Coupling between nerves in teeth

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Electrical synapses in mammalian CNS: Past eras, present focus and future directions

Gap junctions provide the basis for electrical synapses between neurons. Early studies in well-defined circuits in lower vertebrates laid the foundation for understanding various properties conferred by electrical synaptic transmission. Knowledge surrounding electrical synapses in mammalian systems unfolded first with evidence indicating the presence of gap junctions between neurons in various brain regions, but with little appreciation of their functional roles. Beginning at about the turn of this century, new approaches were applied to scrutinize electrical synapses, revealing the prevalence of neuronal gap junctions, the connexin protein composition of many of those junctions, and the myriad diverse neural systems in which they occur in the mammalian CNS. Subsequent progress indicated that electrical synapses constitute key elements in synaptic circuitry, govern the collective activity of ensembles of electrically coupled neurons, and in part orchestrate the synchronized neuronal network activity and rhythmic oscillations that underlie fundamental integrative processes. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

An overview of the dental pulp: its functions and responses to injury

The dental pulp is a unique tissue and its importance in the long-term prognosis of the tooth is often ignored by clinicians. It is unique in that it resides in a rigid chamber which provides strong mechanical support and protection from the microbial rich oral environment. If this rigid shell loses its structural integrity, the pulp is under the threat of the adverse stimuli from the mouth, such as caries, cracks, fractures and open restoration margins, all of which provide pathways for micro-organisms and their toxins to enter the pulp. The pulp initially responds to irritation by becoming inflamed and, if left untreated, this will progress to pulp necrosis and infection. The inflammation will also spread to the surrounding alveolar bone and cause periapical pathosis. The magnitude of pulp-related problems should not be underestimated since their most serious consequence is oral sepsis, which can be life threatening, and hence correct diagnosis and management are essential. Clinicians must have a thorough understanding of the physiological and pathological features of the dental pulp as well as the biological consequences of treatment interventions.

Update on connexins and gap junctions in neurons and glia in the mammalian nervous system

Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.

Multiple action potential waveforms of single units in man as signs of variability in conductivity of their myelinated fibres

Percutaneous microneurography was performed with concentric needle electrodes to record neural activity from myelinated fibres in human peripheral nerves. Template matching techniques were used together with interspike interval analysis and studies on functional class, receptive field characteristics, conduction velocities and other single fibre properties to classify single units. Sometimes the same fibres exhibited different action potentials at the same time. The potentials had some common features, but differed either in their waveform types or only in duration. There was a correlation between the occurrence of the different potential shapes and firing frequency of the studied unit. The outcome of the studies suggested that there was a common denominator which could explain the observations. Most likely, momentary fluctuations in excitability of the myelinated fibres occurring during the relative refractory period or the supernormal period were responsible for the variations in complexity of the studied units due to a partial block of fibre propagation probably caused by the recording electrode. Thus, action potentials deriving from the same axon may not always have the same shapes. Methods for unit classification, such as template matching, are discussed in the light of our findings.

Evidence for slowly conducting afferent fibres innervating both tooth pulp and periodontal ligament in the cat

The presence of afferent nerve fibres branching to innervate both the dental pulp and periodontal ligament was studied in pentobarbitone-anaesthetised cats. Extracellular single nerve-fibre recordings were made from fine filaments split from the proximally cut end of the inferior alveolar nerve. Nerve fibres were identified by bipolar constant-current stimulus pulses applied to the periodontal space via platinum wire electrodes. In each case activation of the nerve fibres was also attempted by monopolar electrical stimulation of the dental pulp via a platinum wire electrode inserted into the dentine. Eleven of 142 C fibres and 4 of 97 A delta fibres identified by electrical stimulation of the periodontal ligament also could be activated by electrical stimulation of the dental pulp. Fourteen of the 142 C fibres identified by electrical stimulation of the periodontal ligament exhibited discrete latency jumps at different suprathreshold stimulus strengths. Eight of them also could be activated by electrical stimulation of the dental pulp. Eight of the 15 fine branching afferent fibres were tested with non-electrical stimuli of both periodontal ligament and dental pulp by application of heat, cold and potassium chloride. Three of the 4 C fibres could be activated with at least one of these stimuli applied to both tissues. In one case receptive fields were located in both periodontal ligament and dental pulp. The remaining 5 slowly conducting fibres were activated only from one type of tissue. The results suggest that a small percentage (6%) of the slowly conducting nerve fibres in the inferior alveolar nerve innervate both the periodontal ligament and the dental pulp. According to their response behaviour they might be involved in nociception.

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)

Gap junctions between odontoblasts revealed by transjunctional flux of fluorescent tracers

Cell communication between odontoblasts was investigated with the use of fluorescent-dye tracers; Lucifer Yellow CH (molecular weight = 457.3), and dextran-Lucifer Yellow CH (average molecular weight = 10,000). Dyes were injected into cell bodies of individual odontoblasts via an intracellular microelectrode or into a group of cells through their processes, and passage to adjacent cells was examined with a fluorescence microscope. Lucifer Yellow CH appeared to diffuse very easily among odontoblasts, while dextran-Lucifer Yellow remained within the injected cell or cells. This efficient migration of Lucifer Yellow CH can be considered a functional manifestation of gap junctions between odontoblasts.

Demonstration of physiological barrier between pulpal odontoblasts and its perturbation following routine restorative procedures: a horseradish peroxidase tracing study in the rat

Vascular injection of the macromolecular tracer, horseradish peroxidase (HRP), was used to study the permeability of the odontoblast cell layer in developing and mature rat molar teeth, and to investigate the effect of cavity preparations on the permeability of this epithelioid cell layer in adult animals. HRP injected into the vascular system of normal animals 28 days of age and older was localized histochemically (from 5 to 90 min after injection) throughout the extracellular spaces of the maxillary dental pulps; however, the tracer did not penetrate beyond the tight junctions at the apical region of the odontoblast cell layer, and was absent from the predentin and dentin. In contrast, HRP injected into very young neonatal animals (e.g., day 3) resulted in free passage of HRP between odontoblasts and into the overlying predentin and dentin. When Class V cavities had been prepared in adult maxillary molars after HRP was injected into the blood stream, HRP reaction product penetrated the predentin and dentin immediately beneath the cavity preparation; however, adjacent, untraumatized areas of predentin and dentin in the operated teeth were devoid of reaction product. These results provide evidence that: (1) a physiological barrier develops between the distal segments of odontoblast cell bodies in normal rat molar teeth between days 15 and 28 of postnatal life, and this barrier prevents the passage of macromolecules from the pulp into the predentin and dentin; and (2) this barrier is perturbed following routine restorative procedures in adult animals.

Afferent C fibre innervation of cat tooth pulp: confirmation by electrophysiological methods

1. The presence of afferent C fibres innervating the lower canine tooth was investigated in Nembutal-anaesthetized cats. 2. Twenty-five single fibres with conduction velocities (CVp) of less than 2.5 m/s, as calculated from the shortest response latency using monopolar electrical stimulation of the tooth, were recorded from the inferior alveolar nerve. In addition, the extradental conduction velocity (CVn) of the fibres was determined by using bipolar electrical stimulation of the trunk of the inferior alveolar nerve. 3. The mean CVp was 1.4 +/- 0.4 m/s (n = 25; range, 0.6-2.4 m/s); the mean CVn was higher, 1.7 +/- 0.9 m/s (n = 25; range, 0.6-4.0 m/s). For 20% of the fibres CVn exceeded 2.5 m/s; these were slowly conducting A delta fibres. For 80% of the fibres, however, the extradental conduction velocity was in the C fibre range. 4. The relationship between CVp (y) and CVn (x) was y = 0.66 + 0.40x, the correlation coefficient being r = 0.85. According to the present results this implies that for a reliable classification of pulpal C fibres (CVn less than or equal to 2.5 m/s) by monopolar tooth stimulation alone, CVp should be less than 1.7 m/s. 5. For twenty-three of the twenty-five fibres, one to three discrete shortenings of the response latency occurred when the intensity of the tooth stimulation was increased. When the nerve trunk itself was stimulated, a constant response latency was measured at all stimulus intensities applied. 6. For twelve fibres tested, the mean rate of electrical stimulation of the tooth, which the response followed with a constant latency, was 4.1 +/- 2.3 Hz (range, 1.5-10.0 Hz). With higher rates of stimulation the response latency increased until the fibres failed to follow each stimulus pulse. 7. Fifteen of the nineteen fibres tested responded to radiant heat stimulation of the tooth they were innervating. The mean temperature threshold was 41.4 +/- 2.7 degrees C (n = 11; range, 37.4 +/- 46.4 degrees C). 8. For eight heat-sensitive pulpal C fibres the receptive field was determined by mechanical stimulation of the exposed pulp tissue. Four C fibres developed a long-lasting on-going discharge after intense mechanical stimulation of the receptive field. 9. The discharge evoked by heat and mechanical stimulation of the tooth occluded the response evoked by simultaneously applied electrical current pulses to the nerve trunk, indicating that the same fibres were activated by both tooth and nerve stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of nerve section on the incidence and distribution of gap junctions in the odontoblast layer of the cat

Gap junctions are numerous in the odontoblast layer of the dental pulp and may link sensory axons to odontoblasts. If these junctions do link axons and odontoblasts, they, together with the axons, should disappear after cutting the pulpal nerves centrally. Under general anesthesia the inferior alveolar nerve on one side of two young adult cats was sectioned. Under general anesthesia the animals were perfused with fixative 56 hours later and the coronal dental pulp prepared for electron microscopy. Ultrathin sections were examined from the level of the pulpal cornu and levels approximately one, two, and three mm below this. The incidence of cell processes and gap junctions was measured at different distances from the pulp predentin junction, and operated and control sides compared. The odontoblast layer at the level of the cornu differed from elsewhere in having, on the control side, a greater density of cell processes and gap junctions and in having clearly recognizable axons approaching to within 5 to 10 micron of the predentin. The only statistically significant changes after nerve section occurred in this layer and consisted of a decline in the incidence of cell processes and of gap junctions that link one cell process to another. There was no significant difference between the operated and control sides in the number of gap junctions linking cell processes to recognizable cell bodies. The odontoblast layer in the pulpal cornu contained substantial numbers of unsheathed axons, many presumably en route to the dentin. These axons may participate in gap junctions that link them to other cell processes, possibly even other axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of the relationship between odontoblast processes and nerve fibres in dentinal tubules of rat molar teeth

Serial ultra-thin sections across tubules were analysed three-dimensionally. There was no difficulty in distinguishing nerve fibres and odontoblast processes. In many tubules, a bundle of naked nerve fibres accompanied the odontoblast process for up to 50 microns located in concavities of the surface of the process (type I relationship). No specialized membrane structures between the plasma membrane of nerves and odontoblast process, such as gap junction or membrane thickening similar to that of synapse, were encountered. In a few tubules, the lamellar cytoplasmic processes of the odontoblast extensively enveloped the terminal part of the nerve fibres (type II relationship). This pattern was found in an animal older than the others in which there were lucent tubules and perhaps arose from peripheral sensory stimuli.

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.

The dentin-predentin complex and its permeability: anatomical overview

This paper provides an anatomical overview of the dentin-predentin complex and its permeability. An unique anatomical feature of this complex is the presence of dentinal tubules which extend peripherally from the odontoblast-predentin junction throughout the thickness of the tissue. The permeability of dentin is a consequence of the presence of these tubules. Thus, this review concentrates on the anatomy of dentinal tubules and, in particular, on those studies which in the last decade have increased our knowledge of this anatomy. The structure, size, and number of dentinal tubules and the relationship of these features to the permeability of the tissue are discussed. This is followed by a description of the contents of dentinal tubules, with particular emphasis being paid to the literature concerning the extent of the odontoblast process.

Coupling of action potential activity between unmyelinated fibers in the peripheral nerve of monkey

Bidirectional coupling of action potential activity occurs between unmyelinated fibers in the normal peripheral nerve of monkey. The site of coupling is near the cutaneous nociceptive receptor associated with one of the fibers. This coupling could be due to an electrical synapse and could provide the basis for the flare associated with the axon reflex.

Membrane junctions between odontoblasts and associated cells. A freeze-fracture study of the human odontoblastic cell layer with special reference to its nerve supply

The relationship between odontoblasts and adjacent cell structures within the odontoblastic cell layer was analyzed by means of the freeze-fracturing technique. Two principal forms of interodontoblastic cell structures were found. The first was tubular or thread-like in appearance, having a general diameter around 0.1-1.0 micron. From morphological criteria these were believed to represent small, unmyelinated nerve fibers. The second type of cell structure found between odontoblasts was more irregular and heterogeneous in outline, and often lamellar or branched. These slender formations sometimes proved to constitute cellular projections from adjacent odontoblasts or neighboring, subodontoblastic fibroblasts. Both the nerve-like fibers and the irregular branched cells between the odontoblasts showed morphological contact areas with odontoblastic cell bodies. At these sites the intracellular distances were reduced, and characteristic gap junctional complexes occurred. Nerve ending specialization or membrane structures indicating the presence of chemical synapses on the odontoblastic cell surface were not observed.

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)

Intercellular junctions in odontoblasts of the rat incisor studied with freeze-fracture

The morphology and distribution of various types of intercellular junctions were investigated in young odontoblasts. Gap junctions were found between odontoblasts as well as between odontoblasts and fibroblasts in the dental pulp. The junctions between odontoblasts were larger and more numerous than those between odontoblast and fibroblast, suggesting that the former may play an important role in regulating cellular activity and the latter may provide a pathway of low electrical resistance between odontoblast and nerve fibres. Irregularly-shaped gap junctions appeared as small aggregations of particles associated with a particle-free area and may indicate that the junction might not yet have been completely assembled. Tight junctions were observed at the distal ends of the young odontoblasts, arranged to form small maculae or faciae occludentes rather than belt-like zonulae. It is therefore not likely that the junction contributes to barrier function in the young odontoblasts. Although structures resembling typical desmosome were recognizable, this type of junction in odontoblasts is properly termed a desmosome-like junction from its morphological peculiarities.

Membrane junctions in the subodontoblastic region. A freeze-fracture study of the human dental pulp

The morphology of the subodontoblastic region in the human dental pulp was analysed using the freeze-fracturing technique. Multiple areas of focal intercellular junctions were found between subodontoblastic fibroblasts (or Höhl's cells). The connections were of the gap junction type. Similar junctions were observed between subodontoblastic cells and small-calibre (0.10-0.20 microns) fibres, presumably nerves. Tight junctions were not observed.

Dental sensory receptor structure in human teeth

Electron microscopy was used to study normal human extracted teeth in order to define the junctions between sensory nerve endings and other cells in external pulp and inner dentin at the crown tip. Two sets of associated cells were found: (1) Connective tissue cells. The pulpal fibroblast newtork made occasional desmosome junctions with the odontoblast newtork, and the cells of each network formed many gap junctions and desmosomes with one another. (2) Nerve endings. The terminal axons formed a succession of appositions with each other or with Schwann cells in the plexus of Raschkow and the cell-free zone, possibly with fibroblasts in the cell-free zone and odontoblast layer, and with odontoblasts in the odontoblast layer, predentin and dentin. The appositions between nerve endings and their companion cells at all levels usually maintained a regular intercellular spacing of at least 15-20 nm. In predentin and dentin, axons could be easily identified by their distinctive vesicles and mitochondria, and they often occurred within clusters of adjacent dentinal tubules; in the odontoblast layer axon identification was much more difficult. Axo-axonic appositions were found in the plexus of Raschkow, the cell-free zone, predentin and dentin; in many cases, bare axons were separated from each other only by a 5-10 nm extracellular space. Dental sensory mechanisms are discussed in relation to these observations.

Coupling between chorda tympani nerve fibres in the cat

Evidence for coupling between a small proportion of chorda tympani nerve fibres has been obtained in cats. Single chorda tympani fibres from which a response was recorded following electrical stimulation of another chorda tympani fibre were taken as being coupled to that fibre. The coupled fibres were associated with between 1 and 4 fungiform papillae on the tongue and they showed responses to gustatory, mechanical and electrical stimulation of those papillae.

The effect of buffer molarity on axonal exposure and axoaxonal apposition in the rat molar pulp

Axons in the rat molar pulp have been examined morphometrically to determine axonal size and the degree of axonal exposure and axoaxonal apposition in tissue fixed by perfusion using 2% glutaraldehyde in cacodylate buffers ranging in molarity form 0.025 M to 0.4 M. Between 31.2% and 45.0% of the axons were incompletely ensheathed. This proportion of axons exposed was linearly related to the buffer molarity (P less than 0.05) and was approximately double that found in more central axons. Between 32.3% and 45.0% of the axons were in contact with other axons. This proportion was not linearly related to buffer molarity but was least ten times higher than that observed in more centrally positioned nerve fibers in the inferior alveolar nerves. Increasing buffer molarity reduced the size of the axons, a relationship not found in the more central axons. It is suggested that axonal exposure and axoaxonal apposition are constant, significant features of pulpal nerve fibers that may be related to the onset and spread of nociceptive activity. The permeability properties of pulpal axons may differ from those of more centrally placed axons.

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.

The incidence of dentinal tubules containing more than one process in the cuspal dentin of cat canine teeth

The incidence of multiple processes within the tubules of the cuspal dentin of the cat's canine has been measured by counting directly from ultrathin sections examined in the electron microscope. The incidence is highly variable. The greatest incidence was found 1 mm below the cuspal dentino enamel junction where up to 75% of tubules in some areas contain more than one process. On average 33.6% of the tubules at this level contain multiple processes. The incidence declines linearly toward the apex with virtually no multiple processes occurring at 4 mm below the tip dentino enamel junction or beyond. Half of the "second processes" contain either mitochondria and/or vesicles. They are restricted to predentin where many of them end and to inner calcified dentin. Their distribution is similar to that described for nerve fibers in other studies. None of the processes participated in specialized junctions.

Microtubule and microfilament populations of cell processes in the dental pulp

An attempt has been made to characterize the nature of the unidentified cell processes participating in gap junctions in the odontoblast layer. In peripheral and pulpal nerves, there is a strong relationship between together with the ratio of microtubules to microfilaments, has been measured and compared for four types of cell processes found in the dental pulp, including those participating in gap junctions. The processes taking part in the gap junctions cannot be distinguished from pulpal axons on the basis of microtubule-to-microfilament ratio nor on the relationship between microtubule and microfilament population and process caliber. While these findings do not prove that the "gap members" are nerve fibers, it does support the hypothesis that the processes taking part in gap junctions in the peripheral dental pulp are nerve fibers.

Abnormal impulse generation in focally demyelinated trigeminal roots

In 12 cats and two Macaca mulatta monkeys, areas of chronic focal injury were induced in the trigeminal root by implantation of a chromic suture in the nerve near its entry into the brain stem. Experimental lesions were examined histologically and electrophysiologically at 1 week, 3 weeks, and 6 weeks after surgery. At 3 weeks following implantation, lesioned nerves showed areas of focal inflammation surrounding implanted sutures with prominent demyelination of adjacent axons. A total of 497 trigeminal units were studied, 304 of which had associated demyelination of the root. In these 304 cells, two types of abnormal impulse generation from areas of focal demyelination were recorded. Reflected orthodromic action potentials and prolonged high-frequency after-discharges following short priming trains of orthodromic stimuli were observed in 23% and 4% of cells, respectively. Ectopic spike initiation from areas of focal demyelination was not dependent upon anatomical continuity of the nerve with the brain stem, was augmented by hyperventilation, and could be eliminated by the intravenous administration of diphenylhydantoin sodium. The relevance of these findings to the pathophysiological mechanisms of pain syndromes involving peripheral nerves in general and the trigeminal system in particular is discussed.

The spread of sensitization of polymodal nociceptors in the rabbit from nearby injury and by antidromic nerve stimulation

1. Ninety-three polymodal nociceptor units with unmyelinated axons were isolated from rabbit sural nerves. Twenty-three were used for control data. These showed normal sensitization on repeated heating of their receptive fields, measured here as a drop in mean heat threshold. 2. Small injuries were made 5 (n = 15) or 10 (n = 12) mm outside the receptive fields of some polymodal nociceptors. This resulted in the development of spontaneous firing and lowered thresholds to heating of the receptive field. 3. Local anaesthetic previously injected into the site of injury blocked this spread of heat sensitization. Previous injection of saline had no effect. 4. Antidromic stimulation of the sural nerve, proximal to the recording site, also resulted in heat sensitization of polymodal nociceptors (n = 10). 5. Possible mechanisms for the spread of sensitization of polymodal nociceptors from nearby injury are discussed. Analogies are drawn between these results and those of Lewis (1935--36) on the spread of cutaneous ;yperalgesia around a skin injury in man.

The extent of the odontoblast process in human dentin

Impacted human third molar teeth were examined, both with the light and electron microscope, to determine the extent of the odontoblast process. The odontoblast process was found to be limited to the inner third of coronal dentin and inner half of mid-root dentin.

Branched afferent nerves supplying tooth-pulp in the cat

1. Recordings have been made from nerve terminals in canine and third incisor teeth of cats. 2. Ninety-three tooth-pulp units in ten cats could be excited either by mechanical or electrical stimulation of adjacent mucous membrane or by electrical stimulation of the pulp of a nearby tooth. 3. Section of the trigeminal nerve centrally did not abolish these responses. 4. Results for twelve out of thirteen units tested with collision techniques indicated that the nerves were branched.

Responses of intradental nerves to electrical and thermal stimulation of teeth in dogs

1. Experiments were carried out to investigate the mechanism whereby thermal stimul excite nerves to produce pain from teeth. 2. Recordings have been made from single fibres dissected from the inferior dental nerve in dogs during thermal stimulation of the lower canine tooth. 3. In preliminary experiments, no units were found with thresholds close to the thresholds for pain in man (45 and 27 degrees C) and subsequently test stimuli of 55 degrees C, applied for up to 15 sec, and 0-5 degrees C were used. 4. Of 117 fibres tested, forty-three responded to cooling but not to heating and nine responded to heating but not to cooling. 5. By applying thermal stimuli direct to the saphenous nerve in cats, it was shown that these responses might have been due to direct excitation of nerves and not to stimulation of specialized receptors. 6. Some units responded to electrical stimulation of the tooth pulp with a latency which decreased abruptly at a critical intensity as the stimulus was increased above threshold. Evidence was obtained which suggested that this was due to branching of the fibres.