Human tongue neuroanatomy: Nerve supply and motor endplates

Abstract

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.

Keywords: Sihler's stain; acetylcholinesterase staining; breathing; genioglossus muscle; hyoglossus muscle; hypoglossal nerve; inferior longitudinal muscle; innervation; lingual nerve; motor endplates; neuromuscular compartments; palatoglossus muscle; silver stain; speech; styloglossus muscle; superior longitudinal muscle; swallowing; tongue; transverse muscle; vertical muscle.

Fig. 1

Entire nerve map of the adult human tongue. Sihler’s stained specimens exhibited similar branching and distribution patterns of the nerves within the tongue. Note that some tongue muscles such as genioglossus (GG) and posterior transverse (T) and vertical (V) muscles were removed to track individual nerves. The hypoglossal nerve (XII) with its branches was located between the lingual (LN) and glossopharyngeal (IX) nerves in the posterior tongue. The diameters of the main trunks of the three nerves were ranked LN > XII > IX. The XII nerve gave off its lateral (l-XII) and medial (m-XII) branches. The l-XII was derived from the XII as a short single (left side in A) or multiple (right side in A and both sides in B) branches. Note that at the level of the vallate papillae (VP) the m-XII split off a bundle of branches to supply the T/V muscles in the anterior two-thirds of the tongue.

Fig. 2

Hypoglossal nerve (XII), lateral (l-XII) and medial (m-XII) branches. A: Schematic drawing of nerve branching on the left hemi-tongue (ventral view) as shown in B. The branching of the l-XII (horizontal oval) and nerve supply pattern of the IL muscle (vertical oval) are illustrated. In this specimen, the lateral nerve branches arise as a group of small separate branches from the main XII nerve (horizontal oval). Five lateral branches innervate the superior longitudinal (SL), styloglossus (SG), hyoglossus (HG), and the lateral belly of the inferior longitudinal (IL) muscle. The continuation of the XII nerve after the lateral branches arise is the medial division of XII (m-XII). Small branches from the posterior part of the m-XII arise at intervals to innervate the genioglossus (GG), the posterior part of the transverse/vertical (p-T/V) muscles, and the medial belly of the IL muscle. The main trunk of the XII continues anteriorly to innervate the anterior transverse/vertical (a-T/V) muscle group. Lingual nerve (LN) is divided into 6 to 7 main branches outside the tongue. Two or three anterior LN branches with numerous twigs travel anteriorly to the tongue tip. Three middle LN branches pass through the tongue muscles to reach the dorsal mucosa. The most posterior LN branch supplies innervation to the IL muscle and communicates with the l-XII. B: Sihler’s stained left hemi-tongue (ventral view). Ventral view of the area of initial XII nerve branching in the posterior aspect of the human tongue. Sihler’s stained nerves are difficult to see in this low-power view so they are marked by white arrows that closely correspond to the nerve branching seen in A. Note that after the XII nerve enters the tongue it gives off multiple l-XII branches. The earlier branches from the m-XII have been dissected away in this specimen. The dotted oval marks the IL muscle that is seen in more detail in D. LA, lingual artery. 3×. C: Magnification of the horizontal oval in B, showing the multiple branches of the l-XII. Note that the l-XII consists of five (1,2,3,4,5) first-order branches. These nerve branches supplied the longitudinally arranged tongue muscles as described in A. 12×. D: Close-up photograph of the region outlined in B and C (vertical oval), showing the innervation of the IL muscle. The IL muscle (vertical oval in A–C) is grossly separable into medial (m-IL) and lateral (l-IL) bellies: the outer boundaries of the whole muscle are marked by dashed lines while the boundary between the two bellies is marked by a dotted line. The m-IL has been retracted further medially to expose the nerve branches that pass between the two bellies. The fifth l-XII branch has multiple tertiary branches that innervate the l-IL, pass between the bellies, and anastomose with the lingual nerve branches (vertical oval on the top). The m-IL is innervated by distinct branch derived from the m-XII (large arrow). In addition, the posterior branch of the LN also supplies innervation to the IL (A, D; small arrows in D). 12×.

Fig. 3

Schematic illustration of the tongue muscles and their motor innervation as revealed by Sihler’s stain. Lateral view of a sagittally sectioned human tongue, showing the topographic locations of the tongue muscles and the branching and distribution of the XII nerve. Note that the XII nerve gives off four lateral branches (l-XII) to supply the superior longitudinal (SL), styloglossus (SG), hyoglossus (HG), and inferior longitudinal (IL) muscles, respectively. The continuation of the XII after the l-XII arises is its medial branch (m-XII). The m-XII gives off branches in the posterior tongue to supply the genioglossus (GG) and IL muscles. At the edge of the HG muscle, the m-XII turns medially and enters the GG at the boundary between oblique (GGo) and horizontal (GGh) compartments. It passes between the two layers of the GG and ascends up to the transverse and vertical (T/V) muscles from where it turns anteriorly. The dotted lines in the GG, HG and SG indicate the locations of the motor endplate bands in these muscles.

Fig. 4

Motor endplate (MEP) bands in the human tongue muscles. A: Schematic of the human tongue (lateral view), showing the locations and patterns of the MEP bands in the styloglossus (SG), hyoglossus (HG), and genioglossus (GG) muscles as revealed by wholemount AChE staining. Note that the horizontal GG (GGh) has two endplate bands, whereas the oblique GG (GGo) has a single band. The MEP bands in the superior longitudinal (SL), transverse and vertical (T/V) muscles are shown in Figures 5 and 7, respectively. B: Wholemount AChE stained SG muscle. 9×. C: Wholemount AChE stained HG muscle. 9×. Note that either SG or HG has a single MEP band which is composed of numerous MEPs (black dots).

Fig. 5

Innervation of the human superior longitudinal (SL) muscle. A: Lateral view of a Sihler’s stained adult human tongue (left side) that has been microdissected to preserve the nerve branches from the main trunk of the XII nerve to the SL muscle. The red arrows track the path of a nerve branch from the main trunk of XII to its terminal branches within muscle fascicles of the SL. Note that the main trunk of the XII enters from lower right. Immediately after entering the tongue the lateral branches (l-XII) arise (horizontal oval). The XII then continues as the medial division (m-XII) (vertical oval). In this specimen, the first branch of the l-XII contributes innervation to the SL and hyoglossus (HG) muscles as well as a connection to the second lateral branch. Large nerve bundles arise from the third lateral branch to innervate the inferior longitudinal (IL) and the styloglossus (SG) muscles. Other nerves to the posterior SL are not seen through their whole course and are marked with red arrowheads. The branches of the lingual nerve (LN) are marked by green arrows. The white interrupted line indicates the midline of the tongue. 3×. B: Close-up photograph of enclosed region in A clearly shows that in addition to the XII the LN also contributes branches (green arrows) to supply the SL. 6×. C: Magnification of a Sihler’s stained specimen, showing the arrangement of the SL muscle fascicles. Note that the fascicle (white arrow) close to the midline (interrupted line) on each side is much larger than others. Green arrows indicate the transected nerve stumps of the LN. 6×. D: Close-up photograph of same specimen shown in C. Note that each of the SL muscle fascicles is innervated by a distinct XII nerve branch (red arrows) which gives off terminal twigs during its course. Also note that the LN branches (green arrows) supply the SL muscle fascicles and communicate with the XII. 12×. E: Wholemount AChE stained SL muscle fascicles, showing that each muscle fascicle exhibited multiple MEP bands. 12×.

Fig. 6

Neuromuscular compartments within the human genioglossus (GG) muscle. A: A midsagittal section of the tongue and mandible, illustrating the attachments of the GG muscle fibers and the locations of the horizontal (GGh) and oblique (GGo) compartments within the muscle. B, base of the tongue; GH, geniohyoid muscle; M, mandible; T, tip of the tongue. B: A Sihler’s stained GG muscle, showing the nerve supply patterns of the GGh and GGo. Note that the GGh is supplied by a separate primary nerve branch (arrow head), whereas the GGo is innervated by several primary nerve branches (arrows) which form a dense nerve plexus. GH, geniohyoid branch; l-XII, lateral division of the XII nerve; M, mandible; m-XII, medial division of the XII nerve. C: A wholemount AChE stained GG muscle, showing the distribution of the motor endplates (MEPs). Note that the GGh has two MEP bands (thick arrows), whereas the GGo has a single MEP band (thin arrow). D: Schematic of C, illustrating the MEP bands in the GG compartments.

Fig. 7

Neuromuscular organization of the transverse (T) and vertical (V) muscle fiber layers in the human middle tongue. A: A frontally sectioned and AChE stained meat slice from the middle tongue, illustrating the alternating fiber layers of the T and V muscles. The anatomical positions of the individual tongue muscles are outlined. The vertically placed arrows indicate the midline. Note that a T muscle fiber layer is shown on the left side, whereas a V muscle fiber layer can be seen on the right side. GG, genioglossus muscle; HG, hyoglossus muscle; IL, inferior longitudinal muscle; SG, styloglossus muscle; SL, superior longitudinal muscle. Note that most of the extra-lingual GG muscle as indicated by the interrupted lines has been removed. 2×. B: High-power magnification photograph of the squared region in A, showing a superficially located V muscle fiber layer and an underlying T muscle fiber layer. 8×. C: High-power view of Sihler’s stained anterior T and V muscles in Fig. 1. Note that the medial division of the XII nerve is split off numerous primary branches (vertically oriented nerve branches shown on the right aspect of the photo) at this level, forming a dense plexus. The anterior T and V muscles are innervated by the nerve branches derived from this plexus. 12×. D: High-power view of an AChE and silver stained V muscle fiber layer. Note that a secondary nerve branch (arrow) which travels across the muscle fibers gives off several twigs to innervate different muscle fascicles. 16×. E: High-power view of an AChE stained V muscle fiber layer. Note that the V muscle fiber layer has a single motor endplate (MEP) band (arrow). 16×. F: High-power magnification of a T muscle fiber layer processed with AChE and silver stain, showing nerve supply pattern. 16×. G: High-power magnification of a T muscle fiber layer stained for AChE. Note that the T muscle fiber layer has two MEP bands (arrows). 12×.

Fig. 8

Photomicrographs of the single muscle fibers teased from the AChE and silver stained muscles in the human middle tongue, illustrating the types of the motor endplates (MEPs). Note that both en plaque and en grappe types of MEPs are present in the tongue muscles. A: An en plaque MEP with a single preterminal branch (arrow) on a vertical muscle fiber. B: An en grappe MEP with multiple preterminal branches (arrows) on a transverse muscle fiber. C: Two MEPs on a single muscle fiber teased from AChE stained horizontal genioglossus muscle. Bar = 50 µm.