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PubMed1988

Postnatal development of the mouse temporal muscle and effects of a fine-grained diet on the muscle spindle.

The postnatal development of the mouse temporal muscle and effects of a fine-grained diet on muscle spindles were investigated. On d 25 after birth, 3 types of extrafusal muscle fibers could be distinguished on the basis of SDH activity, but not on the basis of their diameters. And in 50-d-old mice, 3 types of the muscle fibers had different diameters each other. In intrafusal muscle fibers, the postnatal changes of the SDH activity were similar to those of extrafusal muscle fibers, but the diameter distribution showed unimodal statistical distribution from 10th to 50th postnatal day. On the other hand, in control 180-d-old mice, 80 percent of muscle spindles in the temporal muscle had complete annulospiral endings, but the ratio of muscle spindles with complete annulospiral endings was decreased significantly in mice fed a fine-grained diet. The diet also decreased the diameter of the endings significantly. These results suggest that the main changes in the morphological and functional properties of the mouse temporal muscle occur in during and early after weaning period, and that normal mastication is needed to maintain the homeostatic condition of the mouse temporal muscle.

Animals↗
PubMed1991

Rat muscle spindle immunocytochemistry revisited.

The expression of myosin heavy chain isoforms in muscle spindle fibres has been the subject of a number of immunocytochemical studies, some of them with discordant results. In order to assess whether these discrepancies are due to differences in the specificity and sensitivity of the antibodies used, we have compared the reactivity of rat muscle spindle fibres to two pairs of antibodies presumed to be directed against slow tonic (ALD 19 and ALD 58) and neonatal (NN5) and neonatal/fast (MF30) myosin heavy chains. Adult, developing and neonatally de-efferented muscle spindles from the rat hind limb muscles were studied in serial cross-sections processed for the peroxidase-antiperoxidase method. Important differences in the staining profiles of intrafusal fibres were noted when ALD 19 and ALD 58 were compared. ALD 19 stained the muscle spindle precursors from the seventeenth day in utero, whereas ALD 58 only did so by the twentieth day of gestation. In adult spindles ALD 19 stained the nuclear bag1 fibres along their entire length, whereas ALD 58 did not stain these fibres towards their ends. ALD 19 stained the nuclear bag2 fibres along the A, B and inner C region, but ALD 58 stained these fibres only in the A and the inner B regions. ALD 19 stained some nuclear chain fibres along a short equatorial segment, whereas ALD 58 did not stain the nuclear chain fibres at all. NN5 stained the nascent nuclear bag1 and chain fibre precursors at earlier stages of development than MF30. Clear differential staining between primary and secondary generation of both extra- and intrafusal myotubes was seen with NN5, whereas MF30 stained all myotubes alike. However, in postnatal spindles, MF30 was a very good negative marker of nuclear bag1 fibres. The staining profile of the adult fibres with NN5 and MF30 was rather similar. The staining pattern of neonatally de-efferented bag fibres obtained with ALD 19 and ALD 58 was practically identical and it differed from that of control spindles, confirming that motor innervation participates in the regulation of the expression of slow tonic MHC along the length of the nuclear bag2 fibres, as we have previously shown with ALD 19. The distinct staining patterns obtained with ALD 19 versus ALD 58 and with NN5 versus MF30 reflect differences in antibody sensitivity and specificity. These differences account, in part, for the discrepancies in the results of previous studies on muscle spindles, published by Kucera and Walro using ALD 58 and MF30, and by us using ALD 19 and NN5.

Animals↗
PubMed1978

Modification by previous afferent discharge of the adaptation of frog muscle spindles following an extension.

1. Extension of a frog muscle spindle evoked a discharge of impulses in its sensory axon. The adaptation of the discharge after the dynamic phase of stretching occurred in two phases. At first the impulse train was almost regular and adapted quickly, but later this gave place to a phase of slower adaptation in which the variability of discharge was much increased. 2. The discharge of action potentials by the muscle spindle depressed the response of the receptor to a subsequent extension. This was true whether they were elicited antidromically by afferent stimulation or orthodromically by longitudinal vibration. This depression had its most marked effect on the first phase of adaptation where it greatly increased the rate of adaptation. The second, slower, phase of adaptation was little influenced. 3. The depression increased with the duration and with the frequency of afferent stimulation. It persisted for about 300 msec. 4. The results show that the properties of the spike generating mechanisms in the muscle spindle are modified by previous discharge and that this modification may influence the overall properties of the receptor. 5. The fact that afferent stimulation has different effects on the two phases of adaptation is consistent with the suggestion that the impulse train evoked by extension is derived from more than one spike generator within the muscle spindle.

Action Potentials↗
PubMedDate not supplied

Permeability of muscle spindle capillaries and capsule.

The permeability of capillaries in rabbit muscle spindles, intramuscular nerves, and extrafusal muscle to intraaortically infused horseradish peroxidase (HRP) was studied. The permeability of the capsule cells of the muscle spindles was also examined. Immediately following the infusion of HRP, most extrafusal capillaries contained a dense accumulation of HRP reaction product, which began to escape into the surrounding extracellular space within 2-3 min. In contrast, the concentration of HRP in the intrafusal and endoneurial capillaries was much lower than that in extrafusal capillaries, and HRP was never found outside the former vessels. In addition, HRP never completely penetrated through the spindle capsule from the extracellular into the periaxial space, although it was regularly found between the outer two or three layers of capsule cells. The results indicate that the permeability characteristics of the muscle spindle capillaries and capsule to the tracer protein HRP are very similar to those of capillaries in peripheral nerve.

Animals↗
PubMed2003

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive.

There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system. We examined the validity of this idea in human subjects by recording from muscle spindles in the relaxed ankle and toe extensor muscles during a strong and sustained physiological activation of muscle sympathetic outflow. Unitary recordings were made from 20 primary and 17 secondary muscle spindle afferents via a tungsten microelectrode inserted percutaneously into the peroneal nerve in 10 awake, healthy subjects seated with the legs supported in the extended position. ECG, blood pressure, respiration and calf circumference were also recorded. The majority of the muscle spindles were spontaneously active at rest; a background discharge was induced in four silent spindles by vibrating the tendon. A sustained increase in muscle vasoconstrictor activity, an increase in calf volume and a fall in pulse pressure were produced by subjects performing a 30-40 s maximal inspiratory breath-hold. Despite this strong increase in muscle sympathetic outflow no significant changes occurred in the discharge of either primary or secondary muscle spindle afferents, measured as a change in mean frequency and variability over sequential 5 s epochs and compared with the preceding period of rest. Strong chemoreceptor-driven sympathetic bursts during sustained expiratory breath-holds also failed to modulate the firing of 14 spindle endings. We conclude that a sustained, physiological increase in muscle sympathetic activity causes no detectable change in muscle spindle firing, lending no support to the concept that the sympathetic nervous system can influence the sensitivity of human muscle spindles directly.

Adolescent↗
PubMed1975

Topological distribution of muscle spindles in the human tongue and its significance in proprioception.

In order to obtain numerical data concerning the lingual muscle spindle distribution, the muscle spindles were histologically surveyed on the serial frontal and horizontal sections (stained by hematoxylin-eosin) from the apex to the radix of the tongue. On one side four hundred and sixty-six muscle spindles were counted in seven muscles, the superior longitudinal muscle containing 159 spindles, the genioglossus 80, the transverse 79, the styloglossus 75, the hyoglossus 37, the inferior longitudinal 22 and the vertical 14. The chondroglossus and the palatoglossus are devoid of the spindles. The question of whether the proprioceptive impulses from the lingual spindles lie in the hypoglossus nerve or in the lingual nerve has remained unsolved. This histological confirmation of the pathway and primary cell station of the lingual proprioceptive afferents is the subject in which the authors are greatly interested.

Humans↗
PubMed1989

Physiological properties of tandem muscle spindles in neck and hind-limb muscles.

Although tandem muscle spindle complexes are found in small but significant numbers in most muscles, experimental investigation of their properties has been problematic because of the difficulty of distinguishing their afferents from those of "normal" single spindles. Of particular interest are the afferents from b2c capsules of tandem spindles, which unlike normal spindles contain only a static b2 nuclear bag fibre and some nuclear chain fibres. The absence of a dynamic b1 nuclear bag fibre from b2c spindles has engendered much speculation as to their response properties and their possible role in motor control. We have recently developed a method for the identification of afferents from b2c spindles in electrophysiological experiments, using infusion or topical application of succinylcholine (SCh). SCh causes the contraction of the dynamic b1 and static b2 nuclear bag intrafusal fibres, and paralyses the nuclear chain fibres. Afferents from b2c spindles are characterized by a strong "biasing" of their discharge rate to about 100 impulses per second (i.p.s.) when activated by SCh (reflecting the contraction of the static b2 fibre), while primary afferents from normal b1b2c spindles show a large increase in dynamic sensitivity as well as "biasing" (reflecting the contraction of both dynamic b1 and static b2 bag fibres). Histological examination of tenuissimus spindles activated by SCh has confirmed this relationship between the pattern of activation by SCh and the number of intrafusal nuclear bag fibres in the spindle. In this paper we review the value of SCh as a means of testing spindle afferents for functional inputs from sensory terminals on the nuclear bag fibres, and discuss the properties of b2c afferents from tandem spindles in the context of their possible function.

Animals↗
PubMed1986

Classification of muscle spindle receptors.

The conduction velocities of muscle spindle afferent fibers have a bimodal distribution, and classifications of spindle receptors based on afferent fiber diameter have therefore divided these receptors into two groups, the well known primary and secondary endings. However, measures of spindle function that are likely to be important for kinesthetic sensibility such as dynamic response, adaptation and linear directionality (hysteresis) are distributed rather uniformly. Therefore, from this functional perspective it might be argued that muscle spindle receptors should not be subdivided at all. On the other hand, different receptors demonstrate these properties to varying degrees, and there are simple, linear correlations among log (dynamic response), log (adaptation), linear directionality and conduction velocity. Thus, the receptors can be divided into as many as 5-10 different subpopulations that differ significantly in one or more of these properties.

Adaptation, Physiological↗
PubMed1986

Acute lesion of the intrafusal muscle of muscle spindles: ultrastructural and electrophysiological consequences.

Experiments were performed to functionally separate the neural and muscular elements in muscle spindles. Cat muscle spindles were dissected free in vitro and were transected on both sides of the sensory nerve endings. This resulted in an acute lesion of the intrafusal muscle fibers but spared the sensory endings. Within 45 min, the intrafusal muscle fibers degenerated to the condition in which there were no myofilaments left in the sensory zone and the muscle fiber membrane had begun to disintegrate. During this degeneration period, and afterward, the sensory nerve fibers continued to fire action potentials. However, the postlesion discharge was usually more irregular than normal, and the mean rate of firing was often dramatically increased. In addition, multiplex frequencies of firing were often observed; that is, the interspike intervals were clustered around 2 or more modal values. Computer modeling of 2 action potential initiators competing for access to the same output line suggests that the multiplexing results from at least one of the competitors having a fraction of its spikes aborted. Overall, the results indicate that the nerve endings have an intrinsic capability to function as sensory receptors in the absence of a muscle substrate.

Animals↗
PubMed2006

Functional adaptability of jaw-muscle spindles after bite-raising.

In a previous experiment, we found that masseter muscle spindles show functional plasticity after 5 to 15 days under increased occlusal vertical dimension (iOVD). In the present study, we hypothesized that spindle function would eventually recover if longer observation periods were allowed. Therefore, in this study we investigated changes in masseter muscle spindle function over periods of 1 day to 8 weeks. Masseter muscle-spindle responses to ramp-and-hold jaw stretches were recorded from the mesencephalic trigeminal nucleus in 35 barbiturate-anesthetized female Wistar rats. The rats were previously divided into Control and iOVD groups, and those in the iOVD group received a 2.0-mm composite resin build-up to the maxillary molars. In this condition, there were no statistically significant differences in masseter muscle spindle sensitivity between Control and iOVD in the six- and eight-week subgroups. Our results further indicate a high degree of adaptability in masseter muscle spindle function following changes in OVD.

Adaptation, Physiological↗
PubMed1989

Diversity in expression of myosin heavy chain isoforms and M-band proteins in rat muscle spindles.

The composition of adult rat soleus muscle spindles, with respect to myosin heavy chain isoforms and M-band proteins, was studied by light-microscope immunohistochemistry. Serial sections were labelled with antibodies against slow tonic, slow twitch, fast twitch and neonatal myosin isoforms as well as against myomesin, M-protein and the MM form of creatine kinase. Intrafusal fiber types were distinguished according to the pattern of ATPase activity following acid and alkaline preincubations. Nuclear bag1 fibers were always strongly stained throughout with anti-slow tonic myosin, were positive for anti-slow twitch myosin towards and in the C-region but were unstained with anti-fast twitch and anti-neonatal myosins. The staining of nuclear bag2 fibers was in general highly variable. However, they were most often strongly stained by anti-slow tonic myosin in the A-region and gradually lost this reactivity towards the poles, whereas a positive reaction with anti-slow twitch myosins was found along the whole fiber. Regional staining variability with anti-neonatal and anti-fast myosins was apparent, often with decreasing intensity towards the polar regions. Nuclear chain fibers showed strong transient reactivity with anti-slow tonic myosin in the equatorial region, did not react with anti-slow twitch and were always evenly stained by anti-fast twitch and anti-neonatal myosins. All three intrafusal fiber types were stained with anti-myomesin. Nuclear bag1 fibers lacked staining for M-protein, whereas bag2 fibers displayed intermediate staining, with regional variability, often increasing in reactivity towards the polar regions. Chain fibers were always strongly stained by anti-M-protein. The MM form of creatine kinase was present in all three fiber types, but bag1 fibers were less reactive and clear striations were not observed, in contrast to bag2 and chain fibers. Out of 38 cross sectioned spindles two were found to have an atypical fiber composition (lack of chain fibers) and a rather diverse staining pattern for the different antibodies tested. Taken together, the data show that in adult rat soleus, slow tonic and neonatal myosin heavy chain isoforms are only expressed in the muscle spindle fibers and that each intrafusal fiber type has a unique, although variable, composition of myosin heavy chain isoforms and M-band proteins. We propose that both motor and sensory innervation might be the determining factors regulating the variable expression of myosin heavy chain isoforms and M-band proteins in intrafusal fibers of rat muscle spindles.

Animals↗
PubMed1986

Comparison of muscle spindle concentrations in large and small human epaxial muscles acting in parallel combinations.

A small short muscle acting across a joint in parallel with vastly larger and longer muscles is clearly unable to play more than a minimal mechanical role in such a "parallel muscle combination" (PMC). This research investigates a feed back role for the small muscles of PMCs, proposing a significantly higher muscle spindle concentration therein to be consistent with this role. Epaxial PMCs (semispinalis and multifidus versus rotatores brevis) from the C5-C6, T6-T7, and L4-L5 regions of three 36-week-old male fetuses and two adult cadavers were removed and fixed in Carnoy's fluid. Tissue samples were embedded in paraffin, cut into 10 microns thick sections perpendicular to the muscle's longitudinal axis and stained by Harris' hematoxylin and eosin. Representative tissue sections were projected onto a sterological grid and the percentage volume of spindles determined. Data were analyzed with Student's unpaired t test. In all PMCs, rotators brevis spindle percentage volumes ranged from 4.58 to 7.30 times higher than those of multifidus and semispinalis. Differences in mean spindle percentage volumes between large and small members of all PMCs were significant (P less than .0001). Our findings are consistent with the notion of a "kinesiological monitor" or feedback role for rotatores brevis.

Adult↗
PubMed2000

Effects of calcium on the discharge pattern of primary and secondary endings of isolated cat muscle spindles recorded under a ramp-and-hold stretch.

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of external calcium (Ca(2+)). The outer capsule of the muscle spindle represents an effective diffusion barrier for Ca(2+) ions since activity changes were strong and rapid only if the capsule was partly removed from the sensory region of the receptor. The impulse activity of both primary and secondary muscle spindle endings was lowered by an increase in the external Ca(2+) concentration from 1.8 mM (normal Ringer's solution) to 2.7 mM and raised by a decrease in the Ca(2+) concentration from 1.8 to 0.9 mM. Primary endings were generally more strongly affected than secondary endings. With primary endings the firing rate changed by 23-52% when the external Ca(2+) concentration was altered by 0.9 mM. With secondary endings the discharge frequency changed by 15-24%. The afferent discharge patterns were obtained under repetitive ramp-and-hold stretches and were analyzed with regard to influences of external Ca(2+) ions on the static and dynamic components of the endings' responses. The stretch sensitivity and the adaptive response of both types of ending increased in the low Ca(2+) solution and decreased in the high Ca(2+) solution, but a specific effect on a single component of the responses to stretch was not observed. These findings indicate an overall change in excitability when the external Ca(2+) concentration was varied. The mechanical properties of the receptor were probably not affected since changes in the Ca(2+) concentration did not elicit a contraction or relaxation of the intrafusal muscle fibers. On the one hand, the observed effects can be explained according to the surface potential theory by an indirect influence of extracellular Ca(2+) ions on ion channels of the sensory nerve terminals, with Ca(2+) ions binding to negative charged sites at the endings' outer membrane. On the other hand, the results are consistent with the supposition that Ca(2+) ions act directly on ion channels of the sensory membrane of muscle spindle endings.

Adaptation, Physiological↗
PubMed1985

Postnatal development of the anulospiral endings of Ia fibers in muscle spindles of mice.

The formation of the anulospiral ending of Ia fibers in muscle spindles was investigated in the masseter muscle of developing mice. Before 15 days after birth, the complete anulospiral ending was not observed in almost all of the muscle spindles examined. With the growth of mice, the Ia fiber began to construct the spiral ending, and by the 40th postnatal day after weaning, almost all of the Ia fibers of the muscle spindles had complete coiled endings, though the formation still continued in some spindles. The continuous formation of anulospiral endings for a long period after weaning indicates that muscle spindle morphogenesis may be affected by muscle tension in the masseter muscle due to the movement activated after weaning.

Animals↗
PubMed1999

Physiological evidence for caudal brainstem projections of jaw muscle spindle afferents.

Retrograde transport and intra-axonal labeling studies provide convincing evidence that jaw-muscle spindle afferents project to the caudal medulla by way of Probst's tract. However, functional properties of this caudal projection are not well understood. Extracellular recordings were made in cats at the level of the subnucleus interpolaris (Vi) to identify single units that showed consistent responses to ramp-and-hold stretches of the jaw. In this report, we present data from 20 central units with properties indicating that they received input from trigeminal muscle spindle afferents. All units were activated by gentle palpation of jaw muscles, and none had superficial receptive fields. Two groups of neurons could be defined based on their responses to passive jaw movements. One group (n=12) showed an obvious dynamic response (i.e., a higher level of activity at the onset of stretch than during the hold period). Activity was maintained during the hold phase, and the units stopped firing (unloaded) for a brief period upon jaw closure. The other group (n=8) lacked a dynamic response. Instead, they showed an increase in firing with onset of stretch that was maintained during the hold phase. Thirteen units, which were tested with more than three different jaw stretch speeds and/or amplitudes, were further characterized by analyzing dynamic index (DI) and mean firing rate (MFR) during each phase of the ramp-and-hold movement as well as interspike interval (ISI) variability. All but one unit with a dynamic response showed a speed-sensitivity. In all cases, the MFR was a more sensitive indicator of changes in jaw speed than DI. Neurons in the other group (5/5 tested) showed a high position-sensitivity, i.e., their firing rates varied as a function of amplitude of jaw opening. The percent change in ISI variability for all neurons ranged from 37-84%. The response characteristics of these central neurons were compared to known physiological properties of muscle spindle afferents. The results provided compelling evidence for jaw-muscle-spindle afferent projection onto these neurons. Reconstruction of recording sites showed that medial Vi, and the adjacent reticular formation, are likely recipients for the caudal projections from jaw-muscle-spindle afferents. We suggest that muscle spindle input to this region is well suited for influencing the coordination of motor behavior during feeding and for the integration and processing of kinesthetic information.

Afferent Pathways↗
PubMed1980

Ultrastructural study of a blood-muscle spindle barrier after systemic administration of horseradish peroxidase.

The permeability of the normal muscle spindle capsule to the entrance of an exogenous protein tracer was assessed by the use of horseradish peroxidase (HRP). Adult mice were injected intravenously with a solution of HRP (MW 40,000). After varying intervals, ranging from two to 240 minutes, animals were perfused aortically with fixative, and anterior tibialis muscles were removed, processed, and examined. The results were evaluated by electron microscopic cytochemical techniques, and attention was directed to the movement of tracer and its relationship to the outer and inner capsule of the muscle spindle. HRP was first demonstrated in cytoplasmic vesicles crossing the continuous capillary endothelium supplying the muscle spindle, and was then detected in the interstitial space surrounding the outer capsule. In polar regions, HRP had entered the periaxial space by two minutes, and it was seen in cytoplasmic vesicles of the attenuated outer capsule. Later, HRP abutted the sarcolemma of intrafusal fibers, and by 12.5 minutes there was evidence of tracer in T-tubules of these muscle cells. Whereas at no time was tracer observed traversing any of the numerous intracellular junctions of the capsule, it appeared that polar regions were leaky and open-ended at their distal portions. The sensory equatorial zone was considerably less permeable to the entrance of tracer. Whereas HRP was visible intially in cytoplasmic vesicles of the subjacent capillary endothelium, it was not until 12.5 minutes that racer could be demonstrated within some vesicles of equatorial outer capsule cells. At later times, a small amount of HRP was observed in the equatorial periaxial space, where it was phagocytosed and finally sequestered by cells of the inner capsule as membrane-bound lysosomal deposits. Consequently, at no time was tracer incorported into either intrafusal fibers or their sensory nerve terminals. Thus, in contract to polar regions, the muscle spindle capsule in the equatorial zone appears to be effective in preventing the indiscriminate penetration of HRP from the bloodstream. This suggests dissimilar paths of tracer movement from the microvasculature into sensory and non-sensory regions of the muscle spindle.

Animals↗
PubMed1978

Muscle spindles in the inferior constrictor pharyngis muscle of the crab-eating monkey (Macaca irus).

Pharyngeal muscles were considered to be lacking in spindles. A search was made for the same in the inferior constrictor pharyngis muscle of the crab-eating monkey (Macaca irus). Muscle spindles were found for the first time in this muscle. Transverse serial sections and teased preparations were obtained from this muscle belonging to six such animals, and these tissues were stained by Masson's trichrome and Ranvier's gold method, respectively. Both the methods were successful in revealing these receptors. The potential significance of this finding was discussed in relation to the activities of the pharyngeal muscles and a suggestion for a thorough re-investigation of the so-called spindleless muscles was made.

Animals↗