Phase and frequency coordination between neuron firing as an integrative mechanism of human CNS self-organization.

It was shown by surface electromyography (sEMG) from spinal cord injury and Parkinson's disease patients that FF-type motor units fire repeatedly at a rate of 10 Hz and FR-type motor units with approximately 3 action potentials per impulse train at 4 to 5 Hz. Such oscillatory firing was compared with the firing of alpha1, alpha2, and alpha3-motoneurons, innervating FF FR, and S-type muscle fibres, which fire oscillatory approximately at 10 Hz, 4 to 7 Hz and 1 Hz, as measured earlier with the single nerve-fibre action potential recording method from motoneuron axons. Repeated firings with impulse trains consisted of 1, 2-5, and 20-50 action potentials per impulse train respectively. Oscillatory firing of motoneurons has now been demonstrated in humans with two methods. In spinal cord injury patients it was observed that the FF-type motor units, firing normally at 10 Hz, fired for higher activations also at approximately 20 Hz. A motor program burst was generated by recruiting for the burst time motor units to fire oscillatory. It was further shown that FF-type motor units fired with relative phase and frequency coordination but non-synchronously. The coordination was between motor units firing of the same muscle and between muscles of the right and left arm. With the single-nerve fibre action potential recording method it was found earlier that alpha and gamma-motoneurons and muscle spindle afferents fired in relative coordination. There is indication that relative phase and frequency coordination is an integrative mechanism for the self-organization of the neuronal networks of the human central nervous system (CNS).

Glia engulf degenerating axons during developmental axon pruning.

Developmental axon pruning is widely used in constructing the nervous system. Accordingly, diverse mechanisms are likely employed for various forms of axon pruning. In the Drosophila mushroom bodies (MB), gamma neurons initially extend axon branches into both the dorsal and medial MB axon lobes in larvae. Through a well-orchestrated set of developmental events during metamorphosis, axon branches to both lobes degenerate prior to the formation of adult connections. Here, we analyze ultrastructural changes underlying axon pruning by using a genetically encoded electron microscopic (EM) marker to selectively label gamma neurons. By inhibiting axon pruning in combination with the use of this EM marker, we demonstrate a causal link between observed cellular events and axon pruning. These events include changes in axon ultrastructure, synaptic degeneration, and engulfment of degenerating axon fragments by glia for their subsequent breakdown via the endosomal-lysosomal pathway. Interestingly, glia selectively invade MB axon lobes at the onset of metamorphosis; this increase in cell number is independent of axon fragmentation. Our study reveals a key role for glia in the removal of axon fragments during developmental axon pruning.

Quantitative ultrastructural analysis of glycine- and gamma-aminobutyric acid-immunoreactive terminals on trigeminal alpha- and gamma-motoneuron somata in the rat.

Detailed knowledge of the inhibitory input to trigeminal motoneurons is needed to understand better the central mechanisms of jaw movements. Here a quantitative analysis of terminals contacting somata of jaw-closing (JC) and jaw-opening (JO) alpha-motoneurons, and of JC gamma-motoneurons, was performed by use of serial sectioning and postembedding immunogold cytochemistry. For each type of motoneuron, the synaptic boutons were classified into four groups, i.e., immunonegative boutons or boutons immunoreactive to glycine only, to gamma-aminobutyric acid (GABA) only, or to both glycine and GABA. The density of immunolabeled boutons was much higher for the alpha- than for the gamma-motoneurons. In the alpha-motoneuron populations, the immunolabeled boutons were subdivided into one large group of boutons containing glycine-like immunoreactivity only, one group of intermediate size harboring both glycine- and GABA-like immunoreactivity, and a small group of boutons containing GABA-like immunoreactivity only. The percentage of immunolabeled boutons was higher for JC than JO alpha-motoneurons, the most pronounced difference being observed for glycine-like immunoreactivity. In contrast, on the somatic membrane of gamma-motoneurons, the three types of immunoreactive bouton occurred at similar frequencies. These results indicate that trigeminal motoneurons are strongly and differentially controlled by premotoneurons containing glycine and/or GABA and suggest that these neurons play an important role for the generation of masticatory patterns.

Coupling between serotoninergic and noradrenergic neurones and gamma-motoneurones in the cat.

Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to gamma-motoneurones, and serotonin (5-HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of gamma-motoneurones. Eight gamma-motoneurones were labelled with rhodamine-dextran, and 50 micrometer thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine beta-hydroxylase (DBH) and 5-HT. All the cells were directly and/or indirectly excited by muscle group II afferents from the muscle they innervated and/or other muscles. Appositions between monoaminergic fibres and the labelled somata and dendrites were located with three-colour confocal laser scanning microscopy by examining series of optical sections at 1 or 0.5 micrometer intervals. DBH and 5-HT varicosities formed appositions with the somata and dendrites of all the gamma-motoneurones. The mean packing densities for 5-HT (1.12 +/- 0.11 appositions per 100 micrometer(2) for somata and 0.91 +/- 0.07 per 100 micrometer(2) for dendrites) were similar to the densities of contacts reported for alpha-motoneurones. Monoaminergic varicosities in apposition to dendrites greatly outnumbered those on the somata. The density of DBH appositions was consistently lower - corresponding means were 53% and 62% of those for 5-HT on the somata and dendrites, respectively. It is concluded from an analysis of the distribution and density of varicosities in apposition to the gamma-motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of gamma-motoneurones.

Impulse initiation in the mammalian muscle spindle during combined fusimotor stimulation and succinyl choline infusion.

1. This is a report of observations on the responses of the primary and secondary endings of soleus muscle spindles of the anesthetized cat to the combined effects of the depolarizing neuromuscular blocker succinyl choline (SCh), given intravenously, and fusimotor stimulation. The findings were interpreted in terms of a dual pacemaker model for activity generated in the bag1 intrafusal fiber interacting with activity coming from bag2 and chain fibers. 2. In preliminary experiments it was found, using whole ventral root stimulation at fusimotor strength, that spindle responses to fusimotor stimulation were not blocked by SCh, whereas extrafusal junctions blocked rapidly. In the presence of SCh, fusimotor responses of spindle secondary endings were, on average, slightly larger than their control values before SCh was given, whereas fusimotor responses of primary endings were slightly smaller. 3. A study of the responses of spindle primary endings to stimulation of single dynamic (gamma D) and static (gamma S) axons in the presence of SCh revealed a fundamental difference in behavior. None of the responses to stimulation of gamma D axons (9 gamma D axons with 8 primary endings) showed significant summation with the responses to SCh. By contrast, the 20 gamma S axons studied showed varying degrees of summation with the responses to SCh. The responses of secondary endings to gamma S stimulation in the presence of SCh resembled those of primary endings and gamma S stimulation. 4. To explain these differences it is proposed that the primary ending has two separate sites of impulse initiation, one close to terminals on the bag1 intrafusal fiber (innervated by gamma D axons) and a second close to terminals on the bag2 and chain fibers (innervated by gamma S axons). It is proposed that the maintained increase in spindle firing observed during SCh infusion is the result of a bag2 contracture. The response to gamma S stimulation, contracting bag2 and chain fibers, adds to the SCh response. The degree of summation varies depending on whether the gamma S activates bag2 fibers, chain fibers, or both. The bag1 contracture, together with the effects of gamma D stimulation, acts through a separate pacemaker and therefore does not sum with the steady increase in spindle firing in the presence of SCh. There may be pacemaker switching between the bag1 generator and the bag2 and chain generator. 5. If the model is representative of most spindles containing the three kinds of intrafusal fibers, and the contractions of bag2 and chain fibers generate activity through a common impulse generator, then this bears on the question of the functional independence of the bag2 and chain fiber systems.

Gaba-like immunoreactive terminals on lumbar motoneurons of the adult cat. A quantitative ultrastructural study.

The aim of this ultrastructural study was to analyse quantitatively the distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in axon terminals apposed to somatic and proximal dendritic membranes of cat motoneurons in lumbar column 2. Preembedding immunocytochemistry was used to count the GABAergic terminals contacting profiles of eighteen alpha-and six gamma-motoneurons. Of the 1293 terminals counted on the somatic and proximal dendritic compartments of alpha-motoneurons, 197 were GABAergic. In contrast, a total number of only 62 terminals were counted on gamma-motoneurons, of which 8 were GABAergic. These populations of GABAergic terminals were less numerous than the population of glycinergic terminals observed in a previous study. The morphometric characteristics of GABAergic synapses were analyzed using postembedding immunocytochemistry. Most of the GABAergic terminals contained pleomorphic vesicles (F-type boutons, flattened or pleomorphic vesicles). All terminals presynaptic (P boutons) to large terminals containing sphericle vesicles (M-type boutons, characteristic of alpha-motoneurons), were GABA-immunopositive. These results suggest that there are different distributions of the GABAergic control of excitability on gamma- and alpha-motoneurons. GABA appears to be strongly involved in post-synaptic inhibition of alpha-motoneurons, whereas gamma-motoneurons receive very few GABAergic inhibitory inputs. Morphological correlates of GABAergic presynaptic inhibition were seen on alpha- but not on gamma-motoneurons.

Succinate dehydrogenase activity and soma size of motoneurons innervating different portions of the rat tibialis anterior.

The spatial distribution, soma size and oxidative enzyme activity of gamma and alpha motoneurons innervating muscle fibres in the deep (away from the surface of the muscle) and superficial (close to the surface of the muscle) portions of the tibialis anterior in normal rats were determined. The deep portion had a higher percentage of high oxidative fibres than the superficial portion of the muscle. Motoneurons were labelled by retrograde neuronal transport of fluorescent tracers: Fast Blue and Nuclear Yellow were injected into the deep portion and Nuclear Yellow into the superficial portion of the muscle. Therefore, motoneurons innervating the deep portion were identified by both a blue fluorescent cytoplasm and a golden-yellow fluorescent nucleus, while motoneurons innervating the superficial portion were identified by only a golden-yellow fluorescent nucleus. After staining for succinate dehydrogenase activity on the same section used for the identification of the motoneurons, soma size and succinate dehydrogenase activity of the motoneurons were measured. The gamma and alpha motoneurons innervating both the deep and superficial portions were located primarily at L4 and were intermingled within the same region of the dorsolateral portion of the ventral horn in the spinal cord. Mean soma size was similar for either gamma or alpha motoneurons in the two portions of the muscle. The alpha motoneurons innervating the superficial portion had a lower mean succinate dehydrogenase activity than those innervating the deep portion of the muscle. An inverse relationship between soma size and succinate dehydrogenase activity of alpha, but not gamma, motoneurons innervating both the deep and superficial portions was observed. Based on three-dimensional reconstructions within the spinal cord, there were no apparent differences in the spatial distribution of the motoneurons, either gamma or alpha, associated with the deep and superficial compartments of the muscle. The data provide evidence for an interdependence in the oxidative capacity between a motoneuron and its target muscle fibres in two subpopulations of motoneurons from the same motor pool, i.e. the same muscle.

Ultrastructure of gamma-motoneurons after temporary or permanent interruption of peripheral target contact.

The paradigm of nerve crush, vs. nerve transection and ligation, was used to examine the effects of temporary or permanent interruption of peripheral target contact on the ultrastructure of cat thoracic gamma-motoneurons. The normal, highly ordered ultrastructure of Nissl bodies was lost 8 days after axotomy. Nissl bodies remained disorganised up to 305 days after nerve transection and ligation. In contrast, normal ultrastructural orderliness was restored for many of the Nissl bodies of gamma-motoneurons 64 days following nerve crush. A decrease in the area of the Golgi apparatus was found 64 days following both nerve crush and nerve transection with ligation. Other organelles were unaltered.

Alpha and gamma motoneurons in the peroneal nuclei of the cat spinal cord: an ultrastructural study.

The aim of the present study was to investigate whether ultrastructural features can be used as a guide to identify alpha- and gamma-motoneurons among the intermediate-size neurons of the peroneal motor nuclei. The peroneus brevis and peroneus tertius muscles of adult cats were injected with horseradish peroxidase, and motoneurons labeled by retrograde axonal transport were examined by electron microscopy. In both nuclei, the distributions of cell-body diameters, measured in the light microscope, were bimodal covering the range of 28-84 microns, with a trough around 50 microns. The sample of 25 motoneurons selected for the ultrastructural study included not only large (presumed alpha) and small (presumed gamma) neurons but also intermediate-size cell bodies with diameters in the 40-60 microns range. For each motoneuron, 2-5 profiles were reconstructed from ultrathin sections taken at 6-8 microns intervals. Synaptic boutons were counted and their lengths of apposition were measured. On the basis of three criteria, namely: (1) bouton types present on the membrane, (2) percentage of membrane length covered by synapses, and (3) the aspect of the nucleolus, all the examined motoneurons, including those with intermediate sizes, fell into one of two categories. Fourteen motoneurons, with cell-body diameters in a range of 55-84 microns, were contacted by all types of boutons (mainly S-type with spherical vesicles, F-type with flattened vesicles, and C-type with subsynaptic cistern); the synaptic covering of the somatic membrane was over 40% and the nucleus contained a vacuolated nucleolus. These were considered alpha-motoneurons. Eleven motoneurons, with only S and F boutons, a synaptic covering under 30%, a compact nucleolus and a cell-body diameter ranging between 28 and 50 microns, were considered gamma-motoneurons. No other combination of the three criteria was observed. These results show that unequivocal distinction of alpha- and gamma-motoneurons is possible in the peroneal nuclei, on the basis of morphological differences independent of cell-body size.

Reduced branching and length of dendrites detected in cervical spinal cord motoneurons of Wobbler mouse, a model for inherited motoneuron disease.

The Wobbler mouse (wr) has been proposed as a model for human inherited motoneuron disease (infantile spinal muscular atrophy). The primary defect is thought to be in the motoneurons. Therefore we undertook a survey of the qualitative and quantitative changes occurring in the cervical spinal motoneurons of Wobbler mice during a late stage of the motoneuron disease compared with age- and sex-matched normal phenotype (NFR/wr) littermates. The Rapid Golgi Method was applied. In control and Wobbler mice, four types of neurons were identified according to their dendritic patterns: multipolar, tripolar, bipolar, and unipolar cells. Unipolar cells were observed more often in the Wobbler specimens than the controls and may represent a final stage in the degeneration of other cell types with greater numbers of primary dendrites. Medium (300-999 microns 2) and large (greater than 1,000 microns 2) impregnated neurons (presumably alpha-motoneurons) showed strong indications of cell degeneration, including statistically significant reductions in the measurements for dendritic length, distribution, and branching, as well as the number of spines. In contrast, the small (less than 300 microns 2) neurons showed only mild signs of degeneration, including slight reductions in dendritic length, but no significant differences appeared in the distribution and branching of dendrites, or in the number of spines. Instead, a small increase could be detected in the number of primary and secondary dendritic branches emanating from the small neurons, as well as in the number of dendritic spines. These findings suggest that sprouting may occur to a slight extent. Although previous studies document that swelling with subsequent vacuolation of motoneurons is the predominant feature characterizing the Wobbler disease, the mean soma area (microns 2) calculated for the impregnated neurons of the Wobbler specimens showed no significant difference from the controls. It is hypothesized that the advanced signs of the Wobbler motoneuron disease are primarily reflected in the degeneration of the dendrites and spines on the medium and large alpha-motoneurons. The small neurons (presumably a mixed population of gamma-motoneurons, interneurons, and Renshaw cells) possess dendrites and spines that seem to be less affected, and instead show signs of sprouting.

The size and dendritic structure of HRP-labeled gamma motoneurons in the cat spinal cord.

We report quantitative data obtained from 60 fully reconstructed dendritic trees belonging to eight gamma-motoneurons (gamma-MNs) and six additional gamma-MNs that were not completely reconstructed. The cells were labeled intracellularly with horseradish peroxidase (HRP). These data are compared to measurements from 79 reconstructed dendrites belonging to seven documented alpha-motoneurons (alpha-MNs), supplemented by a larger sample of alpha-MNs labeled intracellularly or by retrograde transport with HRP. As expected from earlier studies, the soma dimensions and total membrane area of gamma-MNs were smaller than those of alpha-MNs. Although gamma-MN dendrites were, on average, slightly but significantly longer than those of alpha-MNs, the former had, on average, smaller diameter stem dendrites, less membrane area, and less profuse branching, and they tended to branch closer to the soma and to terminate farther from the soma. These differences were evident even when subsets of dendrites with similar stem diameters were compared. Some of the anatomical distinctions suggest that gamma-MNs are qualitatively as well as quantitatively different from alpha-MNs, even though the distributions of many of the morphological variables examined showed no abrupt discontinuities between the two motoneuron groups.

Ultrastructure of interneurons within motor nuclei of the thoracic region of the spinal cord of the adult cat.

Alpha (greater than 40 microns) and gamma (less than 30 microns) motoneurons in inspiratory motor nuclei of the thoracic spinal cord of the adult cat were labelled retrogradely by the intramuscular injection of HRP. Small (less than 30 microns) unlabelled neurons within 200-300 microns of labelled motoneurons were analysed qualitatively and quantitatively with both the light and electron microscope. Most of these small unlabelled neurons had inconspicuous nucleoli, wrinkled nuclear membranes, low numbers of nuclear pores, and Nissl bodies which were either small or had the form of an amorphous perinuclear band. Such Nissl bodies were composed primarily of aggregates of polyribosomes within which short fragments of granular endoplasmic reticulum were distributed. Alpha and gamma motoneurons in contrast had prominent nucleoli, smooth-contoured nuclei, more nuclear pores and large, discrete Nissl bodies. Such Nissl bodies were composed primarily of several lamellae of granular endoplasmic reticulum with linear arrays of polyribosomes arranged between individual cisternae. Alpha motoneurons had most synaptic terminals on their cell bodies, gamma motoneurons had least and small unlabelled neurons had intermediate values. Synaptic terminals of the S-, F- T- and C-type were observed on alpha motoneurons, whereas only S- and F-types were observed on gamma motoneurons and small unlabelled neurons. Since they were unlabelled and differed morphologically from both alpha and gamma motoneurons, but were similar to small interneurons described elsewhere in the spinal cord and brain, it is suggested that the small unlabelled neurons located in the external intercostal and levator costae motor pools are interneurons. The functional significance of some of the morphological features which distinguish interneurons from motoneurons is discussed.

An ultrastructural study of the synaptology of gamma-motoneurones during the postnatal development in the cat.

The postnatal development of cat triceps surae gamma-motoneurones, retrogradely labelled with horseradish peroxidase (HRP), was studied light and electron microscopically. The mean diameter of the cell bodies of the gamma-motoneurones increased by about 25% from birth to the adult stage, which was much less than the increase in cell body diameter of alpha-motoneurones (about 45%). Throughout development the only bouton types apposing the gamma-motoneurones were the F- and S-types, with flattened and spherical synaptic vesicles, respectively. Thus, the C-, M- and T-types of boutons seen on a alpha-motoneurones. The number of boutons on the gamma-motoneurone cell bodies seemed to decrease postnatally. This decrease was only moderate for S-type boutons but substantial for F-type boutons. In contrast, the number of boutons on the proximal dendrites appeared to increase and this was most evident for S-type boutons. The mentioned postnatal changes in synaptology were more differentiated with regard to bouton type and part of the neurones under study than what could be inferred from earlier studies on the postnatal development of alpha-motoneurones. These changes also occurred later than in alpha-motoneurones. The relative dominance of F-type boutons with probable inhibitory actions on the immature gamma-motoneurone may explain the previously demonstrated poor encoding of muscle length by muscle spindles during the first postnatal weeks in the kitten.

Possible fusimotor innervation to frog muscle spindles.

In some muscle spindles located in or near the red area of semitendinosus muscle of the bull frog, a fine motor axon innervates the intracapsular compact zone, without any branches onto the extrafusal muscle fibers. The fine motor axon is 3.9 microns in mean diameter (measured in living axon) and 9.8 m X s-1 in mean conduction velocity at 20 degrees C. A ramp-and-hold stretch during 20 Hz repetitive stimulation of the fine motor axon markedly increased the rate of afferent discharges during static phase. Histochemical studies indicated depletion of glycogen in a compact zone along a small diameter intrafusal muscle fiber of the muscle spindles frozen immediately after tetanic stimulation of the fine motor axon. These results suggest that the fine motor axon is a fusimotor system in the frog muscle spindle.

Electron microscopic observations on the synaptology of cat sciatic gamma-motoneurons after intracellular staining with horseradish peroxidase.

Four cat sciatic motoneurons with axon conduction velocities below 30 m/s, and thus considered to be of the gamma-type, were intracellularly labelled with horseradish peroxidase (HRP) and subsequently studied in the electron microscope. The labelled neurons were apposed by synaptic terminals with spherical (S-type) and flattened vesicles (F-type) but not by large terminals with spherical vesicles (M- and C-types) seen on alpha-motoneurons. Quantitative analysis of a complete series of ultrathin sections through one of the neurons showed that the synaptic covering on the cell body (24.2%) was considerably larger than what has been reported for triceps surae gamma-motoneurons, but within the range of values for gamma-motoneurons in the thoracic region of the spinal cord.

A quantitative ultrastructural comparison of alpha and gamma motoneurons in the thoracic region of the spinal cord of the adult cat.

The cell bodies of motoneurons supplying both the levator costae and external intercostal muscles were identified after retrograde labelling with horseradish peroxidase. A quantitative ultrastructural comparison of cell bodies of large (greater than 40 microns) and small (less than 30 microns) diameter revealed that the intracellular appearance of large and small motoneurons was similar. However, small motoneurons had less than half the synaptic terminal frequency or cover of large motoneurons. Furthermore, only synapses of the S- and F-type were seen on small motoneurons, while S- T- F- and C-type terminals were consistently seen on large motoneurons. The variation between individual small motoneurons for various aspects of their synaptic features was more than twice that found for large motoneurons. No correlation between small motoneuronal ultrastructure and cell body diameter was found, although scatter diagrams of synaptic terminal cover against cell body size indicated the presence of two groups of small motoneurons: one with relatively high values for synaptic cover and the other with relatively low values. On the basis of the similarity of their cell body diameters to those of electrophysiologically identified alpha and gamma motoneurons, it is concluded that the large and small motoneurons examined in the present study are alpha and gamma motoneurons respectively. The synaptic difference found between alpha and gamma motoneurons is discussed in relation to both their different functional properties and the different natures of their respective peripheral targets.

Factors that determine the form of neuromuscular junctions of intrafusal fibers in the cat.

The form of terminations of fusimotor (gamma) and skeletofusimotor (beta) axons on intrafusal fibers was analyzed in serial sections of 20 spindles of the cat tenuissimus muscle. Seven synaptic features were assessed either qualitatively or quantitatively from electron micrographs of transverse sections of 184 intrafusal and 30 extrafusal endings. Features were compared among endings that were terminations of gamma or beta axons on different types of intrafusal fiber at different distances from the spindle equator. These comparisons indicated that interactions of several factors, and not the motor axon alone, determine the form of motor endings. Intrafusal muscle fiber type is dominant to the motor axon in regulation of the number and depth of postsynaptic folds. Separation of the influence of the motor axon from the muscle fiber was less clear with respect to the size of ending. Complete expression of the muscle fiber-motor axon interaction reflected by the form of motor endings is dependent upon location of the ending relative to the sensory region. Both depth of the primary synaptic cleft and size of the soleplate of motor endings increased with increasing distance of the ending from the spindle equator. A system of classification of cat intrafusal motor endings that reflects the multiplicity of factors that determine the form of endings, and one that simplifies the current terminology, is proposed.

Reconstruction of the nerve supply to a human muscle spindle.

Distributions of one sensory and 13 motor axons to intrafusal fibers in a human spindle from the biceps brachii muscle were reconstructed from serial, 1 micron thick transverse sections. The primary afferent was distributed predominantly to nuclear bag fibers. Motor innervation in the human spindle was characterized by the presence of shared innervation among different types of intrafusal fiber, long unmyelinated preterminal segments of axon, and numerous short motor endings on both bag1 and bag2 fibers. These neuroanatomical features differ grossly from those in the cat tenuissimus spindles and may reflect a major functional difference.

Morphology and morphometry of motor endings in primate intrafusal fibers.

The pre- and postsynaptic structure of 243 axon terminals of bag 1, bag 2, and chain fibers were studied in cynomolgus monkey skeletal muscle spindles. The motor endings of the biceps and gastrocnemius spindles (long limb muscle) were compared to the motor endings of lumbricals and opponens pollicis (intrinsic hand muscle) spindles. In both muscle groups the only significant difference observed in the presynaptic features was in the presynaptic membrane length. The postsynaptic features of bag 1, bag 2 and chain endings were similar in the long limb muscle spindles. In the intrinsic hand muscle, however, the bag 1 and chain endings showed complex postsynaptic structure which resembled the extra fusal endings while the postsynaptic structure of bag 2 endings was much simpler. From these studies we conclude that the postsynaptic structure of various intrafusal fiber types is dissimilar in different muscles.

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.