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The efferent electrical activity in the phrenic nerve can be quantified in such a way that it gives a good correlation to tidal volume. After administration of the drug benzoctamine this relationship changes: more phrenic nerve activity is needed for the same tidal volume. No changes were found in the neuro-muscular transmission from the phrenic nerve to the diaphragm. There was no alteration in dynamic compliance of the lungs or in airway resistance. The afferent phrenic nerve activity from proprioceptors in the diaphragm did not change. It seems unlikely that respiratory neurons in the brainstem were affected since the sensitivity of the respiratory system to CO did not change. It is known that the tonic fusimotoneuron activity is suppressed at a supraspinal level by benzoctamine. Since intercostal muscles have muscle spindles and the diaphragm hardly has any, the intercostal muscle activity will be affected more than diaphragmatic activity by benzoctamine. This could actually be shown by quantifying the electromyogram of inspiratory external intercostal muscles. The tidal volume regulation is controlled by the vagal feedback loop. In order to reach a certain tidal volume after administration of benzoctamine, the contribution of the diaphragm has to increase because the activity of the intercostal muscles is diminished. [ABSTRACT FROM AUTHOR]

A study was made to see if contractions induced by reflex action were followed by an elevation in muscle afferent activity comparable to the postcontraction sensory discharge (PCSD) that occurs after stimulation of the ventral roots. In lightly anesthetized cats, extension reflexes of the triceps surae muscle in the otherwise denervated leg were induced by contralateral stimulation of the popliteal nerve or footpad. The reflex response was followed by an elevated afferent discharge, which disappeared after a brief stretch, as is characteristic also of the PCSD. Crossed-extension responses in fusimotor activity unaccompanied by contraction of the gross muscle were also succeeded by an elevation in sensory discharge and an increased sensitivity to a vibratory stimulus applied to the tendon. Pinna reflexes had a similar effect. In the presence of the elevated discharge, α-motoneurons demonstrated increased sensitivity to vibration stimuli. It is suggested that the increase in spindle sensitivity may help stabilize the muscle against minor perturbations in length, at least when background fusimotor activity is relatively quiet. [ABSTRACT FROM AUTHOR]

The effect of single shock stimulation, up to 20 × threshold (T), of the sural nerve on the discharges of triceps surae γ-efferents was investigated in decerebrate cats. Units were classified as static (12) or dynamic (7) on the basis of their resting discharge rates (Murphy et al. 1984). All neurones were excited at short latency by sural nerve stimulation and response size was graded with stimulus intensity. Short latency mixed or inhibitory responses were not evident. Although reflex effects first occurred at low stimulus strengths (<-1.5T) in both types of efferent, most responses appeared at higher intensities (> 1.5T). The estimated central delays of the responses of static (3.0 ±1.1 ms, mean± SD) and dynamic (3.4 ± 1.0 ms) γ-motoneurones were not significantly different and are consistent with spinal oligosynaptic pathways. The present results differ from those of the only previous study (Johansson and Sojka 1985) of the short latency responses of triceps surae static and dynamic γ-motoneurones to sural nerve stimulation, in which mixed and inhibitory effects were common in anaesthetised cats. Although differences in recording techniques and γ sampling may account for the apparent disparity between these studies, it is also feasible that a difference in the setting of interneuronal pathways in the two types of preparation is responsible. The results are discussed in relation to the control of γ-motoneurones with particular reference to the 'final common input' hypothesis (Johansson 1981; Appelberg et al. 1983). [ABSTRACT FROM AUTHOR]

The activity in α and γ efferent axon populations and in group I and group II afferent fibre populations innervating a flexor muscle, the sartorius medialis, was observed during spontaneous locomotor movements in the thalamic cat. Multi-unit discharges of each kind of fibre were obtained by electronic sorting of the action potentials from the overall activity of a thin, intact branch of the sartorius medialis nerve. The following results were obtained: (1) The γ-motoneurones have a phasic behaviour characterized by a single discharge period during the hip flexion (swing phase of the step-cycle). (2) The γ-motoneurones are co-activated with the homonymous α-motoneurones. (3) Between rhythmic α and γ discharges, i.e. during the hip extension (stance phase of the step cycle), both α- and γ-motoneurones were normally silent. However, in 5 out of 17 experiments, a few units of the γ population fired at very low frequency. (4) Two observations indicate that the γ-motoneurones that are co-activated with the α-motoneurones by central locomotor commands are predominantly of the static type. In actual locomotion, the rhythmic fusimotor discharges over-compensate the depressor effect on the firing rate of the group II afferents of the unloading of muscle spindles by the active shortening of the parent muscle. In fictive locomotion, when the transmission of the excitation is blocked by selective curarization in alpha skeleto-motor junctions alone, the rhythmic fusimotor discharges elicit in-phase modulations not only of the group I but also of the group II fibres. The group II afferent population consists almost entirely of fibres arising from spindle secondary endings which are located primarily on intrafusal muscle fibres whose contraction is exclusively controlled by static fusimotor motoneurones. In the two experimental circumstances, the analysis of the group I fibre discharge does not allow to decide whether dynamic γ motoneurones are firing or silent during rhythmic γ discharge. (5) The group I and group II afferent discharges during the step-cycle showed two frequency peaks, one static-fusimotor dependent while the contracting muscle shortened during the hip flexion (swing) phase, the other length-change dependent while the relaxed muscle was rapidly stretched during the first part of the hip extension (stance) phase. Then, during the second part of hip extension when the muscle was slowly stretched in the absence of fusimotor drive, the firing rate of the spindle afférents decreased to a low level. The spindle sensory endings during the extension phase showed low dynamic and static responsiveness like deefferented spindles. (6) The results obtained in sartorius medialis (flexor) muscle are discussed in comparison with the results previously obtained in gastrocnemii (extensor) muscles (Bessou et al. 1986). The consequences of the predominant activation of the static or dynamic fusimotor system in functionally different muscles are considered with respect to the proprioceptive or motor role of musclespindles during muscle contraction. [ABSTRACT FROM AUTHOR]

During voluntary movement, muscle spindles of both the agonist and antagonist muscles potentially can supply information about position of the limb. Muscle vibration is known to increase muscle spindle discharge and cause systematic distortions of limb position sense in humans. The following two experiments attempted to examine these contributions by separately vibrating over the triceps and biceps muscles during forearm positioning. In the first experiment, subjects performed a horizontal flexion or extension of the right arm to a mechanical stop randomly positioned at 20, 40 or 60°. Vision was occluded and vibration was applied to the right arm. The perceived position of the right limb was assessed by instructing subjects to simultaneously match the right arm position with the left limb. Vibration of the shortening, agonist muscle had no effect on limb matching accuracy. However, antagonist muscle vibration resulted in a significant overestimation of the vibrated limb position by 6-13°. The procedures for the second experiment were similar to the first, except that movements of the right limb were self-terminated and only flexion movements were performed. A screen was mounted over the arms and subjects were instructed to move the right arm until it was positioned beneath a marker on the screen. Vibration of the shortening agonist muscle had no effect on either the positioning accuracy of the right limb or matching accuracy of the left limb. However, antagonist muscle vibration resulted in significantly shorter movements (6-10°) by the right limb and an overestimation of right limb position by the left, matching limb. These findings support the hypothesis that muscle spindle afferent information from the lengthening antagonist muscle contributes to limb position sense during voluntary movement. [ABSTRACT FROM AUTHOR]

Intracellular recordings were made in the barbiturate-anesthetized cat from single afferent fibres left in continuity with the medial gastrocnemius muscle to document the transmembrane potential changes produced in functionally identified fibres by stimulation of sensory nerves and of the contralateral red nucleus (RN). Fifty five fibres from muscle spindles had conduction velocities above 70 m/s and were considered as from group Ia. Stimulation of group I afferent fibres of the posterior biceps and semitendinosus nerve (PBSt) produced primary afferent depolarization (PAD) in 30 (54%) Ia fibres. Stimulation of the sural (SU) nerve produced no transmembrane potential changes in 39 (71%) group Ia fibres and dorsal root reflex-like activity (DRRs) in 16 (29%) fibres. In 17 out of 28 group Ia fibres (60.7%) SU conditioning inhibited the PAD generated by stimulation of the PBSt nerve. Facilitation of the PBSt-induced PAD by SU conditioning was not seen. Repetitive stimulation of the RN had mixed effects: it produced PAD in 1 out of 8 fibres and inhibited the PAD induced by PBSt stimulation in 2 other fibres. Nine fibres connected to muscle spindles had conduction velocities below 70 m/s and were considered to be group II afferents. No PAD was produced in these fibres by SU stimulation but DRRs were generated in 5 of them. In 23 out of 31 fibres identified as from tendon organs group I PBSt volleys produced PAD. However, stimulation of the SU nerve produced PAD only in 3 out of 34 fibres, no transmembrane potential changes in 30 fibres and DRRs in 1 fibre. The effects of SU conditioning on the PAD produced by PBSt stimulation were tested in 19 Ib fibres and were inhibitory in 12 of them. In 9 of these fibres SU alone produced no transmembrane potential changes. Repetitive stimulation of the RN produced PAD in 3 out of 9 Ib fibres. SU conditioning inhibited the RN-induced PAD. The present findings support the existence of an alternative inhibitory pathway from cutaneous to Ib fibres, in addition to the well known excitatory pathway producing PAD. Possible functional implications of inhibitory actions of cutaneous fibres with the pathways mediating the PAD of group Ia and Ib fibres are discussed. [ABSTRACT FROM AUTHOR]

The pre- and postsynaptic structure of 243 axon terminals of bag 1, bag 2, and chain fibers were studied in cynomolgus monkey skeletal muscle spindles. The motor endings of the biceps and gastrocnemius spindles (long limb muscle) were compared to the motor endings of lumbricals and opponens pollicis (intrinsic hand muscle) spindles. In both muscle groups the only significant difference observed in the presynaptic features was in the presynaptic membrane length. The postsynaptic features of bag 1, bag 2 and chain endings were similar in the long limb muscle spindles. In the intrinsic hand muscle, however, the bag 1 and chain endings showed complex postsynaptic structure which resembled the extra fusal endings while the postsynaptic structure of bag 2 endings was much simpler. From these studies we conclude that the postsynaptic structure of various intrafusal fiber types is dissimilar in different muscles. [ABSTRACT FROM AUTHOR]

Mammalian muscle spindles show persistent after-effects following conditioning stretch or fusimotor stimulation. Most previous observations have been carried out on primary endings of spindles and using dynamic fusimotor stimulation. We report here observations on after-effects produced either by conditioning stretch or by static fusimotor stimulation on the responses of primary and secondary endings to a slow test stretch during which a brief burst of static fusimotor stimulation is applied. We find that the response to the test burst is large if the muscle is kept short after conditioning but it becomes depressed if the muscle is held stretched for 3 s following conditioning. We attribute these effects to the presence of stable cross-bridges between actin and myosin filaments in intrafusal fibres. We conclude that, qualitatively, after-effects using static fusimotor testing are the same as with dynamic fusimotor testing and this must be taken into account when providing an explanation for the phenomenon. [ABSTRACT FROM AUTHOR]

Distributions of motor axons to different types of intrafusal fiber were reconstructed from serial 1-μm thick transverse sections of six poles of muscle spindle in the rat soleus. Motor axons innervated (dynamic) bag fibers, or (static) bag fibers in conjunction with chain fibers. However, approximately forty percent of axons that supplied the spindles synapsed on both bag and bag, or bag and chain fibers. The significance of this co-innervation of dynamic and static intrafusal fibers is discussed relative to the general organization and function of mammalian spindles. [ABSTRACT FROM AUTHOR]

The pattern of γ-efferent innervation of the muscle spindle was investigated in the peroneus tertius muscle of the cat. A number of spindle group Ia afferents were isolated and monitored during tetanic stimulation of isolated γ-axons. The sample was dominated by efferents producing a static fusimotor effect on the afferent. An individual static axon normally projected to less than five but at times as many as seven muscle spindles. Each spindle normally received innervation from three to five static γ-efferents but occasionally up to eight. The number of spindles innervated by a static γ-axon was directly related to the efferent axonal conduction velocity. For each experiment, a probability distribution of the number of fusimotor effects was constructed from all available connections between the muscle spindles and static γ-efferents considered in matrices of four afferents and four efferents. This distribution was then compared to a random binomial distribution. In each of the ten experiments, statistically significant differences ( p<0.01) were noted between the experimental and random distributions. In order to interpret such deviations from the random distributions, similar analyses were performed on various combinations of afferents and efferents in simulated sets displaying grouping of effects. It was concluded that the observed differences between experimental and random distributions may be produced by a grouping of fusimotor innervation limited to a small set of spindles. [ABSTRACT FROM AUTHOR]

Responses of muscle spindles of the iliofibularis muscle of the frog Litoria aurea have been recorded during single shock and repetitive stimulation of single functional motor axons. Repetitive stimulation of axons which innervated slow muscle, and on four occasions, axons which innervated twitch muscle, produced a large increase in the dynamic response of the spindle to a ramp-and-hold stretch. While extrafusal slow muscle did not respond to a single motor volley, some spindles did, especially if at the same time the muscle was being stretched. In an explanation of the effect of muscle stretch on responses of spindles to slow motor volleys it was proposed that stretch acted to reduce the internal motion in muscle fibres produced by a non-uniform distribution of sarcomere lengths. It was proposed that this kind of effect may account for dynamic fusimotor actions in all vertebrate spindles. [ABSTRACT FROM AUTHOR]

Illusions of self motion and aircraft motion are experienced when executing deep knee bends in the high force phases of parabolic flight. The occurrence of such illusions indicates that skeletomotor control is actively calibrated to a 1 g reference level and that departures from this level affect the execution and appreciation of voluntary movements. The origin of the illusory patterns is shown to be understandable in terms of 'mismatches' between efferent control signals and expected patterns of associated muscle spindle activity. It is shown, too, that spindle activity is interpreted within an entire context of spatial information about ongoing and intended motion of the body and whether the body is laden. [ABSTRACT FROM AUTHOR]

The regularity of receptor discharges of passive muscle spindles adapted to a different length of the triceps of the calf of narcotized cats was investigated in the range of average frequencies from 5.6 to 40 impulses/sec. The values of the coefficients of variation of the impulse intervals in discharges of the primary endings are distributed in the range from 2.5 to 18.0% and of the secondary from 1.2 to 12%. The discharges of the primary endings are less variable than the discharges of the primary. Among the latter are found receptors with less, moderately, and more regular discharges at any length of the muscle. The shape of the 'average impulse interval vs. coefficient of variation' relationship is nonlinear and differs for different units. For samples of receptors having approximately the same variability of the impulse intervals the average values of the coefficients of variation decrease significantly with muscle tension and increase of the rate of impulse flow. In some cases a decrease of the values of the coefficients of variation occurs due to a decrease of the dispersion at the same rate of flow and in other cases both parameters change simultaneously. A decrease of the dispersion of the intervals in receptor discharges apparently reflects an increase of the number of uniformly depolarized endings as their tension increases. [ABSTRACT FROM AUTHOR]

Investigation of the dynamic responses of 500 muscle receptors to stretching of the triceps surae muscle of the de-efferented hind limb of anesthetized cats revealed in 23 cases a transient fall in the firing rate (FFR) of muscle spindle receptors while muscle stretching was in progress. FFR arises in the same receptor during stretching of the muscle in different forms and at different speeds, approximately in the same region of stretching (the 'FFR zone'). With an increase in the velocity of muscle stretching the FFR effect is intensified. It is suggested that the FFR effect is due to temporary removal of the load from the area of the spindle where the receptor endings are located. The conditions for this may be created by the nonhomogeneous structure of the extrafusal muscle, and also by the different mechanical properties of the intrafusal muscle fibers. [ABSTRACT FROM AUTHOR]

Experiments on cats with a de-efferented triceps surae muscle showed that some stretch receptors of dynamic type adapt quickly within the first few seconds after stretching of the muscle and its keeping at the new length, whereas other receptors reveal two components of adaptation - fast and slow. Units with different types of adaptation are found in the same muscle; their afferents belong mainly to group I, although there is a zone of overlapping with group II afferents. These differences are manifested at high values of initial stretching of the muscle. It is suggested that the slow component of adaptation is based on a mechanical factor depending on the visco-elastic properties of the tissues in the region of the sensory nerve endings. [ABSTRACT FROM AUTHOR]

Fine structural changes of muscle spindles in the extensor digitorum longus of the gracile axonal dystrophy mutant mouse were studied from 20 to 120 postnatal days. Degenerative nerve endings in muscle spindles were first recognized at 20 postnatal days. The sensory nerve endings were usually swollen with decrease of cell organelles, and the cytoplasm was electron-lucent. At 50 postnatal days, atrophic nerve endings were frequently observed in the narrow spaces between the indented cell membrane of intrafusal muscle cells and the basement membrane. In addition to degenerative and atrophic changes, regenerative axons showing fine sprouts (with or without Schwann cell projections) appeared in the sensory nerve endings at this time. At 80 postnatal days, sensory nerve endings frequently showed dystrophic changes characterized by axonal dilatation with accumulations of neurofilaments, tubulovesicular structures, mitochondria and myelin-like figures. These findings suggest that axonal transport in the sensory nerve endings is impaired in this mutant mouse. Motor nerve endings were usually well preserved and normal structures even at 80 postnatal days. Intrafusal fibrosis, decrease in number of sensory nerve endings and atrophy of intrafusal muscle fibres were clearly recognized by 100 days of age. [ABSTRACT FROM AUTHOR]

Endings of four β skeletofusimotor axons in a spindle of the cat tenuissimus muscle were examined in semithin (1-μm thick) and ultrathin transverse serial sections. Two (dynamic) β axons terminated on the nuclear bag intrafusal muscle fiber and on extrafusal fibers of the dark type. Two (static) β axons terminated on the nuclear chain intrafusal fibers and extrafusal fibers of the intermediate type. The degree of indentation of axon terminals into the muscle surface, thickness of the sole plate and extent of folding of subjunctional membranes differed among intrafusal and extrafusal terminations of the same axon. Endings of β axons on the bag and chain fibers were also morphologically dissimilar. Motor axons may not determine ending morphology. Rather the form and structure of a β bag or chain ending may be determined by the type of intrafusal fiber on which the ending lies and the ending's distance from the primary sensory axon. [ABSTRACT FROM AUTHOR]

The motor nerve supply to cat nuclear bag intrafusal muscle fibers was reconstructed from light and electron microscopy of serial transverse sections of spindles in the tenuissimus muscle. Twenty-six of thirty poles of bag fibers that were examined received motor innervation. Every innervated bag pole received at least one (range 1-3) selective motor axon that supplied this fiber type only. Four of the innervated bag poles (15%) received additional motor supply from a nonselective motor axon that also innervated one nuclear chain fiber in the same spindle pole. The chain fibers co-innervated with bag fibers were among the longest chain fibers although they were shorter than two long chain fibers also present in the spindle poles. In cross-sections stained with toluidine blue they displayed 1-3 equatorial nuclei side by side, and there were fewer inter-myofibrillar granules in their polar regions than in most of the other chain fibers. The endings of nonselective motor axons on the bag and chain fibers were morphologically and ultrastructurally dissimilar. It is suggested that instances of common innervation of the (dynamic) bag fiber and a (static?) chain fiber represent an integral and, presumably, functionally meaningful part of the motor pattern in some cat spindles. [ABSTRACT FROM AUTHOR]

Regeneration of muscle spindles was quantified in a series of orthotopically and heterotopically autografted muscles of pigeons. Significantly fewer spindles relative to numbers of extrafusal fibers were present in grafts than in normal muscles. These results are in marked contrast to observations of free-grafted muscles of rats. A majority of grafts of the metapatagialis, a muscle devoid of spindles, into the site of the anterior latissimus dorsi contained spindles. A few spindles were present in grafts of the extensor digitorum communis, which normally contains many spindles, into the site formerly occupied by the metapatagialis whereas muscle spindles were absent in orthotopic grafts of the metapatagialis muscle. These observations suggest that the spindle-like structures observed in the extensor digitorum communis muscles, which regenerated in the sites of the metapatagialis, were derived from spindles of the donor muscle. Thus muscle spindles in transplanted avian muscle can form by two distinct developmental processes. [ABSTRACT FROM AUTHOR]

Several muscle spindles of the cat tenuissimus muscle were cut in serial, 1-gmm thick transverse sections and stained with toluidine blue in search for long nuclear chain intrafusal muscle fibers. Five complete poles of the long chain fibers were located. Each fiber pole displayed one plate-type motor ending situated in the extracapsular fiber region. The endings were supplied by myelinated motor axons that originated from intramuscular nerve fascicles containing motor axons to extrafusal muscle fibers. One of the endings was innervated by a collateral from a motor axon that supplied an extrafusal end-plate. Ultrastructurally, the long chain endings resembled extrafusal end-plates. They were more complex, in terms of prominence of sole-plate and degree of postjunctional folding, than any other intrafusal ending present in the spindles. The motor endings of the long chain fibers were assumed to be the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat muscle spindles. [ABSTRACT FROM AUTHOR]

Pigeon muscles lacking muscle spindles were grafted into sites which normally have a muscle containing spindles. The reciprocal transplantations were also made. After two to eight months, the graft of the donor muscle without spindles had regenerated into a muscle containing muscle spindles. The reciprocal grafts, muscles containing spindles transplanted to a site lacking spindle innervation, had neither muscle spindles nor remnants of the spindles. These experiments demonstrate that 1) the innervation is required for formation of the spindle; 2) the original spindles do not survive transplantation; and 3) parts of the original spindle are not required for spindle regeneration. [ABSTRACT FROM AUTHOR]

Duck muscle spindles show a large outer capsule enclosing a number of intrafusal muscle fibres which are individually encircled by an inner capsule. The outer capsule consists of a multilayer of 4-6 flattened cells with elongated nuclei, and are usually lined by a basement membrane. They resemble and are linked with perineural cells of the nerve bundle that innervates the spindle. There is overlapping and interdigitation between branching processes of these cells. Some apposing surfaces develop junctional complexes in the form of hemi-desmosomes and desmosomes. The cytoplasmic content shows numerous pinocytotic vesicles, some polyribosomes, lamellated cisternae of rough endoplasmic reticulum and microfilaments. The inner capsule consists of cells showing, at the mid-equatorial region, a large ovoid nucleus and extensive branching of thin and long processes that overlap, forming numerous layers around each intrafusal muscle fibre. Bundles of collagen fibrils in a crescentshaped configuration occur circumferentially between the multilayer of processes and the muscle fibre. Some linkages between the inner and the outer capsule cell processes provide a network that subdivides the large periaxial space into compartments. There is no basement material lining the inner capsule cells and the processes. Some cytoplasmic area occurs around the ovoid nucleus and the cytoplasm varies in density, showing some dilated short profiles of rough endoplasmic reticulum, a few pinocytotic vesicles and microfilaments. [ABSTRACT FROM AUTHOR]

Our experiments addressed the problem of the regulation of the number of mechanoreceptors by sensory axons and/or their peripheral target tissues. According to a previous study (Zelená et al. 1997) white leghorn chickens have more muscle spindles in the plantaris muscle (45.4±7.8; mean ±SD) than the Japanese quail (35.3±4.8) and significantly more Herbst corpuscles in the crural region (380.0±85.0) than the quail (124.9±32.8). Embryonic chick-quail chimeras were therefore used as a model with distinct recombinations of the nerve supply and peripheral tissue for studying the developmental control of these mechanoreceptors. The chick host leg bud was replaced with a quail leg bud of equal age and vice versa on embryonic day 3, prior to the onset of innervation of the periphery. Shortly before hatching the chimeras were sacrificed and muscle spindles and Herbst corpuscles counted. Recombinations of chicken nerves with quail limb buds have shown that the richer nerve supply by chick Ia axons induced a significant increase in the number of muscle spindles in the plantaris muscles (55.5±13.4) of the grafted quail limb. In some instances, a similar increase in spindle numbers was also found in control legs grafted onto hosts of the same species. In the reverse type of chimera where chick embryo legs were grafted onto quail hosts, spindles developed in lower numbers (27.3±3.2). In that case the lower number of Ia axons in quail nerves induced a lower number of spindles in the chicken muscle. The numbers of Herbst corpuscles were, however, low in both types of chimera. Quail legs grafted onto host chick embryos contained 126.8+26.4 corpuscles, presumably due to a restrictive influence of the smaller crural area in the quail. Chick legs grafted onto quail hosts had only 99.6+34.1 crural corpuscles; the target area in chick embryo legs failed to attract more quail axons and/or to induce axonal sprouting. The developmental regulation of the number of the two types of mechanoreceptors examined in our study thus differ. While sensory axons appear to play the dominant role in the development of muscle spindles, their role seems to be restricted by hitherto unknown peripheral factors during the development of Herbst corpuscles. [ABSTRACT FROM AUTHOR]

Short mechanical stretches given to partially activated human abductor digiti minimi muscle (ADM) evoke early (M1) and late (M2) reflex responses. Transcranial magnetic brain stimuli were used to evoke compound muscle action potentials in ADM and hence to estimate motoneuronal excitability at various times after mechanical stimuli. There was no evidence that la volleys produce additional facilitation in motoneurones of muscles which are already voluntarily activated. However, the inhibitory phase between M1 and M2 was associated with a reduction in size of muscle responses from brain stimuli. This may reflect reduced Ia input, polysynaptic Ia inhibition or Renshaw inhibition. [ABSTRACT FROM AUTHOR]

This study is concerned with the morphological recovery process of muscle spindles following a long period of immobilization. The right hindlimbs of rats were fixed with a plaster cast for 4 weeks. Thereafter, four groups of rats were examined by electron microscopy. One group served as the control after the cast was removed. The other three groups were examined after free walking for 4, 8, and 12 weeks, respectively. The muscle spindles (tibialis anterior muscle) of the individual animals were then ultrastructurally analyzed. The morphological alterations (of the outer capsule, intrafusal muscle fibers, and intrafusal nerve components) gradually recovered during free walking and regained almost all normal features in 12 weeks after returning to walking. [ABSTRACT FROM AUTHOR]

The ultrastructure of rat muscle spindles was examined after the anterior tibial muscles had been immobilized in a plaster cast. There was an increase in the number of collagen fibrils and external laminae around the outer capsules and in the intracapsular space 2 weeks after immobilization. The changes in the intrafusal muscle fibers within 4 weeks included disorientation of myofilaments. After 6 weeks, Z bands had become disarranged, and there was vacuolar degeneration of the sarcoplasmic reticulum in some fibers. Myelin sheaths of many of the myelinated nerve fibers (especially the thick ones, which were probably sensory nerve fibers) had degenerated within 2 weeks. These results indicate that immobilization of skeletal muscles affects not only extrafusal muscle fibers but also the structure of the muscle spindle. [ABSTRACT FROM AUTHOR]

Muscle spindles of limb muscles were deefferented in neonatal rats by sectioning ventral roots or by removal of the lumbosacral spinal cord. Ten to 56 days after the operation, muscle spindles were examined in the medial gastrocnemius, extensor digitorum longus and soleus muscles. The differentiation of muscle spindles was not affected by deefferentation. The number of spindles in the investigated muscles was not reduced. Intrafusal fibres increased in number from two at birth to four per spindle on the average, as in normal muscles. The characteristic ultrastructural distinctions of nuclear bag and nuclear chain fibres developed as under normal conditions. However, intrafusal fibres atrophied slowly after fusimotor denervation, their polar zones becoming reduced in diameter by about 25% in comparison with control fibre diameters. Spindle capsules, on the other hand, increased in size and attained diameters comparable with normal spindles, appearing even somewhat distended. As intrafusal fibres degenerate after complete denervation at birth (Zelená, 1957), but differentiate in the absence of fusimotor innervation, it can be concluded that sensory nerve terminals induce and support their development. It is assumed that the morphogenetic influence of sensory terminals is mediated by release and uptake of a trophic substance at the synaptic junction. The occurrence of light and dense core vesicles in the sensory terminals and of coated invaginations and vesicles at both the axonal and plasma membrane speak in favour of such a possibility. [ABSTRACT FROM AUTHOR]

Sensory nerve fibres and nerve endings of lumbrical muscle spindles of cat were investigated with the electron microscope. Spheroidal particles, 200-400 Å in size, identified as glycogen, were found either scattered or accumulated in dense aggregations within spindle receptor afferents. These particle aggregates were generally not demarcated by membranous structures. Occasionally, however, they were surrounded by smooth-surfaced cytomembranes. Glycogen-membrane complexes were also observed within the sensory nerve fibres of muscle spindles. Each complex consisted of a concentric array of smooth-surfaced cisternae alternating with narrow bands of cytoplasmic matrix. The inner cavity of each cisterna, 100-300 Å in width, contained an apparently amorphous material. On the other hand roughly spherical glycogen particles, 200-400 Å in size, appeared aligned in a single row within the narrow band of cytoplasmic matrix interposed between adjacent cisternae. The intimate relationship between glycogen particles and smooth-surfaced endoplasmic reticulum has been described and the role of these smooth membranes in glycogen metabolism has been discussed. Glycogen particles were also found within both primary and secondary sensory nerve endings innervating nuclear-bag and nuclear-chain fibres. In no instance, however, were glycogen-membrane complexes observed at this level. [ABSTRACT FROM AUTHOR]

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In cats anesthetized with halothane the influence of central stimulation upon the dynamic sensitivity of muscle spindles in the flexor digitorum longus muscle was studied. One mesencephalic and one medullary region were found which caused an increase in dynamic spindle sensitivity when stimulated electrically. Histological identification of stimulating points showed these regions to be the caudal pole of the red nucleus and the inferior olivary nucleus respectively. Lesions in the olive decreased rubral effects on the spindles, indicating that the red nucleus and the olive are parts of the same system descending to dynamic fusimotor neurones. Further evidence for this point are given in a following paper ( APPELBERG 1967). [ABSTRACT FROM AUTHOR]

In the preceding paper ( Appelberg and Molander 1967) the caudal part of the red nucleus and parts of the inferior olivary nucleus were shown to cause increased dynamic sensitivity of muscle spindles when stimulated repetitively. The results to be presented will show that single shock electrical stimulation in the caudal part of the red nucleus evoked a field potential in the inferior olivary nucleus. This response seemed to be monosynaptically evoked and was observed only in parts of the olive where repetitive stimulation caused increased dynamic sensitivity of muscle spindles. Stimulation in the red nucleus as well as single shock stimulation in the actual part of the inferior olive also caused a potential in the vermis of the posterior cerebellar lobe. In conditioning - test experiments with the two stimuli the conditioning shock was seen to cause alternating periods of decreased and increased responsiveness in the pathway concerned. The same type of interaction was seen between two responses caused by double shock stimulation in the red nucleus. It is concluded that information from the caudal part of the red nucleus reaches dynamic fusimotor neurones in the spinal cord via a relay in the inferior olivary nucleus; an additional relay in the pathway is also predicted. The cerebellum seems to receive information about ongoing activity in the pathway but mesencephalic stimulation was seen to cause spindle effects also in decerebellated animals. [ABSTRACT FROM AUTHOR]

Hind limb muscles were de-efferented in 19 new-born rats by removal of the lumbosacral spinal cord, with preservation of spinal ganglia and their peripheral branches. The juxtaequatorial and polar zones of muscle spindles were studied in different leg muscles 3 to 9 weeks after the operation in order to establish whether intrafusal fibre types would become differentiated after permanent motor denervation. De-efferented intrafusal fibres developed into distinct ultrastructural fibre types similar to those found in control muscles. The nuclear bag type had confluent myofibrils with ill-defined M lines and relatively few mitochondria. The nuclear chain type had discrete myofibrils with prominent M lines, numerous large mitochondria and a more developed sarcotubular system. The fibre type characteristics were sometimes blurred by disarranged cross striation, but they were clearly discernible in 59 out of 69 de-efferented fibres of 31 spindles investigated in the electron microscope. A sample of 220 de-efferented spindles from leg muscles of 6 rats was examined in the light microscope on transverse sections stained for ATPase activity. The difference in the ATPase activity among intrafusal fibre types was marked in about 70% spindles; in contrast to this, no distinct fibre types could be discerned in the population of extrafusal fibres which were stained rather uniformly. In de-efferented spindles-as in normal control spindlesnuclear chain fibres always exhibited high ATPase activity, whereas one of the nuclear bag fibres had low ATPase and the other either low or medium to high ATPase activity. However, the ATPase activity of de-efferented muscles was generally lower than that of normal muscles. It can be concluded that intrafusal fibres do acquire their fibre type characteristics after fusimotor denervation despite complete deprivation of nerve impulse activity during the postnatal period when intrafusal fibre types differentiate in normal spindles. [ABSTRACT FROM AUTHOR]

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