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

Close apposition and exposure of non-myelinated axons in traumatic neuromas of the human lingual nerve

Peripheral nerve injury is sometimes followed by the development of persistent painful sensory disorders, such as dysaesthesia. The aetiology of these disorders is not clear, but abnormal behaviour of damaged axons at the injury site is likely to be involved. In this study, we quantified some ultrastructural characteristics that may be related to the development of abnormal spontaneous activity, sympathetic interactions, and fibre-to-fibre crosstalk. Using electron microscopy, we have determined the frequency and extent of axonal exposure and close apposition among non-myelinated axons from 34 traumatic neuromas of the human lingual nerve. These specimens were removed at the time of microsurgical nerve repair, and the presence or absence of symptoms of dysaesthesia was determined pre-operatively. Comparisons were also made with eight normal control lingual nerve specimens obtained from patients undergoing organ donor retrieval. More non-myelinated axons showed signs of axonal exposure in traumatic neuromas (26%) than in controls (5%), and exposure was higher in nerve-end neuromas (31%) than in neuromas-in-continuity (22%). In addition, the proportion of the non-myelinated axolemma that was exposed was significantly higher in neuromas (32%) than in controls (21%). The frequency of close apposition between neighbouring non-myelinated axons was also higher in neuromas (11%) than in controls (0.35%). The majority of axons showing signs of exposure or close apposition had diameters <1 microm. These ultrastructural changes may account for some of the altered electrophysiological properties of axons within neuromas. However, no significant correlations were found between these ultrastructural characteristics and the patients' reported symptoms of dysaesthesia.

Neural form of voltage-dependent sodium current in human cultured dental pulp cells

Intradental, i.e. pulpal, cells may play an important part in sensory transduction in teeth, although the cellular mechanisms and the identity of the specific cell types involved are still unclear. Because the majority of cells in dental pulp are derived from neural crest, it seemed likely that these might have the membrane properties of other neural-derived cells found in the peripheral or central nervous system. The patch-clamp recording technique was used to show that cells in explant cultures from human dental pulp contain a voltage-gated, tetrodotoxin-sensitive inward current. Mean activation potential of the current was -42 +/- 2.5 mV and the voltage at half-inactivation was -79.4 +/- 5.3 mV, suggesting a neural-like sodium conductance. In addition, these cells were immunoreactive to glial acidic fibrillary protein, growth-associated protein (GAP-43), and vimentin, further suggesting that dental pulp contains a population of cells with membrane properties similar to neuronal satellite cells. These cells may contribute, either directly or indirectly, to somatosensation in teeth.

Potassium currents in cells derived from human dental pulp

Patch-clamp recording methods were used to monitor ion currents in tissue-cultured cells derived from human dental pulp. Recordings were made in excised, outside-in or whole-cell patches. In single-channel experiments, the majority of patches contained a high-conductance (approx. 140-180 pS) K(+)-selective ion channel. The probability of the channel being in an open state was dependent on membrane potential, internal calcium and negative pressure applied to the cell membrane. Whole-cell recordings were consistent with these findings; in response to step-wise depolarizations of the cell membrane, most displayed a family of outwardly rectifying, barium-sensitive currents. In addition, a number of patches contained a second class of potassium channel of intermediate (approx. 85-100 pS) conductance, which was largely voltage insensitive and independent of calcium concentration. These results suggest that pulp cells contain a high-conductance potassium channel which probably underlies the outwardly rectifying current found at the whole-cell level. Further, the existence of mechanosensitive channels in these cells raises the possibility that the response to mechanical perturbation of dental pulp may be mediated, in part, by direct effects on odontogenic cells.

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.

A morphological study of the collateral reinnervation of the cat's canine tooth

We examined the morphologic characteristics of the collateral reinnervation of the cat's canine tooth pulp. Collateral reinnervation was stimulated in six adult cats by transecting the ipsilateral inferior alveolar nerve and preventing it from regenerating by sealing the ligated nerve inside a nylon tube. The apices of the canine teeth were examined after 15 weeks using electron microscopy and each axon was counted and measured. Compared with contralateral control teeth, the collateral reinnervation consisted of fewer myelinated fibers and in five animals the number of nonmyelinated fibers was also smaller. In one animal the nonmyelinated count on the operated side exceeded that on the control side. The total number of axons was 33.5% of the number in control teeth, considerably fewer than seen after reinnervation by regenerating fibers in previous experiments. The structural characteristics of the collateral fibers were similar to those of regenerating nerve fibers. Compared with controls the myelinated collateral fibers were smaller and had thinner myelin sheaths. There were more nonmyelinated fibers containing only one axon. More nonmyelinated fibers had exposure of part of their axolemma to the extracellular space and there was also an increase in axoaxonal apposition.

Reinnervation of teeth after segmental osteotomy

The reinnervation of 8 canine teeth denervated by segmental osteotomy has been investigated in cats by using electron microscopy. 12 weeks after osteotomy, the mean total number of axons at the canine apices was 36% of that found in normal animals. The myelinated axons were smaller than normal with thinner myelin sheaths. In non-myelinated fibres, there were fewer axons per fibre (Remak bundle) and more fibres containing only one axon. The proportions of myelinated and non-myelinated fibres were normal.

A quantitative study of structural features, synapses and nearest-neighbour relationships of small, granule-containing cells in the rat superior cervical sympathetic ganglion at various adult stages

Groups and sub-groups (clusters) of small granule-containing cells ("small cells") were analysed at 3 and 6 micron intervals and in serial sections, in rats aged 2-13 months. Fully intraganglionic clusters of small cells were all found to receive an incoming ("afferent") innervation, of the order of 3-6 afferent terminals per cell, derived from axons of preganglionic type via multifocal, symmetrical, mainly axosomatic synapses. No evidence was obtained of sharing of preganglionic inputs between small cells and principal neurones. Intraganglionic clusters also regularly gave outgoing ("efferent") synapses of the asymmetrical type, of the order of 2-6 per cell, to intraganglionic nerve elements; 30-50% of these synapses were given from somata, 50-70% from processes of the small cells. Whenever the postsynaptic structure was identifiable these synapses were all found to be given to postganglionic neurones or their dendrites, principally to spine-like processes or slender twigs. In some ganglia a few efferent synapses to other small cells were observed; these were of the symmetrical type. Efferent synapses to nerve profiles resembling chemosensory axon terminals, also of the symmetrical type, were extremely infrequent (fewer than 1% of all efferent synapses) in intraganglionic small cell groups and appeared virtually restricted to glomus-like clusters of small cell, which lay intracapsularly, or in and near the bases of nerves entering or leaving the ganglion. Almost all groups and clusters of small cells were located near to fenestrated capillary vessels, which are not found elsewhere in the ganglion. The implications of possible non-synaptic release of material from small cells via membrane regions not covered by satellite cell cytoplasm, were explored in a nearest-neighbour analysis. These "exposed" regions comprised 1-3% of the small cell surface, a proportion comparable with those engaged in receiving afferent synapses or in giving efferent synapses. The majority of such regions faced toward other nerve profiles (axons and dendrites) ensheathed in satellite cytoplasm (mean 30%), intraganglionic tissue spaces wider than 3 micron (mean, 30%) or other small cells (mean, 14%); 25% faced toward blood vessels, but of these vascularly directed regions, only one fifth (or 5% of the total) on average faced directly toward fenestrated endothelium, the rest being non-fenestrated and/or separated by pericyte processes from the exposed regions of small cell membrane. Thirty-three percent of the small cells in a sample of 242 lay within 2 micron of the nearest blood vessel.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for the persistence of axons at the apex of the cat's lower canine tooth after section of the inferior alveolar nerve

The aim of this investigation was to establish the degree of denervation produced by inferior alveolar nerve section and to provide histological evidence for the presence of pulpal nerve fibres supplying the teeth which do not travel with the inferior alveolar nerve. Four adult cats were used. Each stage of the experiment was carried out under general anesthesia. The left inferior alveolar nerve was exposed and sectioned near the mandibular foramen. After 56 hours and 7 days, respectively, the jaw opening reflex to electrical stimulation of each lower canine was tested. Recordings were made from the left canine during electrical stimulation of the ipsilateral inferior alveolar nerve central and peripheral to the site of section as well as from the ipsilateral and contralateral inferior alveolar nerve during electrical stimulation of the left canine. Recordings were also made from the lingual nerve. After the recordings were completed two animals were perfused 56 hours after inferior alveolar nerve section, two more 7 days after section. Ultrathin sections of the apices of the lower canine teeth were examined in the electron microscope and each nerve fibre photographed. Each axon was examined to determine whether it was degenerating or normal. A jaw-opening reflex could not be elicited by stimulation of the left canine either 2 or 7 days after nerve section, whereas a normal response was evoked by stimulation of the right, control canine. At 2 days small responses could be recorded from the left canine teeth during stimulation of the left inferior alveolar nerve peripheral to the point of section. In one 2-day animal, responses could be recorded in the lingual nerve during stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.