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PubMed2002

Effects of simulated weightlessness on Calbindin D-28K-immunoreactivity in rat soleus muscle spindle.

Objective. To analyze the mechanism involved in muscle spindle deterioration during simulated weightlessness. Method. Using the immunoperoxidase reaction utilizing the ABC (avidin-biotin-complex) method, Calbindin D28K-like immunoreactivity (CaBP-LI) of intrafusal fibres in soleus muscle were detected in 7 d, 14 d tail-suspended rats and compared with control rats. Result. The extrafusal muscle fibres and nerve fibres did not exhibit immunoreactivity to CaBP. CaBP-LI was found in some of the intrafusal muscle fibres in all the muscle spindles. After 14 d suspension, the immunoreactivity to CaBP of the intrafusal fibres decreased markedly. Conclusion. Simulated weightlessness could induce changes in the immunoreactivity of rat soleus muscle spindle to CaBP, which may contribute to the deterioration of muscle spindle.

Animals↗
PubMed1981

[Effects of muscle relaxant E.M.P.P. on afferent discharges of muscle spindle in man--an microneurographic analysis (author's transl)].

Effects of a new muscle relaxant E.M.P.P.(4'-ethyl-2-methyl-3-piperidino-propiophenone . hydrochloride) on afferent discharges of muscle spindle were studied in 10 healthy volunteer subjects. Single unit afferent discharges of muscle spindle were recorded microneurographically from the median or tibial nerve using a tungsten microelectrode with a tip diameter of about 1 micrometer (impedence: about 10 M omega). Effects of an oral administration of 150--300 mg of E.M.P.P. on spontaneous afferent discharges as well as dynamic and static responses of muscle spindle were analysed in the present study. 1. E.M.P.P. suppressed remarkably the frequency of spontaneous afferent discharges of muscle spindle. Meanwhile, mental activations such as speech of mental arithmetic recovered temporarily the frequency of afferent discharges to that before E.M.P.P. administration. 2. E.M.P.P. suppressed the dynamic and static responses of muscle spindle to stretch. It also suppressed the dynamic responses of muscle spindle in the decontraction phase of electrically-induced twitch contractions of the receptor-bearing muscle. Based on these results, it is concluded that E.M.P.P. suppresses the static and dynamic activities of muscle spindle in man. These effects seem to be due to modifications of descending influences from central structures on the static and dynamic gamma motoneurons which innervate the muscle spindle.

Adult↗
PubMed1989

The selectivity of fusimotor innervation in muscle spindles of the rat studied by light microscopy.

Six muscle spindles and three muscle spindle poles from four rat soleus muscles have been sectioned serially at 1 micron intervals to trace the motor innervation by light microscopy. Forty myelinated axons had 92 endings on the intrafusal muscle fibres. 67.5% of these axons supplied a single type of muscle fibre only, 22.5% to dynamic bag1 (Db1) fibres, 15% to static bag2 (Sb2) fibres and 30% to chain fibres. The rest supplied more than one fibre type, 5% supplying the Db1 and one chain fibre, 20% supplying the Sb2 and chain fibres, but 7.5% (three axons) supplied all the fibre types together. Apart from these three axons all the fusimotor axons would be expected to have a clear dynamic or static action on the Group Ia discharge. Whilst for the cat entirely non-specific distribution does not exist, or at least is very rare, since only a small proportion of the rat fusimotor axons were in this category we conclude that fusimotor distribution in rats and cats is essentially similar.

Animals↗
PubMed1971

The responses of frog muscle spindles and fast and slow muscle fibres to a variety of mechanical inputs.

1. The tension in the iliofibularis muscle of frogs was recorded while the muscle was stretched or released. At the same time recordings were made from single spindle afferents in dorsal root filaments. Either large or small motor nerve fibres were stimulated in split ventral root filaments.2. While small motor nerve fibres were stimulated the discharge from muscle spindle afferents was greatly increased by stretching, and greatly reduced by shortening the muscle. This sensitivity to movement was shown even if the movements were small, so that a stretch of 0.2% of the muscle length was sufficient to cause a pronounced increase in the afferent discharge.3. In contrast, during stimulation of the large motor nerve fibres the spindle was much less sensitive to movements with the result that even stretches or releases of the muscle by 1 mm did not cause very large changes in the discharge frequency.4. The tension in slow extrafusal muscle fibres in many ways mirrored the spindle discharge during the stimulation of small motor nerve fibres, for the tension was greatly increased by stretching, even through small distances, and greatly reduced by releasing the muscle. The tension in fast extrafusal muscle fibres was much less changed by such movements, and thus was rather like the spindle discharge during stimulation of large motor nerve fibres.5. As the extrafusal muscle fibres do not directly pull on and excite the spindle afferents, the simplest explanation for the similarities between the muscle tension and the spindle discharge is that the mechanical properties of the intrafusal muscle fibres innervated by the large motor nerve fibres are like those of fast extrafusal muscle fibres, and that the mechanical properties of the small intrafusal fibres are similar to those of slow extrafusal muscle fibres.6. It is shown that the cross-bridge sliding filament mechanism of muscle contraction provides a ready explanation for the differences found between fast and slow muscles, and it is concluded that a most important functional difference between the two sorts of intrafusal muscle fibres is the speed of their contractions, for it is this which determines their contrasting actions on the spindle.7. It was also found that low rates (< 4/sec) of small motor nerve fibre stimulation were often very effective in exciting the spindles. These rates produced rather little extrafusal tension.

Action Potentials↗
PubMed2002

A biorobotic structural model of the mammalian muscle spindle primary afferent response.

A biorobotic model of the mammalian muscle spindle Ia response was implemented in precision hardware. We derived engineering specifications from displacement, receptor potential, and Ia data in the muscle spindle literature, allowing reproduction of muscle spindle behavior directly in the robot's hardware; a linear actuator replicated intrafusal contractile behavior, a cantilever-based transducer reproduced sensory membrane depolarization, and a voltage-controlled oscillator encoded strain into a frequency signal. Aspects of muscle spindle behavior not intrinsic to the physical design were added in control software using an adaptation of Schaafsma's mathematical model. We tuned the response to biological ramp and hold metrics including peak, mean, dynamic index, time domain response, and sensory region displacement. The model was validated against biological Ia response to ramp and hold, sinusoidal and fusimotor inputs. The response with dynamic or static gamma motorneuron input was excellent across all studies. The passive spindle response matched well in five of the nine measures. Potential applications include basic science muscle spindle research and applied research in prosthetics and robotics.

Algorithms↗
PubMed1976

Sensory terminal responses of frog muscle spindle recorded across vaseline gap onto intrafusal muscle fibre.

When electrical activities of the sensory terminal in the frog muscle spindle were recorded across a vaseline gap which was oriented transversely to the intrafusal muscle bundle (IFM-gap), large and small spikes occurred in coincidence with propagated and abortive spikes respectively recorded across the axon-gap. The IFM-gap placed on the receptor region displayed triphasic spikes in which the first small positive deflection occurred approximately 0.2 ms prior to the propagated and abortive spikes. No such deflection was observed when the IFM-gap was placed more than 50 mum away from the receptor region. Increasing the distance from the receptor region to the IFM-gap resulted in an exponential decay in the amplitudes of both the large and small spikes. Regardless of the position of the IFM-gap relative to the receptor region, bimodal distributions in the amplitude histograms of the large spikes, and large variations in the shapes of the small spikes were observed. These results indicate that the IFM-gap responses may represent electrical activity of the non-myelinated branches, along which propagated impulses may arise from at lest two separate origins, while abortive spikes arise from many more sites.

Action Potentials↗
PubMed2002

[Effects of simulated weightlessness on ultrastructure of soleus muscle spindle in rats].

OBJECTIVE: To study changes in ultrastructure of soleus muscle spindle induced by simulated weightlessness in rats. METHOD: Weightlessness was simulated by tail suspension in female rats. The ultrastructure of isolated soleus muscle spindle were observed in 4 d, 7 d, 14 d tail-suspended and 14 d recovered rats. RESULT: Chaotic myofibril, hyperplasia of mitochondria, and enlargement of terminal cisterna were found in the ultrastructure of isolated soleus muscle spindle in 4 d tail-suspended rats. The ultrastructure of isolated soleus muscle spindle in 7 d tail-suspended rats showed more distinct changes, while that in 14 d tail-suspended rats showed obvious retrograde changes. The ultrastructure of isolated muscle spindle in 14 d recovered rats after 14 d tail-suspension were similar to that of control group. CONCLUSION: Simulated weightlessness could induce time-related and reversible changes in ultrastructure of soleus muscle spindle in rats.

Animals↗
PubMed1990

Increase of elastic fibres in muscle spindles of rats following single or repeated denervation with or without reinnervation.

Muscle spindles in the lower lumbrical muscles of rats were studied by transmission electron microscopy following denervation with or without reinnervation. The number and total area of elastic fibres per muscle spindle increased at 3-12 months following various experimental procedures: (1) denervation and reinnervation after a single crush lesion to the sciatic nerve; (2) reinnervation after four-fold repeated crush injuries; and (3) transection and suture of the nerve. The increased number of oxytalan and elaunin fibres, the precursors of mature elastic fibres, within these muscle spindles provided further evidence for their numerical and dimensional increase. An attachment site of elastic fibres at the spindle pole was identified at the inner cells of the outer spindle capsule. The processes of these cells embraced terminating elastic fibres tightly. Attachment of elastic fibres to intrafusal muscle fibres was less conspicuous since they were not similarly embraced but were rather indistinctly, though closely, associated with the basal lamina along longitudinal surface indentations of intrafusal muscle fibres. It is concluded from this series of experiments that muscle spindles, as dynamic mechanoreceptors, maintain their elastic properties even under pathological conditions. The increase of elastic fibres following denervation and reinnervation represents an obviously meaningful reaction that may compensate for loss of tonic properties of muscle spindles without causing stiffness.

Animals↗
PubMed1991

Post-contraction changes in human muscle spindle resting discharge and stretch sensitivity.

The activities of human muscle spindle primary endings were recorded in the lateral peroneal nerve using the microneurographic method. The aim of the study was to test whether voluntary isometric contraction causes any after-effects, first in the resting discharge of muscle spindle primary endings and secondly in their responses to a slow ramp stretch. To investigate the latter point, the initial angular position of the ankle was passively adjusted until the unit fell silent, in order to introduce a delay in the responses to muscle stretch. The results were as follows: (1) most of the units did not exhibit the "post-contraction sensory discharge" reported to occur in numerous animal experiments; this means that the muscle spindle resting discharge was essentially the same before and after isometric voluntary contraction. (2) Isometric voluntary contraction led to changes in muscle spindle stretch sensitivity which resulted in a reduction in the stretch threshold and a decrease in the muscle spindle dynamic sensitivity. These data suggest that the after-effects observed may have been triggered by static fusimotor neurones. The results are discussed with reference to the theory according to which the processing by the CNS of muscular proprioceptive messages deals mainly with signals arising from muscles stretched during movement, and it is concluded that the coactivation of alpha and gamma motoneurones during the contraction facilitates the coding of the parameters of forthcoming stretching movements, by the muscle spindles.

Adult↗
PubMed1978

Lyssavirus infection of muscle spindles and motor end plates in striated muscle of hamsters.

Immunofluorescent, light, and electron microscopy were used to document lyssavirus infection of muscle spindles and motor end plates. Virus particles were seen in the narrow intercellular space between sensory nerve endings and intrafusal muscle fibers; they were also observed budding from intracellular and plasma membranes of the latter. Involvement of motor nerves and motor end plates could only be demonstrated by electron microscopy. In nature, rabies virus invasion of the peripheral nervous system must involve centripetal spread across these junctions.

Animals↗
PubMed2005

The effect of experimental muscle pain on the amplitude and velocity sensitivity of jaw closing muscle spindle afferents.

The effect of experimental muscle pain on the amplitude and velocity sensitivity of muscle spindle primary afferent neurons in the trigeminal mesencephalic nucleus (Vmes) was examined. Extracellular recordings were made from 45 neurons designated as spindle primary- or secondary-like on the basis of their response to ramp-and-hold jaw movements. Velocity sensitivity was assessed in spindle primary-like afferents by calculating the mean dynamic index of each unit in response to three different velocities of jaw opening before and after intramuscular injection with hypertonic saline (HS, 5%, 100 microl). The amplitude sensitivity of all jaw muscle spindle afferents was assessed by calculating the mean firing rate of each unit in response to three different amplitudes of jaw openings during both the open and hold phases of the movement and with best-fit lines obtained, using linear regression analysis, before and after HS injection. The variance of the two regression lines obtained for each unit before and after the injection was compared using the coincidence test, and changes in intercept and slope were determined. Seventy-five percent of the primary-like units and 80% of the secondary-like units presented with changes in static behavior after HS injection. Thirty-six percent of the primary-like units showed changes in dynamic behavior. Injection of isotonic saline (control) did not alter the responses of the spindle afferent to jaw opening. Thus, our results demonstrate that the predominant effect of noxious stimulation was a shift in the amplitude sensitivity of both spindle primary-like and secondary-like afferents and, to a lesser extent, the velocity sensitivity of the spindle primary-like unit. In accordance with earlier studies in the cat hindlimb and neck muscles, these results suggest that the activation of masseter muscle nociceptor alters spindle afferent responses to stretch acting primarily through static gamma motor neurons.

Animals↗
PubMed2002

Sympathetic modulation of muscle spindle afferent sensitivity to stretch in rabbit jaw closing muscles.

Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s(-1) for 45-90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF - SI), dynamic difference (DD = PF - IF) and static difference (SD = SI - IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non-curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non-curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non-curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of 'high' sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain.

Animals↗
PubMed2002

Information-theoretic analysis of de-efferented single muscle spindles.

The information transmission properties of single, de-efferented primary muscle-spindle afferents from the hind limb of the cat were investigated. The gastrocnemius medialis muscle was stretched randomly while recording spike trains from several muscle-spindle afferents in the dorsal root. Two classes of input stimuli were used: (i) Gaussian noise with band-limited flat spectrum, and (ii) Gaussian noise with a more "naturalistic" 1/f(n) spectrum. The "reconstruction" method was used to calculate a lower bound to the information rate (in bits per second) between the muscle spindles and the spinal cord. Results show that in response to the flat-spectrum input, primary muscle-spindle afferents transfer information mainly about high frequencies, carrying 2.12 bits/spike. In response to naturalistic-spectrum inputs, primary muscle-spindle afferents transfer information about both low and high frequencies, with "spiking efficiency" increasing to 2.67 bits/spike. A simple muscle-spindle simulation model was analyzed with the same method, emphasizing the important part played by the intrafusal fiber mechanical properties in information transmission.

Action Potentials↗
PubMed1990

Classification of muscle spindle afferents in the peroneus brevis muscle of the cat.

Muscle-spindle afferents are commonly classified according to their conduction velocity. Under certain conditions such classifications may not be feasible and another form of identification is required. In this study 5 tests, comprising either quantitative or qualitative criteria, have been evaluated as a means of classifying spindle afferents. The choice of these tests was made on the basis of predicted physiological differences arising from the structural variations in the endings. Prior conditioning of the spindles was found to enhance the distinction between the two types of afferent. All the tests generated similar identifications with a maximum of 10% of afferents being classified differently by any two tests.

Animals↗
PubMed1981

The occurrence of "mixed" nuclear bag intrafusal fibers in the cat muscle spindle.

A total of 147 muscle spindles was studied histochemically in serial transverse sections of 42 cat tenuissimus muscle specimens. Nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers were distinguished by the differential staining resulting from the reactions for myosin adenosine 5'-triphosphatase and nicotinamide adenine dinucleotide tetrazolium reductase. The majority of intrafusal fibers were of the same histochemical type at both fiber poles. However, seven muscle spindles contained one nuclear bag fiber each that presented as a bag1 in one pole and as a bag2 in the other pole. These "mixed" nuclear bag fibers were found in spindles that also contained at least one bag1 and one bag2 fiber of equivalent histochemical presentation in both fiber poles. The "mixed" bag fibers displayed differences of apparent fiber diameter and relative polar length between the two fiber poles. The motor innervation pattern, as revealed by staining for cholinesterase, was also dissimilar between the two poles of "mixed" bag fibers. The study indicates that the spindle equatorial region may in some instances serve as a boundary between two morphologically and histochemically different poles of the same intrafusal fiber.

Acetylcholinesterase↗