Morphologic and morphometrical study of the muscle spindle in muscular dystrophy

OBJECTIVE: To determine the morphologic and the morphometrical features of spindles in biopsies of patients with different types of muscular dystrophy and investigate the possible involvement of the spindle in the pathologic process of these diseases. STUDY DESIGN: The following variables were studied in biopsy specimens from 10 patients with Duchenne or Becker dystrophy, 9 with limb-girdle dystrophy, 3 with congenital dystrophy and 3 with facioscapulohumeral dystrophy: diameter and area of spindle; thickness of the capsule; number, diameter and area of intrafusal fibers; and number and area of nuclei. RESULTS: The statistical evaluation of the data showed significant differences regarding the thickness of the capsule, which was greater in patients than in controls, while the diameter and the area of the fibers were all smaller in patients than in controls. The area of nuclei of fibers was increased; this was a common feature for all types of muscular dystrophy. CONCLUSION: These findings indicate that the spindle possibly participates in the pathologic process of different types of muscular dystrophies.     

Development of human cerebellar nuclei. Morphometric study

The morphometric development of the human cerebellar nuclei was examined in 9 fetuses (16-40 weeks of gestation; WG), an infant (2 months old) and 2 adults (16 and 63 years old). With the morphological observation of serial sections of the brain containing the cerebellar nuclei, the authors measured sections to get several morphometric parameters: the volume of nuclear column and number, packing density and cell body area of neurons. Each nucleus (dentate, emboliform, globose and fastigial nucleus) was recognized even at 16 WG. Nerve cells containing Nissl bodies were observed in all nuclei after 23 WG. Degenerative changes were detected in some neurons for every nucleus at 21 and 23 WG. Three stages were observed in the developmental course of nuclear volume and neuronal packing density: the primary or undifferentiated stage at 16 WG, the secondary stage with variability at 21-32 WG and the tertiary stage with monotonous increase (nuclear volume) or gradual decrease (neuronal packing density) after 35 WG. No significant correlation between neuronal number and gestational age was noticed for every nucleus. The analysis of cell body area (neuronal size) demonstrated that the dentate neurons developed after the intermediate or fastigial neurons. It is concluded that there is a critical period between slightly before 20 WG and slightly after 30 WG, matched with the secondary stage in the development of the cerebellar nuclei.     

Enzyme-histochemical studies on the muscle spindle

Summary Detailed studies have been made on the distribution of several enzymes in the muscle spindles of the hand and foot interosseous muscles and M. longissimus dorsi of the rhesus monkey as well as in those of the hand interosseous muscles of the squirrel monkey. The intrafusal muscle fibers (IMF) of the rhesus monkey can be classified into two types by the reaction intensity at the polar regions for adenosine triphosphatases and by the enzymes concerning the carbohydrate metabolism except glucose-6-phosphate dehydrogenase, while the extrafusal muscle fibers (EMF) show three types of reactions for the enzymes of the Embden-Meyerhof pathway and the tricarboxylic acid (TCA) cycle. The IMF and EMF of the squirrel monkey are more variable than those of the rhesus monkey for the glycogen breakdown enzyme. It is possible that the small IMF are more capable of energy production through the TCA cycle than the large IMF and the EMF in both species. The positive cholinesterases reactions are found around the polar regions of the IMF, while only the rim of the equator of the IMF shows monoamine oxidase activity. The pericapsular epithelial cells of the muscle spindle seem to be metabolically similar to the perineural epithelial cells.Visiting scientist from the Department of Anatomy, Tokyo Medical and Dental University, Tokyo, Japan. T. R. Shanthaveerappa in previous publications.     

Morphometric study of the postnatal development of the human adrenal cortex]

Under study was the developmental dynamics of a number of morphometrical parameters of the adrenal cortex of man. The number of glandular cells in the organ was found to considerably decrease at late developmental periods, the size of nuclei in them diminished. These factors are considered to be responsible for the drop of functional activity of the adrenal cortex in elderly and senile age. The increase of the cell size during the growing of the organism is thought to be an important factor of the growth of the adrenal cortex of man.     

Morphometric analysis of the developing mouse soleus muscle

The pattern of organogenesis of the soleus muscle of the 129 ReJ mouse was evaluated quantitatively using spaced, serial, ultrathin sections and computer-assisted morphometric analysis. Muscles from 14-, 16-, and 18-day in utero mice and muscles of 1- and 5-day-old mice were analyzed to determine age-related alterations in the maximal girth and length of the muscle, number of myotubes, cluster frequency, and the lengths and diameters of myotubes. Primary myotubes are found in the muscle at 14 days in utero. There is little de novo myotube formation between 14 and 16 days in utero, this interval being principally one of primary myotube growth and maturation. The interval between 16 and 18 days in utero is marked by extensive secondary myotube formation, with more myotubes being formed during this period than in any period studied. Morphometric data support the hypothesis that secondary generation myotubes use primary myotubes as a scaffold on which they are formed. Morphometric data also confirm the hypothesis that cluster formation and cluster dispersal occur concurrently during the prenatal period. Secondary myotubes continue to form until birth. At birth, the soleus muscle contains the adult number of myofibers. The first 5 days postnatally are marked by myofiber growth and maturation.     

Morphometric affinities of the human shoulder

To analyze differences between apes and monkeys and the affinities of man, we have studied the shoulder girdle of 327 specimens of anthropoid primates. The scapula, clavicle and humerus are viewed as an integrated functional complex on the basis of 18 measurements. Several varieties of multivariate analysis show that man is clearly closer to other hominoids than to the included monkey taxa (whether terrestrial or arboreal, Old World or New World). The marked shoulder differences between apes and monkeys and similarities between apes and man correlate with the muscular anatomy, which in hominoids allows the motions involved in their locomotion and feeding behavior. As the hominid-pongid correspondence in shoulder morphology is especially detailed regarding the functionally important joint surfaces, it is consistent with a fairly recent period of common ancestry and behavior. No hypothetical evolutionary pathway or ancestral form of the human shoulder need look far beyond the model afforded by extant pongids. In contrast with previous studies on the primate shoulder, these results agree with information accumulating from other systems—comparative anatomy, primate behavior, and molecular biology — in suggesting very close relationship between man and extant African pongids.     

Modulation of human muscle spindle discharge by arterial pulsations--functional effects and consequences

Arterial pulsations are known to modulate muscle spindle firing; however, the physiological significance of such synchronised modulation has not been investigated. Unitary recordings were made from 75 human muscle spindle afferents innervating the pretibial muscles. The modulation of muscle spindle discharge by arterial pulsations was evaluated by R-wave triggered averaging and power spectral analysis. We describe various effects arterial pulsations may have on muscle spindle afferent discharge. Afferents could be "driven" by arterial pulsations, e.g., showing no other spontaneous activity than spikes generated with cardiac rhythmicity. Among afferents showing ongoing discharge that was not primarily related to cardiac rhythmicity we illustrate several mechanisms by which individual spikes may become phase-locked. However, in the majority of afferents the discharge rate was modulated by the pulse wave without spikes being phase locked. Then we assessed whether these influences changed in two physiological conditions in which a sustained increase in muscle sympathetic nerve activity was observed without activation of fusimotor neurones: a maximal inspiratory breath-hold, which causes a fall in systolic pressure, and acute muscle pain, which causes an increase in systolic pressure. The majority of primary muscle spindle afferents displayed pulse-wave modulation, but neither apnoea nor pain had any significant effect on the strength of this modulation, suggesting that the physiological noise injected by the arterial pulsations is robust and relatively insensitive to fluctuations in blood pressure. Within the afferent population there was a similar number of muscle spindles that were inhibited and that were excited by the arterial pulse wave, indicating that after signal integration at the population level, arterial pulsations of opposite polarity would cancel each other out. We speculate that with close-to-threshold stimuli the arterial pulsations may serve as an endogenous noise source that may synchronise the sporadic discharge within the afferent population and thus facilitate the detection of weak stimuli.     

Morphometric study of human alveolar ducts based on serial sections

Interactive computerized morphometry was used for the quantitative study of the terminal airway branches (alveolar ducts) that followed the last bronchioles in three human acini. Two normal adult human lungs from the autopsy service were fixed by instillation and serial sections were prepared; three tissues blocks showing a central bronchiole were selected. Primary and secondary alveolar walls were traced and the following parameters were measured: volume, surface area (of primary and secondary septa), curvature (in equivalent radius) for branches of individual generations, and cumulative values starting with the first alveolar duct and moving peripherally. Although branching was dichotomous, we noticed considerable asymmetry in the pattern of branching and number of side branches. The branching trees of alveolar ducts that we studied comprised 6,7, and 10 generations. The average volume of ducts was 0.04-0.13 mm3, the surface area of primary walls ranged from 0.3616 to 0.7931 mm2 and of secondary septal walls from 0.0100 to 0.0647 mm2. The equivalent radius of curvature was between 22.7 and 38.1 microns. Cumulative increases of volume and surface area revealed similarity in the first five generations. Secondary walls represented only 4% (or 8% if 2 sides are considered) of the primary surface area, strengthening the view that alveoli are incompletely developed side chambers secondary to the alveolar ducts.     

Rat muscle spindle immunocytochemistry revisited

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

A numerical simulation of muscle spindle ensemble encoding during planar movement of the human arm

We address the issue of what proprioceptive information, regarding movement of the human arm, may be provided to the central nervous system by proprioceptors located within muscles of this limb. To accomplish this we developed a numerical simulation which could provide estimates of the length regimes experienced by a set of model receptors located within some of the principal muscles of the human arm during planar movement of this limb. These receptors were assumed to have characteristics analogous to those associated with a simple model of muscle spindle signalling of movement. To this end each spindle had proprioceptive ‘channels’ associated with it. These corresponded to primary and secondary spindle afferent fibers which could provide independent afferent output regarding the parent muscle the spindle monitored. The angles of the shoulder and elbow joints attained by subjects performing a task requiring movement of the right arm in a horizontal plane to a static visual target were recorded. For this angular data the lengths and rates of change of lengths experienced by muscle fascicles, and hence the model spindles, during movement were calculated by means of the numerical simulation. The discharge rates of the simulated spindles during the movement were calculated to derive a measure of the depth of modulation, induced by the movement, for each spindle. These values were then summed for all spindles to provide a first-order approximation of spindle ensemble coding of the movement. Significant correlations (0.0001, Spearman's rank order) were found between the resulting ensemble encodings and, in order of significance, the angular velocity of the shoulder joint (), the tangential velocity of the hand (), and the angular velocity of the elbow joint (). Correlations between the angular positions of the shoulder () and elbow () were lower. These findings indicate that the ensemble profiles of the simulated muscle spindles, encode information regarding kinematic parameters of movements related to both intrinsic and extrinsic coordinate systems. This suggests that motor structures capable of deriving such an ensemble encoding would be in a position to perform the sensory-motor transformations between intrinsic and extrinsic frames of reference necessary for controlling movements planned in extrinsic coordinates. Received: 12 August 1994 / Accepted in revised form: 17 June 1996     

The fine structure of the guinea-pig muscle spindle

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

Fine structure of the avian muscle spindle capsule

Summary The capsule of the muscle spindle from the anterior and posterior latissimus dorsi muscles of the adult domestic chicken has been studied with the electron microscope. As in other species, two distinct portions of the spindle capsule are distinguished: an outer capsule and an inner capsule.The outer capsule is structurally similar to and continuous with the perineural epithelium. Outer capsule cells are noted by the abundance of pinocytotic vesicles and a network of 6–7 nm microfilaments. The disposition of these microfilaments is circumferential with respect to the longitudinal axis of the spindle. It is proposed that they may provide a contractile mechanism for the capsule which may be related to the over-all functioning of the spindle during movements of the muscle.The inner capsule is composed of a contiguous network of cells possessing long cytoplasmic processes which envelop the intrafusal fibers and their nerve endings in sensory equatorial regions of the spindle. These cells may elaborate the fibrillar and amorphous extracellular material found in the periaxial spindle space. They also possess modified cilia with a 9+0 microtubular pattern. It is suggested that these cilia may behave as sensory transducers, relating fluid changes in the periaxial space to the intrafusal fiber nerve endings.Capillaries of the non-fenestrated variety commonly traverse the outer and inner portions of the capsule and are usually completely surrounded by tenuous overlapping processes of inner capsule cells. These findings suggest that the spindle capsule plays a role as a metabolically-active diffusion barrier to the entrance of substances from the external milieu.The excellent technical assistance of Ms. S.L. Shinn is gratefully acknowledged.Supported by grants from the Medical Research Council of Canada and the Muscular Dystrophy Association of Canada.     

Molecular determinants of mechanosensation in the muscle spindle

The muscle spindle (MS) provides essential sensory information for motor control and proprioception. The Group Ia and II MS afferents are low threshold slowly-adapting mechanoreceptors and report both static muscle length and dynamic muscle movement information. The exact molecular mechanism by which MS afferents transduce muscle movement into action potentials is incompletely understood. This short review will discuss recent evidence suggesting that PIEZO2 is an essential mechanically sensitive ion channel in MS afferents and that vesicle-released glutamate contributes to maintaining afferent excitability during the static phase of stretch. Other mechanically gated ion channels, voltage-gated sodium channels, other ion channels, regulatory proteins, and interactions with the intrafusal fibers are also important for MS afferent mechanosensation. Future studies are needed to fully understand mechanosensation in the MS and whether different complements of molecular mediators contribute to the different response properties of Group Ia and II afferents.     

Model study of muscle spindle primary endings subjected to dynamic fusimotor activation

An earlier proposed model of the de-efferented muscle spindle mechano-receptors has been developed further to simulate the effects of the dynamic fusimotor ( _D) activation of the group Ia afferents (primary endings).The rate sensitivity of the original second order receptor model might be increased simply by increasing the overall viscous damping of the simulated polar regions of the nuclear bag fibre. However, adequate simulation of the typical time course of the stretch response of the primary endings during _D-activation required a subdivision of the polar regions into an active and a passive part. A reasonable behaviour of the model was obtained by simulating a local contraction covering about 50–90% of the polar regions of the nuclear bag fibres.The ramp response of the model showed a quick rate response component that increased by increasing the rate of the simulated stretch. This component was not significantly influenced by the simulated _D-activation.A slow rate component appeared to increase approximately in the same proportion as the intensity of the simulated _D-activation.The behaviour of the model closely corresponds to that of the biological prototype. The study demonstrates that the electrophysiological effects of activating the dynamic fusimotor fibres are indeed compatible with peripheral mechanical events associated with contraction phenomena within the polar regions of the nuclear bag intrafusal muscle fibres.     

Morphometric evaluations of the human nervous system

It is very important to evaluate and accurately understand the various conditions of the human nervous system. In this review article, we introduce several morphometric reports that are proven to be accurate from the view point of various errors (range of tissue shrinkage ratios, microscopic multiple counting, artifacts of microscopic structures, etc.). We review the following aspects of the selected reports: methodology, developmental research, neuronal differences, gender differences, aging process and miscellaneous (nerve fibers, unmyelinated fibers, in relation to neuropathology, clinical image-analysis and immunohistochemistry).