Quantitation of nerve fibres in the primary and permanent canine and incisor teeth in man

Iontophoresis-Infused Deep Topical Anesthesia and Injectable Local Anesthesia for Dental Procedures Among Pediatric Patients: Performances and Consequences

INTRODUCTION

Exploring routes of needle-free anesthesia has drawn particular attention to the iontophoretic technique. Iontophoresis has a wide range of applications in dentistry, treating hypersensitivity, oral ulcers, non-invasive procedures of deep topical anesthesia, etc. Hence, this research was performed for a comparative assessment of topical anesthesia spray infused via iontophoresis and local anesthesia (LA) infiltration for dental procedures among 5-12-year-old patients.

MATERIALS AND METHODS

A split-mouth, randomized clinical trial was undertaken over two years among study subjects aged 5 to 12 years. They were randomly assigned to one of two groups: the first (Group A - iontophoresis group) received topical anesthesia spray (Lidayn®; Pyrax Polymers, Roorkee, India) applied by iontophoresis, and the second (Group B - LA infiltration group) received local infiltration of 2% lignocaine solution (LignoTer®; Lusture Pharma, Ahmedabad, India), where primary teeth extraction or pulpectomy was performed. The Wong-Baker Facial Pain Rating Scale (WBFPRS) was used for a subjective assessment immediately following anesthesia.

RESULTS

The mean value of current intensity for the extraction procedure was 9.43±0.95 mA, and the duration of application was 1.85±0.80 minutes. The mean value of current intensity for pulpectomy was 9.07±1.34 mA, and the time was 2.40±0.74 minutes. In inter-group comparison, WBFPRS scores were lower in Group A (1.96±1.64) compared to Group B (3.62±1.11), which was statistically significant with p=0.001.

CONCLUSION

Compared to local infiltration, iontophoresis as a non-invasive approach for topical anesthesia was more well-received by pediatric patients.

The use of electrodermal activity in pulpal diagnosis and dental pain assessment

AIMS

To explore whether electrodermal activity (EDA) can serve as a complementary tool for pulpal diagnosis (Aim 1) and an objective metric to assess dental pain before and after local anaesthesia (Aim 2).

METHODOLOGY

A total of 53 subjects (189 teeth) and 14 subjects (14 teeth) were recruited for Aim 1 and Aim 2, respectively. We recorded EDA using commercially available devices, PowerLab and Galvanic Skin Response (GSR) Amplifier, in conjunction with cold and electric pulp testing (EPT). Participants rated their level of sensation on a 0-10 visual analogue scale (VAS) after each test. We recorded EPT-stimulated EDA activity before and after the administration of local anaesthesia for participants who required root canal treatment (RCT) due to painful pulpitis. The raw data were converted to the time-varying index of sympathetic activity (TVSymp), a sensitive and specific parameter of EDA. Statistical analysis was performed using Python 3.6 and its Scikit-post hoc library.

RESULTS

Electrodermal activity was upregulated by the stimuli of cold and EPT testing in the normal pulp. TVSymp signals were significantly increased in vital pulp compared to necrotic pulp by both cold test and EPT. Teeth that exhibited intensive sensitivity to cold with or without lingering pain had increased peak numbers of TVSymp than teeth with mild sensation to cold. Pre- and post-anaesthesia EDA activity and VAS scores were recorded in patients with painful pulpitis. Post-anaesthesia EDA signals were significantly lower compared to pre-anaesthesia levels. Approximately 71% of patients (10 of 14 patients) experienced no pain during treatment and reported VAS score of 0 or 1. The majority of patients (10 of 14) showed a reduction of TVSymp after the administration of anaesthesia. Two of three patients who experienced increased pain during RCT (post-treatment VAS > pre-treatment VAS) exhibited increased post-anaesthesia TVSymp.

CONCLUSIONS

Our data show promising results for using EDA in pulpal diagnosis and for assessing dental pain. Whilst our testing was limited to subjects who had adequate communication skills, our future goal is to be able to use this technology to aid in the endodontic diagnosis of patients who have limited communication ability.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.

Secretome Profiling of Periodontal Ligament from Deciduous and Permanent Teeth Reveals a Distinct Expression Pattern of Laminin Chains

It has been suggested that there are histological and functional distinctions between the periodontal ligament (PDL) of deciduous (DecPDL) and permanent (PermPDL) teeth. Thus, we hypothesized that DecPDL and PermPDL display differences in the constitutive expression of genes/proteins involved with PDL homeostasis. Primary PDL cell cultures were obtained for DecPDL (n = 3) and PermPDL (n = 3) to allow us to perform label-free quantitative secretome analysis. Although a highly similar profile was found between DecPDL and PermPDL cells, comparative secretome analysis evidenced that one of the most stickling differences involved cell adhesion molecules, including laminin subunit gamma 1 (LAMC1) and beta 2 (LAMB2). Next, total RNA and protein extracts were obtained from fresh PDL tissues of deciduous (n = 6) and permanent (n = 6) teeth, and Western blotting and qPCR analysis were used to validate our in vitro findings. Western blot analysis confirmed that LAMC1 was increased in DecPDL fresh tissues (p<0.05). Furthermore, qPCR data analysis revealed that mRNA levels for laminin subunit beta 1 (LAMB1), beta 3 (LAMB3), LAMC1, and gamma 2 (LAMC2) were higher in DecPDL fresh tissues, whereas transcripts for LAMB2 were increased in PermPDL (p<0.05). In conclusion, the differential expression of laminin chains in DecPDL and PermPDL suggests an involvement of laminin-dependent pathways in the control of physiological differences between them.

Axonal Degeneration in Dental Pulp Precedes Human Primary Teeth Exfoliation

The dental pulp in human primary teeth is densely innervated by a plethora of nerve endings at the coronal pulp-dentin interface. This study analyzed how the physiological root resorption (PRR) process affects dental pulp innervation before exfoliation of primary teeth. Forty-four primary canine teeth, classified into 3 defined PRR stages (early, middle, and advanced) were fixed and demineralized. Longitudinal cryosections of each tooth were stained for immunohistochemical and quantitative analysis of dental pulp nerve fibers and associated components with confocal and electron microscopy. During PRR, axonal degeneration was prominent and progressive in a Wallerian-like scheme, comprising nerve fiber bundles and nerve endings within the coronal and root pulp. Neurofilament fragmentation increased significantly during PRR progression and was accompanied by myelin degradation and a progressive loss of myelinated axons. Myelin sheath degradation involved activation of autophagic activity by Schwann cells to remove myelin debris. These cells expressed a sequence of responses comprising dedifferentiation, proliferative activity, GAP-43 overexpression, and Büngner band formation. During the advanced PRR stage, increased immune cell recruitment within the dental pulp and major histocompatibility complex (MHC) class II upregulation by Schwann cells characterized an inflammatory condition associated with the denervation process in preexfoliative primary teeth. The ensuing loss of dental pulp axons is likely to be responsible for the progressive reduction of sensory function of the dental pulp during preexfoliative stages.

Comparative gene expression analysis of the human periodontal ligament in deciduous and permanent teeth

There are histological and functional differences between human deciduous and permanent periodontal ligament (PDL) tissues. The aim of this study was to determine the differences between these two types of tissue at the molecular level by comparing their gene expression patterns. PDL samples were obtained from permanent premolars (n = 38) and anterior deciduous teeth (n = 31) extracted from 40 healthy persons. Comparative cDNA microarray analysis revealed several differences in gene expression between the deciduous and permanent PDL tissues. These findings were verified by qRT-PCR (quantitative reverse-transcription-polymerase chain reaction) analysis, and the areas where genes are expressed were revealed by immunohistochemical staining. The expressions of 21 genes were up-regulated in deciduous relative to PDL tissues, and those of 30 genes were up-regulated in permanent relative to deciduous PDL tissues. The genes that were up-regulated in deciduous PDL tissues were those involved in the formation of the extracellular matrix (LAMC2, LAMB3, and COMP), tissue development (IGF2BP, MAB21L2, and PAX3), and inflammatory or immune reactions leading to tissue degradation (IL1A, CCL21, and CCL18). The up-regulated genes in permanent PDL tissues were related to tissue degradation (IL6 and ADAMTS18), myocontraction (PDE3B, CASQ2, and MYH10), and neurological responses (FOS, NCAM2, SYT1, SLC22A3, DOCK3, LRRTM1, LRRTM3, PRSS12, and ARPP21). The analysis of differential gene expressions between deciduous and permanent PDL tissues aids our understanding of histological and functional differences between them at the molecular level.

Unmyelinated nerve fibers in the human dental pulp express markers for myelinated fibers and show sodium channel accumulations

BACKGROUND

The dental pulp is a common source of pain and is used to study peripheral inflammatory pain mechanisms. Results show most fibers are unmyelinated, yet recent findings in experimental animals suggest many pulpal afferents originate from fibers that are myelinated at more proximal locations. Here we use the human dental pulp and confocal microscopy to examine the staining relationships of neurofilament heavy (NFH), a protein commonly expressed in myelinated afferents, with other markers to test the possibility that unmyelinated pulpal afferents originate from myelinated axons. Other staining relationships studied included myelin basic protein (MBP), protein gene product (PGP) 9.5 to identify all nerve fibers, tyrosine hydroxylase (TH) to identify sympathetic fibers, contactin-associated protein (caspr) to identify nodal sites, S-100 to identify Schwann cells and sodium channels (NaChs).

RESULTS

Results show NFH expression in most PGP9.5 fibers except those with TH and include the broad expression of NFH in axons lacking MBP. Fibers with NFH and MBP show NaCh clusters at nodal sites as expected, but surprisingly, NaCh accumulations are also seen in unmyelinated fibers with NFH, and in fibers with NFH that lack Schwann cell associations.

CONCLUSIONS

The expression of NFH in most axons suggests a myelinated origin for many pulpal afferents, while the presence of NaCh clusters in unmyelinated fibers suggests an inherent capacity for the unmyelinated segments of myelinated fibers to form NaCh accumulations. These findings have broad implications on the use of dental pulp to study pain mechanisms and suggest possible novel mechanisms responsible for NaCh cluster formation and neuronal excitability.

Quantitative ultrastructural analysis of the neurofilament 200-positive axons in the rat dental pulp

INTRODUCTION

Previous studies have suggested that myelinated axons lose their myelin and become thinner in their peripheral course to the target organ. In this study, we investigated the morphologic changes of pulpal myelinated axons between their root portion (radicular pulp) and their terminal area (peripheral pulp).

METHODS

Sections of pulp of the rat upper molar teeth were immunostained for the marker of myelinated axons neurofilament (NF) 200. The proportion of NF200+ myelinated and unmyelinated fibers and their sizes were analyzed by using quantitative electron microscopy.

RESULTS

The axon area, myelin thickness, and fraction of NF200+ myelinated axons of all NF200+ axons were significantly lower in peripheral than in radicular pulp. In addition, large unmyelinated axons were frequently observed in peripheral pulp.

CONCLUSIONS

These results suggest that pulpal innervation originates predominantly from myelinated axons, and the myelinated axons undergo extensive morphologic changes during their course from the radicular to the peripheral pulp.

Peripheral mechanisms of odontogenic pain

In this article, we review the key basic mechanisms associated with this phenomena and more recently identified mechanisms that are current areas of interest. Although many of these pain mechanisms apply throughout the body, we attempt to describe these mechanisms in the context of trigeminal pain.

Local Opioids in the Inflamed Dental Pulp

We have looked for the presence of beta-endorphin and somatostatin in normal and inflamed pulps. In 25 adult rats, under anesthesia, small openings were made into the pulp on the mesial surface of both first molars on one side. One week later all four first molars were removed, half were processed for immunohistochemistry using specific antibodies to beta-endorphin, somatostatin, and CD3, a marker for T lymphocytes. The pulps of the remainder were removed, solubilized, and subjected to enzyme-linked immunosorbent assay for somatostatin and beta-endorphin. Many cells that labeled for beta-endorphin and somatostatin in the injured pulps also stained for CD3. The levels of both beta-endorphin and somatostatin, were higher in the exposed than in the uninjured pulps (t test, p < 0.05). beta-endorphin and somatostatin are both produced in increased amounts in the dental pulp during inflammation attributable, at least in part, to the presence of T lymphocytes producing these substances.

Morphometric difference in the human maxillary nerve fibers between dentulous and edentulous jaw subjects

This study was conducted to quantify the change in the number and size of myelinated nerve fibers of the maxillary nerve with tooth loss in humans. We carried out a morphometric analysis to compare the number and size of myelinated nerve fibers in the human maxillary nerve between four dentulous and four edentulous jaw cases. Our results indicated that the number of axons decreased by approximately 13,000 with tooth loss. The average size of axons remained unchanged, but there was a change in the fiber size distribution, namely the loss of a large number of small-sized axons was accompanied by the total disappearance of small number of large-sized axons.

Is pulpitis painful?

AIM

To determine whether inflamed dental pulps progress to necrosis without pain.

METHODOLOGY

Records of 2,202 maxillary anterior teeth endodontically treated at the University of Michigan were collected. Records of teeth presenting with periapical radiolucencies but no response to vitality tests were examined further to determine, from the history, whether the patient had experienced pain or no pain from the involved tooth.

RESULTS

Approximately 40% of the teeth included gave no history of spontaneous or prolonged pain to a thermal stimulus. No statistically significant differences in the incidence of 'painless pulpitis' were related to either gender or tooth type. Patients aged >53 years experienced 'painless pulpitis' more often than patients <33 years.

CONCLUSIONS

Many teeth appear to progress to pulpal necrosis without the patient experiencing pain attributable to the pulp.

Innervation of human tooth pulp in relation to caries and dentition type

The neural status of carious teeth, particularly those associated with a painful pulpitis, is largely unknown. This study sought to determine differences in the innervation density of human primary and permanent teeth and whether caries or painful pulpitis was associated with anatomical changes in pulpal innervation. Coronal pulps were removed from 120 primary and permanent molars with a known pain history. Teeth were categorized as intact, moderately carious, or grossly carious. Using indirect immunofluorescence, we labeled sections for the general neuronal marker, protein gene product 9.5. Using image analysis, we found permanent teeth to be significantly more densely innervated than primary teeth. While there was no significant correlation with reported pain experience, neural density in both dentitions increased significantly with caries. Analysis of these data suggests that caries-induced changes in neural density may be functionally more important in the regulation of pulpal inflammation and healing than in the processing and perception of dental pain.

On the pulpal nerve supply in primary human teeth: evidence for the innervation of primary dentine

The presence of nerves in human tooth pulp has been recognized for over a hundred years, and the innervation of dentine for about 40 years. These observations have been made in permanent teeth. Very few studies have reported on the innervation of the primary pulp and dentine. The purpose of this study was to describe the innervation of the primary tooth pulp-dentine complex. Ten mature primary teeth (one incisor, six canines and three molars) were used. Immediately following extraction they were divided into three sections using a diamond disc and saline coolant. They were then immersion fixed in a solution of formaldehyde and picric acid dissolved in a phosphate buffer pH 7.4). The teeth were then demineralized for 1-3 weeks in formic acid. Following complete demineralization, 30 microns sections were cut on a freezing microtome. Neural tissue was stained using a specific antibody to calcitonin gene related peptide (CGRP). Sections were mounted on glass slides and examined using light microscopy. No individual nerve fibres were seen in the control sections, suggesting that the method used was specific for CGRP-containing nerve fibres. The primary teeth appeared to be well innervated. Myelinated and unmyelinated nerves were seen. There was a dense but variable subodontoblastic plexus of nerves (plexus of Raschkow) and nerve fibres were seen to leave this to travel towards the odontoblast layer. Most terminated here, but a few penetrated the odontoblast layer to enter predentine and the dentine tubules. The maximum penetration was 125 microns but most terminated within 30 microns of the dentinopulpal junction. The coronal region was more densely innervated than the root. Within the crown the cervical third was the most densely innervated region, followed by the pulp horn and the middle third. In conclusion, this study has demonstrated that mature primary tooth contains a pulp which is well innervated and has many nerve endings terminating in or near the odontoblast layer, with a small number penetrating into dentine.

Immunocytochemical and electron-microscopic features of tooth pulp innervation in hereditary sensory and autonomic neuropathy

Characteristics of the pulpal innervation in teeth obtained from a 4-year-old Asian boy with hereditary sensory and autonomic neuropathy, type II (HSAN) were investigated. Four minimally carious primary teeth were split longitudinally and prepared for either fluorescent immunocytochemistry or electron microscopy. The occurrence and distribution of specific neuropeptides were determined by the use of antisera to calcitonin gene-related peptide (CGRP), substance P (SP), neuropeptide Y (NPY), and vasoactive intestinal polypeptide (VIP). The overall innervation of the pulps was visualized using antiserum to protein gene product 9.5; an antiserum to dopamine beta-hydroxylase was used to identify postganglionic sympathetic fibres. Pulpal innervation in HSAN was notably different from that of normal teeth: in comparison with the controls, HSAN teeth had an overall marked reduction in pulpal innervation with an absence of large nerve bundles and the subodontoblastic plexus. CGRP- and SP-immunoreactivity was absent in HSAN specimens and VIP-immunoreactivity was reduced. However, NPY-immunoreactivity appeared to be increased within certain regions of the pulp/dentine complex. In addition, there was evidence of NPY-immunoreactive fibres extending into dentine, a feature not seen in the controls. Electron microscopy revealed an absence of myelinated nerve fibres and a paucity of unmyelinated fibres. CGRP and SP have a well-established role in nociceptive processing and their absence in the HSAN teeth would seem to correspond with the clinical presentation of marked peripheral sensory deficit, characteristic of this condition. An up-regulation of NPY-immunoreactivity has previously been reported in animal teeth following nerve injury and a similar mechanism may have stimulated increased NPY expression in HSAN teeth, but the functional significance of its presence within dentinal nerves is not known.

Neural elements in dental pulp and dentin

This article addresses the structural and quantitative aspects of human tooth innervation and briefly considers the functions and clinical relevance of tooth axons. The classification of peripheral axons, the pulpal and dentinal innervation, and the theories of dentin sensitivity are discussed. Quantitative studies on tooth innervation are also reviewed. Human premolars receive about 2300 axons at the root-apex of which about 13% are myelinated and 87% are nonmyelinated fibers. Most apical myelinated axons are fast-conducting A delta-fibers with their receptive fields located at the pulpal periphery and inner dentin. These fibers are probably activated by a hydrodynamic mechanism and conduct impulses that are perceived as a short well-localized sharp pain. Most C-fibers are slow-conducting fine sensory afferents with their receptive fields located in the pulp and transmit impulses that are experienced as dull poorly localized and lingering pain. In addition to the nociceptive alarm signaling, the intradental sensory axons may play a regulatory role in the maintenance and repair of the pulpodentinal complex.

Number and size spectra of non-myelinated axons of human premolars

The aim of this study was to determine the number and size of apical non-myelinated (C) axons of healthy human premolars. The material was derived from a large collection of specimens prepared for a previous quantitative investigation on the myelinated (A) axons of human premolars. A total of 16 teeth (six maxillary first and five each of mandibular first and second premolars), removed from adolescents for orthodontic reasons, were used. Root discs of about 0.6 mm thickness were prepared at about 2 mm cervical to the root apex and processed for light and electron microscopy. The number of non-myelinated axons was determined by taking a total census of such fibres that could be identified and reconstructed by standardized composite electron micrographs from each root disc. The measurement of axons was done on a statistically representative sample of axons (n = 1810) using an electronic image processing unit. The 16 teeth had an average of 2000 +/- 1023 non-myelinated axons at the juxta-apical level (range 534-3912). The average diameter of the non-myelinated axons was found to be 0.5 +/- 0.4 microns (range 0.05-2.4 microns).

Teeth and tooth nerves

(1) Although our knowledge on teeth and tooth nerves has increased substantially during the past 25 years, several important issues remain to be fully elucidated. As a result of the work now going on at many laboratories over the world, we can expect exciting new findings and major break-throughs in these and other areas in a near future. (2) Dentin-like and enamel-like hard tissues evolved as components of the exoskeletal bony armor of early vertebrates, 500 million years ago, long before the first appearance of teeth. It is possible that teeth developed from tubercles (odontodes) in the bony armor. The presence of a canal system in the bony plates, of tubular dentin, of external pores in the enamel layer and of a link to the lateral line system promoted hypotheses that the bony plates and tooth precursors may have had a sensory function. The evolution of an efficient brain, of a head with paired sense organs and of toothed jaws concurred with a shift from a sessile filter-feeding life to active prey hunting. (3) The wide spectrum of feeding behaviors exhibited by modern vertebrates is reflected by a variety of dentition types. While the teeth are continuously renewed in toothed non-mammalian vertebrates, tooth turnover is highly restricted in mammals. As a rule, one set of primary teeth is replaced by one set of permanent teeth. Since teeth are richly innervated, the turnover necessitates a local neural plasticity. Another factor calling for a local plasticity is the relatively frequent occurrence of age-related and pathological dental changes. (4) Tooth development is initiated through interactions between the oral epithelium and underlying neural crest-derived mesenchymal cells. The interactions are mediated by cell surface molecules, extracellular matrix molecules and soluble molecules. The possibility that the initiating events might involve a neural component has been much discussed. With respect to mammals, the experimental evidence available does not support this hypothesis. In the teleost Tilapia mariae, on the other hand, tooth germ formation is interrupted, and tooth turnover ceases after local denervation. (5) Prospective dental nerves enter the jaws well before onset of tooth development. When a dental lamina has formed, a plexus of nerve branches is seen in the subepithelial mesenchyme. Shortly thereafter, specific branches to individual tooth primordia can be distinguished. In bud stage tooth germs, axon terminals surround the condensed mesenchyme and in cap stage primordia axons grow into the dental follicle.(ABSTRACT TRUNCATED AT 400 WORDS)

Encoding of the subjective intensity of sharp dental pain

This paper is a review and a discussion of our own pain research over the last decade. It is of a methodological and theoretical character and deals with preparation technique, choice of electrodes, control experiments involving pulpotomy and reliability tests of psychophysical methods for pain measurements, and the neuronal population encoding of sharp dental pain. The electrophysiological recording technique selectively picks up electrical activity induced in pulpal A-delta nerve fibers. The sensation of pain was quantified by means of an intermodal matching technique, finger span (PAS), in combination with sensory verbal descriptors covering a range from very, very weak to maximal pain. When a cold stimulus, ethyl chloride, was applied on the tooth surface a close agreement was demonstrated between intradental A-delta nerve activity (INA) and the sensation magnitude of pain (PAS) with respect to curve amplitude and time course. The high covariation of the neural and perceptual response measures indicated a good internal validity and confirmed also the basic soundness and the applicability of the procedures employed. For the purpose of further analyzing the functional relation of INA to PAS we studied specifically the effect of cold stimuli of different intensity on the integrated nerve response. Only sharp, shooting pain was accepted as a sensorial, perceptual correlate of the intradental A-delta nerve activity. Since an increase in amplitude was generally accompanied by an increase in duration of the responses, the fundamental question was raised how to best describe and characterize the neural and perceptual responses so that they most adequately reflect the information processing of the intensive aspect of sharp dental pain.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative study of the apical nerve fibers of adult and juvenile rat molars

The rat molar has become an important model for studies of interactions between nerves and the pulp-dentin complex, yet there is only limited quantitative information on the number and size distribution of axons entering the roots of this tooth. This study was undertaken to provide such a detailed characterization of the apical innervation of the rat molar. An additional objective was to compare the apical nerve composition of young, recently erupted rat molars with that of mature teeth in order to determine whether there is ongoing maturation of the innervation after the teeth have attained functional occlusion. A complete census was made of the nerve fibers entering the roots of both mature and recently erupted juvenile mandibular first molars in Sprague-Dawley rats. Each of the four roots of the first molars was processed for electron microscopy of thin sections near the apex. The majority of intradental nerve fibers entered the molar via the two larger (mesial and distal) roots. Within the apical root pulp, most, but not all, axons occurred within well-defined fascicles associated with blood vessels. Molars from adult animals (age 4 months) had a mean total of 232 (S.D. = 49, N = 7 teeth) myelinated fibers and 806 (S.D. = 143) unmyelinated axons entering the four roots. Fibers exceeding the A delta size range (circumference > or = 19 microns) accounted for only 4% of the myelinated axons at the apex. Molars from juvenile animals (age 4 weeks) had fewer myelinated fibers (mean 176, S.D. 18, N = 8), but more unmyelinated axons (mean 1,174, S.D. 160) than adults. The mean ratio of unmyelinated axons to myelinated axons was 6.6:1 for juveniles compared to 3.5:1 for adults. Juvenile teeth contained no myelinated fibers that exceeded 19 microns in circumference. These results indicate that the innervation of the rat molar resembles that of teeth of non-rodent mammals in that (1) innervation density is high, (2) there is a high ratio of unmyelinated axons, and (3) most of the myelinated fibers are of thin caliber. Furthermore, it appears that after the molar erupts, maturation of the nerve fiber composition continues with processes that include both a marked decrease in the number of unmyelinated axons and an increase in the number and size heterogeneity of myelinated fibers.

Number and size-spectra of myelinated nerve fibers of human premolars

The primary objective of this study was to determine the number and size of myelinated nerve fibers at the subcervical, midroot and juxta-apical levels of human premolars. Sixty-seven healthy premolars extracted from adolescents were utilized. Root-discs were prepared from the three sites and processed for light and electron microscopy. The myelinated nerve fibers were counted from semithin sections using a sampling microscope. The measurements were taken from composite electron micrographs using an electronic image processing unit. A total of 1883 myelinated axons from seven mandibular second premolars was gauged. The 67 teeth had an average of 312 +/- 149 myelinated nerve fibers at the juxta-apical level (range 18 to 728). The contra- and ipsilateral differences in means among the four groups of premolars were not significant (P > 0.05). The number of nerves increased significantly (P < 0.05) toward midroot and subcervical (P < 0.001) levels in all groups. The average neural diameter was 3.5 + 1.0 microns at the juxta-apical level, and the between-teeth difference in mean was found to be significant (P < 0.01). There was no decline (P > 0.05) in the diameter of myelinated nerve fibers toward midroot and subcervical levels.

Occurrence and distribution of different neurochemical markers in the human dental pulp

The occurrence and distribution of neuronal markers in human premolar and molar pulps were studied immunohistochemically. In the apical and central parts of the pulp, evenly distributed, thick neurofilament-immunoreactive nerve bundles predominated, which in many instances accompanied blood vessels. In the coronal parts, especially in the pulp horns, such nerve bundles formed a subodontoblastic plexus, while thin neurofilament-immunoreactive fibres projected into the odontoblastic region. In the coronal parts of the pulp, thin, varicose, calcitonin gene-related peptide (CGRP)- and occasionally substance P-immunoreactive fibres were observed in the pulp-dentine zone and also in the vicinity of blood vessels. Vasoactive intestinal polypeptide (VIP) fibres were distributed in several nerve bundles, while single VIP fibres were seen projecting into the odontoblastic region as well as in the vicinity of blood vessels. Peptide histidine isoleucine amide (PHI)-immunoreactive fibres showed a similar distribution as VIP, but were less common. Furthermore, neuropeptide Y-immunoreactive fibres occurred occasionally around blood vessels in the inner parts of the pulp. Tyrosine hydroxylase-immunoreactive nerve fibres with a varicose appearance were observed in some nerve bundles, but were also frequently seen around and in blood vessels. In premolar pulps obtained from teeth with open apices a less dense neurofilament innervation was seen in the coronal pulp. However, no apparent difference in the occurrence and distribution of the other neuronal markers was found compared to mature teeth. The human dental pulp, thus, seems to have a rich occurrence of neuropeptides and tyrosine hydroxylase in thin, varicose fibres. However, the distribution of the fibres expressing immunoreactivity to these neuronal markers seems to be sparse in comparison to neurofilament-immunoreactive fibres.

Histometric study of myelinated fibers in the human trigeminal nerve

The trigeminal ganglion, roots and the initial portion of the ophthalmic, maxillary and mandibular nerves were dissected in 3 cadavers, to study the number, area and composition of the fascicles, and the density and diameter spectra of myelinated fibers. The total number of fibers (x 1000) was 26 in the ophthalmic, 50 in the maxillary, and 78 in the mandibular division, 7.7 in the motor root and 170 in the sensory root. In all nerves, the histograms of fiber diameter had a bimodal distribution. Cutaneous and muscle nerve fascicles clearly differed in the fiber density and diameter. The ophthalmic and maxillary nerves (cutaneous) had similar fascicles, and their maximum fiber diameter averaged 14.5 microns. Most fascicles of the mandibular nerve (probably cutaneous fascicles) closely resembled those of the ophthalmic and maxillary nerves, but in some fascicles (probably muscle nerves) the fibers were larger, with a maximum diameter of 19.3 microns. The findings in the three peripheral divisions agree with electrophysiological data about sensory and motor conduction in human trigeminal nerves. The observation that the ophthalmic and maxillary nerves have similar fiber spectra indicates that a special fiber composition does not account for the sparing of the ophthalmic division in trigeminal neuralgia. The absence of very large (A alpha) fibers in the sensory root does not support the view that impulses from muscle spindles are conducted along this root.

Innervation of the tooth pulp by the mesencephalic trigeminal nucleus in the cat: a retrograde horseradish peroxidase study

HRP was applied to the tooth pulp of 8 cats. Six were subjected to postoperative administration of the anti-inflammatory drug, prednisolone, whereas the remaining two were not. In all prednisolone-treated cats, labeled neurons were found in both the mesencephalic trigeminal nucleus and trigeminal ganglion, ipsilaterally. On the other hand, no labeled neurons were observed in the mesencephalic nucleus in cats receiving no steroid.

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.

Innervation of teeth: qualitative, quantitative, and developmental assessment

Anatomical characteristics of tooth innervation provide insights into functional capabilities as well as limitations of this organ. In this review, innervation will be discussed from two major viewpoints. The first part will present distribution of nerve fibers in the tooth; nerve pathways, both autonomic and sensory, will be discussed mostly from a descriptive standpoint. In the second part, quantitation of neural units along key points of the pathways will be presented at milestones in tooth and organism development and aging.

Dental sensory receptors

Teeth are innervated by unmyelinated sympathetic axons, and by unmyelinated and small myelinated sensory axons. Some sensory axons in teeth are terminal branches of larger parent axons, so that conduction from teeth to CNS in trigeminal nerves includes C-fiber, A-delta, and A-beta velocities. Sensory dental axons contain acetylcholine or substance P-like immunoreactivity. The sympathetic axons contain noradrenalin. Other neuropeptides may also be present, such as vasoactive intestinal peptide and serotonin. Dental axons of mature teeth of many species (man, monkey, cat, rodents, fish) are essentially the same, but continuously erupting teeth have smaller and fewer axons. Free sensory nerve endings in mature teeth are found in the peripheral plexus of Raschkow, the odontoblastic layer, the predentin, and the dentin. Free nerve endings are most numerous in those regions near the tip of the pulp horn, where more than 40% of the dentinal tubules can be innervated. Many dentinal tubules contain more than one free nerve ending. Intradentinal axons can extend as far as 0.2 mm into dentin but usually end less than 0.1 mm from the pulp. Some sensory endings also occur along pulpal blood vessels. In continuously erupting teeth nerve endings do not enter the dentin but remain within the pulp. Nerve endings in dentin are labeled by axonal transport. They are therefore as viable and active as the nerve endings in pulp. The axoplasm of the free nerve endings contains organelles typical of other somatosensory receptors. These organelles are most common in the successive beaded regions along the free nerve endings and include mitochondria, clear and dense-core vesicles, multivesicular bodies, profiles of smooth endoplasmic reticulum, and relatively few microtubules and neurofilaments. The beads can vary in size from about 0.2 to 2.0 microns and can have varying amounts of receptor organelles. The interbead axonal regions are thin and contain mainly microtubules and neurofilaments. Nerve endings are associated with companion cells after they leave the coronal nerve bundles; these companion cells include Schwann cells, fibroblasts, and odontoblasts. There is no good evidence of gap junctions or synapses between nerve endings and odontoblasts. Instead, the two cell types form appositions that have a 20-40 nm extracellular cleft and parallel apposed plasmalemmas but no unusual membrane-associated material. No special organelles occur in the odontoblastic cytoplasm at these sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative assessment of neural development in human premolars

The number of nerve fibers entering a tooth gives an indication of the tooth's capacity to perform a sensory function. Nerve fiber development was quantitated from cross sections of the apical portions of 49 erupted human premolars at various stages of root development and in subjects up to 71 years of age. Neural development was incomplete in immature teeth, greatly variable in young mature teeth, and complete in older teeth. Myelinated axons changed in number but not in size during tooth development. There were significantly fewer myelinated axons in teeth with open and parallel apical foramina than in older teeth. Unmyelinated axons did not change significantly in number with development but fewer large axons were found in older teeth. The number of unmyelinated axons enclosed in a single boundary lamina tended to be lower in older teeth. As a physiologic correlate, threshold responses to electrical stimulation were also determined prior to premolar removal. Threshold stimulation decreased significantly with apical foramen maturation. A significant negative correlation was found between the threshold stimulus and the number of myelinated axons in fully developed teeth, but not in immature teeth.

The number and size of axons at the apex of the cat's canine tooth

Using electron microscopy and morphometric analysis the number and size of axons entering the apex of the cat's mandibular canine tooth have been measured. The total number of axons varied from 761 to 1,903 between different animals but the maximum difference between right and left sides of the same animal was 353. From 56 to 79.6% of the axons were nonmyelinated; the difference in proportion between right and left sides never exceeded 6.4%. The mean circumference of myelinated axons ranged from 10.2 to 18.3 micrometers but again the right and left variation was much less and never exceeded 2 micrometers. In one tooth 38.8% of the myelinated axons were larger than 19 micrometers in circumference and thus outside the A delta range. The proportion was much smaller in other teeth but some "large" fibers were always present. Of all the nonmyelinated axons 19.7% showed some degree of axonal exposure to the extracellular space and 1.7% showed ax-oaxonal apposition. A small proportion of nonmyelinated axons showed evidence of apparent degeneration. Comparison of these data with those from studies at more coronal levels suggests that there is considerable branching and narrowing of fibers during their course through the dental pulp and that the degree of axonal exposure and apposition increases considerably. Some of the pulpal fibers are derived from larger axons than are normally associated with pain. The animal to animal variation in the parameters measured is considerable but right and left sides are similar.

Quantitative assessment of intrapulpal axon response to orthodontic movement

Nerve fiber changes are of potential concern during tooth movement. Quantitative and qualitative assessments using electron microscopy of nerve fibers entering the tooth give an indication of change in the remainder of the pulp. Eight healthy mandibular first premolars were selected from four subjects, so that respective pairs had open apices and short-term movement, open apices and long-term movement, closed apices and short-term movement, and closed apices and long-term movement. These were compared to forty-nine untreated control teeth. No significant differences in myelinated or unmyelinated axon number were observed between experimental and control teeth. Altered myelin figures, possibly degenerating, were observed in only a small percentage of axons in teeth moved for a short period. No alterations were observed in teeth moved for long periods. It is concluded that intrapulpal axon alterations are minimal and not progressive with conservative orthodontic tooth movement.

Developmental growth and degeneration of pulpal axons in feline primary incisors

The life history of pulpal axons in primary mandibular incisors was examined by light and electron microscopy in 56 kittens aged from 25 days postconception to 120 days after birth. Cells resembling Schwann cells preceded the first arrival of pulpal axons, 1 week postnatally. myelination was initiated during the second week. Two months after birth the incisors were fully grown and each pulp contained about 100 axons. Between 10 and 20% of these were myelinated and ranged in size from 1 to 5 micrometer. The relation between number of myelin lamellae and axon size appeared nonlinear and differed markedly from that in similarly sized inferior alveolar nerve axons. During the third month many unmyelinated axons showed signs of degeneration. With progress of root resorption an increasing proportion of both unmyelinated and myelinated axons degenerated. In highly resorbed incisors necrotic Schwann cells were associated with degenerating axons and there was a generalized pulpal tissue reaction. In some teeth with advanced root resorption pulpal axons were lacking. A progressive derangement of all pulpal tissue elements continued until shedding during the second half of the fourth month. In incisor nerve branches below the incisor teeth axon degeneration was very limited.

A fine-structural survey of the pulpal innervation in the rat mandibular incisor

The innervation of the rat incisor pulp has been studied using transmission electron microscopy and light microscopy. Transverse sections of mandibular incisor pulp (380-460 gm rats) from numerous positions in the long axis of the tooth were examined systematically in the electron microscopy. Quantitative data on total axon populations were obtained. The nerve fibers were found to pass through the lingual half of the pulp from the apical end to within 2 mm of the incisal tip. Although the nerve fibers were seen to lie amongst the connective tissue cells between the blood vessels, the electron microscopic observations showed that the blood vessels are not innervated. Throughout their pulpal course the nerve fibers showed no trace of perineurial investment. Virtually all the axons were unmyelinated. Total numbers of axons were small (233-328) and peak diameters of 0.3-0.4 microM confirmed the observed immature appearance of the nerve supply. Obvious nerve endings were seldom observed and the axons showed no structural association with odontoblasts. The evidence indicates that, although most axons terminate near the incisal end of the tooth, no specific structure is supplied. The qualitative features of the axons do not suggest autonomic function; however, they are consistent with a sensory role.