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Objective: To investigate the characteristics in the topographic distribution of the muscle spindles in human masticatory muscles., Methods: The muscle spindles of the masticatory muscles on both sides of 3 human corpses fixed in formalin for more than 1 year were observed histologically and analyzed quantitatively using an image analysis system., Results: The mean muscle spindle density was 32.11/g in the masseter, significantly higher in the deep muscular fibers (70.76/g) with high proportion of spindle complexes than in the superficial layers of the muscle belly (P<0.05). The mean spindle density in the temporalis was 5.44/g, higher in the anterior portion (7.44/g) than in the posterior portion (3.78/g, P<0.01). The medial pterygoid had a mean spindle density of 4.43/g., Conclusion: These results indicate a heterogeneous distribution of muscle spindles in the masticatory muscles with different morphologies or in different portions of the same muscle, depending on muscle fiber type and muscular architecture.

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles.

The objectives of this study were to identify the exact location of spindles in jaw-closer muscles of the cat, to count the total number of spindles and to compare their distribution with the distribution of slow extrafusal fibres. The jaw-closer muscle group with all the skeletal attachments intact was fixed in a modified Carnoy solution, decalcified and processed through to wax. Complete series of sections were cut transverse, sagittal and perpendicular to the anterior temporalis muscle. At regular intervals, serial sections were stained by the Weigert-van Gieson method or immunostained for myosin isoforms. Spindle counts were made only from muscle areas where fibres were cut in transverse section, and the spindles were followed individually. The extrafusal fibres were identified by indirect immunoperoxidase staining with antibodies specific for the slow (type I) and fast (type IIM) isoforms of myosin found in jaw-closer muscles. The mean numbers and locations of spindles found were 13 in medial pterygoid (close to inferior border), 123 (one count only) in the deep anterior portion of the temporalis muscle (between the coronoid process of the mandible and the cranium), 50.5 in a small deep zone of superficial masseter anterior to the temporomandibular joint, and 50 in zygomaticomandibularis (the deepest portion of masseter). Most of the spindles were simple spindles. Spindle complexes (4 or more spindle units fused in parallel) were rare and were found only in zygomaticomandibularis and in masseter. Most parts of the jaw-closer muscles had no spindles and contained only fast fibres. Cosegregation of muscle spindles with slow fibres was found in most parts of this muscle group, but the distribution of spindles was more restricted than that of slow fibres.

The frequency and distribution of muscle spindles in the muscles of mastication of the rat have been studied. The average number of spindles found was 110 in the masseter, 75 in the temporalis and 25 in the medial pterygoid muscle, the spindles being predominantly in the medial parts of these muscles. No muscle spindles were found in the lateral pterygoid or posterior digastric muscle, and only isolated spindles were found in a few of the anterior digastric muscles. These differences between the jaw-closing and the jaw-opening muscles are intriguing and ask for further investigation of the role of the muscle spindles in mastication.

The digastric muscle of the rabbit was studied for the presence of muscle spindles. One of the 19 adult digastric muscles we examined contained in excess of 12 spindles scattered throughout the belly. No spindles were found in the contralateral muscle, or in any of the digastric muscles from other animals. Although jaw depressor muscles of most mammals contain few, if any, muscle spindles, their occasional presence suggests that these muscles have a potential for spindle formation.

A method, based on experiment, for more reliable and less time-consuming staining and examination of histological specimens for locating muscle spindles by light microscopy is presented. Sections from the masseter and temporal muscles of the rat were stained with different methods and studied in random order. The method giving the highest number of muscle spindles during scrutiny of the slides (Weigert's iron haematoxylin and van Gieson) is recommended for future studies on the numbers of muscle spindles in a given muscle.

The various parts of the masseter muscle complex (pars superficialis, pars profunda, zygomaticomandibularis, maxillomandibularis) in the rat, guinea-pig, rabbit, cat and macaque monkey were examined to discover whether they showed any relationship between the distribution of muscle spindles and extrafusal fibre types. Intrafusal (spindle) and extrafusal fibre types in masseter were compared with those in limb muscles and were identified by a combination of standard histochemical methods and indirect immunoperoxidase staining with antibodies specific for the various isoforms of myosin characteristic of fibre types in mammalian muscle. In general, the fibre-type properties of intrafusal fibres in masseter resembled those in limb muscle spindles, but the extrafusal fibre-type composition was unlike that in most limb muscles. In the rat masseter, most of the spindles were clustered together in a few very restricted areas. Extensive fusion of the external capsules of adjacent spindles, resulting in the formation of giant spindles, was seen in the cat and monkey masseter; this was sometimes accompanied by the enclosure of extrafusal fibres within the fused spindles. Common to all species, but strongest of all in the rat, was a close association between the distributions of muscle spindles and extrafusal Type I (slow twitch) fibres within the masseter complex. Muscle spindles and Type I fibres were either absent or rarest in the superficial part of masseter, but were most common in the deep layer (pars profunda) or zygomaticomandibularis. The functional significance of these observations is discussed.

The morphology of jaw-muscle spindle afferents in the rat has been studied by intra-axonal injection of horseradish peroxidase. All stained axons were located in the motor root of the trigeminal nerve and could be traced dorsomedially to the vicinity of the trigeminal motor nucleus, where they divided into an ascending branch in the tract of the mesencephalic nucleus and a descending branch in the tract of Probst. Axon collaterals and swellings on fine collateral branches presumed to be synaptic boutons were located in the following regions: the trigeminal motor nucleus, the region dorsal to the trigeminal motor nucleus including the supratrigeminal nucleus, the parvicellular reticular formation immediately caudal to the trigeminal motor nucleus, the reticular formation at the level of the facial nucleus, and the caudal portion of the mesencephalic nucleus. No evidence of a projection to the cerebellum was observed. Boutons were most numerous in the region surrounding the trigeminal motor nucleus, especially dorsally. Here they were not demonstrated in close proximity to counterstained cells, and therefore it was not possible to determine how many of these contacts are located on cells in this region and how many are on the distal dendrites of trigeminal motorneurons. Boutons located within the trigeminal motor nucleus were always confined to a small portion of the nucleus and were significantly larger than those located dorsally. Some boutons were found in close apposition to trigeminal motorneurons and presumably make somatic contacts. These results suggest that jaw-muscle spindle afferents make somatic and proximal dendritic contacts with only a limited number of trigeminal motorneurons and also project to masticatory interneuronal regions dorsal and caudal to the motor nucleus.

Histological examination of the jaw muscles of the Japanese ermine showed that 4 jaw-closing muscles have 13 muscle spindles on one side of the face. The temporal muscle has 99 muscle spindles, 68 being in the anterior vertical and 31 in the posterior horizontal belly. The masseter muscle has 33 muscle spindles, 23 being in the profound and 10 in the superficial belly. The medial pterygoid muscle has 7 muscle spindles and the zygomaticomandibular muscle contains 4 muscle spindles. The lateral pterygoid and the jaw-opening muscles have no spindles.

Muscle-spindle density and size, in relation to extrafusal fibre-type, were examined in different portions of the masseter of six normal males by enzyme histochemistry. A direct relationship between spindle density, intrafusal fibre density and spindle size, and proportion of type I muscle fibres was found in the deep portion. Out of 410 muscle spindles in random samplings, 74 per cent were in the deep portion which is predominantly composed of type I fibres. Spindle density was three times greater than in the other portions, and spindles were commonly clustered, sometimes sharing capsule tissue. The deep masseter spindles were distinctive; they had the largest diameter and the most intrafusal fibres, the fibre density being four times greater than in other portions. Eleven per cent of the spindles, mainly in the deep portion, contained an unusually high number of intrafusal fibres (15 or more fibres). The findings imply that individual portions of the masseter have specialized functions. An especially powerful proprioceptive reflex mechanism is suggested for the deep portion. Thus, the human masseter may be part of an intricate, evolutionarily advanced, motor system also connected with speech.

The occurrence and distribution of muscle spindles was studied in histochemically and conventionally stained serial cross sections of 6-week-old and adult rat masticatory and suprahyoid muscles. Spindles were present in moderate to large numbers in jaw closers, but they were absent in jaw openers and two of four muscles of an accessory suprahyoid group. In jaw closers, 67% or more of the total spindle population was concentrated relatively distant from the temporomandibular joint, in muscle portions which contained large numbers of extrafusal fibers reacting strongly for oxidative enzymes. Because of their location, spindles in these portions should be stretched more and, subsequently, should respond with a greater afferent discharge at any given muscle length than spindles situated nearer to the joint. Spindles in jaw closers, especially the medial pterygoid and deep masseter, often occurred in clusters and complex forms near the terminal branching of intramuscular nerve trunks. No such concentrations were seen in the two muscles of the accessory suprahyoid group that had spindles. The association in jaw closers of spindles with extrafusal fibers high in oxidative enzyme activity is consistent with the view that spindles are the sensory component of a reflex system that recruits these fibers for finely-graded contractions in response to small internal length-changes of the muscle (Botterman et al., '78); however, in jaw openers and two muscles of the accessory suprahyoid group, the absence of spindles, coupled with the presence of large populations of extrafusal fibers high in oxidative enzyme activity, is not easily reconciled with this concept.

In the masticatory muscles, neuromuscular spindles have a very important role in controlling the jaw movement since they act as stretch receptors in skeletal muscles. The continuous intake of fine-grained diet which is easily chewable leads to degeneration of the sensory endings of Ia fibers in many muscle spindles of the mouse masseter muscle in only 120 d after birth.

The occurrence of muscle spindles in the anterior digastric muscle in man was investigated and the fibre calibre spectrum of the corresponding nerve was determined. After removal at autopsy from five individuals of both sexes (aged 23--73), the muscles were stained with Weigert's iron hematoxylin-van Gieson stain and the nerves according to the Alzheimer - Mann - Häggqvist method. Altogether 12 spindles were found in five out of ten muscles. Only muscles from one individual were devoid of spindles. This sparsity or absence was supported by analyses of fibre calibre spectra. The small number of spindles and the fact that they do not occur in all muscles or in all individuals, suggest that they are not an essential source of sensory information.

This study of the masticatory muscles of the primate showed that the temporal muscle contained 107 muscle spindles, 45 in the horizontal portion and 62 in the vertical portion; the masseter muscle contained 70, 58 in the profundus portion and 12 in the superficial portion; the medial pterygoid muscle contained 15; the lateral pterygoid muscle contained 6; and the zygomaticomandibular muscle contained 9. The muscle spindles were located around the coronoid process and mandibular ramus.