Periodontal Nerve Fibers Research Articles

Changes in the Distribution of Periodontal Nerve Fibers during Dentition Transition in the Cat.

The periodontal ligament has a rich sensory nerve supply which originates from the trigeminal ganglion and trigeminal mesencephalic nucleus. Although various types of mechanoreceptors have been reported in the periodontal ligament, the Ruffini ending is an essential one. It is unknown whether the distribution of periodontal nerve fibers in deciduous teeth is identical to that in permanent teeth or not. Moreover, morphological changes in the distribution of periodontal nerve fibers during resorption of deciduous teeth and eruption of successional permanent teeth in diphyodont animals have not been reported in detail. Therefore, in this study, we examined changes in the distribution of periodontal nerve fibers in the cat during changes in dentition (i.e., deciduous, mixed and permanent dentition) by immunohistochemistry of protein gene product 9.5. During deciduous dentition, periodontal nerve fibers were concentrated at the apical portion, and sparsely distributed in the periodontal ligament of deciduous molars. During mixed dentition, the periodontal nerve fibers of deciduous molars showed degenerative profiles during resorption. In permanent dentition, the periodontal nerve fibers of permanent premolars, the successors of deciduous molars, increased in number. Similar to permanent premolars, the periodontal nerve fibers of permanent molars, having no predecessors, increased in number, and were densely present in the apical portion. The present results indicate that the distribution of periodontal nerve fibers in deciduous dentition is almost identical to that in permanent dentition although the number of periodontal nerve fibers in deciduous dentition was low. The sparse distribution of periodontal nerve fibers in deciduous dentition agrees with clinical evidence that children are less sensitive to tooth stimulation than adults.

Immunohistochemical detection of nestin in the periodontal Ruffini endings of the rat incisor

Nestin is an intermediate filament which was first identified in neuroepithelial stem cells. This expression has also been reported in restricted locations in adults. Previous studies have suggested that the periodontal Ruffini endings remain immature in nature even in adulthood. The present study reports on a characteristic expression of immunoreaction for nestin in the periodontal Ruffini endings during postnatal development. RT-PCR analysis detected nestin mRNA in a reverse transcripted cDNA sample from both the rat trigeminal ganglion and periodontal ligament. The nestin immunoreaction existed in the periodontal ligament at postnatal day 3 (PO 3 days), when many spindle-shaped Schwann cells were positive for nestin immunoreaction. At PO 1 week, when periodontal nerve fibers displayed a dendritic fashion, the round cells came to show the nestin immunoreaction. These immunopositive cells were also reactive for S-100 protein and non-specific cholinesterase, indicating that these cells could be categorized as terminal Schwann cells associated with the periodontal Ruffini endings. Some ordinary Schwann cells also exhibited nestin immunoreaction. From PO 2 to 3 weeks, nestin positive terminal Schwann cells increased in number in accordance with the postnatal development of the periodontal Ruffini endings, while this immuno-expression pattern remained unchanged. Nestin immunoreaction was also recognizable in the satellite cells – but never in the neurons – in the trigeminal ganglion throughout this observation period. This immuno-expression pattern suggests that nestin serves as an intermediate filament for mechanical stability in the periodontal Ruffini endings against external stimuli.

A new concept in orthodontics: faster and healthier tooth movement by regularly consuming xyilitol chewing gum

Xylitol was first discovered in the 19th century, it wasn’t until the 1960’s that commercial production was first implemented. Recent studies showed that xylitol chewing gum is beneficial for preventing caries and periodontal disease. Therefore, it is also advantageous for orthodontic treatment, especially the fixed orthodontics patients who have difficulties in acquiring optimal oral health, particularly periodontal health which important in remodeling. However, how consuming xylitol chewing gum may stimulate tooth movement and preventing root resorption is still unclear. It is suggested that chewing activities may stimulate tooth movement, since jaw hypofunction leads to lower mineral apposition and bone function; and narrow periodontal ligament (PDL). These conditions may lead to impaired remodeling process, and increases the susceptibility of root resorption during orthodontic tooth movement. Moreover, since stimulation of the PDL could be mechanoreceptive (i.e. chewing action) or nociceptive (i.e. painful stimulation), periodontal nerve fibers are supposed to play an important role in bone remodeling. It is supported by a study which revealed that during tooth movement, the galanin-containing immunoreactive nerve fibers, a part of primary sensory neurons in the PDL is increasing. Galanin is able to induce osteoclast differentiation that needed for bone resorption in orthodontic treatment. The objective of this study is to elucidate a new concept in using xylitol chewing gum as an excellent media to have a faster and healthier orthodontic movement. Since continuous chewing stimulates the PDL which enhances tooth movement, improves oral health, and prevents root resorption; it is concluded that this concept is possible.

Involvement of brain-derived neurotrophic factor (BDNF) in the development of periodontal Ruffini endings.

The periodontal Ruffini ending has been reported to show immunoreactivity for tyrosine kinase B (trkB), the high-affinity receptor for brain-derived neurotrophic factor (BDNF), in the periodontal ligament of the rat incisor. Furthermore, adult heterozygous BDNF-mutant mice showed malformation and reduction of the periodontal Ruffini endings. To investigate further roles of BDNF in these structures, the development, distribution, and terminal morphology of Ruffini endings were examined in the incisor periodontal ligament of heterozygous and homozygous BDNF mutant mice, as well as in the wild-type littermate by immunohistochemistry for protein gene product (PGP) 9.5, a general neuronal marker. A similar distribution and terminal formation of PGP 9.5-immunoreactive nerve fibers was recognized in the periodontal ligament of all phenotypes at postnatal week (PW) 1. At this stage, the nerve fibers had a beaded appearance, but did not form the periodontal Ruffini endings. At PW2, the heterozygous and wild-type mice started to show ramified nerve fibers resembling the mature shape of periodontal Ruffini endings. At PW3, the Ruffini endings occurred in the periodontal ligament of the wild-type and heterozygous mice. While the Ruffini endings of the wild-type mice appeared either ruffled or smooth, as reported previously, most of these structures showed a smooth outline in the heterozygous mice. The homozygous mice lacked the typical Ruffini endings at PW3. In the quantitative analysis, homozygous mice had the smallest percentages of PGP 9.5-immunoreactive areas at the same postnatal periods, but there were no significant differences between wild-type and heterozygous mice during PW1-3. These findings suggest a possible involvement of BDNF during the postnatal development and, in particular, the maturation of periodontal Ruffini endings. Furthermore, other neurotrophins may play a role in the development and/or early maturation of the periodontal nerve fibers, as indicated by the presence of nerve fibers in the BDNF-homozygous mice.

Effects of neonatal injury of the inferior alveolar nerve on the development and regeneration of periodontal nerve fibers in the rat incisor

Our previous study showed that the migration of terminal Schwann cells occurred in the periodontal ligament of the rat lower incisor following transection of the inferior alveolar nerve (IAN) in the adult animals [Y. Atsumi, K. Matsumoto, M. Sakuda, T. Maeda, K. Kurisu, S. Wakisaka, Altered distribution of Schwann cells in the periodontal ligament of the rat incisor following resection of the inferior alveolar nerve: An immunohistochemical study on S-100 proteins, Brain Res. 849 (1999) 187–195]. The aim of the present study was to investigate the effects of neonatal transection of the IAN on the regeneration of axon elements and Schwann cells in the periodontal ligament of the rat lower incisor. Following transection of IAN at post-natal day 5 (PN 5d), when the numbers of both axon elements and the terminal Schwann cells were very small, regenerating nerve fibers appeared between post-injured days 7 (PO 7d) and PO 14d, and increased in number thereafter gradually. Although the terminal morphologies of regenerated Ruffini endings became identical to those of the adult animals by PO 54d, the number of regenerated PGP 9.5-IR nerve fibers did not recover the adult levels even by PO 56d. A small number of Schwann cells migrated into the shear zone, the border between the alveolus-related part (ARP) and the tooth-related part (TRP), but did not enter into the TRP. Following transection of the IAN at PN 14d or PN 28d, when clusters of apparent terminal Schwann cells could be recognized, axon regeneration started around PO 5d. Individual axon terminals of the regenerating Ruffini endings ramified and became identical to those of the adult animals around PO 28d, but the number of regenerated Ruffini endings was smaller than that of the adult animals. Similar to the adult animals, the migration of Schwann cells into the shear zone and TRP occurred, and disappeared prior to the completion of the axonal regeneration. The present results indicate that the migration of the Schwann cells into TRP during the regeneration of the periodontal nerve fibers following nerve injury to the IAN depends on the maturation of the terminal Schwann cells of the periodontal Ruffini endings, not on post-operative time.

Effects of Different Types of Injury to the Inferior Alveolar Nerve on the Behavior of Schwann Cells during the Regeneration of Periodontal Nerve Fibers of Rat Incisor.

The present study reports on different regeneration patterns of axons and Schwann cells in the periodontal ligament of the rat incisor using immunohistochemistry of protein gene product 9.5 (PGP 9.5) and S-100 protein. Three kinds of injury (transection, crush and segmental resection) were applied to the inferior alveolar nerve. In normal animals, PGP 9.5- and S-100-immunoreactivities were detected in the axons and Schwann cell elements of periodontal Ruffini endings, respectively. They were restricted to the alveolus-related part, occurring only rarely in the tooth-related part and in the shear zone (the border between the alveolus-related and tooth-related parts). Both transection and segmental resection caused the complete disappearance of PGP 9.5-immunoreactive nerve fibers in the periodontal ligament, while a small number of them could be found following the crush injury. Regenerating PGP 9.5-reactive nerve fibers appeared at 5 days and 21 days following the transection and segmental resection, respectively. The regeneration of periodontal nerve fibers completed in a period of 21-28 days and 14-21 days following the transection and crush, respectively, but was not completed even at 56 days following the segmental resection. The behavior of Schwann cells during regeneration was similar after the different nerve injuries; spindle-shaped S-100-immunoreactive cells, presumably Schwann cells, appeared in the shear zone and the tooth-related part. These cells disappeared 5-7 days prior to the completion of the regeneration of axonal elements of the periodontal ligament following the transection and crush. Following the segmental resection, in contrast, spindle-shaped S-100-positive cells disappeared from the tooth-related part at 42 days, although the axonal regeneration of periodontal Ruffini endings proceeded even until 56 days. We thus conclude that the duration of the migration of Schwann cells depends on the state of the regeneration of axons.

Delayed Expression of Calbindin D28k during Regeneration of the Periodontal Ruffini Endings of the Rat Incisor following Injury to the Inferior Alveolar Nerve

Expression of calbindin D28k (CB)-like immunoreactivity (-LI) was compared with that of protein gene product 9.5 (PGP 9.5), a general neuronal marker, in the periodontal ligament of the rat lower incisor following resection of the inferior alveolar nerve (IAN). In normal animals, the periodontal nerve fibers showing PGP 9.5-LI formed either Ruffini endings with expanded arborization or thin free nerve endings in the alveolar half of the ligament. Thick CB-like immunoreactive (-IR) nerve fibers terminated in a dendritic fashion in the same region, but thin CB-IR nerve fibers were rarely detected. During the 3 days following resection of the IAN, most of the PGP 9.5-IR and all CB-IR nerve fibers disappeared. Regenerated PGP 9.5-IR nerve fibers appeared around 7 days after resection, in contrast to the very small number of regenerated CB-IR nerve fibers. Around 21–28 days following resection, the number and terminal morphology of regenerated PGP 9.5-IR nerve fibers were comparable to those observed in normal animals, but the number of regenerated CB-IR nerve fibers was still smaller. The terminal morphologies of these regenerated CB-IR nerve fibers showed less expansion compared with normal animals at these post-injured periods. The number of regenerated CB-IR nerve fibers increased gradually to return to normal by 56 days following injury. The delayed expression of CB in the regenerated periodontal Ruffini endings suggests that the functional recovery of periodontal Ruffini endings occurred after the regeneration of periodontal Ruffini endings had been completed.

Calretinin-like immunoreactivity in the regenerating periodontal Ruffini endings of the rat incisor following injury to the inferior alveolar nerve

Regeneration of calretinin (CR)-like immunoreactive (IR) nerve fibers was investigated in the periodontal ligament of the rat lower incisor following resection of the inferior alveolar nerve (IAN). In addition, the degeneration and regeneration processes of periodontal nerve fibers were examined by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker. In normal animals, the periodontal nerve fibers showing PGP 9.5-like immunoreactivity (LI) formed either periodontal Ruffini endings with expanded arborization and thin free nerve endings in the alveolar half of the ligament. Thick CR-IR nerve fibers also appeared in a dendritic fashion in the same region, but thin CR-IR nerve fibers were rarely observed. Five days following resection of the IAN, a major population of PGP 9.5-IR and all CR-IR nerve fibers disappeared except for some thin PGP 9.5-IR nerves in the periodontal ligament. Regenerated PGP 9.5-IR nerve fibers appeared around 7 days following resection, in contrast to a very small number of regenerated CR-IR nerve fibers. Around 14–21 days following resection, the number and terminal morphology of regenerated PGP 9.5-IR nerve fibers were comparable to those observed in normal animals, but the number of regenerated CR-IR nerve fibers was still smaller than that of normal animals. The number of regenerated CR-IR nerve fibers increased to return to normal by 56 days following injury. The delay of expression of CR–LI in the regenerated periodontal Ruffini endings suggests that functional recovery of periodontal Ruffini endings occurred after the completion of the regeneration of periodontal nerve fibers.

The reinnervation of healing extraction sockets in the ferret.

The objective of this study was to describe the pattern of healing of pulpal and periodontal nerve fibers following tooth extraction. The mandibular canine teeth were, under general anesthesia, removed from one side of nine young ferrets. One week (two animals), one month (four animals), and three months (three animals) later, again under general anesthesia, the animals were perfused with fixative. The mandibles were decalcified, paraffin-embedded, and serially sectioned. Sample sections were stained with hematoxylin and eosin, the remainder with a silver stain for nerves. Serial reconstructions were made of the innervation in control teeth and in the extraction sockets. A grid sampling technique was used for the estimation of the innervation density in different regions at various levels in the healing socket. One week after extraction, although osteoid material was beginning to fill the socket, the pattern of innervation remained similar to that of controls in being restricted to the former location of the periodontal ligament and being densest in the apical third of the socket. One month after extraction, the innervation consisted of many fine axons or small bundles distributed throughout the healing osteoid tissue though still most dense in the periphery of the socket. The overall innervation density increased between one week and one month post-extraction. At three months, as the margins of the socket were becoming obscured, the innervation consisted largely of one or two organized nerve trunks running from the apical to the coronal aspect in either the central or lateral aspects of the socket. At no time was there evidence of neuroma formation.

Identification and characterization of afferent periodontal A delta fibres in the cat.

1. The presence and responsiveness of afferent periodontal A delta fibres was studied in pentobarbitone-anaesthetized adult cats. 2. Extracellular single fibre recordings were made from fine nerve filaments split from the proximally cut end of the inferior alveolar nerve. Periodontal nerve fibres were identified by constant current stimulus pulses applied via platinum wire electrodes inserted into the periodontal space of the lower canine tooth. 3. Of a total of 252 periodontal nerve fibres, 97 (37%) were classified as A delta fibres according to their conduction velocities (CV) (> 2.5 m s-1, < 30 m s-1) as determined by electrical stimulation of the periodontal ligament. The mean (+/- S.D.) conduction velocity was 11.0 +/- 7.7 m s-1 (n = 97; range: 2.6-28.2 m s-1). 4. A good exponential correlation (r = 0.85) was found between the electrical thresholds of the A delta fibres and their conduction velocities. 5. For four A delta fibres a complete stimulus-duration curve was determined. It followed rather well the I = I0/(1-et/tau) law, where I represents the stimulus amplitude, t the stimulus duration, I0 the rheobasic current and tau the time constant. 6. In the intact tooth none of the identified periodontal A delta fibres showed any ongoing activity in the absence of intentional stimulation. 7. The responses of sixteen electrically identified periodontal A delta fibres were tested by mechanical, thermal and chemical stimuli applied to the periodontal space. Seven of nine periodontal A delta fibres tested responded to mechanical forces applied to the tooth from different directions of which none could be activated by slight touch. A rudimentary directional sensitivity was seen. When a human tooth was stimulated by a mechanical stimulus of similar strength the sensation evoked was described as a dull, poorly localized pain. 8. Six periodontal A delta fibres were activated by heat and/or cold and/or chemical stimulation. Two of eight periodontal A delta fibres tested responded to heat and four of six A delta fibres tested responded to cold stimuli applied to the alveolar bone overlying the periodontal ligament; none of them responded to both types of thermal stimuli. Two of seven periodontal A delta fibres tested were activated by a saturated solution of potassium chloride applied locally to the periodontal ligament; two of these responded also to cold. 9. The response behaviour of periodontal afferent A delta fibres observed in this study suggests that they may play a role in periodontal nociception.

Identification and characterization of afferent periodontal C fibres in the cat

The presence of afferent periodontal C fibres was studied in pentobarbitone-anaesthetized cats. Extracellular single-fibre recordings were made from fine nerve filaments split from the proximally cut end of the inferior alveolar nerve. Periodontal nerve fibres were identified by constant-current stimulus pulses applied via platinum wire electrodes inserted into the periodontal space. Of 260 periodontal nerve fibres, 142 (55%) were classified as C fibres according to their conduction velocities (≤ 2.5 m/sec) as determined by electrical stimulation of the periodontal ligament (c.v. p). The mean (± S. D.) c.v. p was 1.2 ± 0.6 m/sec (n = 142; range: 0.3–2.5 m/sec). In addition, the axonal conduction velocity of 14 periodontal C fibres was determined by bipolar electrical stimulation of the trunk of the inferior alveolar nerve (c.v. n). On average the c.v. n was 42% higher than the c.v. p; the mean value was 1.7 ± 0.8 m/sec (n = 14; range: 0.6–3.9 m/sec). Nevertheless, the classification of nerve fibres based on c.v. p proved to be reliable; only 1 fibre had a c.v. p < 2.5 m/sec and a c.v. n > 2.5 m/sec and was therefore reclassified as an Aδ fibre. The responses of 30 electrically identified periodontal C fibres were tested by mechanical, thermal and chemical stimuli applied to the periodontal space. Thirteen of 19 periodontal C fibres tested responded to a strong mechanical force applied to the tooth from different directions while none could be activated by slight touch. A rudimentary directional sensitivity was observed. Eleven of these 13 responsive fibres were also activated by heat and/or cold and/or chemical stimulation. Fifteen of 25 periodontal C fibres tested responded to heat (thermode contact temperature +80°C), and 14 of 19 C fibres tested responded to cold (thermode contact temperature −80°C) stimuli applied to the alveolar bone overlying the periodontal ligament; 9 fibres responded to both types of thermal stimuli. Ten of 20 periodontal C fibres tested were activated by a saturated solution of potassium chloride applied locally to the periodontal ligament; 9 of these responded also to thermal stimuli. The receptive fields of 4 periodontal C fibres determined by probing of the thin bone layer covering the periodontal ligament with a sharp dental probe were located in the lower half of the root area of the tooth. The observed polymodal response behaviour of periodontal afferent C fibres, in particular their high threshold to mechanical stimulation, suggests that they may play a role in periodontal nociception.

Growth-associated protein (GAP-43)-like immunoreactivity in primary and permanent tooth pulp nerve fibers of the cat

GAP-43-like immunoreactivity in developing and mature incisor and canine tooth pulp nerve fibers in the cat was examined with fluorescence immunohistochemistry and pre-embedding immunogold electron microscopy. As expected, pulpal and periodontal nerve fibers in primary teeth aged 2–3 weeks showed strong immunoreactivity. Double-labeling experiments demonstrated that 50–70% of primary pulpal GAP-43-positive nerve fibers showed CGRP-like immunoreactivity. However, in adult permanent teeth the vast majority of pulpal nerve fibers also displayed intense GAP-43-like immunoreactivity both when surrounding pulpal blood vessels and in the subodontoblast/odontoblast region. There was a high degree (90–95%) of simulataneous expression of GAP-43-like immunoreactivity and CGRP-like immunoreactivity in adult permanent pulps. Immunogold GAP-43 labeling was mainly associated with the cytoplasmic side of axonal membranes. However, occasional examples of immunolabeled Schwann cells were also found. High levels of GAP-43 in normal mature permanent pulpal nerves may facilitate neural plasticity after dental wear or injury.

Distribution of calcitonin gene-related peptide and substance P-immunoreactive nerve fibers and their correlation in the periodontal ligament of the mouse incisor.

The distribution of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive (IR) nerve fibers and their correlation in the periodontal ligament of mouse incisors were examined by indirect immunofluorescence. Both CGRP-IR and SP-IR thin nerve fibers were abundant in the apical and middle third of the periodontal ligament. In the lingual portion of the incisal periodontal ligament, these nerve fibers were localized in the alveolar half of the periodontal ligament and were observed as free nerve endings. No CGRP-IR and SP-IR specialized nerve endings, such as Ruffini-like corpuscles, were observed. In the labial periodontal ligament, CGRP-IR and SP-IR nerve fibers ran along the incisal axis. The distribution of CGRP-IR nerve fibers was very similar to that of SP-IR nerve fibers.

Sensory innervation of the periodontal ligament in the incisor and molar of the monkey, Macaca fuscata. An immunohistochemical study for neurofilament protein and glia-specific S-100 protein.

Nervous elements in the periodontal ligament of the monkey, Macaca fuscata were investigated by means of immunohistochemistry for neurofilament protein (NFP) and S-100 protein. Thick nerve bundles with NFP-immunoreactivity entered the periodontal ligament through slits at the bottom of the alveolar socket, whereas thinner bundles, also NFP-immunoreactive, penetrated the ligament from the lateral wall of the alveolar socket. The NFP-immunopositive nerve fibers were densely distributed around the root apex in both incisors and molars. In the upper incisors, another dense distribution of NFP-positive nerves was found in the apical half of the lingual periodontal ligament and in the coronal half of the labial periodontal ligament. The periodontal nerve fibers in the incisors ramified in a dendritic fashion, terminating as free nerve endings within the periodontal ligament. Immunostaining for S-100 protein demonstrated the Schwann sheaths of nerves which showed the same distribution pattern as that of the NFP-immunopositive fibers in the periodontal ligament of the incisors. In molars, immunostaining for NFP and S-100 showed that nerves were numerous in the apical region of the periodontal ligament. However, NFP-immunopositive nerves were fewer than the S-100-positive nervous elements in the apical region in molars. This discrepancy suggests the presence of NFP-immunonegative nerves. Immunostaining for S-100 is thus useful for demonstration of neural elements in the periodontal ligament of molars. The S-100-immunoreactive neural elements in the periodontal ligament were more numerous in incisors than in molars. In addition to the free nerve endings, a small population of coiled nerve endings was observed in the periodontal ligament of molars. Most of the nerve fibers and cell bodies in the trigeminal ganglion showed NFP-immunoreactivity, whereas all in the mesencephalic nucleus of the trigeminal nerve were negative in NFP-reaction. This finding suggests that the periodontal ligament of molars receives a dual innervation from the trigeminal ganglion and mesencephalic nucleus.

A transmission electron-microscope stereological study of the blood vessels, oxytalan fibres and nerves of mouse-molar periodontal ligament

Mandibular, mesiobuccal ligament was sectioned from the alveolar crest to the apex at predetermined levels. Data collected using standard point counting procedures was analysed for main effects due to animal, side of mouth, depth and zone across the ligament. Statistically-significant variations only occurred between different lateral thirds with the middle third containing the most oxytalan fibres and the bone third the greatest vascular and neural volumes. Stereology showed that the ligament was 3–7 times more vascular than other connective tissues. Eighty-eight per cent of the periodontal blood volume was enclosed in vessels with a mean lumenal diameter of 20.9 μm and characterized by a thin endothelial lining and few perivascular cells. These vessels had a surface density of 125.1 cm 2/cm 3. Oxytalan fibres had a length density of 1258 × 10 3cm/cm 3 and a mean caliper diameter of 0.7 μm. Furthermore, 78 per cent of fibres were adjacent to cells, 14 per cent within principal collagen fibres and 8 per cent in the walls of blood vessels. The length density of nerves within the ligament was 255.9 × 10 3cm/cm 3. Unmyelinated axons constituting 95 per cent of periodontal nerve fibres had a mean caliper diameter of 1.4 μm.

Is electrical stimulation of the rat incisor an appropriate experimental nociceptive stimulus?

The purpose of this study was (i) to determine whether or not tooth pulp stimulation in the rat can selectively activate the pulp nerve fibers without excitation of the periodontium and (ii) to decide if the nerve fibers situated in the pulp of the rat's incisor are involved in the nociceptive reactions caused by an intrapulpal stimulation. The experiments were carried out on 20 awake and freely moving Sprague-Dawley rats. Bipolar stimulating electrodes were inserted into the pulp of the left lower incisor and in the right incisor after removal of the pulp. Special cares were taken to avoid, on the right side, direct stimulation of the stump of the apical nerve. The jaw opening reflexes were recorded from the digastric muscles ipsilaterally to the stimulated teeth and the thresholds were compared. Using the same animals, four typical and reproducible nociceptive behavioral reactions caused by a long tooth pulp stimulation were also observed (shock of 0.5 ms at 50 Hz during 1 s). The stimulus intensity was progressively increased, and the threshold of each reaction was recorded. For each of the 20 rats tested, the jaw opening reflex and the nociceptive reactions did not disappear after removal of the pulp, but the threshold of the responses to the stimulation of the nonvital tooth were significantly above the threshold of the responses to the stimulation of the vital incisor. The conclusion was (i) tooth pulp stimulation activates the periodontal nerve fibers in the rat, and (ii) stimulation of the incisor pulp is significant in pain study in the rat because the thresholds of the jaw opening reflex and the nociceptive reactions were increased after the tooth pulp tissue was removed.

Periodontal Sensory Innervation of the Dentition of the Japanese Shrew Mole

The neurohistological investigation of the periodontal sensory innervation in the whole dentition of the Japanese shrew mole showed that the periodontium is much more densely supplied by the free nerve endings at the apical region than at the intermediate one at a ratio of about 8:1 in the upper dentition and 5:1 in the lower dentition. The interdental gingival tissue is innervated by the nerve plexus formed by the nerve fibers deriving mostly from the gingival and a few periodontal nerve fibers. The neurohistology of the tooth is analogous to that of the vibrissa.