Muscle spindle supply to the bovine jaw muscles

The jaw-closing muscles of two bovine fetuses average 1,177 muscle spindles on one side of the face: 549 in the masseter, 433 in the temporalis, 192 in the medial pterygoid, and three in the lateral pterygoid. The jaw-opening muscles have no spindles.     

Topography of muscle spindles and Golgi tendon organs in shoulder muscles of "Monodelphis domestica".

The topography of muscle spindles and Golgi tendon organs in the rotator cuff and surrounding shoulder muscles of a small laboratory marsupial (monodelphis domestica) were studied using light microscopy of serial sections. The shoulder joint of monodelphis has a large degree of freedom of movement allowing this animal to use the upper extremities for a wide range of activities like climbing and manipulating food. Thus, similar to the situation in man the shoulder joint is mainly secured by muscles. Silver stained serial paraffin sections were examined under the light microscope and the distribution of muscle spindles and Golgi tendon organs was reconstructed using three-dimensional image processing. In the two animals examined 113 and 131 muscle spindles respectively were found within the 4 rotator cuff muscles. In addition, 76 and 40 Golgi tendon organs respectively were seen at the musculo-tendinous junctions of these muscles preferentially close to the insertion at the humerus head. Also the surrounding shoulder muscles contain both muscle spindles and Golgi tendon organs in large numbers, but the ratio of Golgi tendon organs per muscle spindle appears to be lower. Number and localization of muscle spindles and Golgi tendon organs suggest, that these receptors are important for both reflex control of shoulder muscle tone as well as monitoring of static position and movement in the shoulder joint.     

Innervation of muscle receptors in the cross-reinnervated soleus muscle of the cat

It has recently been reported (Gregory et al., J. Physiol., 331:367-383, 1982) that cutting a muscle nerve and letting it grow back into the muscle or cross-uniting the muscle with a foreign nerve results in major disruption of the normal response patterns of muscle spindles and tendon organs. Here we report observations on the structure of muscle receptors in cross-reinnervated and self-reinnervated soleus muscles in an attempt to detect abnormalities that might account for their disturbed function. Eight soleus muscles were reinnervated with the extensor digitorum longus nerve for periods up to 449 days and two were self-reinnervated. Following the physiological investigation, the muscle was fixed and stained according to the method of Barker and Ip (J. Physiol., 69:73P-74P, 1963). Spindles and tendon organs were teased from the muscle and photographed. In one cross-reinnervated muscle an attempt was made to isolate all receptors. About two-thirds of the normal number of spindles and tendon organs were found. Three categories of receptor were identified: normal, abnormal, and those having no visible nerve endings. There appeared to be little difference in degree of abnormality of receptors in self- and cross-reinnervated muscles. Of the 180 spindles, 3% were normal, 43% had no visible endings, and 54% had abnormal endings. Of 80 tendon organs, 38% were normally innervated, 33% were without visible innervation, and 29% had abnormal endings. We conclude that following long-term cross-reinnervation and self-reinnervation of soleus there is extensive disruption of the normal innervation pattern of both spindles and tendon organs which could account for their functional abnormalities.     

A quantitative study of muscle spindles and tendon organs in some intrinsic muscles of the hand in the bonnet monkey (Macaca radiata)

The number and density of muscle spindles and tendon organs have been determined in the following intrinsic muscles of the hand of bonnet monkeys: I lumbrical, II lumbrical, abductor digiti minimi, adductor pollicis, and I dorsal interosseous. All these muscles were found to be very rich in muscle spindles (17.6 to 42.31 per gram wet weight) but relatively poor in tendon organs (0.606 to 10.06 per gram wet weight). The lumbricals have very few tendon organs. The possible functional significance of these findings has been discussed.     

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum,Monodelphis domestica

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. Anat. Rec. 251:548–562, 1998. © 1998 Wiley-Liss, Inc.     

The afferent innervation of two infrahyoid muscles of the cat

1. The afferent innervation of the straplike muscles of the infrahyoid region were investigated in two ways. The morphology of spindles and counts of tendon organs were investigated by the gold chloride technique in ten muscles. Spindle counts were made in forty pairs of thyrohyoid and infrahyoid muscles. De-efferenting of the nerves to these muscles was done in three cats and the calibre spectra of the afferent innervation investigated. These were compared with the total counts of fibres in intact nerves.2. In the thyrohyoid, spindles are frequently absent. No tendon organs were seen. In the large infrahyoid (combined sternohyoid and sternothyroid), spindle counts varied from 0 to 20 and the mean spindle count per gram of muscle was 3.5. A maximum of five tendon organs were seen in the muscle. Both spindle and tendon organ counts are low when compared with a limb muscle of similar weight and size.3. In the infrahyoid muscle complex spindles were about equal in number to simple spindles.4. Counts of spindles in the infrahyoid muscle in families of three or more siblings suggest that some families of kittens tend to have higher spindle counts than other families.5. The afferent innervation of the two muscles varied between 21 and 42% of the total fibre population and the fibre diameter spectrum is in keeping with the low counts of encapsulated endings.     

Reducing condylar compression in clenching patients

The two major muscle groups used during clenching activity are the masseter and temporalis muscles. EMG readings of the masseter and temporalis muscles rise significantly during times of macro-clenching. Clenching occurs when the masseter and temporalis muscles contract, pulling the mandible superiorly. The continued contraction of the masseter and temporalis muscles results in compression forces on the teeth and temporomandibular joints. Theoretical joint loading models are utilized to demonstrate the load on the TMJ due to forces generated by the masseter and temporalis muscles. This study measures the EMG readings during bilateral macro-contraction of the masseter and anterior temporalis muscles. An appliance is fabricated to disengage the posterior teeth and a second series of EMG readings are taken to record lowered EMG readings. The vector forces of the reduced EMG's recordings demonstrate reduced condylar compression during macro-clenching.     

Muscle Spindle Composition and Distribution in Human Young Masseter and Biceps Brachii Muscles Reveal Early Growth and Maturation

Significant changes in extrafusal fiber type composition take place in the human masseter muscle from young age, 3–7 years, to adulthood, in parallel with jaw‐face skeleton growth, changes of dentitions and improvement of jaw functions. As motor and sensory control systems of muscles are interlinked, also the intrafusal fiber population, that is, muscle spindles, should undergo age‐related changes in fiber type appearance. To test this hypothesis, we examined muscle spindles in the young masseter muscle and compared the result with previous data on adult masseter spindles. Also muscle spindles in the young biceps brachii muscle were examined. The result showed that muscle spindle composition and distribution were alike in young and adult masseter. As for the adult masseter, young masseter contained exceptionally large muscle spindles, and with the highest spindle density and most complex spindles found in the deep masseter portion. Hence, contrary to our hypothesis, masseter spindles do not undergo major morphological changes between young age and adulthood. Also in the biceps, young spindles were alike adult spindles. Taken together, the results showed that human masseter and biceps muscle spindles are morphologically mature already at young age. We conclude that muscle spindles in the human young masseter and biceps precede the extrafusal fiber population in growth and maturation. This in turn suggests early reflex control and proprioceptive demands in learning and maturation of jaw motor skills. Similarly, well‐developed muscle spindles in young biceps reflect early need of reflex control in learning and performing arm motor behavior. Anat Rec, 2011. © 2011 Wiley‐Liss, Inc.     

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end‐organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle‐spindle capsules; however, information on the other types of end‐organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle‐specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver‐impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle‐spindle afferents in hind‐limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind‐foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike‐initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first‐order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind‐foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo‐distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle‐specific variability. Finally, we present some comparative data on muscle‐spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.     

Adaptation of the masseter and temporalis muscles following alteration in length, with or without surgical detachment

Histochemical properties, muscle fiber cross-sectional area, muscle fiber length, and the oxidative capacity of masticatory muscles of female rhesus monkeys were assessed following alteration in functional length by an intraoral appliance or by detachment of the muscle. Experimental groups received the appliance only (A); the appliance and subsequent detachment of the masseter (AD); the appliance and detached masseter, but with surgical reattachment of the masseter to the pterygomasseteric sling (ADR); no appliance, but detachment and reattachment of masseter (DR); or an appliance which was removed after 24 weeks to study posttreatment responses (PT). Animals were sacrificed and the muscles were studied at intervals from 4 to 48 weeks after initiation of the experimental period. The results of these studies led to the following conclusions: (1) Stretching the masseter and temporalis muscles within physiological limits did not significantly alter the proportion of fiber types, although oxidative capacity of the fibers was reduced. (2) Fibers with "intermediate" myofibrillar ATPase activity were no more prevalent in experimental than control muscles. (3) The cross-sectional area of Type I fibers of masseter muscles decreased following some experimental procedures, indicating that recruitment of these fibers is the most sensitive to altered jaw function. (4) Minimal alteration of muscle capillarity was induced by any of the experimental procedures. (5) The lengths of masseter muscle fibers in Group PT and of temporalis muscle fibers in groups AD and ADR were greater than in control animals.     

Muscle spindle supply to the masticatory muscles in the Japane ermine (Carnivora).

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.     

Variation in histochemical enzyme profile and diameter along human masseter intrafusal muscle fibers

The histochemical enzyme profile of human masseter intrafusal muscle fibers was analyzed in consecutive serial cross sections along the individual fibers. Two hundred intrafusal fibers in 21 muscle spindles were classified. On the basis of equatorial nucleation, myosin ATPase-staining reactions after alkaline and acid preincubations and diameter, four different populations or types of intrafusal fiber were identified: large-diameter alkaline-stable and acid-stable fibers, bag2; two types of fiber with intermediate-diameter, alkaline-labile and acid-labile fibers corresponding to bag1 and alkaline-labile and acid-stable fibers designated as AS-bag1; and small-diameter alkaline-stable and acid-stable (pH 4.6)-acid-labile (pH 4.3) fibers called chain fibers. Regional variability in staining and diameter along the individual fibers was noted. In general, intrafusal fibers showed stronger oxidative reactions than did extrafusal fibers. The enzyme profile of the human masseter intrafusal fibers differed from that of extrafusal fibers in jaw, limb, and trunk muscles and also from that reported for spindles in limb and trunk muscles in man. The result suggests unique properties of human jaw muscle spindles and the jaw motor system.     

Organization of group I activated cells in the main and external cuneate nuclei of the cat: Identification of muscle receptors

Summary Extracellular recording was made from 77 primary afferent fibres, 106 cells in the external cuneate nucleus, and 60 cells in the main cuneate nucleus, all activated by slowly adapting muscle stretch receptors. The nature of the muscle receptors responsible for the activation was determined by various types of receptor stimulation.Primary group I afferents from muscle spindles and tendon organs in distal forelimb muscles showed complete overlap of conduction velocities and thresholds to electrical stimulation. Both types of group I afferents as well as group II muscle spindle afferents were shown to ascend through the dorsal funiculus to the level of the cuneate nuclei.Three groups of cells were identified in the external cuneate nucleus, activated by group I muscle spindle afferents, tendon organ afferents and group II muscle spindle afferents, respectively.Almost all group I activated cells in the main cuneate nucleus, including all 34 cells identified as cuneo-thalamic relay cells, received their afferent input from muscle spindle afferents. Three cells were activated by tendon organ afferents.     

Histochemical characteristics of the masseter and temporalis muscles of the rhesus monkey (Macaca mulatta)

The histochemical characteristics, cross-sectional area and capillary of the skeletal muscle fibers of the anterior and posterior regions of the superficial masseter and the temporalis muscles are described for juvnile and adult rhesus monkeys of both sexes. Slow twitch fatigue resistant (S), fast twitch fatigue resistant (FR) and fast twitch fatigable (FF) fibers were found in varying proportions throughout the muscles; however some fibers with an intermediate myofibrillar ATPase activity were observed in the anterior masseter. No significant differences for any of the variables were found between male and female juveniles for a specific muscle sample site. However, consideable variation was found between juvenile and adult and between adult male and female monkeys in the percentages of different fiber types and the cross-sectional area of fibers in specific regions of the superficial masseter and temporalis muscles. We conclude from these observations that significant differences in funtion exist both within and between the different masticatory muscles of rhesus monkeys. Functional differences may result from the pronounced sexual dimorphism evident in the dentofacial complex of rhesusmonkey.     

Histochemical characteristics of the masseter and temporalis muscles of the rhesus monkey (Macaca mulatta).

The histochemical characteristics, cross-sectional area and capillary of the skeletal muscle fibers of the anterior and posterior regions of the superficial masseter and the temporalis muscles are described for juvenile and adult rhesus monkeys of both sexes. Slow twitch fatigue resistant (S), fast twitch fatigue resistant (FR) and fast twitch fatigable (FF) fibers were found in varying proportions throughout the muscles; however some fibers with an intermediate myofibrillar ATPase activity were observed in the anterior masseter. No significant differences for any of the variables were found between male and female juveniles for a specific muscle sample site. However, considerable variation was found between juvenile and adult and between adult male and female monkeys in the percentages of different fiber types and the cross-sectional area of fibers in specific regions of the superficial masseter and temporalis muscles. We conclude from these observations that significant differences in function exist both within and between the different masticatory muscles of rhesus monkeys. Functional differences may result from the pronounced sexual dimorphism evident in the dentofacial complex of the rhesus monkey.     

Positional relationships between the masticatory muscles and their innervating nerves with special reference to the masseter and zygomaticomandibularis in Suncus murinus

In the present study, we investigated the structure and nerve innervation of the masseter, temporalis and zygomaticomandibularis of Suncus murinus which has no zygomatic arch. Detailed dissection of eight head halves of four S. murinus was performed. In S. murinus, small muscle bundle was observed to be adjoined with the lateral surface of the temporalis. This muscle bundle was completely separated from the masseter. Based on the positional relationships between the muscle bundle and supplying nerves, we conducted that the bundle corresponded to the zygomaticomandibularis of human described in our previous study (Shimokawa et al., 1999). In addition, some differences in the nerve distribution to the masticatory muscles were observed in S. murinus as compared with humans with respect to the following points: 1) The additional supplying branch to the masseter originated from the auriculo-temporal nerve: 2) The common trunk of the masseteric nerve and the nerve to the posterior part of the temporalis penetrated the superior head of the lateral pterygoid. A possible model to account for these differences based on the positional relationships among the muscles and supplying nerves is presented.     

Effects of fine-grained diet on annulospiral endings of muscle spindles in the masseter muscle in developing and adult mice

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.     

Comparative Jaw Muscle Anatomy in Kangaroos,Wallabies, and Rat‐Kangaroos (Marsupialia: Macropodoidea)

The jaw muscles were studied in seven genera of macropodoid marsupials with diets ranging from mainly fungi in Potorous to grass in Macropus. Relative size, attachments, and lamination within the jaw adductor muscles varied between macropodoid species. Among macropodine species, the jaw adductor muscle proportions vary with feeding type. The relative mass of the masseter is roughly consistent, but grazers and mixed‐feeders (Macropus and Lagostrophus) had relatively larger medial pterygoids and smaller temporalis muscles than the browsers (Dendrolagus, Dorcopsulus, and Setonix). Grazing macropods show similar jaw muscle proportions to “ungulate‐grinding” type placental mammals. The internal architecture of the jaw muscles also varies between grazing and browsing macropods, most significantly, the anatomy of the medial pterygoid muscle. Potoroines have distinctly different jaw muscle proportions to macropodines. The masseter muscle group, in particular, the superficial masseter is enlarged, while the temporalis group is relatively reduced. Lagostrophus fasciatus is anatomically distinct from other macropods with respect to its masticatory muscle anatomy, including enlarged superficial medial pterygoid and deep temporalis muscles, an anteriorly inflected masseteric process, and the shape of the mandibular condyle. The enlarged triangular pterygoid process of the sphenoid bone, in particular, is distinctive of Lagsotrophus. Anat Rec, 292:875–884, 2009. © 2009 Wiley‐Liss, Inc.     

The responses of muscle spindles in the kitten to stretch and vibration

Summary Discharges of muscle spindle afferents from the soleus muscle were studied in kittens aged 1–21 days and in adult cats. Vibration applied longitudinally to the tendon elicited one impulse for each cycle of vibration over the range 1–200 Hz for the kittens and up to 450 Hz for the adult. Threshold amplitudes were generally higher in the kitten than in the adult. In response to large ramp and hold stretches applied at long muscle lengths kitten spindles showed rate saturation during the length change. Dynamic index, that is the peak rate during the length change minus the rate at the final length became progressively smaller at longer muscle lengths. No sign of saturation was seen at comparable muscle lengths in the adult. It is suggested that in the newborn the bag₁ intrafusal fibre is not functional and that the dynamic response is produced only by the afferent terminals on the bag₂ fibre. Another difference between kitten and adult was the length sensitivity measured under dynamic conditions. This increased much more steeply with stretch rate in the kitten. One possible explanation for the higher dynamic length sensitivity is a lack of elastic fibres surrounding intrafusal fibres of immature spindles.