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.

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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.

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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.

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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 numerous. 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. In Fig. 8, the spindle has tapered down and connective tissue septa from the capsule separate the intrafusal fibres. Additional sections from this spindle are illustrated in colour in Fig. 1 C and 1 D which are 1 -25 and 1F5 mm respectively from Fig. 4.

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Fig. 6 (for legend see p. 161). Neuromuscular spindles in human lateral pterygoid muscles 163

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cofmaringealph oruevtibrs. Onfthis basis,t rtio ofnds to moto units forthemasseterandtemporalmuscles weres.imilart toa hesr rti inatt hse-oasor gastroneme spindlesto sixfmotormunits, and werewidelyExIm! sidife-re fromathsecihes sinldle-hesumbrcicitals Cromparimoston the latericals of the whereas the Cooeuro(1966)lalsroprdthubrohand, large muscleswiebeietfediothspindlesof the shoulder and thethemathigheuclstiatedare placednumerin the low group (Cooper, 1966). The ratios2m5of and 1a5, determined for two adult lateral pterygoid muscles in this study, fall into Cooper's low group and contrast with the values provided by Freimann (1954) for the jaw-closing muscles of one side, viz. medial pterygoid, 20-3; temporal, 14-7; masseter, 11-2 (deep part) and 7-4 (super- ficial part). Cooper (I1966) also compared the number of spindles with the estimated number of large alpha motor nerve fibres. On this basis, the ratio of spindles to motor units for the masseter and temporal muscles were similar to the ratio in the spindle-poor gastrocnemius, with approximately one spindle to six motor units, and were widely different from those in the spindle-rich lumbricals. Comparison with the lateral pterygoid muscle is not possible at present because the number of large alpha motor nerve fibres has not yet been estimated. Spindles have been clearly demonstrated in all members of the following human muscle groups which were once the subject of controversy; the extrinsic eye muscles Neuromuscular spindles in human lateral pterygoid muscles 165 (Cooper & Daniel, 1949), the intrinsic muscles of the tongue (Cooper, 1953) and the muscles of the larynx (Keene, 1961). Uneven distribution of spindles in many of these muscles has been observed and areas containing spindles may have been missed by other investigators. Honee (1966) found lateral pterygoid spindles exclusively in the mid-portions of the muscle but in the present study a few were also observed in its anterior third. The distribution of lateral pterygoid spindles in Macaca irus (Karlsen, 1969) is similar to that in man, but Christensen (1967) observed them in some cases when examining small portions of the muscle attached to the temporomandibular discs of miniature swine. Studies on the course of the main nerve trunks within these muscles may provide an explanation since Barker & Chin (1960) noted that inmost muscles the spindles were concentrated near them. It is difficult to explain why the lateral pterygoid of the guinea-pig, rat, goat, and rhesus and squirrel monkeys should be devoid of spindles while the rabbit, miniature swine and M. irus possess spindles, because all these animals have relatively well- developed lateral pterygoid muscles and their mandibles perform forward or lateral movements as well as opening and closing actions during the mastication of a variety of diets. In the cat, the vestigial proportions of this muscle, and the limitation of temporomandibular joint movements to an almost complete hinge action (Gill & Grant, 1966) may have contributed to their absence. It is apparent that more search- ing comparative studies are required before species specificity is used to explain some negative findings. Morphology Although the morphology of the adult spindles had several features in common with that of spindles in the human lumbrical and suboccipital muscles described by Cooper & Daniel (1963), their average length (1-5 mm) was shorter and their intrafusal fibre content (1-4) was less. In these respects they were similar to most of the spindles in the small posterior cricoarytenoid muscle described by Grim (1967). However, Honee (1966) observed from two to nine intrafusal fibres in lateral pterygoid spindles. The small number of intrafusal fibres which extended beyond the poles of the capsules and showed peripheral clumping of myofibrils in their mid-regions had some of the features of nuclear bagfibres described by Cooper & Daniel (1963), but no large, closely packed nuclei were observed. The location ofthe orange-yellow stained material in the central cores of a few intrafusal fibres was similar to that of the strange argentophilic substance noted by Cooper & Daniel (1963) in the central tubes of some human spindles. Since all the adult spindles possessed well-defined capsules and those of a 12-year- old child, illustrated by Honee (1966), were many layers thick, it is presumed that the delicate capsules of the fetal lateral pterygoid spindles must thicken later, when chewing begins. Significance in jaw muscle function Experiments on decerebrate cats by Sherrington (1917) and the reported absence ofspindles in thelateral pterygoidmuscle (Freimann,I1954) emphasizedthesignificance of discharges from receptors of the peridental membrane, gingivae, palate and other 166 H. I. GILL parts of the oral mucosa as a mechanism in reversing the jaw-closing action during cyclic jaw movements (Jerge, 1964; Kawamura, 1968). Matthews (1964) stated that ' . . the main function of the muscle spindle is now usually believed to be to play a part in the subconscious nervous control of muscular contraction, both during movement and duringcontraction'. Althoughhuman lateral pterygoid spindles are few in number, their relatively localized distribution may make them sufficiently numerous in the mid-region of the muscle to provide the nervous system with information of considerable importance for the normal functioning of this muscle. Further investigation into their number, morphology and function is required for a better understanding of the physiology of mastication.

SUMMARY 1. Neuromuscular spindles have been identified in each of the five lateral pterygoid muscles used in this study. 2. With suitable material and techniques, it is considered that spindles can be demonstrated in this muscle and that a low value for the ratio of spindles per gram of muscle is a normal feature of lateral pterygoid muscles in man. 3. Some features of adult and fetal lateral pterygoid spindles have been illustrated and described. 4. An understanding of the physiology of this muscle, and in consequence, the nature of the reflex control of mastication is incomplete without consideration of these structures. This study is part of a broad research project dealing with the physiology of mastication, which is under the direction of Professor H. F. Atkinson, and which is supported by a grant from the National Health and Medical Research Council of Australia. The valuable technical assistance provided by Mr M. K. Smith has been appreciated. REFERENCES BARKER, D. & CHmN, N. K. (1960). The number and distribution of muscle spindles in certain muscles of the cat. J. Anat. 94, 473-486. BAUM, J. (1900). Beitrage zur Kenntnis der Muskelspindeln. Anat. Hefte (1 Abt. XLII) 13, 249-305. CHRISTENSEN, L. V. (1967). Muscle spindles in the lateral pterygoid muscle of miniature swine. Archs oral Biol. 12, 1203-1204. CIPOLLONE, L. T. (1897). Ricerche sull'anatomia normale e patologica delle terminazioni nervose nei muscoli striati. Annali Med. nav. colon. (Suppl.) 3, 252. COOPER, S. (1953). Muscle spindles in the intrinsic muscles of the human tongue. J. Physiol. 122, 193-202. COOPER, S. (1960). Muscle spindles and other muscle receptors. In Structure and Function ofMuscle (Ed. G. H. Bourne), Vol. I. New York, Academic Press. COOPER, S. (1966). Muscle spindles and motor units. In Control and Innervation of Skeletal Muscle (Ed. B. L. Andrew). Edinburgh: Livingstone. COOPER, S. & DANIEL, P. M. (1949). Muscle spindles in human extrinsic eye muscles. Brain 72, 1-24. COOPER, S. & DANIEL, P. M. (1963). Muscle spindles in man; their morphology in the lumbricals and the deep muscles of the neck. Brain 86, 563-586. CULLING, C. F. A. (1963). Handbook ofHistopathological Techniques, 2nd edn. London: Butterworth. FRANKS, A. S. T. (1964). Studies on the innervation of the temporomandibular joint and lateral pterygoid muscle in animals. J. dent. Res. 43, 947-948. FREIMANN, R. (1954). Untersuchungen uber Zahl und Anordnung der Muskelspindeln in den Kaumuskeln des Menschen. Anat. Anz. 100, 258-264. Neuromuscular spindles in human lateral pterygoid muscles 167 GILL, H. I. & GRANT, A. A. (1966). The anatomy of the pterygoid region of the cat, goat, sheep and monkey. Aust. J. Zool. 14, 265-274. GRIM, M. (1967). Muscle spindles in the posterior cricoarytenoid muscle of the human larynx. Folia morph. 15, 124-131. HONEE, G. L. J. M. (1966). An investigation on the presence of muscle spindles in the human lateral pterygoid muscle. Ned. Tijdschr. Tandheelkunde (Suppl. 3) 73, 43-48. JERGE, C. R. (1964). The neurologic mechanism under-lying cyclic jaw movements. J. prosth. Dent. 14, 667-681. KARLSEN, K. (1965). The location of motor end plates and the distribution and histological structure of muscle spindles in the jaw muscles of the rat. Acta odont. scand. 23, 521-547. KARLSEN, K. (1969). Muscle spindles in the lateral pterygoid muscle of a monkey (Macacus irus). Archs oral Biol. 14, 1111-1112. KAWAMURA, Y. (1968). Mandibular movement: normal anatomy and physiology and clinical dysfunction. InFacialPain andMandibularDysfunction (Ed. L. Schwartz and C. M. Chayes). Philadelphia: Saunders. KEENE, M. F. LUCAS (1961). Muscle spindles in human laryngeal muscles. J. Anat. 95, 25-29. KUHNE, W. (1863). Die Muskelspindeln. Virchows Arch. path. Anat. Physiol. 28, 528-538. McFARLANE, D. (1944). Picro-Mallory-an easily controlled regressive trichromic staining method. Stain Technol. 19, 29-37. MATTHEWS, P. B. C. (1964). Muscle spindles and their motor control. Physiol. Rev. 44, 219-288. PORTELA-GOMES, F. (1963). L'innervation proprioceptive du muscle pterygoidien externe chez l'homme et chez le lapin. C. r. Ass. Anat. 119, 1093-1097. SHERRINGTON, C. S. (1917). Reflexes elicitable in the cat from pinna, vibrissae, and jaws. J. Physiol. 51, 404-431. SMITH, R. D. & MARCARIAN, H. Q. (1967). The neuromuscular spindles of the lateral pterygoid muscle. Anat. Anz. 120, 47-53. Voss, H. (1936). Ein besonders reichliches Vorkommen von Muskelspindeln in der tiefen Portion des M. masseter des Menschen und der Anthropoiden. Anat. Anz. 81, 290-292.