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The Journal of Neurophysiology Vol. 88 No. 4 October 2002, pp. 1664-1674
Copyright ©2002 by the American Physiological Society
1Division of Neurophysiology, Department of Medical Physiology, The Panum Institute. University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen N, Denmark; 2Institute of Biomedical and Life Sciences, West Medical Building, University of Glasgow, Glasgow G12 8QQ, Scotland; 3The Third Department of Internal Medicine, Division of Neurology, Clinical Neurophysiology Laboratory, Shinshu University School of Medicine, Asahi 3-1-1, Matsumoto 390-8621, Japan; and 4Center for Sensory-Motor Interaction, Aalborg University, 9220 Aalborg, Denmark
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Enríquez-Denton, M., H. Morita, L.O.D. Christensen, N. Petersen, T. Sinkjaer, and J. B. Nielsen. Interaction Between Peripheral Afferent Activity and Presynaptic Inhibition of Ia Afferents in the Cat. J. Neurophysiol. 88: 1664-1674, 2002. It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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It is well known that
synaptic transmission from muscle afferents in the spinal cord can be
depressed by presynaptic inhibition (Eccles et al. 1961
,
1962a; Frank and Fuortes 1957; Rudomin et al. 1998). The pathways mediating presynaptic inhibition have been demonstrated to be under supraspinal control in both cat (Enríquez et al. 1996; Rudomin et al.
1986, 1991) and man (Meunier 1999). This is
probably the basis for the modulation of presynaptic inhibition of Ia
afferents in relation to different motor behaviors, which has been
observed in man (Meunier and Morin 1989; Meunier and Pierrot-Deseilligny 1989). Based on human H-reflex
experiments, it is suggested that, according to the functional
requirements of the motor task, the CNS may increase or decrease the
level of presynaptic inhibition of Ia afferents and thereby the central gain of the monosynaptic stretch reflex (Capaday and Stein
1986; Hultborn et al. 1987; Nielsen and
Kagamihara 1993).
However, data have recently appeared which suggest that the
interpretation of the data from these H-reflex experiments may not be
straightforward. Whereas the soleus H-reflex is depressed in the stance
phase of walking compared with standing, presumably due to presynaptic
inhibition, this is not the case for the monosynaptic stretch reflex
(Sinkjaer et al. 1996). There is also evidence that,
during co-contraction of antagonistic ankle muscles, the H-reflex is
depressed through increased presynaptic inhibition (Nielsen and
Kagamihara 1993), while the monosynaptic stretch reflex is not
(Nielsen et al. 1994). It is thought that the same pathway mediates both reflexes. Why would the electrically evoked H-reflex be depressed by presynaptic inhibition, whereas the
mechanically evoked stretch reflex is not? It has been thought for some
time that the only difference between the two reflexes is their
sensitivity to fusimotor drive. However, Morgan et al.
(1984) observed that conditioning stimulation of gamma dynamic
fibers did not increase dramatically the response in Ia afferents to a
brief tendon jerk and several studies have pointed out that there are
also differences in the composition of the afferent volleys evoked by
the two stimuli and the way that the spinal motoneurones are activated
by these volleys (Burke et al. 1983, 1984; Morita
et al. 1998). Morita et al. (1998) suggested, as
a possible explanation of the different sensitivity of the two reflexes
to presynaptic inhibition, that a mechanical stimulus, which induces
repetitive discharge of the Ia afferents, might lower the sensitivity
of the synapses to presynaptic inhibition compared with the synchronous
Ia discharge underlying the H-reflex. The present experiments were
designed to investigate this hypothesis further in the cat.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The experiments were performed in 19 adult male cats (2.5-3.5
kg). All surgery and experimental protocols were conducted according to
the National Institutes of Health Guide for the care and use of
laboratory animals (National Institutes of Health publication no.
86-23, revised 1985). During surgery, animals were deeply anesthetized
with halothane (
2.5% in a mix of 40% oxygen in
NO2) while, during the actual experiment,
anesthesia was maintained with alpha-chloralose (80 mg/kg). Gas
anesthetics were replaced with a mix of 40% oxygen in air.
Supplemental anesthesia, if required, was given as 5 mg/kg of
pentobarbital (Mebumal, SD Denmark) every 2 h. Adequacy of
anesthesia was determined by assessing that withdrawal reflexes were
absent, that the pupils were constricted, and that the blood pressure
was between 80 and 120 mmHg even during high-intensity electrical
stimulation of cutaneous nerves (Sural, 50T, 200 Hz).
The trachea, two forelimb veins, and one carotid artery were cannulated for gas, fluid administration, and monitoring of the blood pressure. A solution of 0.5 M of glucose and 0.2 M NaHCO3 (5 ml/h) was continuously infused throughout the experiment. If blood pressure dropped below 80 mmHg, infusion of Macrodex (300 mg/h) or Noradrenalin (starting dose 0.02 ml/h) was established. Body temperature was kept within 36-38°C with servo-controlled heating devices.
The longest possible length of the motor nerves medial gastrocnemius (MG) and lateral gastrocnemius plus soleus (LG-S) were dissected free, but left in continuity with the muscles (see Fig. 1). These nerves were mounted separately or together (GS nerves) on flexible hook electrodes (silver wire 0.29 mm diam) for electrical stimulation. In addition, the following nerves were dissected, cut, and mounted for stimulation to identify the motoneurones from which intracellular recordings were made from (see following text): quadriceps, semimembranosus and anterior biceps (SmAB), posterior biceps and semitendinosus (PBSt), flexor digitorum hallucis longus (FDHL), and deep peroneus (DP, tibialis anterior plus extensor digitorum longus).
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After measuring the maximal physiological length of the triceps surae
muscle, the tendon was separated from the calcaneous bone and attached
to a servo-controlled puller (Series 300B Lever System, Cambridge
Technology) (see Enríquez et al. 1996;
Lennerstrand 1968).
A laminectomy was performed to expose the lumbar spinal cord from L7 to L5. All exposed tissues were covered with paraffin oil and kept at 37°C by radiant heating.
Incoming volleys (CDP), elicited by electrical stimulation of the peripheral nerves or by stretching the muscle, were recorded with a platinum ball electrode, which was placed on the spinal cord ventral to the L7 dorsal root; the reference electrode was located in the nearby back muscles. The threshold for electrical stimulation (T) was determined as the minimum voltage intensity capable of producing an incoming volley. The CDP and the EMG signals were amplified with low-noise high-gain differential amplifiers (band-pass filters 1 Hz-10 kHz).
Reflex recording
Reflexes evoked by stretching the triceps surae or by
stimulation of the LG-S nerve were recorded with wire electrodes
(Teflon-coated platinum-iridium 25 µm, Cooner Wire, Chatsworth, CA)
inserted directly into the soleus muscle. These reflexes were elicited by stretching the muscle 2.0 mm from 
4 mm of the maximum
physiological length. The stretches had a triangular wave form with a
rise time of 2-250 ms. It is assumed that these stretch parameters
induce activation of the majority of spindle afferents with low or
negligible activation of afferent fibers from tendon organs
(Enríquez et al. 1996; Lennerstrand
1968; Proske et al. 1992; Wood et al.
1994). Electrically induced reflexes were elicited by
stimulation of the LG-S nerve with pulses of 0.1 ms and intensities of
1.3 T. Stretch and electrically evoked reflexes were alternated with each other and conditioned by preceding electrical stimulation of the
flexor PBSt nerves (1.20-5 T; 5 shocks, 200-300 Hz). It was ensured
that the electrically and stretch-induced reflexes had the same size in
the control situation without conditioning stimulation. This was done
by adjusting the intensity of the electrical stimulation until
responses of the same size as those evoked by the stretch were obtained.
Intracellular recording
After the initial EMG reflex recordings, the preparations were immobilized (pancuronium bromide, Pavulon) and artificially ventilated. Expired PCO2 was continuously monitored and kept at 4% by adjusting the volume of the flowing gas (1800-2000 ml/min). Bilateral pneumothorax and rigid clamping of the L4 vertebra was made to ensure mechanical stability of the preparation for intracellular recording from spinal motoneurones.
Intracellular recording was made from motoneurones identified by
antidromic activation from MG or LG-S nerves with KAc-filled (2 M)
microelectrodes (1.2 µm, 5 M
) or with CNQX-Kac-filled
microelectrodes (0.1 and 2 M, respectively) coupled to a microelectrode
amplifier (Axoclamp 2A, Axon Instruments). Only motoneurones with a
stable membrane potential of at least 50 mV were included in the
analysis. The motoneurones were not characterized according to
motoneurone types. Stretch- and electrically induced excitatory
postsynaptic potentials (EPSPs) were conditioned by preceding
stimulation of the PBSt nerve. The interval between conditioning
stimulation and the test EPSPs was carefully adjusted throughout the
experiments. The effect of the conditioning stimulation was
investigated for stretch-induced EPSPs of different duration as well as
electrically evoked EPSPs evoked by either single stimuli or trains of
stimuli. In all recorded motoneurones it was confirmed that the PBSt
stimulation had no effect on the membrane potential or a small pulse
(same size as the investigated EPSPs; duration, 5 ms) injected through the microelectrode at the investigated intervals.
Data analysis
In all cases, conditioned and unconditioned responses were alternated with each other at a repetition frequency of about 1 Hz. At least 16 recordings of each alternative were obtained. All signals were digitized (10 KHz) and analyzed using the SCRC Data Capture and Analysis Software System (Detillieux G. R., The University of Manitoba, Winnipeg, Manitoba, Canada). EPSPs were measured from base to peak. The area of the EPSPs was also determined in 2- and 5-ms time intervals. Statistically significant differences between control and conditioned potentials were determined using a paired Students t-test. Pooled data from different experiments were compared using a one-way ANOVA and a test of difference in proportions.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Electrically and mechanically induced reflexes are not equally sensitive to presynaptic inhibition
Figure 2 demonstrates that a reflex
elicited by electrical stimulation (H-reflex) of the GS motor nerve
(1.2 T; filled circle) is more sensitive to presynaptic inhibition than
a reflex elicited by stretching the GS muscle (0.25 mm, 125 mm/s; open
circle). In Fig. 2A it is shown that preceding stimulation
of the PBSt nerve (1.2 T, 5 shocks, 225 Hz, 40 ms before) depressed the
electrically evoked monosynaptic reflex (MSR) to 34% of the size of
the control reflex (compare dotted and continuous lines), while the
mechanically evoked MSR was only depressed to 76% of control. Figure
2B shows that the mechanically evoked MSR was significantly
(P < 0.05) less affected than the electrically evoked
MSR for conditioning-test intervals longer than 10 ms. In addition, the
electrically evoked MSR was significantly (P < 0.05)
depressed for 200 ms after the conditioning stimulation, whereas the
mechanically evoked MSR was significantly (P < 0.05)
depressed for only 60 ms. This late long-lasting inhibition can be
explained by presynaptic inhibition (Eccles et al.
1961). At short conditioning-test intervals between 2 and 5 ms,
an equal depression of the H-reflex and the stretch reflex was
observed. The inhibition at these short conditioning-test intervals is
accepted to be postsynaptic in origin (Eccles et al.
1961).
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Figure 2C (from the same experiment as Fig. 2B) illustrates the relationship between the depression of the two reflexes and the intensity of the conditioning stimulation at a constant conditioning-test interval of 40 ms. The electrically evoked reflex was abolished already at an intensity of 1.3 T and was significantly (P < 0.05) more depressed than the stretch reflex up to stimulation intensities of 2.5 T. Similar observations were obtained in all six preparations in which the depressions of the stretch- and electrically induced reflexes were compared. At 1.6 T, conditioning stimulation of PBSt (conditioning-test interval 30-60 ms; 5 shocks, 300 Hz) depressed the electrically evoked reflex to 28.7 ± 33.5% (mean ± SD), whereas the mechanically evoked reflex was only depressed to 50.7 ± 26%. At 2T, a depression to 24.5 ± 32.4% of the electrically evoked reflex and to 49.2 ± 27.6% of the mechanically evoked reflex was observed. At both intensities, the depression of the electrically evoked reflex was significantly larger than the depression of the stretch-evoked reflex (P < 0.05).
Different sensitivity of the first and last components of the EPSPs to presynaptic inhibition
To investigate whether there is a similar differential depression of postsynaptic potentials evoked by mechanical and electrical activation of the monosynaptic reflex pathway, the preparations were paralyzed and intracellular recordings were made from antidromically identified triceps surae motoneurones. Figure 3 illustrates data from an LG-S motoneurone. EPSPs were elicited by electrical stimulation of the GS nerve or by stretching the triceps surae muscle with (continuous line) and without (dashed line) preceding PBSt stimulation. Conditioning stimulation of PBSt (55 ms before, 1.3 T, 5 shocks, 300 Hz) evoked a clear depression of an EPSP evoked by either a single electrical stimulus (Fig. 3A; 1.2 T) to the GS nerve or by a fast stretch (ramp of 0.11 mm; 55 mm/s) to the muscle (Fig. 3B). A depression was also seen for the first of a series of EPSPs elicited by a train of electrical stimuli (27 stimuli; 250 Hz) to the GS nerve (Fig. 3, C and C1) as well as the initial part of the complex EPSP evoked by a slow stretch (0.2 mm, 1.6 mm/s) to the muscle (Fig. 3, D and D1). In contrast, both in the electrically evoked and in the stretch-evoked EPSPs, the last part of the potentials were much less depressed by the PBSt stimulation (Fig. 3, C2 and D2, respectively). Comparing the depression of the first and last EPSPs, a difference of 17% was found for the electrically evoked EPSPs and of 18% for the stretch-induced EPSPs.
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The difference in depression between the first and the last EPSP was
analyzed in 17 motoneurones (see Fig.
4A; left
histograms). To exclude that the different latency of the first and the
last EPSP evoked by the train of stimuli was responsible for the
different effect of the PBSt stimulation, the interval between the
conditioning stimulus and the train of pulses to the GS nerve was
adjusted (in separate trials) to obtain the same interval from the
conditioning stimulation to the last as to the first EPSP (see Fig.
5A). The average difference in
the PBSt-induced depression of the last and the first EPSP in the train
was 11 ± 15% (range 76 to 33%) when the conditioning-test
interval was 25 to 100 ms (1.3 to 2 T). A similar analysis was
performed for the stretch-induced EPSPs in 12 motoneurones (35 measurements; Fig. 4A; right histograms). The
interval from the conditioning stimulation was adjusted in the same way
as for the electrically evoked EPSPs to obtain the same
conditioning-test interval for the last as for the first part of the
stretch-evoked EPSPs. The average difference in the depression of the
last and the first component of the composite stretch-induced EPSP was
6 ± 10% (range 25 to 11%). By the use of a paired
t-test, a statistically significant difference in depression
between the first and last components was found both for the
electrically induced EPSPs (first component: 77 ± 10%, last
component: 88 ± 16%; P < 0.001) and the
stretch-induced EPSPs (first component: 75 ± 15%; last
component: 81 ± 12%; P = 0.002). There was no
difference in the depression of the first EPSP evoked by a train of
electrically stimuli and the first part of the stretch-evoked EPSPs
(compare columns 1 and 3 in Fig. 4A).
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The area of the electrically and mechanically evoked EPSPs was also measured in 2- and 5-ms intervals (Fig. 4B). For the population of EPSPs, only the initial 5 ms of the EPSPs were significantly depressed by PBSt stimulation. For the stretch-induced EPSPs, a tendency toward facilitation was even seen for the part of the EPSP from 5 to 15 ms after its onset (Fig. 4B, open circles).
Time course of the depression of EPSPs by PBSt stimulation
Figures 5A and B show the time course of the effect of PBSt stimulation (5 shocks, 2.5 T, 300 Hz) on the first and last EPSP in a series of EPSPs in a MG motoneurone evoked by a train of stimuli (5 shocks, 200 Hz, 1.2 T) applied to the MG motor nerve. The PBSt stimulation depressed the first EPSP in the train to 70% of the control value, whereas a depression to only 94% was seen for the last EPSP (see also Fig. 4A). This difference in depression was also noted when the interval from the conditioning PBSt stimulation was adjusted to take into account the 22-ms interval between the two EPSPs (compare top and bottom traces in Fig. 5A). As seen from the graph in Fig. 5B, the first EPSP in the train was depressed for all intervals up to around 150 ms, consistent with presynaptic inhibition of MG Ia afferents. However, the last EPSP in the train was not depressed within these intervals after the conditioning stimulation to PBSt.
Size of the EPSP and presynaptic inhibition
One possible explanation for the difference in the effect of PBSt
stimulation on the first and last EPSP in a train is that the EPSPs may
differ in size and that potentials of different size may be differently
sensitive to presynaptic inhibition (Zengel et al.
1983). To address this possibility, an analysis of the relationship between the amplitude of the control EPSP and the effect
of the conditioning stimulation was made. The amplitudes of the test
EPSPs were varied by changing the intensity of the GS stimulation.
Figure 5C shows the relationship between the amplitudes of
the control and conditioned EPSP for two different motoneurones (marked
by squares and circles, respectively). The intensity of the
conditioning PBSt stimulation was 1.6 and 2 T, whereas the conditioning-test intervals were 60 and 34 ms, respectively. For both
motoneurones there was a linear relationship between the control EPSP
and the conditioned EPSP [slope (m) = 0.91, r = 0.99; m = 0.67, r = 0.99, respectively]. A similar relationship was found in all tested
motoneurones, two other GS motoneurones, two flexor digitorum hallucis
longus motoneurones (conditioned by PBSt), one quadriceps motoneurone
(conditioned by deep peroneal stimulation), and one deep
peroneal motoneurone (conditioned by PBSt). In all of them, the slope
varied from 0.67 to 0.89 and the linear correlation factor
(r) was 0.9. This suggests that the amount of presynaptic
inhibition is independent of the size of the test EPSP. The different
sensitivity to presynaptic inhibition of the first and the last EPSP
therefore cannot be explained by their different sizes.
Effect of proceeding group I activity on the sensitivity of EPSPs to presynaptic inhibition
To assess the effect of preceding activity in afferent fibers on the magnitude of presynaptic inhibition, intracellular recordings were performed from 16 antidromically identified motoneurones. Trains of EPSPs were induced either by stimulation of the homonymous or the heteronymous nerve and the effect of PBSt stimulation on the test EPSP was assessed at different intervals after these trains. Figure 6A shows the depression of the test EPSP evoked by the PBSt stimulation when the EPSP was preceded by a train of stimuli to the homonymous or heteronymous nerve. In this MG motoneurone, stimulation of the PBSt nerve (2.5 T, conditioning-test interval: 50 ms) depressed the test EPSP to 71.7% of its control size (3.2 mV). When the test EPSP was preceded by a train of five pulses (200 Hz; 9 ms interval) to the homonymous (MG) nerve, PBSt stimulation depressed the test EPSP to only 81.2% of its control value (3.06 mV as the train induced a reduction of the test EPSP also). Presynaptic inhibition was, in other words, reduced by 9.6% when a train of stimuli to the homonymous nerve preceded the test EPSP. When the test EPSP was applied 18 ms after the train of stimuli, an even larger reduction (by 30%) of presynaptic inhibition was observed (lower group of traces). In contrast, a preceding train of stimuli applied to the heteronymous (LG-S) nerve increased the depression evoked by the PBSt stimulation by 5.1 and 7.2% at the same intervals (right traces). Figure 6B illustrates the change in presynaptic inhibition at all tested intervals after the trains applied to the homonymous (MG, squares) and heteronymous (LG-S, circles) nerves for this motoneurone. At intervals between 8 and 25 ms from the preceding train to the homonymous nerve, a reduction in the presynaptic inhibition by more than 5% was noted, whereas a similar train applied to the heteronymous nerve increased the presynaptic inhibition by a similar amount. Of 16 tested motoneurones, a similar pattern was seen in 10 motoneurones; i.e., a train of stimuli applied to the homonymous nerve decreased presynaptic inhibition, whereas no change or an increase in presynaptic inhibition was seen following a train of stimuli applied to the heteronymous nerve. In two motoneurones, trains applied to both the homonymous and the heteronymous pathway reduced presynaptic inhibition. In the remaining four motoneurones neither stimulation of the homonymous nor the heteronymous nerve had any effect on the amount of presynaptic inhibition.
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Quantitatively, an average reduction of presynaptic inhibition of
7.7 ± 9.2% was found when the test EPSP was evoked 5-10 ms
after a train of stimuli applied to the homonymous nerve for the 16 recorded motoneurones (12 MG and 4 LG-S; 31 measurements; P < 0.001; Fig. 6C). When the test EPSP was
evoked 10-20 ms after the train, a reduction of 9.8 ± 11.5%
in presynaptic inhibition was observed (19 measurements; 15 motoneurones; P < 0.05). At intervals of 20-30 ms a
nonsignificant reduction of 3.8 ± 13.4% (10 measurements, 9 motoneurones) was observed, and at intervals longer than 30 ms the
reduction was 1.1 ± 14.3% (11 measurements, 4 motoneurones;
nonsignificant). In contrast, there were no significant changes in the
amount of presynaptic inhibition when the train of stimuli were applied
to the heteronymous nerve regardless of the interval between the train
and the test EPSP (Fig. 6C). This suggests that the
depression of presynaptic inhibition by preceding afferent activity is
restricted to the afferents from which the test EPSP is evoked and that
the time course of the effect is on the order of 20-30 ms.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In the present study we have confirmed in the
alpha-chloralose-anesthetized cat that soleus H-reflexes are less
depressed by presynaptic inhibition evoked by flexor nerve stimulation
than soleus stretch reflexes as has been demonstrated previously in man
(Morita et al. 1998). In the sample of intracellular
recordings from lumbar motoneurones, we found that the initial part of
the electrically and mechanically evoked EPSPs were equally depressed by presynaptic inhibition, whereas later parts were significantly less
depressed. We finally provided evidence that peripheral afferents, which have been previously activated (within 20 ms), are less sensitive
to presynaptic inhibition. We believe that this observation may partly
explain the different sensitivity of H-reflexes and stretch reflexes to
presynaptic inhibition.
Previous activation of peripheral afferents and differential sensitivity of H-reflexes and stretch reflexes to presynaptic inhibition
The afferent volleys evoked by muscle stretch and by electrical
stimulation of peripheral afferents differ significantly (Burke et al. 1983, 1984). Following electrical stimulation of
peripheral nerves, each afferent discharge only once and the afferent
volley is relatively synchronized with little temporal dispersion.
Following muscle stretch, each afferent may discharge several times at
short intervals (5 ms) and the temporal dispersion of the volley is large (Burke et al. 1983). When the intensity of
electrical stimulation and muscle stretch are adjusted so as to evoke
H-reflexes and stretch reflexes of similar size and shape (cf. Fig. 2),
the underlying afferent volleys evoked by the two stimuli will thus be
very different. Due to the temporal dispersion of the afferent volley
underlying the stretch reflex, a combination of spatial and temporal
summation of EPSPs contributes to its size, whereas spatial summation
is mainly involved in the generation of the H-reflex. Short-lasting composite EPSPs evoked by brief muscle stretches were equally sensitive
to presynaptic inhibition as electrically evoked EPSPs (cf. Fig. 3,
A and B). This was also true for the initial part of the long-lasting slowly rising EPSPs evoked by similar slow stretches as those used to evoke the stretch reflexes in this study.
However, the last part of these EPSPs, as well as the last part of
similarly long-lasting EPSPs evoked by a train of electrical stimuli,
were less depressed by presynaptic inhibition (cf. Fig. 3, C
and D). In these latter cases, temporal and spatial
summation of discharges in the same afferents must have contributed to
the size of the later part of the EPSPs, whereas this is not the case, or at least to a much lower extent, for the initial part of the long-lasting EPSPs and for the short-lasting EPSPs evoked by very brief
stretches and single electrical stimuli. We therefore believe that
previous activity in the afferents and temporal summation may decrease
the effect of presynaptic inhibition and explain the differential
effect of presynaptic inhibition on H-reflexes and stretch reflexes.
Two points are of importance in this relation. First, it has been shown
that reflexes are evoked toward the end of the rising phase of the
composite EPSP (Burke et al. 1984). The stretch reflexes
in this study and in the study by Morita et al. (1998)
were in other words evoked at the point of the EPSPs at which
presynaptic inhibition was shown to have little effect. Second, the
seemingly small difference in the effect of presynaptic inhibition on
the electrically and mechanically evoked EPSPs (Fig. 3) is not in
conflict with the very large difference in the effect of presynaptic
inhibition on H-reflexes and stretch reflexes (Fig. 2). Even small
differences in the size of EPSPs may lead to very pronounced
differences in the size of monosynaptic reflexes, since a small
decrease in the EPSP may bring several motoneurones below their firing
threshold and thereby produce a large decrease in the size of the
reflex (Eccles et al. 1961).
We would also like to point out that differences in the sensitivity to
presynaptic inhibition of the EPSPs following previous afferent
activity is not the only possible contributing mechanism to the
observed differential effect of presynaptic inhibition on stretch
reflexes and H-reflexes. Although we did make sure that the H-reflexes
and stretch reflexes were of similar size, we cannot be sure that the
same motoneurones were activated by the two inputs. However, we find it
unlikely that this should explain the different effect of presynaptic
inhibition on the two reflexes. First, it has been shown that stretches
and electrical peripheral nerve stimuli both recruit motoneurones
according to Henneman's size principle (Harris and Henneman
1979; Lüscher et al. 1989). Second, as
shown by Zengel et al. (1983), presynaptic inhibition
does not seem to be differentially distributed in the motoneurone pool.
Another possibility is that the two different inputs recruit the
motoneurones with a different gain (Kernell and Hultborn 1990). Given the different temporal dispersion of the volleys it would not be surprising if the relation between input and output would differ for the two reflexes. However, as shown in the study by
Morita et al. (1998) and in the present study,
postsynaptic inhibition was equally effective for the two reflexes (cf.
Fig. 2C). This would not be the case if differences in
recruitment gain were responsible for the different sensitivity to
presynaptic inhibition.
A final possibility is that the stretch reflex, rather than the H
reflex, is evoked to a higher extent by afferents other than Ia
afferents and/or pathways other than the monosynaptic Ia pathway.
Neither of the reflexes may be assumed to be purely monosynaptic
(Burke et al. 1984); several other pathways may
contribute to their size, including nonmonosynaptic group I pathways
and group II pathways (Behrends et al. 1983a,b;
Eccles and Lundberg 1959; Jankowska et al.
1981a; Jankowska and McCrea 1983;
Kniffki et al. 1981; Lundberg et al.
1987). These other pathways would mostly influence the later
parts of the EPSPs. It has been demonstrated that group II pathways are
less sensitive to presynaptic inhibition evoked by flexor group I
stimulation than the monosynaptic Ia pathway (Eccles et al.
1962a; Riddell et al. 1995). With the
experiments in the present study we cannot exclude that this may
explain the different sensitivity of the two reflexes. Further
experiments are necessary to address this issue.
Which mechanism explains the reduced sensitivity to presynaptic inhibition after previous peripheral afferent activation?
Regardless of whether the decreased presynaptic inhibition
following previous activation of peripheral afferents is responsible for the differential sensitivity of stretch reflexes and H-reflexes, the findings are of interest in relation to the mechanisms responsible for modulation of presynaptic inhibition. One possible mechanism is
that the train of stimuli applied to the peripheral nerves depressed
transmission in the pathway mediating presynaptic inhibition from
flexor group I afferents. In the cat the presynaptic inhibitory pathway
from flexors involves a series of 
2 interneurones, of which the last
is GABAergic (Curtis et al. 1986; Curtis and
Lodge 1982; Eccles et al. 1962a, 1962b;
Jankowska et al. 1981b; Rudomin et al.
1986) and makes axoaxonic synapses on the axons of the afferent
fibers from the extensors (Rudomin et al. 1998;
Solodkin et al. 1984). It would be a possibility that
the train of stimuli applied to the triceps surae motor nerves resulted
in inhibition of one of the interneurones in this pathway. However,
activation of ankle extensor Ia afferents is rather ineffective in
producing presynaptic inhibition of flexor Ia afferents (Eccles
et al. 1962b; Enríquez-Denton et al., 2000).
Furthermore, we found that only preceding stimulation of the homonymous
motor nerve produced decreased presynaptic inhibition, whereas
preceding stimulation of heteronymous ankle extensor nerve had no
effect (Fig. 6). This would be difficult to reconcile with an effect on
transmission in the presynaptic inhibitory pathway from flexors, since
the different ankle extensors would be expected to produce similar
effects in this relation.
We consequently find it more likely that the observed decreased
sensitivity to presynaptic inhibition following previous afferent activity is due to a mechanism intrinsic to the activated afferents and
synapses. Presynaptic inhibition works by activation of
GABAa and/or GABAb
receptors. Activation of GABAa receptors produces changes in the chloride conductance, which lead to depolarization and
changes in the excitability of the terminals of the afferent fibers
(Rudomin and Schmidt 1999). Activation of
GABAb receptors decreases the calcium influx and
thereby the release of transmitter substance from the terminals
(Edwards et al. 1989; Jiménez et al.
1991; Lev-Tov et al. 1988) without affecting the
potential or excitability of the afferent fibers (Quevedo et al.
1992). Following preceding activity in Ia afferents there is an
accumulation of calcium in the terminals and as a consequence of this
an increased transmitter release probability, which lasts for around
20-50 ms (Borst and Sakmann 1998; Curtis and
Eccles 1960; Cuttle et al. 1998; Peshori
et al. 1998). This corresponds rather well to the period in
which we observed that the effect of presynaptic inhibition on the
EPSPs was decreased (cf. Fig. 6) and suggests that there may be a
causal link between the two observations. As can be seen in the
following numerical example, the ability of presynaptic inhibition to
depress transmitter release from the terminals is simply reduced when
there is more calcium present in the terminals. Assume that a unit
x = 1 of calcium is released by the first presynaptic
action potential. The resulting EPSP will depend on a power such as 4 of the calcium released, so EPSP = x4 = 1, as well. If presynaptic
inhibition reduces the calcium released by 10% (Eccles et al.
1961), then x = 0.9 and EPSP = 0.94 = 0.65. With repetitive stimulation there
will be homosynaptic depression of calcium release, but some residual
calcium from previous pulses will be present in the terminals. Since
the EPSP is about the same, the total calcium will be about the same.
For example, if the residual calcium is 0.3 and the calcium released is
0.7, the EPSP will be the same. However, presynaptic inhibition will
reduce the released calcium to 0.7 × 0.9 = 0.63. The total calcium will then be 0.63 released plus the residual 0.3, i.e., x = 0.93 and the EPSP = 0.934 = 0.75; i.e., it will be less affected by
the presynaptic inhibitory volley. Obviously, a different power than 4 or a different balance than 0.3 and 0.7 can be used and the values
would change somewhat, but the qualitative result will be the same. As
can be seen in this numerical example, as the residual calcium builds
up with repetitive stimulation, the effect of presynaptic inhibition
will decrease. This may thus be the underlying mechanism for the
different effect of presynaptic inhibition on H-reflexes and stretch
reflexes as outlined in the previous section.
Finally, we have demonstrated that the amount of presynaptic inhibition
for a particular motoneurone is constant and independent of the size of
the unconditioned EPSP (Fig. 5). Zengel et al. (1983)
found that, in absolute terms, presynaptic inhibition was small in
motoneurones with small EPSPs (type F), whereas it was large in
motoneurones with large EPSPs (type S). Similar to us, they thus found
a constant relation between the amount of presynaptic inhibition, when
expressed as a percentage of EPSP size, and the size of the EPSP
measured in various motoneurones. This might imply that presynaptic
inhibition is also constant for individual Ia afferents acting on the
same motoneurone. This is indeed what Clements et al.
(1987) found for some motoneurones (their Fig. 5A),
but not for all. When testing EPSPs evoked from activation of several
afferents, as in our study, it would seem likely that such variations
among individual Ia afferents are insufficient to be noticed.
Furthermore, our observation of constant presynaptic inhibition for
EPSPs of different sizes would not be inconsistent with the findings by
Clements et al. if the Ia afferents investigated by them were evenly
distributed in terms of electrical threshold.
Methodological and functional considerations
Comparison of the H-reflex and stretch reflex have previously been
used to infer changes in fusimotor drive as only the latter reflexes
are influenced by the sensitivity of the muscle spindles. However, as
argued already by Burke et al. (1983, 1984), there are
significant differences in the afferent volleys evoked by mechanical
and electrical stimuli and the way that they activate the spinal
motoneurones. The findings in the present paper stress this. As pointed
out by Morita et al. (1998), the different effect of
presynaptic inhibition on stretch reflexes and H-reflexes highlights that the two reflexes represent different ways of "probing" the CNS. Changes in presynaptic inhibition are easily demonstrated with the
use of H-reflexes, but may go unnoticed when using stretch reflexes
(Nielsen et al. 1994; Sinkjær et al.
1996). Functionally, the findings in the present study are
consistent with the idea put forward by Morita et al.
(1998) that the differential effect of presynaptic inhibition
on H-reflexes and stretch reflexes may be seen in relation to the
discharge of Ia afferents. The decreased sensitivity to presynaptic
inhibition lasted around 20-30 ms following previous activation of the
afferents. There will thus be no decrease of the sensitivity to
presynaptic inhibition when Ia afferents discharge slower than around
30-50 Hz. When the Ia afferents discharge at higher frequencies the
sensitivity to presynaptic inhibition will decrease and when they
discharge at 200 Hz (i.e., at 5-ms intervals) this effect will be
maximal. As already mentioned this is the case when a sudden external
muscle stretch is imposed (Burke et al. 1983). As one
possible interpretation of our data, presynaptic inhibition may thus
effectively gate and modulate the normal peripheral feedback evoked in
relation to voluntary movements but have a much less significant effect
on activity evoked by sudden external perturbations. The frequency of
the afferent firing may thus code the effect of presynaptic inhibition
and the internally and externally generated feedback may at the same
time be differentially modulated. Further experiments are clearly
necessary to demonstrate whether this simple control paradigm exists.
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ACKNOWLEDGMENTS |
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We are grateful to Prof. Hans Hultborn for valuable suggestions and comments throughout the study. We express our gratitude to G. R. Detillieux from the SCRC, University of Manitoba for assistance with the use of the SCRC analysis system. We also thank L. Grøndahl, E. Gudbrandsen, B. Sanford, and the late J. Nielsen for expert technical support.
The study was supported by the Danish Health Research Council, The NOVO Nordisk Foundation and the Danish Society for Multiple Sclerosis. M. Enriquez-Denton received a Postdoctoral Scholarship from The Council of Science and Technology (CONACyT) (Mexico).
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FOOTNOTES |
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Address for reprint requests: J. B. Nielsen, Department of Medical Physiology, University of Copenhagen, Panum, Blegdamsvej 3, DK-2200, Copenhagen N., Denmark (E-mail: j.b.nielsen{at}mfi.ku.dk).
Received 19 November 2001; accepted in final form 3 June 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Exp Brain Res
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  J. B. Nielsen Sensorimotor integration at spinal level as a basis for muscle coordination during voluntary movement in humans J Appl Physiol, May 1, 2004; 96(5): 1961 - 1967. [Abstract] [Full Text] [PDF]
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