (B1) and by 4 T electric pulses on SP nerve (B2) in the same motoneuron (A and B are from the same experiment). C: responses of another motoneuron (from the same experiment as the example shown in Fig. 1) to electrical stimulation of SP nerve at strengths indicated in multiples of threshold (T). Same arrangement as in Fig. 1B. All records show the result of subtraction of 5 averaged extracellular traces from 5 averaged intracellular traces.



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Fig. 4. Histograms of the central latencies of the earliest EPSPs and IPSPs recorded in peroneal motoneurons on electrical stimulation of SP nerve (top) and on mechanical stimulation of SP territory by toe compression (bottom). Latencies measured between the onset of cord dorsum potential and the onset of postsynaptic potentials after subtraction of extracellular from intracellullar records.

Inhibitory responses

Motoneurons in which toe compression evoked predominant inhibition were found in the same experiments as those showing purely excitatory responses. IPSPs very rarely appeared on weak stimulations. The skin compression produced by 3 mm disk displacements evoked small EPSPs (under 0.5 mV), while inhibitory potentials appeared for disk displacements of 4-5 mm. Hyperpolarizing current passed through the microelectrode was used to reverse compression-induced IPSPs (Fig. 3A), thereby verifying that they did represent inhibitory phenomena. On average, these IPSPs had smaller amplitudes (0.5-2 mV) than the EPSPs observed in excitatory patterns and very long central latencies, with widely dispersed values, mostly beyond 20 ms (mean latency 37.9 ± 18.4 ms, see Fig. 4). These long latencies could be due to long polysynaptic pathways (possibly involving supraspinal loops) or to asynchronous afferent discharges from mechanically activated sensors. Here again, the polyphasic aspect of IPSPs (Fig. 3, A and B1) reflected the temporal dispersion of afferent discharges from cutaneous receptors in the compressed skin area. However, prolongation of the hold phase of compression did not modify the timing of any component of inhibitory responses, and there was no suggestion of any off-effect. Inhibitory responses were observed in 8 of 43 (21%) motoneurons, all from PB-PT. All eight motoneurons belonged to the sample in which effects of mechanical and electrical stimulations could be compared, and, with a single exception (see Table 1), they also displayed inhibition on electrical stimulation of SP nerve (Fig. 3B2).

Of the 52 peroneal motoneurons that were tested for effects of electrical stimulation, 15 (28%) showed inhibitory responses, all from PB-PT. On graded electrical stimulations, weak pulse trains produced small EPSPs following a single IPSP (Fig. 3C1). Repetitive IPSPs appeared for pulses of at least 2 T strength (Fig. 3C, 2 and 3) with amplitudes in the same range as those of EPSPs. Their central latencies (Fig. 4) were consistently longer (range 5.5-8.5 ms, mean 6.8 ± 0.9 ms) than those of EPSPs. In contrast to the dispersed long latencies of compression-induced inhibitory responses, the distribution of the shorter latencies of IPSPs elicited by electric pulses was restricted within a relatively narrow range (Fig. 4) probably because the synchronous afferent volleys generated by electrical stimulation readily activated inhibitory interneurons. The earliest IPSPs had 1.5 ms longer latencies than the earliest EPSPs, indicating that the inhibitory pathway comprised more steps than the excitatory pathway with at least one supplementary interposed interneuron. On repetitive stimulation, the initial IPSP consistently had a larger amplitude than the subsequent ones (Fig. 3C), as already mentioned in the description of excitatory responses (Fig. 1B). For stimulation strengths above 3 T, inhibitory patterns often displayed two components: IPSPs with a fast time course appeared superimposed on a slow wave of hyperpolarization (Fig. 3, B2 and C3). Occasionally a slow component was also detectable in responses to mechanical stimulation (Fig. 3B1). While fast IPSPs could appear without any slow component, the reciprocal situation was never observed.

Mixed responses

Responses to skin compression showing mixed excitatory and inhibitory potentials occurred in 13 of 43 (28%) peroneal motoneurons (8 of 33 PB-PT, 4 of 7 PL, and 1 of 3 unspecified motoneurons). Mixed responses were met in the same experiments as purely excitatory or inhibitory responses. A characteristic feature of mixed patterns was the fact that EPSPs always appeared first, i.e., for weaker stimuli and with shorter latencies than IPSPs, as shown in Fig. 5A. In motoneurons displaying excitatory responses, increases in strength of mechanical stimulation (within the experimental range) failed to evoke inhibitory components of responses, whereas in motoneurons showing mixed responses, these components appeared on strong compression of toe skin, as could be obtained by changing the initial position of the disk (Fig. 5A). Both EPSP and IPSP components had similar amplitudes and the polyphasic appearance already observed in other patterns of responses.



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Fig. 5. Mixed responses to mechanical and electrical stimulations of SP afferents in PB-PT motoneurons. A: same arrangement as in Fig. 1A. Motoneuron from the same experiment as A and B in Fig. 3. Extension of disk course was the same in the 3 panels, but the starting position of the disk was fixed 1 and 2 mm closer to the foot, respectively, in the middle and right panels. - - -, initial position, P0, of the left panel. B: same arrangement as in Fig. 1B. Motoneuron from the same experiment as the example shown in Fig. 3C. All records show the result of subtraction of 5 averaged extracellular traces from 5 averaged intracellular traces.

Mixed responses could appear on electrical stimulation of SP nerve (Fig. 5B) with EPSPs again displaying lower thresholds and shorter latencies than IPSPs. In all respects, the mixed responses showed the combined characteristics of purely excitatory or inhibitory responses. Altogether, this response pattern was evoked in 17 of 52 (32.5%) motoneurons, including 14 of 40 PB-PT, 2 of 9 PL, and 1 of 3 unspecified motoneurons.

Twelve of the 32 peroneal motoneurons that were tested for both modes of stimulation displayed mixed responses to train stimuli on the SP nerve. With toe compression, only half of these motoneurons showed a similar mixed response (Table 1) while in the other half, purely excitatory responses were observed (Fig. 6). This would be in agreement with the assumptions that mechanical stimulation activates fewer receptors and produces less afferent input than electrical stimulation (see Fig. 1B in Perrier et al. 2000) and that activation of excitatory interneurons requires less input than operation of inhibitory pathways. In other terms, the relatively weak and dispersed volleys elicited by toe compression are sufficient to fire excitatory interneurons but not to pass through the longer chain of inhibitory interneurons.



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Fig. 6. Motoneuron displaying a mixed response to electrical stimulation of SP (A) and an excitatory response to mechanical stimulation of SP cutaneous territory (B). Same arrangements as in Fig. 1. Electrical stimulation at 2.5 T. PB-PT motoneuron from the same experiment as the motoneuron illustrated in Fig. 2. All records show the result of subtraction of 5 averaged extracellular traces from 5 averaged intracellular traces.

A comparison of the distribution of responses to both kinds of stimulation in the 32 motoneuron sample (Table 1) would further support these assumptions: on light toe compression, the total incidence of purely excitatory responses was higher (17 motoneurons) than the incidence of the two other types of response taken together (15 motoneurons).


    DISCUSSION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

A striking observation made in the present study is the correspondence between peroneal motoneuron responses to electrical stimulation of SP nerve and mechanical compression of SP-innervated skin areas. As shown by Table 1, all but one of the motoneurons that were excited by electric pulses were also excited by toe compression, and all but one of the motoneurons that were inhibited by electric pulses were also inhibited during toe compression. Mixed responses were more readily elicited by electric pulses than by mechanical stimulation, but altogether, parallel responses to both modes of stimulation were observed in three quarters of the experimental sample. This result indicates that motoneuron responses to electrical stimulation, as used in most experimental studies, are fairly good predictors of their qualitative responses to more physiological stimuli.

To our knowledge, this is the first comparative study of the effects on hindlimb motoneurons of cutaneous afferents activated by two different modes of stimulation. In previous studies using mechanical stimulation of cutaneous sensors, observations mostly concentrated on mechanical or electromyographical effects (Engberg 1964; Hongo et al. 1990; Kanda et al. 1977; Loeb 1993; Schieppatti and Crenna 1984). Schomburg and Steffens (1986) examined, in decerebrate preparations, the responses of 35 triceps surae motoneurons, 37 posterior biceps-semitendinosus motoneurons, and 20 anterior biceps-semimembranosus motoneurons to light manual strokes of the hairy skin of the foot. In this large sample, a majority of motoneurons were weakly excited by the stimulus and a small fraction displayed inhibitory responses.

Quantitative comparison of electrically and mechanically induced responses is difficult because, in the first place, it is not known whether both modes of stimulation generate exactly the same afferent input. Our assumption, however, is that at least some of the afferent fibers present in the terminal portion of SP were activated by both mechanical and electrical stimuli. Electric pulses of 1-6 T strength will activate the fast-conducting afferent fibers in the SP nerve, and it is known that these fibers supply hair, touch, and pressure sensors in cutaneous areas of the cat foot (Hunt and McIntyre 1960). Whatever may be the case, the amplitudes and time courses of both afferent volleys and postsynaptic potentials depend not only on the number of afferent fibers recruited by each mode of stimulation but also on the duration and synchrony of discharges, two crucial factors for activation of interneurons. The relatively mild mechanical stimuli used in the present experiments were most likely to activate rapidly adapting receptors associated with hairs, known for their low threshold and high dynamic sensitivity, and possibly also some pacinian corpuscles responding to both on and off phases of toe compression but not to the hold phase (Brown and Iggo 1967; Hunt and McIntyre 1960). Slow adapting receptors with low thresholds respond to maintained skin displacements by static discharges following an initial dynamic burst (Burgess et al. 1968). Our mechanical stimuli possibly activated such receptors, but their static discharges, even though they persisted throughout the hold phase of toe compression, were not clearly reflected in either afferent volleys or motoneuron responses. The major components of afferent inputs evoked by toe compression were thus generated during disk movements (mostly during forward movements, but also on release of compression, see Fig. 2), that is, over 40- to 100-ms periods corresponding to 2- to 5-mm disk courses at 50 mm/s. A further cause of dispersion of incoming impulses is the 35- to 85-m/s range of conduction velocities of afferent fibers supplying mechanoreceptors in hairy skin (see Birder and Perl 1994). Given the distance between toes and spinal cord, this might result in a delay of 5 ms between the arrival at the spinal cord of impulses carried by the fastest and slowest fibers. Investigations on discharges of single fibers in dorsal root filaments would be necessary to obtain more data on the exact type of mechanoreceptors activated by both the mechanical and electrical stimuli used in the present study.

Low-threshold effects were uniformly excitatory, whether exerted by electrically or mechanically activated afferents, while inhibitory effects required stronger stimulations. Longer chains of interneurons are interposed in inhibitory than in excitatory pathways, as suggested by longer latencies for IPSPs than for EPSPs (Fig. 4). At each step of an interneuronal chain, a given stimulation is likely to elicit discharges of some interneurons and only subliminal excitation in others. The threshold difference between EPSPs and IPSPs raises questions about qualitative differences between motoneurons displaying different response patterns. It is not certain whether each response pattern actually corresponds to a distinct pattern of connections with SP cutaneous afferents. Possibly, the difference is only quantitative, in the sense that every motoneuron has inhibitory as well as excitatory connections but the range of stimulation strengths used in the present experiments did not allow inhibitory potentials to appear in all the tested motoneurons. This assumption would be in keeping with previous observations suggesting that apparently purely excitatory responses of peroneal motoneurons to single SP pulses in fact masked inhibitory components that could be facilitated by convergence (see Fig. 5 in Kouchtir et al. 1997). The distribution of both excitatory and inhibitory influences from cutaneous afferents to every motoneuron in the peroneal pool (rather than a selective distribution of excitation or inhibition to functional subgroups of motoneurons) would allow modulations of cutaneous reflexes over wide ranges as observed in normal and fictive cat locomotion (see e.g., Duysens and Loeb 1980; Forssberg et al. 1977) and in humans (see e.g., Burke et al. 1991; Tax et al. 1995).

However, the existence of at least two different pathways for motoneuron inhibition by cutaneous inputs is suggested by the observation of two components, fast and slow, in inhibitory potentials. The existence of at least two different pathways for excitation, respectively processing the early and late components of the response (as illustrated in Fig. 2), is suggested by the differences observed in interaction of each component with contraction-induced inputs (see Fig. 4 in Perrier et al. 2000). Further studies of the four pathways would require observations of motoneuronal responses after systematical hyper- or depolarizations of the membrane. This was attempted in the present study, but it has been our experience that maintaining a stable depolarization of peroneal motoneuron membrane throughout the kind of mechanical and electrical stimulation we used is quite difficult because it causes discharges and often leads to the loss of the neuron. Hyperpolarization, on the other hand, readily reversed the fast component of IPSPs (Fig. 3A) but not the slow component. Anatomical evidence for the multiplicity of spinal pathways mediating the transmission of cutaneous inputs has been provided by demonstrations that afferent fibers supplying different types of receptors distribute axon collaterals in different zones of laminae III and IV in the dorsal horn of lumbosacral cord (Brown 1981). A recent example where actions from several excitatory and inhibitory pathways could be traced is the complex organization of premotoneuronal circuitry delivering cutaneous influences to ankle flexor motoneurons during fictive locomotion (Degtyarenko et al. 1996).

A consistent feature of both EPSPs (Figs. 1B and 5B) and IPSPs (Fig. 3C) evoked in peroneal motoneurons by electrical stimulation of SP nerve was the rapid decrease in amplitude between the first and second potential in response to a train of repetitive pulses. Similar decline in amplitude of EPSPs evoked by 10/s stimulation of saphenous nerve in tibialis anterior and semi-tendinosus motoneurons was reported by Leahy and Durkovic (1991; see also Heckman et al. 1992). The reasons for this decline are not obvious; causes such as synaptic depletion, desensitization of membrane receptors or changes in membrane conductance or even inhibition of excitatory interneurons, would be difficult to test under the present experimental conditions. Presynaptic inhibition of cutaneous afferent fibers is known to result from the action of these afferents themselves (Eccles et al. 1963; Schmidt et al. 1967; see the review of Schmidt 1973). A specific system of presynaptic inhibition triggered by impulses in fibers innervating rapidly adapting cutaneous receptors is thought to act precisely on these afferents (Jänig et al. 1968) in a phasic manner that would be compatible with the very short delay of the decline in postsynaptic events. If this was the case, the present observations would suggest that the phasic system of presynaptic inhibition of cutaneous afferents is very efficient because decline of EPSPs occurred with the lowest intensities of stimulation supposed to engage but a limited population of afferent fibers.


    ACKNOWLEDGMENTS

The authors are indebted to Dr. Lee Moore, who kindly revised the English. Our thanks to J.-J. Guilbard and G. Jandeau for outstanding technical help with development of a computer-based stimulator and electro-mechanical devices. The gift of Plasmagel by France Pharma is gratefully acknowledged.

This work was supported by grants from Delegation aux Recherches et Etudes Techniques-DGA (95 062), from CNES (920250), and from the European Commission (ERBCHRXCT 930190). J.-F. Perrier was a DRET-DGA research fellow.


    FOOTNOTES

Address for reprint requests: L. Jami, CNRS EP 1848, Université René Descartes, 45 rue des Saints-Pères, 75270 Paris Cedex 06, France.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 9 August 1999; accepted in final form 23 February 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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