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PubMed1998

Postexercise potentiation of the H-reflex in humans.

UNLABELLED: Post-muscle activation effects on segmental reflexes reveal divergent results dependent upon the manner in which the muscle is activated. Electrically activating triceps surae invokes a potentiation of the Achilles' tendon reflex and the soleus (S) H-reflex termed posttetanic potentiation. In contrast, brief volitional activation produces a subsequent potentiation of tendon reflexes, whereas H-reflexes become depressed. PURPOSE: The present investigation explored the effect of an intense bout of volitional resistance exercise on the S and lateral gastrocnemius (LG) H-reflexes to determine if a potentiation of the H-reflex could be induced with physiological stimuli. METHODS: LG and S H-reflexes were obtained from 10 college age men and women before and after a vigorous bout (eight sets of 10 repetitions) of concentric-eccentric triceps surae exercise. RESULTS: Every subject displayed an initial depression of the LG (P < 0.01) and S H-reflex (P < 0.05) immediately postexercise, consistent with postactivation depression. As a group, there was a significant (P > 0.01) potentiation of the LG H/M ratio following the depression. Five of 10 subjects demonstrated this potentiation, which often lasted 10 min postexercise. The other five subjects displayed a longer and more profound early depression followed by a return to control levels. CONCLUSION: The data suggest that at least two overlapping processes are occurring, a brief depression followed by or superimposed over a longer lasting potentiation. Possible neural mechanisms and implications to strength training are discussed.

Adult↗
PubMedDate not supplied

The cerebellum in maintenance of a motor skill: a hierarchy of brain and spinal cord plasticity underlies H-reflex conditioning.

Operant conditioning of the H-reflex, the electrical analog of the spinal stretch reflex, is a simple model of skill acquisition and involves plasticity in the spinal cord. Previous work showed that the cerebellum is essential for down-conditioning the H-reflex. This study asks whether the cerebellum is also essential for maintaining down-conditioning. After rats decreased the soleus H-reflex over 50 d in response to the down-conditioning protocol, the cerebellar output nuclei dentate and interpositus (DIN) were ablated, and down-conditioning continued for 50-100 more days. In naive (i.e., unconditioned) rats, DIN ablation itself has no significant long-term effect on H-reflex size. During down-conditioning prior to DIN ablation, eight Sprague-Dawley rats decreased the H-reflex to 57% (+/-4 SEM) of control. It rose after ablation, stabilizing within 2 d at about 75% and remaining there until approximately 40 d after ablation. It then rose to approximately 130%, where it remained through the end of study 100 d after ablation. Thus, DIN ablation in down-conditioned rats caused an immediate increase and a delayed increase in the H-reflex. The final result was an H-reflex significantly larger than that prior to down-conditioning. Combined with previous work, these remarkable results suggest that the spinal cord plasticity directly responsible for down-conditioning, which survives only 5-10 d on its own, is maintained by supraspinal plasticity that survives approximately 40 d after loss of cerebellar output. Thus, H-reflex conditioning seems to depend on a hierarchy of brain and spinal cord plasticity to which the cerebellum makes an essential contribution.

Animals↗
PubMed1983

Primitive reflexes in Down's syndrome.

The clinical value and prevalences of three primitive reflexes, i.e. the palmomental, the snout, and the corneomandibular, were examined in 30 institutionalized patients with Down's syndrome and the results were compared with those in a control group. The prevalence of palmomental reflex was clearly higher, i.e. 47% (31,63) in DS as compared to 7% (2,16) in controls (confidence limits, P = 0.9, is given in the parenthesis) (P = 0.0001). The prevalences of the snout reflex in the DS and control group were estimated to be 14% (5,29) and 0% (0,6) respectively (P = 0.02). The corneomandibular reflex was found unreliable in DS, mainly because of lack of cooperation. Neither the palmomental nor the snout reflex correlated well with the degree of mental retardation. No correlation of reflex incidence with age was demonstrated for the palmomental reflex. The prevalence of the snout reflex in DS patients less than 35 years was estimated to be 0% (0,18). This is significantly (P less than 0.05) less than the prevalence of 29% (10,54), found in the group older than 35 years of age. The possible value of the snout reflex as a sign of dementia in Down's syndrome is discussed.

Adult↗
PubMed2006

Suppression of the human spinal H-reflex by propofol: a quantitative analysis.

BACKGROUND: The spinal cord is an important site of anaesthetic action because it mediates surgical immobility. During anaesthesia with volatile anaesthetics, it has been shown that the suppression of the spinal H-reflex correlates with surgical immobility. To evaluate whether the H-reflex could also be a possible candidate for monitoring immobility during propofol anaesthesia, this study assessed the concentration-dependent suppression of the H-reflex by propofol. To discriminate different effect sites, the individual concentration response-curves and the t(1/2ke0) of the H-reflex have been compared with those of two EEG parameters. METHODS: In 18 patients, anaesthesia was induced and maintained with propofol infused using a target-controlled infusion pump at stepwise increasing and decreasing plasma concentrations between 0.5 and 4.5 mg/l. The H-reflex of the soleus muscle was recorded at a frequency of 0.1 Hz. Calculated propofol concentrations and H-reflex amplitude were analysed in terms of a pharmacokinetic-pharmacodynamic (PKPD) model with a sigmoid concentration-response function. RESULTS: For slowly increasing propofol concentrations, computer fits of the PKPD model for H-reflex suppression by propofol yielded the following median parameters: EC50 1.1 (0.8-1.7) mg/l, slope parameter 2.4 (2.0-3.7), and a t(1/2ke0) of 6.7 (2.8-7.5, 25-75% quantiles) min. For the bispectral index, the t(1/2ke0) was 2.2 (1.8-3.1) min and for the spectral edge frequency at the 95th percentile of the power spectrum 2.8 (1.9-3.2) min. CONCLUSIONS: Propofol, unlike sevoflurane, suppresses the spinal H-reflex at concentrations far lower than the C50 skin incision. The differences in t(1/2ke0)-values indicate the presence of different effect compartments for effects on the H-reflex and the EEG.

Analysis of Variance↗
PubMed1989

Evolution of skin reflexes in the first years of life.

The evolution of 28 skin reflexes was recorded. In a group of term infants (group A) the 14 limb reflexes disappeared by about 16 months of age and the 14 predominantly truncal reflexes persisted beyond that age. A second group of healthy infants (group B) were followed until the end of their third year. This study and the following cross-sectional study of healthy children from four to 14 years (group C) showed that the same 14 (limb) skin reflexes disappeared early at about the end of the first year. Another four limb reflexes disappeared during the second and third years. These 18 reflexes formed the group with 'early' disappearance. Nine trunk reflexes, mainly dorsal, disappeared between the fifth and 14th years: these formed the group with 'late' disappearance. Abdominal skin reflexes did not disappear. This information concerning time of disappearance is significant when assessing development and for prognosis, since the persistance or reappearance of reflexes which normally disappear indicates a pathological state.

Adolescent↗
PubMed1994

Investigation of the role of 5-HT3 and 5-HT4 receptors in ascending and descending reflexes to the circular muscle of guinea-pig small intestine.

1. The present study was undertaken to ascertain whether 5-hydroxytryptamine (5-HT) acting at either 5-HT3 or 5-HT4 receptors plays a significant role in motility reflexes in the guinea-pig small intestine. 2. An isolated segment of small intestine was opened along its mesenteric border and pinned, mucosa uppermost, in a three chambered organ bath so that the oral, middle and anal regions of a single preparation could be separately superfused. 3. Conventional intracellular recording methods were used to monitor the responses of the circular muscle in the oral or the anal end chambers when distension was applied in either of the other two chambers or the mucosal villi were compressed in the middle chamber. Drugs were added to the middle chamber. 4. 5-HT3 receptor antagonists (tropisetron, 0.1-10 microM; granisetron, 1 microM and BRL 46470, 1 microM) depressed the ascending excitatory reflex evoked by these stimuli but had no effect on the descending inhibitory reflex. The depression of the excitatory reflex was observed whether the reflex was evoked from the chamber containing the drug or was simply conducted, via interneurones, through this chamber. 5. The 5-HT4 receptor antagonist, SDZ 205-557 (1 microM), had no significant effect on either the ascending or descending reflex pathways. However, 5-HT4 receptors were present as cisapride (0.1 microM) significantly enhanced the ascending excitation without affecting the descending inhibition. This effect of cisapride was converted to a significant depression of the ascending reflex by SDZ 205-557. 6. The results suggest that 5-HT3, but not 5-HT4, receptors play an important role in the ascending excitatory reflex and that these receptors may be on interneurones in the reflex pathway.

4-Aminobenzoic Acid↗
PubMed1996

Spinal 5-HT-receptors and tonic modulation of transmission through a withdrawal reflex pathway in the decerebrated rabbit.

1. In decerebrated, non-spinalized rabbits, intrathecal administration of either of the selective 5-HT1A-receptor antagonists (S)WAY-100135 or WAY-100635 resulted in dose-dependent enhancement of the reflex responses of gastrocnemius motoneurones evoked by electrical stimulation of all myelinated afferents of the sural nerve. The approximate ED50 for WAY-100635 was 0.9 nmol and that for (S)WAY-100135 13 nmol. Intrathecal doses of the antagonists which caused maximal facilitation of reflexes in non-spinalized rabbits had no effect in spinalized preparations. 2. In non-spinalized animals, intravenous administration of (S)WAY-100135 was significantly less effective in enhancing reflexes than when it was given by the intrathecal route. 3. When given intrathecally, the selective 5-HT 2A/2C-receptor antagonist, ICI 170,809, produced a bellshaped dose-effect curve, augmenting reflexes at low doses (< or = 44 nmol), but reducing them at higher doses (982 nmol). Idazoxan, the selective alpha 2-adrenoceptor antagonist, was less effective in enhancing reflex responses when given intrathecally after ICI 170,809 compared to when it was given alone. Intravenous ICI 170,809 resulted only in enhancement of reflexes and the facilitatory effects of subsequent intrathecal administration of idazoxan were not compromised. 4. The selective 5-HT3-receptor blocker ondansetron faciliated gastrocnemius medialis reflex responses in a dose-related manner when given by either intrathecal or intravenous routes. This drug was slightly more potent when given i.v. and it did not alter the efficacy of subsequent intrathecal administration of idazoxan. 5. None of the antagonists had any consistent effects on arterial blood pressure or heart rate. 6. These data are consistent with the idea that, in the decrebrated rabbit, 5-HT released from descending axons has multiple roles in controlling transmission through the sural-gastrocnemius medialis reflex pathway. Thus, it appears 5-HT tonically inhibits transmission between sural nerve afferents and gastrocnemius motoneurones by an action at spinal 5-HT1A-receptors. Spinal 5-HT2A/2C-receptors may mediate a weak inhibition of transmission in the spinal cord, but more convincing evidence was obtained for their involvement in descending facilitatory tone. Further, some of the facilitatory consequences of spinal alpha 2-adrenoceptor blockade may be mediated through 5-HT2 type receptors. Spinal 5-HT3 receptors do not appear to have a major role in tonic modulation of the sural-gastrocnemius medialis reflex.

Animals↗
PubMed1975

Effect of central depressants on the acoustic middle ear reflex in rabbit. A method for quantitative measurements of drug effect on the CNS.

The respective effects of pentobarbital-sodium, enibumal-sodium, urethane, urethane-chloralose and lidocaine on the function of the acoustic middle ear reflex in the rabbit were studied. The response of the middle ear muscles was measured by recording changes in both ears' acoustic impedance when the reflex was elicited by applying pure tone stimuli (2,000 Hz) to the two ears one at a time. In that way both the crossed and the uncrossed reflexes were studied. All the drugs were found to depress the reflex in such a way that a higher sound intensity was required after administration to achieve the same impedance change as before. The effect of the anesthetics was roughly proportional to their known anesthetic power. Lidocaine produced only a slight depression of the reflex. The crossed reflex showed a greater susceptibility to the general anesthetics than did the uncrossed reflex which suggests a greater complexity of the crossed reflex. Because the method of recording the reflex response does not require any surgery and is equally well applicable in unrestrained rabbits and in humans, it is suggested as a way of testing the effect of drugs on the central nervous system.

Acoustic Stimulation↗
PubMed1969

Reflexes to sacral parasympathetic neurones concerned with micturition in the cat.

1. Reflexes to sacral parasympathetic neurones were studied by electrophysiological techniques in decerebrate, in chloralose-anaesthetized, and in chronic spinal cats.2. Excitatory reflexes from pelvic nerve and sacral somatic afferent nerve fibres were present before and after chronic transection of the spinal cord, but the latencies differed markedly. It was concluded that the long-latency reflexes observed when the spinal cord was intact involved long-loop reflexes to the brain-stem. The weak, short-latency reflexes in the chronic spinal cat were never observed when the spinal cord was intact and could be due to reorganized spinal connexions. The short-latency reflexes are probably unimportant in normal micturition.3. Stimulation of afferent fibres in the pelvic or sacral somatic nerves produced short-latency inhibitory post-synaptic potentials (IPSPs) and inhibition of discharges in parasympathetic neurones. This inhibition was due to a spinal reflex.4. A local reflex was demonstrated in the pelvic plexus. This was probably a cholinergic axon reflex, but the remote possibility that it was a local cholinergic reflex involving sensory neurones in the bladder wall has not been excluded.

Animals↗
PubMed1973

Evidence from the use of vibration during procaine nerve block that the spindle group II fibres contribute excitation to the tonic stretch reflex of the decerebrate cat.

1. Experiments have been performed to test the hypothesis that the group II fibres from the secondary endings of the muscle spindle provide an excitatory contribution to the tonic stretch reflex of the decerebrate cat. They have consisted of studying the effect of fusimotor paralysis by procaine, applied to the muscle nerve, on the reflex response to the combined stimuli of stretch (5-9 mm at 5 mm/sec) and of high-frequency vibration (100-150 Hz, 150 mum).2. The reflex response to the combined stimuli was found to be paralysed in two distinct stages which paralleled those of the ordinary stretch reflex described earlier. The two phases of paralysis may be attributed to an early paralysis of the gamma efferents followed by a later paralysis of the Ia afferents and alpha motor fibres. However, the Ia discharges elicited by the combined stimuli, unlike those elicited by simple stretch, should have remained unchanged on gamma efferent paralysis since the Ia firing frequency may be presumed to have been clamped at the vibration frequency by the occurrence of one-to-one ;driving'. The early reduction of the response to the combined stimuli may thus be attributed to the removal of a stretchevoked autogenetic excitatory input other than that long known to be provided by the Ia pathway. This supports the view that the spindle group II fibres have such an action, since their firing will be appropriately reduced on gamma efferent paralysis by removal of their pre-existing fusimotor bias; there is no evidence for the existence of any other group of fibres with the right properties.3. Recording of compound action potentials and of single units confirmed the great sensitivity of the gamma efferents to procaine but showed that the group II fibres were nearly as resistant as the Ia fibres and alpha motor fibres.4. The reliability of one-to-one driving of the Ia discharges by the vibration was tested in control experiments in which the reflex was elicited by an asymmetrical vibratory waveform with a rapid rising phase (1.5 or 1.9 msec at 140 Hz) and a slower falling phase. Recordings from single units showed that the use of this wave form greatly diminished any tendency to double driving (2 spikes/cycle of vibration) during the dynamic phase of stretch and never elicited it during the static phase of stretch when the reflex measurements were made. These ;pulsed' vibrations elicited reflex contractions which were of the same general size and which were paralysed in the same two phases by procaine as those elicited by sinusoidal vibrations. This eliminates the possibility that the early phase of paralysis might have been due to conversion of the pattern of Ia firing from double to single driving on gamma efferent paralysis.5. Wedensky inhibition of the afferent fibres could not be held responsible for the early phase of paralysis.6. The results are taken to strengthen the hypothesis that the spindle group II fibres contribute excitation rather than inhibition to the stretch reflex. The particular support derived from the present experiments is that all measurements of the size of the reflex at various times were made with the muscle at the same length so that the findings cannot be attributed to the tension-length properties of muscle. The detailed mechanism of the excitation, however, remains to be established and certain of the present findings suggest that it may not be a direct one.

Action Potentials↗
PubMed1979

Tremor in the tension developed isometrically by soleus during the tonic vibration reflex in the decerebrate cat.

1. Irregularities in the development of tension during the tonic vibration reflex of the soleus muscle of the decerebrate cat have been analysed into their frequency components. The reflex was recorded isometrically and elicited by longitudinal vibration, normally at 150 Hz. The amplitude of vibration was set so as to elicit a maximal reflex response, suggesting 1:1 driving of the majority of the Ia afferents at the frequency of vibration. 2. The resulting power spectrum regularly showed a well marked tremor peak separated by a trough from any slow irregularities. The predominant frequency of this tremor varied from 4 to 11 Hz in different preparations, with a mean of 7.4 Hz; on average, frequencies within 1.7 Hz on either side contained over half the power of the predominant frequency. Altering the frequency of vibration did not alter the distribution of tremor frequencies. 3. The root mean square value of the tension irregularities, over the range 4-14 Hz, varied from 12 to 110 mN in different preparations (median value, 23 mN); this was superimposed on mean active reflex tensions varying from 2 to 10 N. 4. The 'tremor' due to a single motor unit was estimated from spectral analysis of tetanic contractions of the whole muscle and decreased with increasing frequency of activation. Comparison of the single unit values with the tremor seen during vibration in the same preparations showed that equivalent amounts of tremor to the latter could typically have been produced by the continued synchronous contraction of about five 'average' motor units firing at the predominant tremor frequency. 5. When a tonic stretch reflex was present its tremor frequencies did not differ consistently from those of the tonic vibration reflex. On average, the tremor was smaller for the stretch reflex than for the tonic vibration reflex; the difference was usually slight and might have been related to the stretch refex tension being smaller. 6. Evidence was obtained that the tremor was not due to any insecurity of 1:1 driving of the Ia afferents by the vibration. First, the tremor did not increase when the amplitude of vibration was decreased sufficiently to ensure that the degree of 1:1 driving must have been reduced. Secondly, the introduction of a comparable 'artificial tremor' by sinusoidally oscillating the muscle at low frequency did not produce the e.m.g. response that would have been expected if the applied 'tremor' had been modulating the firing of the Ia or any other group of afferents. 7. It is concluded that the observed tremor cannot be attributed to 'oscillation in the stretch reflex arc', though without prejudice to the role of this mechanism under other conditions and especially when the recording is not isometric. However, the genesis of the tremor has not been established and much of it might result simply from the chance synchronization of motor units that are firing below their tetanic fusion frequency.

Action Potentials↗
PubMed1981

The effect of cerebellectomy on the tonic labyrinth and neck reflexes in the decerebrate cat.

1. Tonic labyrinth and neck reflexes were studied separately and in combination in the decerebrate cat before and after acute cerebellectomy. Reflex effects were observed as changes in the length of the medial head of triceps in both forelimbs. 2. In the decerebrate cat with an intact cerebellum the tonic labyrinth reflexes, elicited by side-up and side-down head rotations, produced asymmetric length changes in the medial head of triceps in both forelimbs, as described by Lindsay, Roberts & Rosenberg (1976). After cerebellectomy, head movements in either direction produced reflex shortenings in the medial triceps in both forelimbs, in contrast to the normal reciprocal reflex length changes. The presence of the cerebellum is thus required for the occurrence of the normal asymmetric labyrinth reflexes. 3. The direction of the neck reflexes remained unchanged by cerebellectomy. 4. It is suggested that the postural disturbances following cerebellar damage may partly result from the change in the form of the tonic labyrinth reflexes, which in combination with the tonic neck reflexes would no longer act in a stabilizing manner on the trunk.

Animals↗
PubMed1981

The effect of knee joint afferent discharge on transmission in flexion reflex pathways in decerebrate cats.

1. Changes in excitability of reflex arcs mediating flexion withdrawal ad crossed extensor reflexes have been examined in decerebrate cats. 2. The excitability of flexion withdrawal and crossed extensor reflexes was shown to be modulated by knee joint position. Flexion withdrawal reflexes were most easily elicited when the knee was extended and crossed extensor reflexes were most easily elicited when the knee was flexed. 3. The modulation of transmission was not confined to reflex pathways to muscles acting at the knee but also included pathways to muscles acting at the hip and ankle, as well as pathways to muscles in the contralateral limb. 4. The changing excitability of reflex pathways caused by movement of the knee joint was unrelated to the stretch applied to muscles acting at the knee and to cutaneous afferent discharge. Modulation of reflex excitability by joint movement was totally abolished by local anaesthesia of the knee joint in an otherwise intact limb. 5. The results of the present experiments indicate that transmission in flexion reflex pathways can be inhibited by knee joint afferent discharge.

Animals↗
PubMed1983

Reflex responses at the human ankle: the importance of tendon compliance.

Subjects with active stretch reflexes responded to an imposed sinusoidal movement of the ankle joint with a reflex force whose amplitude and timing varied widely with changes in the frequency of movement. At some frequency between 6 and 8 Hz, the reflex force tended to offset the non-reflex component of resistance, and thus to reduce the total resistance to movement. At this frequency the reflex response was particularly vigorous, with a deep modulation of electromyogram (e.m.g.) activity and a displacement of the joint stiffness vectors far from their high frequency values. The total resistance to movement might then be small, or it might be zero, or the reflex might actually assist the movement. As the frequency of movement was decreased through this critical range, the timing of the reflex response to movement changed rapidly with an abrupt advancement of the triceps surae e.m.g. signal, and a wide separation of the joint stiffness vectors as they passed close to the origin. This result was attributed to a changing distribution of the movement between the muscle fibres and an elastic Achilles tendon. It was assumed that at most frequencies the muscle fibres resisted extension, so that a major part of the imposed movement went into stretching the tendon; when, however, at 6-8 Hz, the reflex response was so timed as to reduce or abolish the resistance of the muscle fibres, more of the movement would take place in them. The muscle spindles would 'see' this larger movement of the muscle fibres, and generate correspondingly more reflex activity. A simplified model of the muscle-tendon combination behaves in a way that supports this view, and the available information about the human Achilles tendon indicates that it is sufficiently compliant for such an explanation. Therefore, movements imposed on the ankle joint would not necessarily be 'seen' by the muscle spindles, since they would be modified by transmission through a compliant tendon. By assuming a value for the tendon stiffness, it was possible to calculate the course of movements that actually occurred in the muscle fibres and spindles. Records of these spindle movements indicated how some non-linearities might arise.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1991

Dynamic control of muscle stiffness and H reflex modulation during hopping and jumping in man.

1. The objective of the study was to evaluate the functional effects of reflexes on muscle mechanics during natural voluntary movements. The excitability of the H (Hoffmann) reflex was used as a measure of the excitability of the central component of the stretch reflex. 2. We recorded EMG, ground reaction forces and the H reflex in the soleus muscle in humans while landing from a downward jump, during drop jumping and during hopping. The movements were also recorded by high-speed cinematography. 3. The EMG pattern was adapted to the motor task. When landing the EMG in the soleus muscle and in the anterior tibial muscle showed preinnervation and alternating activity after touch down. When hopping there was little preinnervation in the soleus muscle, and the activity was initiated about 45 ms after touch down by a peak and continued unbroken until lift off. In the drop jumps the EMG pattern depended on the jumping style used by the subject. 4. The H reflex in the soleus muscle was strongly modulated in a manner appropriate to the requirements of the motor task. During landing from a downward jump the H reflex was low at touch down whereas while hopping it was high at touch down. During drop jumping it was variable and influenced by the jumping technique. 5. Muscle stiffness in the ankle joint was negative after touch down when landing, but always positive when hopping. 6. It is suggested that during landing the alternating EMG pattern after touch down was programmed and little influenced by reflexes. During hopping reflexes could contribute to the initial peak and the EMG during lift off. 7. The programmed EMG activity and the suppression of the H reflex while landing probably contribute to the development of the negative stiffness and change the muscles from a spring to a damping unit.

Adult↗
PubMed1994

Stretch reflex responses in the human elbow joint during a voluntary movement.

1. The responsiveness of the stretch reflex is modulated during human voluntary limb movements. The influence of this modulation on the limb mechanical properties (stiffness) was investigated. 2. Subjects were taught to replicate accurately a rapid (4.0 rad s-1) targeted elbow flexion movement of 1 rad. From the onset of 12% of the trials a sinusoidal position disturbance (0.05 rad) was superimposed on the normal (trained) movement trajectory. The net joint torque (muscle torque) resisting these stretches was computed from measurements of applied torque, acceleration and limb inertia. Electromyographic (EMG) responses in the triceps brachii (TB), brachialis (Br) and biceps brachii (BB) were monitored. 3. The EMG responses to sinusoidal stretches applied early in the movement were less than those responses to perturbations applied when the arm neared the target (especially in the antagonist muscle TB). These EMG responses caused fluctuations in the resistance to the perturbation (stiffness), as described below. 4. When the perturbation frequency was low (< 4 Hz) the resistance of the elbow muscles to the stretch increased as the arm approached the target (48% increase). In contrast, when the stretch frequency was 7 Hz the resistance decreased by 63%. This decrease can be explained by the increased reflex response, since at 7 Hz the reflex response is probably timed so that it assists, rather than resists, the stretching as a result of loop delays. This reflex timing was confirmed by observing that, after abruptly stopping the sinusoidal stretch, the reflex response persisted for 100 ms and was indeed in a direction that would have reduced the resistance, had the perturbation continued. 5. The time course of the net muscle stiffness was estimated for frequencies ranging from 4 to 8 Hz and for each 40 ms interval a Nyquist plot was constructed, forming a C-shaped curve as frequency was varied. The size of this curve gave a measure of the stiffness resulting from reflex activity. When the arm neared the target this reflexive stiffness reached a maximum, and was probably comparable in size to the intrinsic (non-reflexive) muscle stiffness. Also, in four of the five subjects the viscous component of stiffness at 7 Hz dropped significantly below zero when the arm neared the target, again indicating that at this frequency the reflex was large and acted inappropriately.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1994

Flexibility of lower limb reflex responses to painful cutaneous stimulation in standing humans: evidence of load-dependent modulation.

1. In six human subjects standing without support, the reflex response of the tibialis anterior muscle (TA) was elicited by painful electrical stimulation (500 Hz, 20 ms) of the anterior sole of the foot and analysed by post-stimulus averages of rectified electromyography. The threshold intensity for the reflex response was very close to the subjective pain sensation (mean value, 1.05 times). Estimation of the afferent conduction velocity gave a mean value of 26.5 m s-1, suggesting that a contribution from A delta fibres was necessary to evoke the reflex response. The TA reflex response was then used as the pain test reflex. 2. Changes in the TA excitatory pain reflex response (elicited at 1.2 times the pain threshold) were investigated while the subjects maintained different postures in upright stance. Standing on the ipsilateral leg produced a significant decrease in the reflex response with respect to its value in symmetrical stance (standing on both legs), whereas a significant facilitation was observed when the subject was standing on the contralateral leg. A parallel depression of the response in both limbs was present when the subject maintained an upright stance with the lower limbs abducted. Thus, it was apparent that the TA pain responses decreased as the supporting function of the leg increased. 3. A significant inverse correlation between the load to which the limb was subjected and the size of the reflex response was observed in all subjects. We propose that the load to which the limb is subjected, measured from peripheral mechanoreceptors, is used as a measure of the current supporting function of the limb, on the basis of which the reflex is regulated.

Adult↗
PubMed2005

Phase-dependent and task-dependent modulation of stretch reflexes during rhythmical hand tasks in humans.

Phase-dependent and task-dependent modulation of reflexes has been extensively demonstrated in leg muscles during locomotory activity. In contrast, the modulation of reflex responses of hand muscles during rhythmic movement is poorly documented. The objective of this study was to determine whether comparable reflex modulation occurs in muscles controlling finger motions during rhythmic, fine-motor tasks akin to handwriting. Twelve healthy subjects performed two rhythmic tasks while reflexes were evoked by mechanical perturbations applied at various phases of each task. Electromyograms (EMGs) were recorded from four hand muscles, and reflexes were averaged during each task relative to the movement phase. Stretch reflexes in all four muscles were found to be modulated in amplitude with respect to the phase of the rhythmic tasks, and also to vary distinctly with the tasks being conducted. The extent and pattern of reflex modulation differed between muscles in the same task, and between tasks for the same muscle. Muscles with a primary role in each task showed a higher correlation between reflex response and background EMG than other muscles. The results suggest that the modulation patterns observed may reflect optimal strategies of central-peripheral interactions in controlling the performance of fine-motor tasks. As with comparable studies on locomotion, the phase-dependency of the stretch reflexes implies a dynamically fluctuating role of proprioceptive feedback in the control of the hand muscles. The clear task-dependency is also consistent with a dynamic interaction of sensory feedback and central programming, presumably adapted to facilitate the successful performance of the different fine-motor tasks.

Adult↗