The intramuscular nerve supply of the human lateral cricoarytenoid muscle

The Pathway from Anatomy and Physiology to Diagnosis: A Developmental Perspective on Swallowing and Dysphagia

Dysphagia results from diverse and distinct etiologies. The pathway from anatomy and physiology to clinical diagnosis is complex and hierarchical. Our approach in this paper is to show the linkages from the underlying anatomy and physiology to the clinical presentation. In particular, the terms performance, function, behavior, and physiology are often used interchangeably, which we argue is an obstacle to clear discussion of mechanism of pathophysiology. We use examples from pediatric populations to highlight the importance of understanding anatomy and physiology to inform clinical practice. We first discuss the importance of understanding anatomy in the context of physiology and performance. We then use preterm infants and swallow-breathe coordination as examples to explicate the hierarchical nature of physiology and its impact on performance. We also highlight where the holes in our knowledge lie, with the ultimate endpoint of providing a framework that could enhance our ability to design interventions to help patients. Clarifying these terms, and the roles they play in the biology of dysphagia will help both the researchers studying the problems as well as the clinicians applying the results of those studies.

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.

Human tongue neuroanatomy: Nerve supply and motor endplates

The human tongue has a critical role in speech, swallowing, and respiration, however, its motor control is poorly understood. Fundamental gaps include detailed information on the course of the hypoglossal (XII) nerve within the tongue, the branches of the XII nerve within each tongue muscle, and the type and arrangement of motor endplates (MEP) within each muscle. In this study, five adult human tongues were processed with Sihler's stain, a whole-mount nerve staining technique, to map out the entire intra-lingual course of the XII nerve and its branches. An additional five specimens were microdissected into individual muscles and stained with acetylcholinesterase and silver staining to study their MEP morphology and banding patterns. Using these techniques the course of the entire XII nerve was mapped from the main nerve to the smallest intramuscular branches. It was found that the human tongue innervation is extremely dense and complex. Although the basic mammalian pattern of XII is conserved in humans, there are notable differences. In addition, many muscle fibers contained multiple en grappe MEP, suggesting that they are some variant of the highly specialized slow tonic muscle fiber type. The transverse muscle group that comprises the core of the tongue appears to have the most complex innervation and has the highest percentage of en grappe MEP. In summary, the innervation of the human tongue has specializations not reported in other mammalian tongues, including nonhuman primates. These specializations appear to allow for fine motor control of tongue shape.

Alpha-synuclein pathology and axonal degeneration of the peripheral motor nerves innervating pharyngeal muscles in Parkinson disease

Parkinson disease (PD) is a neurodegenerative disease primarily characterized by cardinal motor manifestations and CNS pathology. Current drug therapies can often stabilize these cardinal motor symptoms, and attention has shifted to the other motor and nonmotor symptoms of PD that are resistant to drug therapy. Dysphagia in PD is perhaps the most important drug-resistant symptom because it leads to aspiration and pneumonia, the leading cause of death. Here, we present direct evidence for degeneration of the pharyngeal motor nerves in PD. We examined the cervical vagal nerve (cranial nerve X), pharyngeal branch of nerve X, and pharyngeal plexus innervating the pharyngeal muscles in 14 postmortem specimens, that is, from 10 patients with PD and 4 age-matched control subjects. Synucleinopathy in the pharyngeal nerves was detected using an immunohistochemical method for phosphorylated α-synuclein. Alpha-synuclein aggregates were revealed in nerve X and the pharyngeal branch of nerve X, and immunoreactive intramuscular nerve twigs and axon terminals within the neuromuscular junctions were identified in all of the PD patients but in none of the controls. These findings indicate that the motor nervous system of the pharynx is involved in the pathologic process of PD. Notably, PD patients who have had dysphagia had a higher density of α-synuclein aggregates in the pharyngeal nerves than those without dysphagia. These findings indicate that motor involvement of the pharynx in PD is one of the factors leading to oropharyngeal dysphagia commonly seen in PD patients.

Sihler's whole mount nerve staining technique: a review

Sihler's stain is a whole mount nerve staining technique that renders other soft tissue translucent or transparent while staining the nerves. It permits mapping of entire nerve supply patterns of organs, skeletal muscles, mucosa, skin, and other structures after the specimens are fixed in neutralized formalin, macerated in potassium hydroxide, decalcified in acetic acid, stained in Ehrlich's hematoxylin, destained in acetic acid, and cleared in glycerin. The unique advantage of Sihler's stain over other anatomical methods is that all the nerves within the stained specimen can be visualized in their three-dimensional positions. To date, Sihler's stain is the best tool for demonstrating the precise intramuscular branching and distribution patterns of skeletal muscles, which are important not only for anatomists, but also for physiologists and clinicians. Advanced knowledge of the neural structures within mammalian skeletal muscles is critical for understanding muscle functions, performing electrophysiological experiments and developing novel neurosurgical techniques. In this review, Sihler's stain is described in detail and its use in nerve mapping is surveyed. Special emphasis is placed on staining procedures and troubleshooting, strengths and limitations, applications, major contributions to neuroscience, physiological and clinical significance, and areas for further technical improvement that deserve future research.

Functional anatomy of the recurrent and superior laryngeal nerve

BACKGROUND AND AIMS

The purpose of this study was to present the current topographic and anatomical knowledge in neurolaryngology, with special regard to laryngeal paralyses as a major complication in thyroid surgery.

PATIENTS AND METHODS

Microscopic anatomical preparation of 22 human hemilarynges was accomplished.

RESULTS

Due to their neuroanatomical courses, the following extralaryngeal nerves may be at risk in thyroid surgery: the external branch of the superior laryngeal nerve, the paralaryngeal part of the vagal nerve, the Ansa Galeni, the trunk of the recurrent laryngeal nerve (RLN) and the delicate branches of the RLN to the posterior cricoarytaenoid muscle. The anterior and posterior branches of the RLN (antRLN and postRLN) are less endangered by thyroid surgery because they are covered by the thyroid cartilage and posterior cricoarytaenoid muscle (PCA), respectively. In contrast, the antRLN is vulnerable if a ventilation tube is dislocated, with cuff-induced pressure to the glottic level.

CONCLUSION

The increased knowledge in neurolaryngology provides the basis for a selective neuromonitoring to lower the risk of laryngeal paralyses after thyroid surgery.

Variability in nerve patterns of the adductor muscle group supplied by the recurrent laryngeal nerve

INTRODUCTION

Accurate knowledge of the nerve supply of each individual muscle is needed to achieve a successful selective reinnervation of the larynx. The aim of the present work was to study the nerve supply of the adductor laryngeal muscles supplied by the recurrent laryngeal nerve.

STUDY DESIGN

Morphologic study of human larynges.

METHODS

The muscular nerve supply was studied in a total sample of 75 human larynges obtained from necropsies (47 males and 28 females, age range from 41-95 years) and examined by careful dissection using a surgical microscope.

RESULTS

The arytenoid muscle received one branch from each recurrent nerve. In 88% of cases, this branch arose in a common trunk with the upper branch of the posterior cricoarytenoid muscle. In 8% of cases, the nerve for the arytenoid muscle also had a branch going to the lateral cricoarytenoid muscle. The arytenoid muscle also received from one to three pairs of branches from the posterior division of the internal laryngeal nerve; these were interconnected ipsi- and contralaterally and were also connected to the two branches coming from the recurrent laryngeal nerve. The lateral cricoarytenoid muscle received from one to six branches from the recurrent nerve, but in 5.8% of cases, it also received a twig from a connecting branch between the recurrent nerve and the external (5.6%) or the internal laryngeal nerves (0.2%). The thyroarytenoid muscle received from one to four branches from the recurrent nerve, but in 5.6% of cases, it also received a twig from a connecting branch between the recurrent nerve with the external (4.6%) or the internal (1%) laryngeal nerves.

CONCLUSION

No abductor or adductor division of the recurrent laryngeal nerve was found in the present study. In 88% of cases, the nerve supply to the arytenoid muscle (adductor) and the posterior cricoarytenoid muscle (abductor) arose from a common trunk, which in 8% of cases, also had a branch to the lateral cricoarytenoid muscle. Furthermore, the high incidence of branches innervating the adductor muscles from connections between the recurrent laryngeal nerve and the internal and external laryngeal nerves led us to reconsider the contribution of these nerves in the supply to this muscle group.

Intralaryngeal neuroanatomy of the recurrent laryngeal nerve of the rabbit

We undertook this study to determine the detailed neuroanatomy of the terminal branches of the recurrent laryngeal nerve (RLN) in the rabbit to facilitate future neurophysiological recordings from identified branches of this nerve. The whole larynx was isolated post mortem in 17 adult New Zealand White rabbits and prepared using a modified Sihler's technique, which stains axons and renders other tissues transparent so that nerve branches can be seen in whole mount preparations. Of the 34 hemi-laryngeal preparations processed, 28 stained well and these were dissected and used to characterize the neuroanatomy of the RLN. In most cases (23/28) the posterior cricoarytenoid muscle (PCA) was supplied by a single branch arising from the RLN, though in five PCA specimens there were two or three separate branches to the PCA. The interarytenoid muscle (IA) was supplied by two parallel filaments arising from the main trunk of the RLN rostral to the branch(es) to the PCA. The lateral cricoarytenoid muscle (LCA) commonly received innervation from two fine twigs branching from the RLN main trunk and travelling laterally towards the LCA. The remaining fibres of the RLN innervated the thyroarytenoid muscle (TA) and comprised two distinct branches, one supplying the pars vocalis and the other branching extensively to supply the remainder of the TA. No communicating anastomosis between the RLN and superior laryngeal nerve within the larynx was found. Our results suggest it is feasible to make electrophysiological recordings from identified terminal branches of the RLN supplying laryngeal adductor muscles separate from the branch or branches to the PCA. However, the very small size of the motor nerves to the IA and LCA suggests that it would be very difficult to record selectively from the nerve supply to individual laryngeal adductor muscles.

Three-dimensional anatomic characterization of the canine laryngeal abductor and adductor musculature

The biomechanics of vocal fold abduction and adduction during phonation, respiration, and airway protection are not completely understood. Specifically, the rotational and translational forces on the arytenoid cartilages that result from intrinsic laryngeal muscle contraction have not been fully described. Anatomic data on the lines of action and moment arms for the intrinsic laryngeal muscles are also lacking. This study was conducted to quantify the 3-dimensional orientations and the relative cross-sectional areas of the intrinsic abductor and adductor musculature of the canine larynx. Eight canine larynges were used to evaluate the 3 muscles primarily responsible for vocal fold abduction and adduction: the posterior cricoarytenoid, the lateral cricoarytenoid, and the interarytenoid muscles. Each muscle was exposed and divided into discrete fiber bundles whose coordinate positions were digitized in 3-dimensional space. The mass, length, relative cross-sectional area, and angle of orientation for each muscle bundle were obtained to allow for the calculations of average lines of action and moment arms for each muscle. This mapping of the canine laryngeal abductor and adductor musculature provides important anatomic data for use in laryngeal biomechanical modeling. These data may also be useful in surgical procedures such as arytenoid adduction.

Sensory nerve supply of the human oro- and laryngopharynx: a preliminary study

To date, the details of human sensory innervation to the pharynx and upper airway have not been demonstrated. In this study, a single human oro- and laryngopharynx obtained from autopsy was processed with a whole-mount nerve staining technique, Sihler's stain, to determine its entire sensory nerve supply. The Sihler's stain rendered all mucosa and soft tissue translucent while counterstaining nerves. The stained specimen was then dissected and the nerves were traced from their origins to the terminal branches. It was found that the sensory innervation of the human pharynx is organized into discrete primary branches that innervate specific areas, although these areas are often connected by small neural anastomoses. The density of innervation varied, with some areas receiving almost no identifiable nerve supply (e.g., posterior wall of the hypopharynx) and certain areas contained much higher density of sensory nerves: the posterior tonsillar pillars; the laryngeal surface of the epiglottis; and the postcricoid and arytenoid regions. The posterior tonsillar pillar was innervated by a dense plexus formed by the pharyngeal branches of the IX and X nerves. The epiglottis was densely innervated by the internal superior laryngeal nerve (ISLN) and IX nerve. Finally, the arytenoid and postcricoid regions were innervated by the ISLN. The postcricoid region had higher density of innervation than the arytenoid area. The use of the Sihler's stain allowed the entire sensory nerve supply of the pharyngeal areas in a human to be demonstrated for the first time. The areas of dense sensory innervation are the same areas that are known to be the most sensitive for triggering reflex swallowing or glottic protection. The data would be useful for further understanding swallowing reflex and guiding sensory reinnervation of the pharynx to treat neurogenic dysphagia and aspiration disorders.

Neuromuscular organization of the canine tongue

The tongue manipulates food while chewing and swallowing, dilates the airway during inspiration, and shapes the sounds of speech in humans. While performing these functions the tongue morphs through many complex shapes. At present it is not known how the muscles of the tongue perform these complex shape changes. The difficulty in understanding tongue biomechanics is partly due to gaps in our knowledge regarding the complex neuromuscular anatomy of the tongue. In this study the motor and sensory nerve anatomy of four canine tongues was studied with Sihler's stain, a technique that renders most of the tongue tissue translucent while counterstaining nerves. An additional tongue specimen was serially sectioned to provide a reference for the muscle structure of the tongue. The hypoglossal nerve (XII) has approximately 50 primary nerve branches that innervate all intrinsic and extrinsic tongue muscles. Two extrinsic muscles, the styloglossus and hyoglossus, are innervated by about three to four branches from the lateral division of the XII. The third extrinsic muscle, the genioglossus, is composed of oblique and horizontal compartments, which receive about ten nerve branches from the medial division of the XII. The intrinsic muscles are composed of many neuromuscular compartments. On each side, the superior longitudinal muscle had an average of 40 distinct muscle fascicles that spanned the length of the tongue. Each of the fascicles is supplied by a nerve branch. The inferior longitudinal muscle had a similar organization. Each of the transverse and vertical muscles is composed of over 140 separate muscle sheets, and every sheet is innervated by a separate terminal nerve. The muscle sheets from the vertical and transverse alternate their orientation 90 degrees throughout the length of the tongue. It is concluded that the intrinsic canine tongue muscles are actually composed of groups of neuromuscular compartments that are arranged in parallel (longitudinal muscles) or in a precise alternating sequence (transverse and vertical muscles). This arrangement suggests that the compartments from the different tongue muscles could cooperate to control the three-dimensional contractile state of their local area. This hypothesis could explain how many different tongue shapes are formed, and is supported by physiologic evidence.

Anatomy of the human internal superior laryngeal nerve

The mucosa of the larynx contains one of the most dense concentrations of sensory receptors in the human body. This sensitivity is used for reflexes that protect the lungs, and even momentary loss of this function is followed rapidly by life-threatening pneumonia. The internal superior laryngeal nerve (ISLN) supplies the innervation to this area, and, to date, the distribution and branching pattern of this nerve is unknown. Five adult human larynges were processed by using Sihler's stain, a technique that clears soft tissue while counterstaining nerves. The whole-mount specimens were then dissected to demonstrate the branching of the ISLN from its main trunk down to the level of terminal axons. The human ISLN is divided into three divisions: The superior division supplies mainly the mucosa of the laryngeal surface of the epiglottis; the middle division supplies the mucosa of the true and false vocal folds and the aryepiglottic fold; and the inferior division supplies the mucosa of the arytenoid region, subglottis, anterior wall of the hypopharynx, and upper esophageal sphincter. Several dense sensory plexi that cross the midline were seen on the laryngeal surface of the epiglottis and arytenoid region. The human ISLN also appears to supply motor innervation to the interarytenoid (IA) muscle. A detailed map is presented of the distribution of the ISLN within the human larynx. The areas seen to receive the greatest innervation are the same areas that have been shown by physiological experiments to be the most sensate: the laryngeal surface of the epiglottis, the false and true vocal folds, and the arytenoid region. The observation that the human ISLN appears to supply motor innervation to the IA muscle is contrary to current concepts of the ISLN as a purely sensory nerve. These findings are relevant to understanding how the laryngeal protective reflexes work during activities like swallowing. The nerve maps can be used to guide surgical attempts to reinnervate the laryngeal mucosa when sensation is lost due to neurological disease.

Neuromuscular organization of the human upper esophageal sphincter

The upper esophageal sphincter (UES) is a key component of swallowing, and yet, its anatomy and function are still incompletely understood. The UES is a functional entity that is composed of three muscles: the cricopharyngeal (CP) muscle, the inferior pharyngeal constrictor (IPC) muscle, and the upper esophageal (UE) muscle. This study compared the anatomy of the three muscles of the UES in nine human autopsy specimens. The variables examined included the pattern of motor end plates (acetylcholinesterase stain), the proportion of fast- and slow-twitch muscle fibers (myofibrillar adenosinetriphosphatase), and the details of their nerve supply (Sihler's stain). The results demonstrated that each variable is different in the three muscles. For example, the IPC muscle is innervated by the pharyngeal plexus, the CP muscle by both the pharyngeal plexus and the recurrent laryngeal nerve (RLN), and the UE muscle by the RLN. The IPC and CP muscles showed distinct motor end plate bands, while the horizontal part of the CP muscle also contained small and randomly scattered end plates. This latter pattern was present throughout the UE muscle. Analysis of the muscle fiber types of the UES revealed a type I (slow) predominance (89%) in the CP and UE muscles and a type II (fast) predominance (62%) in the IPC muscle. However, the IPC muscle is composed of two layers: a fast, thick, outer layer (90% type II) and a slow, thin, inner layer (85% type I). The implications of these findings for the diagnosis and treatment of UES dysfunction will be discussed.

Neuromuscular specializations of the pharyngeal dilator muscles: I. Compartments of the canine geniohyoid muscle

BACKGROUND

Little is known about the structure and innervation of the geniohyoid muscle (GH), which is an important pharyngeal dilator muscle activated in swallowing and respiration.

METHODS

The neuromuscular specializations of the canine GH were studied in detail by using a combination of histological, histochemical, and anatomical techniques. First, hematoxylin and eosin staining, Gomori's rapid one-step trichrome stain, and silver impregnation were used to determine the terminations of muscle fibers and existence of fibrous septa within the muscle (n = 8). Second, myofibrillar ATPase staining was employed to document the muscle fiber type distribution (n = 8). Finally, Sihler's stain (n = 10) and wholemount acetylcholinesterase staining (n = 8) were used to examine the distribution of the nerve supply within the muscle.

RESULTS

The canine GH is divided into rostral and caudal compartments, which are arranged in series and separated by a transverse fibrous septum. Each compartment receives its own primary nerve branch, which supplies a separate motor endplate zone. The rostral compartment is innervated bilaterally, whereas the caudal compartment is innervated ipsilaterally. The rostral compartment was composed of significantly more type I (slow twitch) muscle fibers (56%) than the caudal compartment (25%).

CONCLUSIONS

The canine GH is composed of two in-series neuromuscular compartments rather than a single muscle as traditionally believed. This anatomical finding suggests that these two compartments may function independently under different physiological conditions.

The innervation of the human upper esophageal sphincter

The neuroanatomy and physiology of the human upper esophageal sphincter (UES) has long been controversial. As a result, there has been little progress in diagnosing and treating dysphagias involving this area. In this study, three specimens of the UES obtained from human autopsies were examined by Sihler's stain. This stain clears soft tissue while counterstaining the nerves, thereby allowing nerve supply to each muscle of the UES to be demonstrated. It was found that the nerve supply to each component of the UES is substantially different. The inferior pharyngeal constrictor (IPC) is supplied by a dense linear plexus which is about 1.0-1.5 cm wide and 10 cm long and located about 1.5 cm lateral to the attachment of the IPC on the thyroid lamina. The cricopharyngeal (CP) muscle receives its innervation from below via the recurrent laryngeal nerve (RLN) and from above via the pharyngeal plexus. Neural connections between the RLN and the pharyngeal plexus were observed. Finally, the upper esophagus (UE) is innervated by the RLN. The innervation pattern of each component of the UES suggests functional differences between these muscles. These observations help clarify the innervation of the UES. Accurate knowledge of the neuroanatomy of the UES is necessary for advances in diagnosis and treatment of pharyngeal dysphagia.