Microelectrode recordings from human oral mechanoreceptors

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Functional Connectivity Evoked by Orofacial Tactile Perception of Velocity

The cortical representations of orofacial pneumotactile stimulation involve complex neuronal networks, which are still unknown. This study aims to identify the characteristics of functional connectivity (FC) evoked by three different saltatory velocities over the perioral and buccal surface of the lower face using functional magnetic resonance imaging in twenty neurotypical adults. Our results showed a velocity of 25 cm/s evoked stronger connection strength between the right dorsolateral prefrontal cortex and the right thalamus than a velocity of 5 cm/s. The decreased FC between the right secondary somatosensory cortex and right posterior parietal cortex for 5-cm/s velocity versus all three velocities delivered simultaneously (“All ON”) and the increased FC between the right thalamus and bilateral secondary somatosensory cortex for 65 cm/s vs “All ON” indicated that the right secondary somatosensory cortex might play a role in the orofacial tactile perception of velocity. Our results have also shown different patterns of FC for each seed (bilateral primary and secondary somatosensory cortex) at various velocity contrasts (5 vs 25 cm/s, 5 vs 65 cm/s, and 25 vs 65 cm/s). The similarities and differences of FC among three velocities shed light on the neuronal networks encoding the orofacial tactile perception of velocity.

Face-referenced measurement of perioral stiffness and speech kinematics in Parkinson's disease

PURPOSE

Perioral biomechanics, labial kinematics, and associated electromyographic signals were sampled and characterized in individuals with Parkinson's disease (PD) as a function of medication state.

METHOD

Passive perioral stiffness was sampled using the OroSTIFF system in 10 individuals with PD in a medication ON and a medication OFF state and compared to 10 matched controls. Perioral stiffness, derived as the quotient of resultant force and interoral angle span, was modeled with regression techniques. Labial movement amplitudes and integrated electromyograms from select lip muscles were evaluated during syllable production using a 4-D computerized motion capture system.

RESULTS

Multilevel regression modeling showed greater perioral stiffness in patients with PD, consistent with the clinical correlate of rigidity. In the medication-OFF state, individuals with PD manifested greater integrated electromyogram levels for the orbicularis oris inferior compared to controls, which increased further after consumption of levodopa.

CONCLUSIONS

This study illustrates the application of biomechanical, electrophysiological, and kinematic methods to better understand the pathophysiology of speech motor control in PD.

Generation of the masticatory central pattern and its modulation by sensory feedback

The basic pattern of rhythmic jaw movements produced during mastication is generated by a neuronal network located in the brainstem and referred to as the masticatory central pattern generator (CPG). This network composed of neurons mostly associated to the trigeminal system is found between the rostral borders of the trigeminal motor nucleus and facial nucleus. This review summarizes current knowledge on the anatomical organization, the development, the connectivity and the cellular properties of these trigeminal circuits in relation to mastication. Emphasis is put on a population of rhythmogenic neurons in the dorsal part of the trigeminal sensory nucleus. These neurons have intrinsic bursting capabilities, supported by a persistent Na(+) current (I(NaP)), which are enhanced when the extracellular concentration of Ca(2+) diminishes. Presented evidence suggest that the Ca(2+) dependency of this current combined with its voltage-dependency could provide a mechanism for cortical and sensory afferent inputs to the nucleus to interact with the rhythmogenic properties of its neurons to adjust and adapt the rhythmic output. Astrocytes are postulated to contribute to this process by modulating the extracellular Ca(2+) concentration and a model is proposed to explain how functional microdomains defined by the boundaries of astrocytic syncitia may form under the influence of incoming inputs.

Sensations evoked by microstimulation of single mechanoreceptive afferents innervating the human face and mouth

Intraneural microneurography and microstimulation were performed on single afferent axons in the inferior alveolar and lingual nerves innervating the face, teeth, labial, or oral mucosa. Using natural mechanical stimuli, 35 single mechanoreceptive afferents were characterized with respect to unit type [fast adapting type I (FA I), FA hair, slowly adapting type I and II (SA I and SA II), periodontal, and deep tongue units] as well as size and shape of the receptive field. All afferents were subsequently microstimulated with pulse trains at 30 Hz lasting 1.0 s. Afferents recordings whose were stable thereafter were also tested with single pulses and pulse trains at 5 and 60 Hz. The results revealed that electrical stimulation of single FA I, FA hair, and SA I afferents from the orofacial region can evoke a percept that is spatially matched to the afferent's receptive field and consistent with the afferent's response properties as observed on natural mechanical stimulation. Stimulation of FA afferents typically evoked sensations that were vibratory in nature; whereas those of SA I afferents were felt as constant pressure. These afferents terminate superficially in the orofacial tissues and seem to have a particularly powerful access to perceptual levels. In contrast, microstimulation of single periodontal, SA II, and deep tongue afferents failed to evoke a sensation that matched the receptive field of the afferent. These afferents terminate more deeply in the tissues, are often active in the absence of external stimulation, and probably access perceptual levels only when multiple afferents are stimulated. It is suggested that the spontaneously active afferents that monitor tension in collagen fibers (SA II and periodontal afferents) may have the role to register the mechanical state of the soft tissues, which has been hypothesized to help maintain the body's representation in the central somatosensory system.

Sensory-motor function of human periodontal mechanoreceptors*

Natural teeth are equipped with periodontal mechanoreceptors that signal information about tooth loads. In the present review, the basic force-encoding properties of human periodontal receptors will be presented along with a discussion about their likely functional role in the control of human mastication. Microneurographic recordings from single nerve fibres reveal that human periodontal receptors adapt slowly to maintained tooth loads. Most receptors are broadly tuned to the direction of force application, and about half respond to forces applied to more than one tooth. Populations of periodontal receptors, nevertheless, reliably encode information about both the teeth stimulated, and the direction of forces applied to the individual teeth. Information about the magnitude of tooth loads is made available in the mean firing rate response of periodontal receptors. Most receptors exhibit a markedly curved relationship between discharge rate and force amplitude, featuring the highest sensitivity to changes in tooth load at very low force levels (below 1 N for anterior teeth and 4 N for posterior teeth). Thus, periodontal receptors efficiently encode tooth load when subjects contact and gently manipulate food using the teeth. It is demonstrated that signals from periodontal receptors are used in the fine motor control of the jaw and it is clear from studies of various patient groups (e.g. patients with dental implants) that important sensory-motor functions are lost or impaired when these receptors are removed during the extraction of teeth.

Sensory and motor function of teeth and dental implants: a basis for osseoperception

1. When dental implants are loaded mechanically, a sensation, often referred to as osseoperception, is evoked. The sensory signals underlying this phenomenon are qualitatively different from the signals evoked when loading a natural tooth. In contrast with osseointegrated dental implants, natural teeth are equipped with periodontal mechanoreceptors that signal information about tooth loads. In the present review, the functional properties of human periodontal mechanoreceptors will be presented, along with a discussion about their likely functional role in the control of human jaw actions. 2. Microneurographic experiments reveal that human periodontal mechanoreceptors adapt slowly to maintained tooth loads. Populations of periodontal receptors encode information about both which teeth are loaded and the direction of forces applied to individual teeth. 3. Most receptors exhibit a markedly curved relationship between discharge rate and force amplitude, featuring the highest sensitivity to changes in tooth load at surprisingly low forces (below 1 N for anterior teeth and 4 N for posterior teeth). Accordingly, periodontal receptors efficiently encode tooth load when subjects first contact, hold and gently manipulate food by the teeth. In contrast, only a minority of receptors encodes the rapid and strong increase in force generated when biting through food. 4. It is concluded that humans use periodontal afferent signals to control jaw actions associated with intra-oral manipulation of food rather than exertion of jaw power actions. Consequently, patients who lack information from periodontal receptors show an impaired fine motor control of the mandible.

Encoding of Amplitude and Rate of Tooth Loads by Human Periodontal Afferents From Premolar and Molar Teeth

Microneurographic recordings were obtained from 20 periodontal mechanoreceptive afferents in the inferior alveolar nerve while force profiles of different amplitudes and rates were applied to a premolar or the first molar in the most sensitive direction. The majority of afferents (17/20) showed a hyperbolic relationship between the steady-state discharge rate and the amplitude of the stimulating force, featuring a pronounced saturation tendency. These afferents were also characterized by a similar decline in dynamic sensitivity with increasing amplitude of background force. However, a few afferents (3/20) showed nearly linear stimulus-response relationships and a small decline in dynamic sensitivity with increasing tooth load. Quantitative models developed for all afferents successfully predicted the afferent discharge rates for novel force stimulations. Application of the transfer function to chewing forces predicted that the discharge rates of periodontal afferents rapidly increased at initial tooth contact and continued to discharge as long as the tooth was loaded. However, due to the marked saturation tendencies at higher forces, most periodontal afferents poorly encoded the magnitude of the strong chewing forces. In addition, the discharge rates of a minority of afferents continued to reflect the force profile during high chewing forces. The results revealed that periodontal afferents of posterior teeth were less sensitive at low tooth loads compared with afferents of anterior teeth. During each chewing cycle, periodontal afferents may provide information about the mechanical properties of food shortly after tooth contact that can be used to scale the muscle commands of the upcoming power phase.

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.

Analysis of strain around endosseous dental implants opposing natural teeth or implants

STATEMENT OF PROBLEM

Natural teeth and implants have different force transmission characteristics to bone.

PURPOSE

The purpose of this study was to compare strains induced around a natural tooth opposing an implant with strains around occluding implants under static and dynamic loads.

MATERIAL AND METHODS

Occlusion was created between a natural molar tooth and an implant in 1 side, and 2 implants in the contralateral side of acrylic resin models of both jaws. Strain-gauges were bonded around the neck of the natural tooth and implants, and strains were measured under 75 N and 100 N static axial and lateral dynamic loads in separate load situations using a data acquisition system at sample rate of 1000 Hz. The strain data of the natural tooth and implants were compared for each load by Kruskal-Wallis testing followed by the multiple comparison test (alpha=.05).

RESULTS

Compressive strains were induced around natural tooth and implants as a result of static axial loading, whereas combinations of compressive and tensile strains were observed during lateral dynamic loading. Strains around the natural tooth were significantly lower than the opposing implant and occluding implants in the contralateral side for most regions under all loading conditions ( P <.05). There was a general tendency for increased strains around the implant opposing natural tooth under higher loads and particularly under lateral dynamic loading ( P <.05).

CONCLUSION

Under static and dynamic loads, strain magnitudes around a natural tooth were significantly lower than that of an opposing implant and occluding implants in the contralateral side.

A method for intraoperative microneurographic recording of unitary activity in the trigeminal ganglion of patients with trigeminal neuralgia

The etiology of trigeminal neuralgia appears to be vascular compression of the nerve at the root entry zone. However, the physiologic mechanism of trigeminal neuralgia remains uncertain. To gain insight into the pathophysiology of the disorder, we developed a method for intraoperative microneurographic recordings from the trigeminal ganglion of patients with trigeminal neuralgia. The recordings are performed immediately prior to standard percutaneous trigeminal gangliolysis for pain relief. Spontaneous or evoked single- and multi-unit action potential activity can be recorded and the location of receptive fields determined. The method should facilitate the testing of hypotheses concerning the origin of this unique pain disorder.

Effects of machined/turned, TiO2-blasted and sandblasted/acid-etched titanium oral implant surfaces on nerve conduction: A study on isolated rat sciatic nerves

The purpose of this study was to explore the clinical relevance of the effects of machined/turned, TiO(2)-blasted and sandblasted/acid-etched titanium oral implant surfaces on nerve conduction. Isolated rat sciatic nerves were placed between two suction electrodes in a pyrex bath containing a tyrode solution. Evoked compound action potentials (cAPs) of the nerves were recorded before and after contact with the implants. The mandibular incisors of randomly selected animals were extracted and changes in cAP amplitudes were used as controls. The differences in final cAP values of Astra Tech implants and rat natural teeth were insignificant (P < 0.05), whereas the differences between other groups were significant (P < 0.05). Machined/turned-surface implants did not cause any change in cAPs. A slight decrease in cAPs was observed for TiO(2)-blasted and sandblasted/acid-etched implants, and the natural teeth. The reductions of cAPs in latter groups were not 50% after an application time of 300 min. The cAP changes of nerves contacting TiO(2)-blasted and sandblasted/acid-etched oral implants fall within physiologic limits in vitro. Machined/turned, TiO(2)-blasted, and sandblasted/acid-etched titanium implant surfaces do not lead to irreversible neurotoxic effects.

Receptive field properties of human periodontal afferents responding to loading of premolar and molar teeth

Impulses in 45 single mechanoreceptive afferents were recorded from the human inferior alveolar nerve with permucosally inserted tungsten microelectrodes. All afferents responded to mechanical stimulation of one or more premolar or molar teeth and most likely innervated their periodontal ligaments. For each afferent, isolated "ramp-and-hold" shaped force profiles of similar magnitudes (252 +/- 24 mN; mean +/- SD) were applied to the lower first premolar, the second premolar, and the first molar on the recording side. The tooth loads were applied in six directions: lingual, facial, mesial, and distal in the horizontal plane and up and down in the vertical direction of the tooth. The afferents response during the static phase of the stimulus was analyzed. All afferents were slowly adapting, discharging continuously in response to static forces in at least one stimulation direction. Twenty-nine afferents (64%) were spontaneously active, exhibiting an ongoing discharge in the absence of external stimulation. Stimulation of a single tooth was found to excite each afferent most strongly. The most sensitive tooth (MST) was the first premolar for 23, the second premolar for 13, and the first molar for 9 afferents. About half of the afferent population also responded to loading of one or two more teeth. The response profiles of these afferents indicated that the multiple-teeth receptive fields were due to mechanical coupling between the teeth rather than branching of single afferents to innervate several teeth. The afferent responses to loading the mesial and distal halves of the first molars were very similar. Thus both intensive and directional aspects of the afferent response when loading one side of the tooth was preserved to a great extent when loading the other side. When loading the MST, the afferents typically showed excitatory responses in two to four of the six stimulation directions, i.e., the afferents were broadly tuned to direction of tooth loading. In the horizontal plane, the afferent populations at the premolar teeth expressed no clear directional preferences. The afferents at the molar, however, showed a strong directional bias in the distal-lingual direction. In the vertical plane, there was a preference for downward-directed forces with a gradually decreasing sensitivity distally along the dental arch. The present results demonstrate that human periodontal afferents supplying anterior and posterior teeth differ in their capacity to signal horizontal and vertical forces, respectively.

Orofacial mechanoreceptors in humans: encoding characteristics and responses during natural orofacial behaviors

We used microneurography to characterize stimulus-encoding properties of low-threshold mechanoreceptive afferents in human orofacial tissues. Signals were recorded from single afferents in the infraorbital, lingual and inferior alveolar nerves while localized, controlled, mechanical stimuli were delivered to the facial skin, lips, oral mucosa and teeth. We likewise analyzed activity in these afferents during orofacial behaviors such as speech, chewing and biting. The afferents in the soft tissues functionally resemble four types described in the human hand: hair follicle afferents, slowly adapting (SA) type I and type II afferents and fast adapting (FA) type I afferents. Afferents in the facial skin, lips and buccal mucosa respond not only to contact with environmental objects, but also to contact between the lips, changes in air pressure generated for speech sounds, and to facial skin and mucosa deformations that accompany lip and jaw movements associated with chewing and swallowing. Hence, in addition to exteroceptive information, these afferents provide proprioceptive information. In contrast, afferents terminating superficially in the tongue do not signal proprioceptive information about tongue movements in this manner. They only respond when the receptive field is brought into contact with other intraoral structures or objects, e.g. the teeth or food. All human periodontal afferents adapt slowly to maintained tooth loads. Populations of periodontal afferents encode information about both which teeth are loaded and the direction of forces applied to individual teeth. Most afferents exhibit a markedly curved relationship between discharge rate and force amplitude, featuring the highest sensitivity to changes in tooth load at low forces (below 1 N). Accordingly, periodontal afferents efficiently encode tooth load when subjects first contact, hold, and gently manipulate food by the teeth. In contrast, only a minority of the afferents encodes the rapid and strong force increase generated when biting through food. We conclude, that humans use periodontal afferent signals to control jaw actions associated with intraoral manipulation of food rather than exertion of jaw power actions.

Response properties of periodontal mechanoreceptors in rats, in vitro

Single unit activities of the inferior alveolar nerve evoked by calibrated von Frey stimuli (1.1, 2.9, 7.8, 11.8, and 17.2mN) on the periodontal ligaments of the mandibular molars or incisors were recorded in an in vitro jaw-nerve preparation of Wistar albino rats. The data of 55 (lower incisor) and 100 (lower molars) units were collected in the present study. Both rapidly (RA) and slowly adapting (SA) type units were found in the incisors, and most of these units were innervated by Abeta fibers. While all the units of the molars were of RA types, the innervated fibers of two-thirds (67/100) of the units have been identified as Adelta fibers. The response patterns of the RA type were subdivided into three types (ON, OFF or ON-OFF type) both in the incisors and the molars. While von Frey thresholds of all incisor units were 11.8 mN except one unit that was 7.8 mN, those of the molars varied from 2.9 to 11.8 mN. In the molars, a majority of afferents innervated the periodontal ligaments of more than one tooth. This study suggests that response properties of periodontal mechanoreceptors are different between the incisors and the molars in rats, suggesting that these receptors have different functions in the regulation of mastication.

An in vitro temporomandibular joint-nerve preparation for pain study in rats

A novel in vitro TMJ-nerve preparation was developed to quantitatively study peripheral sensory mechanisms of temporomandibular joint (TMJ). The TMJ region on one side (including mandibular head, disc, retrodiscal tissue and mandibular fossa) of adult Wistar albino rats was excised together with the auriculo-temporal nerve. The block was preserved in a modified Krebs-Henseleit solution saturated with O(2)/CO(2) (95/5%) gas mixture. Using a calibrated von Frey type apparatus, mechanical noxious stimulation was applied directly to various sites within the TMJ region. In addition, thermal and chemical noxious stimuli were also attempted. Stable recordings of single unit activities from the auriculo-temporal nerve could be obtained for as long as 5 h, which was sufficient to analyze the response properties of the TMJ units to various stimuli. This new preparation would be useful for investigating TMJ peripheral sensory mechanisms, especially pain, and potentially makes it possible to reveal neural mechanisms of temporomandibular arthralgia, a syndrome that has recently shown an increased incidence in clinical dentistry.

Low-threshold mechanoreceptive afferents in the human lingual nerve

Intrafascicular multiunit activity and impulses in single mechanoreceptive afferents were recorded from the human lingual nerve with permucosally inserted tungsten microelectrodes. Nylon filaments and blunt glass probes were used for mechanical stimulation of the mucosa of the dorsal surface of the tongue. The innervation territories of nine nerve fascicles were mapped during multiunit recordings. All fascicle fields included the tip of the tongue, suggesting a particularly high innervation density for this area. Thirty-three single mechanoreceptive afferents were isolated and studied. Of these afferents, 22 were characterized by very small mucosal receptive fields (range: 1-19.6 mm2; geometric mean: 2.4 mm2) and responded to extremely low mechanical forces (force threshold range: 0.03-2 mN; geometric mean: 0.15 mN). As such, it was concluded that these "superficial" units terminated near the surface of the tongue. The remaining 11 units responded to probing of large areas of the tongue (> 200 mm2) and exhibited high force thresholds (> or = 4 mN). It was concluded that these "deep" units terminated in the muscle mass of the tongue. Fourteen of the superficial units were classified as rapidly adapting and resembled the fast-adapting type I afferents described for the glabrous skin of the human hand. The rapidly adapting units responded both during the application and removal of, but not during maintenance of, the mechanical stimuli on the receptive field. Two types of slowly adapting responses were observed. One type (characteristic of only 2 units) was characterized by a pronounced sensitivity to force change during the application and removal of the mechanical stimuli and an irregular static discharge during maintenance of the stimulus on the receptive field. In contrast, the other six units exhibited a weak sensitivity to force change, a highly regular static discharge, and spontaneous activity. As such, these two types of slowly adapting units resembled the slowly adapting I and II afferents, respectively, described for the hand. All 11 deep units were slowly adapting, and 7 were, in addition, spontaneously active. The units were not equally sensitive to the application and removal of the mechanical stimuli, suggesting at least two different modes of termination in tongue muscle. The deep units reliably encoded information about tongue movements in the absence of direct contact with the receptive field. In contrast, the superficial units responded vigorously when the tongue was moved to bring the receptive field into physical contact with other intraoral structures.

An in vitro jaw-nerve preparation for oral sensory study in the rat

We have developed an in vitro jaw-nerve preparation in Wistar albino rats, suitable for the quantitative study of peripheral sensory mechanisms in the oral region. The mandible on one side was excised together with the inferior alveolar nerve and was kept in a modified Krebs-Henseleit solution saturated with O2/CO2 (95:5) gas mixture. Recordings were made from single afferent fibers in the inferior alveolar nerve in response to mechanical stimulation of various oral sites (oral mucosa, periodontal ligament and tooth pulp) by calibrated von Frey hairs. In the case of tooth pulp stimulation, heat and chemical (bradykinin) stimuli were also used. Stable unitary recordings could be made for up to 5 h which was long enough to record the activities evoked by various stimuli applied to the oral area. This paper will describe the procedure for making this preparation together with some responses of single units in the inferior alveolar nerve to mechanical, thermal and chemical stimulations applied to this preparation.

Role of periodontal ligament receptors in the tactile function of teeth: a review

The tactile function of the human periodontal mechanoreceptors has mostly been studied by psychophysical approaches. It was concluded that periodontal mechanoreceptors play a major role in the tactile function of teeth. It must be noted however that the interocclusal tactile threshold is not solely determined by periodontal mechanoreceptors but also by pulpal, muscular or articular receptors. While temporomandibular joint receptors play a minor role, muscular receptors are important in the discriminatory ability for a mouth opening of 5 mm and more. To discriminate between the contribution of periodontal and other receptors in the oral tactile function, future studies should use appropriate psychophysical methodologies and well-defined stimulus parameters.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

Directional sensitivity of human periodontal mechanoreceptive afferents to forces applied to the teeth

1. Single-unit impulse activity from thirty-eight mechanoreceptive afferent fibres was recorded in the human inferior alveolar nerve using tungsten microelectrodes. All afferents responded to mechanical stimulation of the teeth and most likely supplied periodontal mechanoreceptors. 2. All afferents showed their highest sensitivity to forces applied to a particular tooth (the lower incisors, the canine or the first premolar). Forces with 'ramp-and-hold' shaped profiles of similar magnitudes were applied to that tooth in the following six directions: lingual, labial, mesial and distal in the horizontal plane, and up and down in the axial direction of the tooth. Both static and dynamic response components were analysed. 3. All afferents were 'slowly adapting' since they discharged continuously in response to static forces in at least one stimulation direction. Twenty-five afferents (66%) were spontaneously active in the sense that they showed an on-going discharge in the absence of external stimulation. 4. Diverse receptive fields were observed. Most afferents (74%) responded to static forces in two or three of the four horizontal directions. Likewise, all units showed excitatory responses to axial loading with a majority (74%) responding in one of the two axial directions and the remainder in both axial directions. Spontaneously active afferents generally decreased their discharge rate when stimulated in directions opposite to the directions exciting the afferent. With regard to population responses, approximately half of the afferents showed excitatory responses to each stimulus direction except for downwards, in which 86% responded. 5. Twenty-three afferents (61%) exhibited the strongest response to forces in one of the horizontal directions. Of those, a majority were most responsive to the lingual direction (52%) and some to the labial direction (30%). Accordingly, the discharge rates during force application averaged over the whole afferent sample were highest in these directions. Of the remaining afferents, most responsive to one of the axial directions, 60% showed their strongest responses to forces in the downward direction. 6. Forty-five per cent of the afferents showed wider receptive fields to the dynamic component of the force stimulation than to the static. The direction of maximal sensitivity, however, remained the same with few exceptions. 7. It was demonstrated that even though individual periodontal mechanoreceptive afferents provide ambiguous information regarding the direction of a force applied to a tooth, populations of such afferents are well suited to give detailed directional information. It is suggested that such information may play an important role for the control of mastication.

A possible mechanism of mechanoreception in Ruffini endings in the periodontal ligament of hamster incisors

The topographical relationship between Ruffini endings and the surrounding collagen fibers in the periodontal ligament of hamster incisors was investigated by means of both immunohistochemistry for neurofilament protein (NFP) and electron microscopy. Periodontal Ruffini endings, a type of stretch receptor, were present exclusively in the alveolar half of the periodontal ligament. Their axon terminals were densely and regularly associated with transverse collagen fibers, possibly forming a mechanoreceptive complex. Since blood sinuses with frequent anastomoses extended throughout the alveolus-related part, the densely innervated collagen bundles were separated from each other by the vascular spaces. Electron microscopic observation of specimens stained with tannic acid revealed a linkage between the axon terminals of the Ruffini endings and the surrounding collagen filaments. The axon terminals were enveloped by multiple layers of the basal lamina, which were penetrated by collagen filaments. The irregularly arranged collagen filaments were sandwiched between electron-dense laminae of the multilayered basal lamina. The possible mechanism of mechanoreception by the periodontal Ruffini endings is discussed on the basis of the immunohistochemical and ultrastructural findings.

Vibrotactile adaptation on the face

Threshold amplitude for vibration is elevated if testing is preceded by extended exposure to a vibratory adapting stimulus of appropriate amplitude and frequency. This phenomenon, previously studied almost exclusively on the hand, is here shown for the first time to occur on the face as well. Adaptation is then used analytically to determine that the two-branched threshold-versus-frequency function obtained on the face by Verrillo and Ecker (1977) represents the activity of two distinct mechanisms. Action spectra of vibrotactile adaptation reveal the presence of both mechanisms even in subjects whose unadapted threshold function (like that reported by Barlow, 1987) shows no sign of duplexity. Finally, the data suggest that on the face (unlike the hand), cross-channel adaptation may occur at high adapting amplitudes.

Mastication and its control by the brain stem

This review describes the patterns of mandibular movements that make up the whole sequence from ingestion to swallowing food, including the basic types of cycles and their phases. The roles of epithelial, periodontal, articular, and muscular receptors in the control of the movements are discussed. This is followed by a summary of our knowledge of the brain stem neurons that generate the basic pattern of mastication. It is suggested that the production of the rhythm, and of the opener and closer motoneuron bursts, are independent processes that are carried out by different groups of cells. After commenting on the relevant properties of the trigeminal and hypoglossal motoneurons, and of internuerons on the cortico-bulbar and reflex pathways, the way in which the pattern generating neurons modify sensory feedback is discussed.

A comparison of pulpal and tactile detection threshold levels in young adults

Pulpal and tactile sensory detection threshold (SDT) values of the maxillary and mandibular incisor and canine teeth were determined and recorded for young adult subjects at three test sessions. A commercially available monopolar pulp-testing device was used to determine pulpal SDT values, and von Frey hairs were used to determine the tactile SDT values. Statistical analysis of the data indicated that the pulpal and tactile test procedures were sufficiently reliable in identifying what is defined as the true SDT value for both parameters. The study confirmed the constancy of these SDT values over days and the independence of the values for jaw, side, and sex. SDT values were influenced, however, by tooth type, with canine teeth displaying higher tactile and pulpal values than the central and lateral incisor teeth. These data should provide a suitable baseline for a longitudinal study to identify the SDT fluctuations known to occur in tooth pulp and dental supporting tissues in a growing human population.

Intrafascicular multi-unit recordings from the human infra-orbital nerve

Intrafascicular micro-electrode recordings were made from the human infra-orbital nerve close to the infra-orbital foramen. The fascicular organization was studied and multi-unit activity from low-threshold mechanoreceptive afferents was recorded during tactile stimuli, vibration and facial movements. Attempts were also made to record C-fibre activity. Innervation zones corresponding to 66 fascicles were mapped with tactile stimuli on facial hairy skin and the red zone of the lip. Most of these fields were located on the upper lip, where they overlapped, indicating a high innervation density. The fields had a median size of 3.8 cm2. Skin indentation evoked dynamic on- and off-responses and a much less pronounced static discharge. The afferent double-peaked responses to an oscillating probe applied to the peri-oral region induced similar grouping of the EMG activity during sustained lip protrusion. Contraction of facial muscles and stretching of the skin evoked on- and off-responses, whereas the static discharge was less pronounced, especially during sustained stretching. The dynamic sensitivity to minor variations in contraction and stretching was high, and during normal facial movements, as in speech, there was a barrage of impulses originating from mechanoreceptors within large facial areas. Functional implications of these sensorimotor interactions are discussed. Sympathetic C-fibre activity, frequently seen in recordings from the supra-orbital nerve, was never encountered in the infra-orbital nerve recordings, indicating a lack of such fibres. Failure to detect afferent C-fibre activity could be explained by methodological difficulties.

Mechanoreceptive units in the human infra-orbital nerve

Eighty-four low-threshold mechanoreceptive afferents innervating facial hairy skin or the red zone of the lip were recorded with micro-electrodes from the human infra-orbital nerve. Based on their responses to skin indentations, the units were classified as slowly or fast-adapting, with small or large receptive fields. The responses to hair movement, skin stretching and contraction of facial muscles were also studied. Both hairy skin and the red zone were innervated by slowly and by fast-adapting units. The innervation density was found to be highest at the corner of the mouth and on the upper lip. Slowly adapting units with small fields in hairy skin were most common and included units responding to sustained hair displacement. These units are suggested to have two types of end-organs, either pilo-Ruffini endings or Merkel cell-neurite complexes. The slowly adapting units with large fields were spontaneously active stretch receptors and may have corresponded to Ruffini corpuscles, although the possibility of other, intramuscular, receptors could not be ruled out. Only one afferent possibly innervated a Pacinian corpuscle. Most mechanoreceptors were also activated by skin stretching or contraction of facial muscles. Many of the slowly adapting units with small fields responded to the onset and release of stretch, whereas their discharge in response to sustained stretching adapted more or less completely. Spontaneously active units had the most sustained stretch response. It is concluded that several types of cutaneous mechanoreceptors can operate as sensitive proprioceptors of importance for facial kinaesthesia and motor control.

Dual somatosensory representation of the periodontium in nucleus ventralis posteromedialis of the cat thalamus

The distribution of periodontal units within the nucleus ventralis posteromedialis (VPM) was studied in urethane-chloralose-anesthetized cats. Recordings were made from a total of 63 periodontal units. Of these, 55 received contralateral input, the rest receiving ipsilateral input. The periodontal units receiving contralateral inputs were located in the medial part of the VPM proper, whereas those receiving ipsilateral inputs were found in the lateral subdivision of the nucleus ventralis posteromedialis parvocellularis (VPMpcl).

Mechanoreceptor activity from the human face and oral mucosa

The feasibility of adopting the microneurography technique (Vallbo and Hagbarth 1968) as a tool to investigate the mechanoreceptive innervation of peri- and intra-oral tissues was explored. Multi-unit activity and impulses in single nerve fibers were recorded from the infraorbital nerve in healthy volunteers. The innervation territories of individual nerve fascicles were mapped. These varied considerably but most fascicle fields comprised the corner of the mouth. Twenty-four single mechanoreceptive units were recorded. Eighteen innervated the skin of the face, and six innervated the mucous membranes of the lips or cheeks. A majority of the mechanoreceptive afferent units were slowly adapting with small and well defined receptive fields. It is suggested that the various slowly adapting responses may originate from two different types of afferent units. No afferents showed response properties similar to typical Pacinian-corpuscle afferents.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

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.

Response fields of the periodontal mechanosensitive units in the superior alveolar nerve of the cat

Response properties of periodontal mechanoreceptor primary afferent fibers recorded from the superior alveolar nerve were studied in the cat. The left maxillary canine tooth was stimulated manually in 8 directions and/or in 24 directions in the horizontal plane by a specially designed stimulator. The responses of 328 slowly adapting units observed were affected by the direction of stimulus. These units were classified into three groups according to the shape of the response field: a broad type (more than 180 degrees), a medium type (90 degrees to 180 degrees), and a narrow type (less than 90 degrees). The groups contained 27 units (8.2%), 284 units (86.6%), and 17 units (5.2%), respectively, and the remaining 10 units (10.0%) were unclassified. The shape of each response field was little changed by changes in the stimulus intensity. Every response field investigated showed a unimodal distribution. These results were different from those of Mei et al. (1975) who reported that the response fields of units recorded from a Gasserian ganglion had generally consisted of two parts.

Contractile activity of the masseter muscle in experimental clenching and grinding of the teeth in man

Six human subjects exercised maximum voluntary tooth clenching and right-sided tooth grinding to determine the onset of fatigue in the right and left masseter muscle. Static and dynamic contractile activity of the two muscles was determined by surface electromyography. Muscle fatigue appeared after about 30 s of isometric contractions (clenching), while 30 s of combined concentric and eccentric contractions (grinding) induced no fatigue. In the right muscle the contractile activity of negative work (eccentric contractions of mandibular laterotrusion) was about 50% of that of positive work (concentric contractions of mandibular mediotrusion). During clenching an increased number of contacting teeth might have facilitated the contractile activity of the two muscles. During grinding the height of the cusps of the working side teeth might have contributed to a decrease of tension production by the right masseter muscle. Non-working side tooth contacts and peripheral receptors might have facilitated the contractile activity of the left masseter muscle during tooth grinding. Static contractile activity of the mandibular elevator muscles produced high levels of isometric tension and led to masseter muscle fatigue in about 30 s. The same duration of dynamic contractile activity, resulting in low levels of tension during positive and negative work, did not induce fatigue.

Influence of experimental interfering occlusal contacts on the activity of the anterior temporal and masseter muscles during submaximal and maximal bite in the intercuspal position

The effects of an intercuspal occlusal interference on the pattern of activity of the anterior temporal and masseter muscles during submaximal and maximal bite, were studied in eleven volunteers with complete, natural dentitions. The results show that, during maximal and submaximal bite an occlusal interference (about 0.5 mm) in the intercuspal position is able to disturb the almost symmetric pattern of muscular activity in the anterior temporal and masseter muscles. Further, the level of muscular activity during maximal bite decreased significantly in all muscles studied. In some subjects, the decrease of muscular activity could still be observed one week after insertion of the interfering contact. After eliminating the interference, the muscular co-ordination pattern improved and the level of muscular activity increased significantly.

The properties of cells in the cat trigeminal main sensory and spinal subnuclei activated by mechanical stimulation of the periodontium

Neurophysiological exploration of the trigeminal sensory complex was done on 42 cats under ketamine anaesthesia, paying special attention to units receiving a periodontal input. Among 492 cells recorded in the trigeminal sensory complex, 73 responded to mechanical stimulation of the periodontium and were precisely localized histologically. Thalamic stimulation was also delivered to the ipsi and contralateral ventro-posterior nucleus to test for antidromic responses. Results of this systematic study were plotted on reference drawings of the full extent of the trigeminal sensory complex.

The distribution of mechanoreceptors in the periodontal ligament of the mandibular canine tooth of the cat

1. Periodontal mechanoreceptor activity has been recorded from fibres in the inferior alveolar nerve. 2. A method has been developed for punctate and electrical stimulation of the periodontal ligament mechanoreceptors through a thin layer of bone overlying the labial aspect of the left mandibular canine tooth root. 3. The distribution of periodontal mechanoreceptors in the labial aspect of the left mandibular canine tooth has been described. 4. All receptors located responded maximally when that part of the ligament in which they lay was in tension. 5. It is suggested that there may be only one type of mechanoreceptor and that the rate of adaptation is dependent on the location of the receptor within the periodontal tissues.

Effects of conditioning vibratory stimulation on pain threshold of the human tooth

The effect of mechanical vibratory stimulation on the human tooth pain threshold has been studied in 4 healthy naïve subjects. Vibration at 10 and 100 Hz was applied to a maxillary incisor and the electrical pain threshold was measured in an adjacent tooth. During the conditioning stimulation the pain threshold increased 1.2-1.8 times the unconditioned values. The present study indicates that mechanical vibratory stimulation applied to human teeth elevates the pain threshold in adjacent teeth.

Effects of sympathetic nerve stimulation on intra-oral mechanoreceptor activity in the cat

1. Micro-electrode recordings were made from intra-oral mechanoreceptor neurones in the trigeminal ganglion and mesencephalic nucleus of the fifth nerve in the cat.2. The effect of cervical sympathetic trunk stimulation on the discharge of the mechanoreceptors to a controlled force was observed.3. Almost half of the mechanoreceptor neurones studied were modulated by sympathetic stimulation. Sympathetic stimulation both decreased the impulse frequency to a controlled force application and raised the threshold in these receptors. The remainder were unaffected by sympathetic stimulation.4. In previous studies involving recordings from either the trigeminal ganglion or mesencephalic nucleus no spontaneously active slowly adapting intra-oral mechanoreceptors have been observed, yet in peripheral alveolar nerve studies many workers have reported spontaneously discharging neurones. In the present study spontaneous activity has been recorded from intra-oral mechanoreceptors both in the trigeminal ganglion and in the mesencephalic nucleus after cutting the cervical sympathetic trunk. The spontaneous activity could be abolished in all cases but one, by sympathetic stimulation. It is suggested that reported spontaneous activity from intra-oral mechanoreceptors could be due to cutting the peripheral nerve and therefore the sympathetic supply, thus removing an inhibitory influence from the sensory unit. The means by which the sympathetic supply inhibits the sensory unit has not been resolved.