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PubMed1978

Reciprocal inhibition during the tonic stretch reflex in the decerebrate cat.

1. The aim of this study was to investigate post-synaptic reciprocal Ia inhibition during the stretch reflex; particularly the extent to which an increased Ia excitation of the Ia inhibitory interneurones will be counteracted by recurrent inhibition from motor axon collaterals. For this purpose we investigated depression of monosynaptic test reflexes antagonist flexors (reciprocal inhibition) during static stretch of quadriceps or triceps surae in unanaesthetized decerebrate cats. 3. With increasing stretch of the extensor muscle there was first a linear augmentation of reciprocal inhibition, but along with the stretch reflex in the extensor a plateau appeared in the inhibition of the flexors, although the extensor stretch reflex (judged by the e.m.g.) increased with further stretching. Within the range of stretching of triceps surae which gave increased stretch reflexes the plateau in the reciprocal inhibition was usually maintained, while during stretching of quadriceps a second phase of augmenting reciprocal inhibition often appeared. Stretch beyond the level which increased the stretch reflex activity gave augmenting reciprocal inhibition both in case of quadriceps and triceps surae. 3. Excitability measurements from central terminals of Ia afferents revealed that the increasing reciprocal inhibition during increasing stretch reflex activity in quadriceps was associated with a primary afferent depolarization in knee flexor Ia afferents; there was no corresponding effect in ankle flexor Ia afferents during stretch reflexes in triceps surae. 4. The primary afferent depolarization evoked in knee flexor Ia afferents by electrical nerve stimulation was then compared with the presynaptic inhibition of knee flexor monosynaptic test reflexes produced by the same stimuli. The results suggest that the second phase of increasing reciprocal inhibition in knee flexors is due to presynaptic inhibition and accordingly that the depth of post-synaptic reciprocal inhibition remains constant at different degrees of stretch reflex activity in both knee and ankle extensors. 5. It is postulated that during increasing stretch reflex activity the increment in Ia excitation and recurrent inhibitio; on to the Ia inhibitory interneurones almost exactly balance each other. It is suggested that recurrent inhibition of Ia inhibitory interneurones may serve as a segmental autoregulatory mechanism to keep 'alpha-gamma-linked reciprocal inhibition' at a constant depth during different levels of agonist activity.

Animals↗
PubMed1984

Fusimotor reflexes in triceps surae muscle elicited by extension of the contralateral hind limb in the cat.

Experiments were performed in thirty-two cats anaesthetized with chloralose. The aim of the study was to investigate the reflex effects of flexion or extension of the contralateral hind limb on ipsilateral fusimotor neurones, to compare these effects with the effects elicited by stretch of the ipsilateral posterior biceps and semitendinosus (p.b.s.t.) muscles (Appelberg, Hulliger, Johansson & Sojka, 1982) and to clarify the interactions between the reflexes elicited from the ipsilateral and the contralateral side. Activity in fusimotor neurones was studied indirectly by recording from primary and secondary muscle spindle afferents of the triceps surae muscle. The mean rate of firing and the modulation of the afferent response to sinusoidal extension of the triceps surae was determined. Control measurements were made with the ipsilateral p.b.s.t. muscles relaxed and the contralateral hind limb in resting position. Tests were made with stretch of the ipsilateral p.b.s.t. and/or extension/flexion of the contralateral hind limb. With extension of the contralateral hind limb 64 out of 210 primary afferents (30.5%) showed predominantly dynamic reflexes (41 out of 134 in spinalized preparations: 30.6%), 25 (11.9%) showed mixed or predominantly static effects (1 spinalized: 0.7%), 121 (57.6%) showed no effect (92 spinalized: 68.7%). Flexion of the limb gave, with only two exceptions, no observable effect. Thirty-three secondary afferents were investigated. Five responded to extension of the contralateral hind limb with excitatory reflex effects. Flexion did not influence the secondary afferents. Mostly the reflex effects were not accompanied by detectable electromyogram (e.m.g.) activity in the ipsilateral triceps (surface recordings), indicating that the reflexes mainly involved gamma-motoneurones. A comparison was made between the reflexes elicited by stretch of the ipsilateral p.b.s.t. and extension of the contralateral hind limb. The percentage of responsive units was higher for the contralateral stimulus. Spinalization almost abolished the statis reflex responses to both ipsi- and contralateral stimulation, and it increased the number of dynamic responses to ipsilateral stimulation. The ipsilaterally elicited reflexes also seemed more dependent upon background activity. Ipsilateral stimulus could facilitate or reduce a contralaterally evoked response, even when the ipsilateral stimulus alone gave no effect. The reflexes could also summate. Quite often combined stimuli changed the character of the reflex from dynamic to static or vice versa.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Origin of the specific H reflex facilitation preceding a voluntary movement in man.

1. In a reaction time situation, the monosynaptic spinal reflex (H reflex) is facilitated before the onset of an electromyographic (EMG) response. The aim of the present investigation was to test if the facilitation can be attributed either to a subliminal depolarization of motoneurones or to an increase of the excitatory effect of the afferent volley reaching the motoneurones. 2. At the onset of an acoustic warning signal, human subjects were required to concentrate on a reaction time task and, in addition, to initiate a steady isometric plantar flexion of medium intensity in both feet. In response to a following visual stimulus, they carried out a ballistic plantar flexion randomly with the right or left foot. At different times after the visual reaction signal, H reflexes were elicited bilaterally. 3. The facilitation of the H reflex was similar in the presence and absence of a steady activation. In addition, the facilitations were similar in absolute amplitude and duration when the stimuli evoking the H reflexes were at threshold intensities, or at an intensity which produced control H reflexes of 60% maximum amplitude. 4. In a second series of experiments, no H reflexes were elicited but the strength of the steady plantar flexion was varied. Premotor time, i.e. the interval between the onset of the visual stimulus and the EMG response, and reaction time, i.e. the interval between the onset of the visual stimulus and the mechanical response, were computed. Neither parameter depended significantly on the intensity of steady flexion and they were the same with steady flexion as without. 5. The rectified EMG records and the torque records were aligned by the end of premotor time. Three-dimensional displays of average activity as a function of time and steady activation level were computed. No activation before premotor and reaction time was detected which could have been related to the H reflex facilitation. 6. The present results suggest that all motoneurones, in particular those being activated during the voluntary contraction, can contribute to the H reflex facilitation before movement onset and that the basis of this facilitation is an enhanced excitatory effect of the afferent volley elicited by the H reflex stimulus. Mechanisms leading to the facilitation could be removal of presynaptic inhibition at I a terminals or facilitation of interneurones intercalated in polysynaptic components of the reflex pathways.

Adult↗
PubMed1990

Effects of muscle history on the stretch reflex in cat and man.

1. This is a report of experiments on cat and man which demonstrate effects of a muscle's previous history of contraction and length changes on the size of the stretch reflex. 2. In adult human subjects the size of the tendon jerk was measured in ankle extensor muscles by tapping the Achilles tendon. Muscle conditioning consisted of a maximum voluntary contraction with the foot dorsiflexed or plantarflexed by 30 deg from the test position, after which the subject was asked to relax while the foot was held still for several seconds before being returned to the test position and a tendon tap given. After a contraction of the lengthened muscle the tendon jerk was smaller than after a contraction of the shortened muscle. 3. The experiment was then repeated, but instead of a tendon jerk an H (Hoffmann) reflex was elicited by transcutaneous electrical stimulation of the tibial nerve in the popliteal fossa. The reflex after a conditioning contraction of the lengthened muscle was larger than after a contraction of the shortened muscle. In other words muscle conditioning produced opposite effects on the tendon jerk and H reflex. 4. These findings were confirmed in cats anaesthetized with chloralose. After a conditioning contraction of triceps surae at a length 5 mm longer than the test length (hold-long) a quick tendon stretch produced a smaller reflex response than following a conditioning contraction with the muscle 5 mm shorter than the test length (hold-short). The reverse trend was seen with a reflex elicited by direct electrical stimulation of the muscle nerve, which stimulates the H reflex. 5. One consequence of a conditioning contraction is that it leads to an alteration of the level of resting discharge of muscle spindles. We propose that the larger tendon jerk after a contraction of the shortened muscle is the result of changes in stretch sensitivity of muscle spindles. The reverse effect on the H reflex we attribute to a rise in the level of resting discharge of muscle spindles, which, we propose, leads to reflex inhibition of motoneurones. 6. We support this conclusion with evidence from an experiment in which the size of the conditioning step was systematically altered. Even quite small hold-short conditioning steps led to depression of the H reflex in man and the monosynaptic reflex in cats. Recordings from single afferents showed that such small steps were also accompanied by a detectable rise in spindle resting discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed2001

The use of Preyer's reflex in evaluation of hearing in mice.

Preyer's reflex, the elicitation of startle response to auditory stimuli, has been widely used for the evaluation of hearing in rodents and other animals. Surprisingly, however, the sensitivity and specificity of Preyer's reflex in the assessment of hearing has not been adequately studied. The aim of this study was to investigate the utility of Preyer's reflex in the evaluation of auditory function in mice. Forty-six adult albino mice on an FVB background with variable hearing loss were used for this study. Two different methods for eliciting a Preyer's reflex were tested: a handclap and a sharp metallic sound. The reflex was considered positive when a rapid movement of the whole body of the animal was clearly noticed. Thereafter, the mice underwent auditory brain stem response (ABR) testing with broadband clicks. The presence or absence of Preyer's reflex was compared with the corresponding ABR thresholds. Five of the 46 animals studied (11%) showed a negative Preyer's reflex, while the remaining 41 animals demonstrated a positive Preyer's reflex. There was no difference between the abilities of the two different stimuli to elicit a Preyer's reflex. The click-evoked ABR thresholds in the test animals varied between 8 and 136 (mean 50) dB sound pressure level (SPL). Preyer's reflex was positive in all animals with an ABR threshold of < or = 76 dB SPL, but was absent in animals with an ABR threshold of > or = 81 dB SPL. Preyer's reflex is effective for identifying profound sensorineural hearing loss in experimental mice, but is insensitive for detecting less severe auditory dysfunction. For definitive hearing assessment, and for defining the hearing thresholds. objective electroacoustical methods such as ABR should be used.

Animals↗
PubMed1986

Amplitude modulation of the soleus H-reflex in the human during walking and standing.

Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the soleus muscle at various phases of the step cycle, using a computer-based analysis procedure. In all subjects tested the amplitude of the H-reflex was strongly modulated in amplitude during the walking cycle and was highest during the stance phase. In many subjects the peak reflex amplitude occurred at about the same time as the peak soleus electromyographic (EMG) activity, but in others it occurred earlier. The form of the reflex variation (i.e., envelope of H-reflex amplitude versus phase in cycle) during the step cycle could also be quite different from that of the EMG produced during stepping. At an equal stimulus strength and EMG level, the H-reflex was always much larger, up to 3.5 X, during steadily maintained contractions while standing than during walking. The large reflexes when subjects were standing are consistent with the control of position required to maintain a stable posture in this task. Similarly, the reflexes during walking are greatest during the stance phase, when they will assist in maintaining the upright position of the body against gravity. The reflexes are smallest during the swing phase when they would oppose ankle flexion. However, since the reflex amplitude is task-dependent (i.e., greater during standing than during walking at the same EMG and stimulus levels) and is not always closely related to the EMG produced during a given task such as walking, the strong modulation of H-reflex during walking is not simply a passive consequence of the alpha-motoneuron excitation level.(ABSTRACT TRUNCATED AT 250 WORDS)

Ankle↗
PubMed1996

Behavior of the H-reflex in humans following mechanical perturbation or injury to rostral spinal cord.

In humans H-reflexes are suppressed during early spinal shock. In animals rostral cord injury results in loss of segmental reflexes within seconds. If H-reflexes persist under general anesthesia, can they be used to monitor the integrity of the rostral cord? In part I of this study, we recorded H-reflexes intraoperatively in 25 patients to elucidate general anesthesia effect. In 23 subjects, H-reflexes were consistently elicited, and within +/- 13% of the normalized group mean amplitude. In part II, we recorded H-reflexes in 31 patients during spinal cord surgery to elucidate H-reflex behavior immediately following rostral spinal cord injury. In 6, abrupt suppression of the H-reflex coincided with cord injury. In 4 of 6, suppression was transient and less than 50% of baseline; none of these patients developed neurological deficits. In 2, suppression exceeded 90% and persisted throughout surgery; both patients developed profound deficits. We conclude that (1) the H-reflex can be consistently elicited under general anesthesia in most patients, (2) rostral cord injury rapidly suppresses the H-reflex, and (3) the degree and duration of H-reflex suppression reflects the severity of the injury.

Adolescent↗
PubMed1995

Endocrine basis for two types of individual differences in lordosis reflex intensity.

We have identified individual differences in two independent types of lordosis reflex intensity. Overall lordosis reflex intensity, averaged across 9.4 +/- 0.99 days (range = 1-18 days), differs among cycling individuals and is a biomarker for the rate and pattern of reproductive aging. Although estrogen is necessary to permit lordosis reflex responding and manifests individual differences throughout the estrous cycle, it was individual differences in progesterone that correlated negatively with the individual variation in overall lordosis reflex intensity; females with intense overall lordosis reflexes had low progesterone throughout the estrous cycle. Independent of their overall reflex intensity, females also displayed individual variation in lordosis reflex intensity on proestrus. Among females that still maintained a cyclic lordosis reflex, those with intense lordosis reflexes on proestrus had high levels of luteinizing hormone (LH) at lights out. Among females that displayed intense lordosis reflexes throughout the estrous cycle (i.e., had entered constant lordosis), those with intense proestrous lordosis reflexes had low levels of LH at lights out on proestrus. Females who had entered constant lordosis tended to have lower overall progesterone than females who still maintained cyclic patterns of lordosis. Thus, constant lordosis was identified as an important component of the changes of reproductive aging.

Animals↗
PubMedDate not supplied

Cough and other reflexes on irritation of airway mucosa in man.

Both human and animal studies show that irritation of airway mucosa elicits a variety of reflex responses such as coughing, apnoea, and laryngeal closure. Most of the information concerning these reflex responses were obtained in anesthetized conditions with little applicability to awake conditions. Various aspects of cough and other reflexes on irritation of the airway mucosa are discussed. Studies on awake humans showed that stimulation of the laryngeal mucosa with a small amount of distilled water during wakefulness causes elicitation of the expiration reflex, cough reflex, and swallowing reflex while other types of responses are scarcely observed. In addition, the duration of these responses is remarkably short. In contrast, the same stimulation causes more variant, prolonged, and exaggerated responses during a light depth of anesthesia. An increase in depth of anesthesia abolishes expiratory efforts such as coughing and the expiration reflex whereas the apnoeic reflex and laryngeal closure reflex are resistant to the depressant effect of anesthesia. Also, the respiratory reflex responses to airway irritation varied, depending on the site of stimulation: both laryngeal and tracheal stimulation cause vigorous respiratory responses whereas bronchial stimulation causes little or no respiratory responses. These results indicate not only that the types and magnitude of reflex responses is greatly modified by the central nervous state but also that the site of stimulation is crucial for determining the pattern of respiratory responses elicited by airway stimulation in humans.

Anesthesia↗
PubMed1996

Supraspinal effects on the fractal correlation in human H-reflex.

In our previous study, 1/f beta-type power spectrum with the spectral exponent beta significantly greater than zero was found in the variability of soleus H-reflex amplitudes. This result indicated that the H-reflex variability was time-correlated owing to fractal characteristics. Furthermore, it was also suggested that the fractal characteristics were generated at the spinal level. The purpose of the present study was to investigate whether the fractal nature of the H-reflex variability was influenced by the loss of supraspinal input. Six healthy normal subjects and seven patients with spinal cord injury participated in this study. Soleus H-reflexes were evoked every 1 s from both legs simultaneously (stimulation intensity: motor threshold) and 1050 successive amplitudes of the H-reflex were recorded. The H-reflex sequence evoked from each leg was analyzed by "coarse graining spectral analysis" to calculate the spectral exponent beta. The value of beta was used to evaluate the level of time-correlation (fractal correlation). Cross-spectral analysis was used to evaluate the degree of synchronization between the H-reflex sequences evoked from both legs. The beta values for normal subjects (0.84 +/- 0.33, left leg; 0.88 +/- 0.34. right leg) were significantly greater (P < 0.001) than those for patients (0.31 +/- 0.18, left leg; 0.32 +/- 0.14, right leg), suggesting that the H-reflex sequences for normal subjects were more time-correlated than for patients. In the frequency range less than 0.2 Hz, the coherence of both legs was high (0.41 +/- 0.14) for normal subjects as compared to 0.20 +/- 0.12 for patients (P < 0.005). In this frequency range, the phase was almost 0 rad for normal subjects, indicating that the H-reflex variabilities of both legs were synchronized. These results suggested that (1) the strong fractal correlation observed in the H-reflex sequences for normal subjects was associated with supraspinal input, and (2) such supraspinal input had equal influence on the reflex arcs of the soleus of both legs.

Adult↗
PubMed1991

Midbrain stimulation inhibits the micturition, defecation and rhythmic straining reflexes elicited by activation of sacral vesical and rectal afferents in the dog.

Inhibition of the micturition, defecation and rhythmic straining reflexes by midbrain stimulation was compared with the inhibition of the jaw-opening reflex caused by tooth pulp stimulation in decerebrate dogs. All of the reflexes were inhibited by stimulation of the dorsal and ventral periaqueductal gray, dorsal raphe nucleus and central tegmental field with similar threshold intensities. After a hemisection of the spinal cord at the C2 segment, the midbrain stimulation still suppressed the micturition reflex as well as field potential changes which were evoked by stimulation of the pelvic nerve and recorded from the lateral funiculus just caudal to the hemisection, but did not influence the discharges of the vesical branch of the pelvic nerve which were elicited by stimulation of the lateral funiculus just rostral to the hemisection. The results suggest that stimulation of the neural elements in the 4 midbrain areas depresses the ascending activities from vesical and colorectal afferents of the pelvic nerve at the spinal level, and consequently inhibits the pelvic nerve reflexes. Systemic methysergide suppressed midbrain inhibition of the jaw-opening reflex, but did not affect the midbrain inhibition of the pelvic nerve reflexes. Systemic naloxone did not influence midbrain inhibition of the pelvic nerve reflexes or the jaw-opening reflex, but enhanced the micturition and rhythmic straining reflexes. Possible roles of the midbrain inhibition of the pelvic nerve reflexes are discussed.

Analgesia↗
PubMed1986

Position dependence of stretch reflex dynamics at the human ankle.

The purpose of this study was to examine the effect of ankle position on the human ankle stretch reflexes during tonically-maintained contractions over most of the range of motion. The ankle was placed at randomly selected mean positions. Target levels of triceps surae (TS) or tibialis anterior (TA) tonic contractions were generated while the ankle was displaced by small amplitude, stochastic perturbations. System identification techniques were used to identify the stretch reflex dynamics at each combination of tonic level and ankle angle. As shown previously, the TS stretch reflex was characterized by an unidirectional, velocity-sensitive impulse response function whereas the TA stretch reflex was characterized by a linear impulse response function between ankle velocity and TA EMG. TS stretch reflexes showed a strong dependence on ankle position while TA stretch reflexes did not. Thus the TS stretch reflex magnitude increased greatly as the ankle was progressively dorsiflexed. In contrast, ankle mean position had only a minor effect on the TA stretch reflex magnitude. Our results indicate that the position-dependent facilitation of the TS stretch reflex is not due to changes in the level of skeletal motoneuron excitability. Rather, this effect may be accounted for by mechanisms that modulate the efficacy of the stochastic ankle perturbation. Such mechanisms could include position-induced: modulation of monosynaptic and polysynaptic afferent inputs to skeletal motoneurons, alterations in the extent of fusimotor drive and changes in the transmission of the joint perturbation to spindle receptors. Such mechanisms are discussed in terms of the differences between TS and TA stretch reflexes. Finally, the functional significance of position-dependent reflex responses are considered.

Adult↗
PubMed1989

Rectoanal reflex induced by H2O thermal stimulation.

To obtain clear evidence of the rectoanal reflex, cold water was injected into the rectum. All patients with a normal reflex had a rectoanal reflex when the water was either 4 degrees C or 17 degrees C. The same reflex was seen when a balloon was used for distention. No reflex was evident when the water was 37 degrees C. At 27 degrees C or 45 degrees C, the reflex was evident occasionally. In patients with Hirschsprung's disease, the rectoanal reflex was absent with balloon distention, and when water at any temperature was injected. As cold water will induce the rectoanal reflex safely and without rectal distention, measurement of the reflex using cold water is useful when the presence or absence of the reflex is doubtful. The authors' results suggest that the receptor related to the reflex is neuronal in origin and not muscular, and that the receptor locates near the mucosa.

Anal Canal↗
PubMed2005

The amplitude modulation of the quadriceps H-reflex in relation to the knee joint action during walking.

Previously the modulation of the quadriceps H-reflex has only been investigated in the initial part of the gait cycle, and it was suggested that the quadriceps H-reflex modulates with relative high reflex gain at heel contact and decreases during the subsequent part of stance (Dietz et al. 1990b). The objectives of the present study was to elaborate on the previous results by increasing the measurement resolution around heel contact and include additional measures in order to relate the H-reflex modulation to the mechanical function of the knee extensors throughout the gait cycle. EMG profiles were measured in quadriceps and the antagonistic hamstring muscles simultaneously with the knee joint kinematics in ten subjects during treadmill walking at preferred speed. H-reflex excitability was measured in vastus lateralis (VL) and rectus femoris (RF) at 11 selected positions during the gait cycle. The resulting excitability curves showed a significant modulation of the quadriceps H-reflex during the gait cycle. The H-reflex amplitude increases shortly after heel contact and reflex inhibition is present in the remaining part of stance and most of the swing phase. The modulation of the quadriceps H-reflex during walking does not follow the classical pattern of reciprocal inhibition between antagonistic muscles. It is suggested that at least during the stance phase the modulation of the quadriceps H-reflex is controlled by presynaptic inhibition. The present results confirm the idea that the excitability of the quadriceps H-reflex is controlled to comply with the different mechanical demands on the muscle during the gait cycle in humans.

Adult↗
PubMed2006

Modulation of soleus H-reflexes during gait in healthy children.

During locomotion spinal short latency reflexes are rhythmically modulated and depressed compared to rest. In adults this modulation is severely disturbed after bilateral spinal lesions indicating a role for supra-spinal control. Soleus reflex amplitudes are large in the stance phase and suppressed in the swing phase contributing to the reciprocal muscle activation pattern required for walking. In early childhood the EMG pattern during gait underlies an age-dependent process changing from co-contraction of agonists and antagonists to a reciprocal pattern at the age of 5-7 years. It is unknown whether at this stage apart from the EMG also reflexes are modulated, and if so, whether the reflex modulation is fully mature or still underlies an age-dependent development. This may give important information about the maturation of CNS structures involved in gait control. Soleus Hoffmann H-reflexes were investigated in 36 healthy children aged 7-16 years during treadmill walking at 1.2 km/h and 3.0 km/h. At 7 years old a rhythmic modulation similar to adults was observed. The H-reflex size during the stance phase decreased significantly with age while the maximum H-reflex (H (max)) at rest remained unchanged. At 3.0 km/h H-reflexes were significantly larger during the stance phase and smaller during the swing phase as compared to 1.2 km/h but the age-dependent suppression was observed at both walking velocities. In conclusion H-reflex modulation during gait is already present in young children but still underlies an age-dependent process independent of the walking velocity. The finding that the rhythmic part of the modulation is already present at the age of 7 years may indicate that the supra-spinal structures involved mature earlier than those involved in the tonic reflex depression. This may reflect an increasing supra-spinal control of spinal reflexes under functional conditions with maturation.

Adolescent↗
PubMed2001

Operant conditioning of rat H-reflex: effects on mean latency and duration.

We are currently studying the mechanisms of operantly conditioned changes in the H-reflex in the rat. Primate data suggest that H-reflex decrease is due to a positive shift in motoneuron firing threshold and a small decrease in the monosynaptic excitatory postsynaptic potential (EPSP), and that increase might be due to change in group-I oligosynaptic (especially disynaptic) input. To further evaluate the possibility of conditioned change in oligosynaptic input, we compared the mean latency (i.e., the average latency of the entire H-reflex) and the duration of control (i.e., pre-conditioning) H-reflexes with those of H-reflexes after up-conditioning or down-conditioning. Up-conditioning was associated with small, statistically significant increases in H-reflex mean latency [+0.11+/-0.05 (+/-SE) ms] and duration (+0.32+/-0.16 ms). The mean latency of the H-reflex increase (i.e., the part added to the H-reflex by up-conditioning) was 0.28+/-0.14 (+/-SE) ms greater than that of the control H-reflex. Down-conditioning had no significant effect on mean latency or duration. While these results indicate that operant conditioning does not greatly change H-reflex mean latency or duration, the effects detected with up-conditioning are consistent with the hypothesis that decreased inhibition, or increased excitation, by homonymous and heteronymous group-I oligosynaptic input contributes to the H-reflex increase produced by up-conditioning. Several other mechanisms might also account for these small effects.

Animals↗
PubMed1999

Effect of clenching levels on heteronymous H-reflex in human temporalis muscle.

Selective stimulation of the masseteric nerve has been shown to elicit a heteronymous H-reflex in the ipsilateral temporalis muscle during voluntary clenching. However, the relation between the electromyographic (EMG) activity of the temporalis muscle and the amplitude of the H-reflex has not been previously described. In the present study, the hypothesis was tested that there would be a positive relationship between the level of EMG activity and the amplitude of the H-reflex. The direct motor response (M-response) in the masseter muscle and the heteronymous H-reflex in the anterior temporalis muscle were successfully elicited by electrical stimulation of the masseteric nerve in 12 of 13 subjects. A new automatic system was used to control the on-line EMG activity and to trigger the stimulus. In a random order, two series of 20 stimuli were delivered at each of four clenching levels (0, 25, 50, and 75% of maximal voluntary contraction). The analysis showed that both the masseteric M-response and the temporalis H-reflex were reproducible within and between series. The amplitude of the temporalis H-reflex increased significantly at higher clenching levels (ANOVA: P=0.003). Clenching at 50% and 75% of the maximal voluntary contraction caused significantly larger amplitudes of the H-reflex than clenching at 25% of the maximal voluntary contraction; at rest, no H-reflex could be recorded. There was a significant correlation between the background EMG activity in the ipsilateral temporalis muscle and the amplitude of the H-reflex (Pearson: r=0.313, P=0.008). These data indicate that the heteronymous H-reflex can be reliably elicited by means of an automatic system for stimulus delivery and that the amplitude of the H-reflex is dependent on the preceding activity of the motoneuron pool.

Adult↗
PubMed2004

Changes in the gain of the soleus H-reflex with changes in the motor recruitment level and/or movement speed.

The behaviour of the soleus H-reflex is considered to be motor task-dependent. However, the speed of movements as well as the motor recruitment level is altered when motor tasks are changed. Therefore it is ambiguous to what extent the motor task-dependent changes found between walking and running, for example, are simply due to changes in these two parameters. The purpose of this study was to investigate how movement speed and motor recruitment level separately influence the soleus H-reflex behaviour when the motor task is unchanged. Soleus H-reflexes were elicited during pedalling at different cadences and crank loads, by which movement speed and muscle recruitment level were modified separately. The H-reflex gain was expressed as the slope of the linear relation between the reflex amplitudes and the background electromyelogram (EMG), and the reflex threshold was expressed by the intercept. The results showed a decrease in reflex gain by 54% ( P=0.001) when the speed of movement was doubled from 40 to 80 rpm (repetitions per minute) without changes in the level of soleus EMG activity. Reflex gain decreased 40% ( P=0.002) when the soleus EMG level was increased by 47% without changing the speed of movement. No significant changes were found in the reflex threshold. We conclude that as speed of movement and motor recruitment level influence the gain of the soleus H-reflex, it is significant that these two parameters are comparable before changes in H-reflexes are stated to be task-dependent.

Adaptation, Physiological↗