Studies of the histochemistry, ultrastructure, motor innervation, and regeneration of mammalian intrafusal muscle fibres

Local muscle pressure stimulates the principal receptors for proprioception

Proprioception plays a crucial role in motor coordination and self-perception. Muscle spindles are the principal receptors for proprioception. They are believed to encode muscle stretch and signal limb position and velocity. Here, we applied percutaneous pressure to a small area of extensor muscles at the forearm while recording spindle afferent responses, skeletal muscle activity, and hand kinematics. Three levels of sustained pressure were applied on the spindle-bearing muscle when the hand was relaxed and immobile ("isometric" condition) and when the participant's hand moved rhythmically at the wrist. As hypothesized to occur due to compression of the spindle capsule, we show that muscle pressure is an "adequate" stimulus for human spindles in isometric conditions and that pressure enhances spindle responses during stretch. Interestingly, release of sustained pressure in isometric conditions lowered spindle firing below baseline rates. Our findings urge a re-evaluation of muscle proprioception in sensorimotor function and various neuromuscular pathologies.

Stretch Receptor and Somatic Dysfunction: A Narrative Review

From its founding by Andrew Taylor Still, MD, DO, through the work of many contributors, one of the cornerstones of osteopathic medicine has been its ability to aid health by promoting neuromuscular homeostasis. As part of the understanding of osteopathic medicine since the time of Still, the proper functioning of stretch receptor organs (SROs) of skeletal muscle have been recognized as having a central role in this homeostasis. In doing so, the complexities of these numerous and vital sensors are described, including recent findings regarding their structure, function, and the nature of their neural connections. In their homeostatic role, SROs conduct information centrally for integration in proprioceptive and autonomic reflexes. By virtue of their integral role in muscle reflexes, they are putatively involved in somatic dysfunction and segmental facilitation. In reviewing some well-established knowledge regarding the SRO and introducing more recent scientific findings, an attempt is made to offer insights on how this knowledge may be applied to better understand somatic dysfunction.

Fibre typing of intrafusal fibres

The first descriptions of muscle spindles with intrafusal fibres containing striated myofibrils and nervous elements were given approximately 150 years ago. It took, however, another 100 years to establish the presence of two types of intrafusal muscle fibres: nuclear bag and nuclear chain fibres. The present paper highlights primarily the contribution of Robert Banks in fibre typing of intrafusal fibres: the confirmation of the principle of two types of nuclear bag fibres in mammalian spindles and the variation in occurrence of a dense M-band along the fibres. Furthermore, this paper summarizes how studies from the Umeå University group (Laboratory of Muscle Biology in the Department of Integrative Medical Biology) on fibre typing and the structure and composition of M-bands have contributed to the current understanding of muscle spindle complexity in adult humans as well as to muscle spindle development and effects of ageing. The variable molecular composition of the intrafusal sarcomeres with respect to myosin heavy chains and M-band proteins gives new perspectives on the role of the intrafusal myofibrils as stretch-activated sensors influencing tension/stiffness and signalling to nuclei.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

[Histochemical study of the sensory endings of muscle spindles in rat longissimus muscles]

Most studies concerning the structure and function of muscle spindles have utilized the hind limbs of experimental animals. However, little is known about muscle spindles of the back muscles. The purpose of this study was to investigate the sensory innervation of muscle spindles of the paravertebral muscle in the rat. The subjects were 10 normal male rats. The longissimus muscles were isolated and frozen in cooled isopentane (-160 degrees C), and serial transverse sections were made with a cryostat. Histochemical preparations were then made using nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) stain and modified Gomori-trichrome stain. The muscle spindles in each segment were identified microscopically by observing the equatorial and polar regions. NADH-TR staining was employed to distinguish nuclear bag1, nuclear bag2, and nuclear chain intrafusal muscle fibers. A total of 20 spindle poles were surveyed. The mean polar length of intrafusal fibers as well as that of each region (A, B, and C) were measured. NADH-TR staining also demonstrated the terminal sites of sensory fibers along intrafusal fibers. All spindle poles surveyed were innervated by secondary sensory fibers in addition to primary sensory fibers. Eight spindle poles were intermediate type muscle spindles that were innervated by one primary sensory fiber and one secondary sensory fiber. Twelve spindle poles were complex type muscle spindles that were innervated by one primary sensory fiber and multiple secondary sensory fibers. The mean length of the A region was 223.1+/-37.9 microm (n=8) for intermediate type spindles and 493.8+/-157.0 microm (n=12) for complex type spindles. The length of the A region was significantly longer in the complex type spindles than in the intermediate type spindles (p<0.001). The results suggest that the innervations of secondary sensory fibers were well developed in the longissimus muscle spindles in the rat. The morphological features of muscle spindles of the longissimus muscle may represent the structural basis for qualitatively different afferent discharges that relate to the characteristic types of locomotion served by paravertebral muscles.

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation(1)

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (+/-0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (+/-0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.

Extrafusal and intrafusal muscle effects in experimental botulinum toxin-A injection

The effects of botulinum toxin-A was compared on both extrafusal and intrafusal muscle fibers in the biceps femoris of Wistar rats. Four days after injection no action potentials were elicited with stimulation single-fiber electromyography on the injected side. Fourteen days after injection, jitter became measurable and these values were increased on the injected side. Extrafusal muscle fibers began to atrophy on the 4th day and this continued to the 14th day postinjection. Atrophy was also evident and progressive in intrafusal muscle fibers. Increased terminal innervation ratios, end-plate spread of cholinesterase, and increased density of very small myelinated fibers in large intramuscular nerves were observed 14 days postinjection. Both extrafusal and intrafusal fibers are cholinergically innervated, and both were progressively affected by botulinum toxin, perhaps varying in degree only. In addition to partial denervation, Botulinum toxin effects in dystonia may also be related to modified spindle afferent discharge.

Location and completeness of reinnervation by two types of neurons at a single target: the feline muscle spindle

Muscle spindles from the tenuissimus muscle of the cat were examined microscopically to assess the precision and completeness of reinnervation of intrafusal muscle fibers by efferent and afferent neurons. Positions of motor and sensory nerve terminals were charted relative to the cross-sectional area enclosed by the outer capsule of the spindle. Profiles of nerve endings were measured for normally innervated and reinnervated spindles. The tenuissimus was deprived of innervation by freezing its nerve, sometimes in conjunction with either spinal ganglion removal or ventral rhizotomy. Sensory and motor terminals occupied separate locales along the length of normal muscle spindles. Nerve terminals of efferent and afferent neurons were located in appropriate positions along the length of spindles when axons of both types of neurons regrew together and when either category of axon regenerated alone. Precise reinnervation of muscle spindles occurred in spite of a diminished diameter of intrafusal fibers. Repopulation of the spindle with motor endings was less complete than that by sensory endings, based on the proportion and size of the regenerated terminals. We conclude that under optimal conditions for axonal regrowth, efferent and afferent neurons reinnervate their respective regions along intrafusal muscle fibers but motor lags sensory reinnervation within the spindle. The mechanism by which positional specificity happens during reinnervation of intrafusal fibers requires neither an interaction between terminals of the two types of neurons nor target cells of normal bulk.

High incidence of multiple-bag fiber muscle spindles in the articularis humeri muscle of the horse

The articularis humeri (AH) muscle of the horse is a small muscle composed of histochemically identified type I and IIA extrafusal fibers and a large number of muscle spindles. A total of 150 complete spindles with both spindle poles available were examined in serial transverse sections. On the basis of myosin ATPase-staining reactions after alkaline and acid preincubations, four types of intrafusal fibers, namely, bag1, bag2, "mixed" bag, and chain fibers, were identified. A high proportion of the spindle population (62.6%) consisted of multiple-bag spindles containing three or more (up to six) bag fibers. Also one-bag-fiber spindles were observed. The one-bag-fiber spindles containing a bag2 fiber could be traced into tandem linkages. "Mixed" bag intrafusal fibers, differing in their ATPase staining profile at the two poles, were found in spindles containing also at least one bag1 and one bag2 fiber. An unusually long extracapsular tract (up to 5,500 microns) of the bag intrafusal fibers was observed.

Neural organization of spindles in three hindlimb muscles of the rat

The neuroanatomical organization of the dynamic (bag1) and static (bag2 and chain) intrafusal systems was compared by light and electron microscopy of serial sections among 71 poles of muscle spindle in soleus (SOL), extensor digitorum longus (EDL), and lumbrical (LUM) muscles in the rat. Eighty-four percent of 195 fusimotor (gamma) axons to the spindles innervated either the dynamic bag1 fiber or the static bag2 and/or chain fibers. Sixteen percent of the gamma axons coinnervated the dynamic and static intrafusal fibers. Some of these nonselective axons were branches of effernts that also gave rise to axons selective to either the dynamic or static types of intrafusal fibers in one or more spindles. Thus activation of individual stem gamma efferents might not have a purely dynamic or purely static effect on the integrated afferent outflow from spindles of a hindlimb muscles in the rat. In addition, primary afferents in all muscles had terminations that cross-innervated the dynamic bag1 and static bag1 and/or chain intrafusal fibers in individual spindles, an arrangement that may enhance the mixed dynamic/static behavior of afferents when different intrafusal fibers are activated concurrent. Spindles of the slow SOL and fast EDL muscles had similar features, whereas differences were observed in the organization of the proximal (SOL and EDL) and distal (LUM) muscles. Spindles in LUM muscles had fewer static intrafusal fibers, a higher ratio of dynamic to static gamma axons, and a higher incidence of skeletofusimotor (beta) innervation to intrafusal fibers than spindles in the SOL or EDL muscles. Thus, the relative contribution of dynamic and static systems to muscle afferent outflow may differ among spindles located in different segments of the rat hindlimb. However, the dynamic and static intrafusal systems of spindle were less sharply demarcated in each of the three hindlimb rat muscles than in the cat tenuissimus muscle.

Contours and distribution of sites that react with antiacetylcholinesterase in chicken intrafusal fibers

Serial cross and longitudinal sections of intrafusal fibers from the intracapsular portions of chicken tibialis anterior muscle spindles were incubated with a monoclonal antibody specific for chicken acetylcholinesterase (AchE) and examined by immunofluorescence for the presence of the enzyme on presynaptic and postsynaptic membranes of neuromuscular junctions. The midequatorial sensory region which lacks organized sarcomeres was negative, but immediately distal to it faintly staining regions of AchE localization were observed on intrafusal fibers. In cross sections at the juxtaequator, the outlines of areas that were positive for AchE were either thin and crescentlike or thick and compact. The distribution of both types of localization continued into the polar region. Toward the more distal polar region, the intensity of sites on the postsynaptic membrane that reacted with the anti-AchE progressively increased. In longitudinal sections, AchE localization was largely limited to two configurations. One was elongate, while the other was more round or oval and often also smaller. Both types might occur on the same, or on different, intrafusal fibers. Examination of silver-impregnated sections revealed the presence of platelike and of traillike axon terminals. The variety of shapes observed on presynaptic and postsynaptic membranes warrants further study to determine whether chicken muscle spindles are innervated by more than one type of motor neuron.

Nonuniform expression of myosin heavy chain isoforms along the length of cat intrafusal muscle fibers

The expression of four myosin heavy chain (MHC) isoforms, avian slow-tonic (ATO) or neonatal-twitch (ANT) and mammalian slow-twitch (MST) or fast-twitch (MFT) in intrafusal fibers was examined by immunocytochemistry of spindles in the tenuissimus muscle of adult cats. The predominant MHCs expressed by nuclear bag fibers were ATO and MST, whereas the MHCs prevalent in nuclear chain fibers were ANT and MFT. The expression of these isoforms of MHC was not uniform along the length of intrafusal fibers. In general, both bag and chain fibers expressed avian MHC in the intracapsular region and mammalian MHC in the extracapsular region. The nonuniform expression of MHCs observed along the length of bag and chain fibers implies that different genes are activated in myonuclei located in the intracapsular and extracapsular regions of the same muscle fiber. Regional differences in gene activation might result from a greater effect of afferents on myonuclei located near the equator of intrafusal fibers then on myonuclei outside the spindle capsule.

Fusimotor mechanisms determining the afferent output of muscle spindles

There is both direct and indirect evidence that stretch activation occurs in the dynamic bag1 fibres of the mammalian muscle spindle and that it is responsible for maintaining the high sensitivity of primary sensory endings in stretches great enough to break the resting actomyosin bonds responsible for the short-range stiffness of muscle fibres. However the direct observations of dynamic bag1 fibre behaviour during stretching were made on damaged fibres and during very slow stretches. Preliminary results of experiments employing faster stretches of intact muscle spindles are reported here. An image processing system is being developed to automate and facilitate analysis of sarcomere movements during stretch, release and activation of intrafusal fibres. Unequivocal evidence confirming the development of stretch activation has not yet been found. Boyd (1986a) believed that static bag2 and chain fibres are controlled by separate populations of static gamma motoneurones, while accepting that there is some degree of common innervation. His evidence and the functional implications are discussed.

Histochemical heterogeneity of intrafusal muscle fibres in slow and fast skeletal muscles of the rat

Intrafusal muscle fibres of the slow soleus (Sol) and fast vastus lateralis (VL) muscles of the rat were studied histochemically. Serial transverse sections were incubated for the localization of succinate dehydrogenase (SDH), alpha glycerophosphate dehydrogenase (GPD) and adenosine triphosphatase (ATPase). The latter was examined further after preincubation in acidic solution held at either low or room temperature (RT). The bag2 intrafusal fibres in both muscles displayed high regular and acid stable ATPase, but low SHD and GPD activities. Bag1 intrafusal fibres showed low to moderate regular ATPase, a regional heterogeneity after RT acid preincubation (low activity in juxtaequatorial and high in polar zones), moderate SDH, but low GPD reactions. In both muscles the chain fibres usually exhibited high ATPase for both regular and cold acid preincubated reactions, but usually low activity after RT acid preincubation; they had high SDH but variable GPD activities. In Sol muscle, however, approximately 25% of spindles contained chain fibres that showed high acid-stable ATPase reaction after both cold and RT acid preincubation. In contrast, chain fibres in some VL spindles had a characteristically low ATPase reaction even after cold acid preincubation. This study, therefore, has delineated the existence of an inherent heterogeneity among chain fibres (with respect to their histochemical reactions) in muscle spindles located within slow and fast muscles and also between those found within populations of either Sol or VL muscle spindles.

Degeneration and alteration of axons and intrafusal muscle fibers in spindles following tenotomy

The effect of tenotomy (cutting of distal and proximal tendons) on the spindles in soleus muscle of adult rats was examined by light and electron microscopy in tissue obtained 1, 2, 3, 4, 5, 6, 7, and 14 days after surgery. Degenerative changes in the spindles progressed with time; also, the degree of alteration observed in a spindle varied with its position in the muscle, i.e., the earliest and most extensive changes occurred in spindles situated near the tendons. The disorganization and breakdown of myofibrils in the intrafusal muscle fibers were observed 3 days after tenotomy and continued with longer periods after surgery. The progression of morphological changes included alteration in fiber outline and the external lamina of the intrafusal muscle fibers and changes in the extracellular environment within the spindle capsule. The intrafusal muscle fibers of some spindles were shortened, apparently due to the loss of their polar ends, and degenerated axons which apparently supplied the intrafusal muscle fibers were also observed. Completely necrotic intrafusal fibers were absent. A possible relationship between the observed morphologic alterations and an adjustment of the spindle to muscle shortening is discussed.

The innervation of tandem muscle spindles in the cat neck

Patterns of innervation were examined in tandem muscle spindles teased from silver-stained muscles of the cat neck. Each tandem spindle was composed of two or more encapsulated receptors linked in series by a shared bag2 fiber. In most tandem spindles, two different types of encapsulation were identified according to differences in their intrafusal fiber content. One type, the b1b2c unit, contained typical bag1, bag2, and chain fibers and was structurally similar to single spindles described in other cat muscles. Each b1b2c unit contained a single primary sensory ending and 1-6 secondary endings. Fusimotor innervation was supplied by many axons. Some fusimotor axons ended in trail ramifications on bag2 and chain fibers, others ended in plates on the bag1 or long chain fiber. The other type of tandem encapsulation, the b2c unit, had only bag2 and chain fibers in its intrafusal fiber bundle. The b2c unit was usually supplied by only one sensory axon that ended on the nucleated part of the intrafusal fiber bundle. This single ending had a more variable terminal morphology than the primary ending in b1b2c units. A few b2c units (3/49) were also supplied by a secondary ending. The fusimotor innervation of the b2c unit was relatively simple. A single pole of the b2c unit was usually supplied by only one to three axons, all ending in trail ramifications. No plate endings were found in b2c units. These morphological specializations suggest that b1b2c and b2c units in tandem spindles differ in both their transductive and fusimotor mechanisms. Thus, the tandem spindle is a specialized structure that may provide additional proprioceptive information beyond that available from single muscle spindles.

Activation of cat muscle spindles by static skeletofusimotor axons

The discharges from primary and secondary spindle endings of the cat peroneus tertius muscle were recorded during stimulation of static skeletofusimotor (static beta) axons at frequencies comparable with the presumed range of motoneuronal firing rates. When stimulated at 20-40/s, static beta-axons exerted typical static actions on the spindles they innervated, including activation of primary endings with reduction of their dynamic sensitivity and activation of secondary endings. For these frequencies, the extrafusal portions of static beta-motor units developed unfused contractions producing oscillations of tension within the muscle. After suppression of extrafusal contractions, the effects of the stimulation of static beta-axons on spindle discharge could persist unaltered, showing that extrafusal events need not interfere with the specific intrafusal actions of static beta-axons. Stimulation of a static beta-axon at 20-40/s often elicited a response of primary endings in which the discharge exactly followed the stimulation frequency, i.e. it was driven 1:1. Purely mechanical excitation of a spindle by unfused extrafusal contractions could also drive its discharge at the stimulation frequency. The persistence of static beta-driving after suppression of extrafusal contraction provided evidence for its intrafusal origin. Driving elicited by static beta-axons could persist during changes in muscle length, but small fluctuations in the delay between each impulse and the preceding stimulus were observed. These fluctuations were clearly related to the changes in muscle length, indicating that although the primary ending discharge remained driven 1:1 at the stimulation frequency, the receptor was not totally insensitive to length changes.

Form and classification of motor endings in mammalian muscle spindles

The presynaptic features of 234 motor endings supplied to cat hindlimb muscle spindles have been studied in teased, silver preparations, and the postsynaptic features of a further 27 endings have been studied in serial, 1 micron thick, transverse sections. In the presynaptic study motor endings received by the three types of intrafusal muscle fibre were compared with the endings supplied to spindles by the various functional categories of motor axon. Three forms of motor ending were found that had significantly different presynaptic features. These forms correspond closely to those previously identified in the literature as p1 (beta), p2 (dynamic gamma) and trail (static gamma). The results of the postsynaptic study showed that the degree of indentation of the intrafusal muscle fibres by motor axon terminals increases with greater distance from the primary ending, irrespective of muscle-fibre type. We conclude that the postsynaptic form of intrafusal motor endings is determined by distance from primary ending and muscle-fibre type. It is not determined by type of motor axon, and cannot be correlated with presynaptic form so as to produce a unified classification of intrafusal motor endings.

Histological study of motor innervation of nuclear bag1 intrafusal muscle fibers in the cat

The nerve supply to spindles of the cat tenuissimus muscle was reconstructed with light and electron microscopy of serial transverse sections. Fifty-two poles of the nuclear bag1 intrafusal muscle fiber were examined for motor innervation. The fiber poles were supplied by 71 myelinated motor axons that either terminated on bag1 fibers exclusively (93%) or coinnervated a chain fiber of the same intrafusal bundle (7%). No axons coinnervated both the bag1 and bag2 fibers. The unmyelinated preterminal segments of the axons were frequently short. Lengths and pre- and postsynaptic features of motor endings on bag1 fibers were variable. These features did not permit reliable classification of the endings into more than one morphological category. Moreover, the terminals of fusimotor (gamma) and skeletofusimotor (beta) axons on bag1 fibers appeared similar in cross-section. The degree of indentation of axon terminals into the surface of bag1 fibers increased with increasing distance from the spindle equator. However, cross-sectional areas of sole plates and axon terminals were relatively constant regardless of distance from the equator. The subjunctional membranes of both gamma and beta bag1 endings were typically smooth in contour. Bag1 endings differed from those on bag2 and typical chain fibers in having a thicker sole plate, frequently indented axon terminals, and unfolded subjunctional membranes. None of the bag1 endings resembled an extrafusal end plate. These observations indicated that (1) the dynamic (bag1) and static (bag2 and chain) intrafusal systems of the cat spindle are under separate motor control, and (2) the type of intrafusal fiber and the distance of the motor ending from the equator have a greater influence on the form and structure of bag1 endings than do supplying axons.

Distribution of skeletofusimotor axons in lumbrical muscles of the monkey

The nerve supply to 25 poles of muscle spindles in the monkey was reconstructed by light microscopy of serial 1-micron thick transverse sections of lumbrical muscles. Twenty of 60 motor axons that supplied the spindle poles were identified as skeletofusimotor (beta). Twenty-eight percent of the spindle poles were innervated by beta axons, in addition to gamma axons. Every beta-innervated spindle pole transected an endplate zone of extrafusal muscle. Most beta axons coinnervated extrafusal fibers rich in mitochondria and the nuclear bag1 or nuclear chain intrafusal fibers. All but two beta axons innervated one type of intrafusal fiber only. The intramuscular organization of beta motor system in lumbrical muscles of the monkey was similar to that of the cat tenuissimus muscle. The function of beta-innervated spindles may be preferentially to monitor mechanical disturbances arising from the activity of extrafusal muscle units with which they share motor innervation.

Glycogen depletion elicited in tenuissimus intrafusal muscle fibres by stimulation of static gamma-axons in the cat

In this study the experimental conditions used to elicit glycogen depletion in tenuissimus intrafusal muscle fibres were different from those used by Barker, Emonet-Dénand, Harker, Jami & Laporte (1976): the tenuissimus was left in situ; several (4-6) static gamma-axons were stimulated together; the blood flow through the muscle was not reduced during the periods of gamma stimulation except in two experiments; very much longer periods (up to 9 h) of intermittent stimulation by bursts at 50-500/s were used. Bag1 and bag2 fibres were identified by their different ATPase activities in the B region. In two experiments with normal circulation, test responses of several primary endings to short periods of stimulation at 50-100/s were still very strong after stimulation of several static gamma-axons for 5 and 9 h, respectively. Glycogen depletion was observed in a large number of chain and bag2 poles but in only one of nineteen bag1 poles examined. In two other experiments with normal circulation, there was a very pronounced reduction of the test responses after stimulation of several static gamma-axons for 7 and 9 h, respectively. Out of twenty-four bag1 poles examined, nineteen exhibited zones of depletion. In an experiment in which stimulation was conducted as in Barker et al. (1976), i.e. with reduction of muscle blood flow during 1 min periods of stimulation at 50-100/s, the primary endings still gave a strong response after fifteen periods of stimulation in contrast with the marked 'fatigue' that was constantly observed in the former study. No depleted intrafusal fibres were found in the spindles of this muscle. In a last experiment, after an initial pattern of stimulation similar to that described above, the new pattern of stimulation, but with periodical reduction of blood flow, was applied, leading to a 'fatigue' of the test responses in 2 h. In the spindles of this muscle three out of ten bag1 poles were depleted. The variability of glycogen depletion in bag1 fibres appears to be linked to the degree of spindle 'fatigue' which may develop after static gamma stimulation. It seems that in 'fatigued' spindles some factor or factors liberated by the contraction of neighbouring fibres may deplete glycogen in bag1 fibres by a non-neural mechanism. When, in spite of a prolonged stimulation of static gamma-axons, no fatigue of the test responses develops, zones of depletion in bag1 fibres--possibly of neural origin--are very rare, although a large proportion of bag2 and chain fibres are depleted.

Multiple-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 300 complete and incomplete cat muscle spindles were examined in serial transverse sections of tenuissimus muscles in search of spindles with more than two nuclear bag intrafusal muscle fibers. Several histochemical and histological stains were used to identify the intrafusal fibers and assess their motor and sensory innervation. About 13% of the spindles contained either three or four bag fibers rather than the usual two. Every multiple-bag-fiber spindle possessed at least one nuclear bag1 and one nuclear bag2 fiber. The supernumerary bag fibers were either another bag1 and/or bag2 fiber, or a mixed bag fiber. The extra bag fibers had the usual morphologic and histochemical properties of cat nuclear bag fibers. All multiple-bag spindles received primary sensory innervation, and most had secondary sensory endings in addition. Their motor pattern was similar in the number, appearance and disposition of intrafusal motor endings to that of the usual two-bag-fiber spindles. Bag fibers of the same kind shared motor nerve supply in three multiple-bag spindles in which tracings of individual motor axons were obtained histologically. It is unclear whether any functional advantage is conveyed to a muscle spindle by its having more than one bag1 and one bag2 fiber.

The ultrastructure of cat fusimotor endings and their relationship to foci of sarcomere convergence in intrafusal fibres

1. Six muscle spindle poles, five from experiments in which foci of sarcomere convergence had been observed during stimulation of fusimotor axons, were serially sectioned for light and electron microscopy. Every somatic motor terminal was studied in ultrathin sections at several levels.2. In all six poles static gamma axons, or presumed static gamma axons, supplying the static bag(2) fibre and/or chain fibres had no terminations on the dynamic bag(1) fibre. In five poles, the dynamic bag(1) fibre was selectively innervated by dynamic gamma or beta axons save in one case where a dynamic gamma axon also innervated one chain fibre.3. Seventy-seven motor endings were of four distinct ultrastructural types: ;m(a) plates' lay superficially on the surface of static bag(2) or chain fibres; ;m(b) plates' were deeply indented into dynamic bag(1) fibres; in ;m(c) plates', found on chain fibres only, the muscle surface was thrown into projecting fingers between which the axon terminals were embedded; one type ;m(d) plate' was found, fully indented into a long chain fibre. A few plates of intermediate form (m(ab)) were variants of m(a) and m(b) plates.4. The muscle membrane beneath both m(a) and m(b) plates was smooth, or had a few wide, shallow folds; m(c) plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the m(d) plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag(1) and static bag(2) fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag(1) fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag(2) fibres; contraction in these fibres is typically mid-polar.6. In some poles local contraction of chain fibres centred on the location of m(c) plates. In others, very localized contraction occurred distal to the sites of m(a) plates. Both m(a) and m(c) plates were never found on the same pole of a chain fibre.7. Dynamic gamma or beta axons end in m(b) plates, probably equivalent to p(2) plates. The concept of distinctly different p(1) and p(2) plates on dynamic bag(1) fibres, supplied by dynamic beta and gamma axons, respectively, is not supported by ultrastructural evidence.8. Some static gamma axons end in multiple m(a) plates which correspond with ;trail endings', or in single large m(a) plates, on static bag(2) or chain fibres. The m(c) plates are the terminations of other static gamma, or occasionally dynamic gamma, axons on chain fibres. Static beta axons probably end in m(d) plates on long chain fibres which may correspond with p(1) plates.9. It is proposed that there are two types of static gamma motoneurone, one terminating in m(a) plates and the other in m(c) plates, possibly directed preferentially towards static bag(2) fibres and chain fibres, respectively.

Histological study of an unusual cat muscle spindle deficient in motor innervation

An unusual muscle spindle innervated by only one somatic motor axon and one primary sensory axon was encountered in a cat tenuissimus muscle cut in serial transverse sections and examined by light and electron microscopes. The motor axon branched to supply the distal poles of the nuclear bag2 and nuclear chain intrafusal muscle fibers through motor terminals of several types. The proximal poles of the bag2 and chain fibers and both poles of the nuclear bag1 fiber were devoid of motor endings. In spite of the limited motor nerve supply of the bag 1, bag2 and chain fibers exhibited the usual morphologic characteristics in terms of the appearance of the equatorial region, relative fiber lengths and diameters, and the number of associated elastic fibers. It appears that motor axons may play only a limited role, if any, in the differentiation and maintenance of the three types of intrafusal muscle fiber in the cat.

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.

A study of motor nerve terminals on cat nuclear bag1 intrafusal muscle fibers using the ChE staining technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. Histochemical staining for "myofibrillar" adenosine 5'-triphosphatase was employed to identify nuclear bag1 intrafusal muscle fibers. Staining for cholinesterases (ChE) was used to demonstrate the termination sites of motor axons along the fibers. Several types of ChE deposits could be distinguished along the bag1 fibers based on intensity of staining and morphological characteristics. Most ChE deposits could be classified as either the "pale" or the "nonpale" plates. Some ChE active areas fitted neither of these two categories. Among 328 ChE "plates" encountered on 192 bag, fiber poles, 197 (60%) were of the "pale" and 27 (8%) of the "nonpale" type with 104 (32%) remaining unclassified. These histochemical observations are discussed with regard to the current structural and functional concepts of motor innervation of the nuclear bag1 fiber. It is suggested that the histochemical (ChE staining intensity) and morphological (length and form) characteristics of bag1 fiber motor endings are not determined solely by the nature of the corresponding motor axons.

A study of sensory innervation to long nuclear chain intrafusal fibers in the cat muscle spindle

The sensory innervation of 46 poles of long chain intrafusal muscle fibers was studied histochemically by staining for NADH-TR in periodic, 8 micron thick transverse sections of cat muscle spindles. Each long chain fiber carried terminals of the primary sensory axon, and 23 of the fiber poles also displayed secondary sensory endings. With the NADH-TR reaction there was no apparent difference in the cross-sectional appearance of sensory endings on the long chain and on other nuclear chain fibers. However, the contact area between the secondary endings and the muscle fiber tended to be shorter on the long chain than on the neighboring chain fibers of shorter polar length. This was also the case for one long chain fiber in which the sensory innervation was examined in serial, 1 micron thick sections stained with toluidine blue. Discharges of the secondary sensory axons in cat spindles may be affected more by contraction of the shorter nuclear chain fibers than by activation of the long chain fibers.

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.

Appearance of sensory nerve terminals in cat muscle spindles stained for NADH-tetrazolium reductase

Cat muscle spindles were studied histochemically in serial transverse sections of the tenuissimus muscle stained for myofibrillar ATPase, cholinesterase or NADH-tetrazolium reductase. The terminal sites of the primary and secondary axons on intrafusal muscle fibers could be demonstrated due to their high NADH-TR activity. This sensory NADH-TR reactivity at the equator and in the juxtaequatorial regions disappeared following spindle chronic de-afferentation, but not after de-efferentation. Spindle poles that carried both primary and secondary sensory endings had a longer periaxial fluid space than poles with primary endings only, and their motor innervation, as determined by staining for ChE, was positioned at the greater distance from the equator. Some of the secondary endings occurred in intrafusal regions that displayed surface fiber ChE activity. The histochemical reaction for NADH-TR represents a simple, rapid and reliable method for studies of the distribution of sensory nerve terminals in the spindle.

The patterns of discharge of static and dynamic gamma motoneurones in the decerebrate rabbit

1. The effect of natural stimulation of the limbs (usually pinching the forepaws) on the discharges of muscle spindle primary and secondary afferents and gamma motoneurones of the gastrocnemius medialis muscle were examined in the precollicular decerebrate rabbit, paralysed with gallamine triethiodide to prevent extrafusal muscle contraction. 2. The afferents showed only excitation in response to natural stimulation. The same stimulus strongly inhibited some gamma motoneurones and, at the same time, predominantly excited others. 3. Since extrafusal muscle contraction was prevented using gallamine triethiodide, intrafusal contraction was responsible for the afferent excitation, and could only have been evoked by those gamma motoneurones excited by natural stimulation. The distribution of effects from gamma motoneurones that were strongly inhibited by natural stimulation could not be deduced from afferent recordings. 4. On the basis of these afferent and efferent recordings, and the known structure of the muscle spindle, gamma motoneurones were identified as static or dynamic. 5. The results are discussed in relation to the patterns of discharge of static and dynamic gamma motoneurones.

The topography of long nuclear chain intrafusal fibers in the cat muscle spindle

Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscle fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag2 fiber. Furthermore, they were usually situated adjacent to the bag1 fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag2 fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.

One-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 150 complete and 139 incomplete single muscle spindles were examined in serial transverse sections of cat tenuissimus muscles in search for spindles lacking one of the two types of nuclear bag intrafusal fiber. Several histochemical reactions were used to type the intrafusal muscle fibers and assess the spindle motor and sensory innervation. One complete spindle lacked a bag1 fiber, and another spindle lacked a bag2 fiber. Several incomplete spindles also lacked bag1 fibers. In addition, ten double tandem spindles contained one capsular unit each that lacked the bag1 fiber, and one triple tandem spindle had two such capsules. All one-bag-fiber spindles had primary sensory innervation, but none had secondary sensory innervation. Their motor innervation was similar to that of the usual two-bag-fiber spindles in the number and disposition of intrafusal motor endings. It is unclear whether the one-bag fiber spindles, either single or tandem-linked, are products of an aberrant spindle development or represent a true anatomical and functional subcategory of the cat muscle spindle.

Histochemical study of an unusual cat intrafusal muscle fiber

A cat tenuissimus muscle spindle that contained two long chain intrafusal fibers in its distal pole is described. One of the fibers (lc1) had a histochemical profile (ATPase, NADH-TR, ChE reactions) of the kind which is characteristic for long chain fibers. The other fiber (lc2) consisted of two separate segments. The inner lc2 segment included the sensory equatorial region and was histochemically normal. The outer lc2 segment carried a motor plate, and did not stain for NADH-TR in the same way as the inner lc2 segment and the lc1 fiber. It is suggested that the unusual enzyme staining properties of the outer lc2 segment stemmed from its lack of sensory innervation, a situation which may have permitted the full expression of influences mediated by its motor nerve supply.

Histochemical profiles of cat intrafusal muscle fibers and their motor innervation

Muscle spindles were examined histochemically in serial transverse sections of cat tenuissimus muscles. The myofibrillar adenosine triphosphatase (ATPase) staining reaction was used to identify nuclear bag1, bag2 and nuclear chain intrafusal muscle fibers. Regional differences in ATPase staining occurred along the bag1 and bag2 fibers but not along the chain fibers. All intrafusal fiber types displayed regional variability in staining for nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR). Motor nerve terminals were demonstrated along the poles of bag1, bag2 and chain fibers by staining for cholinesterase (ChE). There was no consistent spatial correlation between the intensity of regional ATPase staining along the bag fibers and location, number or type of motor endings. However, most ChE deposits occurred in intrafusal fiber regions that displayed the greatest NADH-TR variability. Some fiber poles or whole intrafusal fibers were devoid of any ChE deposits but their ATPase and NADH-TR content was comparable to that of fibers bearing ChE deposits. The observations suggested that motor nerve fibers per se may not play a major role in determining the histoenzymatic content of intrafusal fibers.

Morphological and histochemical differentiation of intrafusal fibres in the posterior latissimus dorsi muscle of the developing chick

Morphological and histochemical differentiation of neuromuscular spindles was studied in the posterior latissimus dorsi (PLD) of the chick during embryonic and post-hatching development. A rapid increase in the number of spindles takes place between the 13th and 15th of embryonic life. By the 15th day in ovo, the spindle capsule appears filled with numerous contiguous cells. Large sensory endings and small primitive motor endings are observed on intrafusal fibres. Ultrastructural observations of the nerve supply of the spindles confirm that each developing spindle receives one thick Ia axon with one to three thin gamma axons. The intracapsular space differentiates by the 17th day of embryonic development. All intrafusal fibres are morphologically of the nuclear-chain type, while two fibre types are distinguished as early as the 14th day of embryonic life, when myofibrillar ATPase activity is demonstrated after acid preincubation. These two histochemical types of intrafusal fibres are also described in the adult. The relation between these two histochemical types and different functional activity of intrafusal fibres is suggested.

Immunohistochemical differences in myosin composition among intrafusal muscle fibres

Mammalian intrafusal fibre types (nuclear chain, nuclear bag1 and nuclear bag2 fibres) are known to differ in their ultrastructure, intensity of the myofibrillar histochemical ATP-ase reaction, type of innervation and time course of contraction. The present study concerns the myosin composition of these intrafusal fibre types in the soleus muscle (mouse) and the extensor digitorum longus muscle (rat). We used an immunohistochemical method with three myosin antisera raised in rabbits: anti chicken pectoral myosin, anti chicken heart myosin (1) and anti chicken heart myosin (2) (= anti chicken heart myosin (1) adsorbed with muscle powder from soleus muscle of guinea pig). The results showed that three intrafusal fibre types differed in their myosin composition. A comparison of intrafusal fibre types with extrafusal fibre types for the histochemical myofibrillar ATP-ase reactivity and the reactivity with myosin antisera showed a resemblance of nuclear chain fibres with extrafusal type II fibres and a difference between nuclear bag1 and nuclear bag2 fibres and all other fibre types.

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.

Histochemical study of long nuclear chain fibers in the cat muscle spindle

Cat intrafusal muscle fibers were examined histochemically in serial transverse sections of tenuissimus muscle spindles. The "myofibrillar" adenosine triphosphatase staining reaction was used to recognize the nuclear bag and the nuclear chain fibers in 309 spindle poles. Poles of 40 nuclear chain fibers extended for 1,000 micrometer or more beyond the termination of the spindle capsule. These long chain fibers stained less intensely for nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) than the typical chain fibers of shorter polar length. In sections stained for cholinesterases (ChE), the extracapsular regions of most long chain fibers displayed one or two short, dense "plate"-type ChE deposits, which may represent the terminals of skeleto-fusimotor axons. In addition, about one-third of the long chain fibers displayed one or more thinner and smaller areas of ChE activity, possibly corresponding to the endings of fusimotor axons. The overall ChE staining pattern of the typical chain fibers was unlike that of the long chains. However, some of the shorter nuclear chain fibers resembled long chain fibers with the NADH-TR reaction, even though their ChE "plates" were located intracapsularly. It is concluded that nuclear chain fibers in the cat spindle form a class of intrafusal fibers with heterogeneous histochemical properties, and that the long chain fibers represent one fiber subtype.

Motor innervation of the cat muscle spindle studied by the cholinesterase technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. "Myofibrillar" adenosine triphosphatase staining reaction was used to identify nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers. Typical chain fibers and long chain fibers were distinguished, the latter extending for more than 1,000 micron beyong the termination of the spindle capsule. Simple "rim" and more elaborate "plate" deposits were demonstrated histochemically along the poles of the typical chain fibers in staining for cholinesterases. They were considered to correspond, respectively, to the trail and plate motor nerve terminals. Most long chain fibers and the majority of nuclear bag fibers had their motor innervation limitd to "plate"-type endings. In addition, faint diffuse cholinesterase staining occurred along the spindle capsule and the surface of some intrafusal fibers. These histochemical observations are discussed with regard to the current concepts concerning the morphological and functional organization of the motor innervation of the cat muscle spindle.

Types of human intrafusal muscle fibers

The histochemical and fine structural profiles of human intrafusal muscle fibers were studied. Spindles were located in freshly frozen specimens taken from biopsied normal external intercostal muscles, and periodic 10- and 50-mum-thick cross sections were processed alternately for enzyme histochemical and electron microscopic examination. Nuclear bag fibers were of two types, bag1 and bag2, histochemically, and they displayed two distinct types of ultrastructure. Nuclear chain fibers were histochemically and ultrastructurally homogeneous. Regional differences in enzymatic staining and ultrastructure occurred along individual intrafusal fibers. Human bag1 and bag2 fibers appear to be analogous to the two types of nuclear bag fiber identified in animal spindles and are considered to have different roles in spindle function. The presence of three types of intrafusal fibers should be taken into account when studying spindle abnormalities in human neuromuscular disorders.

"Fast" and "slow" skeleto-fusimotor innervation in cat tenuissimus spindles; a study with the glycogen-depletion method

The glycogen-depletion method was used to investigate the motor supply to tenuissimus with respect to the presence of fast beta axons and to assess the total proportion of both fast and slow beta-innervated spindles in this muscle. In a first series of 5 expts., groups of motor axons with conduction velocities higher than 85 m/s were repetitively stimulated so as to produce glycogen depletion in the muscle fibres they innervated. The whole muscle was then quick-frozen, serially cut, stained to demonstrate glycogen and examined for intrafusal glycogen depletion. Zones of glycogen depletion were found in 16 of the 46 examined spindles; they were most frequently located in the longest of the chain intrafusal muscle fibres. Since it is known that there are no purely fusimotor axons to tenuissimus with conduction velocities above 50 m/s, it was concluded that beta axons are present among the fastest axons to this muscle. In a second series of 5 expts. as many motor axons as possible with conduction velocities above 60 m/s were stimulated. Zones of glycogen depletion were found in 19 of the 47 examined spindles. They affected chain fibres in about half of the instances and bag1 fibers in the others. As this latter location is characteristic of slow dynamic beta axons, it was concluded that both slow and fast beta axons occur regularly in the motor supply to tenuissimus. beta-innervation is present in at least 40% of tenuissimus spindles with almost no convergence of fast and slow beta axons onto the same spindle.