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PubMed1994

Influences on the gamma-muscle-spindle system from muscle afferents stimulated by increased intramuscular concentrations of arachidonic acid.

There is evidence that static muscular contractions induce a release of arachidonic acid (AA) in the working muscle and that increased concentration of AA in a muscle increases the discharge rate of a subpopulation of groups III and IV muscular afferents. It is also known that activity in groups III and IV muscle afferents may activate gamma-motoneurones to both homo- and heteronymous muscles. The aim of the present study was to investigate if increased concentration of AA in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on five cats anaesthetized with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-12 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 36 MSAs (17 PBSt and 19 GS) were recorded. The responsiveness of the MSAs to injections of AA (0.3-1.0 mg; 0.3-1 ml) was 86% (n = 36) for injections into the arterial supply of the ipsilateral GS muscle and 45% (n = 20) for injections to the contralateral GS muscle. Out of 14 secondary MSAs, only one was unresponsive to ipsilateral AA injections while two of eight were unresponsive to contralateral AA injection. The majority of responses were compatible with predominantly static or mixed dynamic and static fusimotor activation. None of the effects were compatible with inhibition of fusimotor activity. The duration of the effects were usually 2-4 min. However, on some occasions the elevations in MSA activity persisted for up to 1 h. Local anaesthesia of the nerve to the injected muscle always abolished the effects of the injections and control injections of the solution in which the AA was dissolved were ineffective in changing the MSA responses. I.v. injections occasionally induced effects on the MSAs, but such effects were significantly different from those caused by close arterial muscle injections. Thus, increased concentration of AA may excite primary and secondary MSAs from homo- as well as heteronymous muscles, including contralateral muscles, most probably via fusimotor reflexes evoked by activity in chemosensitive muscle afferents.

Afferent Pathways↗
PubMed1995

Influences on the gamma-muscle spindle system from muscle afferents stimulated by increased intramuscular concentrations of bradykinin and 5-HT.

There is evidence that static muscular contractions induce a release of bradykinin (BK) in the working muscle, and that increased concentration of BK and 5-HT in a muscle increases the discharge rate of a subpopulation of group III and group IV muscular afferents. It is also known that activity in group III and IV muscle afferents may activate gamma-motoneurones to both homonymous and heteronymous muscles. The aim of the present study was to investigate whether increased concentration of BK and 5-HT in one muscle may influence the activity in primary and secondary muscle spindle afferents (MSAs) from the chemically affected muscle and from surrounding muscles, via fusimotor reflexes. The experiments were made on six cats anaesthetised with alpha-chloralose. The triceps surae (GS) and the posterior biceps and semitendinosus (PBSt) muscles were subjected to sinusoidal stretches. Simultaneous recordings of 2-11 MSAs from these muscles were made and the mean rate of firing and the modulation for each MSA were determined. Responses of 47 MSAs (26 PBSt and 21 GS) were recorded. The responsiveness of the MSAs to injections of BK (9-100 mg/ml, 0.5-1.0 ml) and 5-HT (25-150 mg/ml, 0.5-1.0 ml) was 89% and 83%, respectively, for injections into the arterial supply of the ipsilateral GS muscle, and 84% and 40% respectively for injections to the contralateral GS muscle. Of 10 secondarŁy MSAs, only one was unresponsive to BK injections, while several MSAs responded to both ipsilateral and contralateral BK injections.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed2002

Increased intra-articular concentration of bradykinin in the temporomandibular joint changes the sensitivity of muscle spindles in dorsal neck muscles in the cat.

The aim of the present study was to investigate to what extent activation of bradykinin-sensitive nerve endings of the temporomandibular joint (TMJ) might induce changes in the muscle spindles output from neck muscles through reflex effects on cervical fusimotor neurones. To this end, 26 muscle spindle afferents (MSAs) emanating from the trapezius and splenius muscles of the anaesthetised cat (alpha-chloralose, initial dosage 60 mg/kg) were recorded during injection of Bradykinin (BK) (12.5-50 microg/ml) in the ipsilateral TMJ. Fifteen (58%) MSAs exhibited statistically significant fusimotor effects to injection of BK into the TMJ. Of the 15, ten MSAs showed a response related to activation of static fusimotor neurones, three MSAs showed a response related to an activation of both static and dynamic fusimotor neurones and two MSAs showed a inhibition of both static and dynamic fusimotor neurones. The control experiments suggests that the effects observed were due to activation of BK sensitive nerve endings in the TMJ. It seems possible that the reflex connections between TMJ nociceptors and the fusimotor-muscle spindle system of dorsal neck muscles might be involved in the pathophysiological mechanisms responsible for the sensory-motor disturbances in the neck region often found on patients with temporomandibular disorders.

Animals↗
PubMed1979

[Decrease in the frequency of muscle spindle discharges during muscle stretching].

When studying responses of 500 muscle receptors to stretching of the m. triceps of the cat deafferented hindlimb, 23 cases revealed a transient decrease in the firing rate (DFR) of muscle spindles' receptors. The DFR occurs in the same receptor at different forms and rates of stretching approximately at the same area of stretching ("the DFR area"). When increasing the rate of stretching, the DFR effect is enhanced. The DFR effect seems to be due to a short-lasting unloading of the spindle area which contains the receptor terminals. The unhomogenous structure of the extrafusal muscle as well as various mechanical properties of the intrafusal muscle fibers can provide necessary conditions for the above effect.

Animals↗
PubMed1988

Structure of the intraspinal projections of single, identified muscle spindle afferents from neck muscles of the cat.

The morphology and frequency of collaterals originating from single afferents supplying primary endings of muscle spindles in dorsal neck muscles have been examined using intra-axonal injections of HRP. Within the segment in which the afferent entered the spinal cord, one collateral was found for every 3.3 mm of stained axon. In contrast, afferents--one of more segments rostral to the segment in which they entered the spinal cord--had fewer collaterals: One collateral was found for every 6.3 mm of stained axon. The branching structure and terminal distribution of the collaterals were generally similar regardless of the muscle from which the afferent originated and the segment in which the collateral was found. Boutons were found in 2 zones: One of these was located in the intermediate zone, within and around the central cervical nucleus, and the other was found in laminae VIII and IX, including the motoneuron nuclei. The ventral termination zone of collaterals in the same segment as their parent axon entered the spinal cord was larger and had more boutons than the same projection of collaterals whose parent axon entered the spinal cord 1 or 2 segments caudal to the segment in which the collateral was found. These results indicate that afferents supplying primary endings of neck muscle spindles are more likely to contact neurons in the same segment in which the afferent enters the spinal cord than in more rostral segments. However, even within the same segment in which the afferent enters the spinal cord, the projection of neck muscle afferents to the ventral horn is less dense than the corresponding projection of hindlimb muscle spindle afferents in the lumbosacral spinal cord.

Animals↗
PubMed2003

Muscle spindles in the deep muscles of the human neck: a morphological and immunocytochemical study.

Muscle spindle density is extremely high in the deep muscles of the human neck. However, there is a paucity of information regarding the morphology and immunoreactivity of these muscle spindles. The objective of this study was to investigate the intrafusal fiber content and to assess the myosin heavy chain (MyHC) composition of muscle spindles from human deep neck muscles. In addition to the conventional spindles containing bag(1), bag(2), and chain fibers (b(1)b(2)c spindle), we observed a number of spindles lacking bag(1) (b(2)c spindle) or bag(2) (b(1)c spindle) fibers. Both bag(1) and bag(2) fibers contained slow tonic MyHCs along their entire fiber length and MyHCI, MyHCIIa, embryonic, and alpha-cardiac MyHC isoforms along a variable length of the fibers. Fetal MyHC was present in bag(2) fibers but not in bag(1) fibers. Nuclear chain fibers contained MyHCIIa, embryonic, and fetal isoforms with regional variations. We also compared the present data with our previous results obtained from muscle spindles in human biceps brachii and the first lumbrical muscles. The allotment of numbers of intrafusal fibers and the MyHC composition showed some muscle-related differences, suggesting functional specialization in the control of movement among different human muscles.

Adenosine Triphosphatases↗
PubMed1989

Prolonged degeneration of muscle spindles in the masseter muscle after treatment of developing mice with the local anesthetic lidocaine hydrochloride.

The effect of lidocaine-HCl on muscle spindles in the masseter muscle of developing mice was investigated. Repeated injections of mice with anesthetic in the short term decreased the diameters of primary endings, intrafusal muscle fibers and outer capsules in the equatorial regions of muscle spindles, and caused a drop in the succinic dehydrogenase activity in intrafusal muscle fibers of the muscle spindles. In addition, the diameters did not recover to the control value even after about 10 weeks following cessation of anesthetic treatment. Thus, the present results suggest that repeated use of lidocaine-HCl in developing animals may cause dysfunction of the skeletal muscles.

Animals↗
PubMedDate not supplied

Electron microscopic study of muscle spindle in human interarytenoid muscle.

Using transmission electron microscopy, the fine structure of the muscle spindle in the interarytenoid muscle was studied. The specimen was made from a fresh adult male larynx which had been surgically removed. The muscle spindle showed the following characteristic features: (1) the capsule was thin, with only a small number of layers of capsular sheet cells; (2) the periaxial space was narrow and there were abundant collagenous fibrils; (3) the sensory endings consisted of irregularly coiled terminal axons with varicose swellings, and some endings penetrated the sarcoplasm; (4) one nuclear chain fiber terminated on the capsule, while the other branched into two bundles; and (5) special sarcoplasmic inclusions, possibly related to the sensory endings, were also observed. These findings suggest that the muscle spindle in the interarytenoid muscle is not only a very sensitive stretch receptor, but also a pressure receptor.

Aged↗
PubMed2003

[Influence of simulated weightlessness on the morphology of nerve endings of muscle spindle in rat soleus muscle].

OBJECTIVE: To study changes in structure and innervation of soleus muscle spindle in rats induced by simulated weightlessness. METHOD: Weightlessness was simulated by tail suspension in female rats. The technique of Fawosky's Staining was used to detect the morphology of nerve endings of soleus muscle spindle in 7 d, 14 d, 21 d tail-suspended and control rats. RESULT: The intrafusal muscle fibers were regular and nerve endings were intact and clear in the soleus spindles of control rats. No obvious change was observed in muscle spindles of 7 d tail-suspended rats. Intrafusal muscle fibers became irregular, with rough and vague nerve endings in 14 d tail-suspended rats. The distinct retrograde change of muscle spindles and nerve endings were observed in 21 d tail-suspended rats. Degeneration and rupture were found in nerve endings, which made it unclear and pale in staining. CONCLUSION: Simulated weightlessness could induce time-related changes in structure and innervation of soleus muscle spindle in rats.

Animals↗
PubMed1981

Formation of muscle spindles in regenerated avian muscle grafts.

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.

Animals↗
PubMed1976

[Muscle spindles in denervated and reinnervated m. soleus of the rat. I. Changes in number, distribution and length of muscle spindles].

After a transient or permanent unilateral denervation of the soleus muscle of the rat the number, distribution and length of muscle spindles were determined. The results were compared with those of the contralateral innervated muscle and with the data received from investigating soleus muscles of normal uninjured rats. Denervation (with or without reinnervation) reduces the number of muscle spindles by nearly 50%. The typical uniform distribution of muscle spindles in the muscle remains almost unchanged. The remaining muscle spindles grow longer. The same findings are observed in the contralateral still innervated muscles, too. Consequently a comparison of the denervated (reinnervated) muscle exclusively with its contralateral muscle does not show different counts of muscle spindles. Therefore in such investigations the contralateral muscle is of dubious value as a mean of control. As regards number, distribution and length of muscle spindles there exist only small differences between the various strains of rats.

Animals↗
PubMed1980

The occurrence of muscle spindles in the masticatory muscles: a methodological study of different staining techniques.

A method, based on experiment, for more reliable and less time-consuming staining and examination of histological specimens for locating muscle spindles by light microscopy is presented. Sections from the masseter and temporal muscles of the rat were stained with different methods and studied in random order. The method giving the highest number of muscle spindles during scrutiny of the slides (Weigert's iron haematoxylin and van Gieson) is recommended for future studies on the numbers of muscle spindles in a given muscle.

Animals↗
PubMed1989

Measurements of muscle stiffness, the electromyogram and activity in single muscle spindles of human flexor muscles following conditioning by passive stretch or contraction.

In experiments on adult human subjects we examined the effect on passive mechanical properties of a muscle by conditioning it with either an isometric contraction or passive muscle extension. The test measurement was the amount of muscle displacement (stiffness) and the accompanying EMG in response to a brief torque pulse. Two muscles were tested, flexor digitorum profundus (FDP) and brachialis. In FDP the discharge of single muscle spindles was recorded as well. After muscle extension and return to the initial length, passive stiffness was less than after an isometric contraction. The changes in stiffness were accompanied by changes in pattern of EMG and in the responses of muscle spindles. It is suggested that in resting muscle there are stable cross bridges between actin and myosin filaments of muscle fibres which largely determine the passive stiffness. Muscle extension leads to detachment of these cross bridges which then re-form at the longer length. Return of the muscle to its starting length leads to development of slack in muscle fibres because, stiffened by the presence of the stable cross bridges, they are unable to shorten. Slack in muscle fibres lowers their measured stiffness. Muscle contraction, on the other hand, will result in any preexisting slack being taken up by the actively shortening muscle fibres, thereby raising muscle stiffness. Stiffness in intrafusal fibres is likely to follow a similar pattern to that in extrafusal fibres, leading to changes in stretch responsiveness of muscle spindles and consequently in the reflex EMG. It is concluded that the changes in stiffness and accompanying reflexes observed in this study are likely to be seen, at least under some conditions, in normal movements.

Adult↗
PubMed1998

Morphometric study of the muscles spindles of the eye muscle in human fetuses between 24-29 weeks.

The studies were carried out on human fetuses with the crow-rump length between 230 to 270 mm (24-29 weeks). The length of recti muscles varies from 12-15 mm in 24th week to 25-28 mm in 29th week. Number of muscle spindles varies from 29-36 in 24th week to 54-59 in 29th week. The length of the muscle spindles is from 30-300 microns in 24th week to 40-450 microns in 29th week. The diameter of the muscle spindles varies from 11-36 microns in 24th week to 15-70 microns in 29th week.

Embryonic and Fetal Development↗
PubMed1980

Number and distribution of muscle spindles in the masticatory muscles of the rat.

The frequency and distribution of muscle spindles in the muscles of mastication of the rat have been studied. The average number of spindles found was 110 in the masseter, 75 in the temporalis and 25 in the medial pterygoid muscle, the spindles being predominantly in the medial parts of these muscles. No muscle spindles were found in the lateral pterygoid or posterior digastric muscle, and only isolated spindles were found in a few of the anterior digastric muscles. These differences between the jaw-closing and the jaw-opening muscles are intriguing and ask for further investigation of the role of the muscle spindles in mastication.

Animals↗
PubMed1991

Interaction of recurrent inhibitory and muscle spindle afferent feedback during muscle fatigue.

Mammalian skeletal motor units have differing properties including their different susceptibility to fatigue. The question discussed in this paper is whether and to what extent proprioceptive feedback via muscle spindles can contribute to shape the firing patterns of motor units so as to minimize their loss of force during fatiguing contraction. The firing of a skeleto-motoneuron dispatches signals which are fed back to the same and homonymous as well as synergistic motoneuron. Two feedback pathways are of concern here: one via the related muscle unit and muscle spindle afferents (proprioceptive path), and one via recurrent motor axon collaterals and Renshaw cells (recurrent inhibitory path). It is suggested that the contraction of a motor unit or a small group of adjacent ones is signalled to the homonymous alpha-motoneurons via proprioceptive afferents, the signal being filtered and enhanced by spinal recurrent inhibition. This is effected by timed correlation of the signals which are propagated through the two feedback loops. The effects of the correlation can be strengthened by (i) topographical order of the feedback connections, (ii) heterosynaptic interaction, and (iii) tendencies towards synchronous discharge between motoneurons. These mechanisms render the possibility more likely that information about the unfused contractions of a muscle unit (or a small group of them), mediated via proprioceptive afferents, play a role in shaping the precise discharge pattern of the innervating motoneuron(s). These mechanisms could be used to optimize the force output of large fatiguing motor units during long activation, during which their activation rates normally decrease (adapt) over time. Our results show that during adapting motoneuron firing Renshaw cells and muscle spindle afferents may show discharge patterns which at least in part are in keeping with such an hypothesis.

Animals↗
PubMed1985

Topographic studies relating distribution of Ia- and gamma-fibres in spinal cord and position of muscle spindles in cat tibialis anterior muscle.

The segmental distribution of 115 Ia- and 115 paired gamma-fibres of the tibialis anterior muscle was studied in anaesthetized cats. All Ia-fibres recorded were found in the lumbar segments L6 and L7, from caudal L6 to middle L7. The paired gamma-axons were also mainly found in these parts of the spinal cord, only 7 gamma-fibres were localized in caudal L7. A total of 70% of all fibres was found in L7. Of the fibres constituting 'muscle spindle units' of the tibialis anterior 92.2% enter the same segment (a 'muscle spindle unit' is here defined as a muscle spindle with its Ia-fibres and one gamma-fibre innervating it). More than that, 88% of the afferent and efferent fibres of muscle spindle units were found in the same part of the segment. For the first time, the position of the muscle spindles was related to the location of their Ia- and gamma-fibres in the spinal cord. In general, the muscle spindles located in the proximal muscle region project to the more cranial part of the spinal cord and the muscle spindles localized distally in the muscle project to the more caudal part of the spinal cord. The topographic pattern of the muscle spindle units is discussed with respect to the topographically arranged monosynaptic reflex loop.

Animals↗