The reinnervation of the tongue and salivary glands after two methods of lingual nerve repair in the cat

Histomorphometric changes in repaired mouse sciatic nerves are unaffected by the application of a scar-reducing agent

Microsurgical repair of transected peripheral nerves is compromised by the formation of scar tissue and the development of a neuroma, thereby limiting the success of regeneration. The aim of this study was to quantify histomorphometrically the structural changes in neural tissue that result from repair, and determine the effect of mannose-6-phosphate (M6P), a scar-reducing agent previously shown to enhance regeneration. In anaesthetised C57-black-6 mice, the left sciatic nerve was sectioned and repaired using four epineurial sutures. Either 100 μL of 600 mm M6P (five animals) or 100 μL of phosphate-buffered saline (placebo controls, five animals) was injected into and around the nerve repair site. A further group acted as sham-operated controls. After recovery for 6 weeks, the nerve was harvested for analysis using light and electron microscopy. Analysis revealed that when compared with sham controls, myelinated axons had smaller diameters both proximal and distal to the repair. Myelinated axon counts, axonal density and size all decreased across the repair site. There were normal numbers and densities of non-myelinated axons both proximal and distal to the repair. However, there were more Remak bundles distal to the repair site, and fewer non-myelinated axons per Remak bundle. Application of M6P did not affect any of these parameters.

An electrophysiological study into the effect of neurotrophin-3 on functional recovery after lingual nerve repair

OBJECTIVE

Neurotrophin-3 (NT-3) is known to ameliorate central changes that result from peripheral nerve injury and may promote regeneration of myelinated axons. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat.

DESIGN

Six months after entubulation repair, with or without the incorporation of NT-3 at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents.

RESULTS

When compared with data from uninjured control animals, both repair groups showed persistent reductions in conduction velocity, receptor sensitivity, spontaneous discharge, proportion of gustatory and thermosensitive units, and rate of salivary secretion. Comparisons between the two repair groups revealed that in the NT-3 group, salivary secretion rate was lower and the activity evoked in the chorda tympani by gustatory or thermal stimuli was lower, but the spontaneous discharge rate was higher. Mechanosensitive units in the lingual nerve had slower conduction velocities but the mechanoreceptive field size, adaptation time and discharge frequency had increased.

CONCLUSIONS

Despite its known trophic role in the lingual somatosensory system, NT-3 did not enhance functional recovery from injury and had a negative effect on the long-term outcome for sensory and autonomic fibres.

A prospective, quantitative study on the clinical outcome of lingual nerve repair

We previously showed in laboratory studies that the most effective method for repair of damaged lingual nerves was by excision of the neuroma, mobilization of the stumps, and direct reapposition with epineurial sutures. We have now undertaken a prospective study in a series of 53 patients treated by this method and have evaluated the outcome by quantifying and comparing the results of tests of sensation before and after operation. The outcome in individual patients was variable. However, pooled data from all patients showed a highly significant improvement in sensation at the final assessment 12 months or more after the repair. The proportion of patients who responded to most or all light touch stimuli increased from 0% to 51% after repair, and the proportion who responded to pin-prick stimuli increased from 34% to 77%. There was no correlation between the final results of any of the tests and the delay before repair. None of the patients regained completely normal sensation and there was no reduction in the number with spontaneous paraesthesia or pain. However, fewer patients tended to bite the tongue by accident and most of them considered the operation worthwhile. These data show that lingual nerve repair is effective in most patients and we suggest that it should be offered to all those who show few signs of spontaneous recovery after injury.

Lesions Of The Inferior Alveolar Nerve Arising From Endodontic Treatment

A lesion of the IAN following endodontic treatment of the lower molars and premolars is not a rare event and presents an uncomfortable situation both for the dental surgeon and the patient. Injury can result on the one hand by direct intrusion of the instrument through the apex into the mandibular canal, and on the other by the filling material which becomes forced into the mandibular canal. In the latter case, a nerve lesion will only result when the filling material contains neurotoxic substances such as paraformaldehyde. With a direct lesion or when forcing of resorbable filling material into the mandibular canal is suspected, one should first employ a wait-and-see approach, because usually the only nerve damage is in the form of neuropraxy or axonotmesis for which there is a high rate of spontaneous regeneration. However, if neurotoxic filling material is introduced into the direct vicinity of the nerves, the mandibular canal should be opened and the filling material should be removed as early as possible. If the filling material is forced directly within the endoneurium between the nerve bundles, the damaged nerve sections must be resected and bridged using transplants from the sural or greater auricular nerves.

Amiloride and vertebrate gustatory responses to NaCl

Amiloride at < or = 1 microM may block epithelial Na+ channels without affecting other cellular mechanisms, and attenuates gustatory responses to lingual NaCl from the chorda tympani nerves (CT) of gerbil, hamster, rhesus monkey, and several strains of laboratory rat and mouse, and from glossopharyngeally innervated frog taste-receptor cells; at 5 microM to 50 microM, also from Wistar rat and mongrel dog CT. Affected units responded more to NaCl than to KCl. Suppression of CT responses to KCl, HCl, NH4Cl, or saccharides also occurred in some mammals, but amiloride did not elicit responses. Taste-dependent behaviors towards NaCl or KCl were altered. DBA and 129/J laboratory mice, and mudpuppy, were unaffected by amiloride. In humans, 10 microM amiloride both produced taste reports and reduced total intensity of NaCl and LiCl by 15-20%. NaCl and LiCl sourness, and KCl and QHCl bitterness declined, but saltiness generally did not change. Effects on sweetness were inconsistent. Amiloride-sensitive gustatory mechanisms were prominent in some mammals, were not necessary for responses to NaCl, and were of minor importance for human taste.

Low-threshold mechanoreceptive afferents in the human lingual nerve

Intrafascicular multiunit activity and impulses in single mechanoreceptive afferents were recorded from the human lingual nerve with permucosally inserted tungsten microelectrodes. Nylon filaments and blunt glass probes were used for mechanical stimulation of the mucosa of the dorsal surface of the tongue. The innervation territories of nine nerve fascicles were mapped during multiunit recordings. All fascicle fields included the tip of the tongue, suggesting a particularly high innervation density for this area. Thirty-three single mechanoreceptive afferents were isolated and studied. Of these afferents, 22 were characterized by very small mucosal receptive fields (range: 1-19.6 mm2; geometric mean: 2.4 mm2) and responded to extremely low mechanical forces (force threshold range: 0.03-2 mN; geometric mean: 0.15 mN). As such, it was concluded that these "superficial" units terminated near the surface of the tongue. The remaining 11 units responded to probing of large areas of the tongue (> 200 mm2) and exhibited high force thresholds (> or = 4 mN). It was concluded that these "deep" units terminated in the muscle mass of the tongue. Fourteen of the superficial units were classified as rapidly adapting and resembled the fast-adapting type I afferents described for the glabrous skin of the human hand. The rapidly adapting units responded both during the application and removal of, but not during maintenance of, the mechanical stimuli on the receptive field. Two types of slowly adapting responses were observed. One type (characteristic of only 2 units) was characterized by a pronounced sensitivity to force change during the application and removal of the mechanical stimuli and an irregular static discharge during maintenance of the stimulus on the receptive field. In contrast, the other six units exhibited a weak sensitivity to force change, a highly regular static discharge, and spontaneous activity. As such, these two types of slowly adapting units resembled the slowly adapting I and II afferents, respectively, described for the hand. All 11 deep units were slowly adapting, and 7 were, in addition, spontaneously active. The units were not equally sensitive to the application and removal of the mechanical stimuli, suggesting at least two different modes of termination in tongue muscle. The deep units reliably encoded information about tongue movements in the absence of direct contact with the receptive field. In contrast, the superficial units responded vigorously when the tongue was moved to bring the receptive field into physical contact with other intraoral structures.

Experimental trigeminal nerve injury

The successful reinnervation of peripheral targets after injury varies with the axonal population of the nerve that is injured and the extent of the dislocation of its central component from the peripheral endoneurial tube. Larger-diameter axons such as those supplying mechanoreceptors recover more readily than narrower axons such as those supplying taste. A complex, bi-directional interaction between lingual epithelium and sprouting nerve results in the redifferentiation of taste buds after denervation. Dentin and the dental pulp provide a strong attraction to sprouting nerves and will become reinnervated from collateral sources if recovery of the original innervation is blocked. The most effective repair technique for transected lingual nerves is one which brings the cut ends together rather than one that provides a temporary bridge. Injuries can result in cell death in the trigeminal ganglion but only if the injury is severe and recovery is prevented. Lesser damage results in chromatolysis and the increased expression of neuropeptides. All nerve injuries bring about changes in the trigeminal nucleus. These occur as changes in receptive field and the incidence of spontaneously active neurons, effects which are consistent with the unmasking of existing afferents. These functional changes are short-lived and reversible. Morphologically, nerve injury results in terminal degeneration in the nuclei and an increased expression of the c-Fos gene and some neuropeptides. Only a chronic constriction injury induces behavioral changes. The adult trigeminal system retains considerable plasticity that permits it to respond successfully to nerve injury. Much remains to be learned about this response, particularly of the trophic factors that control peripheral recovery and the central response to more severe injuries.

A quantitative morphological comparison of cat lingual nerve repair using epineurial sutures or entubulation

Since lingual nerves may be transected during a variety of oral surgical procedures, including third molar removal, we have investigated two possible methods of repair. Quantitative morphological observations were made on feline chorda tympani and lingual nerves proximal and distal to transection injuries repaired either by epineurial suturing or by insertion of the cut ends into a perforated silicon tube. Proximal to the repair, the most prominent difference was an increase in the number of myelinated axons in the lingual nerve following epineurial suturing but not entubulation. Proximal to the repair site, the number of nonmyelinated axons increased in comparison with controls in both chorda tympani and lingual nerves after both procedures, though the difference was statistically significant only in the lingual nerve proximal to entubulation. Distal to the injury, both types of repair showed a reduction in the number, size, and sheath thickness of myelinated axons in comparison with unoperated controls, but the difference in numbers was statistically signIficant only distal to repair by entubulation. The number of non-myelinated axons distal to the repair sites was much higher than that in controls, the difference being greater distal to entubulation repair. There were more axons per Remak bundle distal to entubulation repair than to epineurial suturing, suggesting, perhaps, that fewer axons would ultimately become myelinated. Though the morphological differences between the two repair techniques are not as striking as the parallel electrophysiological differences reported previously (Smith and Robinson, 1995a,b), they are consistent with them and support the conclusion that, for transected lingual and chorda tympani nerves, epineurial suturing is the preferred approach.

An experimental study on the recovery of the lingual nerve after injury with or without repair

The recovery of the mechanosensitive and thermosensitive afferent fibres in the lingual branch of the trigeminal nerve has been studied using electrophysiological techniques in cats after nerve section without repair or after section followed by nerve repair 12 weeks later. In the unrepaired group, recovery was permitted for 24 weeks, and after delayed repair there was a further recovery period of either 12 or 24 weeks. The characteristics of the regenerated fibres were then investigated, and data were also compared with those from normal control animals and from animals which had undergone immediate nerve repair. The results revealed only small differences between the repaired and unrepaired groups and it is concluded that delayed repair of a clean transection site results in only slightly better recovery than leaving the nerve unrepaired. It is also concluded that a 12-week delay before repair has little detrimental effect.

The effect of delayed nerve repair on the properties of regenerated fibres in the chorda tympani

The characteristics of regenerated fibres in the chorda tympani have been investigated in cats after nerve section without repair or after section followed by nerve repair twelve weeks later. In the unrepaired group the animals were allowed to recover for twenty four weeks and after delayed repair there was a further recovery period of twelve or twenty four weeks. The properties of gustatory, thermosensitive and mechanosensitive units and the return of vasomotor and secretomotor responses were then investigated and data compared with that from normal controls and from animals which had undergone immediate nerve repair. After nerve section, integrated whole-nerve activity recorded from the chorda tympani during gustatory or thermal stimulation of the tongue was reduced when compared to controls, but there were only small differences between the repaired and unrepaired groups. Recordings made from single units in the chorda tympani revealed that more units were spontaneously active after repair (P < 0.05) and the gustatory units produced more impulses when stimulated (P < 0.005). Twelve weeks after delayed repair the units had slower conduction velocities than those in the unrepaired nerves (P < 0.001), but by twenty four weeks after repair they were significantly faster (P < 0.05). There was little difference in the level of recovery twelve weeks after immediate or delayed repair. We conclude that delayed nerve repair results in better recovery than leaving the nerve unrepaired and that a twelve week delay before repair has little effect.

An experimental study of three methods of lingual nerve defect repair

PURPOSE

This study compares three methods of lingual nerve defect repair.

MATERIALS AND METHODS

The recovery of the mechanosensitive and thermosensitive afferent fibers in the lingual branch of the trigeminal nerve in cats was studied using electrophysiological techniques 24 weeks after the removal of a segment of lingual nerve and repair of the defect by one of three methods. The nerve gap was closed by either stretching the nerve ends together and repairing under tension, insertion of a sural nerve graft, or use of a freeze-thawed muscle graft. The characteristics of the regenerated fibers were investigated and the data were compared with that from normal control animals.

RESULTS

After nerve repair, integrated whole-nerve activity evoked by thermal (cold) stimulation of the tongue was smaller than in the controls, but there were no differences between the repaired groups. However, recordings made from single axons in filaments dissected from the nerve revealed differences between the groups; the units were less sensitive after either method of grafting than after stretch repair.

CONCLUSIONS

It is concluded that repair of a short gap in the lingual nerve by stretch repair with an end-to-end anastomosis, even with some degree of tension, is followed by better recovery than by grafting. However, where a graft is necessary, a similar level of recovery will result from use of a frozen muscle graft or a sural nerve graft.