Soleus Motoneurons Research Articles

Differential tyrosine kinase C mRNA distribution in extensor digitorum longus and soleus motoneurons in adult rats: effect of axotomy and neurotrophin-3 treatment

Delivery of neurotrophin-3 (NT-3) to severed sciatic nerves results in specific normalization of atrophied fast 2b gastrocnemius muscle fibres, and promotes preferential neuromuscular junction maturation of fast extensor digitorum longus (EDL). To investigate the selective influence on fast muscle reinnervation due to NT-3 delivery, we analyzed tyrosine kinase C (trkC) mRNA differential expression in EDL and slow soleus motor pools of unoperated rats and at 1-week post-axotomy. Motoneurons (Mns) were identified using retrograde tracers. TrkC mRNA quantification was estimated by silver grain counting. TrkC mRNA expression was higher in EDL than in soleus Mns in unoperated rats. Axotomy decreased trkC mRNA expression only in EDL Mns, this downregulation being prevented by NT-3. These results suggest that differential expression of trkC receptor is the morphological correlate of the preferential effect of NT-3 upon EDL Mns.

INTERSESSION RELIABILITY FOR H-REFLEX MEASUREMENTS ARISING FROM THE SOLEUS, PERONEAL, AND TIBIALIS ANTERIOR MUSCULATURE

The Hoffmann reflex (H-reflex) has been widely used throughout neuroscience research, as it allows for the assessment of alpha motoneuron excit ability arising from a specific motoneuron pool. Recently, a protocol has been developed allowing for the simultaneous examination of the soleus, peroneal, and tibialis anterior motoneuron pools elicited from a single peripheral stimulus. In order for this protocol to be useful, the reliability of the measures must be established. The purpose of the current study was to determine the intersession reliability of the soleus, peroneal, and tibialis anterior H-reflexes and their corresponding M-waves elicited from a single stimulus to the sciatic nerve. Ten healthy neurologically sound individuals (age: 23 - 7 yrs; height: 175 - 12 cm; mass: 76 - 22 kg) volunteered to participate in this investi gation. To obtain the measurements, the sciatic nerve was stimulated just prior to its bifurcation into the tibial and common peroneal nerves in the popliteal fossa. A 1-ms square wave pulse was delivered in 0.2V increments until the maximum M wave was seen in each muscle. The maximum H-reflex and M-waves were collected from each muscle and their ratios calculated. Intersession reliability over 2 consecutive days was estimated using intraclass correlation coefficients (ICC [2,1]). Intersession reliability for the soleus Max H, Max M, and H:M ratio were 0.9953, 0.9514, and 0.9747, respectively. The peroneal reliability measurements were as follows: 0.9979 (Max H), 0.9924 (Max M), and 0.9664 (H:M ratio). Intersession reliability was 0.8591, 0.9968, and 0.7810 for the tibialis anterior Max H, Max M, and H:M ratio, respectively. These results indicate that the H-reflex measured from the soleus, peroneal, and tibialis anterior musculature elicited with a single peripheral stimulus to the sciatic nerve is reliable between sessions. This protocol allows the clinician/researcher to reliably investigate the alpha motoneuron excitability of multiple motoneuron pools about the ankle at a single point in time.

Facilitation of the Contralateral Soleus Motoneuron Excitability Induced by Ankle Flexion

Facilitation of the Contralateral Soleus Motoneuron Excitability Induced by Ankle Flexion

Modulation of the soleus H-reflex in prone human subjects using galvanic vestibular stimulation

Objectives: To determine if vestibular-evoked modulation of the soleus H-reflex can be achieved in a muscle that is not being used for postural support. Methods: Ten healthy subjects lay prone while the right leg was supported. In this position soleus H-reflex amplitudes were measured with the head facing forward, coupled with ipsilateral monopolar monaural galvanic stimulation (anode or cathode). To evaluate the interval between the onset of the galvanic stimulus and tibial nerve stimulation, the timing was varied between 0 and 200 ms in 20 ms intervals. A two-way ANOVA and student’s t-test was performed to compare the mean amplitudes of the test and conditioned H-reflexes. Results: Galvanic stimulation significantly modified the amplitude of the H-reflex in a prone lying subject ( P<0.05). Furthermore, the peak inhibitory and facilitatory effect occurred when the galvanic vestibular stimulus was delivered 100 ms prior to the H-reflex stimulus. Conclusions: The results of this study demonstrate that galvanic stimulation can modulate the excitability of the soleus motoneuron pool when the muscle is not being used posturally. This suggests that in certain situations, it may be possible to use this type of vestibular stimulation to examine the integrity of descending vestibulospinal pathways in prone human subjects.

Interposition of a pedicle fat flap significantly improves specificity of reinnervation and motor recovery after repair of transected nerves in adjacency in rats.

Despite highest standards in nerve repair, functional recovery following nerve transection still remains unsatisfactory. Nonspecific reinnervation of target organs caused by misdirected axonal growth at the repair site is regarded as one reason for a poor functional outcome. This study was conducted to establish a method for preventing aberrant reinnervation between transected and repaired nerves in adjacency. Rat sciatic nerve was transected and repaired as follows: epineural sutures of the sciatic nerve (group A, n = 6), fascicular repair of tibial and peroneal nerves respectively (group B, n = 8), and, as in group B, separating both nerves using a pedicle fat flap as barrier (group C, n = 8). As control only, the tibial nerve was transected and repaired (group D, n = 5). Muscle contraction force of the gastrocnemius muscle was significantly higher in group C as compared with groups A and B after 4 months. Muscle weight showed significantly lower values in group A as compared with groups B, C, and D. Histologic examination in group C revealed little growth of axons from the tibial to the peroneal nerve and vice versa. This axon crossing was observed only when gaps between the fat cells were available. These findings were confirmed by a significantly lower rate of misdirected axonal growth as compared with groups A and B using sequential retrograde double labeling technique of the soleus motoneuron pool. We conclude that a pedicle fat flap significantly prevents aberrant reinnervation between repaired adjacent nerves resulting in significantly improved motor recovery in rats. Clinically, this is of importance for brachial plexus, sciatic nerve, and facial nerve repair.

Effect of knee joint effusion on quadriceps and soleus motoneuron pool excitability.

To examine changes in quadriceps and soleus MN pool activity resulting from knee joint effusion over a 4-h period and assess the relationship between the muscles. A repeated measures before-after trial design was used for this study. Eight, neurologically sound volunteers (age 23.3+/-2.1 yr, height 171.8+/-15.9 cm, mass 65.5+/-17.7 kg) participated in this study. An area superomedial to the patella was cleaned and anesthetized, and 30 mL of sterile saline was injected into the knee joint capsule to mimic mechanical joint effusion. The Hoffman reflex (H-reflex) was elicited by applying a percutaneous stimulus to the appropriate nerve and recording the response through surface electromyography. Soleus and vastus medialis H-reflex measures were collected from each volunteer before, at 30 min, 90 min, 150 min, and 210 min intervals over a 4-h period after knee effusion. All soleus H-reflex measures after effusion (30 min 5.89+/-0.92 V; 90 min 6.16+/-0.48 V; 150 min 6.59+/-0.50 V; 210 min 6.70+/-0.56 V) were increased in relation to the preeffusion measure (5.01+/-0.79 V). All vastus medialis H-reflex measures after effusion (30 min 4.23+/-0.94 V; 90 min 4.15 +/-1.11 V; 150 min 4.16+/-0.57 V; and 210 min 4.99+/-1.23) were decreased in relation to the preeffusion measure (5.88+/-1.44 V; P < or = 0.05). Afferent activity from the knee joint capsule resulted in an inhibitory effect on the vastus medialis and a facilitatory effect on the soleus. Facilitation of the soleus in cooperation with other lower extremity musculature could be a mechanism for compensation of the inhibited quadriceps to maintain lower kinetic chain function.

Downhill running preferentially increases CGRP in fast glycolytic muscle fibers.

Calcitonin gene-related peptide (CGRP) is present in some spinal cord motoneurons and at neuromuscular junctions in skeletal muscle. We previously reported increased numbers of CGRP-positive (CGRP+) motoneurons supplying hindlimb extensors after downhill exercise (Homonko DA and Theriault E, Inter J Sport Med 18: 1-7, 1997). The present study identifies the responding population with respect to muscle and motoneuron pool and correlates changes in CGRP with muscle fiber type-identified end plates. Twenty seven rats were divided into the following groups: control and 72 h and 2 wk postexercise. FluoroGold was injected into the soleus, lateral gastrocnemius, and the proximal (mixed fiber type) or distal (fast-twitch glycolytic) regions of the medial gastrocnemius (MG). Untrained animals ran downhill on a treadmill for 30 min. The number of FluoroGold/CGRP+ motoneurons within proximal and distal MG increased by 72 h postexercise (P<0.05). No significant changes were observed in soleus or lateral gastrocnemius motoneurons postexercise. The number of alpha-bungarotoxin/CGRP+ motor end plates in the MG increased exclusively at fast-twitch glycolytic muscle fibers 72 h and 2 wk postexercise (P<0.05). One interpretation of these results is that unaccustomed exercise preferentially activates fast-twitch glycolytic muscle fibers in the MG.

Differential changes in corticospinal and Ia input to tibialis anterior and soleus motor neurones during voluntary contraction in man.

Motor-evoked potentials (MEPs) were recorded in the tibialis anterior and soleus muscles following transcranial magnetic stimulation (TMS) of the motor cortex. In the soleus, the H-reflex amplitude increased with the contraction level to the same extent as that of MEPs, whereas in the tibialis anterior, the H-reflex amplitude increased significantly less than that of MEPs. The latency of the MEPs decreased with contraction, whereas this was not the case of the H-reflexes. In the tibialis anterior, the response probability of single-motor units (SMU) to TMS increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. In the tibialis anterior, the response probability of SMU increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. The short-latency facilitation, presumably monosynaptic of origin, of the soleus H-reflex evoked by subthreshold TMS increased as a function of the plantarflexion force. This was not the case for the heteronymous Ia facilitation of the soleus H-reflex following stimulation of the femoral nerve. It is concluded that the corticospinal input to lower limb motor neurones generated by TMS increases with the level of voluntary contraction, whereas this is true only to a limited extent for the synaptic input from Ia afferents. It is suggested that this reflects changes in the susceptibility of corticospinal cells to TMS during voluntary contraction.

Changes in soleus motoneuron pool excitability after artificial knee joint effusion

Hopkins JT, Ingersoll CD, Edwards JE, Cordova ML. Changes in soleus motoneuron pool excitability after artificial knee joint effusion. Arch Phys Med Rehabil 2000;81:1199-1203. Objective: To compare changes in the magnitude of soleus motoneuron excitability before and over a 4-hour period following artificial knee effusion. Design: Before-after trial. Setting: All measurements were collected in the Sports Injury Research Laboratory, Indiana State University. Participants: Eleven healthy and neurologically sound volunteers (mean age ± SD, 24 ± 3yr; height, 173.2 ± 9.6cm; weight, 72.9 ± 8.7kg) with no history of lower-extremity surgery and no lower extremity pathology in the last year. Interventions: An area superolateral to the patella was cleaned and injected subcutaneously with 2mL of lidocaine for anesthetic purposes. With a second disposable syringe, 25mL of sterile saline was injected through the superolateral knee joint capsule into the joint space to mimic mechanical joint effusion. Main Outcome Measure: Hoffmann's reflex (H-reflex) was elicited by applying a percutaneous stimulus to the tibial nerve in the popliteal fossa. Seven to 12 stimuli were delivered at 20-second intervals with varying intensities to find the maximal H-reflex. The maximal H-reflex was measured five times at the same stimulus intensity with 20-second rest intervals. This measurement was recorded before injection and at 1-hour intervals following the injection for 4 hours. Results: An overall difference between groups was found. Measurements from hours 3 and 4 were significantly higher than the preinjection measurements (p ≤.05). Conclusions: The soleus motoneuron pool was not inhibited as expected. The soleus was facilitated beyond the preinjection level, showing that the quadriceps and soleus do not respond in the same way to artificial knee effusion. Because the quadriceps are normally inhibited during knee effusion, this facilitation could be the result of a compensatory reaction by the soleus in response to inhibited quadriceps. Further studies must be performed to determine the extent and duration of soleus motoneuron pool excitability in relation to quadriceps inhibition elicited by artificial knee effusion. © 2000 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Non-weight-bearing condition arrests the morphological and metabolic changes of rat soleus motoneurons during postnatal growth

Effects of non-weight-bearing (NWB) on morphological and metabolic properties of motoneurons innervating the soleus muscle, a postural muscle, were studied in young growing rats. The soma size and the activity of succinate dehydrogenase (SDH), a mitochondrial enzyme, were examined after hindlimb suspension. The hindlimb suspension began at 21 day of age and lasted for 3 week. The results were compared with 21- and 42-day-old controls. The soleus motoneurons were identified by using a fluorescent retrograde tracer, nuclear yellow. The histochemical SDH activity of soleus motoneurons was not modified during the postnatal growth and after the NWB condition. However, postnatal increases in the soma size and the total SDH activity (SDH activity×soma size) were arrested by the NWB condition. The results indicate that the weight support activity imposed on soleus motor units would be important to postnatal increases in the soma size and the total SDH activity (i.e. mitochondria) of soleus motoneurons.

The effect of baclofen on the transmission in spinal pathways in spastic multiple sclerosis patients

Objectives: To measure the effect of baclofen on the transmission in different spinal pathways to soleus motoneurones in spastic multiple sclerosis patients.Methods: Baclofen was administered orally in 14 and intrathecally in 8 patients. Hmax/Mmax, presynaptic inhibition by biceps femoris tendon tap of femoral nerve stimulation, depression of the soleus H-reflex following previous activation of the Ia afferents from the soleus muscle (i.e. postactivation depression), disynaptic reciprocal Ia inhibition of the soleus H-reflex and the number of backpropagating action potentials in primary afferents, which may be a sign of presynaptic inhibition, were examined.Results: Baclofen depressed the soleus Hmax/Mmax ratio significantly following oral and intrathecal baclofen. None of the two tests of presynaptic inhibition, or the postactivation depression or the disynaptic reciprocal Ia inhibition of the soleus H-reflex were affected by baclofen administration. Also the action potentials of the primary afferents were unchanged during baclofen administration.Conclusions: The antispastic effect of baclofen is not caused by an effect on the transmitter release from Ia afferents or on disynaptic reciprocal Ia inhibition. One possible explanation of the depression of the H-reflex by baclofen is suggested to be a direct depression of motoneuronal excitability.

Posture-related changes in heteronymous recurrent inhibition from quadriceps to ankle muscles in humans.

The possibility was investigated that changes in heteronymous recurrent inhibition (RI) from quadriceps (Q) to soleus (Sol) and tibialis anterior (TA) motoneurons (MNs) occur during postural tasks requiring cocontraction of Q with one of these muscles. Stimulation of the femoral nerve (FN), which elicited a Q H-reflex discharge, was used to activate Renshaw cells. The resulting inhibition of TA and Sol MNs was assessed using three test responses: (1) the rectified and averaged ongoing electromyogram (EMG) activity in TA or Sol; (2) the motor-evoked potential (MEP) elicited by cortical stimulation in these muscles; and (3) the Sol H reflex. The characteristics of the depression (appearance and increase with the conditioning reflex discharge, short central delay and long duration) are consistent with a Renshaw origin. In addition, results obtained in control experiments (no change in the EMG suppression after an ischaemic blockade of group-I afferents from the leg, time course of the FN-induced depression of the MEP similar to that of the ongoing EMG) made a significant contribution from other pathways activated by FN stimulation unlikely. Posture-related heteronymous RI from Q was compared in different postural tasks at matched levels of background EMG activity: voluntary co-contraction of Q and of the relevant ankle muscle while sitting (control situation), postural co-contraction of Q and TA (while leaning backwards during stance), or contraction of Sol with (preparation for hopping) and without (standing on tip of toes and leaning forwards during stance) associated contraction of the Q. During stance, heteronymous RI from Q was reduced to TA (but not to Sol) while leaning backwards and to Sol in preparation for hopping, but not in the other situations. Thus, RI from Q to TA or Sol was specifically decreased when a co-contraction of the Q and of the relevant muscle operating at the ankle was required to maintain bipedal stance. It is argued that this control of Renshaw cells is descending in origin and contributes to selection of the appropriate synergism in various postural tasks.

Changes in transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurons during movement in man.

H reflexes were induced in the human quadriceps muscle by electrical stimulation of the femoral nerve. The reflexes were conditioned by prior stimulation of the inferior soleus nerve. The conditioning stimulus produced an inhibition of long duration (>20 ms). The threshold of this inhibition was at zero soleus motor discharge and the inhibition scaled with soleus motor discharge. It was concluded that the inhibition was a heteronymous recurrent inhibition of quadriceps motor neurons mediated by Renshaw cells which had been activated by soleus motor neuron discharge. This recurrent inhibition declined during voluntary tonic contraction of the quadriceps, falling to zero at around one-third of maximum voluntary contraction. Antagonist contraction and weak co-contraction of the quadriceps and its antagonists did not lead to any significant change in recurrent inhibition. It is concluded that motor commands descending from the brain reduce heteronymous recurrent inhibition during isolated quadriceps muscle contraction, but to a much lesser extent during co-contraction. No evidence was obtained for any descending facilitation of heteronymous recurrent inhibition.

Neurotrophin Modulation of the Monosynaptic Reflex after Peripheral Nerve Transection

The effects of neurotrophin-3 (NT-3) and NT-4/5 on the function of axotomized group Ia afferents and motoneurons comprising the monosynaptic reflex pathway were investigated. The axotomized medial gastrocnemius (MG) nerve was provided with NT-3 or NT-4/5 for 8-35 d via an osmotic minipump attached to its central end at the time of axotomy. After this treatment, monosynaptic EPSPs were recorded intracellularly from MG or lateral gastrocnemius soleus (LGS) motoneurons in response to stimulation of the heteronymous nerve under pentobarbital anesthesia. Controls were preparations with axotomized nerves treated directly with vehicle; other axotomized controls were administered subcutaneous NT-3. Direct NT-3 administration (60 microgram/d) not only prevented the decline in EPSP amplitude from axotomized afferents (stimulate MG, record LGS) observed in axotomy controls but, after 5 weeks, led to EPSPs larger than those from intact afferents. These central changes were paralleled by recovery of group I afferent conduction velocity. Removal of NT-3 4-5 weeks after beginning treatment resulted in a decline of conduction velocity and EPSP amplitude within 1 week to values characteristic of axotomy. The increased synaptic efficacy after NT-3 treatment was associated with enhanced connectivity of single afferents to motoneurons. NT-4/5 induced modest recovery in group I afferent conduction velocity but not of the EPSPs they elicited. NT-3 or NT-4/5 had no effect on the properties of treated motoneurons or their monosynaptic EPSPs. We conclude that NT-3, and to a limited extent NT-4/5, promotes recovery of axotomized group Ia afferents but not axotomized motoneurons or the synapses on them.

Increased activity in the form of endurance training increases calcitonin gene-related peptide content in lumbar motoneuron cell bodies and in sciatic nerve in the rat

The relative content of calcitonin gene-related peptide in lumbar motoneuron cell bodies (semi-quantitative immunohistochemistry) and sciatic nerve was examined in rats who had previously undergone a 16-week period of endurance training on a motor-driven treadmill. Soleus motoneurons were identified in the spinal cord by their fluorescence following injection of FluoroGold into the muscle one week before killing. In sedentary rats, calcitonin gene-related peptide was detectable in 76–90% of motoneurons, with no difference in the proportions of negative cells, or in the mean staining intensity of positive cells, between soleus and neighbouring (presumptive fast hindlimb muscle) unlabelled motoneurons. In endurance-trained rats, the estimated content of calcitonin gene-related peptide was significantly increased (90%) in cell bodies of soleus and neighbouring motoneurons, with no training-induced alterations in the proportions of calcitonin gene-related peptide-positive cells in either sample. The content of the neuropeptide was also significantly higher (37%) in sciatic nerve of endurance-trained rats. Relative accumulation of calcitonin gene-related peptide proximal to a sciatic nerve ligature applied 4 h before, however, was unchanged. The increases in calcitonin gene-related peptide in motoneuron cell bodies and sciatic nerve axons following endurance training may indicate an up-regulation of the synthesis, transport and terminal release of this neuropeptide, which could play a significant role in other morphological and functional adaptations which are known to occur at the neuromuscular junction following this chronic change in activity level.

Presynaptic excitability changes of group Ia fibres to muscle nociceptive stimulation in humans

Tonic muscle nociceptive discharge evoked chemically from the foot extensor digitorum brevis muscle in man produces a depression of Ia excitation and Ib inhibition of the soleus (Sol) motoneurones (Mns). The possibility that both these changes partly result from presynaptic inhibition of Ia fibres projecting to Sol Mns and to interneurones mediating group I non-reciprocal inhibition is tested. Convergence of Ia fibres on these interneurones was deduced from evidence that reducing the excitatory effect of the extensor Ia fibres (by potentiation of their presynaptic inhibition) resulted in Ib disinhibition. Nociceptive-induced potentiation of Ia presynaptic inhibition was deduced from the following congruent results obtained by two independent methods: (1) depression of heteronymous Ia monosynaptic facilitation of the quadriceps muscle to Sol Mns; (2) potentiation of presynaptic inhibition of Sol Ia fibres evoked by mechanical activation of the tibialis anterior primary spindle afferents. It is concluded that nociceptive volleys arising from dorsal foot muscles facilitate the activity of interneurones intercalated in pathways responsible for presynaptic inhibition of Sol Ia fibres. It is also proposed that the same Ia presynaptic inhibition depresses the excitability of interneurones mediating group I non-reciprocal inhibition, thus resulting in Ib disinhibition of Sol Mns.

Long-latency contributions to reciprocal inhibition during various levels of muscle contraction

Reciprocal inhibition is a functional term and refers to the proportional decrease in antagonist motoneuron activity that accompanies an agonist contraction. A condition-test (C-T) H-reflex paradigm (conditioned stimulus applied to the common peroneal nerve; test reflex elicited by posterior tibial nerve stimulation) was used during: (1) rest, (2) a tonic isometric tibialis anterior (TA) contraction at 10% of its maximal voluntary contraction (MVC) and, (3) a TA contraction at 25% MVC. The purpose of the study was to assess whether or not long-latency contributions to reciprocal inhibition of soleus H-reflexes changed with increasing levels of TA contraction. C-T intervals ranged from 5 to 150 ms. Subjects (n=14) had long-latency inhibition at rest (x̄=−35.0±18.7%). This inhibition was enhanced during 10% (x̄=−46.1±17.9%; p=0.17) and 25% MVCs (x̄=−56.3±14.0%; p<0.01). Findings indicate that long-latency contributions to reciprocal inhibition of the soleus motoneuron pool are enhanced with increasing force of TA muscle contraction up to 25% MVC. These results indicate that long-latency contributions to reciprocal inhibition of soleus H-reflexes are not static but rather are task-specific and change in relation to levels of TA muscle activity.

On the effect of chemically activated fine muscle afferents on interneurones mediating group I non-reciprocal inhibition of extensor ankle and knee muscles in humans

In a previous paper it was shown that muscle nociceptive discharge depressed the activity of interneurones mediating group I non-reciprocal inhibition (or Ib interneurones) in humans [A. Rossi, B. Decchi, Changes in Ib heteronymous inhibition to soleus motoneurons during cutaneous and muscle nociceptive stimulation in humans, Brain Res. 774 (1997) 55–61.]. However, since nociceptive discharge depressed the size of the soleus H-reflex (by which Ib inhibition was tested) the question arises as to whether modification of motoneurone membrane conductance per se could depress the size of Ib inhibitory post-synaptic potentials. The results of the present study suggest that the contribution of motoneurone hyperpolarization to Ib disinhibition is negligible and that muscle nociceptive discharge actually depresses the activity of these pathways.

Hypothyroidism in the rat results in decreased soleus motoneurone soma size

Adult female rats were thyroidectomized. After an average of 17 weeks, horseradish peroxidase (HRP) was injected into the right side soleus muscle. Two days later, left side soleus muscle properties were recorded and muscles and spinal cord were removed for further histological measurements. Soleus muscles from hypothyroid rats no longer contained type IIA fibers. Contraction and half-relaxation times of twitches had increased significantly compared to control rats. The average cross-sectional surface areas of HRP-labeled soleus motoneurones from hypothyroid rats were slightly but significantly smaller than those of control rats. A similar decrease in size was found for other (presumed moto-) neurones lying ventrolaterally to the soleus motor nucleus. It is concluded that changes in the soleus muscle fiber composition, as caused by lowered levels of thyroid hormone, are paralleled by corresponding changes in the size of its motoneurones and also of other spinal (moto)neurones.

Reassessment of H-reflex recovery curve using the double stimulation procedure.

We conducted two types of experiments to assess the validity of the H-reflex recovery test, using double stimulation to test soleus motoneuron pool excitability in healthy and spastic subjects. One type dealt with the mechanical effect of the conditioning H reflex on the ankle joint; the other type with the effect of change in reflex size. The mechanical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to marked depression. We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group Ia inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical effect in the FR condition was observed despite the FX condition. The effects of systematic changes on soleus H-reflex size were investigated at conditioning-test intervals of 80 ms, so as to avoid mechanical effects. When conditioning and test reflexes were the same size, the amount of recovery increased as the H-reflex size increased. Comparison of the relation between amount of recovery and H-reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity mentioned in previous literature may have resulted from the fact that relatively greater H reflexes were tested in those studies. In conclusion, the present study indicates that double stimulation is not appropriate for assessing spinal motoneuron pool "excitability increase" in spasticity.