Cooperation of Muscle and Cutaneous Afferents in the Feedback of Contraction to Peroneal Motoneurons

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, Unité 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. Cooperation of Muscle and Cutaneous Afferents in the Feedback of Contraction to Peroneal Motoneurons. J. Neurophysiol. 83: 3201-3208, 2000. Peroneal motoneurons were recorded intracellularly in anesthetized cats during sustained submaximal contractions of peroneus brevis muscle (PB) elicited by repetitive electrical stimulation of motor axons in the distal portion of cut ventral root filaments. Mechanical stimulation of the territory innervated by the superficial peroneal nerve (SP) was applied during contraction to assess the influence of afferents from this territory on the contraction-induced excitation of motoneurons. In 21 peroneal motoneurons in which PB contraction evoked excitatory potentials, a stimulation engaging mechanoreceptors located in the skin around toes was found to either enhance (in 12 motoneurons) or reduce (in 9 motoneurons) the contraction-induced excitatory potentials. Among positive effects, six showed simple summation of the responses to each individual stimulus, suggesting a convergence of afferent pathways on motoneurons. In six other motoneurons, complex interactions were observed, as may result from convergence at a premotoneuronal level. Among negative effects, a single instance was observed of inhibitory facilitation, as may result from convergence of cutaneous and muscular, possibly Ib, afferents on inhibitory interneurons. Several pathways, mediating either facilitory or inhibitory influences, are available for cooperation of muscle and cutaneous input, allowing flexibility of motoneuron activation in different tasks.


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The present study stems from previous work on the feedback of hindlimb muscle contractions in homonymous and synergistic motoneurons of the lumbar segments of cat spinal cord. In ankle extensor motoneurons, contraction-activated muscle afferents from triceps surae evoke inhibitory effects that are quickly suppressed, during a brief tetanus, by a central mechanism involving presynaptic inhibition of Ib afferent fibers (Lafleur et al. 1992; Zytnicki et al. 1990). In contrast, the contraction of peroneus brevis (PB, a muscle producing foot eversion and abduction plus a slight contribution to ankle flexion) (see Lawrence et al. 1993) evokes excitation of peroneal motoneurons (Kouchtir et al. 1995). Part of this contraction-induced excitation was shown to depend on input from spindles activated by skeletofusimotor beta -axons (see Fig. 6 in Kouchtir et al. 1995). Possibly, other inputs from either spindle secondaries or muscle receptors innervated by group II-IV fibers or even from Ib afferents (see the review by Pearson 1993) could also contribute. Whatever its origin, a positive feedback might provide an assistance to PB contraction against large loads but, on the other hand, it could favor contraction-induced contractions. In a recent study, Prochazka et al. (1997) used analytic models to examine the effects of positive force feedback on the control of movement and concluded that it may be appropriate in some muscles and motor tasks but not in others.

In standing postures, or climbing, or in any situation where the feet support body weight, movements elicited by PB contractions entail toe displacement accompanied by skin deformations generating cutaneous afferent inputs in terminal branches of plantar and superficial peroneal (SP) nerves. It is known that cutaneous afferents from the leg and foot can evoke both excitatory and inhibitory effects on the motoneurons of hindlimb muscles (see e.g., Degtyarenko et al. 1996; Dum and Kennedy 1980; Fleshman et al. 1988; Loeb 1993; Moschovakis et al. 1991; Powers and Binder 1985; but see also La Bella et al. 1989). Consistent with these data, electrical stimulation of SP nerve was found to elicit nonuniform effects on contraction-induced excitation of peroneal motoneurons (Kouchtir et al. 1995) and to produce mixed actions on their membrane potentials (Kouchtir et al. 1997). This raises questions about the net effect of cutaneous inputs arising on mechanical skin stimulation and muscle afferent inputs arising during contraction: would the excitatory effects of cutaneous inputs cooperate with muscle inputs to enhance the positive feedback from PB contraction or, on the contrary, would their inhibitory effects counteract this feedback?

In the present series of experiments, mechanical stimulation of SP-innervated areas was applied during PB contractions in an attempt to mimic the contraction-induced variations of skin tension and the pressure exerted on toes while the cat is standing or walking. A substantial fraction of this tension and pressure is normally applied to foot-pads innervated by branches of plantar nerves, but cutaneous receptors in SP territory also contribute to the afferent input generated by this stimulus. Mechanical stimulation of SP territory was found to consistently produce appreciable effects on contraction-induced excitation of peroneal motoneurons. Thus through different pathways, cutaneous afferents can contribute to modifications of contraction-induced motoneuron excitation over a wide range, as required by the constraints of permanently varying postures and gaits.

Preliminary results of this study were reported in abstract form (Perrier et al. 1995).


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Experiments were carried out on five adult cats (2.8-3.5 kg) anesthetized with pentobarbitone sodium (Sagatal, May and Baker, initial dose of 45 mg/kg ip, supplemented by additional intravenous doses of 4 mg/kg given at 2- to 3-h intervals). Criteria for control of the adequacy of anesthesia were: stability of blood pressure and heart rate, myotic pupils and absence of movement in response to ear pinching. Ampicillin (Totapen, Bristol, 250 mg sc) was given at the onset of experiment. The central temperature was maintained at 38°C. Blood pressure was maintained greater than 85 mmHg by infusion, at a rate of 3-12 ml/h, of a 5% glucose solution containing NaHCO3 (1%) and gelatin (14% Plasmagel, Roger Bellon). A catheter allowed evacuation of urine from the bladder.

In the right hindlimb, the PB muscle was dissected without disturbing its nerve and blood supplies, set at maximal physiological length (i.e., the length at which a maximal twitch could be produced), and attached by the tendon to a force transducer (Celaster, compliance 60 µm/10 N force, i.e., the full range of the transducer) connected to an amplifier. The hindlimb was denervated except for PB and the terminal branch of SP known to innervate the skin over the medial and lateral aspects of toes after giving off a few minute twigs for the ankle joint (Reighard and Jennings 1961). The intact nerve to PB and the cut nerve to peroneus longus (PL) were dissected over 2 cm and mounted on stimulating electrodes. The nerve to PB occasionally includes a branch containing a few afferents from extensor digitorum brevis muscle and extramuscular regions of the foot (Lundberg et al. 1975); this branch was carefully searched for and cut, and it was verified that stimulation of the terminal branch of SP did not elicit any muscle contraction. The nerve to peroneus tertius (PT) is short and often difficult to isolate over a length sufficient to rule out spread of stimulation to PB nerve; in four of five experiments, it was therefore cut and associated with PB nerve. Results obtained from separately identified PB and PT motoneurons did not differ from those recorded in motoneurons identified by stimulation of PB and PT nerves together (PB-PT motoneurons). All the exposed tissues, in spinal cord and hindlimb, were covered by pools of mineral oil kept at 38°C.

The lumbosacral spinal cord segments were exposed by a laminectomy, and a filament of the L7 ventral root containing motor axons for PB was cut and placed on stimulating electrodes. The filament size varied among preparations: single pulses produced 50-70% of total muscle twitch force and 20/s trains produced a range of tetanic forces. In a given preparation, a force increase could result from potentiation of contraction, whereas a reduction could be due to fatigue. Stimulation sequences at 20/s lasted 500-800 ms and were separated by 1-s intervals to avoid fatigue. Experiments were discontinued if contractile force fell to less than 30% of initial level.

Conventional glass micropipettes filled with 2 M potassium acetate and 0.6 M potassium chloride were used for intracellular recordings (mixing potassium chloride with acetate helped to keep micropipette impedance in the 4-6 MOmega range). Motoneurons with axons in the intact portion of ventral roots were identified by their antidromic responses to nerve stimulation, whereas those with axons in the cut ventral root filament were identified on their pattern of Ia connections (see Kouchtir et al. 1997). Cord dorsum potentials and afferent volleys were recorded by a silver ball electrode placed on the surface of the spinal cord near the entry of the L7 dorsal root.

The leg was rigidly fixed by means of a longitudinal intratibial pin. The ankle was set at right angle and the distal portion of the foot was immobilized between two vertical posts (Fig. 1A). The dorsal aspect of toes was in contact with one of the posts and the plantar aspect was in contact with a plastic circular disk (area 5 cm2) attached to a servo-length actuator. The starting position of the disk could be varied in a range of 1-3 mm. Forward ramp-and-hold movements of the disk (at 50 mm/s, with 2-5 mm amplitudes and 20- to 200-ms plateau durations), producing a mild compression of the toes between the disk and the dorsal post, were used as mechanical stimulus for receptors in SP territory. The absence of PB muscle movement due to disk motion was ascertained by verifying that the force transducer output did not change during disk displacements. At the end of an experiment, the SP nerve was cut, and it was verified that disk motion did not by itself elicit variations in field potentials, or in motoneuron membrane potentials, that could have been mistaken for synaptic events.



View larger version (13K):
[in this window]
[in a new window]
 
Fig. 1. Experimental set up for mechanical stimulation of cutaneous area innervated by superficial peroneal (SP) terminal branches. A: diagrammatic representation of the cat hindlimb with peroneus brevis (PB) muscle tendon attached to a force transducer (F) and distal portion of foot immobilized between two vertical posts. The post on the dorsal side of the paw has been drawn at some distance from the foot in order not to mask the toes; in fact, the post was in contact with the dorsal aspect of toes. A servo-length actuator commanded forward movement of a plastic disk (arrow) pushed at constant velocity against the plantar aspect of toes, causing toe compression (further description in the text). The trajectory of SP is shown diagrammatically with its branch supplying PB, its cutaneous terminal branch (dashed line), and all other branches cut. B: comparison of afferent volleys recorded from cord dorsum on toe compression (left) and on single pulse electrical stimulation of SP (right). Toe compression was produced by a 3 mm forward movement of disk (direction indicated by arrow in A) at 50 mm/s followed by a 200-ms hold phase and return to initial position at the same velocity. Top: cord dorsum potential recorded during disk movement diagrammatically represented in bottom trace. The strength of the electric pulse (dot) applied on SP nerve near the ankle, was 2.2 times the strength eliciting a just detectable afferent volley (T). All records show averages of 10 successive traces. C: cord dorsum potentials (top) recorded during toe compression by ramp-and-hold movements of the actuator (disk course in bottom traces) before (C1) and 10 min after (C2) subcutaneous infiltration of a 2% solution of lignocaïne hydrochloride around each toe (total dose 2 ml). C3 was recorded 20 min after infiltration of a 2nd dose. All records show averages of 25 successive traces. B and C are from different experiments.

Simultaneous records of motoneuron membrane potential, cord dorsum potential, muscle force and disk course were amplified and fed into a four-channel Nicolet 4094-A digital oscilloscope (20-kHz digitization frequency) performing on-line averaging of responses. Membrane potential, muscle force and disk course were recorded via DC-coupled channels. Successive responses (5-10) were averaged, stored on a floppy disk (Nicolet XF-44 unit) and subsequently displayed on a HP 7550A digital plotter.

On stimulation of cutaneous areas, responses of peroneal motoneurons were recorded intracellularly first. The microelectrode was then withdrawn from the neuron by a few micrometers and responses to the same stimulation were recorded extracellularly. Subtractions of extracellular from intracellular records were performed systematically using a Nicolet program, to eliminate field potentials due to interneuronal activation by cutaneous afferents (Pinter et al. 1982). All records in Figs. 2-5 show the results of these subtractions. The same program was further used to perform algebraic operations such as those illustrated in Figs. 3 and 4.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

This report is based on data from 23 peroneal motoneurons (14 from PB-PT, 6 from PL and 3 that could not be ascribed to a particular pool in the peroneal group) with resting membrane potentials (rmp) between -45 and -80 mV (only 4 motoneurons had rmp less than -50 mV and more than half of the sample had rmp greater than -60 mV). During PB contractions elicited by ventral root stimulation, 21 of these motoneurons displayed excitatory postsynaptic potentials (EPSPs) as previously described by Kouchtir et al. (1995). In two motoneurons, contraction-induced hyperpolarizations appeared instead of EPSPs during PB contraction. They were considered as disfacilitation rather than inhibition because they were not reversed by intracellular injection of hyperpolarizing current.

SP input generated by toe compression

Ramp-and-hold compression of the skin around toes was moderate, unlikely to activate more than a fraction of SP-innervated mechanoreceptors and to elicit synchronous afferent discharges from those it did activate. This is why the afferent volleys recorded from the dorsal surface of the spinal cord during toe compression were much smaller than the volleys elicited by electric pulses applied on the distal portion of SP nerve (Fig. 1B). The main component of compression-induced afferent volleys appeared during the forward movement of the disk, and a smaller component subsequently appeared during the return of the disk to its initial position. During the hold phase, afferent volleys were only exceptionnally visible, probably because mainly slowly adapting receptors were active, producing asynchronous discharges.

With this experimental setup, stimulation affected not only the mechanoreceptors in SP territory (i.e., mainly cutaneous receptors plus a few joint receptors, see METHODS) but also some sensors located in the foot beyond SP receptive field. However, as denervation had suppressed all other afferent pathways, only SP fibers remained available to carry the compression-generated input to the spinal cord. Verification that the afferent volleys recorded on toe compression were due to discharges originating from cutaneous receptors was obtained by observing the effects of a local anesthetic drug (2% lignocaïne hydrochloride) injected subcutaneously around the toes. Afferent volleys were appreciably reduced by a first 2-ml dose of the drug distributed by fractions around each toe (Fig. 1C2), and totally disappeared after a second dose (Fig. 1C3). This control, however, did not give any clue about the specific type of cutaneous receptors activated by the mechanical stimulation we used. The shape of the afferent volleys illustrated in Fig. 1 suggests that discharges from the activated receptors mainly occurred during the forward movement of the disk. It seems a reasonable assumption that skin deformation due to toe compression may excite the various types of fast- and slowly adapting receptors present in hairy skin (i.e., hair follicle receptors, touch corpuscles, and Ruffini or Pacini corpuscles). Finally, we cannot exclude the possibility that some receptors from toe joints were also excited during disk displacements. But as SP branches only provide a fraction of toe-joint innervation, mainly supplied by branches of deep peroneal and plantar nerves, toe-joint afferents were unlikely to represent a major component in the compression-induced input. Whatever may be the case, the mixed population of SP-innervated receptors activated by our stimulation belongs to the population activated during either a maintained posture (i.e., quiet stance or crouched position) or the stance phase of various gaits where peroneal muscles are active (see e.g., Loeb 1993).

Cutaneous effects superimposed on contraction-induced excitation

The relatively small afferent volleys generated by toe compression (Fig. 1B) regularly produced PSPs in peroneal motoneurons (Figs. 2, 3B, 4B, and 5B, middle columns). Maximal amplitudes of mechanically induced postsynaptic events were in a range of 0.5-4 mV (see also Behrends et al. 1983), with inhibitory postsynaptic potentials (IPSPs) usually smaller than EPSPs, whatever the rmp. PSP amplitudes were measured on averaged records of 5-10 successive responses, allowing small synaptic events to emerge from background noise provided they appeared consistently in all the responses. However, PSPs less than 0.5 mV were not taken into account and, whenever possible, two or three sets of averaged responses were recorded to verify the consistency of responses. This could not be done in every motoneuron because it required long-lasting stability of recording conditions not always achieved while successive tests of contraction and toe compression were carried out in each motoneuron.

The central latencies of mechanically induced effects in peroneal motoneurons, as measured on averaged records from the onset of the first component of the afferent volley, varied in wide ranges of 7-20 ms for EPSPs and 9-40 ms for IPSPs (see the companion paper, Perrier et al. 2000, for a systematical study of the effects of toe skin stimulations). While the shortest latencies could indicate oligosynaptic intraspinal pathways, latencies of 20-40 ms might be explained either by long pathways (including supra-spinal loops) or by asynchrony of afferent discharges triggered by mechanical stimulation. With asynchronous afferent inputs, the time required to reach the discharge threshold at each interneuronal relay may be expected to be longer than when incoming volleys are elicited by electrical pulses.

Small as they were, the PSPs elicited by the action of cutaneous afferents activated on toe compression interacted with the contraction-induced responses of peroneal motoneurons. The net effects of mechanical stimulation applied during PB contraction were not uniform. Effects with dominant excitation (i.e., extension of the area circumscribed by the EPSP; Figs. 2A, 4, and 5) were observed in 12 of 21 motoneurons (8 from the PB-PT sample, 2 from the PL sample and 2 motoneurons that could not be ascribed to a specific pool in the peroneal group; among the 12 motoneurons, rmp ranged between -45 and -80 mV). Opposite effects, with dominant inhibition (verified by observation of reversal on intracellular injection of hyperpolarizing current), reducing or even suppressing the contraction-induced excitation, were observed in nine other motoneurons with rmp ranging between -47 and -71 mV (5 from PB-PT and 4 from PL; Figs. 2B and 3). Pure inhibition, however, only occurred in three motoneurons, while in six others (3 from PB-PT and 3 from PL), IPSPs rather appeared mixed with EPSPs, but the net result of these mixed effects still was a reduction of contraction-induced excitation. No correlation was observed between r.m.p. and IPSP amplitudes.



View larger version (14K):
[in this window]
[in a new window]
 
Fig. 2. Simple interaction of excitatory (A) or inhibitory (B) effects of toe compression with contraction-induced excitation of two PB-peroneus tertius (PT) motoneurons recorded in the same experiment. In each panel, from top to bottom, motoneuron membrane potential, disk course, muscle force, and ventral root stimulation at 20/s. Cord dorsum potentials have been omitted for simplicity. A1 and B1: contraction alone; A2 and B2: toe compression alone; A3 and B3: toe compression superimposed over muscle contraction. All records show averages of 5 successive traces.

When considering the sample of peroneal motoneurons examined in a particular experiment, no clear relation could be established between stimulus strength and amplitude of either EPSPs or IPSPs elicited by skin stimulation, i.e., a given length of disk course could elicit relatively large postsynaptic events in one motoneuron and relatively small events in another one with similar rmp. It was difficult to quantify the initial position of the disk and to exactly reproduce the same degree of toe compression in different experiments. A disk course of 3 mm was always found efficient for eliciting postsynaptic potentials in all the examined motoneurons.

In responses of peroneal motoneurons to superimposition of skin stimulation over PB contraction, a commonly observed combination of effects suggested simple summation. In this combination, the net result of algebraic subtraction of the sum of contraction effect alone plus effect of toe compression alone from the effect of combined stimulations was nil, i.e., a noisy line without any significant EPSP or IPSP. This was the case in 14 of 21 instances, whether cutaneous input enhanced (in 6 motoneurons) or reduced (in 8 motoneurons) the contraction-induced excitation of these motoneurons. Examples of simple summation are shown in Fig. 2, with two PB-PT motoneurons recorded in the same experiment. In motoneuron A, the EPSPs elicited by toe compression superimposed over those elicited by contraction, whereas in motoneuron B, the same mechanical stimulation caused inhibition and almost suppressed the contraction-induced EPSPs.

Nonlinear interactions

Group I inhibition, as might be evoked by contraction-activated Ib afferents from PB, and as observed in other pools of motoneurons on stimulation of medial gastrocnemius muscle (Lafleur et al. 1993; Zytnicki et al. 1990) never appeared in any motoneuron of the present sample (see Kouchtir et al. 1995 for similar observations on the scarcity of contraction-induced group I inhibition in peroneal motoneurons). However, as cutaneous afferents are known to converge with Ib afferents in their inhibitory pathways to motoneurons (Lundberg et al. 1977), there was a possibility that a weak contraction-induced inhibition might become facilitated through this convergence when mechanical cutaneous stimulation was associated with contraction. Such occurrence was observed in a single case, illustrated in Fig. 3. In this PB-PT motoneuron, a PB tetanus elicited a series of small EPSPs (less than 1-mV amplitude, they could be detected because averaging of several responses allowed their emergence from background synaptic noise, Fig. 3A) and toe compression evoked a weak inhibitory effect (Fig. 3B). On superimposition of cutaneous stimulation over contraction, a supplement of inhibition became visible (Fig. 3C, arrow), as shown by the result of the algebraic operation indicated in Fig. 3, inset (subtraction of the sum of separate contraction effect, A, plus effect of toe compression alone, B, from the effect of combined stimulations, C), suggesting a convergence of cutaneous and muscle afferent inputs on some inhibitory premotoneuronal interneurons.



View larger version (11K):
[in this window]
[in a new window]
 
Fig. 3. Interaction of compression-induced inhibitory postsynaptic potentials (IPSPs) with contraction-induced excitatory postsynaptic potentials (EPSPs) in a PB-PT motoneuron. A-C: same arrangement as in Fig. 2. Comparison of motoneuron membrane potential in B and C shows the appearance of a supplementary IPSP (arrow) further demonstrated in inset (arrow) by the result of the indicated algebraic operation. All records show averages of 5 successive traces. Inset voltage and time calibrations, respectively, 0.5 mV and 200 ms.

In six other motoneurons, recorded in different experiments, complex effects were revealed by interaction of an excitatory response to cutaneous mechanical stimulation with the contraction-induced excitation, as shown by the instances illustrated in Figs. 4 and 5. Here, toe compression elicited biphasic responses in which the first EPSP, displaying a slow time course, was followed by a fast peak (Figs. 4B and 5B, arrows). When toe compression was applied during contraction, the early component of the response added to the contraction-induced EPSPs, whereas the late peak disappeared (Fig. 4C and inset).



View larger version (16K):
[in this window]
[in a new window]
 
Fig. 4. Complex interaction of compression- and contraction-induced responses to cutaneous stimulation in a PB-PT motoneuron. Same arrangement as in Fig. 3. EPSPs in response to toe compression display a double component with a slow wave followed by a fast peak (arrows in B). The slow wave is slightly enhanced, whereas the fast peak is absent from the response when toe compression is superimposed over muscle contraction (C), as shown by the result of the subtraction indicated in inset (arrows). Inset voltage and time calibrations are the same as in C. Slight increase in contraction force between A and C due to potentiation of unfused tetanus. All records show averages of 5 successive traces.



View larger version (18K):
[in this window]
[in a new window]
 
Fig. 5. Further example of complex interactions of the responses recorded in a PB-PT motoneuron on application of toe compression during PB muscle contraction. A: contraction alone. From top to bottom, cord dorsum potential, motoneuron membrane potential, muscle force and ventral root filament stimulation at 20/s. B: effect of 2 brief compressions. From top to bottom, cord dorsum potential, motoneuron membrane potential and disk course. Arrows, fast peak component of the response. C: same compressions applied during muscle contraction. The fast peak is no longer visible in the response. Potentiation of unfused tetanus explains the increase in contraction force. D: same compressions with initial position of disk 1 mm closer to the plantar aspect of toes (previous position indicated by - - -) and longer course of the disk, both resulting in tighter toe compression. Slightly weaker contraction due to incipient fatigue. In C and D, from top to bottom, cord dorsum potential, motoneuron membrane potential, disk course, muscle force and ventral root filament stimulation. All records show averages of 5 successive traces.

Figure 5 illustrates another instance of a PB-PT motoneuron where the EPSPs elicited by cutaneous stimulation displayed two components (Fig. 5B) the second of which, a fast peak, was distinctly reduced when toe compression was superimposed over contraction (Fig. 5C). A stronger compression of the toes was obtained by moving forward the starting position of the disk and extending its course from 3 to 4 mm. The resulting EPSPs had larger amplitudes than those evoked by a 3-mm disk course (compare Fig. 5, C and D), but the peak component still could not be recognized in the response, again suggesting that the contraction-induced input from muscle included an invisible inhibitory component active on the pathway mediating the fast peak. Here, the fact that the peak component was still lacking from the response to a stronger stimulation (Fig. 5D) would support the assumption of an actual inhibitory suppression.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The present data show that relatively weak cutaneous inputs from distal foot regions can interact in various ways with the positive feedback of PB contraction on peroneal motoneurons. In less than half of the experimental sample, the skin deformation caused by a moderate toe compression evoked a net reduction of contraction-induced excitation whereas in other motoneurons, the positive feedback of contraction was reinforced. Instances of more complex interactions of cutaneous input and contraction feedback were also observed: muscle input generated by contraction could exert two different effects on the response to toe compression, enhancing some components and suppressing others. Our results thus suggest that inputs from mechanoreceptors around toes may be processed in several pathways exerting a range of influences on contraction-induced excitation of peroneal motoneurons, possibly depending on the type of activity in which the animal is engaged. At the positive end of the range, these influences will facilitate the recruitment of motoneurons in sustained efforts, and at the negative end, they will reduce the excitability of motoneurons, thereby preventing any uncontrolled development of contraction-induced contraction (see later).

Nonuniform interactions between compression- and contraction-induced effects on peroneal motoneurons are consistent with our observations on the combination of contraction and electrical stimulation of cutaneous afferents (Kouchtir et al. 1995). In a later study, we found that nearly all the motoneurons innervating peroneal muscles received short-latency excitation from cutaneous afferents in sural or SP nerves, and that, in some motoneurons, the simultaneous stimulation of both nerves facilitated subsequent inhibitory components of the response (see Fig. 5 in Kouchtir et al. 1997). The variability observed in the present experiments might be due to the organization of the spinal networks in which cutaneous information is processed or/and to specific functional heterogeneity of motoneuron populations in peroneal nuclei.

There are very few studies of the central effects of naturally activated cutaneous afferents (Behrends et al. 1983; Engberg 1964; Hongo et al. 1990; Kanda et al. 1977; Schieppatti and Crenna 1984; Schomburg and Steffens 1986), and their results cannot be directly compared with the present observations because of major differences in experimental conditions such as type of anesthesia and/or preparation, intracellular or electromyographic recording, location of cutaneous receptors, and reproducibility of cutaneous stimulations. For instance, in high spinal cats paralyzed with pancuronium bromide, Behrends et al. (1983) found that light stroking of the hairy skin of the foot produced small (<2-3 mV) changes in membrane potential of lumbar motoneurons whereas Kanda et al. (1977) had worked on unanesthetized decerebrate cats and found that pinching the ankle skin (sural nerve territory) increased the electromyographic activity of medial gastrocnemius muscle and reduced that of soleus. Nevertheless a general agreement emerges from these studies, pointing to the coexistence of excitation and inhibition from cutaneous afferents among motoneurons innervating different muscles of the cat hindlimb (see the review by Degtyarenko et al. 1996; Hongo et al. 1990; Leahy and Durkovic 1991; Loeb 1993; Schomburg 1990).

The contraction-induced excitation of PB is largely due to Ia afferents (see Kouchtir et al. 1995) acting via mono- or polysynaptic routes, and cutaneous afferents are known to converge on several kinds of interneurons mediating muscle afferent effects (see the review of Jankowska 1992). Different groups of premotor neurons have been identified as transmitters of cutaneous information onto motoneurons (see e.g., Moschovakis et al. 1991; Schmidt et al. 1988). The shortest segmental pathway includes one interneuron (Degtyarenko et al. 1996; Fleshman et al. 1988; Leahy and Durkovic 1991), that is, a site where convergence can occur. In the present study, the length of transmission pathways could not be inferred from the highly dispersed central latencies of compression-induced effects, but such sites were most probably met with by cutaneous and muscle inputs on their way to motoneurons. In addition, presynaptic inhibition of muscle or cutaneous afferents (see Schmidt 1973) might occur on combined stimulation, contributing to a restriction of transmission of muscle or cutaneous information.

Whether positive or negative, the net outcome of interactions between inputs from PB muscle and from toe skin was found to result from either linear or non linear summation of contraction- and compression-induced effects, that is, interactions occurring at either a motoneuronal or a premotoneuronal level. At least four different pathways for transmission of cutaneous afferent actions were thus involved in the observed responses to combined stimulations: two for simple summation of either excitatory or inhibitory effects from cutaneous afferents with contraction-induced excitation (Fig. 2, A and B) and two for nonlinear interactions (Figs. 3-5). The instances illustrated in Figs. 4 and 5 are particularly striking because in these cases, the excitatory response to toe compression had two components that were transmitted via two separate pathways, one of which went through a site of convergent inhibitory influences from cutaneous and muscle afferents. This complex connectivity of cutaneous afferents led us to attempt a further analysis in which the effects of mechanical and electrical stimulations were systematically compared, as will be reported in the companion paper (Perrier et al. 2000).

In the present sample, no segregation of effects was observed between motoneuronal pools innervating PB-PT and PL muscles. However, functional heterogeneities among peroneal motoneurons might account for the observed variety in modulation of contraction-induced excitation by cutaneous afferents. First, it is known that electrical stimulation of cutaneous afferents from several hindlimb territories produces different effects in type-identified motoneurons of ankle extensor and flexor muscles: inhibition is mostly distributed to motoneurons innervating slow motor units while motoneurons innervating fast units are excited (Burke et al. 1970; Dum and Kennedy 1980; but see Clark et al. 1993). It would have been interesting to identify the type of motor units innervated by the motoneurons we examined for interactions of cutaneous and contraction-induced actions. Unfortunately, this proved unfeasible in most cases because repeated muscle contractions and toe compressions did not favor the mechanical stability of recording conditions, and by the time they were carried out, the conditions often deteriorated, precluding application of the tests required for identification of motor unit types.

Second, the biomechanical functions of peroneal muscles allow participation in both ankle extension and flexion (Lawrence et al. 1993) and possibly, different subsets of motor units are recruited for each action (Hensbergen and Kernell 1992; Kandou and Kernell 1989). This was shown to occur in sartorius muscle during trot, paw-shaking, and scratching (Pratt and Loeb 1991; see also Pratt et al. 1991). Peroneal muscles are known to obey different synergies in different motor tasks (Loeb 1993). During fictive locomotion, PL behaves like a flexor muscle, in synchrony with tibialis anterior while PB and PT act as extensors, in synchrony with soleus (Perrier 1996). Recent observations indicate that, in decerebrate or conscious cat, group I afferents from extensor muscles act to prolong the stance phase (Conway et al. 1987; Pearson and Collins 1993). If afferents from PB take part in this reaction, facilitation of excitation from muscle afferents by inputs from SP areas, however weak, might be useful at this stage. Along the same line, the efficacy of excitation from SP nerve in flexor motoneurons is known to fluctuate during a fictive locomotion cycle (Degtyarenko et al. 1996; Forssberg et al. 1977; Schmidt et al. 1988; Schomburg and Behrends 1978; see also Schomburg et al. 1998 for action of flexor reflex afferents on fictive locomotion). PB and PT activities stop in the flexion phase, and the inhibitory component of SP afferent effects might be useful to suppress the contraction-induced excitation of their motoneurons. More generally, the negative effect of cutaneous input from toes on peroneal motoneurons might help whenever the foot meets any unexpected obstacle or nociceptive stimulus or when foot eversion has to be avoided. On the other hand, pretibial flexors are strongly engaged in stalking crouched postures or in vertical ascension of a tree or a wall, two situations in which it is crucial for the animal to maintain a close contact between foot and support. The excitatory component of effects from toe skin afferents on PB and PT motoneurons would then support the abduction and eversion actions of peroneal muscles in resisting contrary influences of tibialis anterior.

Finally the motor innervation of peroneal muscles is known to include an important component from skeletofusimotor, or beta  motoneurons. In both PB and PT muscles, one third of motor units receive beta  innervation (Emonet-Dénand et al. 1992; Jami et al. 1982). As long as nothing is known of the specific (or nonspecific) inputs received by these motoneurons, the possibility cannot be ruled out that their connections with cutaneous afferents might be different from those of alpha -motoneurons.

Although PB, PT, and PL muscles each possess a complement of tendon organs (Scott and Young 1987) that are readily activated by muscle contraction (see the review of Jami 1992), Ib inhibition is very difficult to demonstrate in peroneal motoneurons (see Kouchtir et al. 1995, 1997). In the present sample tested for combined effects of contraction (generating Ib input) and cutaneous stimulation, a single motoneuron (Fig. 3) displayed a facilitation of inhibitory components, indicating that the contraction-induced afferent input from muscle included some inhibitory influences which required facilitation to become manifest, following the mechanisms of convergence demonstrated by Lundberg et al. (1977). Interactions suppressing an excitatory component (as illustrated in Figs. 4 and 5) might also result from this kind of convergence. The different behaviors of the early and late components of the response suggest that compression-induced excitation reached the motoneuron via two different pathways. Interference of contraction-generated muscle inputs with the effect of toe compression only occurred in the pathway mediating the fast peak, possibly because muscle afferents carried some inhibitory input that interrupted the transmission in this pathway. This explanation could account for the suppression of an excitatory component (here, the peak) from the response to cutaneous deformation when combined with contraction. An alternative explanation might be that the inhibitory input from the contracting muscle was, by itself, too weak to elicit discharges from interneurons mediating motoneuron inhibition. On association of cutaneous stimulation with contraction, convergence of cutaneous and muscle inputs on inhibitory interneurons facilitated their discharge (Lundberg et al. 1977), resulting in suppression of the fast peak from the motoneuron response to cutaneous stimulation. Under this assumption, the contraction-activated afferents carrying inhibitory inputs might originate from contraction-sensitive muscle receptors, i.e., tendon organs.


    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 EP1848, 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

0022-3077/00 $5.00 Copyright © 2000 The American Physiological Society