Neuromuscular Spindles in Human Lateral Pterygoid Muscles

J. Anat. (1971), 109, 1, pp. 157-167 157 With 9 figures Printed in Great Britain Neuromuscular spindles in human lateral pterygoid muscles H. I. GILL School ofDental Science, University of Melbourne, Victoria, 3052, Australia (Received 16 September 1970) INTRODUCTION In the hundred years since Kuhne (1863) described muscle spindles, there have been few reports on their structure and distribution in the muscles of mastication. Baum (1900) and Voss (1936) observed spindles in human temporal, masseter and both pterygoid muscles, but theirpresenceinthelateral pterygoid hasbeenchallenged. Freimann (1954), who examined one muscle, and Smith & Marcarian (1967), who examined five, did not find spindles in their serial sections. However, Portela-Gomes (1963) noted them in both heads, and Honee (1966) counted one to 15 spindles in the mid-portions of each of six muscles from different subjects. Comparatively there is similar confusion, with Cipollone (1897) and Christensen (1967) reporting their presence in rabbit and miniature swine respectively, and Karlsen (1969) counting 13 spindles in one lateral pterygoid from a primate (Macaca irus), whereas their absence has been recorded in the cat and goat (Cooper, 1960), guinea-pig (Franks, 1964), rat (Karlsen, 1965) and rhesus and squirrel monkeys (Smith & Marcarian, 1967). It was therefore decided to examine complete serial sections ofhuman lateral pterygoid muscles to determine whether this controversy was due to technical factors or to real differences in distribution between muscles and individuals. MATERIALS AND METHODS Five human lateral pterygoid muscles were examined, of which four were removed within 24 h post-mortem from adults aged 26-84 years; the fifth was obtained from a formalin-fixed 34-week fetus. An intracranial approach was used to remove a large area of bone from the floor of the right middle cranial fossa and orbital roof. The muscle was then detached from the lateral surface of the lateral pterygoid plate and the pterygoid fovea of the mandible, but the anterior parts of the capsule and meniscus of the temporomandibular joint were left attached. For the fetal specimen, a block dissection to include the lateral pterygoid muscle and adjacent structures was performed. Peterfi's double embedding technique (Culling, 1963) was used for each specimen and the paraffin block was positioned so that the greatest number of 8 ,um transverse sections couldbe obtained (Table 1). Every sixth sectionwas stained bya critical Picro- Mallory method (McFarlane, 1944) for initial routine scanning at x 100magnification. The entire collection consisted of almost 8000 sections, each on a separate slide. 158 H. I. GILL Spindles were examined from pole to pole, their lengths determined and the mid- portion of each spindle marked by placing a small ink dot on the coverslip. This enabled their distribution to be illustrated on longitudinal outlines of the muscles and ensured that no spindle was counted more than once. A B 4 4 4' 100ptm - 4 . Fig. 1. Colour illustrations of transverse sections through muscle spindles of human lateral pterygoid muscles. Picro-Mallory stain. The magnification is constant. (A) The blue laminated capsule of an adult spindle, containing three intrafusal fibres, can be readily distinguished from the extrafusal fibres. (B) A spindle from a 34-week foetus is present in the centre of the field, together with its associated nerve branch. The delicate capsule and small extrafusal fibres are featured. (C) and (D) are from the mid-region of the same spindle illustrated in Figs. 4-9 and are 0-25 mm apart. In (C) there are several nerve fibres inside the capsule, and all three intrafusal fibres show peripheral segregation of their myofibrils. The orange yellow staining reaction seen in one of them is also apparent in the same intrafusal fibre in (D) which also shows a blood vessel to the spindle. OBSERVATIONS Neuromuscular spindles were observed in every muscle investigated (Table 1). The adult specimens were identified by the presence of conspicuous blue-stained, laminated capsules which contrasted with the surrounding red-coloured muscle fibres (Fig. 1 A). Differentiation from small blood vessels and nerves, also cut in transverse section, was facilitated because the bright red intrafusal fibres, yellow erythrocytes and orange myelin were quite distinct. The spindles of the two completely sectioned adult muscles were found chiefly in the middle third, there being only a few in the anterior part and none in the posterior Neuromuscular spindles in human lateral pterygoid muscles 159 Table 1. Summary of results, right lateral pterygoid muscles Spindles Age Sex Comments observed 84 years Male Complete transverse serial sections 18 48 years Male Complete transverse serial sections 11 34-week fetus Female Complete transverse serial sections 2 26 years Male Middle third only serially sectioned. 5 Staining difficulties encountered 73 years Female Random sections from middle third 3 part, where there was more connective tissue as the temporomandibular joint was approached (Fig. 2). The average length of these 29 spindles was 1 5 mm, as determined by direct measurement on the slides. The longest was 3-8 mm and the shortest 0 5 mm. Three spindles were seen in the one field (Fig. 3), but the remainder were more widely scattered: no distinct separation of the muscles into upper and lower heads was evident in these regions. Spindles were situated parallel with the extrafusal fibres and in most cases were located in the septa between muscle bundles. Careful serial sectioning was necessary to show the variations in structure along the length of adult spindles (Figs. 4-9). Sometimes adjacent extrafusal fibres were included in lateral extensions of the capsules near their mid-regions, but in later sections were observed to become free of the capsules. Fig. 2. Diagrams showing the distribution of spindles in one plane on the longitudinal outlines of the right lateral pterygoid muscles of two adults. The spindle lengths are drawn to scale and the meniscus (M) is shown at the posterior end of each muscle. 160 H. I. GILL The intrafusal fibres, which numbered from one to four in the adult spindles, were smaller in diameter than the extrafusal fibres, and most of them terminated within the confines of the capsules. Although it was difficult to trace single intrafusal fibres, some appeared to show irregular changes in diameter, while other thin, short fibres had a constant thickness. Those fibres which extended beyond the poles of the capsule (Figs. 4, 9) showed changes in their orientation before they finally terminated on small connective tissue bundles in the perimysium. Cross-striations of intrafusal fibres were seen in an obliquely cut spindle. Bi . _ _ :~~~~~~~~~~~~~~~~4 = . _ . _. A I k_.:_ ^. '*w pp-D , _ aROOM:OVA Fig. 3. Low-power view ofa transverse section through the mid-region of the right lateral pterygoid muscle of a 48-year-old male. Three spindles are indicated in the one field. In the region ofentry ofthe small nerve trunks, the spindles reached their maximum width, and nerve fibres were closely associated with the intrafusal fibres, some of which showed peripheral segregation of their myofibrils. The central core region of a small number of fibres stained an orange-yellow colour with Picro-Mallory (Fig. IC, D) and a few dark-stained nuclei were present. Evidence of a blood supply to this region of the spindle is shown in Fig. 1 D. The capsule gradually tapered down as the Neuromuscular spindles in human lateral pterygoid muscles 161 polar regions were approached and some nerve fibres were still present, but there was less space around the intrafusal fibres (Fig. 8). Spindles in the fetal lateral pterygoid were more difficult to identify because their capsules consisted of only a few lamellae and their five or six intrafusal fibres were of a similar size to the extrafusal fibres (Fig. 1 B). Better developed spindles were observed in the masseter, temporal, medial pterygoid and tensor veli palatini muscles included in the fetal sections. 50.1 m _~~~~~~~~Y X I Fig. 4 Figs. 4-9. A series of photomicrographs of 8 ptm sections through different regions of a spindle in the lateral pterygoid muscle of an 84-year-old male. Picro-Mallory stain. The scale given in Fig. 4 is used throughout, and the distance along the spindle from Fig. 4 is shown on each photo- micrograph. This spindle was 3-5 mm long and it was associated with four intrafusal fibres, two of which, (X) and (Y), were followed along the entire length of the spindle, and also had extra- capsular portions of approximately 0-3 mm in length at both ends (Figs. 4,9). Near their term- inations in Fig 4, both (X) and (Y) were closely associated with small tufts of connective tissue. Another intrafusal fibre (A), surrounded by its own small capsule, gained entry to the spindle by penetrating the side of the main capsule (Figs. 5-7) and accompanied (X) and (Y) beyond the end of the capsule (Fig. 9) where their cross-sectional areas increased and nuclei were more num- erous. Intrafusal fibre (B), shown in Figs. 5-7, was only O-5 mm in length and had a very short extra-capsular portion at one end. Its diameter increased just before it terminated (Fig. 7), and was an orange-yellow colour with two dark stained nuclei and a small amount of peripherally placed muscle. In the same section (Fig. 7), intrafusal fibre (X) also showed iffegular peripheral clumping of myofibrils and the central core was also of an orange-yellow colour. A smnall nerve trunk (N) containing about eight nerve fibres approached the spindle (Fig. 5) and penetrated the lamellae of the capsule (Figs. 6 and 7) to gain access to the intrafusal fibres.

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