Medium Latency Reflex Research Articles

Comparison of the temporal properties of medium latency responses induced by cortical and peripheral stimulation

Sudden foot dorsiflexion lengthens soleus muscle and activates stretch-based spinal reflexes. Dorsiflexion can be triggered by activating tibialis anterior (TA) muscle through peroneal nerve stimulation or transcranial magnetic stimulation (TMS) which evokes a response in the soleus muscle referred to as Medium Latency Reflex (MLR) or motor-evoked potential-80 (Soleus MEP80), respectively. This study aimed to examine the relationship between these responses in humans. Therefore, latency characteristics and correlation of responses between soleus MEP80 and MLR were investigated. We have also calculated the latencies from the onset of tibialis activity, i.e., subtracting of TA-MEP from MEP80 and TA direct motor response from MLR. We referred to these calculations as Stretch Loop Latency Central (SLLc) for MEP80 and Stretch Loop Latency Peripheral (SLLp) for MLR. The latency of SLLc was found to be 61.4 ± 5.6 ms which was significantly shorter (P = 0.0259) than SLLp (64.0 ± 4.2 ms) and these latencies were correlated (P = 0.0045, r = 0.689). The latency of both responses was also found to be inversely related to the response amplitude (P = 0.0121, r = 0.451) probably due to the activation of large motor units. When amplitude differences were corrected, i.e. investigating the responses with similar amplitudes, SLLp, and SLLc latencies found to be similar (P = 0.1317). Due to the identical features of the soleus MEP80 and MLR, we propose that they may both have spinal origins.

P23-F Re-examining the origin of medium latency reflex response of soleus using single motor units

Background This study is inquiring the origin of the medium latency reflex response (MLR) that occurs in soleus muscle when common peroneal nerve is electrically stimulated. Material and methods In total, 23 SingleMUs from 9 males were obtained. During SMU recruitment, 308 ± 141 stimuli were delivered. We investigated the duration of the inhibitory postsynaptic potential (IPSP) and excitatory postsynaptic potential (EPSP). Results Common peroneal nerve stimulation resulted in M-response at SEMG of TA muscle and MLR in SOL-SEMG as well as accumulation of SMUs between 50 and 120 ms after the stimulus. A reduction in firing probability of SMUs was also observed and the reduction in discharge rate continued much longer than the spike reduction probability in PSTH and CUSUM. This reduction in discharge rate was followed by an increased discharge rate of SMUs. Duration of the IPSP obtained at PSF, 24.5 ± 6.3 ms, and the duration of EPSP, i.e. MLR, in PSF was found as 119.3 ± 54.3 The latency of EPSP was found higher in PSF and SEMG as 68.3 ± 9.9 ms and 74.5 ± 9.7 ms, than PSTH and MMU which were 56.3 ± 4.6 ms and 57.6 ± 4.2 ms, respectively. Conclusions There was a genuine Ia reciprocal inhibition followed by a stretch-induced reflex response in the SOL. The first response was due to the well-known Ia-IPSP pathway connecting antagonist muscles and the second response, the so-called MLR in the literature, was an indirect stretch reflex response caused by the M-response in the TA muscle. This Project (3135) was supported by Akdeniz University Scientific Research Unit .

P24-F Medium latency Soleus reflex responses induced by transcranial magnetic and peroneal nerve stimulation

Background Sudden foot dorsiflexion generates medium and long latency responses in soleus muscle which are thought to be the stretch reflex and have a spinal origin. Peroneal nerve stimulation results in foot dorsiflexion and produces a medium latency reflex in soleus muscle (SOL-MLR). Similarly, transcranial magnetic stimulation (TMS) of motor cortex corresponds to the leg area evokes a response analogous to SOL-MLR, referred to as soleus motor evoked potential 80 (SOL-MEP 80). Material and methods In this study, the origin of SOL-MLR and SOL-MEP 80 were investigated in 15 healthy volunteers using surface electromyography. SOL-MEP 80 was evoked by non-invasive TMS of leg area of motor cortex on the scalp using a round coil. On the other hand, SOL-MLR was obtained through electrical stimulation of the peroneal nerve during the foot was slightly dorsiflexed. Given that the contraction of the tibialis anterior (TA) muscle results in foot dorsiflexion, so, the stretch reflex in the soleus muscle. Therefore, the latency of SOL-MLR and SOL-MEP 80 was compared with the muscle action potential generation in TA muscle with similar methodology, TA direct motor response and TA MEP. Results The latency difference between SOL-MLR and TA direct motor response was 64.0 ± 4.0 ms while the difference was 61.4 ± 5.6 ms between SOL-MEP 80 and TA-MEP. Conclusions Comparable latency features and similar behavior pattern of the responses propose that SOL-MLR and SOL-MEP 80 may have a related origin. This project (3135) supported by Akdeniz University Scientific Research Unit .

Medium latency excitatory reflex of soleus re-examined.

We aimed to study the receptor origin and postsynaptic potential profile of the medium latency reflex (MLR) response that develops in the soleus muscle when common peroneal nerve of antagonist tibialis anterior (TA) muscle is electrically stimulated. To achieve this aim, we electrically stimulated common peroneal nerve and recorded surface electromyography (SEMG) responses of soleus and TA muscles of informed volunteers. Additionally, we recorded intramuscular EMG from the soleus muscle. Stimulation of common peroneal nerve induced a direct motor response (M-response) in the TA and MLR in SEMG of the soleus. Using voluntarily-activated single motor units (SMUs) from the soleus muscle we noted that there were two distinct responses following the stimulus. The first response was a reciprocal inhibitory reflex probably originating from the antagonist muscle spindle primary (Ia) afferents. This was followed by an indirect reflex response activated by the contraction of the TA muscle during the M-response. This contraction generated a rapid acceleration in the direction of dorsiflexion hence inducing a stretch stimulus on soleus muscle. The response of soleus to this stimulus was a stretch reflex. We suggest that this stretch reflex is the main contributor to the so-called soleus MLR in the literature. This study illustrated the importance of using SMUs and also using discharge-rate based analysis for closely examining previously 'established' reflexes.

Variable impact of tizanidine on the medium latency reflex of upper and lower limbs.

Sudden limb displacement evokes a complex sequence of compensatory muscle activity. Following the short-latency reflex and preceding voluntary reactions is an epoch termed the medium-latency reflex (MLR) that could reflect spinal processing of group II muscle afferents. One way to test this possibility is oral ingestion of tizanidine, an alpha-2 adrenergic agonist that inhibits the interneurons transmitting group II signals onto spinal motor neurons. We examined whether group II afferents contribute to MLR activity throughout the major muscles that span the elbow and shoulder. MLRs of ankle muscles were also tested during walking on the same day, in the same participants as well as during sitting in a different group of subjects. In contrast to previous reports, the ingestion of tizanidine had minimal impact on MLRs of arm or leg muscles during motor actions. A significant decrease in magnitude was observed for 2/16 contrasts in arm muscles and 0/4 contrasts in leg muscles. This discrepancy with previous studies could indicate that tizanidine's efficacy is altered by subtle changes in protocol or that group II afferents do not substantially contribute to MLRs.

Habituation behavior of the medium-latency reflex over the anterior tibial muscle after electrical stimulation of the sural nerve

Over human leg muscles, three motor responses (MR) can commonly be elicited, namely short-latency reflex (SLR), medium-latency reflex (MLR), and long-latency reflex (LLR). The MLR is less well understood than SLR and LLR. As the response to subsequent stimuli may be used to characterize central influences of an MR, we were interested, whether the MLR differs from SLR and LLR with respect to its habituation and facilitation behavior. MR were examined over the anterior tibial (TA) muscle at different contraction levels after electrical single or train stimuli (time intervals of 3ms) over the ipsilateral sural nerve. Furthermore, MR were selectively averaged after each of four subsequent stimuli (1Hz, 0.4Hz, trains-of-3). After single stimuli, the peak latency values were 46.2±2.3ms, 88.0±5.8ms (MLR), and 131.7±22.2ms (LLR). All three MR gained similarly strong and significantly in amplitude when up to 10kg of weight was loaded compared with no weight load. After train stimuli, the LLR but not SLR and MLR gained significantly in amplitude as compared with single stimuli. Different to SLR and LLR, the MLR showed significant habituation behavior at a stimulus repetition rate of 1Hz but not of 0.4Hz. Thus, inhibitory interneurons seem to be involved in the MLR pathway.

Acceleration dependence and task-specific modulation of short- and medium-latency reflexes in the ankle extensors

Involuntary responses to muscle stretch are often composed of a short-latency reflex (SLR) and more variable responses at longer latencies such as the medium-latency (MLR) and long-latency stretch reflex (LLR). Although longer latency reflexes are enhanced in the upper limb during stabilization of external loads, it remains unknown if they have a similar role in the lower limb. This uncertainty results in part from the inconsistency with which longer latency reflexes have been observed in the lower limb. A review of the literature suggests that studies that only observe SLRs have used perturbations with large accelerations, possibly causing a synchronization of motoneuron refractory periods or an activation of force-dependent inhibition. We therefore hypothesized that the amplitude of longer latency reflexes would vary with perturbation acceleration. We further hypothesized that if longer latency reflexes were elicited, they would increase in amplitude during control of an unstable load, as has been observed in the upper limb. These hypotheses were tested at the ankle while subjects performed a torque or position control task. SLR and MLR reflex components were elicited by ankle flexion perturbations with a fixed peak velocity and variable acceleration. Both reflex components initially scaled with acceleration, however, while the SLR continued to increase at high accelerations, the MLR weakened. At accelerations that reliably elicited MLRs, both the SLR and MLR were reduced during control of the unstable load. These findings clarify the conditions required to elicit MLRs in the ankle extensors and provide additional evidence that rapid feedback pathways are downregulated when stability is compromised in the lower limb.

Effects of Short and Medium Latency Reflexes of the Plantae Muscle with Ankle Vibration during Sudden Foot Movement

[Purpose] This study examined the influence of signals from ankle joint afferent fibers on the reflexive plantae muscle activity during standing subjects. [Subject] Ten male healthy adults participated in this study. [Methods] The subjects stood with their eyes closed on a movable platform that was moved backward. Vibrators (about 90 Hz) were applied to the medial and lateral malleolus of both legs. The vibrators were turned off in the control condition. In the malleolus vibration (MV) condition, vibration stimulus was applied for one minute before the beginning of trials and was continued during the trials (about six minutes). The short (SLR) and medium latency reflex (MLR) of the flexor digitorum brevis muscle (FDB) electromyogram (EMG) responses of the left leg during the platform translations were measured under the control and MV conditions. [Results] The latencies of the FDB SLR and MLR during MV condition were observed to increase significantly in comparison to the control. The integration EMG response values (iEMG) of the SLR and MLR significantly decreased during MV condition. [Conclusion] Excessive afferent signals from the ankle articular mechanoreceptors may participate in the reduction of the reflexive FDB SLR and MLR activities during standing postural perturbation due to changes in the excitability of inhibitory interneurons.

Influences of H and Medium Latency Reflex Responses of the Soleus Muscle while Receiving with Malleolus Vibration during Upright Standing

Influences of H and Medium Latency Reflex Responses of the Soleus Muscle while Receiving with Malleolus Vibration during Upright Standing

Medium-latency reflex response elicited from the flexor carpi radialis by radial nerve stimulation

The H reflex obtained from the flexor carpi radialis muscle by median nerve stimulation is a well-known monosynaptic reflex. However, the origin of the late responses is still contentious. Radial nerve stimulation was performed through the spiral groove, and EMG recording was obtained from the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles. An M response followed by an F response was achieved from the ECR by radial nerve stimulation; the antagonistic FCR muscle elicited a late response. A total of 25 cases were included in this study. In 22 of these cases, a response with a latency of 40.97±5.35ms was obtained from the FCR by radial nerve stimulation. When extension of the hand was restricted, the response disappeared in five of nine cases. Application of cold markedly suppressed the response and prolonged the latency of the FCR medium-latency reflex response (FCR-MLR). Oral tizanidine considerably suppressed the FCR-MLR response. Two out of eight cases did not exhibit any response. No response could be recorded from a patient with complete amputation of the right hand. The FCR-MLR is the reflex caused by stretching of the FCR muscle from radial nerve stimulation, and it is greatly influenced by group II afferents.

Ankle clonus and its relationship with the medium-latency reflex response of the soleus by peroneal nerve stimulation

Ankle clonus and soleus medium-latency reflex are stretch-induced responses. Clonus is traditionally considered to be the result of oscillation in the group Ia mediated spinal stretch reflex but the soleus medium-latency reflex response originates mainly from the activation of group II afferents.The medium latency reflex response (MLR) was recorded in soleus muscle by peroneal nerve stimulation and clonus beats were recorded in soleus muscle using EMG in 19 spastic patients. The dorsiflexion (DF) and plantarflexion (PF) times of clonus and the half-period were calculated based on accelerometric measurements in 11 patients.The MLR of the soleus was 73.63±8.9ms. The half-period of the clonus was 79.34±12.31ms. The difference between the MLR and half-period was significant. The PF was 71.75±6.73ms, and the DF was 88.63±10.83ms. The difference between the soleus MLR and PF part of the clonus beat was not significant.The PF part of the clonus beat is due to soleus muscle contraction and controlled by the neural part of the oscillation. There may be relationship between the soleus MLR and the PF part of the clonus. Clonus is considered to be the result of oscillations in the group Ia spinal stretch reflex, but there is sufficient time for group II afferents to be involved.

Load Rather Than Length Sensitive Feedback Contributes to Soleus Muscle Activity During Human Treadmill Walking

Walking requires a constant adaptation of locomotor output from sensory afferent feedback mechanisms to ensure efficient and stable gait. We investigated the nature of the sensory afferent feedback contribution to the soleus motoneuronal drive and to the corrective stretch reflex by manipulating body load and ankle joint angle. The volunteers walked on a treadmill ( approximately 3.6 km/h) connected to a body weight support (BWS) system. To manipulate the load sensitive afferents the level of BWS was switched between 5 and 30% of body weight. The effect of transient changes in BWS on the soleus stretch reflex was measured by presenting dorsiflexion perturbations ( approximately 5 degrees, 360-400 degrees/s) in mid and late stances. Short (SLRs) and medium latency reflexes (MLRs) were quantified in a 15 ms analysis window. The MLR decreased with decreased loading (P = 0.045), but no significant difference was observed for the SLR (P = 0.13). Similarly, the effect of the BWS was measured on the unload response, i.e., the depression in soleus activity following a plantar-flexion perturbation ( approximately 5.6 degrees, 203-247 degrees/s), quantified over a 50 ms analysis window. The unload response decreased with decreased load (P > 0.001), but was not significantly affected (P = 0.45) by tizanidine induced depression of the MLR (P = 0.039, n = 6). Since tizanidine is believed to depress the group II afferent pathway, these results are consistent with the idea that force-related afferent feedback contributes both to the background locomotor activity and to the medium latency stretch reflex. In contrast, length-related afferent feedback may contribute to only the medium latency stretch reflex.

Age-related changes of the stretch reflex excitability in human ankle muscles

The purpose of this study was to characterize the effects of aging on the stretch reflex in the ankle muscles, and in particular to compare the effects on the ankle dorsi-flexor (tibialis anterior: TA) and the plantar-flexor (soleus: SOL). Stretch reflex responses were elicited in the TA and SOL at rest and during weak voluntary contractions in 20 elderly and 23 young volunteers. The results indicated that, in the TA muscle, the elderly group had a remarkably larger long-latency reflex (LLR), whereas no aging effect was found in the short latency reflex (SLR). These results were very different from those in the SOL muscle, which showed significant aging effects in the SLR and medium latency reflex (MLR), but not in the LLR. Given the fact that the LLR of the TA stretch reflex includes the cortical pathway, it is probable that the effects of aging on the TA stretch reflex involve alterations not only at the spinal level but also at the cortical level. The present results indicate that the stretch reflexes of each of the ankle antagonistic muscles are affected differently by aging, which might have relevance to the neural properties of each muscle.

Medium-latency reflex response of soleus elicited by peroneal nerve stimulation

A medium-latency response (MLR) has been recorded from soleus during stance and walking, and has been attributed to stretch-evoked volleys in group II afferents. The present paper describes a MLR in soleus evoked by stimulating the deep peroneal nerve, documents its characteristics and addresses its likely origin. The MLR of soleus was recorded in healthy subjects and hemiplegic patients, following electrical stimulation of the deep peroneal nerve at the fibula at rest, during voluntary dorsiflexion, during plantar flexion, during external restraint to the ankle dorsiflexion movement, during limb cooling, during limb ischaemia and 1 h after the ingestion of tizanidine. The dorsiflexion movement of the foot was measured using an accelerometer. During cooling, ischaemia and after tizanidine, changes in the MLR were compared with changes in the soleus H reflex, Achilles tendon reflex and, during cooling, F waves of abductor hallucis. The MLR was facilitated by voluntary dorsiflexion, was suppressed during plantar flexion, disappeared when ankle movement was prevented, and was enhanced in patients with spastic hemiplegia. Cooling delayed the MLR significantly more than the Achilles tendon reflex and the abductor hallucis F wave. During ischaemia the response was significantly less affected than the Achilles tendon reflex and the soleus H reflex. Tizanidine suppressed the MLR, but not the soleus H and tendon reflexes. The latencies and the experiments using cooling, ischaemia and tizanidine implicate soleus group II afferents in the genesis of this response.

Therapeutic Cooling

Stretch reflexes contribute to joint stiffness, but the effects of therapeutic cooling on these reflexes are unknown. Therapeutic cooling is frequently used in knee rehabilitation, for instance, after rupture of the anterior cruciate ligament. Cooling a joint can affect nerve conduction velocity and the function of sensory organs. Such changes in neuromuscular coupling could reduce knee stability and increase the risk of knee injury. The aim of the present study was to evaluate whether there are negative effects of knee cooling on the hamstring short and medium latency reflex responses or on the anterior tibia trajectory after mechanically induced tibia translation. In 15 healthy volunteers, the latency and size of short latency and medium latency reflex responses of hamstring muscles were assessed before and after 20 min of cold therapy of the knee joint as applied in rehabilitation. Reflex responses were evoked by a mechanically induced posterior-anterior tibia translation during standing. Reflexes were recorded by surface electromyography. The distance of anterior tibia motion and its velocity were assessed by potentiometric position transducer. Local cold therapy of the knee did not alter the latency or the size of short or medium latency responses. Also, the extent and the velocity of tibia translation were unchanged after knee cooling. Cold therapy does not seem to adversely influence the spinal reflexes of the hamstrings induced by anterior tibia translation. As neuromuscular coupling was not significantly affected, the cold therapy, as performed in the present study, is not likely to increase the risk of knee injury.

Lack of On-Going Adaptations in the Soleus Muscle Activity During Walking in Patients Affected by Large-Fiber Neuropathy

The aim of this study was to investigate the contribution of feedback from large-diameter sensory fibers to the adaptation of soleus muscle activity after small ankle trajectory modifications during human walking. Small-amplitude and slow-velocity ankle dorsiflexion enhancements and reductions were applied during the stance phase of the gait cycle to mimic the normal variability of the ankle trajectory during walking. Patients with demyelination of large sensory fibers (Charcot-Marie-Tooth type 1A and antibodies to myelin-associated glycoprotein neuropathy) and age-matched controls participated in this study. The patients had absent light-touch sense in the toes and feet and absent quadriceps and Achilles tendon reflexes, indicating functional loss of large sensory fibers. Moreover, their soleus stretch reflex response consisted of a single electromyographic (EMG) burst with delayed onset and longer duration (P < 0.01) than the short- and medium-latency reflex responses observed in healthy subjects. In healthy subjects, the soleus EMG gradually increased or decreased when the ankle dorsiflexion was, respectively, enhanced or reduced. In the patients, the soleus EMG increased during the dorsiflexion enhancements; however, the velocity sensitivity of this response was decreased compared with the healthy volunteers. When the dorsiflexion was reduced, the soleus EMG was unchanged. These results indicate that the enhancement of the soleus EMG is mainly sensitive to feedback from primary and secondary muscle spindle afferents and that the reduction may be mediated by feedback from the group Ib pathways. This study provides evidence for the role of sensory feedback in the continuous adaptation of the soleus activity during the stance phase of human walking.

Galvanic and acoustic vestibular stimulation activate different populations of vestibular afferents.

To deduce whether similar or distinct populations of vestibular afferents are activated by acoustic and galvanic vestibular stimulation by comparing the effectiveness of 'matched' stimuli in eliciting vestibulospinal reflexes. Twelve subjects (5 men, 7 women) underwent individual 'matching' of 2 ms tone burst and galvanic stimuli, using vestibulocollic reflexes so that corrected reflex amplitudes to tone burst and galvanic stimuli were within 10% of each other. These same intensities were then administered using 20 ms durations to determine whether they were equally effective in evoking vestibulospinal responses. Corrected reflex amplitudes for vestibulocollic responses to tone burst and galvanic stimulation were not significantly different for the right (P=0.45) or left (P=0.68) sides. All subjects had vestibulospinal responses to galvanic stimulation (average intensity 4.0 mA for both sides). The short latency (SL) and medium latency (ML) components of the vestibulospinal reflexes were larger after galvanic compared to tone burst stimulation in 11 of 12 subjects (P<0.01). Despite evoking equal-sized vestibulocollic reflexes, there was a clear dissociation between the magnitude of tone burst and galvanic-induced vestibulospinal reflexes. Galvanic stimulation evoked SL and ML reflexes in all subjects. Tone burst stimuli evoked only small SL reflexes and, in most cases, no ML reflexes. Acoustically-evoked vestibulocollic reflexes are likely to be due to saccular excitation. The limited effectiveness of longer tone burst stimuli to evoke ML vestibulospinal reflexes suggests that saccular afferents have, at most, only a minor role in the production of these reflexes. We conclude that galvanic stimulation is more effective in eliciting vestibulospinal reflexes than tone burst stimulation, and that the two methods activate different populations of vestibular afferents.

Selective effects of ageing on vestibular-dependent lower limb responses following galvanic stimulation

Objectives: To examine the influence of ageing on electromyographic (EMG) responses to galvanic vestibular stimulation (GVS) in the lower limbs and to define normal values. Methods: EMG responses to 4 mA/20 ms transmastoid binaural GVS were recorded from the soleus muscles of 70 healthy adults aged 24–85 years. Short (SL) and medium latency (ML) vestibular-dependent reflex latencies and amplitudes were measured from averaged rectified EMG. Side to side differences were examined in 30 subjects, using monaural stimulation, and expressed as an asymmetry ratio (AR). Results: SL reflexes to transmastoid binaural stimulation were absent in 10 of the 25 subjects over the age of 60. ML responses were present in all subjects. SL reflex amplitudes decreased from 24.4±12.4% (mean±SD) in the 3rd decade to 7.6±6.2% in the 8th and 9th decades. The ML reflex amplitudes showed a tendency to increase with age. The mean AR for SL reflex amplitudes increased from 12.1% in the 3rd decade to 80.7% in the 8th and 9th decades but that for ML amplitudes did not change significantly. The average SL and ML response onset latencies were 56.5±7.5 and 97.2±9.4 ms. SL onset latencies were significantly delayed with age. Both SL and ML responses were highly reproducible between experiments. Conclusions: SL and ML responses behave differently in response to age. The decrease in SL reflex amplitudes correlates well with previously described age-related morphological changes. Preservation of the ML reflex may reflect central adaptation to reduced afferent input. SL and ML responses are potentially useful measures of vestibular-spinal function, which may have a role in the assessment of older subjects with dysequilibrium. These reflexes need to be interpreted in the context of age.

Group II muscle afferents probably contribute to the medium latency soleus stretch reflex during walking in humans.

1. The objective of this study was to determine which afferents contribute to the medium latency response of the soleus stretch reflex resulting from an unexpected perturbation during human walking. 2. Fourteen healthy subjects walked on a treadmill at approximately 3.5 km h(-1) with the left ankle attached to a portable stretching device. The soleus stretch reflex was elicited by applying small amplitude (approximately 8 deg) dorsiflexion perturbations 200 ms after heel contact. 3. Short and medium latency responses were observed with latencies of 55 +/- 5 and 78 +/- 6 ms, respectively. The short latency response was velocity sensitive (P < 0.001), while the medium latency response was not (P = 0.725). 4. Nerve cooling increased the delay of the medium latency component to a greater extent than that of the short latency component (P < 0.005). 5. Ischaemia strongly decreased the short latency component (P = 0.004), whereas the medium latency component was unchanged (P = 0.437). 6. Two hours after the ingestion of tizanidine, an alpha(2)-adrenergic receptor agonist known to selectively depress the transmission in the group II afferent pathway, the medium latency reflex was strongly depressed (P = 0.007), whereas the short latency component was unchanged (P = 0.653). 7. An ankle block with lidocaine hydrochloride was performed to suppress the cutaneous afferents of the foot and ankle. Neither the short (P = 0.453) nor medium (P = 0.310) latency reflexes were changed. 8. Our results support the hypothesis that, during walking the medium latency component of the stretch reflex resulting from an unexpected perturbation is contributed to by group II muscle afferents.

Vestibulospinal reflexes: quantitative effects of sensory feedback and postural task.

Vestibulospinal and vestibulocollic reflexes evoked by galvanic stimulation were studied in 20 normal volunteers. In an initial "baseline" study, subjects stood unsupported on a flat surface and a narrow base with their eyes shut and with their heads rotated to the left. The effects of vision, external support and increasing stance width were examined both individually and in combination. In a second series, the effects of the same factors were examined while subjects stood on a compliant surface. Short latency (SL) and medium latency (ML) vestibulospinal reflex responses were evoked using 4 mA/20 ms galvanic vestibular stimulation (GVS) and measured from the right soleus muscle. Vestibulocollic reflexes to short duration galvanic stimulation (4 mA/2 ms) were also measured under similar conditions. Both SL and ML vestibulospinal reflexes decreased significantly and to a similar degree with vision, external support and increasing stance width on a flat surface. On the compliant surface, stance width did not result in a decrease in the ML reflex. Reflex amplitudes further decreased in a non-linear fashion with each additional sensory modality ("factor") that was made available; the degree of attenuation due to the addition of a second and third factor closely approximated the product of the effect of each factor in isolation. Standing on a compliant surface resulted in enhancement of average SL and ML reflexes under all conditions. Vestibulocollic reflexes in contrast showed no significant modulation with vision, external support and stance width. The dissociation between vestibulocollic and vestibulospinal reflexes indicates that the modulation of vestibulospinal reflexes with task occurs proximal to the primary vestibular afferents. Vestibulospinal reflexes were largest when subjects stood on a narrow base, on a compliant surface, deprived of vision and external support, consistent with the importance of vestibular function under these conditions. Although attenuated, vestibulospinal reflexes were preserved in most subjects even when vision and external support were available and a wider stance width was adopted. The combination of different factors caused a multiplicative attenuation of the initial response.