Reflex In Man Research Articles

Can tendon reflexes be elicited by both stretch and vibration in man?

Aim of studyWhen the biceps tendon is tapped, a contraction is elicited in the biceps muscle. This also occurs with tapping of the radial bone, and it has been suggested that vibration is a stimulus for deep tendon reflexes. We investigated whether the normal stimulus for the deep tendon reflex is a sudden stretch, a phasic vibration, or both. Furthermore, we investigated the importance of forearm position for the reflex response in controls and stroke patients.MethodsWe investigated 50 neurological outpatients without clinical signs of neurological disorders in the arms. The biceps tendon and distal radius were tapped with the forearm in the midway (90°), supinated, and pronated positions. In 10 of these patients, the two reflexes were also investigated with quantitative electromyography (EMG) measurements in the 3 positions. Another 10 patients were investigated clinically when stretch of elbow was eliminated and 17 patients were examined when prestretching of the biceps tendon was avoided. Finally, we examined 32 patients that had experienced stroke.ResultsIn 94% (47/50) of patients, after a radial tap, the biceps contraction disappeared in the supinated forearm, and the median peak‐to‐peak amplitude of the surface EMG response (n = 10) decreased from 1.1 to 0.2 mV (p < .01). Elimination of elbow stretch as well as pressure on the biceps tendon did not change the reflex response. In 84% (27/32) of stroke patients, after a radial tap, the biceps contraction persisted in supination in the arm with hyperreflexia.ConclusionThe combined clinical and EMG results are consistent with the concept that the deep tendon reflexes in man can be elicited by both stretch and phasic vibration. Clinicians should be aware that the brachioradial reflex depends on the forearm position.

Influence of the 5‐HT3 receptor antagonist ondansetron on gastric sensorimotor function and nutrient tolerance in healthy volunteers

Serotonin is believed to be involved in the regulation of the gastric accommodation reflex in man however which receptor subtype(s) are involved remains to be elucidated. Eleven healthy subjects (nine men, age 19-30) underwent a gastric barostat and a drinking test after treatment with either placebo or ondansetron (8 mg intravenously). During the barostat protocol an intragastric flaccid bag was stepwise distended (2 mmHg increments 2 min) to determine gastric compliance and sensitivity to distention. Subsequently, the pressure level was set at intra-abdominal pressure +2 mmHg while volume was followed before and after administration of a liquid meal (200 mL; 300 kcal). During the drink test volunteers drank at a rate of 15 mL min(-1) until maximal satiation. Results (mean ± SEM) were compared using t-tests and mixed model analysis. Gastric compliance was not significantly altered by ondansetron (51.5 ± 5.6 vs 49.2 ± 5.2 mL mmHg(-1)), neither were the pressure thresholds for first perception or discomfort. Ondansetron treatment did not affect basal gastric tone (173 ± 14 vs 156 ± 12 mL), neither did it affect the amplitude of the meal-induced relaxation (160 ± 52 vs 131 ± 43 mL) or the maximum volume increase after the meal (264 ± 54 mL vs 234 ± 51 mL). During the drinking test the amount of liquid meal ingested at maximum satiation was significantly increased by ondansetron (784 ± 74 vs 907 ± 64 mL, P < 0.05). These data suggest that 5-HT acting at 5-HT(3) receptors is not involved in the control of gastric sensorimotor function, but contributes to the regulation of hunger and satiation in man.

The effect of bilateral vagal blockade on respiration

Although respiratory regulation has been extensively studied so far, the relation between the vagal nerves and respiratory reflex in man is still open to controversy. This paper describes the effect of bilateral vagal blockade on respiration. 1) Dyspnea due to unilateral bronchial occlusion. This type of dyspnea is caused by extreme depression of the intrabronchial pressure on the occluded side and is accompanied by a rapid increase in respiratory rate and a rapid decrease in tidal volume. This type of dyspnea is relieved by blockade of the ipsilateral cervical vagal nerve. 2) Animal experiments on bilateral lung transplantation, heart-lung transplantation and bilateral cervical vagal blockade. The respiratory changes induced by CO2 inhalation, low O2 inhalation, respiratory stimulant. Loss of blood or fever stimulation in the manipulated dogs were compared with those in normal controls. CO2 inhalation alone produced differences in the respiratory rates and tidal volumes between the two groups. The tidal volume increase under CO2 inhalation was suppressed by the inflation reflex but other afferent vagal nerves seemed to be closely associated with the increased respiratory rate. 3) Vagal nerve reflex in humans. Blockade of the bilateral cervical vagal nerves in an awake state resulted in irregular PETCO2 in cases that had been stable and sometimes produced a pattern of Cheyne-Stokes respiration. Inhalation of 5% CO2 restored the regular rhythm of respiration and enhanced the response to CO2. When subjected to bilateral vagal blockade in an awake state, the patient expressed an unbearbly uneasy dyspneic sensation as if he were pulled into a bottomless abyss. This was demonstrated by the unstable FRC level.

Acute changes in cerebral blood flow with change in portal pressure: evidence for the existence of a hepatocerebral reflex in man?: Rajiv Jalan, University College London Medical School, London United Kingdom; Steven W Olde Damink, Maastricht University, Maastricht Netherlands; Doris N Redhead, Peter C Hayes, Alistair Lee, Royal Infirmary, Edinburgh United Kingdom

Acute changes in cerebral blood flow with change in portal pressure: evidence for the existence of a hepatocerebral reflex in man?: Rajiv Jalan, University College London Medical School, London United Kingdom; Steven W Olde Damink, Maastricht University, Maastricht Netherlands; Doris N Redhead, Peter C Hayes, Alistair Lee, Royal Infirmary, Edinburgh United Kingdom

Vestibulo-autonomic control in man: Short- and long-latency vestibular effects on cardiovascular function

We examined the hypothesis that vestibular signals may exert a rapid control on the heart adjusting cardiovascular function to maintain homoestasis during changes in body posture. Short- and long-latency effects of vestibular stimulation on heart rate (HR), systolic blood pressure (SP), diastolic pressure (DP) and digital blood flow (BF) were studied in fourteen normal controls (NC) and nine labyrinthine-defective subjects (LDS) exposed to abrupt head accelerations. Subjects lay supine with their head suspended in a sling whose 'release' was triggered at delays of 170 ms and 570 ms after an R-spike of the ECG. Release caused the head to fall with an acceleration approximately 0.8 g for approximately 140 ms. Three additional NC underwent head drops at 170 ms, 370 ms, 470 ms and 570 ms to determine response latencies with precision. In NC, the short-latency response to head drops timed 170 to 470 ms after an R-spike was to shorten the time to the next R-spike in comparison to pre-drop heartbeat cycles. Drops at 570 ms delay shortened only the succeeding post-stimulus RR-interval. In LDS, head drops timed 170 ms after a heartbeat failed to shorten the ongoing cardiac cycle. For both delays only the succeeding cardiac cycle was significantly shortened. In all subjects, SP rose slightly by approximately 1-3% and BF decreased by 7-24% after 3-4 heartbeats post-drop. The results are evidence for an excitatory vestibulo-cardiac reflex in man which accelerates heart rate at a latency circa 500-600 ms. SP and BF are affected at longer latencies of several heart beats. A delayed increase of heart rate in response to postural challenge may contribute to the autonomic distress experienced by patients with vestibular loss.

Corticotropin-releasing factor modulates cardiovascular and pupillary autonomic reflexes in man: is there a link to inflammation-induced autonomic nervous hyperreflexia?

In two recent studies, we found autonomic nervous hyperreflexia in subjects with chronic inflammatory diseases, particularly, in those subjects with higher degrees of systemic inflammation. Since corticotropin-releasing factor (CRF) is induced by inflammatory stimuli and acts within the brain to change neuroendocrine and autonomic activity, we investigated CRF modulation of standard autonomic nervous reflexes. Fifteen healthy subjects were administered 100 microg CRF i.v., which led to a short-term increase of heart rate (p < 0.001) and a decrease in systolic and diastolic blood pressure (p < 0.050). These changes were accompanied by a reduction in heart rate variation at rest (p = 0.010) and during the respiratory sinus arrhythmia test (p = 0.019), and a reduction of pupillary latency time (p = 0.038). In further 21 normal subjects we studied the effect of prednisolone treatment on autonomic nervous function (100 mg/d during three days --> CRF reduction), which resulted in an increase of heart rate (p < 0.001), increase of heart rate variation during the respiratory sinus arrhythmia test (p < 0.001), increase in pupillary latency time (p = 0.012), a increase in maximal pupillary area (p = 0.002), and a decrease in diastolic blood pressure (p = 0.002). In conclusion, induction of a low central CRF content by prednisolone leads to a marked hyperreflexia in respiratory sinus arrhythmia and hyporeflexia in the latency time test. It is obvious that CRF can regulate autonomic reflexes. Possibly, central CRF status may also influence autonomic reflexes during chronic inflammation due to chronically changed central CRF concentration.

Site of triggering of the accommodation reflex in man

Site of triggering of the accommodation reflex in man

Characterization of the fundo-antral reflex in man

Characterization of the fundo-antral reflex in man

Mechanisms underlying the effects of remote noxious stimulation and mental activities on exteroceptive jaw reflexes in man

Successive inhibitory, excitatory, inhibitory and excitatory reflexes (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by applying non-painful taps to an incisor tooth, were recorded from the jaw-closing muscles of 15 subjects. The effects on these reflexes of the subjects undertaking mental exercises (MEx) in the form of arithmetic calculations were compared with those of remote noxious stimulation (RNS; application of 3°C to a hand). This was done to investigate whether the previously established effects of RNS were likely to be related to a change in the subject's mental state and/or to direct nociceptive mechanisms. Both MEx and RNS caused increases in EMG activity around the Q–R and S–T transitions of the PSEC, which resulted principally from shortenings of the inhibitory Q and S waves. Reducing the intensity of the tap stimuli, which mimicked condition-induced disinhibition, caused shortenings of the inhibitory waves at latencies similar to the shortenings induced by MEx or RNS. The magnitude of the RNS-induced effect on the ST segment of the PSEC was greater (P<0.01) than that on the QR segment. By contrast, MEx induced similar effects on both segments. Regression analyses were performed for the relationship between condition-induced changes in amplitude of the excitatory waves and their control amplitudes. These analyses were performed to reveal any condition-induced inhibition or facilitation of the tap-induced influences on the motoneurons. Overall, the evidence suggested that: (1) mental exercise induced a similar degree of inhibition of the two tap-induced inhibitory jaw reflexes and a facilitation of the excitatory ones, and (2) remote noxious stimulation induced an inhibition of the second tap-induced inhibitory reflex which was greater than that of the first one, and an inhibition of the first excitatory reflex. Thus, although factors related to altered mental activity could play a role in the modulation of jaw reflexes by RNS, the differences between the effects of MEx and RNS suggest that alternative or complementary mechanisms are also likely to be involved.

Differentiating condition-induced facilitation, inhibition and disinhibition in a complex series of reflexes in an electromyogram

In man, the principal exteroceptive reflexes evoked by intra-oral stimulation involve the jaw-closing muscles and include inhibitory and excitatory responses [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117–125.]. These reflexes can be observed in electromyograms (EMGs) recorded with bipolar surface electrodes. The likelihood that these reflexes play important roles in the integrative actions of the jaw has led to interest in the physiological control mechanisms by which they may be modulated. It has been reported recently that the complex series of jaw reflexes evoked by non-painful tapping on human teeth can be modulated by the application of noxious stimulation to the hand [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, Effects of remote noxious stimulation on exteroceptive reflexes in human jaw closing muscles, Brain Res. 726 (1996) 189–197.] or by exercises which produce a change in mental state [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, The influence of attentional factors on short- and long-latency jaw reflexes in man, Arch. Oral Biol. 41 (1996) 995–998.]. The effects of remote noxious stimuli and mental exercises usually involved transient increases in electromyographic (EMG) activity around the interfaces between the successive inhibitory and excitatory reflexes. As the mechanisms underlying the tap-induced inhibitory and excitatory reflexes may show some temporal overlap [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117–125.], these condition-induced increases in EMG activity could, in the simplest hypothesis, have been due to either (i) a condition-induced inhibition of the tap-induced inhibitory influences on the motoneurones (i.e., disinhibition) and/or (ii) a condition-induced facilitation of the tap-induced excitatory influences underlying the subsequent excitatory reflexes. In the present protocol, we describe how it is possible to differentiate between these different underlying mechanisms. The method includes a regression analysis of the relationship between condition-induced changes in amplitude of a reflex and the reflex amplitude under control conditions after taking account of the effect of chance. The analysis is applied on reflex data pooled from various subjects. Although this method of data analysis is illustrated with trigeminal reflexes, it is potentially of use for other complex extracellular recordings including those in other fields of motor control (e.g., EMGs from muscles other than jaw ones). Themes: Motor systems and sensorimotor integration Topics: Reflex function

The non-competitive N-methyl- d-aspartate-antagonist memantine does not affect segmental mono- and polysynaptic reflexes in man

Studies in rats have shown that the polysynaptic flexor reflex (FR) but not the monosynaptic reflexes are affected by N-methyl- d-aspartate (NMDA) receptor antagonists. Theoretically, the suppression of FR might be caused by an alteration of the spinal nociceptive neurons. To investigate, whether the non-competitive NMDA receptor antagonist memantine interferes with nociception in man, we studied both its effect on pain perception and on FR. In a double-blind study 14 male subjects were randomly assigned to either placebo or memantine (30 mg p.o.) treatment. H-reflex (HR) and FR as well as pain and tolerance threshold were determined prior to and 6 h after drug intake. Contrary to expectations, there were no differential treatment effects either on FR threshold and magnitude or on pain and tolerance thresholds or the HR amplitude.

Facilitation of a nociceptive flexion reflex in man by nonnoxious radiant heat produced by a laser.

Electromyographic recordings were made in healthy volunteers from the knee-flexor biceps femoris muscle of the nociceptive RIII reflex elicited by electrical stimulation of the cutaneous sural nerve. The stimulus intensity was adjusted to produce a moderate pricking-pain sensation. The test responses were conditioned by a nonnoxious thermal (</=40 degrees C) stimulus applied to the receptive field of the sural nerve. This stimulus was delivered by a CO2 laser stimulator and consisted of a 100-ms pulse of heat with a beam diameter of 20 mm. Its power was 22.7 +/- 4.2 W (7.2 mJ/mm2), and it produced a sensation of warmth. The maximum surface temperature reached at the end of the period of stimulation was calculated to be 7 degrees C above the actual reference temperature of the skin (32 degrees C). The interval between the laser (conditioning) and electrical (test) stimuli was varied from 50 to 3, 000 ms in steps of 50 ms. It was found that the nociceptive flexion reflex was facilitated by the thermal stimulus; this modulation occurred with particular conditioning-test intervals, which peaked at 500 and 1,100 ms with an additional late, long-lasting phase between 1,600 and 2,300 ms. It was calculated that the conduction velocities of the cutaneous afferent fibers responsible for facilitating the RIII reflex, fell into three ranges: one corresponding to A delta fibers (3.2 m/s) and two in the C fiber range (1.3 and 0.7 m/s). It is concluded that information emanating from warm receptors and nociceptors converges. In this respect, the present data show, for the first time, that in man, conditioning nonnociceptive warm thermoreceptive A delta and C fibers results in an interaction at the spinal level with a nociceptive reflex. This interaction may constitute a useful means whereby signals add together to trigger flexion reflexes in defensive reactions and other basic motor behaviors. It also may contribute to hyperalgesia in inflammatory processes. The methodology used in this study appears to be a useful noninvasive tool for exploring the thermoalgesic mechanisms in both experimental and clinical situations.

Reduction in renal blood flow following acute increase in the portal pressure: evidence for the existence of a hepatorenal reflex in man?

To investigate the relation between changes in portal haemodynamics and renal blood flow (RBF) in patients with cirrhosis. Twenty patients with cirrhosis and transjugular intrahepatic portosystemic stent-shunts were divided into two groups which were well matched. At routine portography, either changes in unilateral RBF (group I) or changes in cardiac output (group II) before and after shunt occlusion were studied. Blood was obtained from the renal and systemic circulations for the measurement of neurohumoral factors before and after shunt occlusion in group I patients. After shunt occlusion, there was a progressive reduction in unilateral RBF from a mean (SD) of 289 (32) to 155 (25) (-43.5%) (p < 0.001). These changes correlated significantly with the changes in the portal atrial gradient (p < 0.001). There was no significant change in heart rate, mean arterial pressure and right atrial pressure. No significant changes were found in the concentrations of the various neurohumoral factors measured. There was a less notable but significant reduction in the cardiac output (-10.9%) (p = 0.02) unaccompanied by significant reduction in the pulmonary capillary wedge pressure or mean arterial pressure. These results suggest the existence of hepatorenal reflex in man which is important in the regulation of RBF, although other mechanisms may also be contributory.

Effects of remote deep somatic noxious stimuli on a jaw reflex in man

The effects on an inhibitory jaw reflex of activating deep somatic afferent nerves in a remote part of the body (the arm) were studied in 13 humans. Electromyographic recordings were made from the active masseter of the long-latency (mean 42.0 ± 1.1 ms) inhibitory reflex evoked by electrical stimulation of the upper lip. Immediately after a 1-min conditioning period during which the participants compressed a hand-held spring once a second while ischaemia was produced in the arm with an inflated pneumatic cuff, the magnitude of the inhibitory reflex decreased significantly (by 43%). The reflex recovered within 5 min to a magnitude that was not significantly different from its pre-conditioning value. The arm exercise or the ischaemia alone produced no significant changes in the reflex. Furthermore, neither of these last two conditions was reported to be painful, whereas the ischaemic exercise produced pain in all but one participant. It is concluded that activation of remote nociceptive, but not non-nociceptive, deep somatic nerves can modulate jaw reflexes in man.

The effects of shounousui on the three responses of the blink reflex in man.

The effects of shounousui on the three responses of the blink reflex in man.

The influence of attentional factors on short- and long-latency jaw reflexes in man

Reflexes evoked by applying non-painful taps to an incisor tooth were recorded bilaterally from the jaw-closing masseter and temporal muscles of 21 humans. A series of inhibitory, excitatory, inhibitory and excitatory waves (the Q, R, S and T waves) appeared in full-wave rectified and averaged post-stimulus electromyograms. These reflex responses were affected by the participants' levels of attention. When they undertook mental exercises in the form of arithmetic calculations, increases in electro-myographic activity were found around the transitions between the Q and R and the S and T waves. These increases involved principally a shortening of the inhibitory Q and S waves. There was no significant difference between the occurrence of these effects in the QR and ST segments. However, the effects were seen more commonly when the reflexes were evoked by hard (7.4 mN.s) as opposed to soft (3.4 mN.s) taps. It is concluded that, in man, attentional factors can modulate both short- and long-latency jaw reflexes, particularly when these are evoked by higher-threshold afferent nerves.

Suppression of an inhibitory jaw reflex by the anticipation of pain in man

Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 15 human subjects. In control sequences, the reflex had a mean latency and duration (± S.E.M.) of 45.4 ± 1.3 msec and 47.9 ± 2.8 msec, respectively. Significant decreases in the reflex as well as increases in heart rate and anxiety levels assessed by a visual analogue scale, occurred when the subjects were stressed by the anticipation of receiving painful electrical stimuli above the ankle (P < 0.00005; Student's t-tests). During such sequences, the magnitude of the reflex measured by integration of the EMG, was reduced by 47.7 ± 5.6%. This effect involved a reduction in both the duration and depth of the inhibitory wave. It occurred regardless of whether the painful stimuli were applied during or after the recording of the reflex and of whether the baseline activity in the muscle was inadvertently raised or lowered during the stressful sequences. It is concluded that stress induced by the anticipation of pain, can markedly reduce an inhibitory jaw reflex in man by exerting an influence on the reflex pathway prior to the motoneurones.

Reflex inhibition following electrical stimulation over muscle tendons in man.

Electrical stimulation over selected muscle tendons in alert human subjects produced, in each muscle, a reflex inhibition of muscle activity. This inhibition, when maximal, was seen in the surface EMG as an interval of complete electrical silence during a sustained voluntary contraction. The inhibition was clearly visible in single sweeps and in averaged records. Its onset latency and duration were respectively, 56 +/- 4.9 and 46 +/- 11.8 ms in extensor digitorum communis, 71 +/- 6.1 and 46 +/- 10.5 ms in extensor pollicis brevis, 77 +/- 11.2 and 47 +/- 10.5 ms in extensor pollicis longus, 72 +/- 7.3 and 43 +/- 8.6 ms in abductor digiti minimi, and 97 +/- 3.5 and 43 +/- 2.8 ms in tibialis anterior. The inhibitory response was produced at low stimulus intensities (< 10 mA) without electrical (M wave) or mechanical (muscle twitch) signs of direct muscle stimulation. It therefore did not arise from stimulation of la afferents (muscle spindles). The response arose from tendons since it occurred at lowest threshold when stimulation was applied directly over the tendons of the five different muscles studied. At low stimulus intensities, the response declined sharply when the stimulating electrodes were moved to the skin immediately adjacent to the tendons. The response did not arise from skin afferents since it was also presented when stimuli were delivered to the tendon by subcutaneous needle electrodes and it was not reproduced by stimulation of cutaneous nerves in the region of the tendon. In another series of experiments on extensor pollicis brevis, five skin locations were stimulated while overlying the tendon and again while the skin was stretched so that they were lying 0.6-0.8 cm dorsal to the tendon. In these experiments the response was again greatly attenuated when the stimulation was not directly over the tendon, although the same cutaneous sites were stimulated. The inhibition was followed by a pronounced excitatory component (E1) of peak latency 120-140 ms. The results of the study provide evidence for a powerful autogenic inhibitory reflex in man. The evidence is consistent with the possibility that the response arises from Golgi tendon organ afferents.

Early nuchal reflexes in man

Concurrent transcranial magnetic stimulation and electroencephalography (TMS–EEG) is an emerging method for studying cortical network properties. However, various artifacts affect measurement of TMS-evoked cortical potentials (TEPs), especially within 30 ms of stimulation.The aim of this study was to assess the origin and recovery of short-latency TMS–EEG artifacts (<30 ms) using different stimulators and under different experimental conditions.EEG was recorded during TMS delivered to a phantom head (melon) and 12 healthy volunteers with different TMS machines, at different scalp positions, at different TMS intensities, and following paired-pulse TMS. Recovery from the TMS artifact and other short-latency artifacts were compared between conditions.Following phantom stimulation, the artifact resulting from different TMS machines (Magstim 200, Magventure MagPro R30 and X100) and pulse shapes (monophasic and biphasic) resulted in different artifact profiles. After accounting for differences between machines, TMS artifacts recovered within ∼12 ms. This was replicated in human participants, however a large secondary artifact (peaks at 5 and 10 ms) became prominent following stimulation over lateral scalp positions, which only recovered after ∼25–40 ms. Increasing TMS intensity increased secondary artifact amplitude over both motor and prefrontal cortex. There was no consistent modulation of the secondary artifact following inhibitory paired-pulse TMS (interstimulus interval = 100 ms) over motor cortex.The secondary artifact observed in humans is consistent with activation of scalp muscles following TMS. TEPs can be recorded within a short period of time (10–12 ms) following TMS, however measures must be taken to avoid muscle stimulation.

The effects of unilateral and bilateral chewing, empty clenching and simulated bruxism, on the masticatory-parotid salivary reflex in man.

In man, parotid flow has been recorded, using modified Lashley cups, in response to mechanical stimulation of the teeth by repeated chewing and clenching manoeuvres. The rectified and integrated masseter electromyographic activity was used as an indirect measure of the stimulus intensity where necessary. Following the observation that "empty clenching' failed to evoke a parotid salivary response, a series of three experiments has been undertaken to address the following questions. (1) Is a lateral component of force required to evoke a flow during empty clenching? (2) Does contralateral inhibition of salivary secretion explain this observation? (3) What is the threshold for the masticatory-salivary reflex? The results suggest that: (i) empty clenching together with a lateral component of force does not result in a secretion above resting flows; (ii) there is no evidence for the existence of contralateral inhibition of salivary secretion; and (iii) the threshold for the masticatory-salivary reflex is lower than 5% of comfortable chewing forces. We are still unable to offer an explanation for the lack of parotid secretion during empty clenching.