Effectiveness of Focal Muscle Vibration in the Recovery of Neuromotor Hypofunction: A Systematic Review

Adequate physical recovery after trauma, injury, disease, a long period of hypomobility, or simply ageing is a difficult goal because rehabilitation protocols are long-lasting and often cannot ensure complete motor recovery. Therefore, the optimisation of rehabilitation procedures is an important target to be achieved. The possibility of restoring motor functions by acting on proprioceptive signals by unspecific repetitive muscle vibration, focally applied on single muscles (RFV), instead of only training muscle function, is a new perspective, as suggested by the effects on the motor performance evidenced by healthy persons. The focal muscle vibration consists of micro-stretching-shortening sequences applied to individual muscles. By repeating such stimulation, an immediate and persistent increase in motility can be attained. This review aims to show whether this proprioceptive stimulation is useful for optimising the rehabilitative process in the presence of poor motor function. Papers reporting RFV effects have evidenced that the motor deficits can be counteracted by focal vibration leading to an early and quick complete recovery. The RFV efficacy has been observed in various clinical conditions. The motor improvements were immediate and obtained without loading the joints. The review suggests that these protocols can be considered a powerful new advantage to enhance traditional rehabilitation and achieve a more complete motor recovery.

Focal Muscle Vibration Improves Gait in Parkinson's Disease: A Pilot Randomized, Controlled Trial

Background

In Parkinson's disease (PD), gait disorders lead to increased risk of falls and patients' reduced participation and independence. Several observations suggest that a single session of focal muscle vibration (fMV) applied to trunk or lower limb muscles during gait may improve several gait variables in patients with PD. The possible long-term beneficial effects of repetitive sessions of fMV (r-fMV) on gait of patients with PD have been investigated.

Methods

A randomized, controlled trial study has been conducted in an outpatient rehabilitation department. Twenty patients with PD diagnosis have been randomized in two groups: "real" or "sham" r-fMV application to quadriceps and paraspinal muscles in patients with PD. Gait was evaluated with objective gait analysis, and a number of variables, including velocity, step length, stride length, percentage of stance, double support duration, cadence, swing velocity, and step width, have been measured. Gait analysis was performed before and 24 hours and 1 and 3 weeks after r-fMV.

Results

After real, but not sham, r-fMV, patients with PD had significant gait improvement as a result of increased walking velocity and stride length. The r-fMV-induced beneficial after effects lasted at least 1 week after the end of stimulation.

Conclusions

Data emerging from our pilot randomized, controlled trial study suggest that r-fMV may improve gait disorders in patients with PD. r-fMV might be a feasible, safe approach for possibly improving gait disorders in patients with PD and might enhance the impact of specific rehabilitation programs in PD.

Focal muscle vibration as a possible intervention to prevent falls in elderly women: a pragmatic randomized controlled trial

BACKGROUNDS

Different and new approaches have been proposed to prevent the risk of falling of elderly people, particularly women.

AIMS

This study investigates the possibility that a new protocol based on the focal mechanical muscle vibration may reduce the risk of falling of elderly women.

METHODS

A pragmatic randomized controlled triple-blind trial with a 6-month follow-up after intervention randomized 350 women (mean age 73.4 years + 3.11), members of local senior citizen centers in Rome, into two groups: vibrated group (VG) and control group (CG). For VG participants a mechanical vibration (lasting 10 min) was focally applied on voluntary contracted quadriceps muscles, three times a day during three consecutive days. CG subjects received a placebo vibratory stimulation. Subjects were tested immediately before (T0) and 30 (T1) and 180 (T2) days after the intervention with the Performance-Oriented Mobility Assessment (POMA) test. All subjects were asked not to change their lifestyle during the study. CG underwent sham vibratory treatment.

RESULTS

While CG did not show any statistically significant change of POMA at T1 and T2, VG revealed significant differences. At T2, ≈47% of the subjects who completed the study obtained the full score on the POMA test and ≈59% reached the full POMA score.

CONCLUSIONS

The new protocol seems to be promising in reducing the risk of falling of elderly subjects.

Long-lasting effects of neck muscle vibration and contraction on self-motion perception of vestibular origin

OBJECTIVE

To show that neck proprioceptive input can induce long-term effects on vestibular-dependent self-motion perception.

METHODS

Motion perception was assessed by measuring the subject's error in tracking in the dark the remembered position of a fixed target during whole-body yaw asymmetric rotation of a supporting platform, consisting in a fast rightward half-cycle and a slow leftward half-cycle returning the subject to the initial position. Neck muscles were relaxed or voluntarily contracted, and/or vibrated. Whole-body rotation was administered during or at various intervals after the vibration train. The tracking position error (TPE) at the end of the platform rotation was measured during and after the muscle conditioning maneuvers.

RESULTS

Neck input produced immediate and sustained changes in the vestibular perceptual response to whole-body rotation. Vibration of the left sterno-cleido-mastoideus (SCM) or right splenius capitis (SC) or isometric neck muscle effort to rotate the head to the right enhanced the TPE by decreasing the perception of the slow rotation. The reverse effect was observed by activating the contralateral muscle. The effects persisted after the end of SCM conditioning, and slowly vanished within several hours, as tested by late asymmetric rotations. The aftereffect increased in amplitude and persistence by extending the duration of the vibration train (from 1 to 10min), augmenting the vibration frequency (from 5 to 100Hz) or contracting the vibrated muscle. Symmetric yaw rotation elicited a negligible TPE, upon which neck muscle vibrations were ineffective.

CONCLUSIONS

Neck proprioceptive input induces enduring changes in vestibular-dependent self-motion perception, conditional on the vestibular stimulus feature, and on the side and the characteristics of vibration and status of vibrated muscles. This shows that our perception of whole-body yaw-rotation is not only dependent on accurate vestibular information, but is modulated by proprioceptive information related to previously experienced position of head with respect to trunk.

SIGNIFICANCE

Tonic proprioceptive inflow, as might occur as a consequence of enduring or permanent head postures, can induce adaptive plastic changes in vestibular-dependent motion sensitiveness. These changes might be counteracted by vibration of selected neck muscles.

Effects of focal vibration on bone mineral density and motor performance of postmenopausal osteoporotic women

AIM

This randomized double blind controlled study is aimed at determining the effect of repeated vibratory stimuli focally applied to the contracted quadriceps muscles (repeated muscle vibration=rMV) on bone mineral density, leg power and balance of postmenopausal osteoporotic women.

METHODS

The study has been conducted on 40 voluntary postmenopausal osteoporotic women, randomised at 2 groups for rMV treatment and for control. The treatment group underwent rMV (100Hz, 300-500 μm; three applications per day, each lasting 10-minutes, for 3 consecutive days) applied to voluntary contracted quadriceps (VC=vibrated and contracted group). The control group, received a sham stimulation on contracted quadriceps (NV=non vibrated group). Bone mineral density T-score of proximal femur of the participants, was evaluated in two weeks before and 360 days after intervention; body balance and explosive leg power were measured 1 day before, 30 days and 360 days after treatment.

RESULTS

VC group T-score at one year didn't change significantly relative to baseline values (pretreatment: -2.61±0.11, post-treatment -2.62±0.13); conversely in NV subjects T-score decreased significantly from -2.64 ± 0.15 SD down to -2.99 ± 0.28 SD. A significant improvement of balance and explosive leg power was observed only in VC group at 30 and 360 days after the intervention.

CONCLUSION

We conclude that rMV is a safe, short-lasting and non-invasive treatment that can significantly and persistently improve muscle performance and can effectively counteract progressive demineralisation in postmenopausal and osteoporotic women.

Improvement of posture stability by vibratory stimulation following anterior cruciate ligament reconstruction

Surgical reconstruction of the anterior cruciate ligament (ACL) may reduce, but it does not always eliminate, knee and body instability because of a persisting proprioceptive deficit. In order to enhance body stability, a new protocol of treatment has been proposed consisting of mechanical vibration (100 Hz frequency and < 20 microm amplitude) of the quadriceps muscle in the leg that has undergone ACL reconstruction. In our trials, stimulation was performed when the quadriceps muscle was kept isometrically contracted. Treatment was started one month after surgery. Vibration was applied for short periods over three consecutive days. Nine months after treatment, postural stability was re-evaluated with the subjects standing on one leg with open and with closed eyes. The postural stability of the subjects having undergone vibration treatment, standing on the operated leg was significantly improved one day after treatment when evaluated as mean of speed and elliptic area of the center of pressure. The improvement persisted and increased during the following weeks. Peak torques of the operated leg extensor muscles also increased and reached values close to that of the leg, which had not been operated. Conversely, the balance of the untreated subjects standing on the operated leg did not improve and the restoration of the extensor muscle peak torque was poor. It is concluded that short lasting proprioceptive activation by vibration may lead to a faster and more complete equilibrium recovery probably by permanently changing the network controlling knee posture.

Motor performance changes induced by muscle vibration

The possibility that mechanical stimulation of selected muscles can act directly on the nervous system inducing persistent changes of motor performances was explored. On the basis of literature, stimulating parameters were chosen to stimulate the central nervous system and to avoid muscle fibre injuries. A sinusoidal mechanical vibration was applied, for three consecutive days, on the quadriceps muscle in seven subjects that performed a muscular contraction (VC). The same stimulation paradigm was applied on seven subjects in relaxed muscle condition (VR) and seven subjects were not treated at all (NV). Two sessions (PRE and POST) of isometric and isotonic tests were performed separated for 21 days, in all studied groups 7 days before and 15 days after stimulation, whilst an isokinetic test was performed on VC only. In the isometric test, the time of force development showed a significant decrease only in VC (POST vs PRE mean 27.8%, P < 0.05). In the isotonic test, the subjects' had to perform a fatiguing leg extension against a load. In this condition, the fatigue resistance increased greatly in VC (mean 40.3%, P < 0.001), increased slightly in VR and there was no difference in NV. In Isokinetic test, at several angular velocities, significantly less time was required to reach the force peak (mean 20.2% P < 0.05). The findings could be ascribed to plastic changes in proprioceptive processing, leading to an improvement in knee joint control. Such action delineates a new tool in sports training and in motor rehabilitation.

Repeated exposure to pyrrolidine-dithiocarbamate induces peripheral nerve alterations in rats

Pyrrolidine-dithiocarbamate (PDTC), a synthetic compound widely used in cell biological investigations, recently attracted considerable interest as a putative anticancer agent. However, different dithiocarbamates have previously shown to cause neurological symptoms and morphological alterations in peripheral nerves. The purpose of the present study was to determine whether a 15-day oral administration with low doses of PDTC may produce adverse effects in peripheral nerves of rats. Female Wistar rats were assigned to receive PDTC [0.1, 0.5 or 1.0mmol/(kg body weight/day)] by gavage for 15 days. Reduced conduction velocity was observed by electrophysiological analysis in tibial nerves of treated animals, accompanied by a marked decrease in Shwann cell S100-protein expression determined by immunohistochemistry. Electron microscopy evaluation revealed marked myelin degeneration in the fibers of treated animals. In particular, both morphological and electrophysiological data suggested an impairment of large, fast conducting fibers, whereas the smallest and slowest ones remained intact. However, the activity of plasma and liver alkaline-phosphatase, an enzymic marker of hepatic dithiocarbamate toxicity, was not altered by the treatment. The total contents of the redox-active metal copper increased in tibial nerves of treated rats and was accompanied by raised levels of lipid peroxidation products. This finding suggests a role for oxidative stress in the development of PDTC-induced pathological and functional alterations of tibial nerves. The observation that a 15-day treatment with low doses of PDTC causes functional and morphological derangement of peripheral nerves advices against the possible use of this compound as a chemopreventive agent against cancer.

Nonlinear tension summation of different combinations of motor units in the anesthetized cat peroneus longus muscle

The purpose of this study was to examine the linearity of summation of the forces produced by the stimulation of different combinations of type identified motor units (MUs) in the cat peroneus longus muscle (PL) under isometric conditions. The muscle was fixed at its twitch optimal length, and the tension produced by the single MU was recorded during 24- and 72-Hz stimulation. The summation analysis was first carried out for MUs belonging to the same functional group, and then different combinations of fast fatigable (FF) MUs were added to the nonfatigable slow (S) and fatigue resistant (FR) group. The tension resulting from the combined stimulation of increasing numbers of MUs (measured tension) was evaluated and compared with the linearly predicted value, calculated by adding algebraically the tension produced by the individual MUs assembled in the combination (calculated tension). Tension summation displayed deviations from linearity. S and FR MUs mainly showed marked more than linear summation; FF MUs yielded either more or less than linear summation; and, when the FF units were recruited after the S and FR MUs, less than linear summation always occurred. The magnitude of the nonlinear summation appeared stimulus frequency dependent for the fatigable FF and FI group. The relationship between measured tension and calculated tension for each MU combination was examined, and linear regression lines were fitted to each set of data. The high correlation coefficients and the different slope values for the different MU-type combinations suggested that the nonlinear summation was MU-type specific. The mechanisms of nonlinear summations are discussed by considering the consequences of internal shortening and thus the mechanical interactions among MUs and shifts in muscle fiber length to a more or less advantageous portion of single MU length-tension curves.

Relations among motor unit types, generated forces and muscle length in single motor units of anaesthetized cat peroneus longus muscle

The active length-tension curves of identified single motor units (MUs) belonging to peroneus longus muscle (PL) of anaesthetized adult cats were obtained by eliciting isometric single twitches and tetani. The recorded responses were evaluated by measuring the peak tension amplitude and the tension-time area at muscle lengths extending throughout the physiological length range of the muscle (mean 5.5 mm, standard deviation +/- 0.8). The muscle lengths at which each tested MU developed its maximal twitch (Ltw) and tetanic (Lte) tensions were determined and compared with the muscle length (Lo) at which the stimulation of all the alpha-axons, innervating PL and contained in L7 ventral root, developed their maximal twitch tension. The mean of single MU Ltw values was at Lo +1.08 +/- 1.1 mm. Slow MUs showed the longest values of Ltw (Lo +1.6 +/- 1.0 mm). Single MUs stimulated at tetanic frequencies presented their Lte at values shorter than Lo (Lo - 2.8 +/- 1.7 mm). Slow MUs had the shortest Lte (Lo - 3.4 +/- 1.5 mm). For all the units Lte was shorter than Ltw. Ltw and Lte were, respectively, negatively and positively correlated with the developed tension. Optimal length values also appeared to be related to the MU types. The possibility is discussed that the muscle and tendon compliances and the high non-linearities to the applied forces are the main factors which can determine the differences among Lo, Ltw and Lte values. The relationships between MU type and optimal length values are suggested to be, at least partly, an epiphenomenon due to the different contraction strengths of the various MU types.(ABSTRACT TRUNCATED AT 250 WORDS)

Botulinum A toxin effects on rat jaw muscle spindles

Botulinum A toxin (Botox) is used for the treatment of many muscular dystonias. However, the relief of the sustained and abnormal postures induced by Botox administration is not fully explained. In this work the possibility was considered that Botox can produce a block not only at the alpha motor endings, but also at the gamma motor endings, consequently reducing the spindle inflow to the alpha motoneurons, which have a great role in maintaining the tonic myotatic reflex. Jaw muscle spindle discharge was recorded before and after Botox injection in the deep masseter muscle. The drug consistently reduced the spindle afferent discharge. Such an effect is suggested to be direct on gamma endings as: i) muscle tension was not modified by Botox during the recording time; ii) saline administration never changed the spindle discharge. The Botox effect on muscle spindles suggests that the relief from dystonias could be due not only to a partial motor paralysis, but also to a decrease of the reflex muscular tone.

Tetanic tension and muscle length of motor units in cat's peroneus longus

Length-tension curves were constructed by stimulating tetanically single motor units belonging to the peroneus longus muscle. The lengths at which the motor units showed the maximal tension appeared to be shorter than the length at which the whole muscle developed the maximal twitch tension. Moreover, the optimal lengths appeared to be different for the different motor unit types, corresponding to very short muscle lengths for Slow units and to slightly higher lengths for the Fast Fatiguable and the Fatigue Resistant units.

Effects of tetanic contraction of motor units of similar type on the initial stiffness to ramp stretch of the cat peroneus longus muscle

1. The stiffness during the initial portion of a ramp stretch was measured in cat peroneus longus muscle at rest and during maximal tetanic contractions produced by increasing numbers of motor units of the same type [slow (S), fast fatigue resistant (FR), or fast fatigable (FF)]. 2. This initial ramp stiffness was defined as the ratio between tension and length change over the limited range of constant velocity extension during which tension rose linearly with length change. This stiffness was reduced by tetanic contraction of a number of motor units while other units remained inactive. The reduction had different characteristics in contractions produced by S, FR, or FF units. 3. Two brief ramp (triangular) stretches were applied at short intervals to evaluate the contribution of stable cross bridges to the changes in ramp stiffness. When the amplitude of the first stretch exceeded the presumed elastic limits of the stable cross bridges, the second ramp stretch showed a reduction of 20-60% in initial stiffness. This was seen both in passive muscles and in muscles in which several motor units were contracting. 4. When increasing numbers of motor units of the same type were activated, the initial ramp stiffness to the second of a pair of triangular stretches delivered during contraction increased almost linearly with the developed tension. The slope of this increase was 2.5 times steeper for S units than for FR units. This reflects the fact that contraction produced by S units causes a proportionally greater resistance to stretch than that of fast units.

Changes in muscle stiffness produced by motor units of different types in peroneus longus muscle of cat

1. The effects of maximal tetanic contractions of varying numbers of motor units of the same type [slow (S), fast fatigue-resistant (FR), or fast fatigable (FF)] on the mechanical responses to muscle stretch were studied in the peroneus longus muscle of anesthetized cats. 2. Two types of stiffness measurements were made: 1) an average stiffness, defined as the tension change from the beginning to end of a 0.5-mm ramp stretch; and 2) a dynamic stiffness, defined as the ratio of peak-to-peak tension to amplitude of a maintained 85-microns sinusoidal stretch at frequencies of 10-80 Hz. 3. Contractions of slow and fast units elicited different increases in average stiffness. Type S units, although developing much smaller tetanic tensions than fast ones, produced a resistance to stretch comparable with or greater than that of fast units developing much higher tensions. 4. For comparable tetanic tensions, slow units also elicited a greater dynamic stiffness than fast units. During sinusoidal stretch, changes in muscle tension led changes in muscle length during contraction of S units, but the reverse was observed for frequencies 30-50 Hz during contraction of FF units. This suggests that the latter perform oscillatory work on the driving apparatus. 5. Type S units, whose low-threshold motoneurons are the first to be recruited, appear well adapted to play a role in posture and in slow movements because of the resistance they offer to forces tending to change joint position or to oppose the progression of slow movements.

Does stimulation of sympathetic axons elicit an increase in cat spindle afferent discharge detectable by the antidromic collision technique?

Averaged antidromic action potentials of Group I and Group II fibres elicited in cats by stimulation of L7 and S1 dorsal roots were recorded from intact tibial nerves (near the ankle) either in absence of or during repetitive stimulation of the ipsilateral lumbar sympathetic chain. This was done to test the suggestion that stimulation of noradrenergic sympathetic axons may elicit, in spindles of foot muscles, a substantial increase in the firing rate of secondary endings, capable of reducing the size of afferent antidromic volleys by collision with orthodromic impulses. We found that potentials recorded during sympathetic stimulation were identical to those recorded in absence of stimulation. The reduction in size of a component of the compound action potentials led from the intact tibial nerve during stimulation at 10-20 Hz of the sciatic nerve with C strength pulses, as described by Grassi, Filippi and Passatore (Brain Research, 435 (1987) 15-23), was observed in certain conditions of stimulation. However this reduction cannot be ascribed to antidromic collision, because it is still observed after severing the tibial nerve distal to the recording electrode.

Tension development in lumbrical muscles and concomitant increase of activity in A alpha and A beta afferents during sympathetic stimulation in the cat

In anaesthetized and curarized cats electrical stimulation of the lumbar sympathetic trunk, at frequencies within the physiological range, induces the development of a small tension in superficial lumbrical muscles (50-100 mg in different preparations). This effect can be reproduced in anaesthetized and curarized cats by stimulating the peripheral stump of the tibial nerve with parameters adequate to activate the entire C-fibre group, and therefore postganglionic sympathetic fibers. The collision technique was used to evaluate in the tibial nerve possible changes in afferent activity from the skinned foot. It was found that an increase of non-cutaneous activity consistently occurs in A beta and, occasionally, in A alpha fibre groups. Secondary and primary spindle afferents, respectively, belong to these fibre groups. It was also found that: (1) the tension development in lumbrical muscles and the increase of the afferent activity of non-cutaneous origin exhibit similar time courses; (2) the two events are affected in parallel by the same stimulus parameters; (3) both events are abolished by alpha-adrenoceptor antagonists. Therefore the possibility of a sympathetic fusimotor action affecting mainly secondary afferents is suggested and discussed.

Postsynaptic alpha 1- and alpha 2-adrenoceptors mediating the action of the sympathetic system on muscle spindles, in the rabbit

In anaesthetized and curarized rabbits, the cervical sympathetic nerve (CSN) stimulation induces in jaw elevator muscles a tension response which can be mimicked by the intravenous injection of adrenaline, noradrenaline and phenylephrine. This response, previously described and attributed to the contraction of muscle spindle fibres, is entirely mediated by alpha-adrenoceptors. The administration of phenoxybenzamine (2.5-3.5 mg/kg) markedly inhibits the responses to the sympathetic stimulation and to the injection of adrenergic agonists. Rauwolscine (1 mg/kg) reduces the development of tension induced by both CSN stimulation and noradrenaline injection without significantly affecting the response to phenylephrine. These data suggest the presence of postsynaptic alpha 1- and alpha 2-adrenoceptors in intrafusal muscle fibres. Moreover, the possibility that alpha 2-adrenoceptors may also have an extrasynaptic location is entertained.

Sympathetically-induced development of tension in jaw muscles: the possible contraction of intrafusal muscle fibres

In rabbits, cats and rats anaesthetized, curarized, with the skull fixed in a stereotaxic apparatus, the peripheral stump of the cervical sympathetic nerve (c.s.n.) was electrically stimulated at frequencies within the physiological range and the isometric tension was recorded at the lower jaw. In a group of experiments the afferent discharge from the jaw elevator muscle spindles was also recorded, in the mesencephalic nucleus of the fifth cranial nerve. Unilateral stimulation of the c.s.n. induced in jaw elevator muscles of rabbits an increase of tension of 5.5 +/- 0.5 g (latency: 0.5-2 s, time constant: 2.5-5 s) maintained with little or no decrement until the end of stimulation. This response proved not to be secondary to vasomotor changes since: i) approximately half of it was mediated by the fastest conducting component of the c.s.n. fibres, ii) it was not mimicked by a sudden reduction of blood supply to the muscles, iii) it was unaffected by 10 min bilateral occlusion of both the external and the internal carotid arteries. During c.s.n. stimulation the afferent discharge from spindles belonging to jaw elevator muscles exhibited an increase of firing (often preceded by a transient decrease) lasting throughout the stimulation. Also, the position sensitivity of all the spindle afferents tested was modified by the sympathetic stimulation. The results presented are interpreted to suggest that the sympathetic system may induce an intrafusal muscle fibre contraction in jaw elevator muscles. The possible functional implications are also discussed.

Localization of neurons innervating masticatory muscle spindle and periodontal receptors in the mesencephalic trigeminal nucleus and their reflex actions

The mesencephalic trigeminal nucleus was studied in anaesthetized and curarized rabbits by recording the unitary activity through extracellular microelectrodes and identifying the constituent cell types. Two types of units were found, namely primary afferents supplying jaw raising muscle spindles and periodontal or gingival mechanoreceptors. These two groups of neurons exhibited a rostrocaudal somatotopy: the former occupied the entire rostral portion of the nucleus (A7-P2.3; trochlear decussation being taken as an arbitrary 0 level), the latter was located caudally (P3-P4.5) while the somata of both types of afferent fibres were present between P2.2 and P3. No evidence was found for representation of both tendon organs of jaw muscles and joint receptors. Among the units innervating muscle spindles, secondary afferents were largely more numerous than the primary ones. Among periodontal and gingival mechanoreceptor afferents, incisors were the most widely represented, followed by interalveolar gingiva and molars; the axonal conduction velocity ranged between 9 and 40 m/sec and between 8 and 16 m/sec for ipsilaterally and contralaterally projecting neurons, respectively. The motor responses obtained by electrical stimulation of discrete areas of the MTN confirmed the presence of a high degree of segregation between the two different populations of neurons. In fact, jaw raising movements are obtained when stimulating the area within A7 and P2 containing the somata of spindle afferent neurons, while only jaw opening movements are elicited by stimulation of the caudal levels of the nucleus. These data also show that the periodontal neurons whose somata are located in the MTN participate in the jaw opening reflex, just as the more numerous periodontal mechanoreceptors whose somata are located in the Gasser ganglion. Soma-somatic and soma-axon hillock gap junctions were found among the neurons of the MTN, particularly in the caudal third of the nucleus.

Sympathetic modulation of periodontal mechanoreceptors

The effect of the electrical stimulation of the peripheral stump of the cervical sympathetic nerve, at physiological frequencies, was studied on the activity of mechanoreceptors with sensory field around the teeth, in anaesthetized and paralysed animals. In the 33% of the tested units either an increase of the resting discharge rate or an activation of the receptors occurred. The latency of this response ranged between 0.3 and 6 sec; the maximum discharge frequency, which varied widely in different units, was reached within the subsequent 2 to 4 sec and usually outlasted the duration of the stimulation. The possible mechanisms of this sympathetic effect are discussed on the basis of its latency and pattern and of the following further observations: i) its presence after denervation of carotid sinus and glomus, ii) its dependence on the stimulation of preganglionic sympathetic fibres belonging to the groups S1 and S2, iii) the absence of response in the mechanoreceptor units by the temporary occlusion of the ipsilateral common carotid artery.

A dual effect of sympathetic nerve stimulation on jaw muscle spindles

In anaesthetized and paralyzed rabbits, electrical stimulation of the cervical sympathetic nerve at physiological frequencies induces in jaw muscle spindle afferents a short-latency decrease or suppression of discharge. This effect is very stereotyped in pattern and is attributed to direct sympathetic innervation of spindles. It is mediated by preganglionic S1-S2 sympathetic fiber groups. A longer-latency facilitatory effect follows, probably vasomotor in origin and mediated by S3-S4 groups. Both responses are eliminated by administration of alpha-adrenergic blocking agents. The latencies, patterns, thresholds, durations and reproducibility of these responses have been studied and the mechanisms possibly involved are discussed.

Muscle spindle autogenetic inhibition in the extraocular muscles of lamb

The role of extraocular muscle (EOM) proprioceptors on eye motility has been investigated in lambs on "encéphale isolé", by evaluating the tension of EOMs at various lengths and velocities of stretch before and after proprioceptive blocks. The EOM tension, in the absence of proprioceptive input, was higher than in normal conditions. Such an effect occurred at lengthening values greater than 3 mm of stretch from resting muscle length, corresponding to 18 degrees of eye deviation and was dependent on the velocity of the stretch, being more effective at high velocity. The muscle receptors responsible for this effect was determined by comparing the sensitivity to vibratory stimulation of spindles and tendon organs to the amount of inhibition provoked by the same stimulation on an EOM electromyographic activity. The tension inhibition appeared to be correlated to muscle spindle activation. Thus, the presence of muscle spindles can determine a reduction of the tension within the stretched muscles. This result suggests that the EOM length and velocity signals operate moment to moment reduction on the stiffness of the muscle which antagonizes eye displacement, thus facilitating the ocular movements.

On whether there is a direct sympathetic influence on jaw muscle spindles

Electric stimulation of the peripheral stump of cervical sympathetic nerve (0.5-5.0 sec trains at 1 msec, 6-10 V, 0.5-30 HZ) induced in jaw muscle spindle afferents: (1) decrease of discharge frequency with 0.6-2.5 sec latency, lasting 8-13 sec (in 40% of the units) possibly due to direct sympathetic innervation of spindles; and (2) subsequent modest increase of frequency, probably secondary to muscular vasoconstriction.

Epileptogenic stimulation of the cortical masticatory area and the mesencephalic trigeminal nucleus

1. The effects of epileptogenic stimulation of the cerebral masticatory area on the somata of the first-order neurons of masticatory proprioception localized in the mesencephalic trigeminal nucleus (MTN) were studied in curarized, lightly anaesthetized rabbits. 2. Low-frequency stimulations inducing modest cortical after-discharges fired the silent MTN units with latencies of 0.3-0.8 sec, and induced alternate excitatory and inhibitory effects on the active units. On some occasions the tonic afferent discharge was changed into rhythmic bursts, which occurred at the constant frequency of 2.5-3.5/sec for the entire range of stimulation frequencies used, i.e. from 0.3 to 10/sec. 3. High-frequency cortical stimulations eliciting true epileptic seizures induced a transient increase in discharge of the MTN units, followed by a prolonged inhibition. 4. All these effects were attributed to activation of reticular pathways acting on the some of the recorded proprioceptive neurons. Such a modulation of the proprioceptive input elicited by the epileptic masticatory cortex may contribute to the motor effects and to the changes of the masseteric reflex which occur during convulsive seizures.