Muscle Spindle Primary Endings Research Articles

Carbonic anhydrase activity in the peripheral nervous system of rat: the enzyme as a marker for muscle afferents

The distribution of carbonic anhydrase (CA) activity was determined histochemically using Hansson's cobalt phosphate method in cross-sections of peripheral nerves from rats. As the studied nerves contain either efferent, proprioceptive or exteroceptive myelinated fibres, our survey particularly focused on the question whether CA-reactive nerve fibres are functionally related. Intense CA activity was detected in all large diameter (8–12 μm) muscle afferents. The amount of similarly reactive cutaneous afferents was negligibly low (3.6%). Efferent fibres displayed only weak CA activity, which was confined to the small myelinated fibres (3–6 μm). Moderate staining could be assessed in medium-sized (4–11 μm) proprioceptive fibres. The same reactivity occurred in a sizeable percentage (11.4%) of exteroceptive afferents. Their diameters ranged from 4 to 11 μm. These results indicated, that high enzyme activity is found predominantly in large-calibre proprioceptive afferents, which according to Hunt's classification were identified as group IA and IB fibres. Further confirmation for our observations was obtained by demonstrating high levels of enzyme activity in primary nerve endings of muscle spindles (IA fibres) and in axon terminals of Golgi tendon organs (IB fibres) constantly. Finally possible functions for neuronal CA are discussed with respect to its high activity in a functionally related subpopulation of nerve fibres.

Alteration of proprioceptive messages induced by tendon vibration in man: a microneurographic study.

The activities of single proprioceptive fibres were recorded from the lateral peroneal nerve using transcutaneously implanted tungsten microelectrodes. Unitary discharges originating from muscle spindle primary and secondary endings and Golgi tendon organs were identified by means of various physiological tests. The sensitivity of proprioceptors to mechanical vibrations with a constant low amplitude (0.2-0.5 mm) applied at various frequencies to the tendon of the receptor-bearing muscle was studied. Muscle spindle primary endings (Ia fibres) were found to be the most sensitive to this mechanical stimulus. In some cases their discharge could be driven in a one-to-one manner up to 180 Hz. Most of them also fired harmonically with the vibration up to 80 Hz and then discharged in a subharmonic manner (1/2-1/3) with increasing vibration frequencies. Muscle spindle secondary endings (II fibres) and Golgi tendon organs (Ib fibres) were found to be either insensitive or only slightly sensitive to tendon vibration in relaxed muscles. The effects of tendon vibration on muscle spindle sensory endings response to muscle lengthening and shortening induced by imposed constant velocity or sinusoidal movements of the ankle joint were studied. Modulation of the proprioceptive discharge frequency coding the various joint movement parameters was either completely or partly masked by the receptor response to vibration, depending on the vibration frequency. Moreover, vibrations combined with sinusoidal joint movements elicited quantitatively erroneous proprioceptive messages concerning the movement parameters (amplitude, velocity). The sensitivity of the Golgi tendon organs to vibration increased greatly when the receptor-bearing muscle was tonically contracted. These data confirm that vibration is able to preferentially activate the Ia afferent channel, even when the vibration amplitude is low. They define the frequency sensitivity of the muscle spindle primary and secondary endings and the Golgi tendon organs. They also show that the physiological messages triggered by ongoing motor activities undergo a series of changes during the exposure of muscles to vibration.

Monosynaptic projections of single muscle spindle afferents to neck motoneurons in the cat

Monosynaptic connections to dorsal neck motoneurons of the cat from single afferents supplying primary endings of neck muscle spindles were studied using spike-triggered averaging techniques. Single-fiber EPSPs were detected in only 11 of the 112 afferent-motoneuron pairs examined. The average amplitude of single-fiber EPSPs recorded in motoneurons with a membrane potential of greater than -40 mV was 49 microV. Motoneurons receiving functional contacts from a single afferent were confined to a small rostrocaudal zone within the motor nucleus. The low frequency of single-fiber EPSPs in neck motoneurons could not be attributed to the absence of projections to the ventral horn or to damage to either the afferents or motoneurons. Our results suggest, therefore, that single afferents from neck muscle spindles make functional contacts with a small fraction of neck motoneurons, unlike the arrangement seen in more commonly studied hindlimb muscle systems (Henneman and Mendell, 1981).

Structure of the intraspinal projections of single, identified muscle spindle afferents from neck muscles of the cat

The morphology and frequency of collaterals originating from single afferents supplying primary endings of muscle spindles in dorsal neck muscles have been examined using intra-axonal injections of HRP. Within the segment in which the afferent entered the spinal cord, one collateral was found for every 3.3 mm of stained axon. In contrast, afferents--one of more segments rostral to the segment in which they entered the spinal cord--had fewer collaterals: One collateral was found for every 6.3 mm of stained axon. The branching structure and terminal distribution of the collaterals were generally similar regardless of the muscle from which the afferent originated and the segment in which the collateral was found. Boutons were found in 2 zones: One of these was located in the intermediate zone, within and around the central cervical nucleus, and the other was found in laminae VIII and IX, including the motoneuron nuclei. The ventral termination zone of collaterals in the same segment as their parent axon entered the spinal cord was larger and had more boutons than the same projection of collaterals whose parent axon entered the spinal cord 1 or 2 segments caudal to the segment in which the collateral was found. These results indicate that afferents supplying primary endings of neck muscle spindles are more likely to contact neurons in the same segment in which the afferent enters the spinal cord than in more rostral segments. However, even within the same segment in which the afferent enters the spinal cord, the projection of neck muscle afferents to the ventral horn is less dense than the corresponding projection of hindlimb muscle spindle afferents in the lumbosacral spinal cord.

Instability in human forearm movements studied with feed-back-controlled electrical stimulation of muscles.

1. Amplitude-modulated electrical stimulation was applied to the elbow flexors and extensors to produce movements of the forearm in normal subjects. The parameters of the modulating (command) signal were set in isometric trials so as to produce equal and opposite background torques, and equal and supportive torque modulations. 2. Bode plots relating forearm movement to command signal (modulating) frequency showed the muscle-load to have a low-pass characteristic similar to that previously described in the cat, and a slightly larger bandwidth than described previously in man. 3. The transduced forearm signals were fed back to provide the command signal to the stimulators via a filter which mimicked the transfer function of muscle spindle primary endings. In effect this replaced the neural part of the reflex arc with an accessible model, but left the muscle-load effector intact. 4. All six subjects developed forearm oscillations (tremor) when the loop gain exceeded a threshold value. The mean tremor frequency at onset was 4.4 Hz, which was similar to that of the equivalent vibration-evoked tremor (previous paper, Prochazka & Trend, 1988). 5. With the linear spindle model, oscillations tended to grow rapidly in amplitude, and the stimuli became painful. The inclusion of a logarithmic limiting element resulted in stable oscillations, without significant alterations in frequency. This allowed us to study the effect on tremor of including analog delays in the loop, mimicking those associated with peripheral nerve transmission and central reflexes. In one subject, loop delays of 0, 20, 40 and 100 ms resulted in tremor at 4.0, 3.6, 3.0 and 2.1 Hz respectively, as quantified by spectral analysis. 6. By considering separately the phase contributions of the different elements of the reflex arc, including delays, it became clear that muscle-load properties were important in setting the upper limit of tremor frequencies which could conceivably be supported by reflexes. 7. The results support the conclusion of the related vibration study (Prochazka & Trend, 1988), that for moderate levels of background co-contraction, the contribution of stretch reflexes to tremor at the elbow should be sought in the 3-5 Hz range. Exaggerated long-latency reflexes would be expected to reduce these baseline frequencies by 1 or 2 Hz.

Instability in human forearm movements studied with feed-back-controlled muscle vibration.

1. Frequency-modulated vibration was applied to the elbow flexor and extensor tendons to produce reflex movements of the forearm in normal subjects. The modulating (command) signal caused equal and opposite deviations from the 40 Hz carrier frequency so that when flexor vibratory frequency increased, extensor frequency decreased, and vice versa. 2. It is argued that the movements resulted largely from the reflex action of muscle spindle primary afferents whose firing frequency had been 'taken over' and modulated by the vibration. 3. Bode plots relating forearm movements to command signal (modulating) frequency showed the transfer function of the Ia afferent-CNS-muscle-load system to have a low-pass filter characteristic. The phase lag of movement on command increased progressively with command signal frequency, exceeding 180 deg at 3-4 Hz. 4. The transduced forearm movements were fed back to provide the command signal to the vibrators (and thus indirectly to the spindle afferents) via a filter mimicking the dynamic responsiveness of muscle spindle primary endings. Our aim was to 'break into' the reflex arc, and re-route it so that we could artificially vary the gain without significantly altering the dynamics of the pathway. 5. Nearly all subjects developed forearm oscillations (tremor) when the gain exceeded a threshold value. Subjects varied widely in their threshold, though for a given subject the threshold remained fairly constant from day to day. The results suggest that reflexly active individuals may not have a large safety margin with respect to forearm instability. 6. The frequency range of the oscillations observed in seven subjects was 3-8 Hz. The frequencies depended upon the level of flexor-extensor co-contraction, and increased from 3 to 5 Hz at 10% co-contraction to 5-8 Hz at 100% co-contraction. An analysis of the mechanical impedance of the arm provided estimates of tremor frequencies consistent with these results. 7. These unexpectedly low tremor frequencies led us to propose that it may be erroneous to expect stretch reflexes to contribute to forearm tremor in the range 8-12 Hz (e.g. physiological and 'enhanced' physiological tremors). Rather, their contributions should be sought in the range 3-8 Hz (e.g. pathological tremors such as those of Parkinson's and cerebellar disease).

Aftereffects in the responses of cat muscle spindles and errors of limb position sense in man

1. This is a report of two series of experiments, one, observations on muscle spindles in the anesthetized cat, the other, based on the animal observations, concerned with limb-position sense in human subjects. 2. When the cat soleus muscle is contracted at a short length and then after a brief period is brought to an intermediate length, the integrated afferent activity recorded in dorsal roots is maintained at a high level. If the contraction is applied while the muscle is held at a long length, the subsequent level of afferent activity back at the intermediate length is much lower. A similar result is obtained if large amplitude vibration of the muscle is used instead of contraction. Recordings from single afferents confirm that these alterations in discharge could be attributed to the primary and secondary endings of muscle spindles. 3. Following the proposal that the resting discharge of muscle spindles contributes to the sense of limb position in man, we have used a sequence of contractions or periods of vibration of biceps and triceps brachii followed by an arm-matching task to look for errors in limb position consistent with the patterns of muscle afferent discharge observed in the animal experiments. 4. Blindfolded subjects were told to contract biceps of one, the test arm, with the arm flexed. After the arm had relaxed, it was placed by the experimenter at an intermediate angle that the subject was asked to match with the other, indicator arm. Then the subject was asked to contract triceps with the arm extended, and the matching task was repeated. 5. The hypothesis was that contracting a muscle while it was short raised the level of afferent discharge coming from it when the arm was moved to an intermediate position so that the muscle would be perceived as more stretched and the arm felt to be displaced further than it really was. Thus, when biceps was contracted with the arm flexed, this would lead the arm in the intermediate position to be perceived as more extended than it really was. When triceps was contracted with the arm extended, this would lead the arm in the intermediate position to be perceived as more flexed than it really was.(ABSTRACT TRUNCATED AT 400 WORDS)

Fiber size distribution of myelinated and unmyelinated axons in the medial and posterior articular nerves of the cat's knee joint.

The distribution of fiber size in the posterior articular nerve (PAN) and medial articular nerve (MAN) of the cat's knee joint was studied by light and electron microscopy. The myelinated fibers of the PAN ranged from approximately 1 to 18 micron, with maxima at 3-4 micron and 8-9 micron. According to the classification of Boyd and Davey (1968), the PAN contained about 34% Group III fibers, 56% Group II fibers, and 10% Group I fibers. In contrast, the MAN showed a unimodal, skewed distribution, with a range from approximately 1 to 14 micron and a maximum at 3-4 micron. According to the Boyd and Davey classification, the PAN contained about 69% Group III fibers, 30% Group II fibers, and 1% Group I fibers. Unmyelinated fibers examined in the MAN showed a unimodal distribution, with a range of from 0.1 to 1.5 micron and a maximum at 0.4-0.5 micron. To differentiate between afferent and sympathetic fibers, a sympathectomy or ganglionectomy was performed on one side. The fiber size distribution indicated a considerable overlap in the diameter of the afferent and sympathetic unmyelinated fibers. Most sympathetic fibers had a diameter of between 0.8 and 0.9 micron, whereas afferent fibers showed a maximum at 0.3-0.4 micron. When data were combined for myelinated and unmyelinated fibers in the PAN, about 74% of the afferent fibers were found to belong to Groups III and IV; they are thought to terminate in noncorpuscular endings. The other 26% were found to belong to Groups I and II; they terminate in corpuscular receptors and muscle spindle primary and secondary endings.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological changes in the primary sensory neuron following subchronic soman and sarin: Alterations in sensory receptor function

Cats were administered soman or sarin either in a single high dose (1 mg/kg) with pretreatment or in multiple sublethal doses to determine whether these potent organophosphorus agents could produce a delayed neurotoxicity and what, if any, pathophysiological changes occurred in peripheral sensory receptors. Neither soman nor sarin, when administered as a single high dose, produced a delayed neurotoxicity as observed behaviorally for up to 60 days. There were also no observable signs of delayed neurotoxicity when these agents were administered in multiple doses. Functional tests of proprioceptors and mechanoreceptors were performed on the cats which received multiple sublethal doses of either soman or sarin. It was found that the discharge rates of muscle spindle primary endings were depressed while the discharge rates of secondary endings were facilitated following the administration of either soman or sarin. The discharge rates of slowly adapting type 1 mechano-receptors were also depressed. The total number of identified mechanoreceptors was reduced in both the soman- and sarin-treated animals. Conduction velocities of several of the muscle spindle and mechanoreceptor afferents were significantly decreased. The alterations in muscle spindle function may be due to changes in the muscle resulting from acetylcholinesterase inhibition. Another explanation for the changes in both muscle spindle and mechanoreceptor function may be the direct effect of the organophosphorus agents on the afferents themselves, thus altering their excitability.

Effects of ischaemia upon reflex electromyographic responses evoked by stretch and vibration in human wrist flexor muscles.

1. The reflex electromyographic responses evoked in a wrist flexor muscle, flexor carpi radialis (f.c.r.), by forcible extension of the wrist ('stretch') and by vibration of the flexor tendon have been studied in normal subjects. Reflexes were elicited during the maintenance of a low level of voluntary flexor contraction (5% maximum). Stretch regularly produced a relatively prolonged (ca. 100 ms duration) increase in e.m.g. activity which was usually divisible into short-latency (ca. 25 ms, M1) and long-latency (ca. 50 ms, M2) peaks. Vibration produced a single, phasic peak, at short latency, with no sign of an accompanying long-latency wave comparable to the M2 stretch response. 2. Ischaemia was induced by inflation of a blood-pressure cuff around the upper arm and its effects upon the reflex patterns were studied. During ischaemia M1 stretch responses showed a more rapid and pronounced decline than did M2 responses and were abolished before voluntary power was appreciably affected. Vibration-evoked short-latency peaks changed in an essentially parallel manner to M1 stretch reflexes. During recovery from ischaemia M2 reflexes were restored before short-latency responses. 3. The patterns of reflex reductions in e.m.g. upon withdrawal of stimulation were also studied. Such troughs in activity, under non-ischaemic conditions, regularly commenced at short latency and were of relatively small amplitude. The records of several of the subjects, and particularly ones obtained during ischaemia, suggested that release of stretch (with concomitant stretch of antagonists) could elicit an additive, long-latency decline in e.m.g. The existence of any such separate, delayed component was never observed upon termination of vibration. 4. Measurements of changes in the latencies and durations of reflex components, accompanying the progression of ischaemia, indicated that depression of early reflex activity resulted in part from increases in the latencies of these initial peaks but predominantly reflected simultaneous and separate reductions in their amplitudes. 5. The generation of short-latency reflexes by stretch and vibration, both of which stimuli powerfully excite muscle spindle primary endings, and the marked susceptibility of these responses to ischaemia supports their being mediated by group Ia afferents. The contrasting behaviour of M2 stretch responses, both regarding their absence with vibration and their resistance to ischaemia, suggests that they depend crucially upon a separate group of reflex afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Event-related cross-correlations between spike trains illustrated on interactions between motor units and muscle spindle afferents.

A method is presented for computing correlation coefficients of two (or more) output spike trains in temporal relation to one (or more) input even trains. These event-related correlation functions are computed by convolving the output spike trains, represented as point processes, with rectangular pulses of selectable width, and by then calculating linear correlation coefficients for the pairs of amplitude values obtained from the two convolved processes in temporal relation to the input events. The merits of this technique are illustrated on stimulus trains delivered to motor units (MUs) and output spike trains recorded from muscle spindle afferents of the same cat hindlimb muscle. The correlation functions obtained show the temporal course of the correlated firings of the two afferents (mostly Ia afferents from primary muscle spindle endings) as a function of time from MU activation; they are compared with the conventional cross-correlation histograms (CCHs) between afferents and with peri-stimulus time histograms (PSTHs) between stimulus and afferent firing patterns. Stimulus-related cross-correlation functions as displayed here can be calculated for any three spike trains. Possible extensions of the method to larger numbers of input and output channels are also discussed.

Stretch and vibration reflexes of wrist flexor muscles in spasticity.

The surface electromyographic (EMG) reflex responses of the voluntarily contracting flexor carpi radialis evoked by 'stretch' and by tendon vibration have been compared in patients with spasticity of the upper limb, arising from upper motor neuron lesions, and normal subjects. Reflex responses to 'stretch' comprised increases in EMG activity lasting up to 100 ms which were often divided into 'short' and 'long'-latency peaks. The short-latency responses of spastic patients were increased in size compared with those of normal subjects whereas later activity was commonly reduced or absent. In both groups vibration elicited short-latency, essentially phasic responses with activity falling back to or below the background level within 50 ms despite continuing stimulation. These initial reflex responses were exaggerated in the spastics as compared with the normals. In the relaxed state 'stretch' and vibration either failed to elicit reflex responses in normal subjects or reflexes were of small amplitude; in spastic patients both modes of stimulation regularly evoked well developed responses. These findings with 'stretch' and vibration, both of which forms of stimulation powerfully excite primary endings of muscle spindles, support the view that group Ia afferent-mediated reflex action is enhanced in spasticity. The observation that the normal long-latency responses evoked by stretch, which have been attributed to the action of spindle group II afferents (Matthews, 1984a) additionally excited with this stimulus, are depressed in many spastic patients is consistent with reduced group II effects. Observed abnormalities of stretch reflex behaviour did not readily explain the severity of accompanying spasticity of individual patients.

Observations on the primary sensory ending of tenuissimus muscle spindles in the cat.

The arrangement of preterminal and terminal axon branches in the primary sensory endings of cat tenuissimus muscle spindles was studied using whole-mount and serial-section techniques. Although in every case one first-order preterminal branch was supplied exclusively to the bag1 type of intrafusal muscle fibre, the preterminal branching patterns differed considerably in detail. Terminals varied widely in size and location. Their precise form varied according to their position on the intrafusal muscle fibres rather than their relationship to preterminal branches. Terminals derived from separate preterminal branches remained separate and did not fuse with themselves or each other. Individually bag1 fibres had most terminals, chain fibres least. The surface of the muscle fibres were differentially indented by the terminals, least in bag1 fibres and most in chain fibres. The results are discussed in relation to mechanosensory transduction and to the factors involved in determining the form of the primary ending.

Aftereffects in the responses of cat muscle spindles.

Responses have been recorded from primary endings of muscle spindles in the cat soleus muscle. Changes in spindle responsiveness were measured following a period of conditioning that consisted of a series of rapid stretches or of tetanic ventral root stimulation. In the testing procedure the response of a single spindle afferent was recorded to stimulation of a dynamic fusimotor axon during a slow stretch. Changes in gross afferent discharge coming from the muscle were measured by integrating the activity recorded in dorsal roots. If, after conditioning stretches, the muscle was immediately returned to its initial length, the spindle responded to the test fusimotor stimulation with a high-frequency burst of afferent impulses. If the muscle was held stretched for 3 s after conditioning the response to the brief test tetanus was small or "depressed." It has been suggested that conditioning stretches result in detachment of stable crossbridges in intrafusal fibers and that these bridges then reform over the next few seconds at whatever length the muscle happens to have at the time. When it is long, shortening the muscle back to the initial length leads to the development of slack in intrafusal fibers because of the passive stiffness they have acquired from the presence of the stable bridges. Under these conditions a brief test fusimotor tetanus will lead to a depressed response because the slack must first be taken up before a full response can be generated. It was possible to reverse the depression by interposing an extrafusal contraction during the period between the conditioning and test sequences. It is suggested that lateral compression from the contracting extrafusal fibers and the stretch they impose as they relax reduces any intrafusal slack and thereby reduces the depression. A more quantitative measure of intrafusal slack than the test for depression is to determine the delay in onset of the afferent response to a longer fusimotor tetanus. The delay was short a long initial muscle lengths where, if the muscle was left undisturbed, it soon disappeared completely and spontaneously. It is suggested that at long lengths passive tension in the muscle tends to remove any slack in intrafusal fibers and therefore removes any after effects. The rise in resting discharge of muscle afferents after a conditioning tetanus applied to the ventral root ("postcontraction sensory discharge") can be accounted for by the same hypothesis.(ABSTRACT TRUNCATED AT 400 WORDS)

The response of primary muscle spindle endings to random muscle stretch: a quantitative analysis.

In cats the stretch responses of deafferented Ia afferent fibers from gastrocnemius muscle spindles were measured. The response properties were derived by correlating the action potentials to random mechanical stimuli applied to the gastrocnemius muscle. The muscle stretch consisted of band-limited, normally distributed random changes in muscle length. The upper cut-off frequency of the noise stimuli fc (i.e. the -3 dB point of the amplitude spectrum) was varied systematically between 1.5 and 600 Hz and the amplitude of the noise stimuli sigma (i.e. the standard deviation of the distribution of muscle displacements) between 1.5 and 1200 microns. The neuronal responses were analysed by averaging the stimulus signals centered on the Ia action potentials (the "peri-spike average", PSA). The cross-spectrum between stimulus and neuronal response was determined by computing the Fourier expansion of the PSA. Immediately before the action potentials the PSA decreased below mean muscle length and increased thereafter rapidly. As a rule, the action potentials were elicited close to the point of maximum stretch velocity. The cross-spectra revealed a monotonic increase in gain with stimulus frequency, corresponding to a power function with an average exponent of 0.86. The PSA-shape obtained at stimulus cut-off frequencies between 1.5 and 260 Hz remained fairly independent of fc when the abscissa was normalized with 1/fc. In the cross-spectra a maximum gain was always reached near the actual fc-value for upper cut-off frequencies up to 260 Hz. Above 260 Hz maximum gain remained independently of fc at about 260 Hz. The peak-to-peak displacement A of the PSA increased linearly with the logarithm of fc. The shapes of the PSAs and the cross-spectra did not depend on the noise amplitude sigma in the range above 20 microns. A was linearly related to sigma (about 1.6 to 2.4 . sigma for sigma greater than 10 microns and 75 Hz less than fc less than 450 Hz). A unifying mathematical description of the responses, valid for a wide range of stretch parameters, was obtained. We demonstrated that the application of one short run of random muscle stretch of an intermediate noise amplitude and medium cut-off frequency provides substantial information on the spindle stretch response and is recommended for investigations in which stable recordings cannot be obtained for a sufficient length of time.

Evaluation of the muscle relaxant properties of a novel beta-carboline, ZK 93423 in rats and cats.

The muscle relaxant action of ZK 93423 (6-benzyloxy-4-methoxymethyl-beta-carboline-3-carboxylate ethyl ester), a novel beta-carboline with agonistic properties at the benzodiazepine receptor, was examined by assessing its effect on the tonic electromyogram (EMG) activity of the gastrocnemiussoleus (GS) muscle of genetically spastic rats and on ventral root reflexes, presynaptic inhibition and fusimotor activity in the spinal cord of decerebrate cats. ZK 93423 (0.1-10.0 mg kg-1) depressed the tonic EMG activity in mutant rats in a dose-dependent manner. This effect was reversed by the benzodiazepine antagonist Ro 15-1788 (5.0 mg kg-1). Both ZK 93423 (0.5 mg kg-1) and diazepam (0.3 mg kg-1) enhanced the presynaptic inhibition of the GS muscle and associated dorsal root potentials in decerebrate cats in an almost identical manner. The actions of both drugs were reversed by Ro 15-1788 (5.0 mg kg-1). ZK 93423 (0.5 mg kg-1) and diazepam (0.3 mg kg-1) depressed the activity of both static and dynamic fusimotor neurones, as deduced from changes in the afferent responses of muscle spindle primary endings to low frequency (1 s-1) sinusoidal stretching. The depressant action of diazepam (0.3 mg kg-1) was weaker than that of ZK 93423 (0.5 mg kg-1). The actions of both drugs were reversed by Ro 15-1788 (5.0 mg kg-1). ZK 93423 (0.5 mg kg-1) failed to alter the magnitude of monosynaptic ventral root reflexes evoked by electrical stimulation of a flexor and an extensor nerve, whereas diazepam (0.3 mg kg-1) had a depressant effect on both types of monosynaptic reflexes, which was antagonized by Ro 15-1788 (5.0 mg kg-1). Neither ZK 93423 (0.5 mg kg-1) nor diazepam (0.3 mg kg-1) depressed polysynaptic ventral root reflexes evoked by electrical stimulation of a flexor and a cutaneous nerve. The present results demonstrate that ZK 93423 exerts a potent muscle relaxant action due to a specific interaction with benzodiazepine receptors. However, its profile of actions on spinal motor mechanisms is not identical to that of diazepam.

Two types of cat static fusimotor neurones under separate central control?

The effect on the fusimotor system of an injection of the γ-aminobutyric acid (GABA) antagonist picrotoxin into the reticular part of the substantia nigra (SNR) was studied in muscle spindle primary and secondary endings of pretibial flexors in cats anaesthetized with ketamine. Changes in fusimotor action after drug administration were deduced from changes in bias and sensitivity to sinusoidal stretching of the receptor-bearing muscles when compared to the deefferented preperation. Intranigral injection of picrotoxin (2 μg) removed a tonic static fusimotor action from flexor muscle spindle primary endings without affecting the secondary endings in the same muscles. It is concluded that static γ motoneurones of cat flexors are of more than one functional type, and that the central nervous control of the static fusimotor input to cat flexor muscle spindles has selective access to static bag 2 fibres and nuclear chain fibres.

Effect of intrafusal muscle mechanics on mammalian muscle spindle sensitivity

Sensitivity differences between primary and secondary endings of mammalian muscle spindles under various conditions of stretch and fusimotor activation may be due to differences in their respective mechanoelectric transducers or to mechanical properties of the intrafusal muscle supporting those endings. This study of isolated cat muscle spindles examines the strain in individual intrafusal muscle fibers resulting from stretch and fusimotor stimulation. The degree of local stretch occurring at the sensory endings under these conditions was measured. The results support the hypothesis that the sensitivities of primary and secondary endings are quite similar. They are directly related to the local stretch of the underlying muscle which may be altered by changes in muscle stress and stiffness.

Illusory movements produced by electrical stimulation of low-threshold muscle afferents from the hand.

There is continuing controversy as to the role of muscle afferents in kinaesthesia. Moberg (1983) has recently claimed that much of the evidence previously used in favour of a kinaesthetic role for muscle afferents points to a crucial role for cutaneous afferents (remote from the joint but overlying the relevant muscle) and that there is no definite evidence for a contribution to kinaesthesia from 'musculotendinous' receptors. In the present study, trains of electrical stimuli delivered to the ulnar nerve at the wrist (through a surface probe, needle electrode or microelectrode within motor fascicles) produced illusory movements of the fingers and distortions in their position. These illusions occurred in the absence of overt movement (stimuli were 0.5-0.9 times motor threshold) and in the absence of cutaneous paraesthesiae. The velocity of the smooth illusory movements increased with the frequency of stimulation. The direction of the illusory movements was opposite to that produced by shortening of intrinsic muscles of the hand. Short-latency cerebral potentials were recorded from the contralateral scalp during the illusions. These illusions are most simply explained by the perception of discharges in low-threshold muscle afferents, probably those which innervate primary muscle spindle endings. Stimulation of the digital nerves sometimes produced a sensation of extreme position of one of the interphalangeal joints or of an apparent oscillation at the joint. The frequency of oscillation increased as the stimulus frequency increased. These results argue for a direct contribution to kinaesthesia from primary muscle spindle afferents though they do not exclude contributions from nonmuscle afferents.

The after-effects of stretch and fusimotor stimulation on the responses of primary endings of cat muscle spindles.

These experiments explore the after-effects of repetitive movements and dynamic fusimotor stimulation on the responses of primary endings of soleus muscle spindles in the anaesthetized cat. If immediately following a series of conditioning stretch and shortening movements, the muscle was held at the stretched length for 3 s before being returned to its rest length, the subsequent response to a brief dynamic fusimotor tetanus given during a slow test stretch produced only a small increase in spindle firing. If, on the other hand, the muscle was returned to its rest length immediately after the movements, the fusimotor tetanus evoked a much larger afferent burst. This difference in the size of the burst could only be observed if the fusimotor tetanus was given soon after onset of the test stretch. If it was delayed and given at a time when the test stretch passed through the length at which the muscle had been held stretched after the movements, there was no difference in the size of the afferent burst. If following the movements the muscle was held stretched for less than 3 s, the response to the subsequent tetanus was not fully depressed. Once the depressed condition had been achieved, the muscle had to be left undisturbed for up to half an hour before the response had recovered its original fully undepressed size. Conditioning repetitive stimulation of the fusimotor fibre was just as effective as using alternating movements in producing the effects. If the test tetanus, which was normally ten shocks in 50 ms, was made longer, the change in size of the impulse burst became a change in latency of onset of the response to the tetanus. Holding the muscle stretched at the end of the conditioning movements/tetanus produced a delay in onset of the response to the test tetanus without significantly altering its size. These observations have been interpreted as arising from development of stable cross-bridges between actin and myosin filaments in the intrafusal fibres. During repetitive movements or fusimotor stimulation, stable bridges become detached and during the subsequent 3 s they re-attach, at the length at which the muscle is being held after conditioning.(ABSTRACT TRUNCATED AT 400 WORDS)