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. 1999 Mar 1;515(Pt 2):599–607. doi: 10.1111/j.1469-7793.1999.599ac.x

The role of capsaicin-sensitive muscle afferents in fatigue-induced modulation of the monosynaptic reflex in the rat

V E Pettorossi *, G Della Torre *, R Bortolami *, O Brunetti *
PMCID: PMC2269162  PMID: 10050025

Abstract

  1. The role of group III and IV afferent fibres of the lateral gastrocnemious muscle (LG) in modulating the homonymous monosynaptic reflex was investigated during muscle fatigue in spinalized rats.

  2. Muscle fatigue was induced by a series of increasing tetanic electrical stimuli (85 Hz, 600 ms) delivered to the LG muscle nerve. Series consisted of increasing train numbers from 1 to 60.

  3. Potentials from the spinal cord LG motor pool and from the ventral root were recorded in response to proprioceptive afferent stimulation and analysed before and during tetanic muscle activations. Both the pre- and postsynaptic waves showed an initial enhancement and, after a ‘12-train’ series, an increasing inhibition.

  4. The enhancement of the responses to muscle fatiguing stimulation disappeared after L3-L6 dorsal root section, while a partial reflex inhibition was still present. Conversely, after section of the corresponding ventral root, there was only a reduction in the inhibitory effect.

  5. The monosynaptic reflex was also studied in animals in which a large number of group III and IV muscle afferents were eliminated by injecting capsaicin (10 mM) into the LG muscle. As a result of capsaicin treatment, the fatigue-induced inhibition of the pre- and postsynaptic waves disappeared, while the response enhancement remained.

  6. We concluded that the monosynaptic reflex inhibition, but not the enhancement, was mediated by those group III and IV muscle afferents that are sensitive to the toxic action of capsaicin. The afferents that are responsible for the response enhancement enter the spinal cord through the dorsal root, while those responsible for the inhibition enter the spinal cord through both the ventral and dorsal roots.

It has been shown that the muscle fatigue induced by maximal voluntary contraction in humans is associated with a decline in the motoneuron firing rate (Bigland-Ritchie et al. 1983, 1986; Woods et al. 1987). This phenomenon, called ‘muscle wisdom’, optimizes the force-generating capacity in fatiguing muscles by matching motoneuron discharge to changing muscle contractile properties (Marsden et al. 1983). The mechanisms proposed to explain the motor unit discharge decrease may be based on central influences (Brasil-Neto et al. 1994; McKay et al. 1995; Gandevia et al. 1996) or on peripheral muscle signals (Garland et al. 1988; Duchateau & Hainaut, 1993; Macefield et al. 1993).

At the peripheral level, in addition to intrinsic adaptation of the motoneurons (Kernell & Monster, 1982; Spielmann et al. 1993), two other mechanisms have been hypothesized. One is based on the disfacilitation of the α-motoneuron pool, due to a progressive withdrawal of spindle activity mediated by a change in the fusimotor support (Bongiovanni & Hagbarth, 1990; Macefield et al. 1991). The other relies on the signals that are elicited by mechanical and metabolic stimuli during muscle fatigue and run through the myelinated (group III) and unmyelinated (group IV) muscle afferents (Garland, 1991; Hayward et al. 1991). In humans, both of the mechanisms have been supported in different experiments (Garland, 1991; Macefield et al. 1993; Sacco et al. 1997). Moreover, the involvement of small afferents in inhibiting the motor responses during muscle fatigue has been suggested in decerebrated or spinalized animals (Hayward et al. 1988). However, this results from indirect observation and a proprioceptive contribution cannot be excluded (Ljubisavljevic et al. 1992; Ljubisavljevic & Anastasijevic, 1994).

This study aims to provide further information on peripheral signals and muscle afferent fibres that are responsible for the motor unit excitability changes during fatigue. We studied how the monosynaptic reflex is influenced by muscle fatigue in spinalized rats and whether there is a contribution of a subgroup of small muscle afferents that are sensitive to capsaicin in the reflex modulation. As shown in previous studies (Baranowski et al. 1986; Lynn, 1990; Holzer, 1991), capsaicin activates fibres containing neurokinins as neuromediators at a low dose, and destroys them at a high dose; it has therefore been recognized as a selective neurotoxin affecting small diameter sensory afferent neurons containing neurokinins (Jancso et al. 1985; Lynn, 1990; Jancso & Lawson, 1990). Moreover, we have shown that some muscle afferents belonging to groups III and IV can be selectively eliminated by applying capsaicin directly into the muscle or onto its motor nerve (Pettorossi et al. 1994; Della Torre et al. 1996). Therefore, by applying capsaicin into the muscle, small muscle afferents may be selectively eliminated and the role of these fibres in ‘muscle wisdom’ may be demonstrated simply by examining the responses to muscle fatigue after capsaicin deafferentation.

We therefore analysed the monosynaptic reflex of lateral gastrocnemious (LG) during fatiguing muscle stimulation in spinalized rats, and then in animals with LG muscle injected with capsaicin. Finally, the spinal root entry pathway of the fibres responsible for the fatigue effects was also investigated in distinct experiments by cutting L3-L6 dorsal and ventral roots.

METHODS

Surgical procedures

Experiments were performed on forty-two male Wistar rats, weighing 300-350 g. Urethane (1 g kg−1i.p.) was used to induce and maintain anaesthesia. The level of anaesthesia was verified by a stable heart rate and pupillary diameter throughout the experiment. The trachea was cannulated and end-tidal CO2 concentration was monitored. If necessary, the animals were artificially ventilated. The femoral blood pressure was measured and maintained at a constant level within the physiological range. The rectal temperature was maintained close to 37.5°C using a feedback-regulated heating blanket. Under a dissecting microscope, the nerve to LG muscle was isolated in the poplitea fossa and all other hindlimb muscles were denervated. LG muscle and its nerve were covered with mineral oil maintained at 37°C by servo-controlled thermoresistance The Achilles’ tendon was detached from its distal insertion and connected to an isometric strain gauge. The spinal cord was exposed by dorsal laminectomy of T12 through to S1, and L3-L6 dorsal and ventral roots were isolated. Then the animal was fixed to a stereotaxic apparatus by clamping the eleventh thoracic spine and iliac process. The exposed spinal cord was transected at T12 level and the spinal cord was covered with mineral oil maintained at 37°C. All experiments were conducted in accordance with the guidelines of the Institutions’ animal welfare committees.

Stimulation and recording

Figure 1 shows the experimental arrangement. The L5 dorsal root was placed over a pair of platinum stimulating wires, 1 cm from the spinal cord. Stimulation parameters consisted of single 0.18-0.22 μA intensity pulses lasting 0.1 ms, to activate the largest afferent fibres. The extracellular field potentials evoked in the LG motor nucleus were amplified using a Grass P511 amplifier (Quincy, MA, USA) with the filters set at 0.8-1000 Hz, stored and averaged (30 simple sweeps) using an ATMIO 16E10 acquisition and analysis system (National Instrument, Austin, TX, USA). The field potentials were recorded by a microelectrode (1 MΩ tip resistance) inserted in L5 and advanced in a dorso-ventral direction towards the ventral horn to reach the LG motor nucleus. The exact recording position was established as that in which LG contraction was elicited by the lowest current intensity and the highest antidromic potential was recorded following LG motor nerve stimulation. The orthodromic field potential consisted essentially of a small early wave followed by a large, late one (Fig. 1, S1-R1). On the basis of the potential latency and the 12 mm distance between stimulating and recording electrodes, the first wave can be considered as presynaptic (0.3 ms peak latency) and the second wave as postsynaptic (1 ms peak latency). The postsynaptic nature of the late wave was confirmed by the fact that it showed post-tetanic potentiation (nerve stimulation: 300 Hz, 10 s). The late potential was also compared with the antidromic potential evoked by ventral root stimulation (Fig. 1, S2-R1). When considering synaptic delay and conduction time, the timing of the antidromic and orthodromic postsynaptic potentials was substantially similar, so that it is likely that the motoneuron activation was responsible for the postsynaptic event. The motoneuron involvement was also confirmed by recording the responses at the L5 ventral root. At the beginning of each experiment, a stimulus-response curve was established and the strength of stimulus was adjusted to elicit 50% of the response maximum. To measure the effect on the potentials, we evaluated their size by rectifying and integrating the waves within time windows of 0.3-0.8 ms and 1-2 ms for the first and the second waves, respectively.

Figure 1. Schematic drawing of the experimental arrangement.

Figure 1

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A, transverse section of the spinal cord, the dorsal and ventral roots, the LG motor nerve and the LG muscle are depicted. Stimulation points: L5 dorsal root (S1), L5 ventral root (S2) and LG motor nerve (S3). Recording points: LG motor pool in the ventral horn (R1), ventral root (R2) and motor nerve (R3). Td and Tv: section level of the dorsal and ventral roots, respectively. B, evoked potentials are shown. S1-R2, ventral root potential induced by electrical stimulation of dorsal root; S1-R1, pre- and postsynaptic potentials in the ventral horn by dorsal root stimulation; S2-R1, antidromic potential in the LG motor pool.

Muscular contraction and fatigue

Force from the LG muscle before and during muscle fatigue was recorded using a force transducer (F03 Grass, Quincy, MA, USA). At the beginning of the experiments, the optimal muscle length for the maximal twitch force was determined, and all force recordings were then taken at this optimal length. Isometric contractions were elicited by electrical stimulation delivered by bipolar platinum electrodes placed on the isolated nerve of the LG muscle (Fig. 1, S3). The compound action potential of the nerve was recorded simultaneously. Once the twitch threshold was determined, the strength of the stimulus was adjusted to 1.3 times twitch threshold, an intensity that never exceeded the electrical threshold for activating the muscular afferents belonging to groups III and IV. The muscle was fatigued by means of a series of trains applied to the LG motor nerve (a train lasted 600 ms at a fusion frequency of 85 Hz, with a 1 s interval between each train). Each rat underwent six series of increasing numbers of trains (1, 3, 6, 12, 30 and 60), with 60 s intervals between the series.

To evaluate the fatigue effect on the force of the LG muscle, we measured both the tension at the end of the tetanic stimulation and the twitch peak tension elicited 10 s before and after the tetanic stimulations. The time to peak and half-relaxation time of twitch were also measured.

Compound action potential of the LG motor nerve

Bipolar stimulating electrodes were placed on the motor nerve supplying the LG muscle, in the vicinity of its entry point into the muscle. A platinum recording electrode was placed 10 mm centrally from the stimulating cathode (Fig. 1A, S3-R3) for recordings of compound action potential (Fig. 7A). This allowed us to verify the effect on nerve fibres of intramuscular capsaicin injection and to reveal possible motoneuron drop-out following high-frequency stimulation.

Figure 7. Effect of different doses of capsaicin on the compound action potential of the nerve supplying the LG muscle.

Figure 7

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A, typical traces of compound action potentials of the LG motor nerve recorded before (top) and 2 h after (bottom) 10 mM capsaicin injection into LG muscle. The distance between the stimulating cathode and the recording electrode was 10 mm. The arrow indicates the stimulus onset. The horizontal lines indicate the ranges that include the Aα/β (I), Aδ (III) and C (IV) groups, according to Harper & Lawson's classification (Harper & Lawson, 1985). Note the marked reduction of compound action potential waves belonging to group IV and a partial reduction of group III in capsaicin-treated muscle. B, the histogram shows the size reduction of different components of compound action potentials, expressed as a percentage of the pre-injection values, 2 h after applying different doses of capsaicin. Each group of three animals received a different dose of capsaicin. The data refer to means and s.d. from three experiments. □, group I; Inline graphic, group III; ▪, group IV.

Experimental procedure

We studied the effect of LG muscle fatigue in seven untreated rats, eleven rats treated with capsaicin (0.1 ml, 10 mM; Sigma) dissolved in vehicle (10% ethyl alcohol, 10% Tween and 80% normal saline) and five rats treated with vehicle (0.1 ml). Under urethane anaesthesia (1 g kg−1i.p.) solutions were slowly injected into the central portion of the LG muscle using a glass micropipette. Usually the animals were tested 2 h after injection, but in four cases they were tested 24 h (2 rats) and 7 days (2 rats) after capsaicin injection. The monosynaptic reflex and the muscle twitch tension were analysed before and 10 s after each fatiguing stimulation series, while in two cases, tests were also continued for 30 min after the last stimulation of 60 trains.

In five more animals the LG motor nerve was completely severed and the tests were performed by applying the series of stimulation trains both centrally and peripherally to the section of the nerve. In two further groups of five animals each, tests were performed before and after complete section of the dorsal and ventral root, from L3 to L6. The level of the ventral root section was close to its entry point into the spinal cord (0.5 mm apart). This level of section was chosen not to damage the fibres that make a loop in the ventral root and reach the spinal cord through the dorsal root (Shin et al. 1986; Chung & Kang, 1987). The dorsal root was sectioned half-way between the dorsal ganglion and the root entry point. At the end of the experiments the animals were killed by an anaesthetic overdose.

Data analysis

The data represent means ±s.d. from five animals for each experimental protocol and are expressed as a fraction of the control value (the mean of the responses recorded within the first 10 min of each experiment). The results were compared using Student's paired t test. A difference of P < 0.05 was considered to be statistically significant.

RESULTS

Influence of muscle fatiguing stimulation on muscle tension

The effect of progressively increasing series of tetani (85 Hz for 600 ms) on the muscle force was examined in five untreated rats. Tetanic tension began to decline after the ‘6-train’ series and reached 30% of the control value at the end of the stimulation sequences (Fig. 2). Conversely, the tension of twitch showed a significant peak increase after 1-, 3- and 6-train sequences (P < 0.01 and P < 0.001), and only began to decrease after the 12-train sequence to reach about 30% of the initial value after the 60-train sequence (Fig. 2). The twitch contraction time and half-relaxation time showed an elongation, already present after the 3-train sequence.

Figure 2. Effect of repetitive tetanic stimulation on LG muscle mechanical responses.

Figure 2

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A, twitch (left) and tetanic (right) muscle contraction before (C) and after muscle stimulation sequences of 1, 3, 6, 12, 30 and 60 trains. B, the graph shows the effect of repetitive stimulation on peak force, time to peak, half-relaxation time of the twitch, and final tension value of the last tetanic response in untreated and capsaicin-treated LG muscles in five experiments. Data points represent the means and the vertical bars the s.d., expressed as a fraction of the control value; horizontal bars indicate the application period of muscle stimulation series. Note the initial twitch force enhancement, followed by a force reduction.

As the stimulus intensity was not supramaximal for all α-motoneurons, some of them dropped out during repetitive electrical stimulation, leading to an overestimation of the muscle fatigue. However, the compound action potential recorded during the stimulation series was only slightly affected, showing a wave reduction of about 5%.

Influence of muscle fatiguing stimulation on the monosynaptic reflex examined at the level of the ventral horn and ventral root

In the five untreated animals, the field potentials induced by L5 dorsal root stimulation were recorded in the LG motoneuron pool after each tetanic stimulation sequence (Fig. 1, S1-R1). The potentials were recorded where the antidromic potential evoked by ventral root stimulation was maximal (Fig. 1, S2-R1). The size of both pre- and postsynaptic potentials showed a significant enhancement of about 12% following 3- and 6-train sequences (P < 0.005) (Fig. 3), but after the 12-train sequence the responses began to decrease, reaching about 65% of the initial value (P < 0.005) at the end of 60-train series. The extent of the tetanic effect on the pre- and postsynaptic waves was not significantly different and was correlated with the muscle tension decay. In fact, the sizes of potentials during the fatigue test showed a significant positive correlation with muscle tension evaluated from 6- to 60-train sequences (r = 0.954, n = 20, Fig. 4). Following the last stimulation sequence (60 trains), the size of the potentials gradually recovered as muscle force recovered.

Figure 3. Effect of repetitive tetanic LG nerve stimulation on the size of presynaptic and postsynaptic potentials of the LG muscle monosynaptic reflex.

Figure 3

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Pre- (open symbols) and postsynaptic potentials (filled symbols) after motor nerve stimulation series of 1, 3, 6, 12, 30 and 60 trains. Data points represent the means and the vertical bars the s.d. of potential sizes, expressed as a fraction of the control value and collected from five experiments. The horizontal bars indicate the application muscle stimulation series. The continuous line refers to the stimulation of intact LG motor nerve and the dotted line to the stimulation of the central and peripheral stump of the severed nerve. Note in the intact nerve stimulation, the enhancement of the potentials after 3- and 6-train sequences, followed by their reduction after 12-, 30- and 60-train sequences. Conversely, no change was observed when the LG nerve had been severed. On the right-hand side of the figure, the upper potential was recorded from the LG motor pool in the ventral horn (VH), and the lower potential from the ventral root (VR), before (continuous line) and after 6-train (dashed line) and 60-train (dotted line) stimulation sequences.

Figure 4. Percentage change of postsynaptic potential size as a function of LG muscle fatigue.

Figure 4

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Each point indicates the postsynaptic potential size (ordinate) evoked in the ventral horn and the corresponding tetanic end tension (abscissa) of LG muscle during fatiguing stimulation (6, 12, 30 and 60 trains). The values, collected from five experiments, are expressed as a percentage of the control values. The r value of linear regression was 0.954 in controls and 0.168 in capsaicin-treated animals.

In two experiments the responses to L5 dorsal root stimulation were recorded at the level of the corresponding ventral root (Fig. 1, S1-R2). The repetitive muscle stimulation influenced the size of the potential, provoking an initial enhancement and a subsequent reduction similar to that observed in the spinal cord (Fig. 3). This confirms that the effects of muscle fatiguing stimulation occurred at the level of the motoneurons.

The monosynaptic reflex was also examined after cutting the nerve supplying the LG muscle. In this condition, the repetitive electrical stimulation of the central as well as the peripheral stump of LG motor nerve did not induce any significant change in the potential size (Fig. 3).

Effect of fatiguing stimulation on the monosynaptic reflex after dorsal and ventral rhizotomy

In a group of five rats, the effects of LG muscle fatiguing stimulation on the monosynaptic reflex were studied before and after dorsal rhizotomy from L3 to L6 (Fig. 5). After rhizotomy, the enhancement of presynaptic and postsynaptic responses was not observed following 3- and 6-train sequences, but the potential inhibition was still present, albeit to a lesser extent, reaching about 80% of the control value after the 60-train sequence (P < 0.005).

Figure 5. Effect of tetanic repetitive stimulations on the monosynaptic reflex before and after dorsal root section.

Figure 5

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Pre- (open symbols) and postsynaptic potentials (filled symbols) of monosynaptic reflex after tetanic stimulation sequences of 6, 12, 30 and 60 trains (horizontal bars). Dorsal root sections were performed at the L3-L6 level. Data points represent means and s.d. of potential sizes, expressed as a fraction of the control value, in five experiments (before section: continuous line; after section: dotted line). Note the absence of the enhancement effect and the reduction of inhibition after dorsal root section.

Conversely, in another five rats the effects of fatiguing stimulation were analysed before and after ventral rhizotomy (L3-L6). The earlier reflex potentiation induced by the tetanic stimulation was not abolished after rhizotomy, yet the late depression still occurred, but was reduced (Fig. 6).

Figure 6. Effect of tetanic repetitive stimulations on the monosynaptic reflex before and after ventral root section.

Figure 6

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Pre- (open symbols) and postsynaptic potentials (filled symbols) of monosynaptic reflex after tetanic stimulation sequences of 6, 12, 30 and 60 trains (horizontal bars). Ventral root sections were performed at the L3-L6 level. Data points represent means and s.d. of potential sizes, expressed as a fraction of the control value, in five experiments (before section: continuous line; after section: dotted line). Note that the effects were still present after rhizotomy, although the inhibition was to a lesser extent.

Effect of capsaicin on the compound evoked potential of the motor nerve supplying the LG muscle

The compound action potentials of the nerve supplying the LG muscle before and 2 h after injection of different doses of capsaicin into the LG muscle (three animals for each dose used) were compared (Fig. 7). The size of the potential wave associated with group III fibres, corresponding to the Aδ group of Harper & Lawson's classification (Harper & Lawson, 1985), showed a reduction of about 20%, and that associated with group VI (C group) a reduction of about 80%, after high (33 mM) as well as intermediate (10 mM) doses of capsaicin, while no effect was observed at lower dosage (0.22 mM) (Fig. 7). In the present study we used a dose of 10 mM.

Effect of fatiguing stimulation on muscle tension and the monosynaptic reflex after capsaicin injection into LG muscle

Rats were treated with capsaicin (n = 5) and vehicle (n = 5) 2 h before muscle electrical stimulation. Neither capsaicin nor vehicle injections significantly altered the characteristics of the muscle response to nerve stimulation and the effects of the fatiguing stimulation (Fig. 2). Similarly, the field potentials in the LG motor pool evoked by dorsal root stimulation before and after the injections were not modified (Fig. 1, S1-R1). Conversely, the effect of repetitive muscle stimulation on the size of the pre- and postsynaptic waves was remarkably changed after capsaicin treatment. In capsaicin-treated rats, the potentials did not show inhibition in response to 12-, 30- and 60-train sequences and remained enhanced at the same level reached after 3- and 6-train sequences (Fig. 8), so there was no longer a correlation between the muscle fatigue level and potential size (Fig. 4). On the other hand, vehicle-treated groups showed a significant progressive decrease of the potential size after the 12-train sequence, as did the control group. A comparison between vehicle-treated animals and untreated ones did not reveal any significant differences.

Figure 8. Effect of tetanic repetitive stimulations on the monosynaptic reflex after capsaicin injection into the muscle.

Figure 8

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Pre- (open symbols) and postsynaptic waves (filled symbols) of monosynaptic reflex following muscle repetitive activation, 2 h after vehicle (continuous line) and capsaicin (dotted line) injection into LG muscle. Data points represent means and s.d. of potential size, expressed as a fraction of the control value, and horizontal bars indicate the application period of muscle stimulation series of 1, 3, 6, 12, 30 and 60 trains. On the right-hand side of the figure, field potentials recorded in the LG motor pool of capsaicin-treated rats before (continuous line) and after 6-train (dashed line) and 60-train (dotted line) stimulation series are shown. Note that capsaicin fully abolishes the fatigue-induced inhibition of the potentials, without affecting their enhancement.

The effect of capsaicin was also studied 24 h and 7 days after capsaicin injections in four other animals. In these cases, as in the acute preparations, the evoked potentials showed a lack of fatigue-induced inhibition in response to the 12-, 30- and 60-train sequences.

In two experiments the L5 ventral root potential evoked by dorsal root stimulation was recorded after capsaicin injection (Fig. 1, S1-R2). The ventral root response was only enhanced by repetitive stimulation, showing no difference from the postsynaptic event recorded in the LG motor pool.

In another group of five rats the effect of fatigue on the monosynaptic reflex was tested in the contralateral LG muscle, compared with the capsaicin-injected muscle. There were no potential size differences between the contralateral monosynaptic reflex changes and those observed in the vehicle and control groups. We may therefore exclude the possibility that the disappearance of the inhibition in the capsaicin-treated muscle could be a consequence of excitability change in the spinal cord induced by capsaicin deafferentation.

DISCUSSION

In spinalized rats, the repetitive tetanic contractions of the lateral gastrocnemious (LG) influenced the homonymous monosynaptic reflex, initially enhancing and subsequently reducing the field potentials recorded in the LG motor pool. This effect was confirmed by the ventral root response. The early and late waves, which are likely to represent the presynaptic and postsynaptic components of the monosynaptic reflex, underwent quite similar changes during fatiguing stimulation. It can therefore be stated that both the afferent and efferent components of the monosynaptic reflex were affected, but the influence should be greater at the presynaptic level.

Since high-frequency stimulation of the central and peripheral stump of the severed motor nerve did not induce any change in the reflex amplitude, the effect of repetitive muscle stimulation cannot be attributed to the direct electrical activation of the nerve or to metabolites released from the muscle into the blood, but only to signals originating from the muscle and travelling through the muscle nerve. The effect of the tetanic stimulation clearly varied with the changes occurring in the muscle during long-lasting activity. At the beginning, when there was still no significant deficit in the muscle contraction, yet there was a slight tension increase, the reflex amplitude increased. As far as we know, this is the first observation of an increase in the monosynaptic reflex induced by muscle signals during pre-fatigue conditions. When the muscle tension and velocity of contraction fell, the reflex showed a depression, which was positively correlated with the amount of tension deficit. Also, the return of the reflex to the control value after the fatiguing stimulation occurred simultaneously with the muscle force recovery. Therefore, it would seem that peripheral signals from the muscle, which undergoes repetitive fatiguing contractions, may induce changes in the excitability of the motoneurons to adapt their discharge to changes in muscle contractile properties. However, the amplitude of the reflex modulation was less than that required to adjust for the remarkable muscle deficit, suggesting that other mechanisms may contribute to this control. It is also possible that we underestimated the reflex changes, because part of the evoked activity may result from neurons that do not belong to the fatigued muscle, and the spinal excitability may be low in our preparation.

The second finding of this study is that capsaicin-sensitive fibres are responsible for the reflex inhibition induced by muscle fatigue, as it disappeared 2 h after capsaicin injection, but they do not affect the initial reflex enhancement. Since high doses of capsaicin injected into the LG muscle block most of group IV and some group III muscle afferents (Pettorossi et al. 1994), we attribute the disappearance of the reflex reduction to the lack of inhibitory input from these fibres. We can exclude the probability that initial peptide release in the dorsal horn may be responsible for the observed effect, since the reflex depression disappeared even when the animals were tested 24 h and 7 days after capsaicin injection. In addition, in the literature there is no evidence for an enhancement of responsiveness of some afferents that could partially or fully mask the reflex depression. The capsaicin-sensitive fibres may influence the monosynaptic reflex by postsynaptic inhibition of the motoneurons, or by presynaptic inhibition of the afferents impinging upon the motoneurons. The presynaptic inhibition is the most reasonable mechanism that could account for the changes observed in the early wave, since we exclude the occurrence of changes in the axonal threshold or spindle activity, as the potential inhibition was also maintained after nerve transection and dorsal rhizotomy. The presynaptic effect may be mediated by interneurons or may be due to a direct influence of capsaicin-sensitive afferents on the large afferents, since anatomical evidence of a direct synaptic contact between these small fibres and large afferent axons has recently been provided (Della Torre et al. 1996).

As ergoceptors and nociceptors contain neurokinins (Kaufman et al. 1985; Duggan et al. 1991), and capsaicin blocks fibres containing these substances, we suggest that muscle afferents responsible for the fatigue-induced depression may be of an ergoceptive and nociceptive nature. The ergoceptive and nociceptive signals may be imortant in controlling motoneuron excitability during the development of muscle fatigue.

Interestingly, after capsaicin injection, the initial enhancement of the response appears to be maintained at the same level throughout all the tetanic stimulation sequences. This suggests that the peripheral signals leading to the reflex potentiation not only act during the pre-fatigue condition, but also when fatigue is settled. Thus it seems that prolonged activation of the muscle elicits two opposite afferent signals from the muscle, which counteract each other at the level of the monosynaptic reflex. These different signals travel through different groups of muscle afferents: the fibres responsible for the reflex inhibition are capsaicin sensitive and belong to groups III and IV, while the remaining fibres, such as the proprioceptive afferents as well as the capsaicin-insensitive group III and IV afferents, may be responsible for the reflex enhancement.

Moreover, the spinal cord entry root of these opposite signals was different. The integrity of the dorsal root is required for the reflex potentiation, while both dorsal and ventral roots contribute to the reflex depression. Section of the ventral roots partially reduced the reflex depression. This section, performed very close to the root entry point in the spinal cord, eliminated only the afferents that reach the spinal cord via the ventral root. On the other hand, section of the dorsal root abolished the potentiation and reduced the depression. This section eliminated all the afferents except those that enter the spinal cord through the ventral root. Anatomical studies support these latter findings, showing that, besides the dorsal roots, ventral roots also contain fibres of small to medium diameter (Coggeshall et al. 1980; Longhurst et al. 1980), which are sensitive to capsaicin (Kimura et al. 1994; Della Torre et al. 1996). It seems that the effect on the reflex inhibition is surprisingly greater than the number of ventral root afferent fibres suggests (Nam et al. 1990), but the rhizotomy was extended from L3 to L6, and the ventral root afferent fibres may distribute their influence outside its metameric zone and may affect the L5 evoked potentials.

The absence of reflex facilitation after dorsal rhizotomy indicates that the fibres responsible for reflex potentiation enter the spinal cord through the dorsal root. Thus both large and small afferent fibres could be involved in this effect. Fatigue may enhance the receptor sensitivity of the muscle spindle afferents, as suggested by Ljubisavljevic et al. (1992) and Ljubisavljevic & Anastasijevic (1994). Yet it may be also possible that small-diameter fibres, which are not capsaicin sensitive, may facilitate the reflex.

In conclusion, this study suggests that prolonged muscle activity elicits different signals from the muscle itself, which are responsible for the enhancement and depression of the monosynaptic reflex. The facilitatory influence may help to enhance the motor performance and counteract possible fatigue taking place at upper motor levels during the initial period of prolonged muscle activity. Once fatigue is established, the inhibitory effect could prevail, reducing motoneuron discharge to match the motoneuron output to the changing contractile properties of the muscle.

Acknowledgments

This work was supported by CNR and MURST grants. The authors thank Mr D. Bambagioni and Mr E. Mezzasoma for expert technical assistance and Miss H. A. Giles for editorial assistance in preparing the English text.

References

  1. Baranowski R, Lynn B, Pini A. The effects of locally applied capsaicin on conduction in a cutaneous nerve of four mammalian species. British Journal of Pharmacology. 1986;89:267–276. doi: 10.1111/j.1476-5381.1986.tb10256.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Bigland-Ritchie B, Dawson NJ, Johansson RS, Lippold OCJ. Reflex origin for the slowing of motoneurone firing rates in fatigue of human voluntary contractions. The Journal of Physiology. 1986;379:451–459. doi: 10.1113/jphysiol.1986.sp016263. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Bigland-Ritchie B, Johansson RS, Lippold OCJ, Smith S, Woods JJ. Changes in motoneurone firing rates during sustained maximal voluntary contractions. The Journal of Physiology. 1983;340:335–346. doi: 10.1113/jphysiol.1983.sp014765. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Bongiovanni LG, Hagbarth K-E. Tonic vibration reflexes elicited during fatigue from maximal voluntary contractions in man. The Journal of Physiology. 1990;423:1–14. doi: 10.1113/jphysiol.1990.sp018007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Brasil-Neto JP, Cohen LG, Hallett M. Central fatigue as revealed by postexercise decrement of motor evoked potentials. Muscle and Nerve. 1994;17:713–719. doi: 10.1002/mus.880170702. [DOI] [PubMed] [Google Scholar]
  6. Coggeshall RE, Maynard CW, Langford LA. Unmyelinated sensory and preganglionic fibers in rat L6 and S1 ventral spinal roots. Journal of Comparative Neurology. 1980;193:41–47. doi: 10.1002/cne.901930103. [DOI] [PubMed] [Google Scholar]
  7. Chung K, Kang HS. Dorsal root ganglion neurons with central processes in both dorsal and ventral roots in rats. Neuroscience Letters. 1987;80:202–206. doi: 10.1016/0304-3940(87)90654-9. [DOI] [PubMed] [Google Scholar]
  8. De Torre G, Lucchi MN, Brunetti O, Pettorossi VE, Clavenzani P, Bortolami R. Central projections and entries of capsaicin-sensitive muscle afferents. Brain Research. 1996;713:223–231. doi: 10.1016/0006-8993(95)01538-8. 10.1016/0006-8993(95)01538-8. [DOI] [PubMed] [Google Scholar]
  9. Duchateau J, Hainaut K. Behaviour of short and long latency reflexes in fatigued human muscles. The Journal of Physiology. 1993;471:787–799. doi: 10.1113/jphysiol.1993.sp019928. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Duggan AW, Hope PJ, Lang CW, Williams CA. Sustained isometric contraction of skeletal muscle results in release of immunoreactive neurokinins in the spinal cord of the anaesthetized cat. Neuroscience Letters. 1991;122:191–194. doi: 10.1016/0304-3940(91)90855-n. 10.1016/0304-3940(91)90855-N. [DOI] [PubMed] [Google Scholar]
  11. Gandevia SC, Allen GM, Butler GJ, Taylor JE. Supraspinal factors in human muscle fatigue: evidence for suboptimal output from the motor cortex. The Journal of Physiology. 1996;490:529–536. doi: 10.1113/jphysiol.1996.sp021164. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Garland SJ. Role of small diameter afferents in reflex inhibition during human muscle fatigue. The Journal of Physiology. 1991;435:547–558. doi: 10.1113/jphysiol.1991.sp018524. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Garland SJ, Garner SH, McComas AJ. Reduced voluntary electromyographic activity after fatiguing stimulation of human muscle. The Journal of Physiology. 1988;408:547–556. doi: 10.1113/jphysiol.1988.sp017178. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Harper HH, Lawson SN. Conduction velocity is related to morphological cell type in rat dorsal root ganglion neurones. The Journal of Physiology. 1985;359:501–511. doi: 10.1113/jphysiol.1985.sp015573. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Hayward L, Breitbach D, Rymer WZ. Increased inhibitory effects on close synergists during muscle fatigue in the decerebrate cat. Brain Research. 1988;440:199–203. doi: 10.1016/0006-8993(88)91178-x. 10.1016/0006-8993(88)91178-X. [DOI] [PubMed] [Google Scholar]
  16. Hayward L, Wesselmann V, Rymer WZ. Effects of muscle fatigue on mechanically sensitive afferents of slow conduction velocity in the cat triceps surae. Journal of Neurology. 1991;65:360–370. doi: 10.1152/jn.1991.65.2.360. [DOI] [PubMed] [Google Scholar]
  17. Holzer P. Capsaicin: cellular targets, mechanisms of action, and selectivity for thin sensory neurons. Pharmacological Review. 1991;43:143–201. [PubMed] [Google Scholar]
  18. Jancso G, Kiraly E, Joo F, Such G, Nagy A. Selective degeneration by capsaicin of a subpopulation of primary sensory neurons in the adult rat. Neuroscience Letters. 1985;59:209–214. doi: 10.1016/0304-3940(85)90201-0. 10.1016/0304-3940(85)90201-0. [DOI] [PubMed] [Google Scholar]
  19. Jancso G, Lawson SN. Transganglionic degeneration of capsaicin-sensitive C-fiber primary afferent terminals. Neuroscience. 1990;39:501–511. doi: 10.1016/0306-4522(90)90286-d. 10.1016/0306-4522(90)90286-D. [DOI] [PubMed] [Google Scholar]
  20. Kaufman MP, Kozlowski GP, Rybicki KJ. Attenuation of reflex pressor response to muscular contraction by substance P antagonist. Brain Research. 1985;333:182–184. doi: 10.1016/0006-8993(85)90143-x. 10.1016/0006-8993(85)90143-X. [DOI] [PubMed] [Google Scholar]
  21. Kernell D, Monster AW. Motoneurone properties and motor fatigue: an intracellular study of gastrocnemius motoneurones of the cat. Experimental Brain Research. 1982;46:197–204. doi: 10.1007/BF00237177. [DOI] [PubMed] [Google Scholar]
  22. Kimura M, Kishida R, Abe T, Goris RC, Kawai S. Nerve fibers immunoreactive for substance P and calcitonin gene-related peptide in the cervical spinal ventral roots of the mouse. Cell and Tissue Research. 1994;277:273–278. doi: 10.1007/BF00327774. [DOI] [PubMed] [Google Scholar]
  23. Ljubisavljevic M, Anastasijevic R. Fusimotor-induced changes in muscle spindle outflow and responsiveness in muscle fatigue in decerebrate cats. Neuroscience. 1994;63:339–348. doi: 10.1016/0306-4522(94)90028-0. 10.1016/0306-4522(94)90028-0. [DOI] [PubMed] [Google Scholar]
  24. Ljubisavljevic M, Jovanovic K, Anastasijevic R. Changes in discharge rate of fusimotor neurones provoked by fatiguing contractions of cat triceps surae muscles. The Journal of Physiology. 1992;445:499–513. doi: 10.1113/jphysiol.1992.sp018936. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Longhurst JC, Mitchell JH, Moore AB. The spinal cord ventral root: an afferent pathway of the hindlimb pressor reflex in cat. The Journal of Physiology. 1980;301:467–476. doi: 10.1113/jphysiol.1980.sp013218. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Lynn B. Capsaicin: action on nociceptive C fibers and therapeutic potential. Pain. 1990;41:61–69. doi: 10.1016/0304-3959(90)91110-5. 10.1016/0304-3959(90)91110-5. [DOI] [PubMed] [Google Scholar]
  27. Macefield VG, Gandevia SC, Bigland-Ritchie B, Gorman R, Burke D. The firing rates of human motoneurones voluntarily activated in the absence of muscle afferent feedback. The Journal of Physiology. 1993;474:429–443. doi: 10.1113/jphysiol.1993.sp019908. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Macefield VG, Hagbarth K-E, Gorman R, Gandevia SC, Burke D. Decline in spindle support to α-motoneurones during sustained voluntary contractions. The Journal of Physiology. 1991;440:497–512. doi: 10.1113/jphysiol.1991.sp018721. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. McKay WB, Tuel SM, Scherwood AM, Stokic DS, Dimitrijevic MN. Focal depression of cortical excitability induced by fatiguing muscle contraction: a transcranial magnetic stimulation study. Experimental Brain Research. 1995;105:276–282. doi: 10.1007/BF00240963. [DOI] [PubMed] [Google Scholar]
  30. Marsden CD, Meadows JC, Merton PA. Muscular wisdom’ that minimized fatigue during prolonged effort in man: peak rates of motoneuron discharge and slowing of discharge during fatigue. In: Desmedt JE, editor. Motor Control Mechanisms in Health and Disease. New York: Raven Press; 1983. pp. 169–211. [PubMed] [Google Scholar]
  31. Nam SC, Kim KJ, Leem JW, Chung K, Chung JM. Increased number of unmyelinated fibers in the ventral root after peripheral neurectomy in adult rat. Neuroscience Letters. 1990;116:40–44. doi: 10.1016/0304-3940(90)90383-k. 10.1016/0304-3940(90)90383-K. [DOI] [PubMed] [Google Scholar]
  32. Pettorossi VE, Bortolami R, Della Torre G, Brunetti O. Effect of capsaicin in the motor nerve. Experimental Neurology. 1994;128:284–289. doi: 10.1006/exnr.1994.1138. 10.1006/exnr.1994.1138. [DOI] [PubMed] [Google Scholar]
  33. Sacco P, Newberry R, McFadden L, Brown T, McComas MB. Depression of human electromyographic activity by fatigue of a synergist muscle. Muscle and Nerve. 1997;20:710–717. doi: 10.1002/(sici)1097-4598(199706)20:6<710::aid-mus8>3.0.co;2-b. 10.1002/(SICI)1097-4598(199706)20:6<710::AID-MUS8>3.3.CO;2-2. [DOI] [PubMed] [Google Scholar]
  34. Shin HK, Kim J, Nam SC, Paik KS, Chung JM. Spinal entry route for ventral root afferent fibers in the cat. Experimental Neurology. 1986;94:714–725. doi: 10.1016/0014-4886(86)90249-9. 10.1016/0014-4886(86)90249-9. [DOI] [PubMed] [Google Scholar]
  35. Spielmann JM, Laouris Y, Nordstrom MA, Robinson GA, Reinking RM, Stuart DG. Adaptation of cat motoneurons to sustained and intermittent extracellular activation. The Journal of Physiology. 1993;464:75–120. doi: 10.1113/jphysiol.1993.sp019625. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Woods JJ, Furbush F, Bigland-Ritchie B. Evidence for a fatigue-induced reflex inhibition of motoneurone firing rates. Journal of Neurophysiology. 1987;58:125–137. doi: 10.1152/jn.1987.58.1.125. [DOI] [PubMed] [Google Scholar]

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