The chorda tympani degenerates during chronic otitis media: an electron microscopy study.

CONCLUSION: Chorda tympani nerve specimens from ears with chronic inflammatory middle ear disease exhibit structural signs of degeneration. These correlate well with taste disturbance. Simultaneously, they exhibit signs of regeneration, which may explain the ability for taste recovery. OBJECTIVES: The chorda tympani, the major taste nerve, runs uncovered through the middle ear cavity. This situation exposes it to various forms of middle ear pathology. A difference has been noticed regarding taste symptoms pre- and postoperatively between inflammatory and non-inflammatory diseases. The present study aimed to investigate ultrastructural changes of chorda tympani in different forms of inflammatory middle ear disease, such as chronic suppurative otitis media and cholesteatoma, as compared with normal. METHODS: Five chorda tympani specimens were collected from healthy middle ears of patients subjected to surgery for acoustic neuroma, to be used as normal controls, and five from middle ears with chronic otitis media or cholesteatoma, where the nerve could not be saved during the operation. Light and electron microscopy were performed. RESULTS: For all five nerves from diseased ears, microscopy showed a higher percentage of axon and myelin sheath degeneration than in the normal controls. Furthermore, three of the five also exhibited sprouting.

Chorda tympani nerve analysis with electron microscopy in chronic suppurative otitis media.

CONCLUSION: Chronic suppurative otitis media causes some disturbance to the chorda tympani nerve (CTN), which may affect the facial nerve. It is not possible to perform a biopsy of the main truncus of the facial nerve, therefore studies of the CTN might show possible pathologic or physiologic changes of the facial nerve in the future. OBJECTIVES: The specific aim of this study was to investigate the effects of chronic suppurative otitis media on the CTN. METHODS: The tympanic segments of CTNs were collected for ultrastructural investigations in patients with chronic suppurative otitis media who underwent canal wall-down tympanoplasty. The study population comprised 10 patients, 7 males, 3 females; the age range was 16-66 years, and the mean age was 38. Qualitative and semiquantitative evaluations were performed on the specimens of CTN by electron microscopy. RESULTS: Our histopathologic examinations showed that there were changes of varying severity in all the CTNs such as scarcity of unmyelinated nerve fibers, Schwann cell nucleus condensation, scarcity of Schwann cell cytoplasm, adaxonal vacuolation and edema, myelin sheath disintegration, shrunken electron-dense axoplasm, increased collagen fibers, adaxonal circular lamellar complex, interstitial edema, and vacuolation of Schwann cell cytoplasm.

Ultrastructure of primary afferent terminals and synapses in the rat nucleus of the solitary tract: comparison among the greater superficial petrosal, chorda tympani, and glossopharyngeal nerves.

The greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves terminate in overlapping patterns in the brainstem in the rat nucleus of the solitary tract (NTS). There is one region, in particular, that receives overlapping inputs from all three nerves and is especially plastic during normal and experimentally altered development. To provide the requisite data necessary ultimately to delineate the circuitry in this region, we characterized the morphology of the synaptic inputs provided by the GSP, CT, and IX nerves through transmission electron microscopy. Although all three nerves had features characteristic of excitatory nerve terminals, ultrastructural analysis revealed dimorphic morphologies differentiating IX terminals from GSP and CT terminals. IX terminals had a larger area than GSP and CT terminals, and more synapses were associated with IX terminals compared with GSP and CT terminals. Additionally, IX terminals formed synapses most often with spines, as opposed to GSP and CT terminals, which formed synapses more often with dendrites. IX terminals also exhibited morphological features often associated with synaptic plasticity more often than was seen for GSP and CT terminals. These normative data form the basis for future studies of developmentally and environmentally induced plasticity in the rodent brainstem.

Lingual deficits in neurotrophin double knockout mice.

Brain-derived neurotrophic factor (BDNF) and Neurotrophin 3 (NT-3) are members of the neurotrophin family and are expressed in the developing and adult tongue papillae. BDNF null-mutated mice exhibit specific impairments related to innervation and development of the gustatory system while NT-3 null mice have deficits in their lingual somatosensory innervation. To further evaluate the functional specificity of these neurotrophins in the peripheral gustatory system, we generated double BDNF/NT-3 knockout mice and compared the phenotype to BDNF(-/-) and wild-type mice. Taste papillae morphology was severely distorted in BDNF(-/-) xNT-3(-/-) mice compared to single BDNF(-/-) and wild-type mice. The deficits were found throughout the tongue and all gustatory papillae. There was a significant loss of fungiform papillae and the papillae were smaller in size compared to BDNF(-/-) and wild-type mice. Circumvallate papillae in the double knockouts were smaller and did not contain any intraepithelial nerve fibers. BDNF(-/-) xNT-3(-/-) mice exhibited additive losses in both somatosensory and gustatory innervation indicating that BDNF and NT-3 exert specific roles in the innervation of the tongue. However, the additional loss of fungiform papillae and taste buds in BDNF(-/-) xNT-3(-/-) mice compared to single BDNF knockout mice indicate a synergistic functional role for both BDNF-dependent gustatory and NT-3-dependent somatosensory innervations in taste bud and taste papillae innervation and development.

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract.

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST). To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.

[Research of the chorda tympani nerve in cholesteatoma].

OBJECTIVE: To investigate the ultrastructure of the chorda tympani nerve and analyze the taste and facial nerve functions in patients with cholesteatoma. METHODS: 1. The tympanic segments of chorda tympani nerves were collected for ultrastructural investigations in 19 cholesteatoma cases who underwent canal-wall-down tympanoplasty. 2. All these patients received a spatial (regional) taste test preoperatively and postoperatively, respectively. Multiple loci were tested in a given order (front of the tongue, foliate papillae, circumvallate papillae, and soft palate). The solutions used in this study were 1.0 mol/L sodium chloride (salty), 1.0 mol/L sucrose (sweet), 0.032 mol/L citric acid (sour) and 0.001 mol/L quinine hydrochloride (bitter). The analyses of variance was used. 3. House-Brackmann facial nerve grading system was conducted to evaluate the facial nerve function preoperatively and postoperatively. RESULTS: 1. There were obvious ultrastructural damages in all the chorda tympani nerves, such as swelling (100%), disarrangement (100%), vacuoles formation of myelin (89%), edema of Schwann cells (95%), intracytoplasmic vacuoles in Schwann cell (89%) and proliferation of the collagen tissue (89%). 2. Two patients complained of change of taste after operation. The analyses of variance showed that the taste function of the ipsilateral side of tympanoplasty were not statistically altered for each stimulus at each locus (P > 0.05). 3. No facial palsy occurred postoperatively. CONCLUSION: This research suggested that the chorda tympani nerves underwent ultrastructural changes in patients with cholesteatoma. The dissection of chorda tympani nerve would not affect the taste and facial nerve functions.

Morphological and functional study of regenerated chorda tympani nerves in humans.

It is still unclear whether the chorda tympani nerves in humans regenerate after being severed during middle ear surgery, although functional studies have demonstrated recovery of taste 1 to 2 years after surgery. To date, 12 cases of regenerated chorda tympani nerves have been found in our series of patients during secondary surgery. The regenerated nerves of 3 cases of the 12 were removed as samples during secondary surgery to detect regenerated myelinated axons. All regenerated nerves were in the submucosal connective tissue layer of the reconstructed eardrum. In the regenerated nerves, myelinated nerve fibers existed in a small fascicle or in connective tissue, but the number of myelinated axons was low compared with that in normal subjects ( 1.752 +/- 78; n = 3), and the distribution was sparse. The total number of regenerated myelinated axons varied from 141 (8.3%) to 979 (55.9%). From a functional study using electrogustometry, incomplete recovery of electrogustation was observed in all 3 cases before secondary surgery, suggesting that chorda tympani nerves actually regenerate in the middle ear and do function.

Synaptic relationships between the chorda tympani and tyrosine hydroxylase-immunoreactive dendritic processes in the gustatory zone of the nucleus of the solitary tract in the hamster.

The toxic lectin ricin was applied to the hamster chorda tympani (CT), producing anterograde degeneration of its terminal boutons within the gustatory zone of the nucleus of the solitary tract (NST). Immunocytochemistry was subsequently performed with antiserum against tyrosine hydroxylase (TH), and the synaptic relationships between degenerating CT terminal boutons and either TH-immunoreactive or unlabeled dendritic processes were examined at the electron microscopic level. Degenerating CT terminal boutons formed asymmetric axodendritic synapses and contained small, clear, spherical synaptic vesicles that were densely packed and evenly distributed throughout the ending, with no accumulation at the active synaptic. The degenerating CT terminated on the dendrites of TH-immunoreactive neurons in 36% (35/97) of the cases. The most frequent termination pattern involved the CT and two or three other inputs in synaptic contact with a single immunoreactive dendrite, resulting in a glomerular-like structure that was enclosed by glial processes. In 64% (62/97) of the cases, the degenerating CT was in synaptic contact with unlabeled dendrites, often forming a calyx-like synaptic profile that surrounded much of the perimeter of a single unlabeled dendrite. These results indicate that the TH-immunoreactive neurons of the gustatory NST receive direct input from the CT and taste receptors of the anterior tongue and that the termination patterns of the CT vary with its target neuron in the gustatory NST. The glomerular-like structure that characterizes many of the terminations of the CT provides an opportunity for the convergence of several functionally distinct inputs (both gustatory and somatosensory) onto putative dopaminergic neurons that may shape their responsiveness to the stimulation of the oral cavity.

Recovery of chorda tympani nerve function following injury.

The chorda tympani (CT) nerve carries taste information from the anterior tongue to the brain stem. Injury to the chorda tympani may result in loss or distortion of taste information. This study examined changes occurring in the hamster peripheral taste system during recovery from injury. The hamster chorda tympani nerve was crushed in the middle ear and the animals were allowed to survive from 2 to 16 weeks. At 2 weeks, CT fibers had degenerated distal to the crush site. Up to 16 weeks after crush, there were 67% fewer myelinated fibers in regenerated nerves than in controls. The mean area of the Ca(2+)-ATPase-stained core of the fungiform taste buds was significantly smaller than in controls 2 weeks after injury, but recovered to control values by 4 weeks. Electrophysiological responses to taste stimuli were recorded from the chorda tympani distal to the injury. No responses were seen after 2 weeks; weak and unstable responses were seen after 3 weeks. By 4-8 weeks, relative responses to taste stimuli were similar to control responses, but the variability of the responses to sucrose was significantly greater than that in controls. The frequency of responses to the water rinse following taste stimuli, particularly sucrose, was also greater in the regenerated nerves. The abnormal electrophysiological responses to sucrose may be the result of the differential rate of return of fiber types and/or the transduction mechanisms. In some ways, recovery of the peripheral gustatory system after damage to the chorda tympani nerve recapitulates the later stages of taste bud development.

Persistence and calcium-dependent ATPase staining of denervated fungiform taste buds in the hamster.

Some fungiform taste buds in the hamster have been previously shown to persist for indefinite periods when deprived of their gustatory, chorda tympani (CT), innervation or both their CT and their trigeminal, lingual nerve, innervation (CT-L). The properties and numbers of persisting fungiform taste buds were examined 1 or 3 weeks after permanent CT or combined CT-L nerve cuts. The purpose was to reveal the status of taste buds at a time (3 weeks) when regenerating nerve fibres would normally be expected to reinnervate the epithelium. Denervated taste buds retain many normal characteristics including the pattern of histochemical staining for ectocalcium-dependent ATPase (Ca-ATPase). Taste-bud cells (including basal cells) have an intensely Ca-ATPase stained core surrounded by lightly stained peripheral cells. The Ca-ATPase stain was used to help identify and to define the size of the taste-bud core in denervated taste buds. Following CT-L or CT denervation most taste buds persisted; however the size of the taste-bud core was dramatically reduced. Fungiform taste buds differed in size based on their location in one of three tongue regions. The percentage decrease in size after denervation was also region specific and about the same for CT-L or CT cuts, suggesting that trigeminal fibres have no trophic effect on taste buds. However, trigeminal denervation caused a reduction in the number of persisting taste buds relative to CT denervation alone, which may be due to damage because of the loss of somatosensation.

Ultrastructural analysis of the chorda tympani nerve in facial paralysis from different etiologies.

Twelve chorda tympani segments, removed from as many patients (7 females, 5 males) affected by otosclerosis (3, as controls) and from either idiopathic (4), or traumatic (3) or herpetic (2) facial paralysis, were examined by a transmission electron microscope, in order to verify chordal ultrastructural changes, the gravity of neural damage in the various types of facial paralysis and the role played by chorda tympani on the etiopathogenesis of the idiopathic form. The segments were obtained, while performing a stapedectomy in otosclerotic patients and, in facial palsy, while performing chordal neurotomy or direct decompression of the II and III portio of the VII nerve. The obtained data showed no noteworthy alterations concerning myelinated and unmyelinated nerve fibers and chordae stroma in otosclerotic patients, confirming their reliability as controls. In facial paralysis, both normal ultrastructures and alterations were noticed, even though polymorphic: unravelling and dissociation of the lamellae sheaths up to a point of degeneration with a more or less complete resorption of myelin, accumulation of dense bodies and electron-dense formation, particularly in unmyelinated fibers, vacuoles in myelinated fibers, intracytoplasmic vacuoles in Schwann cells and, in herpetic forms, an occasional presence of inflammatory cells. Such results demonstrate that, in any type of facial paralysis, the ultrastructure of the chorda tympani is constantly involved, probably due to main trunk involvement. In the idiopathic forms the alterations of the chorda look like to degenerative alterations caused by traumato-ischemic events, rather than to cytopathic lesions induced by herpetic virus.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary sodium chloride deprivation throughout development selectively influences the terminal field organization of gustatory afferent fibers projecting to the rat nucleus of the solitary tract.

In order to determine whether the developing central gustatory system responds to altered sensory experience, terminal fields of the chorda tympani nerve (CT) within the nucleus of the solitary tract (NTS) in control, NaCl deprived, and rats in which CT taste responses "recovered" from NaCl deprivation were investigated via anterograde transport of HRP. Rats fed a low sodium diet (0.03% NaCl) from the third day of gestation to at least 35 days postnatal exhibited both abnormally distributed and irregularly shaped CT terminal fields. Specifically, the dorsal zone of the field was the smallest in controls whereas it was the largest in deprived rats, occupying more medial and caudal territory within the nucleus. The portion of the field immediately ventral to the dorsalmost zone was characterized by a compact, oval shape in control rats and an irregular, broad configuration in deprived rats. Although it has been observed that deprivation-induced changes in the neurophysiology of the CT are reversible, the central morphological alterations reported here remain abnormal. Restoration of 1.0% NaCl in the diet at 28 days postnatally, for at least 60 days, did not result in normal CT terminal fields. The pattern of the field in rats "recovered" from NaCl deprivation was comparable to that found in deprived rats, and the size of the field was three times that found in control and deprived rats. The terminal fields of another nerve containing gustatory afferents, the lingual-tonsilar branch of the glossopharyngeal nerve (LT-IX), were studied for comparison. Interestingly, the pattern of the LT-IX field was not altered by sodium deprivation. The relative size and topography of the LT-IX fields in deprived rats were similar to controls. Thus, sodium deprivation appears to alter selectively the anatomical organization of the CT. Differences in vulnerability between the CT and LT-IX terminal fields may derive from differences in the responsiveness of these nerves to NaCl, and/or to differences in the timing of early neural events.

Mechanisms of middle ear aeration: anatomic and physiologic evidence in primates.

Proper aeration is a prerequisite for normal middle ear function in all terrestrial mammals. Our previous studies in primates provided anatomic evidence of neural circuits between the middle ear, brain, and eustachian tube by which central respiratory neurons can control middle ear aeration. Yet mechanisms that regulate middle ear aeration remain poorly understood. This study extends our research by examining maturation of these neural circuits, and investigating their underlying physiology. Ultrastructural examination of tympanic nerves, the afferent limb of the neural circuit, in an age-graded series of cynomolgus monkeys (Macaca fascicularis) showed substantial differences between newborn, young, and adult animals. These included a twofold increase in average myelin thickness, and greater than threefold increase in the ratio of myelinated to unmyelinated fibers from newborn to adult animals. These marked developmental changes may translate into functional differences in regulation of middle ear aeration in young animals, and possibly explain the extraordinarily high incidence of middle ear disease in early childhood. In physiologic experiments, bilateral electromyographic responses were recorded from eustachian tube muscles, the efferent limb of the neural circuit, in adult monkeys after ipsilateral stimulation of the tympanic nerve. Response latencies were 9 to 28 msec, similar to those of other multisynaptic bilateral brainstem reflexes. These physiologic data strongly suggest a concept of active control of middle ear aeration by respiratory neurons in the brain.

Electron microscopic study of the effect of capsaicin on the mouse chorda tympani nerves.

Studies were made to determine whether this nerve contains capsaicin-sensitive fibres. Capsaicin (50 mg/kg, subcutaneously) was injected into 2 animals on day 2 after birth and into one animal on days 2 and 3 after birth. Both chorda tympani of these and 4 control mice were later excised. The constituent fibres of 3 of the capsaicin-treated and all 8 control nerves were then analysed. The myelinated fibres in 3 chorda tympani of treated and control animals were measured, and the unmyelinated axons in Schwann cells were counted from electron-micrograph montages of the entire nerve. Normal chorda tympani contained about 600 nerve fibres, 55% myelinated and 45% unmyelinated. Capsaicin-treatment did not change the constituent fibres nor the size distribution of the myelinated fibres. Thus no capsaicin-sensitive, nociceptive fibres were found in the mouse chorda tympani. Capsaicin does not destroy the neurones of the geniculate ganglion and parasympathetic, presynaptic fibres. Therefore, gustation and secretion of saliva are not influenced by capsaicin.

Facial nerve biopsy for etiologic clarification of Bell's palsy.

Electron microscopic findings of biopsy specimens of the intratemporal facial nerve from four selected patients with Bell's palsy are described, and the results of clinical examinations and surgical findings are presented. In all specimens, wallerian degeneration of various degrees was demonstrated. In one specimen, extravascular erythrocytes, and in another, lymphatic infiltrations were noted. On the basis of the present investigation and reported temporal bone histopathologic evidence, the causes of Bell's palsy are concluded to be multiple and to be vascular, inflammatory, or degenerative.

Myelination in the chorda tympani of the postnatal rat: a quantitative electron microscope study.

We determined the course of myelination of the chorda tympani in rats aged from 4- to 30-days-old, the interval of the most rapid developmental changes in neurophysiological taste responses and behavioral discrimination among chemical stimuli. The overall number of axons in rats aged from 16- to 30-days-old and in mature 120-day-old animals were the same and averaged 1,500. By 30 days, rats had 80% of the total number of myelinated axons observed in adults, but the average thickness of the myelin sheath per neuron and the proportion of the total cross-sectional area that were only about 60% of adult values. Observed increases in myelination closely parallel decreasing response latencies of single chorda tympani fibers to tongue stimulation with salts.

Chorda tympani nerve fibers in man.

Four normal human chorda tympani nerves were studied for quantitative data of nerve fibers. The average total number of nerve fibers was 5360 (range, 4941-6020). The average number of unmyelinated fibers was 1835 (range, 1518-2083), or 34% of the total. The diameters of myelinated fibers ranged from 1 to 7 microns, peaking at 2 microns. Most of the unmyelinated axons had a diameter between 0.5 and 2 microns. The importance of the quantitative evaluation of the chorda tympani nerve fibers is stressed in reporting structural changes.