INTRAMUSCULAR BRANCHING OF FUSIMOTOR FIBRES

Morphological analysis of neck muscle nerves and neurons in cats

Previous studies have failed to show morphological differences between neck muscle alpha and gamma motor fibers or alpha and gamma motoneurons. The present study aimed to investigate the morphological features of neck muscle motor nerves and motoneurons in cats. To determine the morphological features of peripheral motor fibers, the value of the outer contours of each fiber was converted into a perfect circle after ganglionectomy to remove sensory fibers, and the fiber diameters were calculated based on their circumferences. The sizes of neck motor fibers in the peripheral nerves had an evident bimodal distribution into small and large fiber groups, as depicted in histograms. The sizes of small and large motor fibers ranged from 2 to 12 µm and from 12 to 40 µm, respectively. The small fiber group is likely to correspond to gamma motor fibers and the large fiber group to alpha motor fibers. The morphological features of neck muscle motoneurons sectioned in the horizontal plane were examined using the horseradish peroxidase (HRP) retrograde labeling technique. The diameters of the biventer cervicis and complexus motoneurons had bimodal distributions. The inflection point between the small and large diameter population was 28 µm for the biventer cervicis and 26 µm for the complexus. We also observed that larger neurons displayed more dendrites. In conclusion, we could identify morphological differences likely to correlate with alpha and gamma motoneurons in both neck muscle peripheral nerves and neck motoneurons.

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Ephaptic transmission from type ii afferents to static γ and β efferents causes complex repetitive discharge: An hypothesis

INTRODUCTION

Complex repetitive discharges (CRDs) are thought to result from depolarization of a single denervated muscle fiber, followed by ephaptic spread to adjacent fibers. This leads to cyclic spread of the depolarization to produce a recurrent discharge. Another explanation is suggested.

METHODS

CRDs were recorded with single and multiple electromyographic needles longitudinal to muscle fibers in 39 neuropathy patients.

RESULTS

The mean frequency of CRDs was 26 Hz, mean number of negative spikes was 5.4, and blocking of spikes occurred in 53% of CRDs. In multi-needle recordings most CRDs were local, but propagation of the discharge was sometimes observed.

CONCLUSIONS

The prevailing hypothesis of CRDs cannot explain local CRDs. Type II afferents of bag2 and chain fibers branch widely in the juxtaequatorial region of muscle spindles where they may intermingle with motor terminals. Ephaptic transmission from type II afferents to static γ and β efferents may cause CRDs and fix the CRD frequency.

Fusimotor innervation in the cat

The motor innervation of cat spindles was examined in hindlimb muscles using a variety of techniques employed in light and electron microscopy. Observations were made on teased, silver preparations of 267 spindles sampled from the peroneal, flexor hallucis longus, and soleus muscles, hereafter referred to as the PER/FHL/SOL series. The γ innervation. Trail endings are almost invariably present, and innervate both bag and chain muscle fibres. Trail fibres accounted for 64.6 to 74.8% of the total fusimotor supply to samples of spindle poles in the PER/FHL/SOL series, the mean number of fibres per pole varying from 2.7 to 5.0 in the different muscles, and the mean number of ramifications (areas of synaptic contact) per fibre being 3.7. By contrast, the p₂innervation of a spindle pole generally consists of a single fibre supplying only one plate. In the above samples p(2) fibres accounted for 4.1 to 28.0% of the total fusimotor supply, and the mean number of fibres per pole varied from 0.3 to 1.2 in the different muscles. Ninety per cent of p(2) plates innervate bag fibres. The α innervation. The structure of p₁plates as seen in both light and electron microscopy compares very closely with that of extrafusal plates. After nerve section p₁plates degenerate at the same time as extrafusal plates, being the first of the three types of fusimotor ending to disappear. The frequency of the p₁innervation is similar to that of the p₂innervation. In the same samples of PER/FHL/SOL spindle poles as above p₁ fibres accounted for 6.0 to 28.8% of the total fusimotor supply, the mean number of fibres per pole varying from 0.25 to 2.1 in the different muscles. The majority of p₁ fibres enter a pole to terminate in one plate only. Seventy-five per cent of the plates innervate bag fibres. The three types of fusimotor ending are thus not selectively distributed to the two types of intrafusal muscle fibre. All three types of fusimotor fibre may branch within the spindle so as to innervate both bag and chain fibres. Bag fibres receive both types of plate ending as well as trail endings. Most chain fibres receive trail endings only; the rest receive either a p₁or a p₂plate innervation in addition, 25% of the p₁and 10% of the p₂innervation being distributed to chain fibres. The significance of this nonselective innervation is interpreted as indicating that the type of contraction elicited by stimulating a fusimotor fibre depends upon the type of ending initiating it rather than upon the type of muscle fibre executing it. Reasons are given for concluding that the dynamic response is controlled via the p₁and p₂plates, and that the static response is controlled by the trail endings. The participation of the α fibres in mammalian fusimotor innervation, previously regarded as a vestigial feature, proved to be widespread in the muscles studied and more prevalent in fast muscles (FHL, peroneus digiti quinti) than slow (soleus). A low frequency of p₁innervation is offset by a high frequency of p₂(as in peroneus longus), and vice versa (as in FHL). It is unlikely that collaterals from slow α fibres innervating type B muscle fibres are wholly responsible for the high frequency of the p₁innervation in FHL, and it is suggested that collaterals may also be derived from fast α fibres innervating type C muscle fibres. The possibility of there being some motor fibres of α conduction velocity and with an exclusively fusimotor distribution is also taken into account.

Alpha, beta and gamma motoneurons: functional diversity in the motor system's final pathway

Since their discovery in the late 19th century our conception of motoneurons has steadily evolved. Motoneurons share the same general function: they drive the contraction of muscle fibers and are the final common pathway, i.e., the seat of convergence of all the central and peripheral pathways involved in motricity. However, motoneurons innervate different types of muscular targets. Ordinary muscle fibers are subdivided into three main subtypes according to their structural and mechanical properties. Intrafusal muscle fibers located within spindles can elicit either a dynamic, or a static, action on the spindle sensory endings. No less than seven categories of motoneurons have thereby been identified on the basis of their innervation pattern. This functional diversity has hinted at a similar diversity in the inputs each motoneuron receives, as well as in the electrical, or cellular, properties of the motoneurons that match the properties of their muscle targets. The notion of the diverse properties of motoneurons has been well established by the work of many prominent neuroscientists. But in today's scientific literature, it tends to fade and motoneurons are often thought of as a homogenous group, which develop from a given population of precursor cells, and which express a common set of molecules. We first present here the historical milestones that led to the recognition of the functional diversity of motoneurons. We then review how the intrinsic electrical properties of motoneurons are precisely tuned in each category of motoneurons in order to produce an output that is adapted to the contractile properties of their specific targets.

Presynaptic inhibition evoked by muscle contraction

1. Contractions of flexor and extensor muscles of the knee and ankle were used to investigate presynaptic inhibition at the spinal level.2. Contractions evoked dorsal root potentials, and increased the excitability of the central terminals of group Ia, Ib and low threshold cutaneous primary afferent fibres.3. The monosynaptic reflexes recorded in response to stimulation of flexor or extensor muscle nerves were depressed, in the presence of strychnine hydrochloride 0.1 mg/kg I.V., by the contractions.4. It is suggested that these presynaptic inhibitory effects are largely due to the activation of Golgi tendon organs by contraction.

Gamma motor neurons express distinct genetic markers at birth and require muscle spindle-derived GDNF for postnatal survival

BACKGROUND

Gamma motor neurons (gamma-MNs) selectively innervate muscle spindle intrafusal fibers and regulate their sensitivity to stretch. They constitute a distinct subpopulation that differs in morphology, physiology and connectivity from alpha-MNs, which innervate extrafusal muscle fibers and exert force. The mechanisms that control the differentiation of functionally distinct fusimotor neurons are unknown. Progress on this question has been limited by the absence of molecular markers to specifically distinguish and manipulate gamma-MNs. Recently, it was reported that early embryonic gamma-MN precursors are dependent on GDNF. Using this knowledge we characterized genetic strategies to label developing gamma-MNs based on GDNF receptor expression, showed their strict dependence for survival on muscle spindle-derived GDNF and generated an animal model in which gamma-MNs are selectively lost.

RESULTS

In mice heterozygous for both the Hb9::GFP transgene and a tau-lacZ-labeled (TLZ) allele of the GDNF receptor Gfralpha1, we demonstrated that small motor neurons with high Gfralpha1-TLZ expression and lacking Hb9::GFP display structural and synaptic features of gamma-MNs and are selectively lost in mutants lacking target muscle spindles. Loss of muscle spindles also results in the downregulation of Gfralpha1 expression in some large diameter MNs, suggesting that spindle-derived factors may also influence populations of alpha-MNs with beta-skeletofusimotor collaterals. These molecular markers can be used to identify gamma-MNs from birth to the adult and to distinguish gamma- from beta-motor axons in the periphery. We also found that postnatal gamma-MNs are also distinguished by low expression of the neuronal nuclear protein (NeuN). With these markers of gamma-MN identity, we show after conditional elimination of GDNF from muscle spindles that the survival of gamma-MNs is selectively dependent on spindle-derived GDNF during the first 2 weeks of postnatal development.

CONCLUSION

Neonatal gamma-MNs display a unique molecular profile characterized by the differential expression of a series of markers - Gfralpha1, Hb9::GFP and NeuN - and the selective dependence on muscle spindle-derived GDNF. Deletion of GDNF expression from muscle spindles results in the selective elimination of gamma-MNs with preservation of the spindle and its sensory innervation. This provides a mouse model with which to explore the specific role of gamma-fusimotor activity in motor behaviors.

A brief historical note on the classification of nerve fibers

This is a brief review of the literature focused on the articles that formed the basis for the classification of the nerve fibers. Mention is also made to the origin of the nomenclature of the different motoneurons (alpha, beta and gamma).

Analysis of motor fibers in the communicating branch between the cervical nerves and the spinal accessory nerve to innervate trapezius in the rat

The communicating branch between the ventral rami of cervical nerves and the spinal accessory nerve (SAN) has been reported to also send motor fibers to supply the trapezius. However, the motor fiber type of the communicating branch and its peripheral distribution are still unclear. To determine the fiber elements within the branch and its peripheral distribution of the motor fibers in the trapezius, the anterograde tracing method was used in this study. The results show that a few a motor end plates from the communicating branch were observed on the extrafusal fibers, while in the muscle spindle the motor elements from the communicating branch were distributed to the polar portions of the intrafusal fibers. These results indicated that the motor fibers passing through the communicating branch to supply the trapezius are mainly y motor fibers, with some a motor fibers. Moreover, the a and y motor fibers from the communicating branch were observed in the clavotrapezius, acromiotrapezius and the rostral part of spinotrapezius. These findings also correlate with the clinical observation indicating that even when the spinal accessory nerve is injured, the trapezius is still capable of slight movement.

Power-law for axon diameters at branch point

BACKGROUND

Axon calibers vary widely among different animals, neuron classes, and even within the same neuron. What determines the diameter of axon branches?

RESULTS

We pursue the hypothesis that the axon caliber has evolved to minimize signal propagation delays, while keeping arbor volume to a minimum. For a general cost function, we show that the optimal diameters of mother and daughter branches at a bifurcation satisfy a power law. The derivation relies on the fact that the axon conduction speed scales as a power of axon diameter. Although available data are consistent with the law, there is a large spread in the data. Future experimental tests will determine whether this spread is due to biological variability or measurement error.

CONCLUSIONS

Minimization of arbor volume and signal propagation delay may have been an important factor in the evolution of the brain.

On the branching of motoneurons

Innervation ratios were estimated for motor units supplying superficial lumbrical muscles of the cat after assigning units to one of three types, FF/FI, FR, and S, and estimating their axonal conduction velocity, based on published data. Similarly, unit tensions were converted to muscle fiber number using published values for specific tension and fiber cross-sectional area. A relation was established between axonal area of cross-section and number of fibers innervated which was matched reasonably well by a model of a branching motoneuron with preterminal diameter of 4.28 microm and a daughter-to-parent ratio of 1.48 at each level of branching. It is proposed that the main features of the model are likely to apply to all motoneurons.

The ratio of pre- to postganglionic neurons and related issues in the autonomic nervous system

The motor outflow of the autonomic nervous system (ANS) is differentiated into two major divisions, parasympathetic (PSNS) and sympathetic (SNS). Both are organized hierarchically into pre- and postganglionic levels, but classically the two divisions have been assumed to differ in their ratios of pre- to postganglionic neurons. The PSNS been characterized as having lower ('one-to-few') ratios, whereas the SNS has been described as possessing higher ('one-to-many') ratios. These patterns have been assumed to measure differing divergences of the outflows. In this review, a ratio of pre- to postganglionic neurons is called a ratio index, and the idea that the PSNS and SNS have characteristically different ratio indexes and divergences is called the ratio rule. The putative differences in the ratio indexes of the two divisions - as well as Fulton's influential proposal that they form one of the bases of contrasting functional capacities of the PSNS and SNS - have been widely accepted for nearly for nearly three quarters of a century. A survey of the original observations yielding the concept of the ratio rule as well as the more recent estimates of pre- and postganglionic numbers, however, challenges both the generality and the adequacy of the ratio rule and indexes. The originally formulated differences between the PSNS and SNS represent an overgeneralization since they were based on observations of only two ganglia, the ciliary ganglion in the PSNS and the superior cervical ganglion in the SNS. Furthermore, these original estimates were based on limited samples and were subject to a number of counting artifacts. A survey of the literature suggests that ratio indexes vary much more within each ANS division than they do between the two divisions. When ganglia other than the ciliary and superior cervical are examined, the two divisions of the ANS have broad, largely overlapping ranges of ratio indexes. Additionally, other PSNS-SNS pairs can be found in which the relative sizes of their respective indexes are completely contrary to the ratio rule. For a given ganglion, there are substantial differences in the ratio index between species, between individuals of the same species, and between stages of development in the same species. Furthermore, both divisions of the ANS have wide and largely overlapping ranges of physiological effects varying from specific to diffuse, from local to widespread. Finally, the ratio index measure ignores the degree of convergence found in different ganglia, and it is insensitive to the fact that many ganglia have multiple functionally distinct motor neuron pools, each with separate inputs varying in their degrees of divergence and/or convergence. Thus ratio indexes do not differentiate the PSNS from the SNS, and conclusions based on such putative distinctions are questionable.

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.

Muscle-spindle distribution in relation to the fibre-type composition of masseter in mammals

The various parts of the masseter muscle complex (pars superficialis, pars profunda, zygomaticomandibularis, maxillomandibularis) in the rat, guinea-pig, rabbit, cat and macaque monkey were examined to discover whether they showed any relationship between the distribution of muscle spindles and extrafusal fibre types. Intrafusal (spindle) and extrafusal fibre types in masseter were compared with those in limb muscles and were identified by a combination of standard histochemical methods and indirect immunoperoxidase staining with antibodies specific for the various isoforms of myosin characteristic of fibre types in mammalian muscle. In general, the fibre-type properties of intrafusal fibres in masseter resembled those in limb muscle spindles, but the extrafusal fibre-type composition was unlike that in most limb muscles. In the rat masseter, most of the spindles were clustered together in a few very restricted areas. Extensive fusion of the external capsules of adjacent spindles, resulting in the formation of giant spindles, was seen in the cat and monkey masseter; this was sometimes accompanied by the enclosure of extrafusal fibres within the fused spindles. Common to all species, but strongest of all in the rat, was a close association between the distributions of muscle spindles and extrafusal Type I (slow twitch) fibres within the masseter complex. Muscle spindles and Type I fibres were either absent or rarest in the superficial part of masseter, but were most common in the deep layer (pars profunda) or zygomaticomandibularis. The functional significance of these observations is discussed.

Plantar motoneuron columns in the rat

In the rat, the numbers and locations of motoneurons innervating the short plantar muscles of the hindlimb (supplied by the medial and lateral plantar nerves, as well as a branch of the sural nerve) were determined by using both horseradish peroxidase (HRP) and fluorochromes as retrograde labels. Topographical organization within the plantar motor nucleus was examined by exposing individually the cut ends (encapsulated in low melting-point paraffin) of medial plantar, lateral plantar, and sural nerves to HRP. In addition, double-labeling experiments were conducted in which the medial plantar nerve was labeled with one fluorochrome (either true blue or diamidino yellow) and the lateral plantar nerve with another. The plantar motor pool is located in the extreme dorsolateral portion of the ventral horn, usually concentrated in the fifth lumbar (L5) spinal segment. Labeled motoneurons extended caudally into the sixth lumbar (L6) segment and rostrally into portions of the fourth lumber (L4) segment. Motoneurons of the medial plantar, lateral plantar, and sural nerve have overlapping territories. Sural motoneurons (about 70 cells per side) are generally confined to L5, medial plantar motoneurons (about 180 cells per side) tend to be concentrated in caudal L5 and rostral L6, whereas the lateral plantar motoneurons (about 310 cells per side) extend throughout the entire length of the plantar motor pool. The distribution of motoneuronal cell size is unimodal (mean cross-sectional area = 610 +/- 150 microns2). Cell bodies of plantar motoneurons tend to have similar geometries in all three major planes of sectioning. In all, the combined plantar plus sural nerve population amounts to about 560 motoneurons on each side of the spinal cord. On the basis of these data, and those published by others, the innervation of the small muscles of the foot accounts for about 25% of the motor axons carried by the entire sciatic nerve.

The localization of axon branchings in two muscle nerves of the rat. A contribution to motor unit topography

The distribution of fiber branching in the proximal intramuscular motor nerves of the gracilis and gastrocnemius muscles of the rat was studied in whole-mount preparations of teased nerves. Branchings of nerve fibers were clustered at fascicular divisions. Such concurrence of fascicular and fiber branchings determines the dispersion of the single muscle fibers belonging to a motor unit. The distribution of fiber branching reveals the wiring pattern of muscle fibers. These patterns differed for the gracilis and gastrocnemius muscles in correspondence with their functional organization.

Distribution of skeletofusimotor axons in lumbrical muscles of the monkey

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

The sensory and motor innervation of muscle spindles in cat tail dorsolateral muscles

The sensory and motor innervation of a sample of 202 muscle spindles from the dorsolateral tail muscles of eight cats were studied by silver and gold staining techniques. The range of diameters of Group Ia afferent nerve fibres to primary (P) sensory endings was 2.0-14.0 micron with a peak at 8.5 micron. The range of Group II fibres to secondary (S) sensory endings was 2.0-8.5 micron with a peak at 5.0 micron. In a sample of 234 spindle capsules, 171 (84.2%) were single capsule spindles and 31 (15.8%) were tandem spindles. With respect to the sensory endings of single capsule spindles, 19.7% showed only a P-ending, 32.5% had an additional S-ending and 15.8%, 3.0% and 2.1% had two, three and four S-endings, respectively. In the tandem spindle capsules, 0.8% of the larger proximal capsules showed only a P-ending, 5.1% had an additional S-ending and 4.7%, 2.6% and 0.4% had two, three and four S-endings, respectively. In the 31 tandem spindles, one of which had a triple capsule, there were 14 types of sensory ending combination in the proximal and distal capsules. The most common (44.4%) innervation was by a P-ending only, while 27.0% had an additional S-ending and 17.5%, 9.5% and 1.6% had two, three and four S-endings, respectively. Of the distal capsules, 92.9% showed only a P-ending. There were three types of motor nerve ending, namely the p1-plate, the p2-plate and the trail (tr) ending, in spindle polar regions. Spindle poles without a motor innervation also occurred. There were 16 types of motor ending combination in the two polar regions of muscle spindles. The most common type of combination was the trp2-ending (41.8%), receiving an average of 5.8 fusimotor fibre branches and the next common was the tr-ending only (38.7%), innervated by an average of 3.2 fibres. The least common combination was with the p1-plate ending (9.5%), receiving an average of 6.9 fibres. The mean number of fusimotor fibre branches per spindle pole was 4.2.

Patterns of fusimotor innervation by gamma-efferents in cat peroneus tertius

The pattern of gamma-efferent innervation of the muscle spindle was investigated in the peroneus tertius muscle of the cat. A number of spindle group Ia afferents were isolated and monitored during tetanic stimulation of isolated gamma-axons. The sample was dominated by efferents producing a static fusimotor effect on the afferent. An individual static axon normally projected to less than five but at times as many as seven muscle spindles. Each spindle normally received innervation from three to five static gamma-efferents but occasionally up to eight. The number of spindles innervated by a static gamma-axon was directly related to the efferent axonal conduction velocity. For each experiment, a probability distribution of the number of fusimotor effects was constructed from all available connections between the muscle spindles and static gamma-efferents considered in matrices of four afferents and four efferents. This distribution was then compared to a random binomial distribution. In each of the ten experiments, statistically significant differences (p less than 0.01) were noted between the experimental and random distributions. In order to interpret such deviations from the random distributions, similar analyses were performed on various combinations of afferents and efferents in simulated sets displaying grouping of effects. It was concluded that the observed differences between experimental and random distributions may be produced by a grouping of fusimotor innervation limited to a small set of spindles.

The effect of digital nerve stimulation on recruitment order of motor units in the first deep lumbrical muscle of the cat

The normal recruitment order of EMG spikes of the first deep lumbrical muscle of the cat's hindpaw, usually seen during cortical stimulation, pad pinch and weak plantar nerve stimulation, was temporarily reversed after stimulation of the medial digital nerve of the foot at 50 Hz for 2 min, and normal order was recovered in 2 to 10 min. The longer the period of stimulation of the medial digital nerve was, the longer the time for recovery. Alteration of order is repeatable and reversible after an interval of more than 15 min. After prolonged medial digital nerve stimulation, EMG responses to pad pinch and plantar nerve stimulation were facilitated. Combination of some stimuli (e.g. cortical stimulation and plantar nerve stimulation, pad pinch and plantar nerve stimulation, plantar nerve and medial digital nerve stimulation) also produced reversal of recruitment order during the period of stimulation. A functionally single motor nerve fiber to the first deep lumbrical muscle was isolated from the motor nerve axons in L7 and S1 of the spinal cord of cats, and physiological properties of pairs of motor units whose recruitment order was temporarily altered by plantar nerve stimulation after prolonged stimulation of the medial digital nerve of a foot were examined. Motor units with large action potentials were more facilitated than motor units with small action potentials after prolonged stimulation of the medial digital nerve. The former motor units showed fast contraction time, large twitch tension, low resistance to fatigue and presence of sag-behavior. The latter motor units showed slow contraction time, small twitch tension and high resistance to fatigue. High threshold motor units with middle-sized surface EMG records were recruited after motor units with large potentials had been recruited; motor units with middle-size action potentials were beta motor axons branching to both intrafusal and extrafusal muscle fibers. The beta motor axons also showed fast conduction velocity and the presence of sag-behavior.

Distribution, density and size of muscle receptors in cat tail dorsolateral muscles

Images

The sizes of motoneurons supplying hindlimb muscles in the mouse

Motoneurons supplying identified muscle groups in the mouse spinal cord were labelled by retrograde transport of horseradish peroxidase. The size of motoneurons was estimated by measuring perimeter and cross-sectional area at the level of the nucleolus for the following seven major muscle groups: quadriceps femoris, adductors and gracilis, gluteal musculature, hamstring muscles, posterior crural musculature, anterolateral crural musculature and intrinsic musculature of the foot. The qualitative observation of two size ranges of motoneuron was supported by the measurements. Frequency distribution histograms of motoneuronal cross sectional area were bimodal for all motoneuronal groups except for the foot musculature. The population parameters and proportions for the six bimodal histograms were estimated by the method of maximum likelihood. It was found that the mean area of the small neuron component, which were presumed to be gamma motoneurons, was similar for the six bimodal systems. In contrast to this the mean area of the large neuron component, presumed to be alpha motoneurons, was found to be different for the six bimodal systems; motoneurons supplying more proximal muscles showed a larger mean area than those supplying distal muscles. The mean area of both components was unaffected by survival time and this was interpreted as indicating that changes in survival time did not label greater numbers of small or large motoneurons. The proportion of motoneurons in the small neuron component was found to vary from 9 to 27%.

Identifications of the intrafusal endings of skeletofusimotor axons in the cat

Direct identification of the endings of skeletofusimotor (beta) axons has been made in muscle spindles deprived for their gamma innervation by degeneration. Hindlimb muscles were prepared in which 1--5 fast-conducting motor axons were left intact while the rest of the motor supply was cut and allowed to degenerate for a period of 7 days. In 3 experiments a single beta axons survived supplying tenuissimus, and in 2 experiments beta axons were among 4 or 5 surviving axons that supplied superficial lumbrical and abductor digiti quinti medius muscles. Motor endings identified as p1 plates were found in teased, silver preparations of all experimental muscles, a total of 35 such plates being located in 15 spindles. The plates were all supplied to bag1 fibres. The experiments show that if a spindle innervated by a beta axon is deprived of its gamma supply by degeneration the motor endings that remain intact are p1 plates.

The structure and source of lingual proprioceptors in the monkey

The proprioceptive innervation of the tounge has been investigated in the Cynamolgus monkey by silver impregnation methods following unilateral section of lingual, hypoglossal, and cervical nerves. Muscle spindles were constantly present in the intrinsic and extrinsic muscles. They varied greatly in number, averaged half the length of lumbrical spindles, and showed an unusual arrangement of chain fibre nuclei. Other, inconstant proprioceptors included tendon endings, Ruffini endings, Pacinian corpuscles, paciniform and lamellated endings. Topologically, the endings other than spindles were extra-muscular, so that the overall pattern of proprioceptive innervation resembled that of skeletal muscle in general. Lingual nerve section was without apparent effect on the proprioceptors. Section of the hypoglossal nerve at its point of entry into the tongue caused severe depletiion of ipsilateral proprioceptors and of fusimotor nerves. In the anterior tongue there was evidence of transmedian overlap by efferent and afferent axons contained in the hypoglossal nerve. Hypoglossal section at the skull base caused degeneration of fusimotor nerves but not of proprioceptors. Section of (a), the connexion of C2-C3 ventral rami with the hypoglossal, together with section of (b), the ramus descendens hypoglossi, coused depletion of lingual proprioceptors; again there was evidence of transmedian overlap. Procedures (a) or (b) alone had a lesser effect. It was concluded that lingual proprioceptive afferent fibres occupy the distal hypoglossal nerve, leaving it in the ramus descendens and in the C2-C3 connexion to enter the spinal cord via nerves C2 and C3.

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

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

Myelinated nerve fiber supply and muscle spindles in the respiratory muscles of cat: quantitative study

The present study was undertaken to provide quantitative data on the myelinated fibers of the phrenic and intercostal nerves and the number of spindles in the main respiratory muscles of the cat. The myelinated component of the phrenic and intercostal nerves was studied in the cat. Histograms of sequency distributions as a function of nerve fiber diameter were established for normal nerves. Certain nerves were then examined 35 to 40 days after excision of the dorsal spinal ganglia. The muscle spindles of the corresponding muscles were counted and localized, and, on the basis of several morphological criteria, were classified with those usually described in the interosseous muscles. The study of the nerves, as that of the spindles, demonstrates clear differences of proprioceptive innervation among the respirator muscles. The lateral part of the diaphragm and the Triangularis sterni have practically no spindles. The external muscles of the first thoracic spaces are very rich in spindles. Respiratory muscles can be ranged in an almost continuous manner between these two extremes.

Types of intra- and extrafusal muscle fibre innervated by dynamic skeleto-fusimotor axons in cat peroneus brevis and tenuissimus muscles, as determined by the glycogen-depletion method

1. The types of intra- and extrafusal muscle fibre innervated by dynamic skeleto-fusimotor (beta) axons were determined by using a modification of the glycogen-depletion method of Edström & Kugelberg (1968) combined with histochemical tests for various enzyme reactions. A single beta axon was prepared in each of the experiments, which were carried out on six peroneus brevis and two tenuissimus muscles. 2. The intrafusal distribution of dynamic beta axons is almost exclusively restricted to bag1 fibres. The bags fibre was depleted in each of twenty-four beta-innervated spindle poles; the only fibres of a different type depleted intrafusally were a bag2 fibre in one pole and a long chain in another. 3. Depletion in the bag1 fibres was usually restricted to one zone in one pole, generally in a mid-polar location. 4. The extrafusal muscle fibres depleted by dynamic beta axons belong to the slow oxidative type as defined by Ariano, Armstrong & Edgerton (1973). The number of such fibres in each motor unit could not be accurately determined, but is almost certainly small. 5. The slow oxidative muscle fibres innervated by dynamic beta axons were not depleted over their entire length. Since there is no reason to assume that they are not twitch fibres, it would seem that the localized depletions result from the conditions required to obtain glycogen depletion, i.e. long periods of motor stimulation applied during the occlusion of the muscle's blood supply. Under similar experimental conditions depletion of glycogen was also restricted to portions of fibres in fast oxidative-glycolytic motor units, but extended over most of the length of the fibres in fast glycolytic units.

Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic gamma axons, dynamic beta axons, or static gamma axons in ventral root filaments was observed and recorded on still and moving film. 2. Most spindles were controlled by one dynamic gamma axon (sometimes a beta axon) and three static gamma axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only. 3. Dynamic gamma and beta axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static gamma axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'. 4. Static gamma axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'. 5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static gamma axons, frequently by static gamma axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic gamma axons and on one occasion only by a static gamma axon controlling a nuclear bag fibre alone. 6. The conduction velocities of dynamic gamma and static gamma axons overlapped extensively, though dynamic gamma axons were absent from the lower end, and static gamma axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities. 7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972). 8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

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

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 development 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.

Properties of motor units in the first deep lumbrical muscle of the cat's foot

Isometric contractions of single motor units were studied in the first deep lumbrical muscle of the cat's hind-foot. Motor units with short twitch contraction times (15-20 msec) generally differed from those with longer ones (23-50 msec; contraction time measured in unpotentiated twitches) in showing (1) a greater maximum tetanic tension, (2) a smaller resistance to fatigue, (3) more post-tetanic potentiation of twitch tension, and (4) no post-tetanic occurrence of repetitive activity in response to single nerve stimuli (such "post-tetanic repetitive activity" was seen in several of the slower units). The ratio between unpotentiated twitch tension and maximum tetanic tension was similar for units with brief and long contraction times. The peak-to-peak amplitude of a single motor unit spike, recorded with gross electrodes, tended to be directly proportional to the maximum tetanic tension of the same motor unit.

Recruitment and firing rate modulation of motor unit tension in a small muscle of the cat's foot

Maintained contractions were elicited in the first deep lumbrical muscle of the cat's foot by electrical stimulation of the contralateral motor cortex or, reflexly, by pinching of the foot pad. The discharges of all significant motor units of the muscle were monitored by electromyography, and contractions of the various motor units were observed in isometric recordings of muscle tension. Over a wide range, muscle tension could be enhanced by an increased intensity of pad pinching or cortical stimulation. This increase in muscle tension was caused by a recruitment of new motor units as well as by an increase in the firing rate of already active motor units. The latter mechanism was clearly of great importance. Pad pinching or cortical stimulation could sometimes cause the muscle to produce a tension close to that of a maximum tetanic contraction. This was several times greater than the mean tension that would have been caused by motor unit recruitment alone (i.e. by the motor units firing at their minimum steady rate). Cortical stimulation as well as pad pinching commonly recruited weak units more easily than stronger ones of the same muscle. The recruitment order obtained in response to pad pinching often differed, however, in various details from the recruitment caused by cortical stimulation.

Skeleto-fusimotor axons in the hind-limb muscles of the cat

1. Motor axons supplying various hind-limb muscles of the cat (flexor hallucis lingus, peroneus brevis, peroneus digiti quinti, tibialis anterior, soleus and tenuissimus) were identified as skeleto-fusimotor or beta axons because their repetitive stimulation elicited both the contraction of extrafusal muscle fibres and an increase in the rate of discharge of spindle primary endings which perisited after selective blockade of extrafusal neuromuscular junctions. 2. The conduction velocity of these axons ranged from 39 to 92 m/sec. 3. Of seventy-six beta axons, seventy-two had a dynamic action on the sensitivity to velocity of stretching of primary endings, four had a static action. 4. The dynamic action of six beta axons was observed only after the contraction of extrafusal muscle fibres was selectively suppressed. 5. Tendon organs can be activated by beta motor units.