The reflex responses of single motor units in human lower lip muscles to mechanical stimulation

Mentalis muscle related reflexes

The mentalis muscle (MM) arises from the incisive fossa of the mandible, raises and protrudes the lower lip. Here, we aim to characterize responses obtained from MM by supraorbital and median electrical as well as auditory stimuli in a group of 16 healthy volunteers who did not have clinical palmomental reflex. Reflex activities were recorded from the MM and orbicularis oculi (O.oc) after supraorbital and median electrical as well as auditory stimuli. Response rates over MM were consistent after each stimulus, however, mean latencies of MM response were longer than O.oc responses by all stimulation modalities. Shapes and amplitudes of responses from O.oc and MM were similar. Based on our findings, we may say that MM motoneurons have connections with trigeminal, vestibulocochlear and lemniscal pathways similar to other facial muscles and electrophysiological recording of MM responses after electrical and auditory stimulation is possible in healthy subjects.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Optimal placement of bipolar surface EMG electrodes in the face based on single motor unit analysis

Locations of surface electromyography (sEMG) electrodes in the face are usually chosen on a macro-anatomical basis. In this study we describe optimal placement of bipolar electrodes based on a novel method and present results for lower facial muscles. We performed high-density sEMG recordings in 13 healthy participants. Raw sEMG signals were decomposed into motor unit action potentials (MUAPs). We positioned virtual electrode pairs in the interpolated monopolar MUAPs at different positions along muscle fiber direction and calculated the bipolar potentials. Electrode sites were determined where maximal bipolar amplitude was achieved and were validated. Objective guidelines for sEMG electrode placement improve the signal-to-noise ratio and may contribute to reduce cross talk, which is particularly important in the face. The method may be regarded as an important basis for improving the validity and reproducibility of sEMG in complex muscle areas.

Modulation of the trigeminofacial pathway during syllabic speech

The human orofacial system is richly endowed with low-threshold, slowly adapting mechanoreceptors that respond to self-generated movements and external loads. The functional linkage between these afferents and the recruitment of motor units in the lower face during the dynamics of speech is unknown. Mechanically evoked activity in the orbicularis oris muscles was studied in young human female adults (N=10) during a lip force recruitment task associated with the repetition of the nonsense speech utterance "ah-wah." This speech task involved the recruitment of perioral motor units against an elastic load. A skin contactor probe coupled to a servo-controlled linear motor delivered punctate ipsilateral mechanical inputs (25 ms duration, 1800 mum displacement) to the glabrous surface of the upper lip in order to index the modulation and specificity of the compound trigeminofacial response as a function of speech force recruitment threshold (Ft). Modulation of the early (Ft=0.2 N) and later (Ft=1.0 N) components of the evoked perioral response was found at the two force thresholds. Beginning at approximately 60 ms post-stimulus, a significant suppression response was found among lower lip EMG recording sites and its magnitude was greatest when the mechanical perturbation occurred during the early phase of lip force recruitment. Variation in the lip force trajectories was manifest by a greater difference in net interangle force associated with lip perturbations indexed to the early Ft. This was interpreted to reflect the operation of a feedforward mechanism which may play a more significant role during an evolving speech action. Thus, the application of servo-controlled mechanosensory inputs effectively indexed the excitability of the facial motor nucleus during production of a simple speech phrase. Future studies are needed to explore mechanisms of short-term adaptation and trigeminofacial modulation during propositional speech in health and disease.

Inhibitory reflexes in human perioral facial muscles: A single-motor unit study

OBJECTIVE

To describe the reflex responses evoked by trigeminal stimulation in perioral facial motor units (MUs) in humans.

METHODS

We recorded single motor units (MUs) from perioral muscles performing three movements: elevation of the upper lip (levator labii superioris muscle--LLS), protrusion of the lips (orbicularis oris muscle--OOr) and depression of the lower lip (depressor anguli oris and depressor labii inferioris muscles--DAO/DLI) with concentric needle electrodes. MUs were tested during constant voluntary activation with non-painful cutaneous electrical stimuli applied to the mental or supraorbital nerves and intraorally. Analysis was performed with peristimulus histograms and cumulative sum.

RESULTS

Eighty MUs were sampled from 17 subjects. Cutaneous stimulation induced inhibition of discharge in 100% of the lip-depressor MUs, inhibition in 65-70% of LLS MUs and in 25% of OOr MUs. Mean latency of inhibition was of 35+/-12ms. Intraoral stimulation produced an equivalent percentage of inhibitory or facilitatory effects with no difference among the three muscles.

CONCLUSIONS

Reflex responses to cutaneous stimulation identify a completely inhibitory (DAO/DLI), a mainly inhibitory (LLS) and a mixed (OOr) pattern in perioral muscles.

SIGNIFICANCE

A purely inhibitory trigemino-facial reflex is present in lip-lowering muscles with potential use in clinical practice.

Topographical Characteristics of Motor Units of the Lower Facial Musculature Revealed by Means of High-Density Surface EMG

The objective of this study was to systematically characterize motor units (MUs) of the musculature of the lower face. MU endplate positions and principal muscle fiber orientations relative to facial landmarks were identified. This was done by the analysis of motor unit action potentials (MUAPs) in the surface electromyogram. Thirteen specially trained, healthy subjects performed selective contractions of the depressor anguli oris, depressor labii inferioris, mentalis, and orbicularis oris inferior muscles. Signals were recorded using recently developed, 0.3-mm thin and flexible high-density surface electromyography (sEMG) grids (120 channels). For each subject and each muscle and for different low contraction levels, representative MUAPs ("MU fingerprints") were extracted from the raw sEMG data according to their spatiotemporal amplitude characteristics. We then topographically characterized the lower facial MUs' endplate zones and main muscle fiber orientations on the individual faces of the subjects. These topographical MU parameters were spatially warped to correct for the different sizes and shapes of the faces of individual subjects. This electrophysiological study revealed a distribution of the lower facial MU endplates in more or less restricted, distinct clusters on the muscle often with eccentric locations. The results add substantially to the basic neurophysiologic and anatomical knowledge of the complex facial muscle system. They can also be used to establish objective guidelines for placement of conventional (surface or needle) EMG electrodes as well as for clinical investigations on neuromuscular diseases affecting the facial musculature. The localized endplate positions may also indicate optimal locations for botulinum toxin injection in the face.

Reflex responses of human lip muscles to mechanical stimulation during speech

The role played by reflex pathways in the production of movement has been a significant issue for motor control theorists interested in a wide variety of motor behaviors. From studies of locomotion and chewing, it appears that gains in reflex pathways can be altered so that activity in these pathways does not produce destabilizing responses during movement. In speech production, recent experimental evidence has been interpreted to suggest that autogenetic lip reflexes (perioral reflexes) are suppressed during sustained phonation or speech production. The present study was conducted to assess the effects of phonation, direction of movement, and ongoing speech production on reflex responses of lip muscles. The present results suggest, in contrast to earlier work, that this reflex pathway is not suppressed or absent because the amplitude of the observed response depends upon the activation levels of the various muscles of the lower lip and, therefore, indirectly on the nature of the gesture the subject is instructed to produce.

Mechanosensory modulation of perioral neuronal groups during active force dynamics

The spatiotemporal organization of the mechanically evoked perioral sensorimotor response was sampled from five normal females using a custom-designed linear motor operating under force feedback. Electromyographic activity was sampled from the superior and inferior segments of the orbicularis oris muscle during the production of a visually guided ramp-and-hold lip-rounding task. Brief mechanical inputs of approximately 0.45 N delivered to the left upper lip during the ramp-and-hold task produced a composite myogenic response characterized by phases of excitation and suppression. Modulation of the primary excitatory component (R1) of the mechanically evoked perioral response was found to be highly dependent upon the rate of force recruitment (1 N/s vs. 4 N/s) and the phase of force recruitment (20% vs. 50% vs. 80% of 1 N end-point force). Modulation of later occurring inhibitory (S1) and excitatory (R2) potentials were also found to be dependent upon differences in the rate and phase of force recruitment. The organization of the perioral sensorimotor response is considered in relation to speech motor control and the dynamic organization of neuronal groups subserving perioral sensorimotor activity.

Patterns of coordinated lower facial muscle function and their importance in facial reanimation

Coordinated patterns of lower facial muscle activity have been observed using 8-channel electromyography (surface electrodes in a bipolar configuration) on 11 healthy volunteers to provide a picture of the patterns of muscle activity during function. Measurements of integrated electrical activity were made to allow comparison of different muscle groups during active, active-against-resistance, and passive movements. A contraction reflex was assessed by an electromechanical device and noted in 8 of 11 subjects. Lower facial movements were found to involve simultaneous bilateral activity in all the muscle groups tested bilaterally and to require a balance between dilator and constrictor forces. The significance for facial reanimation surgery is that it is desirable to reconstruct balanced constrictor and dilator forces, the latter having vector pulls upwards, downwards and laterally to reconstruct the normal mechanism of lower facial movement.

Reflex responses of lip muscles in young and older women

The perioral reflex in response to innocuous mechanical stimulation of the lip vermilion was studied in 20 young and 20 older women. Responses to stimuli at the right and left sides of both the upper and lower lips were recorded. Results show significant specificity of response, especially for upper lip sites. Reflex response at the site of stimulation was greatest in amplitude and shortest in latency, followed by response at sites ipsilateral to the site of stimulation. Younger subjects showed greater localizing tendency than older subjects. Stimulation was significantly less likely to produce a reflex response in the older group. When reflex responses did occur, they were significantly lower in amplitude and longer in latency than the responses of the younger group. Nonetheless, reflex responses were common in both groups, with responses at the site of stimulation occurring 78% of the time in older women and 90% of the time in younger women. Every participant showed at least one reflex response to lip stimulation. Results suggest decreasing complexity of synaptic drive to the perioral system in old age but also show that reflexive response does not deteriorate completely, remaining an available element for motor control in normal older women.

Orolaryngeal reflex responses to changes in affective state

Previous research has shown that the eye-blink startle reflex can be modulated by changes in affective state. The purpose of the present study was to determine whether human perioral and trigemino-laryngeal reflexes are sensitive to affective state changes. Impetus for this study comes from theories suggesting that orolaryngeal reflexes may be modulated by affective states, and as such influence voice and speech production. Subjects were 24 classically trained female sopranos (21-35 years). Each produced a pursing lip posture while sustaining the continuant /m/ at 440 Hz and at a comfortable voice intensity level. Simultaneously subjects were shown an aversive, pleasant, or neutral slide (experimental conditions) or no slide (control condition) and received unanticipated, servo-controlled mechanical taps to the midline upper lip. Perioral responses were recorded bilaterally from the orbicularis oris inferior (OOI) muscle using surface electromyography (EMG). Trigemino-laryngeal responses were obtained indirectly by measuring changes in the voltage analog of the voice fundamental frequency (VF0). Reflex responses were detected by smoothing and signal-averaging the VF0 and rectified EMG signals. Response magnitude and latency measures were compared across the affective valence and no-slide conditions. Statistically significant differences were not observed between conditions for the magnitude or temporal measures of either reflex. Significant differences, independent of affective valence, were observed between right and left early excitatory perioral response magnitudes. Differences between the startle and orolaryngeal reflexes, as well as the implications of these findings for speech motor control, are discussed.

Activation of lip motor units with variations in speech rate and phonetic structure

The purpose of this study was to describe the activation characteristics of lip-muscle single motor units in relation to speech rate and phonetic structure. Repeated experiments were carried out on three adult subjects from whom recordings of lower lip EMG and two-dimensional displacement were obtained. Single motor unit recordings were obtained from the orbicularis oris inferior (OOI), depressor labii inferior (DLI), and mentalis (MENT) muscles. Subjects' tasks involved repeating CV syllables at 1 to 4 syllables per second (syl/sec). The distribution of interspike intervals and corresponding firing rates were obtained on 11 motor units. The firing rates of OOI and MENT motor units increased as syllable rate changed from 1 to 3 syl/sec, but firing rates tended to be equivalent at 3 and 4 syl/sec. DLI and tonic motor units showed little or no modulation in their firing rates with speech rate. Firing rate data and related observations on lip movement and EMG spike count levels suggest that distinct neuromechanical processes control lip movements at low and high speech rates. Both kinematic and EMG data support the expectation that phonetic structure has its greatest effects on lip opening compared to lip closing movements in CV syllables. OOI and MENT activation levels tended to be highest for /p/ productions compared to /w/ and /f/. This may be related to the requirements for complete lip closure and elevated levels of intraoral pressure for production of /p/.

Transitional properties of the mechanically evoked perioral reflex from infancy through adulthood

The organization of motor responses in the orbicularis oris muscle following the delivery of punctate mechanical inputs to vermilion skin of the lips was studied in a group of young infants, school-age children, and adults during periods of voluntary lip muscle activation. A specially designed multi-point array skin contactor, coupled to a position-servo controlled linear motor, was highly effective in driving the early component of the perioral reflex (R1). Overall, the evoked R1 response obtained from the infant was of variable amplitude relative to the children and adults, lacked response specificity, and occurred at a longer latency. This brainstem mediated sensorimotor action appears to take on several characteristics of the adult form by the age of 12. The emergence and maturation of mechanically evoked perioral reflexes is discussed in relation to the acquisition of motor skills, including speech and smiling.

Cutaneous oral-motor reflexes of children with normal and disordered speech

Cutaneous reflex responses in the human jaw-closing system to low-threshold, mechanical stimulation were investigated in 30 four- to six-year-old children with normal speech and in seven children with disordered speech. Responses of these children were smaller and less frequently observed than those of a seven- to eight-year-old group in an earlier study, suggesting that cutaneous oral-motor reflexes may develop with the acquisition of speech motor skills. Preliminary observations in a very small sample of speech-impaired children suggest that future work should explore the hypothesis that development of cutaneous oral reflexes is delayed in this group. The results of this and other studies suggest that human oral-motor reflexes undergo a pattern of continuous change throughout the lifespan.

Firing rate interactions among human orbicularis oris motor units

Groups of human motor units from the same muscle exhibit joint fluctuations in firing rate during voluntary muscle contraction. In an effort to determine whether similar behavior would be observed in a muscle lacking muscle spindles, motor unit firing behavior was examined in the human orbicularis oris inferior (OOI) during mild voluntary effort. Motor unit activity was recorded with a quadrifilar needle inserted in the OOI. Firing occurrences were identified using a motor unit decomposition procedure. Cross-correlation of motor unit firing rates revealed a tendency for motor unit firing rates to covary, although the effect was somewhat more variable than that observed previously in other skeletal muscles. There was also a statistically significant tendency for pairs of motor units to fire at simultaneous or near-simultaneous (+/- 5 ms) intervals (synchronization). Firing rate variability in OOI motor units was not significantly different (p > .05) from that observed in the FDI. Thus, the present results suggest that the common drive of human motor unit activity may not depend on the presence of muscle spindles.

The control of orofacial movements in speech

Rapid, complex movements of orofacial structures are essential to produce the sounds of speech. A central problem in speech production research is to discover the neural sources that generate the control signals supplied to motoneurons during speaking. Speech movement production appears to share organizational principles with other motor behaviors; thus speech movements probably arise from an interaction of centrally generated command signals with sensory information. That speech movements are ultimately linked to the perception of language, however, has led many investigators to suggest that speech movement control involves unique features, features that may be linked to abstract linguistic units.

Modulation of mechanically evoked perioral reflexes during active force

Mechanically evoked activity in orbicularis oris inferior and mentalis muscles was studied in humans during active lip force generation. A specially designed multipoint array skin contactor, coupled to a position servo-controlled linear motor, was used to deliver precise mechanical imputs to the lip vermilion. The array size of the skin contactor was systematically varied to quantify the effects of spatial summation on the amplitude and time course of the early component (R1) of the perioral reflex. For normal young adults, significant positive trends were found for the amplitude of R1 sampled from orbicularis oris inferior and mentalis muscle recording sites as contactor array size increased. Increasing skin contactor size from 2 to 16 points was also effective in shortening the latency of R1 by 3-5 ms.

Gain and spatial characteristics of human lip-muscle reflexes

Mechanically evoked early and late excitatory reflexes (E1 and E2) and suppression responses (S) were studied in human lip muscle. Acceleration pulses were applied at 30-150 m/s2 independently to the upper and lower lips during lip rounding and lower-lip depression postures, and to both lips during a lip-press posture. E1 responses were prevalent during lip rounding and press gestures and S responses during lip depression. Reflex magnitudes were well correlated with stimulus acceleration for the 3 response components, with E1 responses showing the strongest association. The slopes of linear equations relating reflex and stimulus magnitudes, i.e. reflex gains, for E1 and E2 responses were highly variable across subjects. This variability was partially related to subject gender, females showing larger reflex gains. Two novel findings on the spatial aspects of lip-muscle reflexes are that: (1) S responses in lower-lip muscle are more prevalent to upper-lip versus lower-lip stimulation, and (2) E1 and E2 gains in lower-lip muscle are larger for stimulation of the lower lip compared to stimulation of the upper lip. Further testing suggested that this latter pattern differs with form of stimulation, with a laterally directed sliding stimulus on the upper lip producing predominant effects in both the upper- and lower-lip muscle, and an indenting stimulus producing the largest reflexes in muscle of the stimulated lip.

Spatial organization of human perioral reflexes

Reflex responses recorded from the upper and lower divisions of the human orbicularis oris muscle were studied as a function of the site of stimulation. Stimuli were applied to 11 sites, ranging from the glabrous skin of the upper and lower lip vermilion borders to the hairy skin of the cheek. Highly localized, innocuous mechanical stimuli were created by displacing a servo-controlled probe over the surface of the perioral skin. Reflex response amplitude was strongly dependent on the site of stimulation. Stimulation of some sites, for example the ipsilateral corner of the mouth, the chin, and cheek, produced no responses, whereas stimulation of other sites, particularly the ipsilateral vermilion borders, produced large reflex responses. Changes in response amplitude as a function of stimulation site were the same for the upper lip and lower lip muscle recordings, with the largest responses at both recording sites produced by stimulation of the ipsilateral upper lip vermilion border. These results suggest that the upper and lower divisions of orbicularis oris share common synaptic drive, at least from inputs generated via reflex pathways, and that the upper vermilion border may be more densely innervated with mechanoreceptors than the lower. The latter hypothesis was supported by an additional experiment examining two-point discrimination thresholds for the glabrous skin of the upper and lower lips. Two-point thresholds were significantly smaller for the upper compared with the lower lip vermilion border.

Mechanical frequency detection thresholds in the human face

Afferent information originating from mechanoreceptors located within perioral facial skin is considered to play an important role in the development and maintenance of fine motor skill associated with the production of speech. To date, little quantitative information is available on the frequency sensitivity of the face to controlled sinusoidal displacements, especially in areas of tissue that change shape during speech. The purpose was to determine the mechanical frequency detection thresholds of select skin sites in the face and hand using a psychophysical procedure. Mean frequency detection thresholds for all facial skin sites were significantly higher than thresholds for the forefinger. Further, pacinian-type frequency sensitivity, characteristic of the finger, was absent in the face. Finally, vibratory stimuli may be useful in the assessment of skin sense in patients with suspected involvement of the trigeminal system. A case study is presented to highlight the quantitative nature of this psychophysical procedure.