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PubMed2001

Electromyographic reflex responses to mechanical force, manually assisted spinal manipulative therapy.

STUDY DESIGN: Surface electromyographic reflex responses associated with mechanical force, manually assisted (MFMA) spinal manipulative therapy were analyzed in this prospective clinical investigation of 20 consecutive patients with low back pain. OBJECTIVES: To characterize and determine the magnitude of electromyographic reflex responses in human paraspinal muscles during high loading rate mechanical force, manually assisted spinal manipulative therapy of the thoracolumbar spine and sacroiliac joints. SUMMARY OF BACKGROUND DATA: Spinal manipulative therapy has been investigated for its effectiveness in the treatment of patients with low back pain, but its physiologic mechanisms are not well understood. Noteworthy is the fact that spinal manipulative therapy has been demonstrated to produce consistent reflex responses in the back musculature; however, no study has examined the extent of reflex responses in patients with low back pain. METHODS: Twenty patients (10 male and 10 female, mean age 43.0 years) underwent standard physical examination on presentation to an outpatient chiropractic clinic. After repeated isometric trunk extension strength tests, short duration (<5 msec), localized posteroanterior manipulative thrusts were delivered to the sacroiliac joints, and L5, L4, L2, T12, and T8 spinous processes and transverse processes. Surface, linear-enveloped electromyographic (sEMG) recordings were obtained from electrodes located bilaterally over the L5 and L3 erector spinae musculature. Force-time and sEMG time histories were recorded simultaneously to quantify the association between spinal manipulative therapy mechanical and electromyographic response. A total of 1600 sEMG recordings were analyzed from 20 spinal manipulative therapy treatments, and comparisons were made between segmental level, segmental contact point (spinous vs. transverse processes), and magnitude of the reflex response (peak-peak [p-p] ratio and relative mean sEMG). Positive sEMG responses were defined as >2.5 p-p baseline sEMG output (>3.5% relative mean sEMG output). SEMG threshold was further assessed for correlation of patient self-reported pain and disability. RESULTS: Consistent, but relatively localized, reflex responses occurred in response to the localized, brief duration MFMA thrusts delivered to the thoracolumbar spine and SI joints. The time to peak tension (sEMG magnitude) ranged from 50 to 200 msec, and the reflex response times ranged from 2 to 4 msec, the latter consistent with intraspinal conduction times. Overall, the 20 treatments produced systematic and significantly different L5 and L3 sEMG responses, particularly for thrusts delivered to the lumbosacral spine. Thrusts applied over the transverse processes produced more positive sEMG responses (25.4%) in comparison with thrusts applied over the spinous processes (20.6%). Left side thrusts and right side thrusts over the transverse processes elicited positive contralateral L5 and L3 sEMG responses. When the data were examined across both treatment level and electrode site (L5 or L3, L or R), 95% of patients showed positive sEMG response to MFMA thrusts. Patients with frequent to constant low back pain symptoms tended to have a more marked sEMG response in comparison with patients with occasional to intermittent low back pain. CONCLUSIONS: This is the first study demonstrating neuromuscular reflex responses associated with MFMA spinal manipulative therapy in patients with low back pain. Noteworthy was the finding that such mechanical stimulation of both the paraspinal musculature (transverse processes) and spinous processes produced consistent, generally localized sEMG responses. Identification of neuromuscular characteristics, together with a comprehensive assessment of patient clinical status, may provide for clarification of the significance of spinal manipulative therapy in eliciting putative conservative therapeutic benefits in patients with pain of musculoskeletal origin.

Adolescent↗
PubMed1999

A simulation study of reflex instability in spasticity: origins of clonus.

Clonus is defined as an involuntary rhythmic muscle contraction that generally occurs in people who have sustained lesions involving descending motor pathways in the neuraxis, and is usually accompanied by other signs of reflex hyperexcitability such as spasticity. This paper hypothesizes that clonus arises when two conditions occur simultaneously: 1) the reflex pathway contains long delay times (implying innervation of distal limb muscles, exacerbated when these muscles display slow twitch properties) and 2) the excitability of the motoneurons is enhanced. This paper tested this dual hypothesis by developing a computer model representing the ankle reflex pathway. This model included the ankle muscles, afferent and efferent pathways, and a monosynaptic spinal link between spindle afferents and motoneurons. Simulations show that as the motoneuron current threshold was reduced (reflecting increased excitability of spinal motoneurons), normal reflex responses became unstable and oscillations developed similar to those observed in spastic patients. In parallel, when we choose reflex delay times typical for distal leg muscles in man, system stability is poor, and oscillations occur readily with increasing motoneuron excitability. As simulated pathway delays are reduced, oscillatory behavior is also reduced, and usually damps out. Conversely, as simulated reflex delays are increased, oscillations increase in amplitude and do not decay. Finally, these two phenomena interact, so that increasing motoneuron excitability will induce reflex oscillations for intermediate loop delays. These findings support the hypothesis that unstable oscillatory behavior, such as the oscillations observed in clonus, will occur when the motoneuron excitability increases in a reflex pathway containing long delays. This change in excitability is mediated by a reduction in motoneuron firing threshold, rather than by an increase in feedback gain. Furthermore, we demonstrate that sustained oscillations occur readily through self reexcitation, which reduces the need to propose that a "central oscillator" must be involved in generating clonus.

Algorithms↗
PubMed2000

Estimation of intrinsic and reflex contributions to muscle dynamics: a modeling study.

This work evaluated system identification-based approaches for estimating stretch reflex contributions to muscle dynamics. Skeletal muscle resists externally imposed stretches via both intrinsic stiffness properties of the muscle and reflexively mediated changes in muscle activation. To separately estimate these intrinsic and reflex components, system identification approaches must make several assumptions. We examined the impact of making specific structural assumptions about the intrinsic and reflex systems on the system identification accuracy. In particular, we compared an approach that made specific parametric assumptions about the reflex and intrinsic subsystems to another that assumed more general nonparametric subsystems. A simulation-based approach was used so that the "true" characters of the intrinsic and reflex systems were known; the identification methods were judged on their abilities to retrieve these known system properties. Identification algorithms were tested on three experimentally based models describing the stretch reflex system. Results indicated that the assumed form of the intrinsic and reflex systems had a significant impact on the stiffness separation accuracy. In general, the algorithm incorporating nonparametric subsystems was more robust than the fully parametric algorithm because it had a more general structure and because it provided a better indication of the appropriateness of the assumed structure.

Algorithms↗
PubMed1995

Evidence that enteric motility reflexes can be initiated through entirely intrinsic mechanisms in the guinea-pig small intestine.

Although motility reflexes can be elicited in the intestine in vivo after all neural connections with the central nervous system are cut, or in vitro in isolated intestinal segments, it is not proven that the cell bodies of the primary sensory neurons for these reflexes are in the intestinal wall. It is feasible that the nerve cells are in dorsal root ganglia and that axon reflexes are involved in the initiation of the reflexes. We have examined reflexes in segments of guinea-pig intestine in which extrinsic denervation, 9-11 days before the intestine was removed, and isolation of the intestine in vitro were combined. The experimental segments were isolated from extrinsic inputs by severing nerves in the mesentery and those running in the gut wall that entered the segment. The effectiveness of denervation was confirmed histochemically. Ascending and descending reflexes were evoked by mucosal distortion or distension and responses were recorded by intracellular microelectrodes in the circular muscle. Reflex responses recorded after denervation were no different to those recorded from control tissue. It is concluded that, in the small intestine of the guinea-pig, cell bodies of primary sensory neurons for mucosal and probably for distension reflexes are intrinsic to the organ.

Animals↗
PubMed2004

Comparison of the concentration-dependent effect of sevoflurane on the spinal H-reflex and the EEG in humans.

BACKGROUND: It has been shown that spinal reflexes such as the H-reflex predict motor responses to painful stimuli better than cortical parameters derived from the EEG. The precise concentration-dependence of H-reflex suppression by anaesthetics, however, is not known. Here we investigated this concentration-response relationship and the equilibration between the alveolar and the effect compartment for sevoflurane. METHODS: In 26 patients, the H-reflex was recorded at a frequency of 0.1 Hz while anaesthesia was induced and maintained with sevoflurane at increasing and decreasing concentrations. Population pharmacodynamic modelling was performed using the NONMEM software package, yielding population mean parameters as well as indicators of interindividual variability. RESULTS: Suppression of H-reflex amplitude occurred at lower concentrations (mean EC(50) 1.04 +/- 0.10 vol%, SE of NONMEM estimate) than the effect on either BIS or SEF(95) of the EEG (mean EC(50) 1.55 +/- 0.08 and 1.72 +/- 0.18 vol%, respectively), and exhibited a higher interindividual variability. The concentration-response function for the H-reflex was also steeper (mean ë 2.83 +/- 0.25). In addition, the equilibration between alveolar and effect compartment was slower for the H-reflex (mean k(e0) 0.15 +/- 0.01 min(-1)) than for BIS or SEF(95) (mean k(e0) 0.22 +/- 0.02 and 0.41 +/- 0.05 min(-1)). CONCLUSION: The differences in EC(50) and slope of the concentration-response relationships for H-reflex suppression and the EEG parameters point to different underlying mechanisms. In addition, the differences in time constant for equilibration between alveolar and effect compartment confirm the notion that immobility is caused at a different anatomic site than suppression of the EEG.

Adult↗
PubMedDate not supplied

Comparison of the reflex effects of arterial baroreceptors and cardiac receptors on the heart rate of conscious rabbits.

The reflex effects on heart rate (HR) of presumptive cardiac receptors have been differentiated from those of the arterial baroreceptors in conscious rabbits by inflating cuffs on the ascending aorta, descending aorta, inferior vena cava and pulmonary artery. When the outflow from the right ventricle was progressively impeded the accompanying increase in HR was entirely explicable by unloading of the arterial baroreceptors. As the outflow from the left ventricle was progressively impeded there was an initial increase in HR due to unloading of the arterial baroreceptors, followed by a progressive decline. This decline was attributed to a reflex arising from vagally-innervated receptors in the left side of the heart. The threshold of this cardiac receptor-HR reflex occurred at a higher level of mean aortic pressure than that at which the effects of the arterial baroreceptors on HR were maximal. Cholinergic (vagal) efferent nerves were responsible for two-thirds of the decline in HR caused by the reflex. The properties of the cardiac receptor-HR reflex were altered by drugs that affect myocardial contractility. Isoprenaline raised the mean aortic pressure at which the threshold occurred, but lowered the corresponding level of left ventricular end-diastolic pressure. Propranolol virtually abolished the reflex, even though left ventricular end-diastolic pressure reached a high level. It is concluded that in the conscious rabbit the arterial baroreceptor reflexes normally prevent the threshold of the cardiac receptor-HR reflex from being attained.

Animals↗
PubMed1997

Excitatory and inhibitory motor reflexes in the isolated guinea-pig stomach.

1. We have described and analysed the movements of the isolated stomach during distension by correlating intragastric pressure with video recordings, and investigated the presence of intrinsic inhibitory and excitatory reflexes. 2. Isolated guinea-pig stomachs, placed in an organ bath, were slowly distended with Krebs solution using a syringe pump via a cannula through the pylorus. The changes in intragastric pressure during cycles of distension were monitored by pressure transducers connected to both oesophageal and pyloric cannulae. The resistivity of the gastric wall (change in pressure with volume, delta P/delta V) and the amplitude and frequency of phasic pressure events were calculated from pressure recordings. 3. The movements of the stomach were also recorded onto videotape. The motion of the gastric wall during distension cycles was analysed to establish the patterns of contractions, their propagation and the distribution of fluid in the stomach. During filling, fluid was preferentially accommodated in the fundus. Propagating (peristaltic) contractions, often starting in the fundus, moved aborally towards the pylorus. The peak of the phasic pressure event was observed when a contraction reached the orad antrum. As it reached the pylorus, intragastric pressure was at its minimum. 4. During the initial phase of distension, intragastric pressure increased steeply. Tetrodotoxin and hyoscine reduced both the resistivity and amplitude of phasic pressure events. Hexamethonium had a similar effect. Thus distension appears to activate an excitatory reflex pathway, involving nicotinic ganglionic transmission. This reflex increases wall tension and enhances myogenic peristaltic contractions. 5. In control preparations, with larger distension volumes, the intragastric pressure decreased, despite the continued infusion of Krebs solution. L-NAME and apamin abolished this drop in pressure, indicating that gastric enteric inhibitory mechanisms prevail at larger distension volumes. After blockade of the excitatory reflex, hexamethonium antagonized the inhibitory response, indicating that activation of inhibitory mechanisms involves nicotinic transmission, probably on enteric inhibitory motoneurons. 6. Both the excitatory and inhibitory reflexes in the isolated stomach operate within a physiological range of gastric volumes. The excitatory reflex predominates at small distension volumes, leading to large phasic propagated contractions that mix the contents and may lead to emptying of the stomach. The inhibitory reflex, described previously as adaptive relaxation, can maximally relax the stomach and is activated preferentially at higher distension volumes to accommodate the contents. The interplay of these reflex pathways in the isolated stomach produces a rich repertoire of gastric movements. 7. The isolated stomach preparation, used with a combination of kinematic, kinetic and pharmacological methods, provides a highly suitable means of investigating the mechanisms of gastric motility.

Animals↗
PubMed1999

Release of nitric oxide within the coeliac plexus is involved in the organization of a gastroduodenal inhibitory reflex in the rabbit.

1. The coeliac plexus can organize a gastroduodenal inhibitory reflex without action potentials. The involvement of the nitric oxide-cGMP pathway in this reflex was investigated in the rabbit on an in vitro preparation of the coeliac plexus connected to the stomach and duodenum. Intraluminal duodenal pressures were measured with water-filled balloons. Gastric distension inhibited duodenal motility, thus characterizing a gastroduodenal inhibitory reflex organized by the coeliac plexus. 2. L-Arginine, superfused at the coeliac plexus level, enhanced this reflex, whereas Nomega-nitro-L-arginine (L-NOARG) or 2-(4-carboxyphenyl)-4,4,5,5 tetramethylimidazoline-1-oxyl-3-oxide (carboxy PTIO) reduced or abolished it. Moreover, diethylamine/nitric oxide complex superfused at the coeliac plexus level inhibited duodenal motility in the absence of gastric distension. 3. The effects of nitric oxide were mediated through the activation of guanylyl cyclase, as 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ) reduced or abolished the gastroduodenal inhibitory reflex, whereas zaprinast enhanced it. Moreover, 8-bromo-cGMP and cGMP, superfused at the coeliac plexus level, inhibited duodenal motility in the absence of gastric distension. 4. On the other hand, when perfused at the visceral level, L-NOARG, propranolol plus phentolamine, and guanethidine did not affect the reflex. Thus, neither nitric oxide nor noradrenaline could be the transmitters released at the muscular level to induce this reflex. 5. Our study demonstrates that the gastroduodenal inhibitory reflex, which is organized by the coeliac plexus without action potentials, is induced by the release within the plexus of nitric oxide acting on the cGMP pathway. These results provide new insights into the control of digestive motility by the prevertebral ganglia.

Action Potentials↗
PubMed2001

H-reflex modulation during passive lengthening and shortening of the human triceps surae.

1. The present study investigated the effects of lengthening and shortening actions on H-reflex amplitude. H-reflexes were evoked in the soleus (SOL) and medial gastrocnemius (MG) of human subjects during passive isometric, lengthening and shortening actions performed at angular velocities of 0, +/-2, +/-5 and +/-15 deg s(-1). 2. H-reflex amplitudes in both SOL and MG were significantly depressed during passive lengthening actions and facilitated during passive shortening actions, when compared with the isometric H-reflex amplitude. 3. Four experiments were performed in which the latencies from the onset of movement to delivery of the stimulus were altered. Passive H-reflex modulation during lengthening actions was found to begin at latencies of less than 60 ms suggesting that this inhibition was due to peripheral and/or spinal mechanisms. 4. It is postulated that the H-reflex modulation seen in the present study is related to the tonic discharge of muscle spindle afferents and the consequent effects of transmission within the Ia pathway. Inhibition of the H-reflex at less than 60 ms after the onset of muscle lengthening may be attributed to several mechanisms, which cannot be distinguished using the current protocol. These may include the inability to evoke volleys in Ia fibres that are refractory following muscle spindle discharge during rapid muscle lengthening, a reduced probability of transmitter release from the presynaptic terminal (homosynaptic post-activation depression) and presynaptic inhibition of Ia afferents from plantar flexor agonists. Short latency facilitation of the H-reflex may be attributed to temporal summation of excitatory postsynaptic potentials arising from muscle spindle afferents during rapid muscle lengthening. At longer latencies, presynaptic inhibition of Ia afferents cannot be excluded as a potential inhibitory mechanism.

Adult↗
PubMed2001

Transcranial magnetic stimulation and stretch reflexes in the tibialis anterior muscle during human walking.

Stretch of the ankle dorsiflexors was applied at different times of the walking cycle in 17 human subjects. When the stretch was applied in the swing phase, only small and variable reflex responses were observed in the active tibialis anterior (TA) muscle. Two of the reflex responses that could be distinguished had latencies which were comparable with the early (M1) and late (M3)components of the three reflex responses (M1, M2 and M3) observed during tonic dorsiflexion in sitting subjects. In the stance phase a single very large response was consistently observed in the inactive TA muscle. The peak of this response had the same latency as the peak of M3, but in the majority of subjects the onset latency was shorter than that of M3. The TA reflex response in the stance phase was abolished by ischaemia of the lower leg at the same time as the soleus H-reflex, suggesting that large muscle afferents were involved in the generation of the response. Motor-evoked potentials (MEPs) elicited in the TA by transcranial magnetic stimulation (TMS) were strongly facilitated corresponding to the peak of the stretch response in the stance phase and the late reflex response in the swing phase. A similar facilitation was not observed corresponding to the earlier responses in the swing phase and the initial part of the response in stance. Prior stretch did not facilitate MEPs evoked by transcranial electrical stimulation in the swing phase of walking. However, in the stance phase MEPs elicited by strong electrical stimulation were facilitated by prior stretch to the same extent as the MEPs evoked by TMS. The large responses to stretch seen in the stance phase are consistent with the idea that stretch reflexes are mainly involved in securing the stability of the supporting leg during walking. It is suggested that a transcortical reflex pathway may be partly involved in the generation of the TA stretch responses during walking.

Adult↗
PubMed2005

Reflex seizures in patients with malformations of cortical development and refractory epilepsy.

PURPOSE: Malformations of cortical development (MCDs) are usually highly epileptogenic, and their hyperexcitability could facilitate the occurrence of reflex seizures. We sought to characterize reflex seizures in patients with MCDs and refractory epilepsy. METHODS: Clinical, electrographic, and neuroimaging data were reviewed in eight patients with MCDs who had reflex seizures reproduced during presurgical evaluation. RESULTS: All eight patients had both reflex and spontaneous seizures. In six, however, drop attacks or axial myoclonic seizures occurred only upon specific sensory stimulation. Reflex seizures were induced by more than one type of stimulus in most patients, but anatomofunctional correlations could usually be invoked. Six patients had significant intellectual impairment. Surgical resection controlled seizures in two patients. CONCLUSIONS: Reflex seizures in patients with MCDs may be medically refractory and may often manifest as drop attacks or axial myoclonus. Surgical resection of focal lesions can bring reflex seizures under control. Putative mechanisms related to the relatively low frequency of reflex seizures in MCDs are discussed.

Adolescent↗
PubMed1994

Blinking and the release reflexes: are they clinically useful?

OBJECTIVE: To review the clinical utility of blinking and the release reflexes, palmomental, glabellar, grasp, and snout, as an important part of the bedside neurological examination. DESIGN: Articles published from 1966 to 1993 that addressed blinking or the release reflexes were identified by searching the MEDLINE database. Thirty-seven references provided clinically useful and applicable information. RESULTS: There is controversy regarding the optimal method of clinical elicitation of these reflexes. The release reflexes are frequently seen as an incidental abnormality in the elderly. A positive grasp reflex, especially if asymmetrical, is sensitive but not specific for abnormal brain function. Lack of habituation is more important than the mere presence or absence of a reflex. The brainstem circuits involved in the regulation of blinking and the release reflexes are discussed, as are their abnormalities in a variety of clinical settings. CONCLUSION: Blinking and the release reflexes have clinical utility in the diagnosis and bedside clinical assessment of diseases as diverse as frontal lobe lesions, hydrocephalus, multiple sclerosis, Parkinson's disease, Alzheimer's disease and other dementias, falls in the elderly, ageing, HIV-encephalopathy, schizophrenia, tumors in the region of the sylvian aqueduct and recovery from head trauma.

Blinking↗
PubMed1995

H-reflex modulation during gait in multiple sclerosis patients with spasticity.

The aim of the present study was to quantitate the Hoffmann (H-) reflex modulation in the soleus muscle of spastic multiple sclerosis patients during walking at different stimulus strengths, and to quantitate the influence of the excitation level of the soleus muscle on the H-reflex. The H-reflex modulation was quantified by the minimum H-reflex in the swing phase and the modulation index introduced by Yang et al. (1). Stimulus strengths were matched between subjects, and only stimuli intensities, which reflect modulation in the H-reflex with true neurophysiological origin, were used. Based on this approach, we found that the reflex modulation is impaired in moderate spastic patients at both a strong and a weak afferent input to the motoneuron pool in consistency with studies on severe spastic patients. This study concludes that the H-reflex modulation in the spastic patients is only related to the excitation level of the muscles, whereas both the excitation level and other factors than the excitation level are known to influence the H-reflex modulation in healthy subjects.

Adult↗
PubMed1977

Reflex elicitation thresholds in senile dementia.

Forty-eight female patients with the diagnosis of senile dementia and 20 elderly healthy women were studied concerning thresholds for orienting responses to intermittent light, the eyelid reflex, and hand withdrawal on electrical pain stimulation. A U-shaped relationship between degree of mental deterioration and reflex thresholds was predicted from two assumptions: (1) that reflex thresholds are progressively increased following the general blunting of cognitive processes, and (2) that active inhibition of certain overt behaviour takes place in patients with little deterioration (as well as in healthy people). The U-shaped relationship was only obtained for the eyelid reflex, and support was only obtained in connection with the orienting response for the assumption that some active inhibition of overt behaviour normally takes place. A further hypothesis was that thresholds for protective-defensive reflexes would not be substantially increased in the most deteriorated cases. This expectation was confirmed only for the hand withdrawal reflex. It is concluded that for the orienting response and for the eyelid reflex there is some relation, although the form is unclear and a rationale is lacking, between the thresholds and degree of mental deterioration. It is further concluded that the threshold for the hand withdrawal reflex is unrelated to the degree of mental deterioration. The inertia of the senile demented patients is stressed as well as their undifferentiated reactions. It is suggested that cognitive defects may partly explain these deficiencies.

Aged↗
PubMed1978

Observations on stretch reflexes in lumbar back muscles of the cat.

Reflex responses to brief muscle stretch and to electrical stimulation of dorsal roots were studied in cat longissimus and iliocostalis muscles by myographic and electromyographic recordings in spinal and anesthetized preparations. Brief stretch applied simultaneously to both muscles, by pulling at an isolated segment of the iliac bone, elicited contractions only in the central region of longissimus, composed of slowly contracting fibers, the time to peak tension varying between 70 and 100 ms. No reflex responses were observed in the faster contracting parts of this muscle or in the iliocostalis under the stimulation conditions used. The total reflex time usually varied from 4.5 to 7 ms which is shown to correspond to intra-spinal conduction times, approximately between 2 and 5 ms. Only exceptionally was a central reflex delay corresponding to a monosynaptic transmission observed. Reflexes evoked by dorsal root simulation (L4, L5) have a central conduction time similar to those elicited by adequate stimulation. The possibility that the central pathway for stretch reflexes in longissimus may involve more than two neurons is considered. In decerebrate preparations the central region of longissimus displays a prominent tonic stretch reflex. With the longissimus in situ the magnitude of the reflex is highly dependent on the position of the lumbar spine relative to the pelvic girdle.

Anesthesia↗
PubMed1987

Seventy years of the Bainbridge reflex.

The discovery of the Bainbridge reflex 70 years ago, of a tachycardic response to a rise in central venous pressure, stimulated a lot of interest in this and other cardiovascular reflexes. The mechanoreceptors that elicit the reflex are located at the junction of the right atrium and caval veins or at the junctions of the pulmonary veins and the left atrium. The Bainbridge reflex is controversial, however, because its existence cannot always be demonstrated. Intravenous infusions, which usually elicit a reflex tachycardia, sometimes cause a bradycardic response. This paper reviews the history of the studies associated with the reflex. Results are reported, which demonstrate that the chronotropic response to i.v. infusions depends upon the resulting change in aortic diameter; bradycardia is evoked by infusions leading to a rise in aortic baroreceptor activity through increases in aortic diameter, volume or pressure; tachycardia follows whenever the infusion fails to trigger the baroreflex. The importance of the Bainbridge reflex as a counterbalance to the baroreceptor reflex is discussed.

Animals↗
PubMed1990

Sympathetic activity influences the vascular axon reflex in the skin.

The interaction between changes of skin blood flow evoked by centrally mediated reflexes and local axon reflexes was studied in healthy subjects. Axon reflexes were evoked on the dorsum of the hand by transcutaneous electrical stimulation and reflex changes of blood flow by changes of ambient temperature, deep breath and emotional changes of blood flow by changes of ambient temperature, deep breath and emotional stress. Skin blood flow was measured by two laser-Doppler flowmeters, the probes of which were situated 6-8 mm from the stimulating electrode (monitoring net flow responses) and several centimetres away (monitoring generalized reflex responses only). The axon reflex responses were markedly diminished by body cooling but did not change during body warming. In warm subjects, a deep breath and emotional stress caused transient reductions of the flow response evoked by the electrical stimulation. Regional anaesthesia of the nerve(s) innervating the stimulated skin area led to marked increases of axon reflex responses in cold subjects, but no changes occurred in warm subjects. The anaesthesia also eliminated the transient flow reductions evoked by deep breaths and emotional stress. Since the applied stimuli are known to change skin sympathetic activity, it is concluded that sympathetic (presumably vasoconstrictor) impulses destined for the skin may reduce axon reflex responses.

Adult↗
PubMed1969

Evidence that the secondary as well as the primary endings of the muscle spindles may be responsible for the tonic stretch reflex of the decerebrate cat.

1. The size of the tonic stretch reflex of the soleus or gastrocnemius muscle of the decerebrate cat has been compared with the size of the reflex contraction elicited in the same muscle by high-frequency vibration applied to its tendon.2. On the assumption that vibration preferentially excites the primary endings of the muscle spindles it may be used to estimate the relation between the reflex response and the frequency of the Ia input to the spinal cord. On this basis, the increase in tension evoked by increasing extension is too great to be explained by the increase in Ia input with extension previously found on single fibre recording in comparable preparations.3. When vibration was superimposed on stretch reflexes elicited by different extensions, the size of the additional contraction elicited by the vibration remained approximately constant. If the stretch and vibration reflexes both depended entirely upon the Ia pathway, then occlusion between them would have been expected instead of the simple summation which was found.4. The absence of occlusion was not due to variation of the contractile strength of the muscle with its extension. This was shown by finding that the reflex contraction of soleus produced by stimulating the medial gastrocnemius nerve also remained the same size when elicited at different lengths of the muscle.5. The reflex effects were studied of superimposing alternate stretches and releases of 0.2 mm, on extensions of several mm. The small stretches elicited responses which were larger than expected from the response to large stretches, and which were approximately the same size at different mean lengths of the muscle.6. It is concluded that the tonic stretch reflex of the decerebrate cat cannot readily be explained solely by the increase in Ia discharge produced by stretching, as usually believed. Instead, it is suggested that the group II afferent fibres from the secondary endings of the muscle spindle also play an important part in its production.

Animals↗