Fast-conducting skeletofusimotor axons supplying intrafusal chain fibers in the cat peroneus tertius muscle

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.

Glycogen depletion in intrafusal fibres in rats during short-duration high-intensity treadmill running

AIM

The recruitment patterns of the intrafusal and extrafusal fibres in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats were investigated during brief-intensity exercise by assaying their glycogen content histochemically.

METHODS

Six adult male rats were assigned to each of four groups that ran up a 6 degrees incline on a motor-driven treadmill, at 40 m min(-1) for either 0.5, 1, 2, or 4 min. Six adult male rats in the control group did not run. Extrafusal and intrafusal fibres were classified by myosin ATPase staining. Optical densities for glycogen content were evaluated in serial periodic acid Schiff (PAS) stained-sections from the B and C regions of intrafusal fibres.

RESULTS

The glycogen content of type IIA fibres in the SOL and EDL muscles decreased significantly in the early phase of exercise whereas the glycogen content of type I fibres in these muscles decreased later than that of type IIA fibres. The glycogen content of bag2 fibres decreased after 1 min of exercise in the SOL muscle and after 2 min of exercise in the EDL muscle. On the other hand, the glycogen content of bag1 and chain fibres decreased significantly after 2 min in the SOL muscle but not in the EDL muscle.

CONCLUSION

The results suggest that during brief-intensity exercise, as the glycogen content of type IIA fibres is reduced earlier than that of type I fibres, bag2 fibres are most important early in this type of exercise.

Afferent-inherent regulation of myosin heavy chain isoforms in rat muscle spindles

Whether afferents exert their morphogenetic influence on spindles through release of trophic factors at intrafusal fiber junctions or via participation in proprioceptive pathways which modulate the motor activity to muscles was investigated by comparing myosin heavy chain (MHC) expression in intrafusal fibers after ablation of afferents (deafferentation, or DA) to the extensor digitorum longus (EDL) of adult rats or after ablation of the corresponding central processes of afferents to the spinal cord (central-process ablation, or CPA). DA and CPA elicited an exaggerated pedal plantarflexion, and hypertrophy of the EDL concomitant with atrophy of the soleus in the affected hindlimb. Frequencies and patterns of expression of seven MHCs expressed by intrafusal fibers in CPA muscles were indistinguishable from normal rats. However, frequencies and patterns of expression of several MHCs were abnormal following DA. Thus factors transported anterogradely from afferents to intrafusal fibers may regulate MHC expression in intrafusal fibers.

Comparison of skeleto-fusimotor innervation in cat peroneus brevis and peroneus tertius muscles

1. The skeleto-fusimotor or beta innervation was compared in cat peroneus brevis and peroneus tertius muscles, which differ in their composition of fatigue-resistant motor units; the slow (S) units predominate in brevis and the fast units (FR) in tertius. 2. In four brevis muscles, of thirty-four beta-axons (from a total of 114 axons supplying extrafusal muscle fibres) twenty-nine were dynamic (beta D) and only five static (beta S). In contrast, in three tertius muscles, of twenty-five beta-axons (from a total of 82 axons) twelve were static and thirteen dynamic. 3. In a population of thirty-five brevis and thirty tertius spindles, the proportion of beta D-innervated spindles was greater in the brevis (68.5%) than in the tertius (50%) whereas that of beta S-innervated spindles was greater in the tertius (40%) than in the brevis (17.1%). In a population of thirty-two brevis and twenty-seven tertius spindles in which the presence of bag1 fibres was deduced from the existence of a dynamic innervation, the proportion of spindles innervated by beta D-axons was 80% in the brevis and 62% in the tertius. 4. In both muscles, the number of beta D effects was greater than that of beta S effects. beta S-axons were rarely found to supply more than one spindle whereas beta D-axons supplying more than one spindle (up to four) were common. Spindles were often coinnervated by beta D- and beta S-axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Sprouting of fusimotor neurones after partial denervation of the cat soleus muscle

Normally, gamma motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of gamma motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving alpha motoneurones. In addition, in four experiments axons conducting within the gamma range, on stimulation, produced measurable tension. In one experiment, stimulation of one such gamma axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The gamma axon was therefore a fusimotor axon. The effect observed on stimulation of the gamma axon suggested a largely dynamic action. Other examples of gamma axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of gamma axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles gamma motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such gamma axons were similar to those of ordinary alpha motor units.

A study of glycogen depletion and the fibre-type composition of cat skeleto-fusimotor units

1. We have used the glycogen-depletion technique, combined with myofibrillar ATPase (mATPase) staining for muscle fibre type, to study the fibre-type composition of four skeleto-fusimotor (beta) units in cat peroneus tertius, namely, one beta dynamic (beta d) unit and three beta static (beta s) units. 2. Depletion of glycogen was observed in serial cross-sections of thirty-four beta-unit extrafusal muscle fibres of various types traced from origin to insertion. No fibre was depleted of glycogen throughout its length; depletion was restricted to a number of zones, usually about five. Oxidative (type I) and oxidative-glycolytic (type IIA) fibres were depleted for a significantly greater proportion of their total length than glycolytic IIB fibres. 3. The fibre-type composition of the beta d unit was determined by tracing its fibres from end to end. The muscle unit consisted of one intrafusal bag1 fibre and ninety-three extrafusal muscle fibres comprising seventy-six type I fibres, eleven IIC fibres, and six fibres that changed from IIC to I during the course of their length (IIC/I fibres). The extrafusal fibre-type composition was thus 81.7% I plus 18.3% IIC and IIC/I. 4. The three beta s units (beta s1, beta s2, beta s3) were all fast-contracting and fatigued rapidly. Identification of their extrafusal fibre types, made in 1 mm2 areas sampled from different parts of each unit, gave mixed compositions as follows: beta s1, IIB + 6.7% IIA; beta s2, IIB + 5.8% IIA; beta s3, IIB + 29.9% IIA. The intrafusal component of each unit included either one or two long chain fibres. 5. In a discussion of the results, the fact that the continuous stimulation of extrafusal muscle fibres does not deplete them of glycogen throughout their length is examined in relation to the work of others who have assumed that it did. With regard to the finding of mixed extrafusal fibre types in the beta units, a distinction is drawn between minimal (around 5%) and moderate mixing. It is suggested that minimal mixing may occur in any motor unit as the outcome of endplate degeneration with foreign replacement, but that moderate mixing indicates an on-going process of conversion from one fibre type to another which in the adult may prove to occur only among beta units.

The distribution of static gamma-axons in the tenuissimus muscle of the cat

1. The distribution of static gamma-axons within and among muscle spindles of the tenuissimus muscle has been studied in the anaesthetized cat, on the basis of the effects on the responses of primary endings when bag2 or chain fibres or both are activated by static gamma-stimulation. 2. Locations of spindles were marked for subsequent histological analysis using teased, silver-impregnated preparations. 3. Static effects were classified into: (i) biassing; (ii) driving; or (iii) indeterminate categories. 4. Critical correlations established that the biassing type was produced by bag2 activity, either alone or in combination with chain fibres, whereas the driving type was produced by chain fibres active alone. Indirect evidence suggested that indeterminate effects were produced by bag2 and chain fibres active together. 5. The static gamma-axons showed some differential distribution according to their conduction velocities: faster-conducting axons were likely to be more widely distributed among spindles but less likely to innervate chain fibres alone than were more slowly conducting axons. 6. The results are discussed in terms of their possible functional and developmental significance.

Discharge behaviour of feline gamma-motoneurones following induction of an artificial myositis

The study was undertaken to test the widely held hypothesis that a painful lesion of the skeleto-motor system leads to an increase in the neuromuscular component of muscle tone by activating gamma-motoneurones in the affected region. In chloralose-anaesthetized cats, artificial myositis was induced in the lateral gastrocnemius-soleus (LGS) muscle and several hours later the impulse activity was recorded from single gamma-motoaxons supplying the medial gastrocnemius (MG) muscle. Under the conditions of the study, the majority of the fusimotor neurones had a resting activity and could be readily excited by natural stimuli. In contrast to the assumptions of the working hypothesis, the gamma-motoneurones in the myositis animals were not activated but showed a strong inhibition; both resting activity and excitability by electrical and natural stimuli were decreased. Additional recordings from fusimotor neurones of a flexor muscle (tibialis anterior, TA) demonstrated that in the preparation used, the behaviour of the flexor gamma-motoneurones was different from extensor ones in that the former usually had no resting activity and did not respond to natural stimuli. The only discernible effect of a myositis of the LGS muscle on the TA gamma-motoneurones was a decrease in their electrical reflex threshold. The results of the study do not support the assumption that a painful muscle lesion is followed by an activation of the gamma-loop that leads to an increase in muscle tone. Instead, the data may offer an explanation for the weakness and--in chronic cases--the reflex atrophy of lesioned muscles.

Motor nuclei of peroneal muscles in the cat spinal cord

The cat peroneal muscles have been used in numerous investigations dealing with the physiological properties of motor units, muscle spindles, and Golgi tendon organs. This report presents a study of the organization of peroneal motor pools in the cat spinal cord by means of retrograde axonal transport of horseradish peroxidase from individual muscles to the corresponding motoneurons. The motor nuclei of peroneus longus (PL), peroneus brevis (PB), and peroneus tertius (PT) muscles formed thin columns in the lateral part of the ventral horn in spinal segments L6-S1. In the transverse plane, the PT and PL nuclei occupied, respectively, dorsolateral and ventromedial positions, with PB nucleus in an intermediate position overlapping with the other two nuclei. Measurements of cell body diameters allowed identification of alpha and gamma subgroups in peroneal motoneuron populations. The average numbers of motoneurons were about 96 alpha and 60 gamma in PL, 75 alpha and 54 gamma in PB, and 34 alpha and 23 gamma in PT. Comparison with data from electrophysiological studies indicated that whole populations of motoneurons were labeled in each motor nucleus. The proportions of gamma motoneurons were the same, and cell bodies of gamma motoneurons had similar sizes in the three peroneal populations. In contrast, alpha motoneurons were significantly smaller in PB than in the two other pools, in keeping with the fact that PB contains a proportion of slow motor units larger than the two other muscles. In large samples of homonymous motoneurons, the numbers of first-order dendrites correlated linearly with motoneuron sizes.

Activation of cat muscle spindles by static skeletofusimotor axons

The discharges from primary and secondary spindle endings of the cat peroneus tertius muscle were recorded during stimulation of static skeletofusimotor (static beta) axons at frequencies comparable with the presumed range of motoneuronal firing rates. When stimulated at 20-40/s, static beta-axons exerted typical static actions on the spindles they innervated, including activation of primary endings with reduction of their dynamic sensitivity and activation of secondary endings. For these frequencies, the extrafusal portions of static beta-motor units developed unfused contractions producing oscillations of tension within the muscle. After suppression of extrafusal contractions, the effects of the stimulation of static beta-axons on spindle discharge could persist unaltered, showing that extrafusal events need not interfere with the specific intrafusal actions of static beta-axons. Stimulation of a static beta-axon at 20-40/s often elicited a response of primary endings in which the discharge exactly followed the stimulation frequency, i.e. it was driven 1:1. Purely mechanical excitation of a spindle by unfused extrafusal contractions could also drive its discharge at the stimulation frequency. The persistence of static beta-driving after suppression of extrafusal contraction provided evidence for its intrafusal origin. Driving elicited by static beta-axons could persist during changes in muscle length, but small fluctuations in the delay between each impulse and the preceding stimulus were observed. These fluctuations were clearly related to the changes in muscle length, indicating that although the primary ending discharge remained driven 1:1 at the stimulation frequency, the receptor was not totally insensitive to length changes.

Form and classification of motor endings in mammalian muscle spindles

The presynaptic features of 234 motor endings supplied to cat hindlimb muscle spindles have been studied in teased, silver preparations, and the postsynaptic features of a further 27 endings have been studied in serial, 1 micron thick, transverse sections. In the presynaptic study motor endings received by the three types of intrafusal muscle fibre were compared with the endings supplied to spindles by the various functional categories of motor axon. Three forms of motor ending were found that had significantly different presynaptic features. These forms correspond closely to those previously identified in the literature as p1 (beta), p2 (dynamic gamma) and trail (static gamma). The results of the postsynaptic study showed that the degree of indentation of the intrafusal muscle fibres by motor axon terminals increases with greater distance from the primary ending, irrespective of muscle-fibre type. We conclude that the postsynaptic form of intrafusal motor endings is determined by distance from primary ending and muscle-fibre type. It is not determined by type of motor axon, and cannot be correlated with presynaptic form so as to produce a unified classification of intrafusal motor endings.

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.

Histological identification of (static) skeletofusimotor innervation to a cat muscle spindle

Two skeletofusimotor (beta) axons were identified histologically in cat tenuissimus muscle. The axons supplied the distal poles of 3 intrafusal muscle fibers of the nuclear chain type and two extrafusal fibers of the fast twitch type. The observations strengthen the case for the existence of static beta-innervation to cat muscle spindles.

Physiological identification of static beta axons in primate muscle

Skeletofusimotor (beta) axons exerting a static action on spindle primary endings were identified in the peroneus brevis and tertius muscles of the cynomolgus monkey. In most of the identified static beta motor units, the extrafusal portion was of the fast-contracting fatiguable type.

Ultrastructural study of motor endings on long chain intrafusal fibers in muscle spindles of the cat

Five complete poles of long nuclear chain intrafusal fibers of cat tenuissimus muscle spindles embedded in a plastic medium were cut in serial, 1 micron thick transverse sections and stained with toluidine blue. Each fiber pole displayed one plate-type motor ending positioned outside the encapsulated part of the spindle. Ultrastructurally, the endings resembled extrafusal end-plates and were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other motor ending present in the spindles. They were identified as the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat spindles.

Histochemical evidence for the existence of skeletofusimotor (beta) innervation in the primate

A total of ten alpha motor axons which innervated the peroneus brevis muscle were isolated in two cynomolgus monkeys. In each experiment, the isolated alpha axons were stimulated collectively to deplete glycogen from their muscle units. The muscle was then frozen quickly , cut serially, and stained for glycogen. Of the 52 muscle spindles that were examined, zones of glycogen depletion were found in the intrafusal fibres of 32 spindles. The glycogen-depleted motor units included both fast-twitch and slow-twitch types. Depleted zones were observed in all three types of intrafusal muscle fibres. It was concluded that skeletofusimotor (beta) efferents were among the stimulated motor axons. This finding constitutes the first anatomical evidence for the existence of beta innervation in the primate.

Analysis of bimodal interspike interval histograms of primary muscle spindle endings in active triceps surae muscles of cats

A feature frequently observed in interspike interval histograms (IH) derived from stationary Ia fibre discharge patterns was a bimodality suggesting a mixed population composed of two sub-populations of possibly different origins. To verify this hypothesis a separation of these subpopulations was attempted by assuming a Gaussian distribution for the longer intervals. The success of such a separation depended on the distinctness of the histogram peaks and the relative sizes of each mode. Both factors in turn depended on the overall muscle length and, hence, on the mean spindle discharge rate. The smaller intervals were then isolated from the spike trains and submitted to further statistical analysis. By averaging overall muscle tension with respect to the smaller intervals, the latter could often be linked to tension trajectories which were of higher amplitude and faster time course than the trajectories obtained by averaging with respect to the rest of the spikes. In some cases, auto-correlation histograms of isolated small intervals as well as serial correlograms computed for all intervals indicated that some muscle spindle afferents responded sensitively to activity of single motor units. This would confirm recent results of Cameron et al. (1980).

A study of sensory innervation to long nuclear chain intrafusal fibers in the cat muscle spindle

The sensory innervation of 46 poles of long chain intrafusal muscle fibers was studied histochemically by staining for NADH-TR in periodic, 8 micron thick transverse sections of cat muscle spindles. Each long chain fiber carried terminals of the primary sensory axon, and 23 of the fiber poles also displayed secondary sensory endings. With the NADH-TR reaction there was no apparent difference in the cross-sectional appearance of sensory endings on the long chain and on other nuclear chain fibers. However, the contact area between the secondary endings and the muscle fiber tended to be shorter on the long chain than on the neighboring chain fibers of shorter polar length. This was also the case for one long chain fiber in which the sensory innervation was examined in serial, 1 micron thick sections stained with toluidine blue. Discharges of the secondary sensory axons in cat spindles may be affected more by contraction of the shorter nuclear chain fibers than by activation of the long chain fibers.

A quantitative study of skeletofusimotor innervation in the cat peroneus tertius muscle

1. Physiological tests were used to identify skeletofusimotor or beta axons to the cat peroneus tertius muscle in order to assess the proportion of beta axons in the motor supply to this muscle. 2. Static beta axons (beta S) were identified by: (a) observation of a delay between the complete block of extrafusal contraction and the failure of spindle activation upon prolonged stimulation, (b) increase of spindle excitation with stimulation frequencies above that eliciting maximal extrafusal contraction, (c) observation of 'unfused' frequencygram of spindle primary afferent discharge during stimulation of the axon at frequencies above that eliciting complete fusion of extrafusal contraction and (d) static action exerted on the response of the spindle afferent to ramp stretch. 3. Dynamic beta axons (beta D) were identified by the persistence of spindle activation after selective block of extrafusal neuromuscular junctions and by their dynamic action on spindle primary endings. 4. The actions of 116 motor axons (conduction velocity 56-104 m/sec) on ninety-five spindle afferents (fifty-seven from primary and thirty-eight from secondary endings) were examined in ten experiments. Thirty-six beta axons (31% of the total sample) were identified: twenty-four beta S (conduction velocity 69-104 m/sec) and twelve beta D (conduction velocity 56-91 m/sec). 5. Twenty (35%) primary endings were activated by a beta S and sixteen (28%) by a beta D axon. Nineteen (45%) secondary endings were activated by a beta S and five (13%) by a beta D axon. Convergence of beta D and beta S axons on the same spindle occurred in 10% of instances. beta-innervated spindles were also supplied by gamma axons. 6. Most of the beta S motor units were of the fast-fatigue resistant (FR) type, with a few units of the fast-fatigable (FF) type, and nearly all the beta D motor units were of the slow (S) type.

The topography of long nuclear chain intrafusal fibers in the cat muscle spindle

Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscle fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag2 fiber. Furthermore, they were usually situated adjacent to the bag1 fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag2 fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.

Histochemical study of an unusual cat intrafusal muscle fiber

A cat tenuissimus muscle spindle that contained two long chain intrafusal fibers in its distal pole is described. One of the fibers (lc1) had a histochemical profile (ATPase, NADH-TR, ChE reactions) of the kind which is characteristic for long chain fibers. The other fiber (lc2) consisted of two separate segments. The inner lc2 segment included the sensory equatorial region and was histochemically normal. The outer lc2 segment carried a motor plate, and did not stain for NADH-TR in the same way as the inner lc2 segment and the lc1 fiber. It is suggested that the unusual enzyme staining properties of the outer lc2 segment stemmed from its lack of sensory innervation, a situation which may have permitted the full expression of influences mediated by its motor nerve supply.

Evolving views on the internal operation and functional role of the muscle spindle

Images

Histochemical study of long nuclear chain fibers in the cat muscle spindle

Cat intrafusal muscle fibers were examined histochemically in serial transverse sections of tenuissimus muscle spindles. The "myofibrillar" adenosine triphosphatase staining reaction was used to recognize the nuclear bag and the nuclear chain fibers in 309 spindle poles. Poles of 40 nuclear chain fibers extended for 1,000 micrometer or more beyond the termination of the spindle capsule. These long chain fibers stained less intensely for nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR) than the typical chain fibers of shorter polar length. In sections stained for cholinesterases (ChE), the extracapsular regions of most long chain fibers displayed one or two short, dense "plate"-type ChE deposits, which may represent the terminals of skeleto-fusimotor axons. In addition, about one-third of the long chain fibers displayed one or more thinner and smaller areas of ChE activity, possibly corresponding to the endings of fusimotor axons. The overall ChE staining pattern of the typical chain fibers was unlike that of the long chains. However, some of the shorter nuclear chain fibers resembled long chain fibers with the NADH-TR reaction, even though their ChE "plates" were located intracapsularly. It is concluded that nuclear chain fibers in the cat spindle form a class of intrafusal fibers with heterogeneous histochemical properties, and that the long chain fibers represent one fiber subtype.

Motor innervation of the cat muscle spindle studied by the cholinesterase technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. "Myofibrillar" adenosine triphosphatase staining reaction was used to identify nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers. Typical chain fibers and long chain fibers were distinguished, the latter extending for more than 1,000 micron beyong the termination of the spindle capsule. Simple "rim" and more elaborate "plate" deposits were demonstrated histochemically along the poles of the typical chain fibers in staining for cholinesterases. They were considered to correspond, respectively, to the trail and plate motor nerve terminals. Most long chain fibers and the majority of nuclear bag fibers had their motor innervation limitd to "plate"-type endings. In addition, faint diffuse cholinesterase staining occurred along the spindle capsule and the surface of some intrafusal fibers. These histochemical observations are discussed with regard to the current concepts concerning the morphological and functional organization of the motor innervation of the cat muscle spindle.

Histophysiological observations on fast skeleto-fusimotor axons

In three instances, repetitive stimulation of a single fast skeleto-fusimotor axon supplying cat tenuissimus muscle was observed to activate a secondary ending belonging to a precisely located spindle. Prolonged repetitive stimulation of these beta-axons elicited glycogen depletion in the intrafusal muscle fibres of the located spindles and in extrafusal fibres. The intrafusal depletion affected the longest of the chain fibres in two instances, and the bag1 and bag2 fibres in the third instance. In one experiment, the discharge from the primary ending of the located spindle was also recorded. The fast beta-axon (it supplied the longest chain fibre in the spindle) had a static action on the primary ending. The extrafusal muscle fibres of the 3 fast beta-motor units belonged to the fast oxidative-glycolytic type.

"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.