Effects of electrical stimulation of the lateral habenular nucleus and lateral hypothalamus on unit activity in the upper brain stem

In decerebrated unanaesthetized cats the reflex mechanical response of the jaw and the reflex EMG activity in digastric and temporalis muscles were recorded following electrical stimulation of the exposed infraorbital nerve. Single shocks elicited rapid transient jaw opening associated with digastric activity (5–6 msec latency) and simultaneous weak temporalis activity; under isotonic conditions this was followed by temporalis activity (12–20 msec latency) corresponding to jaw movement. Reflex jaw opening elicited by a moderate strength stimulus was converted into jaw closing if it was elicited during an opening movement; reflex opening in response to a strong stimulus was not reversible. Reflex jaw closing was also elicited at rest by short trains of stimuli individually subthreshold for digastric activity; closing was characterised by temporalis activity with 5–6 msec latency. Maintained opening was elicited by trains of afferent volleys in slowly conducting fibres and was characterised by temporalis inhibition alone. The results were explicable on the basis of separate afferent pathways for digastric excitation, temporalis excitation and temporalis inhibition.     

Tongue and jaw muscle activities during chewing and swallowing in freely behaving rabbits

To study the function of the tongue and the coordination among jaw, tongue, and hyoid muscles during chewing and swallowing, we recorded the electromyographic activities from the masseter (Mas), digastric (Dig), mylohyoid (Myl), thyrohyoid (Thy), genioglossus (Gg) and styloglossus (Sg) muscles as well as jaw movement trajectories in the freely behaving rabbit. Three phases were identified in the chewing cycle (fast- and slow-closing and opening phases). The Gg (main tongue protruder) was active synchronously with the Dig during opening. The Sg (tongue retractor) showed two peaks in each cycle, one in the opening phase and the other in the closing phase. The latter may have a role in retracting the tongue during jaw closing. The co-contraction of the antagonists (i.e. Gg and Sg) during opening may contribute to shape the tongue to be appropriate to gather the foodstuff. In the swallowing cycle, five phases were identified, two in the closing phase and three in the opening phase. Regression analysis revealed that swallowing cycles had a longer cycle duration than that of the chewing cycles due to an extra phase (a pause) inserted in the opening phase, where there was a small co-activation in the jaw opening and closing muscles. The findings suggest that the swallowing center affects masticatory center in the central nervous system, and may also support the view that the masticatory burst timing begins with the Dig activities in the middle of the opening phase.     

Tonic stretch reflexes in lip, tongue and jaw muscles

Despite considerable speculation it remains unclear as to whether stretch reflexes perform a functional role in speech articulator muscles. Recent research, however, has shown that long loop stretch reflex mechanisms are brought into play during voluntary contraction of limb muscles and a functional role in oscillatory damping has been suggested. It was decided, therefore, to use a method and a technique of analysis similar to that used in limb muscles to search for tonic stretch reflex (TSR) responses in lip, tongue and jaw muscles during sustained voluntary contraction. The term 'action TSR' is used to differentiate stretch reflex responses measured from voluntary activity from those measured at rest. Simultaneous electromyogram (EMG) recordings were taken from the lip, tongue and jaw musculature in normal, stutterer and cerebral spastic subjects. Subjects were instructed to hold the appropriate articulator in a fixed position while the experiment applied an irregular, continuously changing, stretching force. The stretch and EMG signals were analyzed using a cross correlation and spectral analysis technique. This provided a sensitive means of detecting any EMG fluctuations which covaried with applied stretch and might therefore be classified as reflex. No suggestion of such action TSR responses could be found in lip or tongue muscles of any of the subjects tested, including the cerebral spastic subjects with dysarthric speech. It is therefore concluded that action TSR mechanisms are not operative in control of lip and tongue muscles in man. Furthermore, dysarthric speech in cerebral spasticity cannot be attributed to exaggerated tone of lip and tongue muscles resulting from hypersensitivity of TSR mechanisms. In contrast, clear action TSR responses were demonstrable in jaw closing muscles while in jaw opening muscles, small amplitude responses were detected but were not substantial in comparison with background activity. Since the action TSR is present in jaw and limb muscles, but absent in lip and tongue muscles, the suggestion of a functional role of this reflex in damping mechanical oscillations associated with inertial loads is further supported.     

Investigation of an unusual,high‐frequency jaw tremor with coherence analysis

Normal physiological tremor of the jaw has a frequency of 6 to 8 Hz. A patient is described with jaw tremor at frequencies of 12 Hz during jaw movement and 15 Hz when the jaw was relaxed. The 15 Hz tremor was driven by synchronous, bilateral bursts of activity in the temporalis and masseter muscles, which alternated with digastric bursts. Coherence analysis indicated the tremor was highly correlated with both opening and closing muscle activity, and that the opening and closing muscles were about 180° out of phase. The existence of two tremors with different, nonphysiological peak frequencies and the influence of attention, relaxation, and movement in switching from one tremor frequency to the other, suggest that more than one generator may be operating. © 2007 Movement Disorder Society     

Plantar flexor stretch reflex responses to whole body loading/unloading during human walking

Numerous animal and human studies have shown that afferent information from the periphery contributes to the control of walking. In particular, recent studies have consistently shown that load receptor input is an important element of the locomotion control mechanism. The objective of this study was to investigate the contribution of load receptor feedback to the compensatory stretch reflex response. We examined the contribution of load receptor feedback to the magnitude of the short and medium latency components of the ankle plantar flexor stretch reflex responses following an unexpected dorsiflexion perturbation during human walking. Three body load conditions were investigated: normal body load, a 30% increase in body load, and a 30% decrease in body load. Healthy subjects walked on a treadmill at approximately 3.6 km/h with the left ankle attached to a portable stretching device. Dorsiflexion perturbations (8 degrees; 350-425 degrees/s) were generated during the late stance phase of gate (approximately 400 ms following heel contact). Electromyographic activity was recorded from the soleus, tibialis anterior, medial gastrocnemius, rectus femoris, and biceps femoris muscles using bipolar surface electrodes. Stretch reflex responses were observed in the soleus and gastrocnemius muscles for all of the body load conditions; however, increasing or decreasing the body load did not affect the timing and magnitude of the responses. This study provides evidence that load receptor input does not contribute strongly to the corrective response of the stretch reflex in the plantar flexor muscles during walking.     

Inverse activity of masticatory muscles with and without trismus: a brainstem syndrome.

Clinical and EMG findings in 10 cases of intrinsic brainstem lesions are reported with paradoxical activity of jaw closing muscles during jaw opening, with and without trismus. In five cases with trigeminal anaesthesia, the inverse activity of jaw closers is interpreted as a manifestation of disturbance in the central programming of mastication in the motor trigeminal area of the brainstem. Stretch reflex mechanisms and disinhibition of the trigeminal motor neurones play no part in the origin of inverse activity. The distinct brainstem syndrome can only be detected by EMG and the special clinical features.     

Forelimb responses to cutaneous nerve stimulation during locomotion in intact cats

The reflex responses of forelimb muscles to electrical stimulation of the cutaneous superficial radial nerve were recorded during treadmill locomotion in chronically implanted cats. In brachialis and cleidobrachialis (muscles which are active mainly during the swing phase) the responses were maximal during the swing phase and minimal or absent during the stance phase. In the long head of triceps which is active mainly during the stance phase, responses were also minimal during stance and maximal during swing. It is concluded that, for some muscles, the period of maximal reflex responsiveness can be out of phase with the period of the step cycle during which they are normally active.     

Effects of sensory deafferentation on glucose metabolism of muscles during locomotion

We measured the uptake of a radiolabeled analogue of glucose ([3H]FDG) into muscles during treadmill walking so as to determine whether sensory deafferentation of a limb affects the muscles' metabolic response to the exercise. The muscles on the deafferented side took up less tracer than those in the intact side with exercise, but not at rest. Extensor muscles were more affected than flexors by deafferentation. This agrees with the proposal that "reflex" inputs adjust the level of recruitment of motor units by central pattern generators.     

TVR and vibration-induced timing of motor impulses in the human jaw elevator muscles.

In order to investigate myotatic reflex involvement in jaw muscle control, an analysis was made of the motor responses induced by mechanical vibration (120-160 Hz) of the jaw elevator muscles in healthy subjects. As seen in torque measurements and mean-voltage electromyographic (EMG) recordings, the vibration caused involuntary reciprocal changes in jaw muscle tone, the contraction force increasing in jaw elevators and decreasing in antagonistic jaw opening muscles. This tonic vibration reflex (TVR) elicited from the jaw elevators exhibited many characteristics similar to those previously described for limb muscle tonic vibration reflexes: it varied in strength from one subject to the next independently of the briskness of the jaw elevator tendon jerks; it had a gradual onset with successive recruitment of jaw elevator motor units firing largely out of phase with one another and at rates much lower than the vibration frequency; it was susceptible to voluntary control--when allowed visual feed-back from the torque meter all subjects were able to suppress the TVR and keep mean contraction force constant. The results indicate that with respect to the tonic motor response to sustained inflow in the Ia afferent nerve fibres, the jaw elevators do not differ markedly from other skeletal muscles. Independently of whether a TVR was present or not, the vibration caused a timing of the motor unit discharges in the jaw elevators that could not be controlled voluntarily and that showed up in gross EMG recordings as a marked grouping of discharges synchronous with each wave of vibration. A similar but less distinct grouping of the gross EMG pattern was seen in limb muscles exposed to vibration, the dispersion increasing with the peripheral conduction distances of the reflex arcs. It is suggested that contrary to the TVR, which depends on the sustained mean level of the Ia afferent input, the timing phenomenon depends, like the tendon jerk, on the degree of synchrony in the afferent Ia volleys. Monosynaptic projections may well be involved in the dynamic timing of motor discharges during tonic firing, but this does not imply that the TVR or the tonic stretch reflex is dependent upon such projections.     

Grasping in the pigeon (Columba livia): final common path mechanisms

A combination of cinematographic and denervation procedures were used to analyse the mechanisms involved in the adjustment of gape size during grasping in the pigeon. Gape size was found to vary directly with seed size and to reflect the operation of two variables, jaw opening velocity and jaw opening duration. Effects upon duration are mediated, indirectly, by the effect of seed size upon head height, which, in turn, controls the velocity of head descent. The data suggest that the control of gape during grasping may involve two different effector systems (jaw muscles, neck muscles). Analysis of the displacement of individual jaws (maxilla, mandible) during grasping indicates that both opener muscles take part in the control of gape. Denervation experiments (motor nerve section) identified these opener motoneurons as contributors to the final common path for the opening phase of grasping. A comparison of the kinematics of pecking/grasping in pigeons and reaching/grasping in humans reveals a number of similarities in the topography and spatiotemporal organization of these behaviors.     

Normalization reduces the spatial dependency of the jaw‐stretch reflex activity in the human masseter muscle

The jaw‐stretch reflex is the short‐latency response in the jaw‐closing muscles after a sudden stretch. The hypothesis whether normalization of the jaw‐stretch reflex amplitude with respect to prestimulus electromyographic (EMG) activity will make the amplitude more independent of the location of the electrodes over the masseter muscle was tested. A 5 × 6 electrode grid was used to record the jaw‐stretch reflex from 25 sites over the right masseter muscle of 15 healthy men. The results showed that there was a significant site dependency of the prestimulus EMG activity and the reflex amplitude. High cross‐correlation coefficients were found between the spatial distribution of mean prestimulus EMG activities and reflex amplitude. When the reflex amplitude was normalized with respect to the prestimulus EMG activity, no site dependency was found. In conclusion, normalization of the jaw‐stretch reflex amplitude by the prestimulus EMG activity strongly reduces its spatial dependency. Muscle Nerve, 2010     

An analysis of mandibular movement trajectories and masticatory muscle EMG activity during drinking in the guinea pig

Electromyographic (EMG) activity of the anterior digastric, lateral pterygoid, and deep masseter muscles as well as the associated jaw movements during drinking were studied in the awake guinea pig. Drinking was characterized by rhythmic, vertically directed jaw movements with little or no associated lateral movements. The jaw opening phase of each cycle was associated with bilaterally synchronized EMG activity in the digastric and lateral pterygoid muscles, and the jaw closing phase with bilaterally synchronized activity in the masseter muscles. The mean EMG burst durations (+/- 1 S.E.) in the digastric and masseter muscles were 164.2 +/- 14.93 ms and 94.3 +/- 26.44 ms, respectively. The digastric muscle EMG burst duration was significantly correlated with drinking cycle time and with masseter muscle EMG onset; on the other hand, masseter muscle EMG burst duration was not correlated with cycle time. These patterns of EMG activity and jaw movement trajectories are similar to those induced by apomorphine in the ketamine-anesthetized guinea pig.     

Long term follow up of a hemimasticatory spasm

OBJECTIVES: To describe the clinical and neurophysiological findings in a case of hemimasticatory spasm (HMS) followed during 14 years of evolution. MATERIAL AND METHODS: A woman suffered from very frequent paroxysmal episodes of painful involuntary occlusion of the jaw. Neurophysiological studies were performed at the 3, 12 and 14 years after the onset of symptoms. They included a needle electromyographic (EMG) evaluation of the main jaw closing and opening muscles, the jaw reflex (JR), the masseteric silent period (MSP) and the masseteric inhibitory reflex (MIR). RESULTS: Clinical symptoms remained unchanged throughout the period of observation. Conventional EMG never disclosed neurogenic signs. Voluntary closure of the jaw systematically provoked an abnormal activity with muscle cramps characteristics, restricted to the left masseter muscle. Left JR response was normal in the first evaluation and became delayed and of reduced amplitude in the second. The MSP and MIR were abolished on the left side during the spasmodic episodes whereas they were strictly normal out of them. The MIR abnormalities showed the characteristic pattern of an efferent lesional type. CONCLUSIONS: Hemimasticatory spasm probably is the consequence of an abnormal trigeminal hyperexcitability likely induced by the demyelinating lesion of its peripheral motor pathway. The main neurophysiological abnormalities may persist unmodified over a long course of the disease and allow the differential diagnosis of HMS from oromandibular dystonia and temporomandibular dysfunction (TMD).     

Masseter, digastric and omohyoidal responses from weak mechanical oral stimuli in the chewing rat

In rats anesthetized with ketamine, effects of jaw jerks (rise time 5 ms, strength 70 mN) applied in the open direction were studied both in the quiescent preparation and during rhythmic jaw-opening and jaw-closing movements (induced by mechanical stimulation of the palate). The left masseter, posterior and anterior digastric and omohyoid muscles were fitted with electrodes for recording electromyographic responses. In the quiescent state a jaw jerk causes responses in all recorded muscles. In the rhythmic preparation masseter responses persisted but digastric and omohyoidal responses were suppressed. This suppression was most prominent when jerks were applied during closing. However, responses in digastrics and omohyoid persisted when masseter nerves and periodontal afferents of the lower incisors were blocked with xylocaine. The suppression of digastric and omohyoidal responses therefore originates--at least partly - from masseter and periodontal afferents. When the periodontal afferents alone were blocked with xylocaine only the response in the opening phase persisted. The suppression of digastric and omohyoidal responses during opening are therefore reflexes from periodontal receptors. These observations support the concept that oral activity is controlled by two relatively independent systems. The first program-oriented system generates the basic movements whereas the second, environment-oriented, system has access to the program for correcting perturbations. There is evidence that muscles poorly supplied with muscle spindles (digastrics and omohyoid) act as pure effectors. Modulation of their activity is prompted by exteroceptors and muscle receptors in the masticatory muscles.     

Cortical facilitation of cutaneous reflexes in leg muscles during human gait

During human gait, cortical convergence on sural nerve reflex pathways was investigated by means of transcranial magnetic stimulation (TMS) of the cortex in five phases of the step cycle during human walking on a treadmill. Muscular responses to paired electrical and magnetic stimulation were compared with the linear summation of the individual stimuli. For both the tibialis anterior (TA) and biceps femoris (BF) muscles, the averaged data of four subjects showed a significant facilitation mainly in the swing phase of the step cycle. It is suggested that facilitation of corticospinal input onto cutaneous reflex pathways is enhanced specifically in these periods of the step cycle.     

H-reflex modulation during walking in spastic paretic subjects.

Hoffman (H) reflexes were elicited from the soleus muscle during treadmill walking in 21 spastic paretic patients. The soleus and tibialis anterior muscles were reciprocally activated during walking in most patients, much like that observed in healthy individuals. The pattern of H-reflex modulation varied considerably between patients, from being relatively normal in some patients to a complete absence of modulation in others. The most common pattern observed was a lack of H-reflex modulation through the stance phase and slight depression of the reflex in the swing phase, considerably less modulation than that of normal subjects under comparable walking conditions. The high reflex amplitudes during periods of the step cycle such as early stance seems to be related to the stretch-induced large electromyogram bursts in the soleus in some subjects. The abnormally active reflexes appear to contribute to the clonus encountered during walking in these patients. In three patients who were able to walk for extended periods, the effect of stimulus intensity was examined. Two of these patients showed a greater degree of reflex modulation at lower stimulus intensities, suggesting that the lack of modulation observed at higher stimulus intensities is a result of saturation of the reflex loop. In six other patients, however, no reflex modulation could be demonstrated even at very low stimulus intensities.     

Reflex changes in the masticatory muscles with load perturbations during chewing hard and soft food

Reflex changes in masticatory muscles were investigated in naturally behaving rabbits while chewing soft food (bread) and hard food (raw rice). To study peripheral control mechanisms of mastication, reflex changes in masticatory muscles were correlated with the jaw movement trajectories. When the hard food was tested, the closing muscle was activated isometrically and the antagonist activity was evident during closure. Under such conditions, the so-called masseteric inhibitory periods (MIPs) and digastric short bursts (DSBs) were found in the closing phase, which was not the case with soft food. The reflex changes in the masticatory muscles were similar to those in an unloading reflex or in a reflex after tooth tap. Precise comparison of the EMG and the movement orbit showed that DSB with MIP was preceded by a trough in the closing velocity which bottomed at 8 ± 1 ms (mean ± S.D.:n = 9) before the DSB onset. These results suggest that the DSB with MIP may be a reflex change generated by periodontal mechanoreceptor stimulation after the upper and lower teeth come together with hard food, which have a role in regulatory mechanisms of mastication when the teeth are suddenly loaded by hard food.     

Changes in soleus H‐reflex during walking in middle‐aged,healthy subjects

Introduction: To assess the effect of aging on stretch reflex modulation during walking, soleus H‐reflexes obtained in 15 middle‐aged (mean age 56.4 ± 6.9 years) and 15 young (mean age 23.7 ± 3.9 years) subjects were compared. Methods: The H‐reflex amplitude, muscle activity (EMG) of the soleus and tibialis anterior muscles, and EMG/H‐reflex gain were measured during 4‐km/h treadmill walking. Results: The normalized H‐reflex amplitude was lower in the swing phase for the middle‐aged group, and there was no difference in muscle activity. EMG/H‐reflex gain did not differ between groups. Conclusions: H‐reflex amplitude during walking was affected by aging, and changes during the swing phase could be seen in the middle‐aged subjects. Subdividing the 2 age groups into groups of facilitated or suppressed swing‐phase H‐reflex revealed that the H‐reflex amplitude modulation pattern in the group with facilitated swing‐phase H‐reflex may be influenced by aging. Muscle Nerve 51: 419–425, 2015     

Functional differentiation of hypoglossal motoneurons during the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat

Activity of hypoglossal motoneurons was studied in relation to the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat. Tongue protrudor and retrusor motoneurons were antidromically identified by stimulation of the medial and the lateral hypoglossal nerve branches, respectively. About 80% of protrudor motoneurons excited during the opening phase of the rhythmical jaw movements with or without prior excitation. Most of them did not respond to amygdaloid or cortical stimulation. Fifty-three percent of retrusor motoneurons were excited during the closing phase and 31% discharged around the peak opening. The majority of them and a few protrudor motoneurons responded to amygdaloid or cortical stimulation with similar latencies. Activities of the medial and the lateral hypoglossal branches corresponded with the discharge patterns of respective motoneurons. Tongue protrusion and retrusion appeared to slightly precede jaw opening and closing, respectively during the rhythmical jaw and tongue movements. This was probably due to activity of the brain stem pattern generator.     

Sensory-to-motor transformations in the vestibular system

The vestibulo-ocular reflex is a compensatory reflex that results in eye movements that are 180 degrees out of phase with movements of the head but that match head velocity. Because of these reflex eye movements that are equal, but opposite to head movement, the viewed object remains on the fovea of the retina during head movement, thus resulting in visual acuity that is not degraded by visual image slip on the retina. This reflex is compensatory over a large spectrum of head movements in any plane of space. This is accomplished by a spatial and temporal transformation of the input from the vestibular semicircular canals to the motoneurons that innervate the extraocular muscles. The reflex is a three-neuron arc. The middle leg of the reflex is accomplished by secondary vestibular neurons whose axons branch to innervate more than one extraocular muscle. These secondary neurons thus program an eye movement rather than the contraction of a single extraocular muscle. These programmed eye movements that match the plane of the particular semicircular canal that is the input to the reflex constitute the spatial transformation. Primary vestibular afferents innervating the semicircular canals have a broad range of response dynamics that either lead, lag or are in phase with head velocity. The predominant vestibular primary afferent input to the middle leg of the reflex, the same secondary neurons as mentioned above, is parcellated so that afferents more in phase with head velocity predominate.(ABSTRACT TRUNCATED AT 250 WORDS)