Properties of the lingual and LVP branches of the glossopharyngeal nerve

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Innervation of human soft palate muscles

Our objective was to determine the branching and distribution of the motor nerves supplying the human soft palate muscles. Six adult specimens of the soft palate in continuity with the pharynx, larynx, and tongue were processed with Sihler's stain, a technique that can render large specimens transparent while counterstaining their nerves. The cranial nerves were identified and dissection followed their branches as they divided into smaller divisions toward their terminations in individual muscles. The results showed that both the glossopharyngeal (IX) and vagus (X) nerves have three distinct branches, superior, middle, and inferior. Only the middle branches of each nerve contributed to the pharyngeal plexus to which the facial nerve also contributed. The pharyngeal plexus was divided into two parts, a superior innervating the palatal and neighboring muscles and an inferior innervating pharyngeal constrictors. The superior branches of the IX and X nerves contributed innervation to the palatoglossus, whereas their middle branches innervated the palatopharyngeus. The palatoglossus and palatopharyngeus muscles appeared to be composed of at least two neuromuscular compartments. The lesser palatine nerve not only supplied the palatal mucosa and palatine glandular tissue but also innervated the musculus uvulae, palatopharyngeus, and levator veli palatine. The latter muscle also received its innervation from the superior branch of X nerve. The findings would be useful for better understanding the neural control of the soft palate and for developing novel neuromodulation therapies to treat certain upper airway disorders such as obstructive sleep apnea.

Neuromuscular specializations within human pharyngeal constrictor muscles

OBJECTIVES

At present it is believed that the pharyngeal constrictor (PC) muscles are innervated by the vagus (X) nerve and are homogeneous in muscle fiber content. This study tested the hypothesis that adult human PCs are divided into 2 distinct and specialized layers: a slow inner layer (SIL), innervated by the glossopharyngeal (IX) nerve, and a fast outer layer (FOL), innervated by nerve X.

METHODS

Eight normal adult human pharynges (16 sides) obtained from autopsies were studied to determine 1) their gross motor innervation by use of Sihler's stain; 2) their terminal axonal branching by use of acetylcholinesterase (AChE) and silver stain; and 3) their myosin heavy chain (MHC) expression in PC muscle fibers by use of immunocytochemical and immunoblotting techniques. In addition, the specialized nature of the 2 PC layers was also studied in developmental (newborn, neonate, and senescent humans), pathological (adult humans with idiopathic Parkinson's disease [IPD]), and comparative (nonhuman primate [adult macaque monkey]) specimens.

RESULTS

When nerves IX and X were traced from their cranial roots to their intramuscular termination in Sihler's-stained specimens, it was seen that nerve IX supplied the SIL, whereas branches of nerve X innervated the FOL in the adult human PCs. Use of AChE and silver stain confirmed that nerve IX branches supplying the SIL contained motor axons and innervated motor end plates. In addition to distinct motor innervation, the SIL contained muscle fibers expressing slow-tonic and alpha-cardiac MHC isoforms, whereas the FOL contained muscle fibers expressing developmental MHC isoforms. In contrast, the FOL became obscured in the elderly and in the adult humans with IPD because of an increased proportion of slow muscle fibers. Notably, distinct muscle fiber layers were not found in the human newborn and nonhuman primate (monkey), but were identified in the 2-year-old human.

CONCLUSIONS

Human PCs appear to be organized into functional fiber layers, as indicated by distinct motor innervation and specialized muscle fibers. The SIL appears to be a specialized layer unique to normal humans. The presence of the highly specialized slow-tonic and alpha-cardiac MHC isoforms, together with their absence in human newborns and nonhuman primates, suggests that the specialization of the SIL maybe related to speech and respiration. This specialization may reflect the sustained contraction needed in humans to maintain stiffness of the pharyngeal walls during respiration and to shape the walls for speech articulation. In contrast, the FOL is adapted for rapid movement as seen during swallowing. Senescent humans and patients with IPD are known to be susceptible to dysphagia; and this susceptibility may be related to the observed shift in muscle fiber content.

The transforming growth factor-beta 3 knock-out mouse: an animal model for cleft palate

The recent report of a transforming growth factor-beta 3 (TGF-beta 3) knock-out mouse in which 100 percent of the homozygous pups have cleft palate raised the question as to the potential usefulness of these animals as a model for cleft palate research. The specific aim in this study was to carefully document the anatomy of the cleft palate in the TGF-beta 3 knock-out mice as compared with wild type controls. Special attention was paid to the levator veli palatini muscle, the tensor veli palatini muscle, and their respective innervation. Because the TGF-beta 3 knock-out is lethal in the early perinatal period and because the heterozygotes are phenotypically normal, polymerase chain reaction was required to genotype the animals before mating. Time-mated pregnancies between proven heterozygotes were then delivered by cesarean section at gestational day 18.5 to prevent maternal cannibalism of homozygote pups. All delivered pups were killed and their tails processed by polymerase chain reaction to verify genotype. The heads were then fixed and sectioned in axial, coronal, or sagittal planes. Sections were stained with hematoxylin and eosin or processed for immunohistochemistry with nerve specific protein gene product 9.5 and calcitonin gene-related peptide antibodies. Sections were analyzed in a serial fashion. Nine wild type control animals were analyzed along with nine TGF-beta 3 knock-out homozygotes. Time matings between proven heterozygotes yielded wild type pups, heterozygote pups, and homozygote knock-out pups in the expected mendelian ratios (28 percent to 46 percent to 26 percent; n = 43). The results demonstrated 100 percent clefting in the homozygous TGF-beta 3 knock-out pups. Complete clefting of the secondary palate was seen in four of nine and incomplete clefting was seen in five of nine. The levator veli palatini and tensor veli palatini muscles were demonstrated coursing parallel to the cleft margin in all cleft mice. The orientation of these muscles differs from the normal transverse sling of the levator veli palatini muscle and the normal palatine aponeurosis of the tensor veli palatini muscle at the soft palate in control animals. Innervation of the levator veli palatini muscle by cranial nerve IX and the tensor veli palatini muscle by cranial nerve V were demonstrated in both cleft and control animals by use of immunohistochemistry with nerve-specific antibodies. Demonstration of a teratogen-free, reproducible animal model of clefting of the palate with a known, single-gene etiology is an important step in the systematic understanding of a congenital defect whose multifactorial etiology has hampered previous research efforts. This study presents a detailed anatomic description of such a model, including a description of the muscular anatomy and the innervation of the muscles of the palate. Because of early perinatal mortality, this model has limited applications for postnatal studies.

Main trajectories of nerves that traverse and surround the tympanic cavity in the rat

To guide surgery of nerves that traverse and surround the tympanic cavity in the rat, anatomical illustrations are required that are topographically correct. In this study, maps of this area are presented, extending from the superior cervical ganglion to the otic ganglion. They were derived from observations that were made during dissections using a ventral approach. Major blood vessels, bones, transected muscles of the tongue and neck and supra and infrahyoid muscles serve as landmarks in the illustrations. The course of the mandibular, facial, glossopharyngeal, vagus, accessory and hypoglossal nerves with their branches, and components of the sympathetic system, are shown and discussed with reference to data available in the literature. Discrepancies in this literature can be clarified and new data are presented on the trajectories of several nerves. The course of the tympanic nerve was established. This nerve originates from the glossopharyngeal nerve, enters the tympanic cavity, crosses the promontory, passes the tensor tympani muscle dorsally, and continues its route intracranially to the otic ganglion as the lesser petrosal nerve after intersecting with the greater petrosal nerve. Auricular branches of the glossopharyngeal and of the vagus nerve were noted. We also observed a pterygopalatine branch of the internal carotid nerve, that penetrates the tympanic cavity and courses across the promontory.

Central distribution and peripheral functional properties of afferent and efferent components of the superior laryngeal nerve: morphological and electrophysiological studies in the rat

The central distribution of the afferent and efferent components of the superior laryngeal nerve (SLN), which in the rat is ramified into the three branches of the rostral branch (R.Br), middle branch (M.Br), and caudal branch (C.Br), was examined after application of horseradish peroxidase conjugated with wheat germ agglutinin (HRP-WGA) to the proximal cut end of each branch. In addition, the afferent and efferent neural activities of each branch were recorded to investigate the functional properties. The present study provided several new findings as to the distribution of each branch and the functional properties of the SLN. The following conclusions were drawn: 1) the R.Br, containing only afferent fibers projecting to the ipsilateral lateral region of the nucleus of the solitary tract (NST), extends between slightly below the obex and the region approximately 0.6 mm rostral from the obex, and it corresponds to the interstitial subnucleus of the NST; 2) the M.Br, innervating the cricothyroid muscle, contains only efferent fibers originating ipsilaterally from the motoneurons localized within the ambiguus nucleus (Amb) and in the area ventrolateral to the Amb; and 3) the C.Br, which innervates the inferior pharyngeal constrictor muscle, contains both efferent and afferent fibers. HRP-WGA-labeled cells are distributed within both the Amb and the dorsal motor nucleus of the vagus nerve, ipsilateral to the injection site. Afferent proprioceptive fibers project to the ipsilateral interstitial subnucleus of the NST. The present results provide evidence that each branch of the SLN has distinctive functional properties and contributes to the laryngeal functions.

Proprioceptive representation of the levator veli palatini muscle in the solitary nucleus of the rat

The levator veli palatini muscle is innervated by motoneurons of the glossopharyngeal nerve, which are located within the ambiguus nucleus; however, little is known about the afferent fibers of this muscle. A horseradish peroxidase study was conducted in rats following injection into the levator veli palatini muscle branch to reveal the location and the distribution of dendrites of the afferent fibers of the muscle. Terminal labels were densely distributed in the lateral region of the solitary nucleus, which receives afferents of the glossopharyngeal nerve, ipsilateral and contralateral to the injection site. The relationship of the levator veli palatini muscle with respiration was suggested by the localization of labeled terminals at sites where the afferents from the respiratory organs project densely, and by the demonstrated proprioceptive role of the afferent fibers passing through the muscle spindles contained in the levator veli palatini muscle.

Tactile-evoked response of sensory fibers in buccal and submandibular regions of the rat

Evoked neural responses to tactile stimulation were recorded electrophysiologically from the mechanoreceptive afferent fibers innervating the buccal and submandibular regions of Wistar rats anesthetized with sodium thiopental. Miniature probes 200 microns in diameter were used, and data analysis was performed on the mechanosensitivity of responses to tactile stimulation in the areas innervated by the mental, mylohyoid, auriculotemporal, and cervical nerves. Mechano-sensitivity of each area showed a characteristic distribution of slowly adapting (SA), rapidly adapting (RA), C-fiber (CF), and hair follicle (HF) units in individual receptive fields. The density of the SA units was high in the areas innervated by the mylohyoid and auriculotemporal nerves. The CF units were concentrated in the small dome in the area of the mylohyoid nerve and the auriculotemporal nerve, as shown by a significant response to the dynamic features of stimulation. Estimation of the current needed for tactile acuity suggests an important role of the SA fibers in the areas innervated by the auriculotemporal, mylohyoid, and cervical nerves.

Muscle spindle distribution in the levator veli palatini muscle in the rat

We previously reported that the levator veli palatini muscle (LVP) in the rat is innervated by the glossopharyngeal nerve. The LVP positioned between the mouth and nasopharynx, has important roles in respiration, swallowing, and speech. Muscle spindles, structures scattered through skeletal muscles, appear to function like miniature strain gauges, sensing the degree of tension in the muscle. Muscle spindles were demonstrated in the rat's LVP in our neurophysiologic and histologic studies. We think the stretch of LVP modulates the rapid movements of the LVP by the proprioceptive component of the muscle spindle. Our results infer it is important to protect the innervation and the muscle spindles of the LVP from damage during any surgical dissection of the soft palate musculature.