In recent years dysphagia or age-induced functional reductions in ingestion have become relevant to clinical issues, and oropharyngeal movements from mastication to swallowing are being closely examined to solve these problems. The mechanism of dysphagia is thus being clarified.

It is well known that the pharyngeal constrictor muscles are related to swallowing. The pharyngeal constrictor muscles that form the posterior wall of the pharynx consist of three muscles, namely, superior, middle, and inferior pharyngeal constrictor muscles [1–8]. Previous anatomical studies have sought to ascertain the morphologic and muscular properties of the inferior pharyngeal constrictor muscle adjacent to the esophagus [9–11]. In those studies, movements of this muscle were discussed from the perspective of anatomical structures. On the other hand, the superior pharyngeal constrictor muscle adjacent to the oral cavity shows a complicated morphology with four origins and one insertion. This muscle can be divided into four parts based on the origin: the pterygopharyngeal part which originates from the pterygoid hamulus of the sphenoid bone; the buccopharyngeal part which originates from the pterygomandibular raphe; the mylopharyngeal part which originates from the mylohyoid line; and the glossopharyngeal part which originates from the tongue [4, 7, 8, 12]. An anatomical study on this muscle was reported by Howland and Brodie [13]. They observed the pterygomandibular raphe at the buccopharyngeal part and described it as “the sphenoid tendon between the jaws.” Also, Brand and Isselhard [6] reported that the raphe is a tendinous band and Williams [4] described the raphe as tendinous fibers. Sicher [14] reported that the raphe forms a tendinous structure, and, except for this intermediate area, a continuous sphincter muscle-like band is formed. However, Gaughran [15] reported that the pterygomandibular raphe is missing in all cases, and the raphe that had been described was an anatomical artifact. Therefore, the conclusion does not yet become clear.

With respect to the other parts of the superior pharyngeal constrictor muscle, the morphology of the glossopharyngeal part has been reported in recent years [16]. The morphology of the pterygopharyngeal part and the mylopharyngeal part, however, has remained unclear. In other words, there are few studies on the total picture of the superior pharyngeal constrictor muscle.

The aim of this study was to clarify the morphologic characteristics of the superior pharyngeal constrictor muscle by gross anatomical observation, which was conducted to ascertain the origins, insertion, and muscle alignment of the four parts.

Materials and Methods

Thirty-seven cadavers of Japanese adults (24 males and 13 females; age range = 53–91 years, mean = 73 years) provided for anatomical practice to the Department of Anatomy at Tokyo Dental College were used. The cadavers were fixed with 10% formaldehyde.

Dissection of the Superior Pharyngeal Constrictor Muscle

The facial muscles, except the buccinator muscle, masticatory muscles, zygomatic arch, and part of the mandibular ramus, were removed. Tissue on the posterior side of the cervical vertebrae was also removed. The superior pharyngeal constrictor muscle was then observed from the facial side (Fig. 1). Next, the head was sliced into median sagittal sections, and the oral mucosa was removed. The superior pharyngeal constrictor muscle was then observed from the oral side.

Fig. 1.
figure 1

Superior pharyngeal constrictor muscle as observed from the facial side. SPC = superior pharyngeal constrictor muscle, B = buccinator muscle, M = mandible, SG = styloglossus muscle.

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Gross Anatomical Observation of the Superior Pharyngeal Constrictor Muscle

Morphology of the Origin

  1. (1)

    Pterygopharyngeal part. Morphology of the origin of the muscle at the pterygopharyngeal part was observed from the facial side.

  2. (2)

    Buccopharyngeal part. Morphology of the origin of the muscle at the buccopharyngeal part was observed from the facial side.

  3. (3)

    Mylopharyngeal part. Morphology of the origin of the muscle at the mylopharyngeal part was observed from the oral side.

  4. (4)

    Glossopharyngeal part. Morphology of the origin of the muscle at the glossopharyngeal part was observed from the oral side.

Morphology of the Insertion

The insertion of the pharyngeal constrictor muscles at the posterior pharyngeal wall was observed from the facial side.

Histological Observation of the Superior Pharyngeal Constrictor Muscle at the Buccopharyngeal Part

The superior pharyngeal constrictor muscle at the buccopharyngeal part (which originates from the fourth and sixth branchial arches) comes in contact with the buccinator muscle (which originates from the second branchial arch), which has a different embryologic origin. To clarify this relationship, macroscopic and histologic analyses were performed. After macroscopic observation, the buccinator muscle and superior pharyngeal constrictor muscle at the buccopharyngeal part were removed in a single mass. The mass was fixed again in 10% formaldehyde and embedded in paraffin according to conventional methods. Then, 8-μm serial sections parallel to the alignment of muscle fibers from facial to oral sides were prepared. These sections were subjected to Azan staining and then observed under light microscopy.

Results

Although 24 male and 13 female cadavers were used as the subjects in this study, no gender differences in morphology of the four parts of the superior pharyngeal constrictor muscle were identified. Therefore, the numbers of subjects belonging to each type at the origin and the insertion of the superior pharyngeal constrictor muscle were counted without taking gender into consideration.

Gross Anatomical Observation of the Superior Pharyngeal Constrictor Muscle

Morphology of the Origin

  1. (1)

    Pterygopharyngeal part. The most superior area of the superior pharyngeal constrictor muscle corresponded to the pterygopharyngeal part. The muscle at this part originated between the pterygoid hamulus of the sphenoid bone and the posterior margin of the medial pterygoid plate of the sphenoid bone. Immediately after the origin, the muscle gradually narrowed and passed the lateral inferior area of the pharyngobasilar fascia. This muscle then passed along the posteroinferior part of the pharyngobasilar fascia. In almost all specimens analyzed in the present study, the abovementioned origin and alignment were seen, and no marked differences were noted (Fig. 2)

  2. (2)

    Buccopharyngeal part. The superior pharyngeal constrictor muscle at the buccopharyngeal part merged with the superior pharyngeal constrictor muscle at the pterygopharyngeal part originating superiorly and the mylopharyngeal part originating inferiorly to form the lateral and posterior walls of the epipharynx (Fig. 1).

    Morphology of the origin of the muscle at the buccopharyngeal part was divided into three types: type A, membranous connective tissue from superior to inferior areas (Fig. 3); type B, membranous only in superior area (Fig. 4); and type C, complete lack of membrane from superior to inferior areas (Fig. 5). The 54 samples consisted of 7 type A cases (13.0%), 10 type B cases (18.5%), and 37 type C cases (68.5%).

  3. (3)

    Mylopharyngeal part. The superior pharyngeal constrictor muscle at the mylopharyngeal part originated from the posterior area of the mylohyoid line on the lingual side of the retromolar pad (Fig. 6). The morphology of the origin of the muscle at this part was divided into two types: type A, tip of the origin on the mylohyoid line (Fig. 7) and type B, tip of the origin away from the mylohyoid line (Fig. 8). The 44 samples consisted of 32 type A cases (72.7%) and 12 type B cases (27.3%).The muscle of the mylopharyngeal part merged with the muscle of the buccopharyngeal part at the upper margin of the retromolar pad immediately after the origin and formed the lateral and posterior walls of the pharynx. In addition, the muscle at this part after the origin was thinner than at the other three parts until merging with the muscle at the buccopharyngeal part.

  4. (4)

    Glossopharyngeal part. The superior pharyngeal constrictor muscle at the glossopharyngeal part originated from the root of the tongue in all cases. The palatoglossus muscle, which originates from the palatine aponeurosis, inserted at the superior area of the origin of the muscle at this part. At the lateral inferior margin of the origin of the muscle at this part, the styloglossus muscle was aligned in parallel. The muscle at this part formed the root of the tongue with the palatoglossus and styloglossus muscles (Fig. 9). Immediately after the origin, the muscle at this part either interlaced or merged with the muscle at the mylopharyngeal part near the palatine tonsil. The state of connection of the two parts could be divided into two types: interlaced type with muscle fibers in both parts interlaced and indistinguishable, and merged type with muscle fibers in both parts merged without interlacing. The 43 samples consisted of 28 interlaced type cases (65.1%) and 15 merged type cases (34.9%) (Figs. 10, 11).

Fig. 2.
figure 2

The superior pharyngeal constrictor muscle at the pterygopharyngeal part as observed from the facial side. The muscle at the pterygopharyngeal part originates between the pterygoid hamulus of the sphenoid bone and the posterior margin of the medial pterygoid plate of the sphenoid bone. 1 = superior pharyngeal constrictor muscle at the pterygopharyngeal part, S = sphenoid bone.

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Fig. 3.
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Type A as observed from the facial side. The origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part comprises membranous connective tissue from the superior to inferior areas. 2 = superior pharyngeal constrictor muscle at the buccopharyngeal part, B = buccinator muscle, arrow = membranous.

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Fig. 4.
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Type B as observed from the facial side. Only the superior area of the origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part is membranous. The inferior area of the superior pharyngeal constrictor muscle at the buccopharyngeal part transitionally originates from the buccinator muscle. 2 = superior pharyngeal constrictor muscle at the buccopharyngeal part, B = buccinator muscle, arrow = membranous.

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Fig. 5.
figure 5

Type C as observed from the facial side. The origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part completely lacks membrane from the superior to inferior areas. From the superior to inferior areas of the origin, the superior pharyngeal constrictor muscle at the buccopharyngeal part transitionally originates from the buccinator muscle. 2 = superior pharyngeal constrictor muscle at the buccopharyngeal part, B = buccinator muscle.

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Fig. 6.
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Superior pharyngeal constrictor muscle as observed from the oral side. 3 = superior pharyngeal constrictor muscle at the mylopharyngeal part, 4 = superior pharyngeal constrictor muscle at the glossopharyngeal part, M = mandible, MH = mylohyoid muscle, SG = styloglossus muscle.

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Fig. 7.
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Morphology of the origin of the superior pharyngeal constrictor muscle at the mylopharyngeal part (type A). The tip of the origin of the superior pharyngeal constrictor muscle at the mylopharyngeal part lies on the mylohyoid line. 3 = superior pharyngeal constrictor muscle at the mylopharyngeal part, M = mandible, MH = mylohyoid muscle, arrow = tip of the origin.

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Fig. 8.
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Morphology of the origin of the superior pharyngeal constrictor muscle at the mylopharyngeal part (type B). The tip of the origin of the superior pharyngeal constrictor muscle at the mylopharyngeal part lies away from the mylohyoid line. 3 = superior pharyngeal constrictor muscle at the mylopharyngeal part, M = mandible, MH = mylohyoid muscle, arrow = tip of the origin.

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Fig. 9.
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Morphology of the origin of the superior pharyngeal constrictor muscle at the glossopharyngeal part. The superior pharyngeal constrictor muscle at the glossopharyngeal part forms the root of the tongue with the palatoglossus and styloglossus muscles. 4 = superior pharyngeal constrictor muscle at the glossopharyngeal part, PG = palatoglossus muscle, SG = styloglossus muscle.

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Fig. 10.
figure 10

Enlarged image of the superior pharyngeal constrictor muscle at the glossopharyngeal and mylopharyngeal parts (interlaced type). The superior pharyngeal constrictor muscle at the glossopharyngeal and mylopharyngeal parts interlace immediately after the origin. 3 = superior pharyngeal constrictor muscle at the mylopharyngeal part, 4 = superior pharyngeal constrictor muscle at the glossopharyngeal part.

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Fig. 11.
figure 11

Enlarged image of the superior pharyngeal constrictor muscle at the glossopharyngeal and mylopharyngeal parts (merged type). The superior pharyngeal constrictor muscle at the glossopharyngeal and mylopharyngeal parts merge immediately after the origin. 3 = superior pharyngeal constrictor muscle at the mylopharyngeal part, 4 = superior pharyngeal constrictor muscle at the glossopharyngeal part.

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Morphology of the Insertion

The superior pharyngeal constrictor muscle originated from four parts that merged immediately after the origin and formed the superior lateral wall of the pharynx. This muscle then passed the upper margin of the stylopharyngeus muscle and merged with the opposite side of the superior pharyngeal constrictor muscle to form the insertion.

Morphology of the insertion of the muscle could be divided into two types: type A, membranous connective tissue (Fig. 12) and type B, merging of right and left pharyngeal constrictor muscles (Fig. 13). The 37 cases consisted of 21 type A cases (56.8%) and 16 type B cases (43.2%). When observing the pharyngeal constrictor muscle from the posterior direction, the middle and inferior pharyngeal constrictor muscles aligned obliquely and inserted, while the superior pharyngeal constrictor muscle aligned horizontally and inserted. The dorsal inferior area of the superior pharyngeal constrictor muscle was covered by the middle pharyngeal constrictor muscle.

Fig. 12.
figure 12

Morphology of the insertion of the superior pharyngeal constrictor muscle as observed from the posterior direction (type A). The posterior wall of the pharynx comprises membranous connective tissue. SPC = superior pharyngeal constrictor muscle, MPC = middle pharyngeal constrictor muscle, SP = stylopharyngeus muscle, arrow = membranous.

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Fig. 13.
figure 13

Morphology of the insertion of the superior pharyngeal constrictor muscle as observed from the posterior direction (type B). The posterior wall of the pharynx comprises right and left superior pharyngeal constrictor muscles. SPC = superior pharyngeal constrictor muscle, MPC = middle pharyngeal constrictor muscle, SP = stylopharyngeus muscle.

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Histological Observation of the Superior Pharyngeal Constrictor Muscle at the Buccopharyngeal Part

The three morphologic types (type A: membranous connective tissue from superior to inferior areas; type B: membranous only in superior area; and type C: complete lack of membrane from superior to inferior areas) were subjected to Azan staining and analyzed histologically.

The results showed that type A consisted of connective tissue fibers with sporadic muscle fibers (Fig. 14), type B consisted of rough connective tissue fibers in the superior area and transverse muscle fibers in the inferior area (Fig. 15), and type C displayed fewer connective tissue fibers and was composed of the buccinator muscle and the superior pharyngeal constrictor muscle. These two muscles were not continuous and were separated (Fig. 16).

Fig. 14.
figure 14

Histologic image of origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part (type A, Azan stain). Muscle fibers are sporadically observed among rough connective tissue fibers.

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Fig. 15.
figure 15

Histologic image of the origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part (type B, Azan stain). Rough connective tissue fibers are apparent in the superior area, and transverse muscle fibers are seen in the inferior area. 2 = superior pharyngeal constrictor muscle at the buccopharyngeal part, B = buccinator muscle.

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Fig. 16.
figure 16

Histologic image of the origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part (type C, Azan stain). Muscle fibers of the superior pharyngeal constrictor muscle at the buccopharyngeal part and buccinator muscle are not continuous and were separated. 2 = superior pharyngeal constrictor muscle at the buccopharyngeal part, B = buccinator muscle.

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Discussion

The superior pharyngeal constrictor muscle of the pterygopharyngeal part formed the most superior area of the pharynx. Some reports have the muscle originating from the pterygoid hamulus of the sphenoid bone [5–8, 12]. However, the present study shows that the muscle originates over a broad area of the posterior margin of the medial pterygoid plate of the sphenoid bone.

Morphology of the origin of the superior pharyngeal constrictor muscle at the buccopharyngeal part could be divided into three types: type A, membranous connective tissue from superior to inferior areas; type B, membranous only in the superior area; and type C, complete lack of membrane. The muscle at this part originates from the pterygomandibular raphe, which is also the origin of the buccinator muscle. In past reports, there were different descriptions of the pterygomandibular raphe such as “the sphenoid tendon between the jaws,” a tendinous band, tendinous fibers, and a tendinous structure [4, 6, 13, 14]. On the other hand, Gaughran [15] stated that the pterygomandibular raphe was missing in all his cases, and the raphe that had been described was an anatomical artifact. In the present study, no tendon was observed. Gaughran also stated that the buccinator and superior pharyngeal constrictor muscles are continuous and form the buccinatopharyngeus muscle, and the existing concept of the raphe must be disregarded. This is the same as type C observed by the gross anatomical study. However, the histologic study showed that type C was not continuous and was separated. Shimada and Gasser [17] stated that the pterygomandibular raphe of fetuses was divided into the buccinator and superior pharyngeal constrictor muscles by a broad fascia, and the morphology of the raphe changed after birth. The Shimada and Gasser report and the present results suggest that functions such as mastication, swallowing, and articulation develop after birth, and because the superior pharyngeal constrictor muscle at the buccopharyngeal part and buccinator muscle act together, the morphology of the pterygomandibular raphe changes to suit particular functions. According to Casey [12], the superior pharyngeal constrictor muscle forms a sphincter muscle with the buccinator and orbicularis oris muscles and performs important functions in the first phase of swallowing. The results of the present study showed that between the superior pharyngeal constrictor muscle at the buccopharyngeal part and the buccinator muscle, there are differences in the presence and absence of membrane, but no bone is present and both muscles attach to mucosa. Based on these findings, the superior pharyngeal constrictor and buccinator muscles appear to work together during swallowing in terms of morphology. It seemed that a series of smooth movements from mastication to swallowing results from the superior pharyngeal constrictor, buccinator, and orbicularis oris muscles becoming like a single sphincter muscle.

The morphology of the origin of the superior pharyngeal constrictor muscle at the mylopharyngeal part could be divided into two types, with the tip of the origin on the mylohyoid line in one type and not on this line in the other. The origin of the muscle at this part is described as the mylohyoid line in anatomy textbooks [4, 7]. However, the present study clarified that the tip of the origin deviates from the mylohyoid line in some cases. While two types were observed, the muscle at this part originated from the mandible in both cases. This suggests that when the mandible moves during swallowing, the muscle at this part is also involved.

The origin of the superior pharyngeal constrictor muscle at the glossopharyngeal part along with the palatoglossus and styloglossus muscles forms the root of the tongue. Immediately after the origin, the muscle at this part either interlaces or merges with the muscle at the mylopharyngeal part. While these two types were observed, it is difficult to assume that the muscles at the glossopharyngeal and the mylopharyngeal parts act independently from the perspective of anatomical structures, and both parts seem to act together. The muscle at the glossopharyngeal part would play an important role in sending the bolus from the root of the tongue to the pharynx.

The morphology of the insertion of the superior pharyngeal constrictor muscle displayed two types, with membranous connective tissue in one type and merging of the right and left pharyngeal constrictor muscles in the other. The findings of Shimada and Gasser [18] were comparable to the present results. While these two types were observed, both were formed by soft tissue. Insertion of the superior pharyngeal constrictor muscle is thus shaped so that the pharynx can freely dilate and contract during swallowing. At the insertion, the area made of the superior pharyngeal constrictor muscle was almost at the same height as the soft palate. While the middle and inferior pharyngeal constrictor muscles were aligned obliquely, the superior pharyngeal constrictor muscle was aligned horizontally. This suggests that the insertion of the superior pharyngeal constrictor muscle is involved in Passavant’s ridge, which is formed during nasopharyngeal closure during swallowing.

The results of the present study show that the superior pharyngeal constrictor muscle attaches to the buccinator muscle (which plays an important role in mastication) with mucosa, and it originates from the mandible and root of the tongue. These findings showed that the oral cavity and the pharynx are surrounded by a series of muscles such as the buccinator and superior pharyngeal constrictor muscles and that the muscles surrounding the circumference of the oral cavity and the pharynx act together during mastication and swallowing. Therefore, the morphology of the superior pharyngeal constrictor muscle enables a smooth transition from the lingual stage to the pharyngeal stage during ingestion.