Rat muscle spindle immunocytochemistry revisited

Summary The expression of myosin heavy chain isoforms in muscle spindle fibres has been the subject of a number of immunocytochemical studies, some of them with discordant results. In order to assess whether these discrepancies are due to differences in the specificity and sensitivity of the antibodies used, we have compared the reactivity of rat muscle spindle fibres to two pairs of antibodies presumed to be directed against slow tonic (ALD 19 and ALD 58) and neonatal (NN5) and neonatal/fast (MF30) myosin heavy chains. Adult, developing and neonatally de-efferented muscle spindles from the rat hind limb muscles were studied in serial cross-sections processed for the peroxidase-antiperoxidase method. Important differences in the staining profiles of intrafusal fibres were noted when ALD 19 and ALD 58 were compared. ALD 19 stained the muscle spindle precursors from the seventeenth day in utero, whereas ALD 58 only did so by the twentieth day of gestation. In adult spindles ALD 19 stained the nuclear bag₁ fibres along their entire length, whereas ALD 58 did not stain these fibres towards their ends. ALD 19 stained the nuclear bag₂ fibres along the A, B and inner C region, but ALD 58 stained these fibres only in the A and the inner B regions. ALD 19 stained some nuclear chain fibres along a short equatorial segment, whereas ALD 58 did not stain the nuclear chain fibres at all. NN5 stained the nascent nuclear bag₁ and chain fibre precursors at earlier stages of development than MF30. Clear differential staining between primary and secondary generation of both extra- and intrafusal myotubes was seen with NN5, wheras MF30 stained all myotubes alike. However, in postnatal spindles, MF30 was a very good negative marker of nuclear bag₁ fibres. The staining profile of the adult fibres with NN5 and MF30 was rather similar. The staining pattern of neonatally de-efferented bag fibres obtained with ALD 19 and ALD 58 was practically identical and it differed from that of control spindles, confirming that motor innervation participates in the regulation of the expression of slow tonic MHC along the length of the nuclear bag₂ fibres, as we have previously shown with ALD 19. The distinct staining patterns obtained with ALD 19 versus ALD 58 and with NN5 versus MF30 reflect differences in antibody sensitivity and specificity. These differences account, in part, for the discrepancies in the results of previous studies on muscle spindles, published by Kucera and Walro using ALD 58 and MF30, and by us using ALD 19 and NN5.     

Influence of neonatal motor denervation on expression of myosin heavy chain isoforms in rat muscle spindles

In order to evaluate the effects of fusimotor elimination on the expression of myosin heavy chain (MHC) proteins in intrafusal fibres, we compared the muscle spindles in hind limb muscles of 3- to 6-week-old rats de-efferented at birth with those of their litter-mate controls. Serial sections were labelled with antibodies against slow tonic, slow twitch, fast twitch and neonatal MHC isoforms, against synaptophysin, the neurofilament 68 kD subunit and laminin. We found that de-efferented intrafusal fibres differentiated, as in normal spindles, into nuclear bag and bag fibres both containing predominantly slow MHC, and nuclear chain fibres that contained fast and neonatal MHC. In both de-efferented and control intrafusal fibres the same MHCs were stained; the degree and extent of staining, however, varied. Both types of de-efferented bag fibres displayed a high content of slow tonic and slow twitch MHC along most of the fibre length, in contrast to the prominent regional variation in control bag fibres. In their encapsulated regions, the de-efferented bag fibres were more similar to each other in their reactivity to anti-fast twitch and anti-neonatal MHC antibodies than the control bag fibres. In these aspects they resembled more closely the bag fibres of newborn rats. The differences might be due to an arrest of "specialization" in the regional expression of the different MHC isoforms. Chain fibres developed MHC patterns identical to those of control spindles with all the antibodies used, even though they differentiated from the beginning in the absence of motor innervation. The structural differentiation of the capsule and sensory innervation in de-efferented muscle spindles, as shown by anti-laminin, anti-synaptophysin and anti-neurofilament staining, did not differ from the controls. We conclude, in agreement with previous studies, that the sensory innervation plays a key role in inducing and supporting the differentiation of intrafusal fibres and the specific expression of their MHC. However, we also show that motor innervation and/or muscle function seem to be necessary for the diversity in the expression and distribution of different slow and fast MHC isoforms in the bag and bag fibres.     

Examination of chronically de-efferented cat muscle spindles for cholinesterase activity

Summary Cat tenuissmus muscles were deprived of motor nerve supply for three months by sectioning of the appropriate ventral spinal roots. Muscle spindles were located in the chronically de-efferented muscles and examined histochemically in serial transverse sections. Staining for nicotinamide adenine dinucleotide tetrazolium reductase showed that the spindle sensory innervation was preserved. The de-efferented intrafusal muscle fibers retained their differential staining with the reaction for myosin adenosine triphosphatase. However, all cholinesterase-active areas that are normally found along nuclear bag and nuclear chain intrafusal fibers demonstrated loss of the enzyme activity in the chronically de-efferented spindles. It is concluded that all histochemically demonstrable cholinesterase activity within the cat muscle spindle is dependent upon the continuous presence of motor innervation.     

Influence of neonatal motor denervation on expression of myosin heavy chain isoforms in rat muscle spindles

Summary In order to evaluate the effects of fusimotor elimination on the expression of myosin heavy chain (MHC) proteins in intrafusal fibres, we compared the muscle spindles in hind limb muscles of 3- to 6-week-old rats de-efferented at birth with those of their litter-mate controls. Serial sections were labelled with antibodies against slow tonic, slow twitch, fast twitch and neonatal MHC isoforms, against synaptophysin, the neurofilament 68 kD subunit and laminin. We found that de-efferented intrafusal fibres differentiated, as in normal spindles, into nuclear bag₁ and bag₂ fibres both containing predominantly slow MHC, and nuclear chain fibres that contained fast and neonatal MHC. In both de-efferented and control intrafusal fibres the same MHCs were stained; the degree and extent of staining, however, varied. Both types of de-efferented bag fibres displayed a high content of slow tonic and slow twitch MHC along most of the fibre length, in contrast to the prominent regional variation in control bag fibres. In their encapsulated regions, the de-efferented bag fibres were more similar to each other in their reactivity to anti-fast twitch and anti-neonatal MHC antibodies than the control bag fibres. In these aspects they resembled more closely the bag fibres of newborn rats. The differences might be due to an arrest of specialization in the regional expression of the different MHC isoforms. Chain fibres developed MHC patterns identical to those of control spindles with all the antibodies used, even though they differentiated from the beginning in the absence of motor innervation.The structural differentiation of the capsule and sensory innervation in de-efferented muscle spindles, as shown by anti-laminin, anti-synaptophysin and anti-neurofilament staining, did not differ from the controls.We conclude, in agreement with previous studies, that the sensory innervation plays a key role in inducing and supporting the differentiation of intrafusal fibres and the specific expression of their MHC. However, we also show that motor innervation and/or muscle function seem to be necessary for the diversity in the expression and distribution of different slow and fast MHC isoforms in the bag₁ and bag₂ fibres.     

The effect of neonatal deafferentation or deefferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Summary Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles wereexcised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.     

The effect of neonatal deafferentation or defferentation on myosin heavy chain expression in intrafusal muscle fibers of the rat

Muscle spindles were either deafferented or deefferented by selectively severing the sensory or motor nerve supply to neonatal soleus muscles of rats at a time when spindles are formed but when intrafusal muscle fibers are structurally and immunocytochemically immature. Experimental muscles were excised two months after nerve section. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles. Only intrafusal fibers bound these antibodies in intact soleus muscles. The deefferented spindles exhibited a pattern of ALD58 and MF30 binding similar to that of normal adult intrafusal fibers, whereas deafferented intrafusal fibers were unreactive with the two antibodies. Thus intact sensory innervation is essential for myosin heavy chain expression in intrafusal muscle fibers during postnatal development of rat spindles.     

Morphometric studies on tenuissimus muscle spindles in the cat

Muscle spindles were studied histochemically in serial transverse sections of 42 cat tenuissimus muscle specimens. Staining for myofibrillar adenosine triphosphatase was employed to identify nuclear bag 1, nuclear bag 2, and nuclear chain intrafusal muscle fibers. The nuclear chain fibers were further subdivided into three categories according to their polar length and the intensity of their staining for nicotinamide adenine dinucleotide tetrazolium reductase. A total of 430 spindle poles were surveyed. The mean spindle content of bag 1, bag 2, and chain fibers was established. The mean polar length of intrafusal fibers as well as that of the intracapsular and extracapsular spindle regions was determined. A cholinesterase (ChE) staining technique was used to demonstrate the termination sites of motor axons along intrafusal fibers. Two types of circumscribed ChE deposits. The "rim" and the "plate," occurred on the fibers. The nuclear chain fibers usually carried both the ChE rims and plates, while most nuclear fibers displayed only the plates. The ChE plates were assessed in term of their appearance, staining intensity, length, and location along the fibers. The mean number of ChE plates found along the fibers was established for each of the various intrafusal fiber types. These histochemical observations are discussed with regard to the current concepts of cat spindle morphology and motor innervation. The results suggest a degree of predictability in the spindle fiber content and in the distribution of motor nerve terminals along intrafusal muscle fibers, at least in the tenuissimus muscle.     

The response of fast and slow nuclear bag fibres and nuclear chain fibres in isolated cat muscle spindles to fusimotor stimulation, and the effect of intrafusal contraction on the sensory endings.

1. The mechanical behaviour of intrafusal muscle fibres during fusimotor stimulation and passive stretch was observed directly in muscle spindles isolated from the cat tenuissimus muscle. 2. Mammalian intrafusal muscle fibres are of three functional types. Most spindles contain one slow nuclear bag fibre, one fast nuclear bag fibre, and four or five nuclear chain fibres. 3. Contraction in slow nuclear bag fibres is characterized by a long latency and very slow initial velocity, whereas the latency for the other intrafusal fibres is short and the inital velocity rapid. The mean time for maximum contraction (at 75 Hz to 100 Hz) and relaxation is significantly longer for slow nuclear bag fibres (0-8s) than for other intrafusal fibres (0-5 s). The contraction time of fast nuclear bag fibres is sometimes longer than that of nuclear chain fibres but the mean values are not significantly different; a difference in the time to attain 90% contraction is more obvious. 4. At low stimulation frequencies (10 Hz) contraction in slow nuclear bag fibres and in most fast nuclear bag fibres is smooth whereas nuclear chain fibres exhibit marked oscillations. Single stimuli elicit small local twitches in nuclear chain fibres and occasionally in fast nuclear bag fibres but produce no visible effect in slow nuclear bag fibres. 5. Maximum contraction of slow and fast nuclear bag fibres at body temperature is attained at a stimulation frequency of 75 Hz to 100 Hz, whereas a frequency of 150 Hz or more is required for maximum contraction of nuclear chain fibres. At 50 Hz at body temperature contraction in nuclear bag fibres is at least half the maximum, whereas in many spindles nuclear chain fibres show only a very small contraction at this frequency. 6. Contraction in slow nuclear bag fibres occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Weak contraction extends the primary sensory spiral by a small amount (2%-8%) at a low velocity (5%-10%s-1). When the fibre is passively stretched the spiral opens and then creeps back to about 75% of the extension at the end of the stretch due to yielding in the poles of fibre; creep is complete in 0-5s to 2-5s. 7. Contraction in fast nuclear bag fibres also occurs at one or two discrete foci, most of which lie in the intracapsular region beyond the end of the fluid space. Shortening of sarcomeres at the foci and extension of the sensory spiral are, however, up to eight times greater (up to 25%) than in slow nuclear bag fibres, and the velocity of stretch of the spiral is three to eight times greater (25%-40%s-1). Fast nuclear bag fibres exhibit little or no creep following passive stretch. 8. Contraction in the nuclear chain fibre bundle is localized to the intracapsular region, centered on a point in the intracapsular region between 0-9 mm and 1-6 mm from the spindle equator. Maximal contraction stretches primary and secondary sensory endings by 15% to 20%, at 30% to 40% s-1...     

Intrafusal fibre types in rat limb muscle spindles: morphological and histochemical characteristics.

Morphological, histochemical and ultrastructural characteristics of intrafusal fibre types were studied in rat muscle spindles. The existence of three intrafusal fibre types, namely the typical bag, the intermediate bag and the chain fibres was confirmed. Intrafusal fibres differ in diameter, length and number of nuclei in the equatorial zone. Histochemically, typical bag fibres exhibit both alkali- and acid-stable ATPase activity and low SDH activity. Intermediate bag fibres possess low alkali-stable ATPase activity; after acid-preincubation, however, they have low activity only in the juxtaequatorial region, whereas in the polar zones they exhibit high acid-stable ATPase activity. The SDH activity varies from moderate to high. The chain fibres exhibit high alkali-stable and low acid-stable ATPase and high SDH activity in the extensor digitorum longus muscle, whereas in the soleus muscle the acid-stable ATPase activity varies from a low one to a high one, either among individual chain fibres in one spindle, and/or repeatedly along the fibre length. Since there are regional differences in morphological characteristics and in staining properties of intrafusal fibres, a reliable identification of intrafusal fibre types can only be achieved by an analysis of serial sections.     

Intrafusal fibre types in rat limb muscle spindles

Summary Morphological, histochemical and ultrastructural characteristics of intrafusal fibre types were studied in rat muscle spindles. The existence of three intrafusal fibre types, namely the typical bag, the intermediate bag and the chain fibres was confirmed. Intrafusal fibres differ in diameter, length and number of nuclei in the equatorial zone. Histochemically, typical bag fibres exhibit both alkali-and acid-stable ATPase activity and low SDH activity. Intermediate bag fibres possess low alkali-stable ATPase activity; after acid-preincubation, however, they have low activity only in the juxtaequatorial region, whereas in the polar zones they exhibit high acid-stable ATPase activity. The SDH activity varies from moderate to high. The chain fibres exhibit high alkali-stable and low acid-stable ATPase and high SDH activity in the extensor digitorum longus muscle, whereas in the soleus muscle the acid-stable ATPase activity varies from a low one to a high one, either among individual chain fibres in one spindle, and/or repeatedly along the fibre length.Since there are regional differences in morphological characteristics and in staining properties of intrafusal fibres, a reliable identification of intrafusal fibre types can only be achieved by an analysis of serial sections.     

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.     

Differentiation of supernumerary fibres in neonatally deefferented rat muscle spindles

Abstract. The myofibrillar ATPase (mATPase) activity and the pattern of expression of several myosin heavy chain (MHC) isoforms and of M-protein (Mr 165000) were studied in serial cross sections of neonatally deefferented 5- to 8-week-old rat hindlimb muscle spindles with supernumerary intrafusal fibres. In a sample of 5- to 6-week-old neonatally deefferented muscle spindles cut through the A region, the average number of intrafusal fibres per spindle was 8.4 in comparison to 4.2 in control spindles. Parent fibres extended throughout the whole encapsulated portion of the spindle, whereas supernumerary fibres were found only in the A region. The diameters of the supernumerary intrafusal fibres varied from less than 1 μ up to 10 μ approximately. On the basis of the mATPase activity and the pattern of expression of MHC isoforms and of M-protein, the vast majority of the supernumerary fibres could be classified as nuclear bag₂, bag₁ or chain fibres. However, some supernumerary fibres with small diameters exhibited features that did not fit any of the three known intrafusal fibre types. Two major processes, namely fibre splitting versus activation and fusion of satellite cells, might account for the formation of supernumerary fibres. The data presented suggest the existence of at least two types of intrafusal satellite cells. One type of satellite cell is related to the nuclear bag fibres and gives rise to myotubes which, if they have sensory innervation, can express slow tonic MHC and, therefore, differentiate into a phenotype similar to that seen in nuclear bag fibres. The other type of satellite cells form myotubes which attain a fast phenotype similar to that seen in nuclear chain fibres irrespective of the presence or absence of sensory innervation.     

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7-21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag1 and bag2 fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.     

Unusual intrafusal fibres in human muscle spindles

We have studied the morphology and pattern of expression of myosin heavy chain (MHC) isoforms of intrafusal fibres in a human first lumbrical muscle. Each intrafusal fibre type, namely nuclear bag1, nuclear bag2 and nuclear chain fibres, had a distinct MHC composition and distribution of different MHC isoforms along the whole length of intrafusal fibres. However, most muscle spindles analyzed also contained one or several intrafusal fibres exhibiting an extrafusal or mixed pattern of immunoreactivity which did not correspond to any of the described intrafusal fibre types. We conclude that the latter fibres do not represent new intrafusal fibre types, but their morphology and expression of MHC merely reflects the differences in their innervation owing to their unusual localization at the edge or outside the axial bundle of intrafusal fibres.     

The fine structure of the guinea-pig muscle spindle

Summary Nuclear bag and nuclear chain intrafusal fibres are present in guinea-pig muscle spindles. Unlike muscle spindles in other species two types of nuclear chain fibre seem to be present. The electron microscopical appearance of one type of nuclear chain fibre is similar to that of nuclear bag fibres.It is suggested that under tension the nuclei of small nuclear bag fibres become sufficiently displaced to form nuclear chain-like fibres. The frequent occurrence of fibres which combine some of the properties of both nuclear bag and nuclear chain fibres indicates the possible occurrence of a third type of intrafusal fibre.The sensory innervation of guinea-pig muscle spindles is similar to that of the cat and the rat. Three types of motor nerve ending which could be classified according to the complexity of their subneural apparatus were seen.     

Histochemistry of rat intrafusal muscle fibers and their motor innervation.

Muscle spindles were followed in serial transverse sections of freshly frozen rat soleus muscles. Adenosine triphosphatase (ATPase) histochemical staining reaction was used to identify nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers. Regional differences in ATPase staining occurred along bag1 and bag2 fibers but not along chain fibers. Bag1 fibers displayed ultrastructural heterogenity when their intra- and extracapsular regions were compared. Simple "diffuse" and more elaborate "plate" motor nerve terminals were demonstrated histochemically along the poles of bag1 and bag2 fibers by staining for cholinesterase. One motor terminal of the "plate" appearance was present on a chain fiber pole. There was no consistent spatial correlation between the intensity of regional ATPase staining along the nuclear bag fibers and the location, number and type of motor endings. Other factors, such as intrafusal fiber sensory innervation and regional differences in active and passive functional recruitment of nuclear bag fibers during muscle activity, may contribute to the ATPase staining variability along the intrafusal fibers.     

Diversity in expression of myosin heavy chain isoforms and M-band proteins in rat muscle spindles

The composition of adult rat soleus muscle spindles, with respect to myosin heavy chain isoforms and M-band proteins, was studied by light-microscope immunohistochemistry. Serial sections were labelled with antibodies against slow tonic, slow twitch, fast twitch and neonatal myosin isoforms as well as against myomesin, M-protein and the MM form of creatine kinase. Intrafusal fiber types were distinguished according to the pattern of ATPase activity following acid and alkaline preincubations. Nuclear bag1 fibers were always strongly stained throughout with anti-slow tonic myosin, were positive for anti-slow twitch myosin towards and in the C-region but were unstained with anti-fast twitch and anti-neonatal myosins. The staining of nuclear bag2 fibers was in general highly variable. However, they were most often strongly stained by anti-slow tonic myosin in the A-region and gradually lost this reactivity towards the poles, whereas a positive reaction with anti-slow twitch myosins was found along the whole fiber. Regional staining variability with anti-neonatal and anti-fast myosins was apparent, often with decreasing intensity towards the polar regions. Nuclear chain fibers showed strong transient reactivity with anti-slow tonic myosin in the equatorial region, did not react with anti-slow twitch and were always evenly stained by anti-fast twitch and anti-neonatal myosins. All three intrafusal fiber types were stained with anti-myomesin. Nuclear bag1 fibers lacked staining for M-protein, whereas bag2 fibers displayed intermediate staining, with regional variability, often increasing in reactivity towards the polar regions. Chain fibers were always strongly stained by anti-M-protein. The MM form of creatine kinase was present in all three fiber types, but bag1 fibers were less reactive and clear striations were not observed, in contrast to bag2 and chain fibers. Out of 38 cross sectioned spindles two were found to have an atypical fiber composition (lack of chain fibers) and a rather diverse staining pattern for the different antibodies tested. Taken together, the data show that in adult rat soleus, slow tonic and neonatal myosin heavy chain isoforms are only expressed in the muscle spindle fibers and that each intrafusal fiber type has a unique, although variable, composition of myosin heavy chain isoforms and M-band proteins. We propose that both motor and sensory innervation might be the determining factors regulating the variable expression of myosin heavy chain isoforms and M-band proteins in intrafusal fibers of rat muscle spindles.     

Myosin heavy chain composition of muscle spindles in human biceps brachii.

Data on the myosin heavy chain (MyHC) composition of human muscle spindles are scarce in spite of the well-known correlation between MyHC composition and functional properties of skeletal muscle fibers. The MyHC composition of intrafusal fibers from 36 spindles of human biceps brachii muscle was studied in detail by immunocytochemistry with a large battery of antibodies. The MyHC content of isolated muscle spindles was assessed with SDS-PAGE and immunoblots. Four major MyHC isoforms (MyHCI, IIa, embryonic, and intrafusal) were detected with SDS-PAGE. Immunocytochemistry revealed very complex staining patterns for each intrafusal fiber type. The bag(1) fibers contained slow tonic MyHC along their entire fiber length and MyHCI, alpha-cardiac, embryonic, and fetal isoforms along a variable part of their length. The bag(2) fibers contained MyHC slow tonic, I, alpha-cardiac, embryonic, and fetal isoforms with regional variations. Chain fibers contained MyHCIIa, embryonic, and fetal isoforms throughout the fiber, and MyHCIIx at least in the juxtaequatorial region. Virtually each muscle spindle had a different allotment of numbers of bag(1), bag(2) and chain fibers. Taken together, the complexity in intrafusal fiber content and MyHC composition observed indicate that each muscle spindle in the human biceps has a unique identity.     

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Summary Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7–21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag₁ and bag₂ fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.     

Muscle spindle receptors

Summary The depolarization of the sensory terminals of muscle spindle primary endings is studied in terms of a simplified core conductor model of the system. The terminals branching from a group Ia afferent fibre have different geometrical structures, depending on whether they innervate the nuclear bag or the nuclear chain intrafusal fibres of a muscle spindle.The depolarization of the two different structures as a function of space and time is analysed by digital simulation technique. The studies indicate that the tapered core conductor model of the bag-fibre terminals responds faster than a uniform system similar to the terminals innervating the nuclear chain fibres. The steady state response, on the other hand, appears to supply a more effective depolarization in the case of a uniform system with otherwise equivalent parameters.The results suggest that the bag fibre terminals are best suited for converting dynamical stimuli into an electrical signal, whilst the chain fibre terminals seem to yield a more effective steady state response. The results are discussed in relation to the functional properties of the mammalian muscle spindles and in relation to the earlier proposed models of the mechanical system represented by the two types of intrafusal muscle fibres.