Geniculate neuralgia and facial nerve sensory systems

Neuroscience of taste: unlocking the human taste code

Since antiquity human taste has been divided into 4-5 taste qualities. We realized in the early 1970s that taste qualities vary between species and that the sense of taste in species closer to humans such as primates should show a higher fidelity to human taste qualities than non-primates (Brouwer et al. in J Physiol 337:240, 1983). Here we present summary results of behavioral and single taste fiber recordings from the distant South American marmoset, through the Old World rhesus monkey to chimpanzee, the phylogenetically closest species to humans. Our data show that in these species taste is transmitted in labelled-lines to the CNS, so that when receptors on taste bud cells are stimulated, the cell sends action potentials through single taste nerve fibers to the CNS where they create taste, whose quality depends on the cortical area stimulated. In human, the taste qualites include, but are perhaps not limited to sweet, sour, salty, bitter and umami. Stimulation of cortical taste areas combined with inputs from internal organs, olfaction, vision, memory etc. leads to a choice to accept or reject intake of a compound. The labelled-line organization of taste is another example of Müller's law of specific nerve energy, joining other somatic senses such as vision (Sperry in J Neurophysiol 8:15-28, 1945), olfaction (Ngai et al. in Cell 72:657-666, 1993), touch, temperature and pain to mention a few.

Physiology of Taste Processing in the Tongue, Gut, and Brain

The gustatory system detects and informs us about the nature of various chemicals we put in our mouth. Some of these have nutritive value (sugars, amino acids, salts, and fats) and are appetitive and avidly ingested, whereas others (atropine, quinine, nicotine) are aversive and rapidly rejected. However, the gustatory system is mainly responsible for evoking the perception of a limited number of qualities that humans taste as sweet, umami, bitter, sour, salty, and perhaps fat [free fatty acids (FFA)] and starch (malto-oligosaccharides). The complex flavors and mouthfeel that we experience while eating food result from the integration of taste, odor, texture, pungency, and temperature. The latter three arise primarily from the somatosensory (trigeminal) system. The sensory organs used for detecting and transducing many chemicals are found in taste buds (TBs) located throughout the tongue, soft palate esophagus, and epiglottis. In parallel with the taste system, the trigeminal nerve innervates the peri-gemmal epithelium to transmit temperature, mechanical stimuli, and painful or cooling sensations such as those produced by changes in temperature as well as from chemicals like capsaicin and menthol, respectively. This article gives an overview of the current knowledge about these TB cells' anatomy and physiology and their trigeminal induced sensations. We then discuss how taste is represented across gustatory cortices using an intermingled and spatially distributed population code. Finally, we review postingestion processing (interoception) and central integration of the tongue-gut-brain interaction, ultimately determining our sensations as well as preferences toward the wholesomeness of nutritious foods. © 2021 American Physiological Society. Compr Physiol 11:1-35, 2021.

Changes in transient receptor potential channels in the rat geniculate ganglion after chorda tympani nerve injury

We reported differential expression of the transient receptor potential vanilloid 1 (TRPV1), the transient receptor potential ankyrin 1 (TRPA1), and the (TRPM8) in the geniculate ganglions (GGs) of naive rats. In medical practice, the chorda tympani nerve (CTN) is injured in some patients during middle-ear surgery, and results in tongue numbness and taste disorder. We investigated changes in the expression of these receptors in GGs after CTN injury. In naive-rat GGs, 11.4, 11.8, and 0.5% of neurons were found to express the TRPV1, the TRPA1, the TRPM8, respectively. At 3 days after CTN injury, 5.2 and 4.0% of activating transcription factor 3-immunoreactive neurons, considered as injured neurons, were found to express the TRPV1 and the TRPA1, respectively. Among activating transcription factor 3-immunonegative neurons, considered as uninjured neurons, 3.9 and 3.8% were found to express the TRPV1 and the TRPA1, respectively. The TRPM8 was not detected in GGs after CTN injury. We found decreased mRNA levels of the TRPV1 and the TRPA1 in all neurons, as well as in uninjured neurons of ipsilateral GGs after CTN injury. CTN injury changes the gene expression in GGs and may have effects on the tongue.

Recurrent facial taste neurons of sea catfish Plotosus japonicus: morphology and organization in the ganglion

This study investigated the morphology of the recurrent facial taste neurons and their organization in the recurrent ganglion of the sea catfish Plotosus japonicus. The recurrent ganglion is independent of the anterior ganglion, which consists of trigeminal, facial and anterior lateral line neurons that send peripheral fibres to the head region. The recurrent taste neurons are round or oval and bipolar, with thick peripheral and thin central fibres, and completely wrapped by membranous layers of satellite cells. Two peripheral nerve branches coursing to the trunk or pectoral fin originate from the recurrent ganglion. The results presented here show that the trunk and pectoral-fin neurons are independently distributed to form various sizes of groups, and the groups are intermingled throughout the ganglion. No distinct topographical relationship of the two nerve branches occurs in the ganglion. Centrally, the trunk and pectoral-fin branches project somatotopically in the anterolateral and intermediate medial regions of the trunk tail lobule of the facial lobe, respectively.

Persistence of attacks of cluster headache after trigeminal nerve root section

Cluster headache is a strictly unilateral headache that occurs in association with cranial autonomic features. We report a patient with a trigeminal nerve section who continued to have attacks. A 59-year-old man described a 14-year history of left-sided episodes of excruciating pain centred on the retro-orbital and orbital regions. These episodes lasted 1-4 h, recurring 2-3 times daily. The attacks were associated with ipsilateral ptosis, conjunctival injection, lacrimation, rhinorrhoea and facial flushing. From 1986 to 1988, he had trials of medications without any benefit. In February 1988, he had complete surgical section of the left trigeminal sensory root that shortened the attacks in length for 1 month without change in their frequency or character. In April 1988, he had further surgical exploration and the root was found to be completely excised; post-operatively, there was no change in the symptoms. From 1988 to 1999, he had a number of medications, including verapamil and indomethacin, all of which were ineffective. Prednisolone 30 mg orally daily rendered the patient completely pain free. Sumatriptan 100 mg orally and 6 mg subcutaneously aborted the attack after approximately 45 and 15 min, respectively. He was completely anaesthetic over the entire left trigeminal distribution. Left corneal reflex was absent. Motor function of the left trigeminal nerve was preserved. Neurological and physical examination was otherwise normal. MRI scan showed a marked reduction in the calibre of the left trigeminal nerve from the nerve root exit zone in the pons to Meckel's cave. An ECG-gated three-dimensional multislab MRI inflow angiogram was performed. No dilatation was observed in the left internal carotid artery during the cluster attack. Blink reflexes were elicited with a standard electrode and stimulus. Stimulation of the left supraorbital nerve produced neither ipsilateral nor contralateral blink reflex response. Stimulation of the right supraorbital nerve produced an ipsilateral response with a mean R2 onset latency of 36 ms and a contralateral response with a mean R2 onset latency of 32 ms. Lack of ipsilateral vessel dilatation makes the role of vascular factors in the initiation of cluster attacks questionable. With complete section of the left trigeminal sensory root the brain would perceive neither vasodilatation nor a peripheral neural inflammatory process; however, the patient continued to have an excellent response to sumatriptan. The case illustrates that cluster headache may be generated primarily from within the brain, and that triptans may have anti-headache effects through an entirely central mechanism.

Expression of activating transcription factor 3 and growth-associated protein 43 in the rat geniculate ganglion neurons after chorda tympani injury

The purpose of this study was to evaluate the degree of damage in the geniculate ganglion and its target organ as a result of chorda tympani (CT) injury. We performed unilateral transection of the rat CT and examined expression of the activating transcription factor 3 (ATF3), a neuronal injury marker, and the growth-associated protein 43 (GAP-43), a regeneration-associated molecule. The mean proportion of ATF3-immunoreactive (ir) neurons in the geniculate ganglion was approximately 32% at 3 days after CT injury, but these neurons were never detected in the naive ganglion. Using in situ hybridization, the mean percentage of GAP-43 mRNA-labeled neurons (signal : noise ratio > or = 10) was observed to have increased significantly to approximately 60% for 1-7 days after CT injury, while that in the naive ganglion was < 15%. The results of morphological studies using scanning electron microscopy and immunohistochemistry indicated that atrophic change and reduction of protein gene-product 9.5-ir fibers in the denervated papillae, mainly in the intragemmal region, were observed after CT injury. Increase in GAP-43 mRNA, suggesting CT axonal regeneration, may have a role in recovery from taste disorders. However, this regenerative process may be involved in abnormal activity in the axotomized neurons or the adjacent intact neurons and so one must not disregard the existence of injured geniculate ganglions when considering the treatment of diseases that cause CT injury.

Presence of gastrin-releasing peptide in neurons of the geniculate ganglion in rat and man

Gastrin-releasing peptide (GRP) was demonstrated by immunohistochemistry and high-performance liquid chromatography to be present in the great majority of neurons in the geniculate ganglion of rat and man. This was in contrast to findings in the sphenopalatine ganglion of rat, where only little GRP was found, and in the otic, trigeminal, glossopharyngeal-vagal and internal carotid ganglia, where no GRP-containing cells could be demonstrated. The peptide was not detected by immunohistochemistry in nerve fibers within structures innervated from the ganglion (tongue, soft palate, retroauricular skin and facial musculature). The retrograde axonal tracer True blue accumulated in these neurons of rat after application not only in the tongue and soft palate but also in the retroauricular skin.

Geniculate neuralgia: the surgical management of primary otalgia

Intractable, unexplained deep-ear pain presents a rare, albeit significant problem in otolaryngological and neurosurgical practice. The authors review their experience with 18 cases of primary otalgia during the past 15 years. A total of 31 surgical procedures were performed. Seventeen patients had sequential rhizotomies and one patient had microvascular decompression alone. Based on the clinical diagnosis, the nerves sectioned were singly or in combination: the nervus intermedius (14 patients), geniculate ganglion (10 patients), ninth nerve (14 patients), 10th nerve (11 patients), tympanic nerve (four patients), and chorda tympani nerve (one patient). Microvascular decompression of the involved nerves was undertaken in nine patients, in whom vascular loops were discovered. Adhesions (six patients), thickened arachnoid (three patients), and benign osteoma (one patient) were other intraoperative abnormalities noted. The overall success of these procedures in providing pain relief was 72.2%, and the mean follow-up period was 3.3 years (range 1 month to 14.5 years). There was no surgical mortality. Expected side effects were: decreased lacrimation, salivation, and taste related to nervus intermedius nerve section, and transient hoarseness and diminished gag related to ninth and 10th nerve section. Four patients developed sequelae consisting of sensorineural hearing loss, vertigo, and transient facial nerve paresis. One patient had a cerebrospinal fluid leak and another developed aseptic meningitis as postoperative complications. Except when primary glossopharyngeal neuralgia is the working diagnosis, a combined posterior cranial fossa-middle cranial fossa approach is recommended for adequate exploration and/or section of the fifth, ninth, and 10th cranial nerves as well as the geniculate ganglion and nervus intermedius.

Central afferent connections and origin of efferent projections of the facial nerve in the chicken (Gallus gallus domesticus)

The central afferent connections and origin of efferent projections of the facial nerve in the adult domestic chicken were studied by anterograde and retrograde transport of horseradish peroxidase from the geniculate ganglion. Ipsilateral afferent projections were traced caudal to the level of entrance of the facial nerve and into tractus solitarius (TS), located dorsomedial to the spinal trigeminal nuclear complex. At several rostrocaudal levels in the medulla, fibers exited from TS and terminated in n. sensorius N. facialis (SVII), and nn. ventrolateralis anterior (Vla) and intermedius anterior (Ia) solitarii. Some axons were followed to n. presulcalis anterior (Pas) solitarii. A separate component terminated in subnucleus interpolaris (ip) of n. descendens nervi trigemini or its medially adjacent reticular formation either by exiting from TS or coursing caudally through the trigeminal complex from entering facial rootlets. Another diffuse component of facial axons ascended dorsally and rostrally from the level of entrance of the facial nerve; these projections dissipated in the pons--some on the dorsomedial border of n. principalis N. trigemini (PrV). Ipsilateral efferent projections were traced through the main genu of the facial nerve to retrogradely labelled somata of pars dorsalis (FMd), pars intermedia (FMi), and pars ventralis (FMv) of n. motorius nervi facialis. A separate group of smaller, multipolar, and spindle-shaped cells (8-25 microns) wedged between n. olivaris superioris (OS) and the caudal end of FMv, rostrally, and extending caudally in ventrolateral medulla were labelled. These small cells contrast with the larger (21-45 microns), oval, round, and multipolar somata of FMv and may correspond, in part, to a parasympathetic n. salivatorius (Sal).