Denervation of the painful temporomandibular joint

The successful management of temporomandibular joint (TMJ) pain remains elusive. Often the initial relief of pain is complicated by recurrence of the symptoms. This time frame suggests that the pain may be related to neuromas of the nerves that innervate the TMJ. In 2003, an anatomic description of the innervation of the TMJ suggested that denervation of this joint might be the appropriate treatment for pain resistant to traditional forms of therapy. In January, 2005, this approach was used to treat recalcitrant left TMJ pain in a 21-year-old woman with congenital hearing loss who had recurrent dislocations of her TMJ articular disc. She previously had two arthroscopic surgeries and one open attempt to treat her TMJ pain. The last failed TMJ surgery created a painful neuroma that prevented her from wearing her hearing aid. A medial and lateral denervation of the TMJ joint was done. The successful results of this surgery are presented at one-year follow-up. The technical considerations of this approach and risk to the facial nerve are discussed.

Somatosensory function following painful repetitive electrical stimulation of the human temporomandibular joint and skin

Temporomandibular disorders (TMD) are common pain problems in the population with uncertain pathophysiology and mechanisms. The aim of this experimental study was to: (1) Establish an experimental pain model using electrical stimuli to describe characteristics of nociception from the human temporomandibular joint (TMJ) and overlying skin. (2) Test the hypothesis that there would be sex-related differences in TMJ sensitivity. Forty-three healthy subjects (24 men and 19 women) participated. Using two unipolar needle electrodes into the skin (above the TMJ) in one session or into the TMJ in the other session, sensory detection threshold (SDT), pain detection threshold (PDT), and summation threshold (SumT) were measured, before and after repetitive electrical stimulation. Painful repetitive electrical stimulation was applied for 20 min with individually adjustment of the intensity of the stimuli to keep the pain rating around five on a 0-10 cm visual analogue scale (VAS). Sensitivity to tactile and pin-prick stimuli were assessed at 11 sites around the TMJ using two von Frey nylon filaments (5.16 and 84.96 g), as well as pressure pain threshold (PPT) and pressure pain tolerance (PPTOL) before the stimulation, after 20 min of stimulation and finally 15 min after the end of stimulation. Numerical rating scale (NRS) from 0 to 100 was used to rate the intensity of applied von Frey filaments. SDT, PDT, and SumT were higher in the TMJ than in the skin. These three measures increased after painful repetitive stimulation for 20 min (de-sensitization). In contrast to this effect, a hypersensitivity to pin-prick stimuli was detected around the TMJ area on the stimulated side after 20 min of electrical stimulation in the TMJ, but not in the skin. A bilateral hyposensitivity to tactile stimuli was detected after skin and TMJ stimulation. PPT and PPTOL did not show a significant change over time. Except for lower TMJ PPTOLs in women than men there were no significant sex-related differences in mechanical or electrical measures. The present findings indicate differences in the elicitation of hypersensitivity following repetitive electrical stimulation of skin and deep tissues. The mechanisms underlying these findings are not clear but differences in the induction of long-term potentiation and depression is a possibility. From a clinical point of view, the lack of sex differences in most of the used measures indicates that the higher prevalence of women than men amongst patients with persistent TMJ pain problems not entirely can be ascribed to a higher sensitivity of the TMJ. Further studies will examine the somatosensory sensitivity of patients with TMJ pain problems.

Capsaicin receptor expression in the rat temporomandibular joint

Experimentally, temporomandibular joint (TMJ) nerve units respond to capsaicin, which is used clinically to treat TMJ pain. However, the existence of capsaicin receptors in the TMJ has not previously been clearly demonstrated. Immunohistochemical analysis has revealed the presence of transient receptor potential vanilloid subtype 1 (TRPV1) expression in the nerves and synovial lining cells of the TMJ. TRPV1-immunoreactive nerves are distributed in the synovial membrane of the joint capsule and provide branches to the joint compartment. The disc periphery is supplied by TRPV1 nerves that are mostly associated with small arterioles, and occasional nerves penetrate to the synovial lining layer. Double immunofluorescence has shown that many TRPV1-immunoreactive nerves are labeled with neuropeptide calcitonin gene-related peptide, whereas few are labeled with IB4-lectin. The results provide evidence for the presence of TRPV1 in both nerves and synovial lining cells, which might thus be involved in the mechanism of nociception and inflammation in the TMJ.

Enhanced excitability of rat trigeminal root ganglion neurons via decrease in A-type potassium currents following temporomandibular joint inflammation

The aim of the present study was to investigate the effect of temporomandibular joint inflammation on the excitability of trigeminal root ganglion neurons innervating the temporomandibular joint using a perforated patch-clamp technique. Inflammation was induced by injection of complete Freund's adjuvant into the rat temporomandibular joint. The threshold for escape from mechanical stimulation in the temporomandibular joint-inflamed rats was significantly lower than that in control rats. Fluorogold labeling was used to identify the trigeminal root ganglion neurons innervating the site of inflammation. When voltage-clamp (V(h)=-60 mV) conditions were applied to these Fluorogold-labeled small diameter trigeminal root ganglion neurons (<30 mum), voltage-dependent transient K(+) current densities were significantly reduced in the inflamed rats compared with controls. In addition, the voltage-dependence of inactivation of the voltage-dependent transient K(+) current was negatively shifted in the labeled temporomandibular joint-inflamed trigeminal root ganglion neurons. Furthermore, temporomandibular joint inflammation significantly reduced the threshold current and significantly increased action potential firings evoked at two-fold threshold in the Fluorogold-labeled small trigeminal root ganglion neurons. Application of 4-aminopyridine (0.5mM) to control trigeminal root ganglion neurons mimicked the changes in the firing properties observed after complete Freund's adjuvant treatment. Together, these results suggest that temporomandibular joint inflammation increases the excitability of trigeminal root ganglion neurons innervating temporomandibular joint by suppressing voltage-dependent transient K(+) current via a leftward shift in the inactivation curve. These changes may contribute to trigeminal inflammatory allodynia in temporomandibular joint disorder.

Inflammation alters sodium currents and excitability of temporomandibular joint afferents

Inflammation-induced changes in voltage-gated sodium currents (I(Na)) in primary afferent neurons may contribute to hyperexcitability and pain. The present study was designed to test the hypothesis that persistent inflammation of the temporomandibular joint (TMJ) increases I(Na) in TMJ afferents. Acutely dissociated retrogradely labeled TMJ afferents were studied using whole-cell patch clamp techniques three days following Complete Freund's Adjuvant-induced inflammation of the TMJ. Inflammation was associated with a decrease in tetrodotoxin (TTX)-sensitive Na+ conductance and no significant change in slowly inactivating TTX-resistant Na+ conductance. However, inflammation increased the excitability of TMJ afferents. These results suggest that changes in ion channels other than those underlying TTX-sensitive and the slowly inactivating TTX-resistant Na+ conductance are likely to account for the inflammation-induced increase in the excitability of TMJ afferents.

Evidence for the involvement of endogenous ATP and P2X receptors in TMJ pain

Evidence is accumulating which supports a role for ATP in the initiation of pain by acting on P2X receptors expressed on nociceptive afferent nerve terminals. To investigate whether these receptors play a role in temporomandibular (TMJ) pain, we studied the presence of functional P2X receptors in rat TMJ by examining the nociceptive behavioral response to the application of the selective P2X receptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) into the TMJ region of rat. The involvement of endogenous ATP in the development of TMJ inflammatory hyperalgesia was also determined by evaluating the effect of the general P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on carrageenan-induced TMJ inflammatory hyperalgesia. Application of alpha,beta-meATP into the TMJ region of rats produced significant nociceptive responses that were significantly reduced by the co-application of lidocaine N-ethyl bromide quaternary salt, QX-314, (2%) or of the P2 receptor antagonist PPADS. Co-application of PPADS with carrageenan into the TMJ significantly reduced inflammatory hyperalgesia. The results indicate that functional P2X receptors are present in the TMJ and suggest that endogenous ATP may play a role in TMJ inflammatory pain mechanisms possibly by acting primarily in these receptors.

Jaw stretch reflexes in children

The substantial morphological transformations that occur during human development present the nervous system with a considerable challenge in terms of motor control. Variability of skilled motor performance is a hallmark of a developing system. In adults, the jaw stretch reflex contributes to the functional stability of the jaw. We have investigated the response properties of the jaw stretch reflex in two groups of young children and a group of young adults. Response latencies increased with development, and all age groups produced stimulus-magnitude-dependent increases in reflex gain and resulting biting force. Reflex gain was largest for the older children (9-10 years), yet net increases in resulting biting force were comparable across age groups. These data and earlier experiments suggest that oral sensorimotor pathways mature throughout childhood in concert with the continued acquisition of complex motor skills.

Variations in the anatomy of the auriculotemporal nerve

The common representation of the auriculotemporal nerve is either that of a single posterior branch of the mandibular nerve or of two roots that envelope the middle meningeal artery. Our observation in the anatomy of the auriculotemporal nerve on 32 dissections (16 cadaveric heads) of the infratemporal fossa included: one specimen with four roots (3.1%), three specimens with three roots (9.4%), 12 specimens with two roots (37.5%), and 16 specimens with one root (50%). Furthermore, a connecting nerve branch was observed between auriculotemporal and inferior alveolar nerves in four specimens, and in another auriculotemporal nerve case, between the upper and lower roots. In the cadaver of a 70-year-old male, a four-rooted auriculotemporal nerve variation was found. These four branches lay to the posterior, combined at the posterosuperior of the maxillary and superficial temporal arteries and formed a ganglion-like knot. From this knot, four branches stemmed and ran to the temporomandibular joint, external acoustic meatus, zygoma, and parotid gland. The knot was larger and thicker than expected; thus, it was removed and stained with haematoxylin-eosin (H&E) and S100 for histological studies. This structure was not a true ganglion but a structure formed by fusion of nerve fibers.

Alterations of the rat temporomandibular joint in functional posterior displacement of the mandible

Functional malocclusion that induces posterior condylar displacement may affect the remodeling processes of the temporomandibular joint structures. We tested the hypothesis that intermittent posterior condylar displacement due to functional malocclusion traumatizes condylar cartilage and joint innervated nerve fibers. Thirty-nine eight-week-old Wistar rats were used. To induce functional posterior condylar displacement, guiding appliances were attached to maxillary incisors of 24 rats for four, seven, and 14 days. Fifteen normal rats served as controls. Sections were stained with hematoxylin and eosin or processed for immunohistochemistry of protein gene product 9.5 and growth-associated protein-43 (GAP-43). Functional posterior condylar displacement led to a diminution in proliferative cells, reduction in cartilage width, and re-expression of GAP-43-immunoreactive nerve fibers. These results indicate that intermittent posterior condylar displacement due to functional malocclusion causes dysfunctional remodeling of condylar cartilage and nerve injury.

Convergence of nociceptive information from temporomandibular joint and tooth pulp afferents on C1 spinal neurons in the rat

The aim of the present study was to test the hypothesis that there is a convergence of afferent inputs from the temporomandibular joint (TMJ) on C1 spinal neurons responding to electrical stimulation of the tooth pulp (TP). In 14 pentobarbital anesthetized rats, the extracellular single unit activity of 31 C1 spinal neurons and the amplitude in a digastric muscle electromyogram (n = 31) increased proportionally during 1.0-3.5 times the threshold for the jaw-opening reflex (JOR). Of 31 C1 spinal neurons responsive to TP afferents, 28 (approximately 90%) were also excited by electrical stimulation of the ipsilateral TMJ capsule. All neurons tested were divided into three categories of nociceptive specific, wide dynamic range and non-responsive as to their responsiveness to mechanical stimuli (pin prick and touch) of the somatic receptive field (skin of the face, neck, jaw and upper forearm) and TMJ capsule. Nineteen (68%) of 28 C1 spinal neurons received nociceptive information from C fibers of the TMJ capsule. These results suggest that there is a convergence of noxious information from the TMJ and TP afferents on the same C1 spinal neurons, which importantly contribute to pain perception from the TMJ region.

The deep subfascial approach to the temporomandibular joint

PURPOSE

The aim of the present study was to improve safety and visibility in the surgical approach to the temporomandibular joint (TMJ). In spite of the development of a myriad of surgical approaches to the TMJ, the facial nerve remains at risk for damage. For this reason the authors present an additional, safe, surgical approach to avoid injuries of the facial nerve during TMJ surgery, termed "the deep subfascial approach."

PATIENTS AND METHODS

Two resident surgeons, under supervision of the Department Chief, made 29 surgical exposures of the condyle, safely using the deep subfascial approach. This surgical approach is carried out by means of a modified preauricular incision followed by the subfascial dissection. Dissection passes under both layers of the deep temporalis fascia.

RESULTS

Neither permanent nor temporary motor loss of the frontal branch of the facial nerve was observed after surgery. It has not been necessary to ligate the superficial temporal vessels except the posterior branch of the superficial temporal artery. Aesthetic results were excellent in all cases.

CONCLUSION

The operation is technically facile, surgically uncomplicated, and clinically effective. It is our opinion that the deep subfascial approach to the TMJ represents a safe method to avoid injury of the facial nerve. We suggest this surgical procedure as a routine and safe approach to the TMJ and to the zygomatic arch, especially in second procedures or in delayed treatments.

[The temporo-mandibular articulation]

The standing posture of humans has created both morphological and functional adaptations in the temporo-mandibular joint and the masticatory function. This biped state is the one of the most important characteristic of human evolution. It is furthermore the agent determining most of the functional changes in the whole body. This survey will be carried out in several levels including, a descriptive anatomy, biomechanics, radiological imaging, functioning in the articulation of TMJ. The descriptive anatomic picture will be obtained by the traditional dissection techniques. 20 TMJ joints are dissected from 10 cadavers: 7 cadavers, 65-75 year old, 3 cadavers, 60-65 year old. The x-rays are lateral view and the subjects of the radiological imaging are young's, adults and olds: 1, 3 y-old Male; 1, 7 y-old Female; 1, 14 y-old Female; 10, 19-23 y-old Male; 1, 26 y-old Female; 1, 34 y-old Male; 1, 75 y-old Female. The anatomic elements in the TMJ well resembled the ones described in the literature of the capsule, the ligament, the masticator muscles (masseter, temporal, medial and lateral pterygoids). The temporo-mandibular ligament proved to be difficult to separate from the capsule in some of the specimens. Sometimes it was not always found after a dissection.

VR1-, VRL-1- and P2X3 receptor-immunoreactive innervation of the rat temporomandibular joint

Immunohistochemistry for vanilloid receptor subtype 1 (VR1), vanilloid receptor 1-like receptor (VRL-1) and P2X3 receptor was performed in the rat temporomandibular joint (TMJ). Blood vessels in the articular disk and capsule, the synovial membrane and the fibrous tissue around the condylar process were innervated by VR1- or P2X3 receptor-immunoreactive (ir) nerve fibers. However, VRL-1-immunoreactivity (ir) could not be detected in the TMJ. Retrograde tracing and immunohistochemical methods revealed that 25%, 41% and 52% of TMJ neurons in the trigeminal ganglion (TG) exhibited VR1-, VRL-1- and P2X3 receptor-ir, respectively. VR1-ir TMJ neurons were mostly small to medium-sized, whereas VRL-1- and P2X3 receptor-ir TMJ neurons were predominantly medium-sized to large. In addition, 73%, 28% and 44% of VR1-, VRL-1- and P2X3 receptor-ir TMJ neurons, respectively, coexpressed calcitonin gene-related peptide (CGRP)-ir. The present study suggests that the TMJ has abundant nociceptors which respond to vanilloid compounds, protons, heat and extracellular ATP.

Nerve terminals extend into the temporomandibular joint of adjuvant arthritic rats

The innervation of the temporomandibular joint (TMJ) has attracted particular interest because of the close association with complex mandibular movement. Although the pathological changes of disk innervation may have a crucial role in the development of TMJ pain, the innervation of the TMJ disk by experimentally induced arthritis has rarely been examined in detail. Arthritic rats were induced by injection with 0.1ml solution of Complete Freund's adjuvant (CFA). We investigated three-dimensional distribution of nerve fibers in the TMJ disk using immunohistochemistry for protein gene product-9.5 (PGP-9.5) and calcitonin gene-related peptide (CGRP) in naive and arthritic rats. To clarify the possible role of nerve growth factor (NGF) and its receptor on changes in peripheral innervation of the TMJ, the expressions of trkA and p75 receptor in trigeminal ganglia were examined. Although PGP-9.5 and CGRP immunoreactive (ir) fibers were seen in the peripheral part of the TMJ disk, they were not seen in its central part. The total length and the length density of PGP-9.5 ir and CGRP ir nerve fibers increased in arthritic rats. The innervation area of fibers proliferating in the rostro-medial part merged with that of fibers in the rostro-lateral part in the arthritic rats. In addition, the ratio of trkA- and p75-positive small- and medium-sized cells increased in trigeminal ganglia. It is assumed that increasing innervation of the TMJ disk may be important for the pathophysiology of TMJ pain. NGF and its receptors are likely involved in pathological changes of the TMJ disk.

Sex differences in rabbit masseter muscle function

The proportions of fibers of different phenotypes in the rabbit masseter muscle differ strikingly in adult males and females. Muscles from females contain similar proportions of small fibers that express both the slow/beta and cardiac alpha myosin heavy chain (MyHC) isoforms and larger fibers containing the IIa MyHC isoform. In muscles from males, nearly 80% of fibers are of the IIa phenotype. To evaluate the functional significance of these sex differences, we used finely graded intramuscular microstimulation to study the contractile properties of masseter motor units in >6-month-old male and female rabbits. Twitch forces and torques in males were significantly greater in magnitude than those of females. Greater proportions of units that produced larger forces/torques were encountered in the males. The same motor units produced force or torque more rapidly in males than in females, principally because units in which twitch rise times were >22 ms were found only in females. The forces applied to the mandible and the torques generated about the right mandibular condyle were studied during cortically evoked rhythmic activation of the masticatory muscles. The overall range of torque rise times and the magnitudes of the peak torques did not differ between sexes. The mean rise time was significantly shorter and the mean peak torque was significantly greater in males. We conclude that sex differences in fiber phenotype proportions are reflected in sex differences in motor unit properties and in the function of these motor units during rhythmic activation.

Effects of experimentally induced inflammation on temporomandibular joint nociceptors in rats

Response properties of nociceptors in the temporomandibular joint (TMJ) and surrounding area under experimental inflammation were investigated using an in vitro TMJ-nerve preparation in the rat. Nociceptive units (receptor and innervating nerve fiber) were classified into the following subtypes: Adelta-high-threshold mechanonociceptor (HTM), Adelta-polymodal nociceptor (POLY), C-HTM and C-POLY. In the inflamed joint, mechanical thresholds tended to be lower; however, the reaction to bradykinin was not identified as clearly as in control. Experimentally induced inflammation increased the proportion of heat-sensitive units and lowered heat threshold significantly. These results suggest that inflammation may sensitize nociceptors in the temporomandibular joint, and cause hyperalgesia and allodynia.

The anatomical relationship between the position of the auriculotemporal nerve and mandibular condyle

Head, neck, face, and ear pains are commonly associated with disorders of the temporomandibular joint (TMJ). Several theories have been proposed regarding the functional relationship of the TMJ and the associated structures, and how they might contribute to certain painful conditions. This study was conducted to determine the anatomic relationship of the auriculotemporal nerve to the middle meningeal artery and the mandibular condyle. Forty human cadaver temporomandibular joints were dissected to locate the precise position of the auriculotemporal nerve to the mandibular condyle. The study findings revealed a significant variation in the relationship of the auriculotemporal nerve to the middle meningeal artery. The auriculotemporal nerve was found to be between 10-13 mm inferior to the superior surface of the condyle and 1-2 mm posterior to the neck of the condyle. The nerve was not found to be in a position that would likely create an entrapment with adjacent tissues. These findings may assist the clinician to locate the most appropriate injection site for an auriculotemporal nerve block.

Coeruleotrigeminal suppression of nociceptive sensorimotor function during inflammation in the craniofacial region of the rat

Descending action from the locus coeruleus (LC) on the trigeminal sensorimotor function was evaluated in a rat model of oral-facial inflammation. For the induction of oral-facial inflammation, mustard oil (20% solution in 20microl mineral oil) was injected into the region of the temporomandibular joint (TMJ). One week before testing, rats received bilateral lesions of the LC using a cathodal current. The electromyogram (EMG) threshold, which is the threshold intensity for the onset of EMG activity of the masseter muscle evoked by pressure on the TMJ region, was used in the present study as an indicator of the trigeminal sensorimotor function. Following mustard oil injection, in the LC-lesioned rats, EMG thresholds significantly decreased at 30min, which lasted up to 240min. In contrast, EMG thresholds in the LC-intact rats returned to the level before injection after 180min. Systemic naloxone (1.3mg/kg, i.v.) produced a further decrease of EMG thresholds in both the LC-intact and LC-lesioned rats. Under the existence of naloxone, EMG thresholds in the LC-lesioned rats were significantly lower than those of the LC-intact rats. These results suggest that oral-facial inflammation activates the coeruleotrigeminal modulating system and that an action of this system is independent of the opioid depressive mechanism.

Response properties of TMJ units in superficial laminae at the spinomedullary junction of female rats vary over the estrous cycle

Neurons responsive to stimulation of the temporomandibular joint (TMJ) region were recorded from superficial laminae at the trigeminal subnucleus caudalis/upper cervical cord (Vc/C(2)) junction region of cycling female rats under barbiturate anesthesia. To determine if receptive field (RF) properties or sensitivity to algesic chemicals of TMJ units vary over the estrous cycle, animals were selected from proestrous (high estrogen) or early diestrous (low estrogen) stages. More than 90% of TMJ units from each group received convergent nociceptive input [wide dynamic range (WDR) or nociceptive specific (NS)-like] from facial skin. The cutaneous high-threshold RF areas of WDR units from proestrous rats were 30% larger than diestrous units, while RF areas of NS units were similar. Bradykinin (BK, 0.1-10 microM) injection into the TMJ region excited a high percentage of units (>80% of total) from both groups in a dose-related manner. However, BK-evoked response magnitude (R(mag), +140%) and duration (+64%) were greater for proestrous than diestrous units. Both WDR and NS-like TMJ units of proestrous females displayed enhanced BK-evoked R(mag) values and response duration. Glutamate or mustard oil excitation of TMJ units was not affected by stage of the estrous cycle. Several TMJ units from proestrous and diestrous females were activated antidromically from the contralateral posterior thalamus, indicating that projection and nonprojection units were included in the sample population. These results were consistent with the hypothesis that factors related to stage of the estrous cycle modify the processing of deep craniofacial inputs by superficial dorsal horn neurons at the spinomedullary junction, a key region for the initial integration of sensory signals from the TMJ.

Clinical implications of the innervation of the temporomandibular joint

The successful management of temporomandibular joint (TMJ) pain remains elusive. Often, the initial relief of pain is complicated by recurrence of the symptoms. This time frame suggests that the pain may be related to neuromas of the nerves that innervate the TMJ. The current study attempted to define the innervation pattern of the TMJ as identified in 16 embalmed and 8 fresh-frozen specimens. In each specimen, the auriculotemporal nerve, a branch of the mandibular portion (V3) of the trigeminal nerve, was found to innervate the lateral capsule of the TMJ. In 75% of the specimens, the masseteric nerve, a branch of the maxillary portion (V2) of the trigeminal nerve, was found to innervate the anteromedial capsule of the TMJ. In 33% of the specimens, there was a branch coming through the mandibular notch to innervate the anteromedial capsule that was not from the masseteric nerve; this nerve is believed to have passed through the lateral pterygoid muscle after leaving V2. These consistent patterns of innervation of the TMJ suggest that diagnostic nerve blocks can be done to determine the pain pathway in these patients. It is suggested that if the nerve blocks are successful, TMJ denervation may be a future method of pain relief in patients with recalcitrant or recurrent TMJ pain.

Auriculo-temporal nerve. Clinicopathological relevance to facial-maxillary practice

Anatomically, the auriculo-temporal nerve (ATN), a peripheral branch of the trigeminal nerve is in close relation to the parotid gland, neck of the mandible, temporal vessels, and sphenomandibular ligament. Fine branches of the ATN are at risk of division during surgical intervention of the parotid gland or condyle of the mandible. "Frey's syndrome", or abnormal gustatory sweating, may be a consequence of injury to the branches of the auriculo-temporal nerve. This uncommon syndrome can also occur with fractures of the temporo-mandibular joint (TMJ), due to the anatomic proximity of the nerve to the bone.

Topical review: sleep bruxism and the role of peripheral sensory influences

Sleep bruxism (SB) is an unusual orofacial movement described as a parafunction in dentistry and as a parasomnia in sleep medicine. Since several peripheral influences could be involved in sleep-wake regulation and the genesis of rhythmic jaw movements, the authors have reviewed the relevant literature to facilitate understanding of mechanisms possibly involved in SB genesis. Various animal and human studies indicate that during either wakefulness or anesthesia, orofacial sensory inputs (e.g., from periodontium, mucosa, and muscle) could influence jaw muscle activity. However, the role of these sensory inputs in jaw motor activity during sleep is unclear. Interestingly, during sleep, the jaw is usually open due to motor suppression; tooth contact most likely occurs in association with sleep arousal. Recent physiologic evidence supports an association between sleep arousal and SB; a sequential change from autonomic (cardiac) and brain cortical activities precede SB-related jaw motor activity. This suggests that the central and/or autonomic nervous systems, rather than peripheral sensory factors, have a dominant role in SB genesis. However, some peripheral sensory factors may exert an influence on SB through their interaction with sleep-wake mechanisms. The intent of this review is to integrate various physiologic concepts in order to better understand the mechanisms underlying the genesis of SB.

Temporomandibular joint nociception: effects of capsaicin on substance P-like immunoreactivity in the rabbit brain stem

AIMS

To specify the regions of the brain stem that are characterized by changes in substance P (SP)-like immunoreactivity following activation of capsaicin-sensitive afferents innervating temporomandibular joint (TMJ) tissues in New Zealand rabbits.

METHODS

Capsaicin, an activator of small-diameter unmyelinated and thinly myelinated nociceptive afferent fibers, was administered unilaterally to the right TMJ of experimental animals. Another group received vehicle solution and served as controls. The animals were sacrificed 6 hours post-treatment through transcardial perfusion. Their brain stems were removed and sectioned, and SP-like immunoreactivity was assessed in serial horizontal sections.

RESULTS

A decrease in brain stem SP-like immunoreactivity occurred ipsilateral to capsaicin application. This reduction was primarily localized in brain stem regions that correspond to the trigeminal main sensory nucleus, as well as subnucleus oralis, interpolaris, and caudalis of the trigeminal spinal tract nucleus.

CONCLUSION

The present study revealed central nervous system changes following TMJ capsaicin treatment in rabbits.

[Distribution of neuropeptide Y-like immunoreactive fiber in rat temporomandibular joint]

OBJECTIVE

This investigation aimed at explore the total distribution of neuropeptide Y-like immunoreactive (NPY-LI) fibers and their changes post-trauma in rat temporomandibular joints (TMJs).

METHODS

Six groups of rats were killed individually before trauma, 1, 3, 7, 14 and 28 days after trauma. TMJs were extracted totally, and the avidin-biotin-peroxidase complex method and image analysis were employed to detect NPY-LI fibers in frozen sections of TMJs.

RESULTS

NPY-LI fibers were distributed extensively in TMJs, except the central disc band and bone, and they were mainly located around blood vessels, especially arteries. The densities of fibers in the six groups were 160.4 +/- 27.5, 95.8 +/- 16.4, 88.6 +/- 14.5, 114.3 +/- 17.0, 135.0 +/- 20.7, 158.6 +/- 19.5 (unit:mm2).

CONCLUSION

NPY-LI nerve fibers are distributed extensively in the periphery of blood vessels of TMJs and densities changed dynamically when TMJs were impacted. NPY may play an important role in pathologic change of TMJ by regulating local blood circulation.

Intrathecal administration of 5-HT(3) receptor agonist modulates jaw muscle activity evoked by injection of mustard oil into the temporomandibular joint in the rat

The effect of intrathecal administration of the 5-HT(3) receptor agonist 2-methyl-5-hydroxytryptamine (2m-5HT) on jaw muscle activity evoked by mustard oil (MO) injection into the temporomandibular joint of anesthetized rats was examined. One microgram or 100 microg of 2m-5HT significantly enhanced or suppressed jaw muscle responses, respectively. Pre-administration of tropisetron, a 5-HT(3) receptor antagonist, attenuated the effect of 2m-5HT. These results indicate that activation of 5-HT(3) receptors can modulate trigeminal nociceptive responses.