Electron microscopic quantitations of feline primary and permanent incisor innervation

Comparison of nerve fibers in the deciduous first and second molar teeth: an in vitro study

INTRODUCTION

Sensory nerves of the tooth pulp have been evolved in both deciduous and permanent teeth for conducting possible damages to the teeth. Although it has been reported that the deciduous tooth's pulp is histologically similar to permanent teeth, the difference in the density of nerves between the first and second deciduous molars has been reported in no studies.

AIM

The aim of the study was to investigate the histological assay of pulpal nerves and assess the relationship between diameter and density of nerve fibres in the first and second deciduous molar teeth.

MATERIALS AND METHODS

In this in vitro study, 15 deciduous first and 16 deciduous second extracted from molar teeth belonging to children aged from 5 to 8 years were studied. After fixation, the decalcified teeth were sectioned and then stained with silver solution and calcitonin gene-related peptide (CGRP) antibody. The mean diameter of myelinated fibres, un-myelinated fibres, and density of peripheral network fibres of dental pulp were examined.

RESULTS

The results showed that the mean diameter of myelinated and non-myelinated fibres and density of nerve fibres were significantly more in the second molars (4.269, 2.328 µm, 2.6 fibres per unit area) compared to the first molar teeth (3.793, 2.093 µm, 1.8 fibres per unit area) (P < 0.032, 0.019, 0.003, respectively). An important finding in this study, which is reported for the first time, is the presence of fatty cells (adipocytes) in pulp tissue of some of the first deciduous molars as well as the second molars.

CONCLUSIONS

Based on the results of the current study, differences between nerve fibres of first and second molar teeth are significant. It could be concluded that the less sensitivity to pain of the first deciduous molars compared with the second deciduous molars, is due to the lower nerve density of this tooth.

Neurochemical properties of dental primary afferent neurons

The long belief that dental primary afferent (DPA) neurons are entirely composed of nociceptive neurons has been challenged by several anatomical and functional investigations. In order to characterize non-nociceptivepopulation among DPA neurons, retrograde transport fluorescent dye was placed in upper molars of rats and immunohistochemical detection of peripherin and neurofilament 200 in the labeled trigeminal ganglia was performed. As the results, majority ofDPA neurons were peripherin-expressing small-sized neurons, showing characteristic ofnociceptive C-fibers. However, 25.7% of DPA were stained with antibody against neurofilament 200, indicating significant portion of DPA neurons are related to large myelinated Aβ fibers. There were a small number of neurons thatexpressed both peripherin and neurofilament 200, suggestive of Aδ fibers. The possible transition of neurochemical properties by neuronal injury induced by retrograde labeling technique was ruled out by detection of minimal expression of neuronal injury marker, ATF-3. These results suggest that in addition to the large population of C-fiber-related nociceptive neurons, a subset of DPA neurons is myelinated large neurons, which is related to low-threshold mechanosensitive Aβ fibers. We suggest that these Aβ fiber-related neurons might play a role as mechanotransducers of fluid movement within dentinal tubules.

Long term histological response of hemisectioned exposed primary pulps. An in vivo study

THE AIM of this study was to analyze the pulp behavior 17 hemisectioned primary second mandibular molars, exposed into the oral environment. The mesial crown and root portions were extracted after 8 months and analyzed histologically.

RESULTS

The cardinal signs such as pain, sensitivity and necrosis were not found in any of these teeth with the exception of one case which had a previous restoration. The formation of pulp polyps, pulp calcifications and pulp obliteration were seen as a normal physio-pathological response.

CONCLUSIONS

Exposed pulps, reacted forming pulp polyps and in a similar fashion to exfoliating primary teeth but in an accelerated manner.

Striatal Inhibition of Nociceptive Responses Evoked in Trigeminal Sensory Neurons by Tooth Pulp Stimulation

The noxious evoked response in trigeminal sensory neurons was studied to address the role of striatum in the control of nociceptive inputs. In urethane-anesthetized rats, the jaw opening reflex (JOR) was produced by suprathreshold stimulation of the tooth pulp and measured as electromyographic response in the digastric muscle, with simultaneous recording of noxious responses in single unit neurons of the spinal trigeminal nucleus pars caudalis (Sp5c). The microinjection of glutamate (80 ηmol/0.5 μl) into striatal JOR inhibitory sites significantly decreased the Aδ and C fiber–mediated–evoked response (53 ± 4.2 and 43.6 ± 6.4% of control value, P < 0.0001) in 92% (31/34) of nociceptive Sp5c neurons. The microinjection of the solvent was ineffective, as was microinjection of glutamate in sites out of the JOR inhibitory ones. In another series of experiments, simultaneous single unit recordings were performed in the motor trigeminal nucleus (Mo5) and the Sp5c nucleus. Microinjection of glutamate decreased the noxious-evoked response in Sp5c and Mo5 neurons in parallel with the JOR, without modifying spontaneous neuronal activity of trigeminal motoneurons ( n = 8 pairs). These results indicate that the striatum could be involved in the modulation of nociceptive inputs and confirm the role of the basal ganglia in the processing of nociceptive information.

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)

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.

Pain sensation during cold stimulation of the teeth: differential reflection of A delta and C fibre activity?

Cold stimuli of varying intensities were randomly applied to upper middle incisors of 12 healthy young subjects for a mean duration of 2 min by individually adapted thermodes the temperatures of which ranged from +30 degrees C to -30 degrees C. The subjects were asked to rate the magnitude of their pain sensations during application of the stimuli by means of a linear potentiometer according to a category scale. After each stimulus, they were asked to describe the quality of their pain sensations. Cold stimulation of the teeth evoked pain sensations were reproducible that in subsequent trials and could be graded according to stimulation intensity. Below certain individually different threshold thermode temperatures the onset of a stimulus was followed, after a short latency (1.6 +/- 1 sec), by a sharp and shooting pain sensation which immediately decreased after reaching its maximum value while the stimulus was still present. The mean maxima of the pain intensities were correlated to the thermode temperature. In general, this first pain component was followed by a second one (latency: 29.9 +/- 6.3 sec) with a lower threshold temperature, less of an increase in rate and lower magnitude. This was described as a dull, burning pain which was difficult to localize. The human pain ratings are compared to recordings of intradental nerve fibres in the cat and, under the assumption that the response behaviour of human pulpal nerve fibres is comparable to that of the cat, we hypothesize that the first pain component is evoked by intradental A delta fibres exhibiting their typical phasic response behaviour and firing during the initial steep temperature decrease. After some seconds, intradental temperature reached values sufficient to evoke C-fibre activity associated with the second pain component.

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.

Immunohistochemical analysis of pulpal innervation in developing rat molars

The beginning of pulpal innervation was examined in these developing teeth. Mandibular first molars removed from newborn to 7-day-old rats were cryosectioned and nerve fibres were localized by the peroxidase-antiperoxidase technique using a neuropeptide-specific (200 kDa) antibody. Some pulps from 5- and 7-day-old rats were also examined by conventional electron microscopy. In newborn to 4-day-old rats, molars in the initial stages of the dentine deposition were innervated in the follicles but not in the pulps. In molars from 5-day-old rats, nerve fibres were found in the pulps of 11 out of 16 samples. The fibres were mainly located along the blood vessels in the basal part of the pulp, with some arborizations. In rats of 6 and 7 days old, nerve fibres were found in all the pulps examined. These had gradually extended deep into the cuspal area and were increasingly arborized with increasing age. Nerve fibres were also found along the basal laminae of blood vessels in some dental pulps from 5-day-old rats when examined by electron microscopy. At 7 days, nerve fibres were composed of bundles of axons, some of which were covered with Schwann cell processes and basal laminae. These observations indicate that the innervation of rat molar dental pulps begins after the start of the deposition of enamel, in animals of around 5 days of age, which is at the same stage as in mouse molars, as others have shown by a silver-staining method.

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.

Periapical innervation of the ferret canine and the local retrograde neural changes after pulpectomy

The amputation of the dental pulp severs a population of axons that are predominantly in the A delta and C fiber size range and are principally involved in nociception. Local periapical neuromas, if they are formed after pulpectomy, may be the sites of spontaneous nervous activity that may, in some circumstances, be involved in the genesis of chronic pain. The periapical tissues from the mandibular canines of four ferrets were examined 3 months after pulpectomies. Silver-stained paraffin sections were examined in three dimensions at the light microscope level. Ultrathin sections were examined at the electron microscope level. Compared with contralateral and independent controls, the principal changes were the loss of the periodontal plexus around the root apex, the extension of damage well below the apical foramen, and the persistence of inflammation 12 weeks postoperatively. While a somewhat disorderly mass of nerve fibers develops subapically, the arrangement has only some of the features usually associated with neuromas.

Medialis dorsalis thalamic unitary response to tooth pulp stimulation and its conditioning by brainstem and limbic activation

In nembutalized cats tooth pulp stimulation (TPS) was effective in exciting 18% of medialis dorsalis (MD) thalamic units. Facilitation of spontaneous MD unitary discharge followed high frequency stimulation of the lateral amygdala (25%), dorsal hippocampus (22%), mesencephalic reticular formation (20%) and septal nuclei (17%). Conditioning high-frequency stimulation of limbic and reticular structures, strongly facilitated the MD unitary responses to TPS. None of the thalamic neurons involved in nociception seems to project to the cerebral cortex. The conditioning effect on MD response of limbic and reticular stimulation suggests that these central structures may be involved in the modulation of the nociceptive input.

Intracellular responses of raphe magnus neurons during the jaw-opening reflex evoked by tooth pulp stimulation

Neurons in the nucleus raphe magnus (RM) may play an important role in the modulation of nociception. To determine how RM neurons are activated during a nociceptive reflex, the intracellular responses of raphe neurons were studied during the jaw-opening reflex (JOR) elicited by tooth pulp shock in lightly anesthetized cats. Tooth pulp stimulation produces reflex EMG activation of the digastric muscle at a latency of 7-11 ms, resulting in jaw opening. Tooth pulp shock that elicits the JOR also produces an EPSP in a subset of raphe neurons. This EPSP consists of an early small depolarization that occurs at a latency of 10-15 ms followed by a larger depolarization at a latency of 20-60 ms. In all cases the latency to EPSP is longer than the latency to digastric EMG onset. Electrical stimulation of the 4 paws elicits oligosynaptic EPSPs in the same cells at a latency of 16-20 ms. Electrical train stimulation of the midbrain periaqueductal gray region (PAG) suppresses the JOR. Single shock stimulation at the same PAG sites that suppress the JOR evokes monosynaptic EPSPs in the large majority of raphe neurons recorded. In all cases, the threshold for EPSP is below the threshold for suppression of the JOR. The EPSP amplitude is a direct function of PAG stimulus intensity and there is temporal summation of EPSPs evoked by paired PAG shocks. At condition-test intervals of 40-90 ms, train stimulation of PAG suppresses the tooth pulp-evoked EPSP in raphe neurons. The threshold for EPSP suppression occurs at a PAG stimulation intensity below that required for suppression of the JOR. The present findings provide evidence that RM neurons may play an important role in the modulation of the tooth pulp-evoked JOR, but only after the initial withdrawal reflex has occurred.

Influence of changes of tooth temperature on reflex and central activity evoked by stimulation of tooth pulp afferents

The importance of the temperature of the dentine was studied in teeth prepared for electrical stimulation. During experiments with the mouth open, the temperature of teeth covered by cement was normal. The digastric EMG and the brainstem--evoked response following electrical stimulation of the tooth pulp as well as the threshold for eliciting a jaw-opening response remained constant throughout prolonged experiments. However, heat produced by the cement used to fixate the tooth electrodes could have damaged the tooth if the dentine temperature had exceeded 45 degrees C. A careful preparation of the tooth pulp by repeated application of thin layers of cement allowed an adequate preparation without damage to tooth pulp afferents.

Is the jaw-opening reflex a valid model of pain?

Tooth pulp shock does not produce only pain; low intensity stimulation results in a non-painful sensation that is termed pre-pain. In animals low intensity tooth pulp shock does not evoke escape behavior; the similarity of the animal escape/detection threshold ratio with the human pain/pre-pain threshold ratio is evidence that pre-pain and pain may be present in animals as in humans. Both pre-pain and pain may arise from the activation of a common afferent modality. The TP-JOR does not correlate with the degree of pain experienced under all conditions. The TP-JOR threshold is at or near the sensory detection threshold, at stimulation intensities which evoke pre-pain. Under normal conditions both the magnitude of the TP-JOR response and the degree of pain experienced increase with increasing stimulation intensity. The TP-JOR and the tooth pulp-evoked pain are affected in parallel by sensory habituation and both appear to relay in the rostral trigeminal complex. There are no cases where the TP-JOR is suppressed and pain is still experienced from tooth pulp shock; the suppression of the TP-JOR may therefore be an accurate index of analgesia. However, in humans treatments that produce analgesia have not been shown to produce suppression of the TP-JOR. Thus, the TP-JOR that persists following analgesic treatments is not a reliable index of either analgesia or pain.

A pilot study of the clinical problem of regionally anesthetizing the pulp of an acutely inflamed mandibular molar

The diplomates of the American Association of Endodontists were surveyed regarding difficulties in regionally anesthetizing acutely painful mandibular molar teeth. Frequently patients developed evidence of adequate regional block anesthesia but experienced pain when the dentist attempted access. A pilot projection was undertaken to determine if a scientific basis existed for this problem. With the animal under general anesthesia, the mandibular nerve of a cat was isolated and then regionally blocked. A stimulating electrode was implanted in a cuspid tooth and a receiving electrode in the cortex of the brain. When a state of inflammation was created in the tooth in the presence of a regional block, there was an increasing cortical response to this stimulation. It was concluded that the pilot project offered a realistic technique for the study of this clinical entity.

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.

Pontomedullary raphe neurons: monosynaptic excitation from midbrain sites that suppress the jaw opening reflex

The intracellular responses of pontomedullary raphe neurons to midbrain stimulation were studied in chloralose anesthetized cats. Electrical stimulation of the periaqueductal gray region at sites that suppressed the nociceptive jaw opening reflex evoked monosynaptic excitatory postsynaptic potentials in the majority of raphe neurons recorded. Intracellular labeling of studied cells demonstrated that recordings were obtained primarily from large and medium sized neurons in nucleus raphe magnus and the rostral nucleus raphe obscurus. These results provide further evidence that analgesia induced by periaqueductal gray stimulation may result in part from the direct activation of caudal raphe neurons.

Sensory innervation of periodontal ligament of rat molars consists of unencapsulated Ruffini-like mechanoreceptors and free nerve endings

The trigeminal ganglion (TG) of adult rats was injected with 3H-amino acids to label periodontal receptors by axonal transport; 20-24 hours after injection, samples of molar ligament were prepared for autoradiography and electron microscopy. Four types of neurites labeled from TG were found in the avascular ligament fiber regions: large, complex, Ruffini-like endings, lacking a capsule, but with finger extensions touching ligament collagen; smaller Ruffini-like endings, lacking a capsule and neural fingers; free bundles of unmyelinated axons; and free, small, myelinated axons. The vascular channels plus associated loose connective tissue that perforate the ligament contained labeled preterminal ensheathed axons, small Ruffini endings, and free unmyelinated or small myelinated axons. The incidence of labeled endings was about 5 X greater next to the lower third of the root than in the upper two-thirds or beneath the root. The TG myelinated axons (diameter range 2-15 microns) entered the ligament in sheathed nerve bundles; these branched to form numerous small preterminal axons that were surrounded by a periaxonal fluid space and a perineurial sheath. Terminal axons branched from nodes of Ranvier, left the preterminal chamber, and followed an extended branching course through the collagen fibers. Large, complex Ruffini-like endings had numerous mitochondria and were partially covered by special lamellar Schwann cells and complex basal lamina; vesicles and multivesicular bodies were found near exposed regions of the receptor. Smaller Ruffini-like endings lacked neural fingers and had a simpler structure and less elaborate Schwann cells. The structure of Ruffini-like endings was highly varied; thus a structural continuum may exist from the largest, most complex to the smallest, simplest Ruffini-like receptor. The TG unmyelinated axons entered the ligament in ensheathed bundles; they then branched into free bundles that were found in the avascular ligament or near blood vessels. No encapsulated receptors were found.

Spatial summation of pre-pain and pain in human teeth

The purpose of this work was to investigate the relation between the sensations of pain and 'pre-pain' evoked by stimulation of teeth in human subjects. Electrical pulses of progressively increasing amplitude, generated by a computer-controlled stimulator, were applied to 1 or 2 teeth, and the subjects responded by indicating the nature of the resulting sensation. Pre-pain and pain could be readily and rapidly distinguished by all 11 subjects (response latency about 0.4 sec). Both sensations had stable thresholds with relatively small variance (S.D. 10-15% of threshold value) for a given subject. Subjects characterized the stimuli as indifferent or unpleasant, localized, and brief. By using special stimulation strategies (termed 'optimal trajectories') for exciting 2 teeth simultaneously, spatial summation for pre-pain was demonstrated in most subjects and for pain in almost all subjects. Spatial summation of pre-pain resulted in pain rather than in more intense pre-pain. These results are consistent with both the dual modality (separate afferent fibers for pre-pain and pain) and the single modality hypotheses (single type of afferent fibers) of tooth pulp sensibility, but favor single modality innervation.

Perception of pulpal pain as a function of intradental nerve activity

The purpose of the present investigation was to find neurophysiological correlates of pain perception. The magnitude and time course of perceived pain was successfully related to the neural discharge evoked by rapid cooling of the tooth surface in 6 dental patients whose lower incisors were to be extracted for prosthodontic reasons. Two cavities were prepared on the facial surface of human lower incisors. The cavities were deepened using hand driven instruments until the pulp was visible through a thin layer of dentin. A metal tube was placed in contact with amalgam on each cavity bottom and fixed in place by composite filling material. The tubes were connected to standard equipment for electrophysiological recordings by a flexible circuit. The magnitude of perceived pain was assessed by a cross-modality matching to finger span in combination with sensory verbal pain descriptors and magnitude estimation. The striking agreement between the integrated nerve activity, probably of the A delta type and pain perception, is of great importance from the methodological point of view since it strongly argues in favor of the appropriateness of the techniques applied here to elucidate the neural substrate of some types of nociception and also to evaluate various means of relieving such pain.

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)

Electron microscopic demonstration of horseradish peroxidase-tetramethylbenzidine reaction product as a method for identifying sensory nerve fibers in the rat tooth pulp

The purpose of the present investigation was to determine if the horseradish peroxidase (HRP) technique could be used as a method for labeling sensory nerve fibers (specifically, tooth pulp afferents) for detailed ultrastructural analyses. HRP injected into the trigeminal ganglion of adult rats was taken up by ganglion cell bodies and transported anterogradely to their peripheral endings in the dental tissues. Following perfusion-fixation, the teeth were decalcified in EDTA, sectioned, reacted for HRP activity according to the tetramethylbenzidine (TMB) technique, and processed for electron microscopy. The HRP-TMB reaction product was clearly visible within most of the axons in the dental pulp, appearing as conspicuous, rectangular shaped aggregates of fine rods or needles.

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.

Non-pain and pain sensations evoked by tooth pulp stimulation

This study investigated the quality and magnitude of sensations evoked by electrical tooth pulp stimulation. Detection threshold (the minimum current intensity that evoked a sensation) and pain threshold were determined for tooth pulp stimuli varying in frequency from 5 to 500 Hz. The effect of frequency and intensity of tooth pulp stimulation on the magnitude of sensations was assessed using visual analog scales and verbal descriptor scales. Detection thresholds were stable over experimental sessions and independent of the frequency of the stimulating current. Pain threshold varied as a function of frequency with a minimum value at 100 Hz. Stimuli that evoked non-pain sensations at low frequencies evoked pain sensations when frequency was increased from 5 to 100 Hz. Subjects were able to scale non-pain sensations over a range of stimulus intensities and frequencies. The lowest currents evoked sensations that were non-painful and were of constant magnitude despite changes in the frequency of stimulation. Higher stimulus currents evoked sensations that were non-painful at low stimulus frequencies and painful at high stimulus frequencies. Sensation magnitude at each stimulus intensity increased as a function of frequency. Temporal summation occurred in proportion to stimulus intensity. These findings suggest that the non-pain sensations evoked in tooth pulp are mediated by a distinct population of afferents that are not involved in the coding of pain. High frequency stimulation that increased the discharge rate of the lowest threshold pulpal afferents resulted in no summation of non-pain sensation and never produced pain. However, high frequency stimulation evoked greater magnitude sensations at higher stimulus currents, indicating that central summation mechanisms were critical for higher threshold afferents signaling more intense non-pain and pain sensations.

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.

Axon number and size distribution in the developing feline inferior alveolar nerve

The number and size distribution of axons in the developing feline inferior alveolar nerve (IAN) were examined by electron microscopy. Seven cat fetuses and thirty kittens and cats aged from 25 days post conception (dpc) to 11 years were used. The total number of IAN axons increased from 4,400 to 16-17,000 between 25 and 40 dpc, and then decreased to about 13,000 before birth. This level was maintained up to at least 11 years. Myelinated axons first appeared by 45 dpc and constituted 28% at birth. The young adult proportion of about 45% myelinated axons was established at 2 months. In the old adult (11 years) 55% of the IAN axons were myelinated. Size measurements showed that unmyelinated axons had diameters of 0.1-0.5 micrometer at 25 dpc. From 55 dpc and on the size range extended from 0.1 micrometer to 1 micrometer. The size range of myelinated axons was 1-4.5 micrometers at birth and 1-8 micrometers 2 months postnatally. A bimodal size distribution first appeared by 2 months, and the range was 1-13 micrometers from 6 months and on. The findings were compared with age-related changes in the primary and permanent dentitions.

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.

Autoradiographic demonstration of ipsilateral and contralateral sensory nerve endings in cat dentin, pulp, and periodontium

In order to determine the location of sensory nerve ending in cat teeth, 3H-proline and 3H-leucine were injected into the left trigeminal ganglion of eight cats aged 6.5-10 months; 24 hours was allowed for axonal transport of radioactive protein to dental nerve endings, and the endings were then detected by autoradiography. The pulps of most ipsilateral (left) teeth contained some labeled axons. These axons ended in the odontoblastic layer and predentin of roots and crown; at the tip of the pulp horn of each cusp, nerve endings also extended as far as 150 micrometer into dentinal tubules. Labeled nerve endings were extremely rare in contralateral (right) teeth; only one tooth of 83 studied (eight cats) contained heavily labeled axons, and one other had faintly labeled axons. Both labeled contralateral teeth were central maxillary incisors. Their labeled axons were unbranched in the root and arborized in the crown to end among odontoblasts and many adjacent dentinal tubules. Labeled periodontal nerve endings were most numerous in the apical one-third of the ligament, with some endings extending as far as the gingiva. The nerve endings in the periodontal ligament were often clustered and appeared to end freely between the collagen bundles; their radioactivity varied in the same way as that of pulp nerves in the adjacent root.

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.

Altered pain perception in hypertensive humans

Sensory and pain thresholds to electrical stimulation of tooth pulps were measured in normotensive and essential hypertensive unmedicated human subjects. In both adult and young subjects there was a significant correlation between blood pressure and pain sensitivity: hypertensives had a higher threshold for sensation of pain in the tooth-pulp test than normotensive controls. An interrelationship between blood pressure and pain regulation is suggested.

Vasodilatation in the dental pulp produced by electrical stimulation of the inferior alveolar nerve in the cat

The effects of nerve stimulation on blood flow were studied in the dental pulp of anesthetized cats. Changes in iodide disappearance rate (k-value) from dentinal cavities were used to determine changes in pulpal blood flow. Electrical stimulation of the distal end of the cut inferior alveolar nerve after alpha-adrenoceptor blockade (phentolamine, 3 mg/kg) consistently resulted in a rapid increase in disappearance rate. The first stimulation produced the greatest response (an average increase in k-values of 60%) and repeated stimulations showed a successive attenuation in response, the fourth stimulation usually having no effect. A progressive decrease in resting k-values was observed after the first stimulation, indicating an impaired exchange function of the capillary vessels. Systemic pretreatment with propranolol (0.5--1 mg/kg), atropine (3 mg/kg), mepyramine (3 mg/kg) and cimetidine (3 mg/kg) did not influence the rapid increase in k-values produced by the nerve stimulation. The experiments show that vasodilatation in the cat dental pulp produced by stimulation of the inferior alveolar nerve is not mediated by common efferent vasodilatory mechanisms and strengthen the hypothesis that the sensory nerve axon reflex mechanism is involved.

Some anatomical and electrophysiological properties of tooth-pulp afferents in the cat

1. The responses of nerves which could be excited by electrical stimulation of the canine tooth-pulp were recorded from the trigeminal ganglion. 2. Preliminary experiments showed that the responses of mandibular canine pulpal afferents were recorded from the postero-lateral part of the trigeminal ganglion whereas the responses of maxillary canine pulpal afferents were recorded from the central part of the ganglion. 3. The conduction velocity differed in various parts of these nerves; the conduction velocity inside the tooth tended to be slower than that from the tooth to the trigeminal ganglion; the mean conduction velocity in the part of the nerve peripheral to the trigeminal ganglion was faster than that in the part central to the ganglion, and in the central part the conduction velocity decreased caudally. 4. Collision experiments combining tooth stimulation and stimulation of sites in the trigeminal sensory nuclear complex were carried out to investigate the projections of individual pulpal afferents. 5. The results of these experiments showed that some tooth-pulp afferents bifurcate and project to both the main sensory nucleus and the nucleus caudalis. For other pulpal afferents only a projection to the main sensory nucleus or the nucleus caudalis could be demonstrated. 6. Pulpal afferents that bifurcated had faster conduction velocities than those for which no bifurcation could be shown.

Degeneration of unmyelinated axons in the dental root canal induced by 6-OH-dopamine

The pulpal nerve fibres of feline incisors were examined ultrastructurally after i.v. administration of 6-OH-dopamine. The presence of degenerating unmyelinated fibres at this site provides conclusive morphological evidence that sympathetic fibres enter the dental pulp.

A quantitative analysis of the innervation of the pulp of the cat's canine tooth

Although the cat's canine tooth has become the accepted model for the electrophysiological study of dental sensory mechanisms no examination of its innervation has been carried out at the electron-microscopical level. This study looked at the number and size distribution of both myelinated and non-myelinated fibers in the crown of the cat's canine. The material examined was prepared by routine methods and the measurements taken from electron-microscopical montages of pulpal cross-sections. The measurements were made using a Quantimet 720 image analysis system. In one complete cross-section 3,470 fibers were counted. Eighty-one percent of these were non-myelinated with a modal diameter of 0.35 micron. The modal diameter of the myelinated fibers was 2.5 micron. The relative preponderance of non-myelinated fibers increased from core to periphery. The largest myelinated fibers were concentrated in the core. The fiber size distribution was similar in the single complete and two partial sections examined. It is concluded that all the fibers in the crown of the cat's canine would be contained in the A-delta and C groups and that the strikingly large number of fibers present suggests that the peripheral pulp has a dense innervation, many of the cell processes found there being unsheathed axons.

Localization of substance P-like immunoreactivity in nerves in the tooth pulp

The occurrence of substance P (SP)-like immunoreactivity was studied in dental pulps of the cat. In untreated animals SP-positive fibres were found in all areas of the pulp. Most fibres were seen in central parts of the pulp but they were also observed in relation to the odontoblasts. Single, possibly unmyelinated, or fine caliber fibres or small bundles of them were seen running close to large non-fluorescent myelinated nerves, to blood vessels or without any obvious association with either of these structures. Fourteen days after transection of the inferior alveolar nerve no SP-positive fibres were observed in pulps on the denervated side. Transection of the cervical sympathetic ganglion did not change the occurrence of SP-positive fibres. The results indicate the existence of at least two types of afferent fibres in the dental pulp of the cat. Since the tooth pulp has been demonstrated to give rise only to pain sensation when stimulated, the results give morphological support for a role of SP neurones in pain transmission.

Sensation evoked by bipolar intrapulpal stimulation in man

Bipolar intrapulpal stimulation was applied to human teeth using the same procedure as in animal experiments. The effects of variation of stimulus parameters on the quality of sensation were studied. A prepain sensation exists which cannot be explained by diffusion of the stimulus to periodontal tissues. When the intensity of stimulation is increased, the prepain sensation is gradually replaced by a pinprick sensation. With long, high intensity stimulation, an acute long lasting very painful sensation appears. To evoke a pinprick sensation the best stimulation seems to be a 50 msec train (0.5 msec, 300 Hz, 0.5 mA). Longer train duration and a higher intensity of current are necessary to evoke a long lasting, acute very painful sensation. Since the exclusively Adelta and C nerve fiber content of the dental pulp is well documented and since it is possible to avoid current diffusion outside the dental pulp cavity, the tooth pulp implantation seems to be a good technic for studying pain, as long as the investigator uses adequate stimulation.

Development of neural elements in apical portions of cat primary and permanent incisor pulps

Pulp development was studied with light and electron microscopy in apical portions of cat primary and permanent incisors at various stages of development in order to qualitatively assess hypothesized differences during maturation. Primary and permanent tooth pulps passed through similar stages up to when the primary teeth began to resorb. At that point permanent tooth pulps continued to develop in complexity relative to myelinated axons and developed thin epineurial sheaths. Although the stages were similar in several regards, the time needed by the permanent tooth pulps to reach each stage was considerably greater than for the primary tooth pulps. It is of interest that the young permanent teeth had only a small portion of their final complement of nerves in spite of their relatively complete development. Individual axons, especially myelinated axons, appeared more immature in younger primary and permanent teeth. This suggests a less mature sensory capability for erupting primary teeth as compared to fully developed primary teeth as well as for erupting and young permanent teeth compared to old permanent teeth.