Muscle spindles in muscle control. I. Afferent outflow from frog muscle spindles in open and closed loop modes.
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Animals↗
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Effects of changes in pH on the afferent impulse activity of isolated cat muscle spindles.
Muscle spindle activity has been shown to decrease in the sustained contracting muscle. The effect has been assumed to result from a declining fusimotor drive. Since accumulation of metabolites including H(+), lactate and CO(2) might also affect the receptor in the fatiguing muscle, the impulse activity of muscle spindles isolated from the cat tenuissimus muscle was characterized under varying degrees of extracellular pH, thus excluding any effect on fusimotor activity, blood supply and extrafusal muscle fibers. The isolated receptor was exposed to bathing fluids of pH 6.4, 7.4 and 8.4, and afferent discharge activity was recorded from the spindle nerve. Both primary and secondary endings responded similarly to changes in pH. Resting discharge frequency usually decreased with decreasing pH and increased with increasing pH. A sudden break-off in activity was observed with about 40% of primary endings and about 30% of secondary endings at pH 6.4. Experiments with slow stretch stimulation indicated that this effect was caused by a rising threshold of firing at the encoder site of the endings. With brief ramp-and-hold stretches, we tested the effects of changes in pH on the dynamic and static sensitivity of primary and secondary endings. When pH was reduced from 7.4 to 6.4, the initial burst activity at the beginning of the ramp phase increased in primary and secondary endings and the dynamic response increased in secondary endings, demonstrating that the dynamic properties of muscle spindle endings were usually augmented in the acidic milieu. The static properties rose as well because the static index of both types of ending increased significantly. By contrast, dynamic and static properties of both primary and secondary endings decreased significantly, when pH was increased from 7.4 to 8.4. The amplitude of tension that was measured during the passive stretch stimuli very slightly decreased in the acidic solution and very slightly increased in the alkaline solution. The decrease in the resting discharge activity at low pH supports those previous observations, which demonstrate a reduced peripheral input from muscle spindle afferents to the spinal motor nuclei during fatigue in the isometric contracting muscle. The present finding indicates that an attenuated afferent discharge is not only caused by a decreasing central activation of gamma-motorneurons, but may additionally be supported by a direct effect of protons on the muscle receptor itself. The accompanying augmentation of stretch sensitivity is suggested to correspond to the well-known increase in physiological tremor during exhaustive exercise.
Animals↗
Morphometry and histoenzymology of the hamster tenuissimus and its muscle spindles.
Muscle spindles and extrafusal fibers in the tenuissimus muscle of mature golden Syrian hamsters were studied morphologically and quantitatively using several light microscopic techniques. Muscle spindles were identified in serial-transverse frozen-sections of whole muscles stained with hematoxylin and eosin. Five tenuissimus muscles were examined from origin to insertion, and the locations of individual receptors were plotted in camera-lucida reconstructions. Spindles were found in proximity to the main neurovascular bundle in the central core of each muscle. A range of 16-20 receptors was noted per muscle. The mean muscle spindle index (the total number of spindles per gram of muscle weight) was 503 and the average spindle length was 7.5 mm. Oxidative enzyme and myosin adenosine-triphosphatase (ATPase) staining profiles were also evaluated in the intrafusal and extrafusal fibers in each muscle. Even numbers of type I and type IIA extrafusal fibers were distributed homogeneously throughout all muscle cross-sections. Histochemical staining patterns varied along the lengths of the three intrafusal fiber types. Nuclear chain fibers possessed staining properties similar to the type IIA extrafusal fibers and exhibited no regional variations. Bag1 fibers displayed staining variability, particularly when treated for myosin ATPase under acid preincubation conditions. Some spindles were isolated under darkfield illumination and then either treated with 7-nitrobenz-2-oxa-1,3-diazole (NBD)-phallacidin to detect filamentous actin by fluorescence microscopy, or prepared for conventional scanning electron microscopy (SEM). By fluorescence microscopy, a registered actin banding-pattern was observed in the sarcomeres of the intrafusal fibers, and variations in the intensity of banding were noted amongst different fibers. SEM revealed punctate sensory nerve endings that adhered intimately to the surfaces of underlying intrafusal fibers in the equatorial and juxtaequatorial regions. By transmission electron microscopy (TEM) these endings appeared crescent-shaped and were enveloped by external laminae. Each profile contained numerous mitochondria and cytoskeletal organelles. The high spindle density observed in this muscle suggests that the hamster tenuissimus may function in hindlimb proprioception.
Animals↗
Location and completeness of reinnervation by two types of neurons at a single target: the feline muscle spindle.
Muscle spindles from the tenuissimus muscle of the cat were examined microscopically to assess the precision and completeness of reinnervation of intrafusal muscle fibers by efferent and afferent neurons. Positions of motor and sensory nerve terminals were charted relative to the cross-sectional area enclosed by the outer capsule of the spindle. Profiles of nerve endings were measured for normally innervated and reinnervated spindles. The tenuissimus was deprived of innervation by freezing its nerve, sometimes in conjunction with either spinal ganglion removal or ventral rhizotomy. Sensory and motor terminals occupied separate locales along the length of normal muscle spindles. Nerve terminals of efferent and afferent neurons were located in appropriate positions along the length of spindles when axons of both types of neurons regrew together and when either category of axon regenerated alone. Precise reinnervation of muscle spindles occurred in spite of a diminished diameter of intrafusal fibers. Repopulation of the spindle with motor endings was less complete than that by sensory endings, based on the proportion and size of the regenerated terminals. We conclude that under optimal conditions for axonal regrowth, efferent and afferent neurons reinnervate their respective regions along intrafusal muscle fibers but motor lags sensory reinnervation within the spindle. The mechanism by which positional specificity happens during reinnervation of intrafusal fibers requires neither an interaction between terminals of the two types of neurons nor target cells of normal bulk.
Animals↗
Spaced serial section analysis of the avian muscle spindle.
Muscle spindle ultrastructure of the extensor digitorum communis of pigeons was studied using spaced serial sections. The intrafusal fibers extended well beyond the ensheathing capsule, after which they became disoriented and often fused with each other before terminating on the connective tissue of extrafusal fibers. Several extracapsular fibers contained macrofibrils which were about 0.1 micron in diameter and contained several dozen smaller (10 nm) subunits. Intrafusal fibers commonly formed close attachments with one another for short or extended (240 micron) lengths. A basal lamina was absent between regions of pairing, and a myosatellite cell lay at the border of the coupled region. Several fibers could be coupled together in a single cross section; fibers coupled together, separated, and either recoupled or became associated with other fibers along the length of a spindle. Profiles of sensory terminals and sensory satellite cells alternated to form a smooth-contoured surface over most of the fiber cross section in the equatorial region. The sensory terminals contained many mitochondria, lysosomes, and clear and dense core microvesicles. Both the terminals and sensory satellite cells formed desmosomelike junctions with the intrafusal fiber. A crescentic collagen sheath covered that portion of the fiber cross section containing the sensory terminal-satellite cell complex. Inner capsule cells surrounded the entire assembly in the equatorial region. The basal lamina thicknesses differed over the naked intrafusal fiber compared to that portion covered by the sensory terminal or sensory satellite cell. The thickness was more than doubled over the latter regions, indicating that the basal lamina over these areas was a product of the fused intrafusal fiber and sensory terminal and/or sensory satellite cell basal laminae. These are discussed in terms of intrafusal fiber degeneration and regeneration.
Animals↗
The topography of long nuclear chain intrafusal fibers in the cat muscle spindle.
Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscle fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag2 fiber. Furthermore, they were usually situated adjacent to the bag1 fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag2 fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.
Adenosine Triphosphatases↗
A novel muscle spindle containing muscle fibres devoid of sensory innervation in the posterior cricoarytenoid muscle of the normal adult guinea pig.
A novel muscle spindle composed of five thin intrafusal muscle fibres and four thick muscle fibres was encountered in the posterior cricoarytenoid muscle of the normal adult guinea pig. Four intrafusal muscle fibres of the five were innervated by the annulospiral type of sensory ending, but lacked a distinct aggregation of equatorial nuclei such as bags and chains. On the other hand, one intrafusal fibre and four thick fibres between the layers of the spindle capsule absolutely lacked sensory innervation and were almost similar in the fine structural features to neighbouring extrafusal fibres. These fibres are considered to fail to receive the sensory innervation in an early stage of development, and to retain an undifferentiated feature of intrafusal fibre in the intracapsular space into adulthood or to follow the course of extrafusal differentiation between the layers of the spindle capsule. It is suggested therefore that this muscle spindle may be primitive.
Animals↗
Muscle spindles in muscle control. II. Analysis of muscle servo model.
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Amphibians↗
Muscle spindles in muscle control. 3. Analysis of adaptive system model.
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Adaptation, Physiological↗
Muscle spindles in muscle control. IV. Correlation with muscle dynamics.
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Animals↗
Ultrastructural observations on muscle spindles in extraocular muscles of pig.
Human and some mammals such as the sheep, goat and domestic and wild pigs have more or less muscle spindles in the extrinsic eye muscles, especially the domestic pigs having abundant muscle spindles (Matsuyama, 1987). The muscle spindles play a large role in maintaining the stable visual posture of the eyeballs. To define the morphological properties of the muscle spindles relative to the eye movement, the ultrastructure of the spindles was investigated in 6 extraocular muscles of the pigs by electron microscopy. The muscle spindles in the pig extraocular muscles consist of 4 to 5 intrafusal muscle fibers, one of which is nuclear bag fiber and 3 to 4 are nuclear chain fibers. The outer capsule is thin, composing of few layers, and the inner capsule ramifying to enwrap the individual fiber, accompanied by the medullated and unmedullated nerve fibers and blood capillaries. The nuclear bag fiber, 14 micron in diameter, is innervated by the atypical annulospiral sensory terminals and the chain fiber by the typical annulospiral terminal packed with mitochondria and microvesicles. The intrafusal fibers are innervated by the flower-spray sensory terminals anchoring deeply into the sarcoplasma, having abundant neurotubules and few mitochondria. The gamma motor end-plates have a relatively smooth synaptic cleft with a width of 70 nm and synaptic boutons containing few synaptic vesicles, sometimes, revealing a shallow fold of postsynaptic sarcolemma and abundant synaptic vesicles. The alpha motor end-plates reveal a relatively smooth synaptic cleft with a width of 80 nm, sometimes with a rough postsynaptic infolding, and boutons containing few synaptic vesicles and small-sized mitochondria. The satellite cells are innervated by the sensory terminals in various ways. The muscle spindles in the pig extraocular muscles are found to be much simpler in structure than those in the other antigravity muscles of the body. Their ultrastructure seems to reflect the morphological adaptation relative to the eyebal movement.
Animals↗
Five different types of centrally nucleated muscle fibres in man: elemental composition and morphological criteria. A study of myopathies, fetal tissue and muscle spindle.
Muscle biopsy material from patients with two types of benign hereditary myopathy was explored with electron probe X-ray microanalysis performed on thick cryosections observed in a scanning electron microscope equipped for the analyses. The elemental spectra thus obtained were related to the ratio between cross-sectional areas of centrally located nuclei and corresponding muscle fibres. Fetal myotubes and intrafusal muscle fibres were examined by the same methods. One of the myopathies was characterized by fibres resembling myotubes ('myotubular myopathy') and exhibited elementary spectra with i.a. higher sodium and chlorine and lower potassium signals than normally found. These spectra have a tinge of those found in fetal muscle and in extrafusal muscle fibres. The other type of myopathy had less specific morphological characteristics and exhibited different pathological changes in the muscle fibres and i.a. central nuclei in a considerable number of fibres. These nuclei were, however, smaller in relation to the cross-sectional area of the corresponding fibres than in fetal myotubes, fibres in the myotubular myopathy and in the intrafusal muscle fibres. Elemental spectra of muscle fibres in the latter myopathy conformed to the normal state. We conclude that central nuclei of muscle fibres is a pathological phenomenon which can mirror an arrest in fibre development as well as a muscle fibre degeneration. An arrest in the development is suspected in myopathic muscle fibres with large central nuclei and an elevated content of sodium chlorine in the sarcoplasm.
Adolescent↗
Representation of wrist joint kinematics by the ensemble of muscle spindles from synergistic muscles.
Proprioceptive information about movement is transmitted to the central nervous system by a variety of receptor types, which are widely distributed among the muscles, joints, and skin. Muscle spindles are known to be an important and reliable source of information for the perception of movement kinematics. Previous studies that focused on the characteristics of single muscle spindle firing patterns have left the impression that each receptor fires in relation to a number of kinematic variables, leaving the following question unanswered: what role is played by the ensemble of muscle spindles within the same muscle or within synergistic muscles? The study described in this paper addressed whether the perception of joint position and velocity is based on the net input of muscle spindles residing in all synergistic muscles crossing a joint. Normal human adults performed a motor coordination task that required perception of joint velocity and dynamic position at the wrist. The task was to open the left hand briskly as the right wrist was passively rotated in the flexion direction through a prescribed target angle. In randomly occurring trials, the tendons to three muscles [extensor carpi radialis (ECR), extensor carpi ulnaris (ECU), and extensor digitorum (ED)] were vibrated either individually or in different combinations during the performance of the motor task. Tendon vibration is known to distort muscle spindle firing patterns, and consequently, kinesthesia. By comparing performance errors with and without tendon vibration, the relative influences of muscle spindles residing in ECR, ECU, and ED were quantified. Vibration of the individual ECR, ECU, or ED tendons produced systematic undershoot errors in performance, consistent with the misperception of wrist velocity and dynamic position. Performance errors were larger when combinations of, rather than individual, muscle tendons were vibrated. The error resulting from simultaneous vibration of ECR and ECU was roughly equal to the sum of the errors produced by vibration of the individual tendons. These effects of vibrating synergistic tendons at the wrist suggest that kinesthesia is derived from the integrated input of muscle spindles from all synergistic muscles.
Adult↗
Alterations in information transmission in ensembles of primary muscle spindle afferents after muscle fatigue in heteronymous muscle.
This study showed that fatigue of the ipsilateral medial gastrocnemius muscle caused a clear-cut reduction in the ability of ensembles of primary muscle spindle afferents from the lateral gastrocnemius muscle to discriminate between muscle stretches of varying amplitude. The results were probably caused by reflex-mediated effects from chemosensitive group III and IV afferents onto the gamma-motoneurons projecting to lateral gastrocnemius muscle spindles. The experiments were conducted on seven cats anaesthetized with alpha-chloralose and a total of 41 primary muscle spindle afferents from the lateral gastrocnemius were registered. Afferents were simultaneously recorded in ensembles of three to 10 afferents. A method based on principal component analysis and algorithms for quantification of stimulus discrimination in ensembles of muscle afferents was used prior to, immediately following and five or more minutes after muscle fatigue had been induced to the ipsilateral medial gastrocnemius muscle. It is well established that the primary muscle spindle afferents play an important role in proprioception and kinaesthesia. Therefore the decrease in the accuracy of the information transmitted by ensembles of primary muscle spindle afferents caused by fatigue in an ipsilateral muscle implies concomitant effects on proprioception and kinaesthesia.
Animals↗
Activity of muscle spindles during muscle twitch and clonus in normal and spastic human subjects.
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Adult↗
Number, length and localization of muscle spindles in recti muscles of the eyeball in human fetuses.
Studies were performed on recti muscles of the eyeball, isolated from human fetuses in 14th to 24th week of their development. Number of muscle spindles and their length increased with progressing age of the fetuses. Most numerous muscle spindles were present in the rectus inferior muscle. The spindles were spread along the entire length of the muscle. Somewhat more numerous spindles were present in the half of the muscle belly positioned from the side of original attachment (24th week). Spindles were localized in the peripheral part of the muscle.
Embryonic and Fetal Development↗
Muscle spindles in extraocular muscles of human infants.
The capacity of muscle spindles in adult human extraocular muscles (EOM) to provide effective proprioception was questioned on the grounds of their peculiar morphology. Their appearance could be attributable to ageing and to test this possibility examples of infant muscle spindles have been examined. Forty encapsulated structures from five extraocular muscles removed post mortem from 4 infant patients aged 6 days, 5, 23 and 30 months were examined by means of light microscopy using serial transverse sections. Seven of them were identified as false spindles. The remaining 33 structures, identified as spindles, contained a total of 175 intrafusal fibres varying from 2 to 12 (mean: 6) in each. 130 of these fibres (74.3%) were of nuclear chain type. Unequivocal evidence of bag fibres was not found. Spindles lacked or had a limited equatorial expansion, and the inner capsule was incomplete and irregularly shaped. 45 (25.7%) of the intrafusal fibres had extrafusal features with large diameters, peripherally placed nuclei, no equatorial modification and without associated sensory nerve terminals. Serial sections revealed that a majority of the nuclear chain fibres were interrupted, fragmented or terminated abruptly, and most spindles contained at least one incomplete fibre. These observations show that the atypical features observed in adult human EOM spindles are also present in infants and are therefore not attributable to ageing.
Child, Preschool↗
[Influence of 100 Hz sinusoidal vibration on muscle spindle afferents of soleus muscles in suspended situation rat].
OBJECTIVE: To study influence of 100 Hz sinusoidal vibration on muscle spindle afferents discharges of rat soleus muscles in simulated-weightlessness situation. METHOD: The tail-suspended rat model was used to simulate weightlessness, and 100 Hz sinusoidal vibration was performed by a vibrator. Unit activity was recorded electrophysiologically from the centrally cut filaments of the spinal dorsal roots innervating muscle spindles of the rat soleus muscle; then observation on the changes in afferent discharges from muscle spindle in a rat soleus muscles were made after 7 d. RESULT: 1) Compared with control muscle spindle afferents discharges of rat soleus muscles decreased after 7 d tail-suspension (P<0.05). 2) Compared with suspension the afferent discharges from soleus muscle spindles increased after 7 d tail-suspension with 100 Hz sinusoidal vibration. CONCLUSION: Sinusoidal vibration (100 Hz; 300 micrometers) can selectively activate muscle spindles. Muscle spindle afferent input of the suspended rats increased after the application of 7 d 100 Hz sinusoidal vibration.
Afferent Pathways↗