The human medial pterygoid muscle: Attachments and distribution of muscle spindles.

Published descriptions about the sites of origin of the human medial pterygoid muscle vary and there are few reports on the distribution and density of muscle spindles in this muscle. We aimed to: (1) determine the extent of anatomical variability in the origins and insertions of the superficial and deep heads of the human medial pterygoid muscle and (2) determine the extent of variation in the distribution of spindles in the two heads of the muscle. Thirty-nine human cadaver hemi-heads were dissected and the attachments of the medial pterygoid muscle examined. The whole muscle was removed, weighed, cut into segments and embedded in wax for light microscopy. Sections were stained with Weigert-Van Gieson stain and scanned into digital images. Spindles were manually counted. In seven specimens, the deep head of the muscle arose from the medial surface of the lateral pterygoid plate and the pterygoid fossa. In 28 specimens, the origin extended onto the lateral surface of the medial pterygoid plate. There were abundant muscle spindles in the middle of the muscle, slightly fewer 1 cm toward the insertion, significantly fewer 1 cm toward the origin, and few or no spindles near the origins of the superficial and deep heads or near their insertion. In conclusion, firstly, this study shows that in 72% of the specimens examined, the origin of the medial pterygoid is wider than conventionally described in anatomical textbooks. Secondly, the segmental distribution of muscle spindles is described for the first time. Clin. Anat. 30:1064-1071, 2017. © 2017 Wiley Periodicals, Inc.

[Morphological characteristic of a motor unit afferent limb in hypokinesia].

Neuromuscular spindles (NMS) of soleus muscle and their vascular bed was studied in 250 mature male Wistar rats following long-term (30, 60, 120 and 240 days) hypokinesia. Material was studied using Kulchitsky, Mason and Ranson histological staining, electron microscopic and histochemical methods (demonstration of succinate dehydrogenase). It was found that initial changes involved the decrease of the lumen of precapillary arterioles (by 25.4%) and blood capillaries (by 29.1%). These changes closely correlated with the increase of the subcapsular space width (by 34.7%), the decrease of NMS capsule thickness (by 48.7%) and equatorial zone diameter (by 17.9%). Activity of succinate dehydrogenase in the intrafusal muscle fibers was also decreased. As a result of disintegration of the majority of gamma-axon terminals, NMS cross sectional area was decreases by 75.6%. Thus, during hypokinesia initial changes were detected in the sources of NMS blood supply, while their structural remodelling involved destructive alterations.

Analyses of muscle spindles in the soleus of six inbred mouse strains.

Adult muscle size and fibre-type composition are heritable traits that vary substantially between individuals. We used inbred mouse strains in which soleus muscle mass varied by an order of magnitude to explore whether properties of muscle spindles can also be influenced by genetic factors. Skip-serial cross-sections of soleus muscles dissected from 15 male mice of BEH, BEL, C57BL/6J, DUH, LG/J and SM/J strains were analysed for number of muscle spindles and characteristics of intrafusal and extrafusal fibres following ATPase staining. The BEL and DUH strains determined the range of: soleus mean size, a 10-fold difference from 2.1 to 22.3 mg, respectively; the mean number of extrafusal fibres, a 2.5-fold difference from 497 to 1249; and mean fibre-cross-sectional area, three-fold difference, e.g. for type 1 fibres, from 678 to 1948 µm². The range of mean proportion of type 1 fibres was determined by C57BL/6J (31%) and DUH (64%) strains. The mean number of spindles per muscle ranged between nine (LG/J) and 13 (BEL) (strain effect P < 0.02). Genetic correlations between spindle count and muscle weight or properties of extrafusal fibres were weak and not statistically significant. However, there was a strong correlation between the proportion of spindles with more than one bag2 fibre and the proportion of extrafusal fibres that were of type 1, and strain-dependent variation in the numbers of such spindles was statistically significant. The numbers of intrafusal fibres per spindle ranged from 2 to 8, with the most common complement of four found in 75.6% of spindles. There were no significant differences between the strains in the mean numbers of intrafusal fibres; however, the variance of the number was significantly less for the C57BL/6J strain than for any of the others. We conclude that abundance of muscle spindles and their intrafusal-fibre composition are substantially determined by genetic factors, which are different from those affecting muscle size and properties of the extrafusal fibres.

γ-Aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on mesencephalic trigeminal neurons that innervate jaw-closing muscle spindles in the rat: ultrastructure and development.

Unlike other primary sensory neurons, the neurons in the mesencephalic trigeminal nucleus (Vmes) receive most of their synaptic input onto their somata. Detailed description of the synaptic boutons onto Vmes neurons is crucial for understanding the synaptic input onto these neurons and their role in the motor control of masticatory muscles. For this, we investigated the distribution of γ-aminobutyric acid (GABA)-, glycine-, and glutamate-immunopositive (+) boutons on Vmes neurons and their ultrastructural parameters that relate to transmitter release: Vmes neurons that innervate masseteric muscle spindles were identified by labeling with horseradish peroxidase injected into the muscle, and immunogold staining and quantitative ultrastructural analysis of synapses onto these neurons were performed in adult rats and during postnatal development. The bouton volume, mitochondrial volume, and active zone area of the boutons contacting labeled somata (axosomatic synapses) were similar to those of boutons forming axoaxonic synapses with Vmes neurons but smaller than those of boutons forming axodendritic or axosomatic synapses with most other neurons. GABA+ , glycine+ , and glutamate+ boutons constituted a large majority (83%) of all boutons on labeled somata. A considerable fraction of boutons (28%) was glycine(+) , and all glycine+ boutons were also GABA+ . Bouton size remained unchanged during postnatal development. These findings suggest that the excitability of Vmes neurons is determined to a great extent by GABA, glycine, and glutamate and that the relatively lower synaptic strength of axosomatic synapses may reflect the role of the Vmes neurons in modulating orofacial motor function.

Influence of 14-day hind limb unloading on isolated muscle spindle activity in rats.

During hind limb unloading (HU), the soleus is often in a shortened position and the natural physiological stimulus of muscle spindles is altered, such that muscle spindle activity also changes. Using isolated spindle conditions, the present study investigates the electrophysiological activity and ultrastructure of muscle spindles following HU. Results show that muscle spindle discharges fall into either of two main patterns, single spikes or spike clusters in shortened positions, with a steady frequency of 18-38 spikes/s (mean 29.08 +/- 2.45) in an extended position. Following 14-day HU, afferent discharge activity was significantly altered in soleus muscle spindles. Duration of individual spikes was significantly prolonged, from 0.54 +/- 0.05 ms for control rats to 1.53 +/- 0.25 ms for rats in the HU group. In a shortened position, regular rhythm afferent discharges were obviously depressed, and the majority of muscle spindles became silent, while in an extended position, the discharges remained continuous but with decreased frequency. Results also show that the ultrastructure of muscle spindles experience degenerative changes during HU. Altered muscle spindle afference could possibly modify the activity of motor neurons and further affect the activity of extrafusal fibers.

A further observation of muscle spindles in the extensor digitorum longus muscle of the aged rat.

We observed three novel muscle spindles in the extensor digitorum longus muscle of the aged (20 months) rat. Two muscle spindles of the three contained thin muscle fibers lacking sensory innervation between the layers of the spindle capsule and within the periaxial space, respectively. The other one contained sensory-innervated thin muscle fibers with an indistinct equatorial nucleation between the layers of the spindle capsule. These findings suggest that the occurrence of thin muscle fibers may be intimately related to the degeneration and regeneration of extrafusal muscle fibers during aging and that these newly formed thin muscle fibers may often fail to receive sensory innervation.

Fine structural study of the innervation of muscle spindles in the internal oblique muscle of the abdominal wall in the adult mouse.

We examined by electron microscopy the innervation of muscle spindles in the internal oblique muscle of the mouse abdominal wall. In the equatorial region, in addition to the sensory innervation on individual intrafusal muscle fibers, sensory cross terminals were often observed between nuclear chain fibers. In the area from the juxtaequatorial region to the polar region, nuclear bag fibers were supplied by trail and plate-type motor endings, while nuclear chain fibers were innervated by sensory endings, being probably secondary sensory endings. From these findings, it is clear that the innervation patterns differ between two types of intrafusal muscle fibers.

Ultrastructural and molecular biologic comparison of classic proprioceptors and palisade endings in sheep extraocular muscles.

PURPOSE: To analyze and compare the structural and molecular features of classic proprioceptors like muscle spindles and Golgi tendon organs (GTOs) and putative proprioceptors (palisade endings) in sheep extraocular muscle (EOMs). METHODS: The EOMs of four sheep were analyzed. Frozen sections or wholemount preparations of the samples were immunohistochemically labeled and analyzed by confocal laser scanning microscopy. Triple labeling with different combinations of antibodies against neurofilament, synaptophysin, and choline acetyltransferase (ChAT), as well as alpha-bungarotoxin and phalloidin, was performed. Microscopic anatomy of the nerve end organs was analyzed by transmission electron microscopy. RESULTS: The microscopic anatomy demonstrated that muscle spindles and GTOs had a perineural capsule and palisade endings a connective tissue capsule. Sensory nerve terminals in muscle spindles and GTOs contained only a few vesicles, whereas palisade nerve terminals were full of clear vesicles. Likewise, motor terminals in the muscle spindles' polar regions were full of clear vesicles. Immunohistochemistry showed that sensory nerve fibers as well as their sensory nerve terminals in muscle spindles and GTOs were ChAT-negative. Palisade endings were supplied by ChAT-positive nerve fibers, and the palisade complexes including palisade nerve terminals were also ChAT-immunoreactive. Motor terminals in muscle spindles were ChAT and alpha-bungarotoxin positive. CONCLUSIONS: The present study demonstrated in sheep EOMs that palisade endings are innervated by cholinergic axons exhibiting characteristics typical of motoneurons, whereas muscle spindles (except the polar regions) and GTOs are supplied by noncholinergic axons. These results raise the question of whether palisade endings are candidates for proprioceptors in EOMs.

Cellular structures involved in the transport processes and neuroglial interactions in the crayfish stretch receptor.

In order to explore neuroglial relationships in a simple nervous system, the ultrastructure of crayfish stretch receptor, which consists of only two sensory neurons enveloped by satellite glial cells, was studied. Neuronal Golgi complex was oriented such that its output trans-Golgi network usually faced the bundles of microtubules within the neuronal cytoplasm and very rarely to the outer membrane. Therefore, it participates mainly in the processing of proteins transported along microtubules to distal neuron parts rather than those transported to glial cells. Structural triads of submembrane cisterns-vesicles-mitochondria were involved in formation of glial protrusions into the neuronal cytoplasm. The double-wall vesicles within the neuron body were the captured parts of such glial protrusions. Glial protrusions and double-wall vesicles facilitated the neuroglial transport and large-scale delivery of the glial material into the neuron. The neuroglial transport could also be performed by diffusion across the intercellular space. These data indicate the significant neuroglial exchange with cellular components.

Scanning electron microscopic observation of the sensory region in the frog muscle spindle.

The equatorial sensory region of muscle spindles in the fourth toe extensor digitorum longus muscle of the adult frog was examined by scanning electron microscopy. Segments of this thin and long muscle after fixation were longitudinally cut with a razor blade and then treated with an HCl-hydrolysis method to remove connective tissues. Cells of the inner capsule extended thin and flattened cytoplasmic processes, showing a sieve-like appearance. Some specimens after a partial disruption of the inner capsule reevaluated at the fine structural level that numerous sensory terminals with varicose swellings longitudinally arranged along each intrafusal muscle fiber.

Novel muscle spindles containing muscle fibers devoid of sensory innervation in the extensor digitorum longus muscle of aged rats.

We examined the structural features of muscle spindles at the equatorial and juxtaequatorial regions in the extensor digitorum longus muscle of adult (12 months) and aged (25 months) rats. In aged muscle spindles, the lamellated layers of the spindle capsule were a little increased in number compared to those in the adult ones. Two novel muscle spindles were observed in the aged muscle. In one muscle spindle, the spindle capsule contained four thin intrafusal muscle fibers invested by the inner capsule and two muscle fibers between the layers of the spindle capsule. Serial semithin sections revealed that the latter lacked the investment of the spindle capsule at the polar region. The other muscle spindle contained four intrafusal muscle fibers: two thin sensory-innervated muscle fibers invested by the inner capsule and two thick muscle fibers similar in structural features to neighboring extrafusal muscle fibers and lacking sensory innervation within the wide periaxial space. These findings suggest that two muscle fibers between the layers of the spindle capsule may be invested by the newly formed capsular cells during aging, while two thick fibers within the periaxial space may fail to receive the sensory innervation during the early development and follow the course of extrafusal fiber differentiation.

Ultrastructural and biochemical changes of the medial pterygoid muscle induced by unilateral exodontia.

The aim of the present study was to investigate the histological, biochemical and ultrastructural effects of occlusal alteration induced by unilateral exodontia on medial pterygoid muscle in guinea pigs, Cavia porcellus. Thirty (n=30) male guinea pigs (450g) were divided into two groups: experimental-animals submitted to exodontia of the left upper molars, and sham-operated were used as control. The duration of the experimental period was 60 days. Medial pterygoid muscles from ipsilateral and contralateral side were analyzed by histological (n=10), histochemical (n=10), and ultrastructural (n=10) methods. The data were submitted to statistical analysis. When the ipsilateral side was compared to the control group, it showed a significantly shorter neuromuscular spindle length (P<0.05), lower oxidative metabolic activity, and microvessel constriction, in spite of the capillary volume and surface density were not significantly different (P>0.05). In the contralateral side, the neuromuscular spindles showed significantly shorter length (P<0.05), the fibers reflected a higher oxidative capacity, the blood capillaries showed endothelial cell emitting slender sprouting along the pre-existing capillary, and significantly higher blood capillary surface density, and volume density (V(v)=89% Mann-Whitney test, P<0.05). This finding indicated a complex morphological and functional medial pterygoid muscle adaptation to occlusal alteration in this experimental model. Considering that neuromuscular spindles are responsible for the control of mandibular positioning and movements, the professional should consider if these changes interfere in the success of clinical procedures in medical field involving stomatognathic structures.

Muscle proprioceptive feedback and spinal networks.

This review revolves primarily around segmental feedback systems established by muscle spindle and Golgi tendon organ afferents, as well as spinal recurrent inhibition via Renshaw cells. These networks are considered as to their potential contributions to the following functions: (i) generation of anti-gravity thrust during quiet upright stance and the stance phase of locomotion; (ii) timing of locomotor phases; (iii) linearization and correction for muscle nonlinearities; (iv) compensation for muscle lever-arm variations; (v) stabilization of inherently unstable systems; (vi) compensation for muscle fatigue; (vii) synergy formation; (viii) selection of appropriate responses to perturbations; (ix) correction for intersegmental interaction forces; (x) sensory-motor transformations; (xi) plasticity and motor learning. The scope will at times extend beyond the narrow confines of spinal circuits in order to integrate them into wider contexts and concepts.

[Ultrastructure of spindles in m. soleus of rats after repeated simulation of the weightlessness effects].

Postponed for a long time repeated simulation of the weightlessness effects in rats by tail-suspension increases in the amount of intrafusal muscle fibers in m. soleus spindles, reflecting presumably elevation of the sensitivity of mechanoreceptors.

[The absence of support loading on the rat m. soleus leads to an increase in the number of intrafusal muscle fibres in muscle spindles].

The ultrastructure of muscle spindles (incapsulated mechanoreceptors of stretch of extrafusal muscle fibres) of m. soleus in adult Wistar rats after repeated unloading of support on hind limbs with preservation of support loading on fore limbs has been studied by transmissing electron microscopy. It was shown that, along with muscle spindles with the ordinary number of intrafusal muscle fibres (four), m. soleus contains spindles with an increased number of intrafusal fibers (five to six). It was assumed that the increase in the number of intrafusal muscle fibers is due to the proliferation of their satellite cells.

Neurotrophin-3 ameliorates sensory-motor deficits in Er81-deficient mice.

Two factors, the ETS transcription factor ER81 and skeletal muscle-derived neurotrophin-3 (NT3), are essential for the formation of muscle spindles and the function of spindle afferent-motoneuron synapses in the spinal cord. Spindles either degenerate completely or are abnormal, and spindle afferents fail to project to spinal motoneurons in Er81 null mice; however, the interactions between ER81 and NT3 during the processes of afferent neuron and muscle spindle development are poorly understood. To examine if overexpression of NT3 in muscle rescues spindles and afferent-motoneuron connectivity in the absence of ER81, we generated myoNT3;Er81(-/-) double-mutant mice that selectively overexpress NT3 in muscle in the absence of ER81. Spindle reflex arcs in myoNT3;Er81(-/-) mutants differed greatly from Er81 null mice. Muscle spindle densities were greater and more afferents projected into the ventral spinal cord in myoNT3;Er81(-/-) mice. Spindles of myoNT3;Er81(-/-) muscles responded normally to repetitive muscle taps, and the monosynaptic inputs from Ia afferents to motoneurons, grossly reduced in Er81(-/-) mutants, were restored to wild-type levels in myoNT3;Er81(-/-) mice. Thus, an excess of muscle-derived NT3 reverses deficits in spindle numbers and afferent function induced by the absence of ER81. We conclude that muscle-derived NT3 can modulate spindle density and afferent-motoneuron connectivity independently of ER81.

Expression of vesicular glutamate transporter 1 immunoreactivity in peripheral and central endings of trigeminal mesencephalic nucleus neurons in the rat.

The major neuronal components of the trigeminal mesencephalic nucleus (Vmes) are primary afferent neurons that convey proprioceptive information from the cranioorofacial regions. In the present study, we examined expression of vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, in the primary afferent neurons of the Vmes (Vmes neurons) in neonatal and adult rats. VGLUT1 immunoreactivity was detected in the cell bodies of Vmes neurons in neonatal rats younger than 11 days old, but not in older rats. However, in situ hybridization signals for VGLUT1 mRNA were detected in both neonatal and adult rats. No VGLUT2 immunoreactivity was detected in Vmes neurons of neonatal or adult rats. VGLUT1 immunoreactivity was also seen in the peripheral sensory endings on the equatorial regions of intrafusal fibers of muscle spindles in the masseter muscles in both neonatal and adult rats. In adult rats injected with cholera toxin B subunit (CTb) into the masseter nerve, central axon terminals of Vmes neurons were identified on masseter motoneurons within the trigeminal motor nucleus (Vm) by transganglionically and retrogradely transported CTb. VGLUT1-immunopositive axon terminals in close apposition to CTb-labeled Vm motoneurons were also detected by dual-immunofluorescence histochemistry for VGLUT1/CTb. Electron microscopy after dual immunolabeling for VGLUT1/CTb by the VGLUT1/immunoperoxidase and CTb/immunogold-silver methods further revealed synaptic contact of VGLUT1- and CTb-immunopositive axon terminals upon CTb-labeled neuronal profiles within the Vm. These data indicate that VGLUT1 is expressed in both the central axon terminals and the peripheral sensory endings of Vmes neurons, although no VGLUT1 immunoreactivity was detectable in the cell bodies of Vmes neurons in adult rats.

Proprioception in the extraocular muscles of mammals and man.

This article summarizes the authors' previous studies on proprioceptors in extraocular muscles (EOMs) of mammals and man. They report on muscle spindles in the EOMs of man, Golgi tendon organs in the EOMs of even-toed ungulates, and palisade endings in the EOMs of the cat. Muscle spindles: Muscle spindles are present in the EOMs of some mammals and in the EOMs of man. Compared with muscle spindles in other skeletal muscles, those in human EOMs exhibit structural differences. These structural differences may indicate a special function. Golgi tendon organs: Golgi tendon organs are absent in human EOMs. Golgi tendon organs exhibiting a specific morphology are present in the EOMs of even-toed ungulates. Their high number and rich innervation indicate functional importance. Palisade endings: Palisade endings are nervous end organs confined to the EOMs of mammals and man. It is assumed that these organs have a proprioceptive function. The authors show that palisade endings are immunoreactive for antibodies against choline acetyltransferase. Neuromuscular contacts, if present in palisade endings, are alpha -bungarotoxin positive as well. Taken together, these results show that palisade endings exhibit molecular characteristics of effector organs.

Immunohistochemical localization of alpha(1a)-adrenoreceptors in muscle spindles of rabbit masseter muscle.

The expression of alpha(1a)-adrenoreceptors (alpha(1a)-ARs) within the muscle spindles of rabbit masseter muscle was investigated. The alpha(1a)-ARs were detected by immunohistochemical fluorescent method and examined along the entire length of 109 cross serially sectioned spindles. The sympathetic fibers were visualized by the immunofluorescent labeling of the noradrenaline synthesizing enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). In order to recognize the intrafusal muscle fiber types, antibodies for different myosin heavy chain isoforms (MyHCI) were used. TH and DBH immunolabeled nerve fibers have been observed within the capsule lamellar layers, in the periaxial fluid space and close to intrafusal muscle fibers. The alpha(1a)-ARs were detected on the smooth muscle cells of the blood vessels coursing in the muscle and in the capsule lamellar layers or within the periaxial fluid space of the spindles. Moreover, at the polar regions of a high percentage (88.1%) of muscle spindles a strong alpha(1a)-ARs immunoreactivity was present on the intrafusal muscle fibers. In double immunostained sections for alpha(1a)-ARs and MyHCI it was evidenced that both bag, and nuclear chain fibers express alpha(1a)-ARs. The receptors that we have detected by immunofluorescence may support a direct control by adrenergic fibers on muscle spindle.

Sensory control of extraocular muscles.

The role of sensory receptors in eye muscles is not well understood, but there is physiological and clinical evidence for the presence of proprioceptive signals in many areas of the central nervous system. It is unclear which structures generate these sensory signals, and which central neural pathways are involved. Three different types of receptors are associated with eye muscles: (1) muscle spindles, (2) palisade endings, and (3) Golgi tendon organs, but their occurrence varies wildly between species. A review of their organization shows that each receptor is mainly confined to a morphologically separate layer of the eye muscle. The palisade endings - which are unique to eye muscles, are associated with the global layer; and they have been found in all mammals studied so far. Their function is unknown. The muscle spindles, if they are present in a species, lie in the orbital layer, or at its junction to the global layer. Golgi tendon organs appear to be unique to artiodactyls (i.e., sheep and goats, etc.); they lie in an outer distal marginal layer of the eye muscle, called the "peripheral patch layer" in sheep. The specific association between palisade endings and the multiply innervated type of muscle fibers of the global layer has led to the hypothesis that together they may act as a sensory receptor, and provide a source of central proprioceptive signals. But other interpretations of the morphological evidence do not support this role.