Beverly Bishop

Effect of gum hardness on chewing pattern

Chewing rhythms are set by a putative central pattern generator whose output is influenced by sensory feedback. In this study we assessed how an altered feedback imposed by changing the hardness of a gum bolus modifies the timing of chewing, the maximal gape, and the activity in the masseter muscle on the chewing side. Ten adult subjects with no orofacial dysfunction chewed a standard piece of soft or hard gum for at least 3 min in random order. Vertical jaw movements were recorded with a kinesiograph and activity of the masseter muscle was recorded and integrated from surface EMG electrodes. The subjects sat in a dental chair and viewed a video lecture to distract their attention from chewing; they were instructed to chew on the right molars. Cycle-by-cycle analysis showed that 9 of the 10 subjects chewed the hard gum more slowly than the soft with no significant change in gape. The increases in cycle duration were due to changes in the duration of the opening and occlusal phases. The duration of closing was not significantly changed even though the duration and level of masseter activity were both significantly increased. We conclude that gum hardness by altering proprioceptive feedback modifies the output of the masticatory central pattern generator in such a way that the temporal aspects of chewing and the output of the masseteric motor pool are affected.

Effect of Biting Force on the Duration of the Masseteric Silent Period

The relationship of biting force to the duration of the masseteric silent period was studied. After introduction of a bite transducer in ten individuals, the jaw jerk reflex was elicited at specific decreases in biting force. No significant alteration in the duration of the silent period was observed.

Contribution of Periodontal Receptors to the Masseteric Silent Period

Quadrant local anesthesia was sequentially administered to the maxillary and mandibular teeth and periodontium in three healthy individuals. After anesthesia the duration of the silent period was shortened. Total anesthesia of all quadrants abolished the silent period in every individual, demonstrating that sensory impulses from periodontal receptors provide a major source of inhibition, and disfacilitation or active inhibition from intact muscle receptors is insufficient to produce a silent period.

Mandibular movements and jaw muscles' activity while voluntarily chewing at different rates

As a way of learning about the motor control of chewing, we studied how well a subject could voluntarily chew in time with a metronome and defined the changes in the spatial and temporal aspects of the chewing pattern with changes in chewing rate. Timing and extent of mandibular movements were assessed in nine adults from Kinesiograph recordings; timing and level of activity in digastric and both masseter muscles were determined from surface EMGs. Each subject chewed gum in time with a metronome set randomly at 46, 100, 160 beats per minute or at a frequency close to his automatic chewing rate. Cycle-by-cycle analysis showed that subjects varied in their ability to keep pace with the metronome. When chewing at high frequencies, six subjects reduced gape, three did not. Contralateral deflection in opening, when present, was significantly reduced when chewing at high frequencies and this decrease was independent of vertical gape. Durations of opening, closing, and occlusal phases decreased in proportion to the decrease in total cycle duration. Burst duration of diagastric activity decreased about 29% compared with a 77% decrease in cycle duration over all chewing rates. At low frequencies onset of digastric activity occurred after onset of opening. At high frequencies digastric onset preceded opening. Burst durations of both masseter muscles decreased in concert with the decrease in cycle duration. Termination of activity in both masseters was synchronous and always occurred 100 +/- 20 ms prior to the next opening. Therefore, we conclude that (i) individuals vary in ability and strategy for controlling chewing rate voluntarily and (ii) with increases in chewing rates the shortening of burst duration for the digastrics is significantly less than for the masseters.

Effects of chewing frequency and bolus hardness on human incisor trajectory and masseter muscle activity.

Nine adults with no orofacial dysfunctions were instructed to chew a standardized piece of soft or hard gum on the right side in time with a metronome set at 46, 100 or 160 beats/min. Jaw movements were recorded with a Myotronics kinesiograph and masseter electromyograms were detected with surface electrodes. The chewing patterns on either gum were not significantly different in any of their spatial or temporal aspects, in mean or peak opening or closing velocities, or in the timing or level of activity in either masseter at any of the three chewing frequencies. These findings suggest that during metronome-paced chewing the change in sensory feedback resulting from a change in gum hardness exerts little or no effect on either the spatial or temporal aspects of masticatory motor output.

Discharge of abdominal muscle α and γ motoneurons during expiratory loading in cats

Expiratory responses were elicited in abdominal alpha and gamma motoneurons of anesthetized cats by continuous positive airway pressure, tracheal occlusion, lung inflation, or step-changes in expiratory airflow resistance. Abdominal alpha motoneurons were silent during quiet breathing but became rhythmically active whenever expiration was opposed. In addition, the responses of abdominal alpha motoneurons to an increase in expiratory airflow resistance included an increase in discharge frequency, an earlier firing time of individual neurons, recruitment of successively larger motor fibers, and an increased duration of discharge. Abdominal gamma motoneurons discharged continuously during quiet breathing and an increased frequency of discharge during the expiratory phase of the respiratory cycle was evident in approximately one-third of the fibers. This respiratory modulation was enhanced, or initiated if absent, by imposing a load during expiration. Bilateral cervical vagotomy abolished both the respiratory modulation of abdominal gamma motoneurons and expiratory activity in abdominal alpha motoneurons indicating the importance of supraspinal structures. Coactivation of abdominal alpha and gamma motoneurons during active expiration also suggests that a segmental reflex involving abdominal muscle spindles may be capable of providing automatic compensation for changes in expiratory load. In conclusion, both vagal and dorsal root information appear to contribute to the proprioceptive control of abdominal muscle expiratory activity.

Phasic stretch reflex of the abdominal muscles

This analysis of the abdominal stretch reflex (ASR) evoked by a tap to the abdomen was designed to explore how abdominal motoneurons process signals from respiratory and nonrespiratory sources. We recorded surface EMGs from the external and internal oblique muscles in standing subjects. Amplitudes of the abdominal stretch reflex varied despite constant tap forces, but strong taps evoked a larger reflex than weak taps. Trunk rotation toward the recording side, or voluntary contraction of the external and internal oblique muscles increased the reflex amplitudes, whereas contralateral rotation reduced their occurrence. An abdominal stretch reflex during a voluntary contraction was followed by a silent period of 40 to 80 ms. Often a late wave followed a reflex by 20 to 40 ms. Amplitudes during breathholds at residual lung volume were larger than those evoked during a breathhold at functional residual capacity, suggesting that abdominal stretch reflex amplitudes are inversely proportional to static lung volume. During quiet breathing the reflex amplitude reached a maximum slightly before end-expiration and decreased progressively to a minimum close to end-inspiration. During rebreathing, background abdominal activity was augmented with highest activity in late expiration. Abdominal stretch reflex amplitudes continued to wax and wane in phase with respiration, and the maximal reflex occurred progressively earlier in expiration. In summary, the abdominal stretch reflex reflects strong control from abdominal muscle spindles, lung proprioceptors, and chemoreceptors. The relative contributions of these inputs need to be determined.

Effects of whole-body rotation on masseteric motoneuron excitability

Vestibular stimulation is a popular clinical treatment for enhancing the excitability of spinal motoneurons innervating trunk and limb muscles, but whether vestibular stimulation can also influence trigeminal motoneurons is not known. We determined whether or not vestibular stimulation evoked by rotation of a seated subject would modify the excitability of masseteric motoneurons. The amplitude and frequency of occurrence of masseteric compound action potentials evoked by standard chin taps provided measures for assessing masseteric motoneuron excitability. Eleven healthy adults with no orofacial or otologic disorders served as subjects. Each sat in a motorized dental chair with his head stabilized by a halo head-piece so that chair rotation caused labyrinthine excitation. The frequency (3/s) of chin taps and their impact force were maintained constant by microcomputer control. After each tap, a 16-ms sample of EMG recorded from surface electrodes over the right masseter was digitized and stored for subsequent visual inspection. Only compound action potentials meeting rigorous criteria in terms of latency, amplitude, duration, and waveform were accepted as responses. The mean frequencies of occurrence and the mean amplitudes of the responses showed wide variability. Histogram displays of every response for each subject, however, revealed enhanced output from the masseteric motoneuron pool during the decleration and postrotation phases. In subjects not immediately retested this enhancement was persistent but decayed during the next 5 min. In five subjects the experiment was repeated after 1 min. The changes in response variables during phases 3 and 4 were significantly less than on the first trial, suggesting habituation. These results provide quantitative evidence that the dynamic input from vestibular ampullary receptors in response to rotation enhances masseteric motoneuron output.

Identification and assessment of factors contributing to variability of the jaw jerk

Unlike limb monosynaptic reflexes, the jaw jerk reflex ( JJR ) is extremely variable. We studied 35 healthy adults to determine the relative contributions of extrinsic and intrinsic factors underlying this variability. Each subject sat in a dental chair with his head and chin securely stabilized. Chin taps, delivered by a solenoid-driven plunger, were quantified with a piezo -transducer. The reflex response was recorded from surface electrodes over the right masseter muscle. A nasal thermistor signalled phases of respiration. Five of the 35 subjects had no reflex when relaxed, but during 15 degrees neck extension or voluntary contraction of the platysma muscle, a JJR appeared. The amplitude of the reflex varied considerably from trial to trial in all but one subject. A small component of this variability was due to minute changes in tap force despite head and chin stabilization and stimulus uniformity. Mean amplitudes of the reflex tended to increase with increases in tap force, but variability was large indicating intrinsic fluctuations in motoneuron excitability. Voluntary contraction of the platysma muscle and 15 degrees neck extension reliably enhanced the reflex. The JJR showed negligible respiratory modulation during quiet breathing. The reflexs variability in and among subjects precludes the use of the JJR as an index of masseteric motoneuron excitability. Our findings suggest that branchial motoneurons innervating the masticatory muscles receive far more diverse and fluctuating inputs than do somatic motoneurons innervating limb muscles.

NEURAL REGULATION OF ABDOMINAL MUSCLE CONTRACTIONS

The abdominal muscles are the chief expiratory muscles of the body, and yet they do not participate in normal speaking or breathing. However, during laughing, singing, or playing wind instruments, it is the finely adjusted contraction of the abdominal wall that helps control the rate of exhalation at lung volumes below resting end expiratory levels.l.2 The abdominal muscles differ from the other respiratory muscles in that they participate in r i m y different kinds of motor activities. They resemble the antigravity muscles of the limbs in their postural role of bending the trunk and supporting the viscera against gravity. They resemble the respiratory muscles of the chest wall in their respiratory and circulatory activities of emptying the lungs and aiding venous return Of all skeletal muscles, none has a wider variety of functions and, as a consequence, probably no .motoneurone pool has a more complex motor control system.