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PubMed1995

Neomyogenesis in neonatally de-efferented and postnatally denervated rat muscle spindles.

The ultrastructure of muscle spindles de-efferented by the extirpation of the lumbosacral spinal cord at the age of 2 days and subsequently deprived of their sensory innervation by the section of the sciatic nerve at 3-4 weeks of age was studied in serial sections of 2-month-old rat hindlimb muscles. De-efferentation leaves the primary sensory neurons and their peripheral axons intact and capable of inducing the muscle spindle morphogenesis during the critical period of their development. In de-efferented and subsequently denervated muscle spindles, new supernumerary intrafusal muscle profiles (SIPs) appeared in the muscle spindle A region. They were formed in intimate spatial relation with the original intrafusal muscle fibres (IMFs) predominantly from activated satellite cells derived from both nuclear bag (larger diameter) and nuclear chain fibres. SIPs, however, lacked the typical nuclear accumulations, as well as other ultrastructural distinctions present in control IMFs. The majority of differentiated SIPs separated from original IMFs, whereas the less differentiated SIPs were usually closely apposed to the surface of the parent IMFs and both were covered by the common basal lamina. In some spindles, the original IMFs and/or new SIPs at different stages of their differentiation were found together and they formed clusters of variable shape and composition. In the majority of clusters, all profiles seemed to be isolated along their entire length, although in few clusters, occasional cytoplasmic connections of variable length between intrafusal profiles were found. This result is important for the interpretation of the forthcoming study of expression of muscle spindle-specific myosin heavy chain isoforms in denervated SIPs in rat muscle spindles gradually deprived of their motor and sensory innervation.

Animals↗
PubMed1992

Development of the muscle spindles in the human fetal oblique muscles of the eyeball.

The development of muscle spindles in human oblique eye muscles was investigated in 22 fetuses aged 12 to 24 weeks. The muscle spindles appear at 12th week as 1-2 intrafusal fibers surrounded by capsule. With advancement of the development the number of intrafusal myofibers increases. In 14th week the capsule is composed of 2 layers and in 17th week the external capsule appears.

Embryonic and Fetal Development↗
PubMed1982

[On the fine structure of muscle spindle terminal in the lumbricalis muscles of the rat after section of the nerve (author's transl)].

The nerve endings in muscle spindles from lumbricalis muscles of the hindpaw of the rat were studied by electron microscopy from 10 h to 7 days after section of the sciatic nerve. No sensory endings were found after 5 days and no motor endings after 3 days. Early changes in sensory endings included accumulation of lamellar bodies, mitochondrial swelling or shrinkage, an edematous appearance of the cytoplasm and disruption of membranes. The predominant way of removal was phagocytosis by activated mesenchymal cells; other endings were incorporated into the sarcoplasm as electron dense bodies. Early changes in motor endings included aggregation or swelling of synaptic vesicles, swelling or shrinkage of mitochondria, edematous change of the cytoplasm and disruption of membranes. The predominant way of removal was incorporation into the cytoplasm of adjacent Schwann cells and degradation to phagolysosomes. Sites of removed sensory endings could be identified by basement membrane reduplication, while the intact subsynaptic apparatus of removed motor endings was either loosely covered by Schwann cell processes, or by fibrillar material, or appeared empty.

Animals↗
PubMed1994

Fibrillin and elastin networks in extrafusal tissue and muscle spindles of bovine extraocular muscles.

PURPOSE: Bovine extraocular rectus muscles were examined to map the distribution of elastin and fibrillin in extrafusal tissue and muscle spindles. METHODS: Immunohistochemical techniques and immunolocalization were employed to pin-point the placement of molecules relative to muscle fibers. RESULTS: Strands containing elastin and fibrillin surrounded all extrafusal fibers. They also covered the external surface of intrafusal fibers, more extensively at the equator than at the pole. Within strands elastin was placed in the center, whereas fibrillin was located in microfibrils on the periphery. CONCLUSIONS: The wide distribution in extrafusal tissue of elastin and fibrillin suggests that they are factors in determining the mechanical properties of extraocular muscles. Their placement in proximity to individual intrafusal fibers should affect the viscoelastic properties of these fibers and, thus, influence the dimensions of the afferent discharge.

Animals↗
PubMed1978

Myelinated nerve fiber supply and muscle spindles in the respiratory muscles of cat: quantitative study.

The present study was undertaken to provide quantitative data on the myelinated fibers of the phrenic and intercostal nerves and the number of spindles in the main respiratory muscles of the cat. The myelinated component of the phrenic and intercostal nerves was studied in the cat. Histograms of sequency distributions as a function of nerve fiber diameter were established for normal nerves. Certain nerves were then examined 35 to 40 days after excision of the dorsal spinal ganglia. The muscle spindles of the corresponding muscles were counted and localized, and, on the basis of several morphological criteria, were classified with those usually described in the interosseous muscles. The study of the nerves, as that of the spindles, demonstrates clear differences of proprioceptive innervation among the respirator muscles. The lateral part of the diaphragm and the Triangularis sterni have practically no spindles. The external muscles of the first thoracic spaces are very rich in spindles. Respiratory muscles can be ranged in an almost continuous manner between these two extremes.

Animals↗
PubMed1997

Phasic and tonic stretch reflexes in muscles with few muscle spindles: human jaw-opener muscles.

We investigated phasic and tonic stretch reflexes in human jaw-opener muscles, which have few, if any, muscle spindles. Jaw-unloading reflexes were recorded for both opener and closer muscles. Surface electromyographic (EMG) activity was obtained from left and right digastric and superficial masseter muscles, and jaw orientation and torques were recorded. Unloading of jaw-opener muscles elicited a short-latency decrease in EMG activity (averaging 20 ms) followed by a short-duration silent period in these muscles and sometimes a short burst of activity in their antagonists. Similar behavior in response to unloading was observed for spindle-rich jaw-closer muscles, although the latency of the silent period was statistically shorter than that observed for jaw-opener muscles (averaging 13 ms). Control studies suggest that the jaw-opener reflex was not due to inputs from either cutaneous or periodontal mechanoreceptors. In the unloading response of the jaw openers, the tonic level of EMG activity observed after transition to the new jaw orientation was monotonically related to the residual torque and orientation. This is consistent with the idea that the tonic stretch reflex might mediate the change in muscle activation. In addition, the values of the static net joint torque and jaw orientation after the dynamic phase of unloading were related by a monotonic function resembling the invariant characteristic recorded in human limb joints. The torque-angle characteristics associated with different initial jaw orientations were similar in shape but spatially shifted, consistent with the idea that voluntary changes in jaw orientation might be associated with a change in a single parameter, which might be identified as the threshold of the tonic stretch reflex. It is suggested that functionally significant phasic and tonic stretch reflexes might not be mediated exclusively by muscle spindle afferents. Thus, the hypothesis that central modifications in the threshold of the tonic stretch reflex underlie the control of movement may be applied to the jaw system.

Electromyography↗
PubMed1983

Muscle spindle supply in the pig masticatory muscles.

This histological study was designed to define the anatomical background of the neuromuscular control mechanism involved in the non-ruminant omnivorous mastication, focusing on the muscle spindle supply to each muscle belly of the jaw muscles of 3 pig fetuses (near term). The jaw-closing muscle averaged 427 muscle spindles, 566 spindles, 703 spindles on one side of the face respectively: 160, 203, 240 in the temporalis, 122, 160, 248 in the masseter, 128, 154, 193 in the medial pterygoid, and 16, 49, 21 in the zygomaticomandibularis. No spindles were in the lateral pterygoid and the jaw-opening muscles.

Animals↗
PubMed1989

Endocytosis and transcytosis by the capsule cell of vertebrate muscle spindles.

The vertebrate muscle spindle has been observed to be ionically and biochemically isolated from the surrounding muscle fibers by the spindle capsule. We have explored the possibility that the capsular cells are endocytically active and can transport both small molecules and macromolecules into the capsular space. Transcytosis (the endocytic transport of extracellular substances across a cell) through the capsule cell layer was examined with muscle spindles of snake, rat, and cat using fluorescent markers for fluorescence microscopy and horseradish peroxidase (HRP) and ferritin for electron microscopic examination. The fluorescent markers were actively taken up by capsule cells, making it easy to locate the spindle capsular region of spindles among extrafusal fibers by their strong fluorescence. Ferritin and HRP were used to identify the pathway of transcytosis by electron microscopy. These markers were found in endocytic vesicles of capsule cells, in the narrow space between capsule layers and in the capsular space, indicating that the marker was transferred to the capsular space by the pinocytic activity of capsule cells. Scattered cells in the capsule of cat muscle spindles appeared to take up fluorescein isothiocyanate (FITC)-coupled beta-glucuronidase by a receptor-mediated process. The uptake was sensitive to temperature and [Ca2+], and specifically inhibited by yeast mannan. By electron microscopy with dilute HRP (10 micrograms/ml) this specific uptake was by isolated cells in the interlamellar space. The functional significance of the above findings is discussed.

Animals↗
PubMed1982

The number and distribution of muscle spindles in human intrinsic postvertebral muscles.

Serial transverse sections of intrinsic postvertebral muscles of a 21 weeks old human fetus were studied after staining with haematoxylin and eosin, Van Gieson's stain with Weigert's haematoxylin, and Glees' silver impregnation technique. Muscle spindles have been demonstrated throughout the entire length of the intrinsic postvertebral muscles. The total numbers of spindles on the right and left sides were determined. Using a special technique of graphic reconstruction, the locations of spindles within these muscles were plotted. Regional differences in the numbers and distribution of spindles were noted. The significance of these findings has been discussed.

Humans↗
PubMed2003

Mechanical effect of muscle spindles in the canine external intercostal muscles.

High-frequency mechanical vibration of the ribcage increases afferent activity from external intercostal muscle spindles, but the effect of this procedure on the mechanical behaviour of the respiratory system is unknown. In the present study, we have measured the changes in external intercostal muscle length and the craniocaudal displacement of the ribs during ribcage vibration (40 Hz) in anaesthetized dogs. With vibration, external intercostal inspiratory activity increased by approximately 50 %, but the respiratory changes in muscle length and rib displacement were unaltered. A similar response was obtained after the muscles in the caudal segments of the ribcage were sectioned and the caudally oriented force exerted by these muscles on the rib was removed, thus suggesting that activation of external intercostal muscle spindles by vibration generates little tension. Prompted by this observation, we also examined the role played by the external intercostal muscle spindles in determining the respiratory displacement of the ribs during breathing against high inspiratory airflow resistances. Although resistances consistently elicited prominent reflex increases in external intercostal inspiratory activity, the normal inspiratory cranial displacement of the ribs was reversed into an inspiratory caudal displacement. Also, this caudal rib displacement was essentially unchanged after section of the external intercostal muscles, whereas it was clearly enhanced after denervation of the parasternal intercostals. These findings indicate that stretch reflexes in external intercostal muscles confer insufficient tension on the muscles to significantly modify the mechanical behaviour of the respiratory system.

Airway Obstruction↗
PubMed1988

Superimposing noise linearizes the responses of primary muscle spindle afferents to sinusoidal muscle stretch.

Experiments were conducted in anaesthetized and spinalized cats to measure the extent to which the non-linear response of Ia afferent fibers to sinusoidal muscle stretch as expressed by the peristimulus-time-histograms, PSTHs, can be transformed into a linear one by means of the superposition of random stretch ("mechanical noise"). The gastrocnemius muscles of one hind leg were stretched and the response to sinewave muscle stretch (amplitudes between 0.01 and 4.0 mm, frequencies between 0.1 and 20 Hz) were investigated while band-limited mechanical noise was superimposed on the sinewave stretch. The random stretch upper cut-off frequency was varied between 60 and 300 Hz; the displacements were normally distributed. The noise amplitude sigma, i.e. the standard deviation of the displacement distributions, was varied systematically between 0.002 and 0.4 mm. Mechanical noise was very effective in raising the mean discharge rate. Added to the sinusoidal stretch it prevented the cessation of firing during the release phase of the stretch cycle, or at least reduced the duration of discharge pauses, i.e., a linearization occurred. In general, the larger the noise amplitude, the more the amplitude of the fundamental harmonic component was attenuated and the phase lead reduced. Apart from this rule the particular combination of superimposing small noise (sigma less than 0.02 mm) on small sinewave stretch (A less than 0.02 mm) could enhance the depth of sinusoidal modulation of cycle histograms (compared with responses to pure sinusoids). Linearizing the sinewave response by additional noise allowed the estimation of frequency response characteristics in the otherwise non-linear range of amplitudes (sinewave amplitude 0.5-1.0 mm).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗