Comparisons in the structure of avian muscle spindles

The evolution of muscle spindles

Muscle spindles are stretch-sensitive mechanoreceptors found in the skeletal muscles of most four-limbed vertebrates. They are unique amongst sensory receptors in the ability to regulate their sensitivity by contraction of the intrafusal muscle fibres on which the sensory endings lie. Muscle spindles have revealed a remarkable diversity of functions, including reflex action in posture and locomotion, contributing to bodily self awareness, and influencing wound healing. What were the circumstances which gave rise to the evolution of such complex end-organs? We argue that spindles first appeared in early amniotes and only later in frogs and toads. This was considered an example of convergent evolution. Spindles in amphibians and reptiles are characterised by their simple structure, pointing to key features essential for spindle function. Spindle sensitivity in amphibians and reptiles is controlled by intrafusal fibre contractions evoked by branches of motor axons supplying extrafusal muscle. Modern phylogenetic evidence has revised our views on the origin of birds, placing them closer to the dinosaurs than had previously been thought. Birds are the only group, other than mammals, which has a dedicated fusimotor innervation of spindles, another example of convergent evolution, given the widely different origins of the two groups. One factor that may have played a role here was that both groups are endotherms, allowing motor control to develop further in an optimal internal environment. This, as well as other changes within the spindle, has led to the astonishing sophistication of motor control observed especially in many modern mammals.

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 microm and 4.5 to 18.5 microm, 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.

Fiber-type compositions in postnatal chicken muscle spindles with low intrafusal fiber counts and their developmental significance

The fiber-type composition of postnatal chicken leg muscle spindles with from one to four intrafusal fibers was examined in sections incubated with monoclonal antibodies against fast and slow myosin heavy chains. In monofibral spindles the lone intrafusal fiber was almost always fast. In duofibral spindles usually one slow and one fast fiber were present. Trifibral spindles most often displayed two fast and one slow fiber, whereas quadrofibral receptors characteristically contained two slow and two fast fibers. Earlier results showed that the primary intrafusal myotube in nascent spindles has almost always a fast myosin heavy chain profile and that the proportion of slow myotubes and fibers increases as intrafusal fiber bundles grow in size. Data from postnatal chicken leg muscles collected here suggest that up to the first four fibers this proportional increase can be largely accounted for if consecutive intrafusal fibers arise in a fast-slow-fast-slow sequence. The late recognition during myogenesis of primary intrafusal myotubes and their fast myosin heavy chain profiles warrant exploring if nascent chicken muscles spindles are first seeded by fast fetal myoblasts.

Type and regional diversity in the distribution of myosin heavy chains in chicken intrafusal muscle fibers

BACKGROUND

Chicken intrafusal fibers were classified on the basis of their myosin heavy chain (MHC) composition, which was compared to that of mammalian nuclear bag and nuclear chain types.

METHODS

Immunoreactivities of intrafusal fibers from leg muscles of 8-week-old chickens were evaluated in serial cross-sections after incubation with monoclonal antibodies against slow-twitch, slow-tonic, or fast-twitch MHC and fast muscle C-protein.

RESULTS

Four categories of slow intrafusal fiber could be distinguished on the basis of differential expression of slow-twitch and slow-tonic MHC. Segregation into types was most evident at the motor axon supplied pole, followed by the sensory region of the equator. Fiber types were least distinct at the juxtaequator where sensory and motor axons meet. Intrafusal fibers negative for slow myosins reacted with anti-fast myosins. Fast fibers were best viewed as a single group without subdivisions. Immunostaining for fast muscle C-protein paralleled in large part reactivities for neonatal/fast MHC, indicating that proteins other than MHC are useful fiber type markers.

CONCLUSIONS

Despite regional changes along the length of intrafusal fibers and some variation within fiber types, the concept of separate MHC-based fiber types was valid as long as typing of fibers was restricted to the proximal polar region. Comparisons of MHC profiles revealed similarities between chicken fast intrafusal fibers and mammalian nuclear chain fibers and between some chicken slow intrafusal fibers and mammalian nuclear bag fibers.

Development of chicken intrafusal muscle fibers

The first sign of developing intrafusal fibers in chicken leg muscles appeared on embryonic day (E) 13 when sensory axons contacted undifferentiated myotubes. In sections incubated with monoclonal antibodies against myosin heavy chains (MHC) diverse immunostaining was observed within the developing intrafusal fiber bundle. Large primary intrafusal myotubes immunostained moderately to strongly for embryonic and neonatal MHC, but they were unreactive or reacted only weakly with antibodies against slow MHC. Smaller, secondary intrafusal myotubes reacted only weakly to moderately for embryonic and neonatal MHC, but 1-2 days after their formation they reacted strongly for slow and slow-tonic MHC. In contrast to mammals, slow-tonic MHC was also observed in extrafusal fibers. Intrafusal fibers derived from primary myotubes acquired fast MHC and retained at least a moderate level of embryonic MHC. On the other hand, intrafusal fibers developing from secondary myotubes lost the embryonic and neonatal isoforms prior to hatching and became slow. Based on relative amounts of embryonic, neonatal and slow MHC future fast and slow intrafusal fibers could be first identified at E14. At the polar regions of intrafusal fibers positions of nerve endings and acetylcholinesterase activity were seen to match as early as E16. Approximately equal numbers of slow and fast intrafusal fibers formed prenatally; however, in postnatal muscle spindles fast fibers were usually in the majority, suggesting that some fibers transformed from slow to fast.

Sensory and motor innervation of bird intrafusal muscle fibers

1. Most bird muscle spindles are supplied by only one primary afferent. 2. Secondary afferents occur irregularly. 3. Sensory terminals are covered by a basal lamina and a collagenous sheath. 4. Two types of motor terminal are recognized which can be referred to specific types of intrafusal fiber. 5. The sensory and motor innervation of bird intrafusal fibers is less understood than that of mammalian intrafusal fibers.

The avian muscle spindle

The literature on the morphology and physiology of the avian muscle spindle is reviewed, with emphasis placed on the period from 1960 to 1991. Traits similar to or different from mammalian spindles are recognized. Apart from receptors with low intrafusal fiber counts, bird spindles contain two or three types of intrafusal fiber. Unlike that of mammals, the equatorial fiber structure in birds does not lend itself to classification into nuclear bag and nuclear chain types. Avian intrafusal fibers are separable into types based on differences in myosin heavy chain composition and motor innervation, but apportionment of these fiber types to individual spindles is more variable in birds than in mammals. There is morphological evidence in birds for the existence of both gamma and beta innervation; however, confirmation of these systems by physiological experiments is at best sketchy. A general lack of physiological data is currently the greatest drawback to a better understanding of how the avian receptor works, and what role it plays in sensorimotor integration.

Functional morphological interpretation of the distribution of muscle spindles in the jaw muscles of the mallard (Anas platyrhynchos)

The morphology and distribution of muscle spindles of jaw and tongue muscles in the mallard were examined in serial transverse sections of single muscles and in horizontal sections of a whole head. Our observations on spindle morphology are in agreement with previous descriptions of spindles in birds. Some spindles differ in their innervation and the pattern of intrafusal muscle fibers. The spindles of individual adductor and pterygoid muscles are distributed unevenly. Some adductor muscles lack spindles, whereas those of other muscles are confined to limited areas. Jaw opening muscles and extrinsic tongue muscles lack spindles. The stretch of extrafusal muscle fibers could be estimated from the difference in sarcomere length for birds with the beak open and closed. Not all muscle fiber groups are stretched evenly over the whole range of jaw opening. Only those fiber groups that are continuously stretched during jaw opening contain spindles.

Axon contacts and acetylcholinesterase activity on chicken intrafusal muscle fiber types identified by their myosin heavy chain composition

Muscle spindles of 8-week old chicken tibialis anterior muscles were examined to determine if specific intrafusal fiber types were also characterized by differences in motor innervation. Incubation with a monoclonal antibody against myosin heavy chains permitted the identification of strongly reactive, moderately reactive and unreactive intrafusal fibers. The innervation of each fiber type was evaluated in silver-impregnated sections, and in sections incubated with a monoclonal antibody against acetylcholinesterase. There was no acetylcholinesterase activity at the midequator of any fiber. At the juxtaequator and at the pole strongly reactive fibers typically exhibited fewer axon contacts and less acetylcholinesterase activity than unreactive and moderately reactive fibers. Differences were also recognized at neuromuscular junctions in the size and shape of acetylcholinesterase-positive sites. At the juxtaequator and at the pole strongly reactive fibers and moderately reactive fibers displayed significantly more small, dot-like acetylcholinesterase sites than unreactive fibers. On the contrary, the greatest number of larger, stout sites was found on unreactive fibers and the least number on strongly reactive fibers. Moderately reactive fibers took an intermediate position. The results indicate that myosin heavy chain-based chicken intrafusal fiber types are also set apart by differences in innervation.

Presence in chicken tibialis anterior and extensor digitorum longus muscle spindles of reactive and unreactive intrafusal fibers after incubation with monoclonal antibodies against myosin heavy chains

Cross and longitudinal sections from the encapsulated portions of chicken tibialis anterior and extensor digitorum longus muscle spindles were examined to determine whether their intrafusal fibers were a structurally homogeneous or heterogeneous population. The techniques used were the histochemical actomyosin (mATPase) reaction, and fluorescence immunohistochemistry employing two monoclonal antibodies, CA-83 and CCM-52, that are specific for myosin heavy chains. After incubation with antibody CCM-52, intrafusal fibers fluoresced either strongly or weakly to moderately. Antibody CA-83 was even more selective. In addition to identifying the strongly reactive category, it clearly separated the remaining fibers into unreactive and moderately reactive groups. As a whole, after incubation for mATPase, pH 9.6 preincubation, unreactive fibers stained darker than strongly reactive fibers. Moreover, the cross-sectional area of the unreactive fibers was significantly larger than that of the strongly reactive fibers. In the average-size muscle spindle with six intrafusal fibers, there were four unreactive fibers and two strongly reactive fibers. In about one-third of the receptors examined, one moderately reactive fiber was present. Taken together, the data indicate that intrafusal fibers of chicken tibialis anterior and extensor digitorum longus muscles are not structurally homogeneous. The observed variations can be better explained in terms of different fiber types than of continuous gradients within one type of fiber.

Arrangement of cytoskeletal filaments at the equator of chicken intrafusal muscle fibers

The organization of the cytoskeleton at the equator of chicken intrafusal fibers was examined with immunofluorescence light microscopy, using monoclonal antibodies against myosin heavy chains, desmin, actin and tropomyosin. Actin was localized in the cytosol and in equatorial nuclei, while myosin heavy chains, desmin and tropomyosin were only observed in the cytosol. Although all four proteins were present at the equator and at the pole, the fluorescence produced after incubation with the different antibodies varied considerably between the two regions. Staining at the pole was in the form of striations, but at the equator it was non-striated and more uniform. The observed fluorescent patterns suggest that at the equator filaments are assembled into looser arrays than in the sarcomeres of the pole. A flexible cytoskeleton at the equator would be an appropriate substrate for distorting the affixed sensory endings during an applied stress.

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.

Distribution of connective tissue proteins in chick muscle spindles as revealed by monoclonal antibodies: a unique distribution of brachionectin/tenascin

The distribution of chick muscle spindles of eight connective tissue proteins (collagen types I, IV, V, and VI, laminin, heparan sulfate, fibronectin, and brachionectin/tenascin) was examined by immunofluorescent histochemistry. Intrafusal fibers were surrounded by layers of collagen type VI and fibronectin, and by an external lamina containing collagen type IV, laminin, and heparan sulfate. Most of these layers displayed a different pattern of staining at the sensory region of the equator than at the polar region. The crescent-like sheath that caps each intrafusal fiber and sensory terminal at the equator was strongly positive for collagen type I and weakly positive for collagen type V. The outer spindle capsule contained laminin, heparan sulfate, collagen types IV and VI, brachionectin/tenascin, fibronectin, and to a lesser degree also collagen types I and V. Brachionectin/tenascin had the narrowest distribution of any of the connective tissue macromolecules studied. It was found only in the outer capsule and in the coverings of blood vessels and nerves associated with the outer capsule.

Intrafusal muscle fibers of duck muscle spindles

Serial transverse paraffin sections of intrafusal muscle fibers of spindles from the extensor pollicis and the extensor digitorum communis of ducks show that only one type of intrafusal muscle fiber exists, based on the mid-equatorial nucleation pattern, diameter, and length. Although the overall range in fiber diameter at the mid-equatorial region is between 4.2-20.0 microns, the average caliber is 10.4 +/- 3.18 microns (S.D.) for spindles of the extensor pollicis and 9.3 +/- 2.11 microns (S.D.) for spindles of the extensor digitorum communis muscles. The range in spindle length for the extensor pollicis is 290-2,090 microns, average 1,120 +/- 569 microns (S.D.), and for the extensor digitorum communis 1,160-2,500 microns, average 1,745 +/- 367 microns (S.D.). The range in number of fibers per spindle for the extensor pollicis muscle is 5-12, average 8.2, and for the extensor digitorum muscle it is 1-11. In the extensor digitorum communis, there appear to be two groups, based on fiber number. Spindles of one group have a range of 5-11 fibers per spindle with an average of 7.2, whereas the second group has a range of 1-4 with an average of 2.7 fibers per spindle. The second group of spindles constitutes 52.5% of the 40 spindles studied, and of these 7.5% were monofibril spindles, 15.0% difibril, 17.5% trifibril, 12.5% quadrifibril spindles.

Morphology and histochemistry of the ambiens muscle of the red-eared turtle (Pseudemys scripta)

Six fiber types have been described in the ambiens muscle of red-eared turtles. These include one slow oxidative type, two fast oxidative types, two fast oxidative and glycolytic types, and one fast glycolytic type. Fiber types are non-randomly distributed throughout cross sections of the muscle. There is a decreasing gradient of oxidative staining and an increasing gradient of glycolytic staining along an axis from the superficial to deep regions of the muscle. The slow oxidative fibers are predominantly located within one or two fascicles of the superficial surface of the muscle. The fast glycolytic fibers are predominant in deep fascicles. In contrast to previous reports of histochemically monotypic intrafusal fibers in turtle muscle, ambiens muscle spindles have been observed containing one to eleven intrafusal fibers, including two fiber types. Fiber diameter and area are consistently smaller than observed in most extrafusal fibers. Spindles are predominantly located in superficial and cranial fascicles of the ambiens muscle and are located in regions characterized by extrafusal fibers with high oxidative activity.

Spaced serial section analysis of the avian muscle spindle

Muscle spindle ultrastructure of the extensor digitorum communis of pigeons was studied using spaced serial sections. The intrafusal fibers extended well beyond the ensheathing capsule, after which they became disoriented and often fused with each other before terminating on the connective tissue of extrafusal fibers. Several extracapsular fibers contained macrofibrils which were about 0.1 micron in diameter and contained several dozen smaller (10 nm) subunits. Intrafusal fibers commonly formed close attachments with one another for short or extended (240 micron) lengths. A basal lamina was absent between regions of pairing, and a myosatellite cell lay at the border of the coupled region. Several fibers could be coupled together in a single cross section; fibers coupled together, separated, and either recoupled or became associated with other fibers along the length of a spindle. Profiles of sensory terminals and sensory satellite cells alternated to form a smooth-contoured surface over most of the fiber cross section in the equatorial region. The sensory terminals contained many mitochondria, lysosomes, and clear and dense core microvesicles. Both the terminals and sensory satellite cells formed desmosomelike junctions with the intrafusal fiber. A crescentic collagen sheath covered that portion of the fiber cross section containing the sensory terminal-satellite cell complex. Inner capsule cells surrounded the entire assembly in the equatorial region. The basal lamina thicknesses differed over the naked intrafusal fiber compared to that portion covered by the sensory terminal or sensory satellite cell. The thickness was more than doubled over the latter regions, indicating that the basal lamina over these areas was a product of the fused intrafusal fiber and sensory terminal and/or sensory satellite cell basal laminae. These are discussed in terms of intrafusal fiber degeneration and regeneration.

An exceptionally high density of muscle spindles in a slow-tonic pigeon muscle

Histochemical and histological observations on the tiny wing muscle, M. coracotriceps, of the pigeon revealed a remarkably high density of muscle spindles (14,582 +/- 2,302/g of muscle)--approximately 15 times the highest densities hitherto reported for any muscle. Furthermore, all of the extrafusal fibers of this muscle were of the slow-tonic variety. This unique muscle probably functions as a mechanoreceptor extremely sensitive to changes in its own length.

Differences in muscle spindle structure between pigeon muscles used in aerial and terrestrial locomotion

Muscle spindle density (number of spindles per gram of muscle) of all 29 muscles of the forearm and leg of the domestic pigeon was evaluated by counting receptors in van Gieson-stained serial cross sections. Extra- and intra-fusal fiber-type profiles were determined from histochemical preparations. Muscles of the leg had on the average significantly more avian slow-twitch oxidative extrafusal fibers (22.5 vs. 0.8%) and slower contraction times than muscles of the forearm, but fiber-type profiles and gross actions of muscles showed no consistent relation to the relative abundance of receptors. Differences in intrafusal fiber-type composition among spindles were sought because of their potential effect on the quality of the afferent discharge. The number of intrafusal fibers per spindle was on the average significantly less (4.57 vs 5.99) in the muscles of the leg than in those of the forearm; and of spindles with the same number of intrafusal fibers, those in the leg had smaller periaxial spaces. Distribution of intrafusal fiber types identified with the myofibrillar adenosine triphosphatase reaction differed among spindles of varying sizes. An acid- and alkali-labile type occurred most frequently (P = 0.05) in spindles with one to three intrafusal fibers, and an acid-labile and alkali-stable type was most often seen (P = 0.05) in spindles with 4 to 7 intrafusal fibers. The smaller receptors were more abundant in the leg, while the larger ones were about equally distributed between the two extremities. Muscle fibers with dimensions that sometimes approached small extrafusal fibers were present in about 3% of the axial bundles examined, most of them in the forearm. The selective morphological variation of avian muscle spindles may represent the structural basis for qualitatively different afferent discharges that relate to the characteristic types of locomotion served by the two extremities.

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.

Variations in intrafusal fiber size within histochemically identified types of intrafusal fibers in the pigeon

The myofibrillar adenosine triphosphatase reaction with acid preincubation allowed the identification of three types of intrafusal fibers in pigeon flexor carpi ulnaris muscle spindles. Measurements of cross-sectional areas at the polar region showed much overlap in fiber size among populations of each type.