Ultrastructural localization of non-specific cholinesterase activity in rat muscle spindles

Electron microscopical localization of non-specific cholinesterase activity was studied in the encapsulated part of rat hindlimb muscle spindles. After incubation of the muscle tissue in a medium containing butyrylthiocholine bromide as substrate and BW284c51 as the specific inhibitor of acetylcholinesterase, a distinct electron-dense precipitate corresponding to non-specific cholinesterase activity was found along the whole length of muscle spindles. The richest source of non-specific cholinesterase activity were the motor end-plates present in the polar and juxtaequatorial regions. Much smaller amounts of reaction deposits were found at the secondary sensory terminals in the juxtaequatorial zones. The primary sensory terminals in the equatorial zone contained only low amounts of the reaction product. A fine homogeneous reaction product was localized in the narrow spaces between Schwann cell processes or in gaps between the Schwann cell, and axonal and muscle membranes. A granular precipitate was localized on the basal lamina in the synapse region of motor terminals or covering Schwann cell processes and sensory terminals with adjacent intrafusal muscle fibres. Our results suggest that most of non-specific cholinesterase in muscle spindles is synthesized by the Schwann cells; but a small amount can also be synthesized by fibroblast-like cells forming the inner capsule of muscle spindles. Non-specific cholinesterase thus coexists with acetylcholinesterase at motor end-plates, but is single at sensory terminals. The function of non-specific cholinesterase in sensory receptors is still not clear. It seems most probable that non-specific cholinesterase in muscle spindles may play a role in the maintenance of the external milieu around nerve endings, especially in the sensory region.     

The effect of the local anaesthetic bupivacaine on the muscle spindle of rat

Summary The effects of a long-lasting local anaesthetic, bupivacaine, in combination with hyaluronidase, on the intrafusal muscle fibres of adult rat muscle spindles have been investigated by light and electron microscopy. Necrotic changes in the axial bundle are present within 4 h of the combined drug injection and are widespread by 2 days. The equatorial nucleation of the spindle is lost owing to the necrosis of the myonuclei. Satellite cells, however, appear to survive these changes, even where the underlying muscle fibres are grossly necrotic. Sensory nerve terminals undergo necrosis during this period and plate-type fusimotor nerve terminals withdraw from the degenerating muscle fibres.Macrophage infiltration and early regeneration of the axial bundle is apparent by the third day. Myoblasts first appear at the periphery of the affected muscle fibres, but decrease in number as regeneration proceeds. By the end of the third week, regeneration of the muscle component is complete and re-innervating fusimotor nerve terminals established. The encapsulated regions of the spindle remain abnormal, without recognizable bags or chains of nuclei, although the muscle fibres differ both in size and ultrastructure. Regenerating sensory axons make contacts with the intrafusal bundle that differ in their configuration and ultrastructure from normal terminals.The production of enucleated spindles is discussed in the light of the morphogenetic influences of the spindle nerve terminals, as well as the possible use of such a model in discovering the function of nuclear bags and nuclear chains.     

Fine structural changes of muscle spindles in the gracile axonal dystrophy mutant mouse

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

Muscle spindles in rheumatoid arthritis

Summary Ultrastructural features of muscle spindles were studied in biopsy material from 100 patients suffering from classical rheumatoid arthritis. Thickening of the outer capsule, increased amount of extracellular ground substance within the inner capsule, and marked thickening of the basement membrane of capillary blood vessels supplying the muscle spindles were observed. Chronic inflammatory cells and macrophages were present within the spindles. Changes affecting the intrafusal muscle fibres were also seen. They were manifest as atrophy and degeneration of the intrafusal muscle fibres, absence of the specialised junctional complexes, and of the intercellular bridges, microladders and satellite cells. It is suggested that the changes affecting the intrafusal muscle fibres are probably secondary. Damage to the myelinated nerves was present, while the sensory and motor nerve endings were well preserved.Temporary Research Fellow the Rotterdam Centre for Rheumatic Disease Present address: Division of Pathology, Department of Obstetrics and Gynaecology, University of Chicago, Chicago, Illinois 60637, USA     

Effects of the infraorbital nerve-transection on the formation of Ia fibers in muscle spindles of the masseter muscle in developing mice

The effect of the transection of the infraorbital nerve on d 20 after birth on the formation of the annulospiral ending of Ia fibers of muscle spindles in the masseter muscle was investigated in developing mice. On the 20th d after the operation, there were significant differences between control and treated animals in the ratio of muscle spindles with non-coiled endings to all muscle spindles, and the ratios of muscle spindles with non-coiled endings in the masseter muscle on the operated and sham-operated sides were about equal to each other. On the 30th d after the operation, the ratio for the masseter muscle on the operated side was approximately twice that for the one on the sham-operated side. The ratio for the operated side was about equal to that for 15-d-old control mice, and about twice that for 20-d-old control animals. Thus, the present results indicate that disorder of sensory inputs following the transection of the infraorbital nerve leads to degeneration and/or inhibition of the formation of annulospiral endings in muscle spindles of the masseter muscle, and suggest that disorder of the masticatory mechanism may be caused by the degeneration and/or inhibition of the formation of the endings in many muscle spindles of the masseter muscle.     

Innervation of developing intrafusal muscle fibers in the rat

The chronology of development of spindle neural elements was examined by electron microscopy in fetal and neonatal rats. The three types of intrafusal muscle fiber of spindles from the soleus muscle acquired sensory and motor innervation in the same sequence as they formed—bag₂, bag₁, and chain. Both the primary and secondary afferents contacted developing spindles before day 20 of gestation. Sensory endings were present on myoblasts, myotubes, and myofibers in all intrafusal bundles regardless of age. The basic features of the sensory innervation—first-order branching of the parent axon, separation of the primary and secondary sensory regions, and location of both primary and secondary endings beneath the basal lamina of the intrafusal fibers—were all established by the fourth postnatal day. Cross-terminals, sensory terminals shared by more than one intrafusal fiber, were more numerous at all developmental stages than in mature spindles. No afferents to immature spindles were supernumerary, and no sensory axons appeared to retract from terminations on intrafusal fibers. The earliest motor axons contacted spindles on the 20th day of gestation or shortly afterward. More motor axons supplied the immature spindles, and a greater number of axon terminals were visible at immature intrafusal motor endings than in adult spindles; hence, retraction of supernumerary motor axons accompanies maturation of the fusimotor system analogous to that observed during the maturation of the skeletomotor system. Motor endings were observed only on the relatively mature myofibers; intrafusal myoblasts and myotubes lacked motor innervation in all age groups. This independence of the early stages of intrafusal fiber assembly from motor innervation may reflect a special inherent myogenic potential of intrafusal myotubes or may stem from the innervation of spindles by sensory axons.     

Motor and sensory innervation of muscle spindles in the neonatal rat

Summary Neural and muscular elements of three muscle spindles from the soleus muscles of 4-day-old rats were reconstructed by electron microscopy of skip-serial transverse ultrathin sections. Each spindle contained four encapsulated intrafusal fibers, including a minimum of one bag₁, one bag₂ and one chain fiber. The fibers were innervated by unmyelinated motor and sensory axons. The primary and secondary afferents approached the spindles as single axons and terminated on the central region of the intrafusal fibers. Single profiles of terminal axons occupied the sites of sensory neuromuscular junctions, similar to adult sensory endings. No morphological features suggested retraction of afferents from 4-day postnatal spindles. Motor axons approached spindles tightly packed in bundles of 5–20 axons and terminated in the juxtaequatorial and polar regions of both bag and chain fibers. Multiple profiles of terminal axons were visible for each intrafusal motor ending. More motor axons innervated 4-day postnatal spindles and a greater number of axon terminals were visible in immature intrafusal motor endings than in adult spindles. The data suggest that postnatal maturation of motor innervation to intrafusal fibers involves the elimination of supernumerary motor nerve inputs. Synapse elimination in the development of the fusimotor system might represent a mechanism whereby individual axons adjust the number of spindles they innervate.     

Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron microscopy to determine whether morphological or quantitative differences existed between these sensory receptors. Differences were found in the density, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsular components. The slow ALD possessed muscle spindles which were evenly distributed throughout the muscle, whereas in the fast PLD they were mainly concentrated around the single nerve entry point into the muscle. The muscle spindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5±2.0 vs 55.4±2.0 respectively, n=6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters varied from 5.0 to 16.0 μm and 4.5 to 18.5 μm, respectively. Large diameter intrafusal fibres were more frequently encountered in spindles of the PLD. Unique to the ALD was the presence of monofibre muscle spindles (12.7% of total spindles observed in ALD) which contained a solitary intrafusal fibre. In muscle spindles of both the ALD and PLD, sensory nerve endings terminated in a spiral fashion on the intrafusal fibres in their equatorial regions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive in polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity with respect to the number of intrafusal fibres per spindle, range of intrafusal fibre diameters and development of their inner capsules. It is postulated that the differences in muscle spindle density and structure observed in this study reflect the function of the muscles in which they reside.     

Lyssavirus infection of muscle spindles and motor end plates in striated muscle of hamsters

Summary Immunofluorescent, light, and electron microscopy were used to document lyssavirus infection of muscle spindles and motor end plates. Virus particles were seen in the narrow intercellular space between sensory nerve endings and intrafusal muscle fibers; they were also observed budding from intracellular and plasma membranes of the latter. Involvement of motor nerves and motor end plates could only be demonstrated by electron microscopy. In nature, rabies virus invasion of the peripheral nervous system must involve centripetal spread across these junctions.With 6 Figures     

Morphogenesis of rat muscle spindles after nerve lesion during early postnatal development

Summary The influence of innervation on muscle spindle morphogenesis has been investigated in rat hind-limb muscles by sectioning the sciatic nerve, with suture of the stumps, at various postnatal stages. After nerve section at 4 or 7 days of age a proportion of spindles survived during the denervation phase and developed, during the subsequent reinnervation phase, into atypical structures. The reinnervated spindles were recognized by the presence of a limiting capsule but lacked the characteristic distinction of equatorial and polar regions. The intrafusal fibres were fewer than normal and were indistinguishable in size and fine structure from extrafusal fibres; they had a single motor endplate and lacked sensory nerve terminals. In reinnervated muscles of animals operated at 13 and 22 days of age there was a progressive tendency towards a restoration of normal spindle structure and innervation. These findings indicate that muscle spindle morphogenesis is profoundly altered by nerve lesion at early developmental stages, apparently as a result of inadequate sensory reinnervation. This study also shows that the differentiation of intrafusal fibres is dictated by their specific pattern of innervation and is not intrinsically predetermined.     

Ultrastructural identification of the primitive muscle spindle in the Xenopus laevis larvae

Summary The initial formation of muscle spindles was studied with electron microscopy using the toe muscle of Xenopus laevis. At the larval stage 57 (Nieuwkoop and Faber 1967), muscle spindles were first identified primarily by the presence of sensory endings associated with a thin bundle of myotubes, e.g. intrafusal (IF) myotubes which were partly invested by a single cellular layer. The number of IF myotubes per spindle was 5 to 6; the adult complement. IF-and extrafusal (EF) myotubes were almost identical in their size and structure. A few thinner IF myotubes with scaree myofibrils were also present. The reticular zone had been undeveloped. Sensory endings were smaller in size and in number per spindle than those in the adult, forming irregular beaded chains with occasional tubular expansions. The endings and IF myotubes were rarely in direct contact, being frequently interposed by a satellite cell and its process. Incipient fusimotor endings were widely distributed from the juxta-equatorial to the polar region. Large cored vesicles resembling the neurosecretory vesicles occurred in sensory and motor endings as well as in intramuscular nerve fibers. The vesicles may be involved in the neuronal influence upon the spindle differentiation.The results were compared with the formative process of mammalian spindles.     

Intramembrane structure of the sensory axon terminals in bullfrog muscle spindles

Much physiologic and morphologic research has been done into the sensory mechanism of the frog muscle spindle. However, no freeze-fracture study has described in detail the shape and intramembrane structure of the nonmyelinated sensory axon terminals of the frog muscle spindle. In this study, muscle spindles were isolated from the red part of bullfrog semitendinous muscles. Chemically fixed spindles were subjected to freeze fracturing. The sensory axon endings were reconstructed, and the size and density of intramembrane particles (IMPs) were measured along the sensory nerve endings. The axon terminals had four distinctive parts: parent trunks (>0.5 μm in diameter), primary branches (0.15–0.5 μm), terminal branches (<0.1 μm), and varicosities (0.02–0.5 μm). IMPs ranged from 5 nm to 21 nm in diameter and were present in the intramembrane space of the plasma membrane all throughout the nonmyelinated sensory nerve endings. Mean IMP sizes in the protoplasmic face (PF) and the external face (EF), respectively, were 8.1 nm and 8.4 nm in the parent trunks, 8.8 nm and 8.8 nm in the primary branches, 9.4 nm and 9.0 nm in the varicosities, and 8.7 nm and 8.7 nm in the terminal branches. Mean IMP size in the PF was smallest in the parent trunk and largest in the varicosity. Mean IMP densities (numbers of IMPs per μm²) in the PF and the EF, respectively, were 2,500 and 700 in the parent trunks, 2,200 and 500 in the primary branches, 1,700 and 400 in the varicosities, and 1,000 and 300 in the terminal branches. Density decreased with the tapering of the axon terminal, with IMPs distributed evenly in the PF and the EF. The characteristic intramembrane structure of sensory nerve endings is discussed. Anat. Rec. 252:340–354, 1998. © 1998 Wiley-Liss, Inc.     

Function and structure of atypical muscle spindles after neonatal nerve crush in rats

Summary During the early postnatal period, the differentiation and maturation of muscle spindles in the rat is still dependent on their sensory innervation. When a nerve is crushed during this period, most spindles in the denervated muscles degenerate and after reinnervation only occasional spindles of atypical structure are to be found in these muscles. We determined the basic functional properties of these atypical spindles in adult rats and attempted to correlate them with their structural characteristics. The discharge rates of 13 afferent units from the soleus or lateral gastrocnemius muscles were evaluated in response to stretch. These units were capable of a slowly adapting response to 2–4 mm stretches. Their mean discharge frequencies at any point of the ramp-and-hold stretch were, however, on an average 50% lower than normal values. The conduction velocities of afferents from the atypical spindles were in the range of 10–40 m/s. Histological examinations revealed that 90% of the atypical muscle spindles found in the soleus or lateral gastrocnemius muscles had only 1 or 2 intrafusal fibres without any nuclear accumulations as compared to four intrafusal fibres in normal muscle spindles in the rat. The proportional decrease of the discharge rate in both the dynamic and static part of the response of these atypical spindles could be due to the decreased synaptic area between the sensory terminals and the intrafusal fibres and/or to altered structural properties of the intrafusal fibres.     

An electrophysiological analysis of responses from lizard muscle spindles

1. Stretch receptor discharges were recorded in the lizard Tiliqua nigrolutea. Responses were identified as coming from muscle spindles by their ;in parallel' behaviour during a twitch.2. Recordings from spindles in the iliofibularis muscle which contains many single innervated fibres revealed spindles which discharged on the rising phase of the muscle contraction. This ;in series' response increased in frequency and duration at high resting tensions on the muscle and was attributed to a specific intrafusal contraction.3. The ;in series' discharge could be induced by stimulation of only one small filament of the muscle nerve. This suggested a focal intrafusal motor innervation.4. Responses from spindles in the semitendinosus muscle, which contains many multiple innervated fibres provided examples of an ;in series' response which could be produced by stimulation of several muscle nerve filaments. This was interpreted as being an example of multiple intrafusal motor innervation.5. Spindles with short or elongated sensory regions were subjected to ;ramp' stretches. The presence of distinct phasic and tonic responses from the two types was confirmed and related to the behaviour of mammalian spindles.     

Structures in sensory region of snake spindles and their displacment during stretch.

1. Structures within the sensory region of short- and long-capsule snake muscle spindles have been visualized using differential interference contrast microscopy. Profiles seen with Nomarski microscopy have been identified by electron microscopy of the same preparations. 2. Sensory nerve terminals, nuclei and other cytoplasmic inclusions in the intrafusal fiber, collagen bands, and capsular cells may be seen in the living preparation. 3. The length changes of various elements within the sensory region in response to stretch of the spindle have been measured using high-speed ciné photomicrography. This has been corrleated with the impulse response from sensory endings in short-and long-capsule spindles. 4. Short-capsule spindles, which have a high dynamic sensitivity, show length changes in the sensory region in response to ramp-and-hold stretch, which are not parallel to the changes in impulse frequency. The implications for mechanical models of spindle behavior are discussed.     

Responses of muscle spindles following a series of eccentric contractions

To investigate the effects of eccentric exercise on the signalling properties of muscle spindles, experiments were done using the medial gastrocnemius muscle of cats anaesthetised with 40 mg/kg sodium pentobarbitone, i.p. Responses were recorded from single afferent nerve fibres in filaments of dorsal root during slow stretch of the passive muscle and during intrafusal contractions at a range of lengths, before and after a series of eccentric contractions. The sensitivity to slow stretch was measured as the average firing rate between muscle lengths 10.5 and 9.5 mm shorter than the physiological maximum (L_m), during stretch at 1 mm/s over the whole physiological range. The mean sensitivity of both primary and secondary spindle endings increased slightly, but not significantly, after a series of 20–150 eccentric contractions consisting of a 6 mm stretch, at 50 mm/s, to a final length of between L_m –7 mm and L_m, during stimulation of the whole muscle or sometimes of single fusimotor fibres. Discharges were recorded from primary endings during fusimotor stimulation at 100–150 pulses/s, and from secondary endings during static bag intrafusal contractures produced by i.v. injection of 0.2 mg/kg succinyl choline. Spindle responses were recorded, over a range of muscle lengths, in steps covering the whole physiological range. About half of the responses showed a peak in the relation between length and net increase in firing rate, while the remainder either progressively increased or progressively decreased over the physiological range. No large or consistent changes were seen after the eccentric contractions. It is concluded that the intrafusal fibres of muscle spindles are not prone to damage of the kind seen in extrafusal fibres after a series of eccentric contractions.     

Palisade endings in extraocular eye muscles revealed by SNAP-25 immunoreactivity

Palisade endings form a cuff of nerve terminals around the tip of muscle fibres. They are found only in extraocular muscles, but no definite evidence for their role in eye movements has been established. Palisade endings have been reported in all species so far investigated except the rat. In this study we demonstrate that antibodies against SNAP-25, the synaptosomal associated protein of 25 kDa, reliably visualize the complete motor, sensory and autonomic innervation of the extraocular muscles in human, monkey and rat. The SNAP-25 antibody can be combined with other immunofluorescence procedures, and is used here to study properties of palisade endings. With SNAP-25 immunolabelling putative palisade endings are identified in the rat for the first time. They are not well branched, but fulfil several criteria of palisade endings, being associated with non-twitch fibres as shown by double labelling with 'myosin heavy chain slow-twitch' antibodies. The putative palisade endings of the rat lack alpha-bungarotoxin binding, which implies that these synapses are sensory. If palisade endings are sensory then they could function as an eye muscle proprioceptor. They seem to be a general feature of all vertebrate eye muscles, unlike the other two extraocular proprioceptors, muscle spindles and Golgi tendon organs, the presence of which varies widely between species.     

Changes in muscle spindle activity of the chronically de-efferented gastrocnemius of the rat

Summary The response to stretch of chronically de-efferented muscle spindles was studied in the rat gastrocnemius muscle 6 days (short-term) and 48 days (long-term) after ventral rhizotomy respectively. Chronic de-efferentation enhances both the dynamic and static component of the response. There was no difference between the short-term and long-term de-efferentation, i.e. the response to stretch was as high several days after de-efferentation as after 1–2 months, when the muscle had undergone considerable atrophy. The response of both spindle primaries and secondaries was enhanced, but more so in the latter group. This suggests that the probable site of these changes after ventral root section lies in the polar regions. Early adaptation of de-efferented group I endings remained unaltered throughout the whole experimental period. Group II and III afferents, on the other hand, exhibited less adaptation than those from control muscles. Fibrillation activity of intrafusal muscle fibres does not seem to be involved, since quinidine sulphate in doses suppressing fibrillations in extrafusal muscle fibres does not eliminate the enhanced response. Ultrastructural changes at the site of sensory nerve terminals on the receptor cells of the spindle caused by chronic de-efferentation are probably responsible.This work was performed during tenure of a Visiting Professorship at the University of Utah. The authors wish to acknowledge gratefully the helpful criticism and advice of Prof. C. Eyzaguirre throughout the experiments and the technical help of Messrs. M. Steadman, J. Fisher, T. O'Leary and P. Lenoir. This work was-supported by Grants NS 05244 and NS 05666 from the U.S. Public Health Service.     

Two factors responsible for the development of denervation hypersensitivity

1. Innervated adult skeletal muscle is sensitive to acetylcholine at the end-plate region only. After denervation the entire muscle membrane becomes chemosensitive. The period of greatest increase in sensitivity in rat soleus muscles following section of the sciatic nerve in the thigh is between 48 and 72 hr post-operatively.2. Direct electrical stimulation was found to prevent the onset of the development of denervation hypersensitivity during the first 2-3 days after nerve section. Thereafter, electrical stimulation only reduced the sensitivity of denervated muscles to acetylcholine (ACh).3. The period of greatest increase in sensitivity follows loss of transmission and degeneration of the nerve terminals. Once this degeneration is under way, electrical stimulation is no longer as effective in preventing the development of denervation hypersensitivity.4. Hypersensitivity is also seen in muscles on which a small piece of thread or degenerating nerve has been placed. Hypersensitivity following these procedures declines within a few days, unlike denervation hypersensitivity which persists until innervation is restored.5. The present results suggest that activity alone cannot prevent the development of hypersensitivity in the presence of degenerating nerve fibres, or muscle damage. Activity does however counteract increased sensitivity. It is suggested that two factors interact to produce denervation hypersensitivity; the presence of degenerating nerve tissue and concomitant cellular changes bring about changes in the muscle fibre membrane causing it to become hypersensitive; and the loss of muscle activity, resulting in the persistence of hypersensitivity until innervation is restored.     

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

Summary 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 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.