Soleus Muscle Spindles Research Articles

Knee movements cause changes in the firing behaviour of muscle spindles located within the mono-articular ankle extensor soleus in the rat.

We recently showed that within an intact muscle compartment, changing the length of one muscle affects the firing behaviour of muscle spindles located within a neighbouring muscle. The conditions tested, however, involved muscle lengths and relative positions that were beyond physiological ranges. The aim of the present study was to investigate the effects of simulated knee movements on the firing behaviour of muscle spindles located within rat soleus (SO) muscle. Firing from single muscle spindle afferents in SO was measured intra-axonally for different lengths (static) and during lengthening (dynamic) of the lateral gastrocnemius and plantaris muscles. Also, the location of the spindle within the muscle was assessed. Changing the length of synergistic ankle plantar flexors (simulating different static knee positions, between 45 and 130°) affected the force threshold, but not the length threshold, of SO muscle spindles. The effects on type II afferents were substantially (four times) higher than those on type IA afferents. Triangular stretch-shortening of synergistic muscles (simulating dynamic knee joint rotations of 15°) caused sudden changes in the firing rate of SO type IA and II afferents. Lengthening decreased and shortening increased the firing rate, independent of spindle location. This supports our prediction that the major point of application of forces exerted by connections between adjacent muscles is at the distal end of SO. We conclude that muscle spindles provide the CNS with information about the condition of adjacent joints that the muscle does not span.

Neural interactions between transspinal evoked potentials and muscle spindle afferents in humans

The objective of this study was to establish neural interactions between transspinal evoked potentials (TEPs) and muscle spindle group Ia afferents in healthy humans. Soleus H-reflexes were assessed following transspinal stimulation at conditioning-test (C-T) intervals that ranged from negative to positive 100 ms. TEPs were recorded from the right and left ankle/knee flexor and extensor muscles, and their amplitude was assessed following stimulation of soleus muscle spindle group Ia afferents at similar C-T intervals. Transspinal conditioning stimulation produced a short-latency, long-lasting soleus H-reflex depression. Excitation of muscle spindle group Ia afferents produced depression of ipsilateral ankle TEPs and medium-latency facilitation of the ipsilateral knee TEPs. At specific C-T intervals, the soleus H-reflex and ipsilateral ankle TEPs were summated based on their relative onset and duration. No changes were observed in the contralateral TEPs. These effects were exerted at both peripheral and spinal levels. Both transspinal and muscle spindle group Ia afferent stimulation produce long-lasting depression of the soleus H-reflex and TEPs, respectively. Transspinal stimulation may promote targeted neuromodulation and can be utilized in upper motoneuron lesions to normalize spinal reflex hyper-excitability and alter excitation thresholds of peripheral nerve axons.

Quantitative input-output relationships between human soleus muscle spindle afferents and motoneurons

A method is described that, for the first time, allows instantaneous estimation of the Ia fiber input to human soleus motoneurons following electrical stimulation of the tibial nerve. The basis of the method is to determine the thresholds of the most and least excitable 1a fibers to electrical stimulation, and to treat the intervening thresholds as having a normal distribution about the mean; the validity of this approach is discussed. It was found that, for the same Ia fiber input, the percentage of soleus motoneurons contributing to the H (Hoffmann)-reflex differed considerably among subjects; when the results were pooled, however, there was an approximately linear relationship between Ia input and motoneuron output. Weak extension of the great toe diminished the soleus motoneuron reflex discharge in all but 2 of 16 subjects; the results for weak ankle plantarflexion were less consistent, but overall, there was a reduction in soleus motoneuron output also. The methodology should provide new insights into disorders of movement and tone, especially as it permits estimates of motoneuron depolarization to be made. NEW & NOTEWORTHY Assuming a normal distribution of Ia fiber thresholds to electrical stimulation and using the H-reflex, we determined for the first time an Ia input-α-motoneuron output relationship for the human soleus muscle. The relationship varies greatly among subjects but, overall, is approximately linear. Minimal contraction of a toe muscle alters the relationship dramatically, probably due to presynaptic inhibition of Ia fibers. Drawing on the literature, we can calculate changes in α-motoneuron membrane potential.

The relationship between the sensory responses to ankle-joint loading and corticomotor excitability

ABSTRACTPurpose: Maintaining joint stability is dependent on the ability of the nervous system to sense and react to potentially injurious loads. In attempts to understand the neurophysiologic mechanisms underlying joint stability, this afferent and efferent activity has been quantified separately at the cortical, segmental and peripheral levels using various electrophysiologic techniques in vivo. However, no studies have attempted to quantify sensory and motor activation at multiple levels of the nervous system in a single subset, to understand potential adaptations for optimizing joint stability.Materials and Methods: Muscle spindle afferent activity and sensory cortex event-related desynchronization were quantified during ankle-joint loading; and motor excitability was assessed through transcranial magnetic stimulation and the Hoffmann reflex in a subset of 42 able-bodied individuals. Microneurography and electroencephalography were used to collect the muscle spindle afferent and sensory cortex activation, respectively, as joint load was applied using an ankle arthrometer. Separately, motor-evoked potentials were obtained from the tibialis anterior (TA) and soleus (SOL) using transcranial magnetic stimulation over the motor cortex, and compared to the reflexive responses evoked via sciatic nerve electrical stimulation.Results: Correlation coefficients revealed significant correlations between the motor threshold of the soleus and early muscle spindle afferent activity (r = −0.494) and early cortical event-related desynchronization (r = 0.470), as well as tibialis anterior motor-evoked potential size and late muscle spindle afferent activity (r = 0.499).Conclusions: The results of this study highlight the nervous system's capability to offset motor output based on the volume of sensory input at the segmental and cortical levels.

Influence of 14-day hind limb unloading on isolated muscle spindle activity in rats

During hind limb unloading (HU), the soleus is often in a shortened position and the natural physiological stimulus of muscle spindles is altered, such that muscle spindle activity also changes. Using isolated spindle conditions, the present study investigates the electrophysiological activity and ultrastructure of muscle spindles following HU. Results show that muscle spindle discharges fall into either of two main patterns, single spikes or spike clusters in shortened positions, with a steady frequency of 18-38 spikes/s (mean 29.08 +/- 2.45) in an extended position. Following 14-day HU, afferent discharge activity was significantly altered in soleus muscle spindles. Duration of individual spikes was significantly prolonged, from 0.54 +/- 0.05 ms for control rats to 1.53 +/- 0.25 ms for rats in the HU group. In a shortened position, regular rhythm afferent discharges were obviously depressed, and the majority of muscle spindles became silent, while in an extended position, the discharges remained continuous but with decreased frequency. Results also show that the ultrastructure of muscle spindles experience degenerative changes during HU. Altered muscle spindle afference could possibly modify the activity of motor neurons and further affect the activity of extrafusal fibers.

Effect of hindlimb unloading on resting intracellular calcium in intrafusal fibers and ramp-and-hold stretches evoked responsiveness of soleus muscle spindles in conscious rats

The present study was to investigate and evaluate the dynamic changes of calcium homeostasis of soleus muscle spindle for the exploration of the potential mechanisms of muscle spindle degeneration induced by hindlimb unloading. We systematically observed the changes in immunoreactivity of calbindin D28K (CaBP-D28K), intracellular resting calcium in intrafusal fibers of soleus muscle spindle, and the responsiveness of muscle spindles to ramp-and-hold stretches after short- and long-term (3, 7, 14 d) hindlimb unloading. The immunoreactivity of CaBP-D28K started to decrease after 7-d hindlimb unloading, while its decrease was obviously different compared with the control group 14 d following the hindlimb unloading. The resting calcium concentration was increased significantly at 3 d, and reached the peak level 14 d after the hindlimb unloading. The responsiveness of muscle spindles, assessed by investigating Index3-L(2), Index3-L(3), and Index5, to ramp-and-hold stretches started to decrease during the period of 7-d hindlimb unloading. All Indexs, in particular Index3-L(3) and Index5, were significantly decreased at 14 d after the hindlimb unloading. The data suggest the disturbance of calcium homeostasis in intrafusal fibers during the exposure to hindlimb unloading might gradually influence the structure and function of muscle spindles.

Adaptation of rat soleus muscle spindles after 21 days of hindlimb unloading

Spindle discharges are affected by muscle unloading, and changes in passive stiffness of the muscle-tendon unit may contribute to the changes in spindle solicitation. To test this hypothesis, we determined the spindle sensitivity from electroneurograms of the soleus nerve, and, concomitantly, we measured the incremental passive muscle tension. Both measurements were done from ramp and hold stretches imposed to the soleus muscle after the Achilles tendon was severed. The ratio between the spindle sensitivity and the passive stiffness gave a "spindle efficacy index" (SEI). The experiments were conducted on control rats (C, n = 12) and on rats that had undergone hindlimb unloading (HU, n = 12) for 21 days. The muscle threshold lengths for electroneurogram to discharge (neurogram length, Ln) and for detecting passive tension (slack length, Ls) were determined, and, when these lengths differed, the stretches were imposed at these two initial lengths. The contralateral muscles were used to count muscle spindles and spindle fibers (ATPase staining) and to identify MyHC isoforms by immunostaining. Ln and Ls values were identical for the C muscles, while after HU, Ln was significantly shorter than Ls, which indicated that spindle afferents were more sensitive since they discharged before any passive tension was developed by the soleus muscle. At Ln, spindle sensitivity and passive stiffness did not differ for C and HU muscles. Consequently, when calculated at this relatively short initial muscle length, the SEI was maintained (or even slightly increased) after HU. This held under dynamic conditions (ramp phase of the stretch) and under static conditions (hold phase of the stretch). At Ls, the dynamic and static incremental stiffness values increased significantly after HU. Under dynamic conditions, the spindle sensitivity also increased after HU but to a less degree than incremental stiffness, which led to a significant decrease in SEI. Under static conditions, the spindle sensitivity presented a high increase, and, consequently, SEI was not modified. These functional changes were associated with structural adaptations: HU did not alter the total number of muscle spindles, but the number of spindles containing three nuclear chain fibers increased significantly. The main change in intrafusal MyHC content concerned the slow type I MyHC isoform. In conclusion, after a period of muscle unloading, the spindle discharges were maintained or even enhanced in several experimental conditions. This may be due to a better transmission of the external stretch to muscle spindles through stiffer elastic structures but also to own muscle spindle adaptations which reinforce the spindle sensitivity, notably under static conditions.

Effects of hypodynamia-hypokinesia on the muscle spindle discharges of rat soleus muscle.

The aim of this study was to determine whether Ia and II fiber discharges of soleus muscle spindles were modified after a 14-day period of hypodynamia (absence of weight bearing) and hypokinesia (reduction of motor activity). Fifty-one and 38 afferent fibers were studied, respectively, in control and hypodynamia-hypokinesia (HH) groups. Under deep anesthesia (pentobarbital, 30 mg/kg), a L3-L6 laminectomy was performed. Unitary potentials from the L5 dorsal root were recorded in response to ramp-and-hold stretches applied at two stretch amplitudes (3 and 4 mm) and four stretch velocities (6, 10, 15, and 30 mm/s) and to sinusoidal stretches applied at four stretch amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz). In both animal groups, the Ia fibers showed higher dynamic index values, smaller linear range, and higher vibration sensitivity than the II fibers. They also exhibited a pause in their discharges during the stretch release contrary to II fibers, which displayed no pause in their responses. After HH, our results showed that for both fiber types all parameters measured under ramp-and-hold stretches (except the static sensitivity) were significantly increased and under sinusoidal stretches, the vibration sensitivity increased, and the response amplitude only increased at 0.12-mm stretch amplitude. The linear range of Ia afferents was limited to 0.12 mm, whereas it was unchanged for the II fibers. After HH, the stretches could be better transmitted to the muscle spindles, probably resulting from changes in passive mechanical properties of the soleus.

Characterization of spindle afferents in rat soleus muscle using ramp-and-hold and sinusoidal stretches.

The discharge properties of 51 afferents were studied in the rat soleus muscle spindles. Under deep anesthesia using a pentobarbital sodium solution (30 mg/kg), a laminectomy was performed and the right L(4) and L(5) dorsal and ventral roots were transected near their entry into the spinal cord. In situ, the minimal (L(min)) muscle length [3 +/- 0.08 (SE) cm] of the soleus was measured at full ankle extension. Unitary potentials from the L(5) dorsal root were recorded in response to ramp-and-hold stretches applied at 3 mm (S3) and 4 mm (S4) amplitudes and four stretch velocities (6, 10, 15, and 30 mm/s), sinusoidal stretches performed at four amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz), and vibrations applied at 50-, 100-, and 150-Hz frequencies. These two kinds of stretches were performed at three different muscle lengths (L(min+10%), L(min+15%), and L(min+20%)), whereas vibrations were applied at L(min+20%) muscle length. Conduction velocity of the fibers was calculated but did not allow to discriminate different fiber types. However, the mean conduction velocity of the first fiber group (43.3 +/- 0.8 m/s) was significantly higher than that of the second fiber group (33.9 +/- 0.9 m/s). Three parameters allowed to differentiate the responses of primary and secondary endings: the dynamic index (DI), the discharge during the stretch release from the ramp-and-hold stretches, and the linear range and the vibration sensitivity from sinusoidal stretches. The slope histogram of the linear regression based on the DI and the stretch velocity was clearly bimodal. Therefore the responses were separated into two groups. During the stretch release at a velocity of 3 mm/s, the first response group (n = 26) exhibited a pause, whereas the second (n = 25) did not. The linear range of the second ending group (0.12-1 mm) was broader than that of the first (0.12-0.25 mm). The first ending group showed a higher sensitivity to high-vibration frequencies of small amplitude than the second. In comparison with the literature, we can assert that the first and the second ending groups corresponded to the primary and secondary endings, respectively. In conclusion, our study showed that in rat soleus muscle spindles, it was possible to immediately classify the discharge of Ia and II fibers by using some parameters measured under ramp-and-hold and sinusoidal stretches.

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (±0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (±0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.

Vibration sensitivity of cat muscle spindles at short muscle lengths.

Experiments are described in which the vibration sensitivity was tested for primary and secondary endings of soleus muscle spindles of the anaesthetised cat. The vibratory stimulus was applied longitudinally to the muscle tendon and, after locating the site of the spindle in the muscle, applied transversely to muscle fibres directly overlying the spindle. All measurements were made with the muscle slack, at 20 mm shorter than its maximum physiological length (Lm-20 mm). Spindles were separated into two groups: spontaneous spindles, which maintained background activity at this length, and silent spindles, which did not. Two forms of muscle conditioning were used: hold-long, in which the muscle was stretched by 5 mm, contracted and then returned to the test length, and hold-test, in which a conditioning contraction was given at the test length. After hold-test, most spindles responded to longitudinal vibration; after hold-long, most did not. This difference in responsiveness was attributed to the presence or absence of slack in intrafusal and extrafusal fibres, due to the muscle's thixotropic property. When the vibration was applied transversely, at a site directly overlying the spindle, responses of silent spindles continued to show thixotropic behaviour, whereas responses of spontaneous spindles were almost independent of the form of muscle conditioning. It is proposed that the ability of spontaneous spindles to maintain background activity at short muscle lengths is due to connective tissue or elastic fibre links between the sensory ending and the spindle capsule. The vibratory stimulus, applied transversely, reaches the sensory ending via these strands which, as non-muscle tissue, do not alter their mechanical state as a result of muscle conditioning.

Regulation of soleus muscle spindle sensitivity in decerebrate and spinal cats during postural and locomotor activities.

1. In order to study fusimotor control in reduced preparations, soleus muscle spindle afferents were recorded in premammillary decerebrate cats (n = 15) during crossed extensor reflexes and, after spinalization, during locomotion produced by either clonidine or L-beta-3,4-dihydroxyphenylalanine (L-DOPA). The soleus muscle was oscillated sinusoidally (0.25 mm, 4 Hz) and the afferent mean firing rate and modulation were calculated. An increase in firing rate was assumed to arise from activity in dynamic gamma-motoneurones (dynamic gamma-drive) when associated with an increase in modulation to stretching, and in static gamma-motoneurones (static gamma-drive) when modulation decreased. 2. At rest in all preparations the firing rate and modulation in primary muscle spindle afferents were generally much higher than after de-efferentation (ventral root section), suggesting a predominant dynamic gamma-drive. Clonidine decreased and even eliminated this presumed resting gamma-drive in many afferents, both in the decerebrate (7 of 8) and the spinal (6 of 18) state. This effect on gamma-drive may account, at least in part, for its suppressive effect on spasticity in humans. 3. When locomotion commenced in clonidine-treated spinal cats, primary afferents generally fired with much higher mean rates (+121%) and lower sensitivities (-32%), suggesting a large increase in static gamma-drive (possibly accompanied by a small decrease in dynamic gamma-drive). These high rates were usually maintained tonically throughout the step cycle. However, a third of the afferents were silenced during locomotor contractions, and de-efferentation had no significant effect on their firing rates. Thus, for some spindles alpha-activity can occur without significant gamma-drive. 4. During locomotion in L-DOPA-treated spinal cats the inferred static gamma-drive only occurred phasically, coactivated with the EMG, though it could precede the EMG by 100-500 ms. In the flexion phase both the afferent rate and modulation were lower than before locomotion, suggesting a lack of effective gamma-drive. 5. Crossed extensor reflexes in decerebrate cats also produced a substantial increase in primary afferent firing rate (+187%) and decrease in sensitivity (-37%), again suggesting increased static gamma-drive (n = 18). This gamma-drive was largely independent of EMG activity and often occurred without alpha-activity. The mean firing rate of secondary muscle spindle afferents increased significantly during locomotion (with L-DOPA) and crossed extensor reflexes, again indicating increased static gamma-drive. Clonidine reduced or eliminated the gamma-drive in seven of eight afferents during crossed extensor reflexes. 6. In conclusion, although there are some common features, such as a predominant static gamma-drive in all walking preparations, the pattern of static and dynamic gamma-drive is not closely linked to alpha-activity under the conditions studied. As well as gamma-drive without alpha-activity, we have shown for the first time that alpha-motoneurones can be activated without significant gamma-drive to many spindles during behavioural tasks.

Glycogen depletion of the intrafusal fibers in a mouse muscle spindle during prolonged swimming.

This study investigated the recruitment of different types of intrafusal fibers during prolonged swimming at 60-75% of VO2max. We used 56 male adult mice and examined depletion of glycogen in soleus (Sol) and extensor digitorum longus (EDL) muscle spindles by visual inspection and a newly developed optical scanning method. More than 80% of all spindles from six mice consisted of four fibers: one type I nuclear bag (bag1) fiber, one type II nuclear bag (bag2) fiber, and two nuclear chain fibers. Glycogen content was estimated in muscle fibers from groups of six mice that had rested or swum for either 0.5, 1, 2, 4, or 8 h. The optical scanning intensity of periodic acid Schiff (PAS)-stained sections was correlated with their biochemically determined glycogen content (r = 0.93). Both methods showed fundamentally the same result: each type of intrafusal fiber has its own typical recruitment pattern during exercise. In the initial phase (0-0.5 h), glycogen depletion was largest in nuclear bag1 fibers and insignificant in the bag2 and chain fibers. With the bag1 fibers having become fatigued, nuclear bag2 fibers mainly took over during the middle phase (2-4 h). During the last phase (4-8 h), only the glycogen content of chain fibers decreased significantly (4-8 h). There were significant correlations between the recruitment pattern of bag1 and extrafusal type I fibers in both Sol and EDL, between nuclear bag2 and type IIa fibers in Sol, and between nuclear chain and type IIb fibers in EDL. This suggests that, during moderately intense exercise, glycogen depletion occurs first in the slow, then the intermediate, and, finally, the fast intrafusal fibers.

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.

The responses of secondary endings of cat soleus muscle spindles to succinyl choline.

This report describes the effects of succinylcholine (SCh) on the secondary endings of cat soleus muscle spindles and attempts to explain them in terms of the action of the drug on intrafusal fibres. All but 2 of 41 secondary endings studied in detail showed a significant response to a single intravenous injection of 200 micrograms kg-1 SCh. This consisted of a rise in the resting rate or development of a resting discharge if the spindle had previously been silent and an increase in the response to stretch. The increases in the responses to stretch were weaker than those observed for primary endings of spindles, but were much larger than those of tendon organs, which showed very little effect with this concentration of drug. The response to SCh showed two features consistent with its action being mediated via an intrafusal muscle fibre contraction rather than a direct depolarising action on the afferent nerve ending. In the presence of SCh, secondary endings were able to maintain a discharge during muscle shortening at rates, on average, more than 5 times greater than under control conditions. Secondly, the increase in spindle discharge produced by SCh showed a length dependence similar to that for fusimotor stimulation. Further support for the action of SCh being principally via an intrafusal fibre contraction was provided by the observation that its effects were abolished by the neuromuscular blocker gallamine triethiodide.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of intrafusal neuromuscular junctions of cat muscle spindles with gallamine

This is a report on the resistance to block of the motor terminals on intrafusal fibres of cat soleus muscle spindles using the drug gallamine triethiodide (Flaxedil). To minimize diffusion barriers and to permit accurate measurements of time courses, rather slow rates of gallamine infusion were used (0.15 mg min-1). The main finding made was that after gallamine infusion, when extrafusal tension had dropped to half, all dynamic fusimotor effects and eight of twenty static effects had fallen to 40% or less of their control value. The remaining static effects persisted at 60-80% of their control value. Where fusimotor fibres were stimulated together with one or two skeletomotor fibres, the influence of the skeletomotor axons was significant only after spindle biasing had fallen to low levels. When gallamine infusion was stopped extrafusal tension returned to control levels within 20-75 min, depending on the length of the block, while fusimotor responses did not fully recover within the recording period of up to 150 min. The combination for some fusimotor responses of an early fall and a late recovery when compared with extrafusal tension, suggested a greater sensitivity of these endings to the drug. A comparison of spindle responses to the drug succinyl choline (SCh) and to fusimotor stimulation in the presence of gallamine showed that SCh responses were rapidly reduced by gallamine and had a long recovery time course, as were some fusimotor responses. From this it is argued that fusimotor effects with a high sensitivity to gallamine blockade were associated with nuclear bag fibre contractions and the more resistant effects with nuclear chain fibre contraction. It is generally believed that intrafusal neuromuscular junctions are more resistant to neuromuscular blockers than extrafusal junctions. The present experiments provide evidence to the contrary for some intrafusal junctions. Since muscle relaxants are often used in general anaesthesia it is interesting to speculate about the recovery of function of proprioceptive reflexes and of kinaesthesia during the immediate post-anaesthetic period, in view of the large difference in recovery time for transmission at intrafusal and extrafusal junctions.

Correlating resting discharge with small signal sensitivity and discharge variability in primary endings of cat soleus muscle spindles

1. In a previous report we proposed that primary endings of cat soleus muscle spindles can be separated into two kinds. One kind, called by us silent endings, at muscle lengths shorter than Lm -10 (maximum body length -10 mm), fell silent after a 5 mm shortening step. Spontaneous endings, on the other hand, were able to resume a resting discharge after a brief pause at all muscle lengths down to Lm -20. This report examines further differences between the two kinds of endings. 2. There were consistent differences in the muscle length dependence of the maintained level of resting discharge of the two kinds of endings, measured after a conditioning contraction or a contraction followed by a shortening step. The resting discharge of spindles with spontaneous endings, after both forms of conditioning increased progressively with length. For silent endings, after a conditioning contraction, resting discharge fell slightly at longer lengths. 3. Discharge variability, measured at a number of muscle lengths, showed a dependence both on mean interimpulse interval and on spindle type, being higher in silent than spontaneous spindles. 4. Small signal sensitivity was measured with the use of 1 Hz sinusoidal stretches applied longitudinally to the tendon. Sine wave amplitude was adjusted to give a 30% depth of modulation of the resting discharge. Spontaneous endings were consistently less sensitive to the stretches than silent endings at all muscle lengths. Average sensitivities, measured over a range of lengths between Lm -4 and Lm -20 mm were 0.30 imp.s-1.microns -1 for spontaneous endings and 0.66 imp.s-1.microns -1 for silent endings.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle history dependence of responses to stretch of primary and secondary endings of cat soleus muscle spindles.

1. Responses were recorded from both primary and secondary endings of soleus muscle spindles in the anaesthetized cat during slow stretches of the muscle after conditioning contractions at different lengths. 2. After a 5 mm length step and a fusimotor-strength contraction given at the longer length, on return to the initial length the response to a slow test stretch (0.5 mm s-1) showed a change in slope midway through the stretch which was attributed to taking up of slack in intrafusal fibres. 3. The onset of the change in slope during the test stretch depended on the size of the conditioning step. With no conditioning length change, just a fusimotor-strength contraction, there was no slope change and the response consisted of an initial burst followed by a maintained high rate of discharge. 4. Following a conditioning length step, the point of onset of the slope change during the test stretch could be altered by stimulating single identified fusimotor fibres to the spindle. Stimulating some static axons produced large changes in the stretch response while other static axons and dynamic axons had only small effects. 5. Many secondary endings showed a delay in onset of their response to a test stretch, dependent on the size of the preceding conditioning step, signalling the presence of slack in much the same way as the primary endings. Other secondary endings, however, appeared to have stretch responses that were largely independent of muscle conditioning. 6. Muscle history-independent responses of secondary endings were associated with low axonal conduction velocities. It is proposed that secondary endings which remain unaffected by muscle conditioning lie on more distal regions of nuclear chain fibres in the S2-S5 position. Here they are stimulated during both the take-up of slack and the subsequent direct stretch of the intrafusal fibres.

Two kinds of resting discharge in cat muscle spindles

1. The behavior of primary endings of cat soleus muscle spindles was studied during shortening steps carried out at different muscle lengths. 2. Spindles were of two kinds: one, silent spindles, whose afferents fell silent after the shortening, at least over part of the range of lengths tested. The second, spontaneous spindles, resumed firing at all lengths. 3. For silent spindles, the duration of the silent period, measured at lengths where they did recover a resting rate, depended directly on muscle length and became shorter at longer lengths. This is what would be expected if the slack introduced in the spindle by the shortening step was removed more rapidly at longer lengths by the higher passive tension. For spontaneous spindles, on the other hand, the duration of the silent period after the shortening was largely independent of muscle length and depended on the spindle's rate of firing immediately before the shortening. 4. At intermediate lengths the discharge of slack spontaneous spindles remained unaffected by an isometric muscle contraction. It was therefore not possible to produce a pause in the discharge, behavior normally taken as typical of spindles. The discharge could be interrupted by the contraction if this was combined with a large shortening movement. 5. It is proposed that when intrafusal fibers are slackened by a shortening step, the resting discharge in spontaneous spindles is generated by a maintained depolarization of the annulospiral ending resulting from extension of the terminal coils by forces from within the receptor. A shortening contraction compresses the spirals to interrupt the discharge. The sensory endings of silent spindles remain below threshold until the spirals have been opened out sufficiently by external stretch.

The dependence of the response of cat spindle Ia afferents to sinusoidal stretch on the velocity of concomitant movement.

1. The responses of de-efferented soleus muscle spindle primary afferents to 1 Hz sinusoidal stretches, which were superimposed on triangular background movements of intermediate amplitude (1.2 mm, half peak-to-peak) and widely varying speed, were recorded in anaesthetized cats. 2. Compared with control responses to the same sinusoids applied at fixed mean muscle length, the sensitivity to small (50 and 100 microns, half peak-to-peak), but not to large (1000 microns), sinusoidal movements was dramatically reduced during concomitant stretching, unless the background movements were extremely slow (well below 0.005 resting lengths per second). 3. For small stretches (50 and 100 microns) the reduction of sensitivity caused by background movement depended on the speed of this movement. For the highest velocity studied (1.6 mm s-1, corresponding to about 0.03 resting lengths per second) sensitivity dropped to below 10% of the control values. 4. The functional implication is that the sensitivity of spindle Ia afferents to small irregularities of voluntary movements (of any but the slowest speeds) may well be very much lower than it has hitherto been inferred from the striking sensitivity to minute disturbances at fixed mean muscle length. The present finding clearly puts extra demands on the gain of any spinal or central reflex actions of the sensory feedback from spindle afferents. 5. The effect is interpreted in terms of the widely accepted cross-bridge hypothesis of spindle small-movement sensitivity. The result suggests that in de-efferented intrafusal muscle fibres, which are subjected to imposed stretches, connected cross-bridges (conveying a friction-like property to the contractile fibre poles) may exist not only in a state of permanent attachment, but also in a dynamic equilibrium between stretch-induced detachment and reattachment. Indirect evidence further suggests that the duration of this disruption and reattachment cycle is of the order of 1 s.