Model-based prediction of fusimotor activity and its effect on muscle spindle activity during voluntary wrist movements

Muscle spindles provide critical information about movement position and velocity. They have been shown to act as stretch receptors in passive muscle, however, during active movements their behavior is less clear. In particular, spindle responses have been shown to be out-of-phase or phase advanced with respect to their expected muscle length-sensitivity. Whether this apparent discrepancy of spindle responses between passive and active movements is due to fusimotor (γ-drive) remains unresolved, since the activity of fusimotor neurons during voluntary non-locomotor movements are largely unknown. We developed a computational model to predict fusimotor activity and to investigate whether fusimotor activity could explain the empirically observed phase advance of spindle responses. The model links a biomechanical wrist model to length- and γ-drive-dependent transfer functions of type Ia and type II muscle spindle activity. Our simulations of two wrist-movement tasks suggest that (i) experimentally observed type Ia and type II activity profiles can to a large part be explained by appropriate, i.e. strongly modulated and task-dependent, γ-drive. That (ii) the empirically observed phase advance of type Ia or of type II profiles during active movement can be similarly explained by appropriate γ-drive. In summary, the simulation predicts that a highly task-modulated activation of the γ-system is instrumental in producing a large part of the empirically observed muscle spindle activity for voluntary wrist movements.     

The functional role of different neural activation profiles during precision grip: An artificial neural network approach

A dynamic and recurrent artificial neural network was used to investigate the functional properties of firing patterns observed in the primary motor (M1) and the primary somatosensory (S1) cortex of the behaving monkey during control of precision grip force. In the behaving monkey it was found that neurons in M1 and in S1 increase their firing activity with increasing grip force, as do the intrinsic and extrinsic hand muscles implicated in the task. However, some neurons also decreased their activity as a function of increasing force. The functional implication of these latter neurons is not clear and has not been elucidated so far. In order to explore their functional implication, we therefore simulated patterns of neural activity in artificial neural networks that represent cortical, spinal and afferent neural populations and tested whether particular activity profiles would emerge as a function of the input and of the connectivity of these networks. The functional implication of units with emergent or imposed decreasing activity was then explored.Decreasing patterns of activity in M1 units did not emerge from the networks. However, the same networks generated decreasing activity if imposed as target patterns. As indicated by the emerging weight space, M1 projection units with decreasing patterns are functionally less involved in driving alpha motoneurons than units with increasing profiles. Furthermore, these units did not provide significant fusimotor drive, whereas those with increasing profiles did. Fusimotor drive was a function of the (imposed) form of muscle spindle afferent activity: with gamma (fusimotor) drive, muscle spindle afferents provided signals other than muscle length (as observed experimentally). The network solutions thus predict a functional dichotomy between increasing and decreasing M1 neurons: the former primarily drive alpha and gamma motoneurons, the latter only weakly alpha motoneurons.     

Somatosensory unit input to the spinal cord during normal walking

Chronic recording techniques in freely walking cats have been used to sample unitary activity from most large myelinated afferent classes. Cutaneous mechanoreceptors are highly sensitive and generate regular activity patterns predictable from their modalities. Knee joint afferents can fire briskly midrange locomotory movements but appear to be influenced by factors other than joint angle. Golgi tendon organs generate activity consistent with sensitivity to active muscle tension. Muscle spindle afferents do not appear to conform to any single functional pattern for all muscles. It is suggested that degree and rate of stretch are sensed by spindles (possibly under dynamic fusimotor bias) in extensor muscles which normally undergo isometric or lengthening contractions whereas rapidly modulated static fusimotor activity is employed to preserve spindle activity during the rapidly shortening contractions of flexor muscles. Both patterns may be represented in different spindles of bifunctional, biarticular muscles such as rectus femoris and sartorius.     

Different fusimotor reflexes from the ipsi- and contralateral hind limbs of the cat assessed in the same primary muscle spindle afferents

Experiments were performed in forty-five cats anaesthetized with alpha-chloralose. The aim of the study was to investigate a sample of primary muscle spindle afferents from triceps muscle with respect to their fusimotor reflex control from ipsi- as well as contralateral hind limb. Primary muscle spindle afferents of the triceps surae muscle were recorded from the mean rate of firing and the modulation of the afferent response to sinusoidal stretching of the triceps surae muscle was determined. Test measurements were made during tonic stretch of the ipsilateral PBSt, contralateral PBSt, contralateral triceps muscle or during extension of the intact contralateral hind limb. Control measurements were made with ipsi- and contralateral PBSt as well as contralateral triceps muscles relaxed and with contralateral hind limb in resting position. The occurrence and types of fusimotor effects were assessed by comparing test to control responses. The main finding of the present investigation was the great variability in type and size of the fusimotor effects evoked by different ipsi- and contralateral reflex stimuli. Both ipsi- and contralateral stimulations gave rise to predominantly dynamic, predominantly static or mixed static and dynamic fusimotor reflexes. In the same preparation, a given reflex stimulus often caused different reflex responses in different triceps surae primary spindle afferents. In the same afferent unit, different reflex stimuli usually produced fusimotor effects which differed from each other in type and/or size. In general, contralateral whole limb extension and stretch of contralateral PBSt muscles were more potent as reflex stimuli than stretch of the ipsilateral PBSt muscle. Stretch of the contralateral triceps surae muscle was, but for a few afferent units, ineffective as reflexogenic stimulus. It is concluded that the individualized receptive profiles of the primary muscle spindle afferents, which have been postulated in earlier investigations where the effects of different stimuli have been investigated on different cell populations, still seems to hold good when the stimuli are tested on the same units. The individuality of the receptive profiles of gamma-motoneurones is discussed in relation to different motor control hypotheses.     

Dynamic Neural Network Models of the Premotoneuronal Circuitry Controlling Wrist Movements in Primates

Dynamic recurrent neural networks were derived to simulate neuronal populations generating bidirectional wrist movements in the monkey. The models incorporate anatomical connections of cortical and rubral neurons, muscle afferents, segmental interneurons and motoneurons; they also incorporate the response profiles of four populations of neurons observed in behaving monkeys. The networks were derived by gradient descent algorithms to generate the eight characteristic patterns of motor unit activations observed during alternating flexion-extension wrist movements. The resulting model generated the appropriate input-output transforms and developed connection strengths resembling those in physiological pathways. We found that this network could be further trained to simulate additional tasks, such as experimentally observed reflex responses to limb perturbations that stretched or shortened the active muscles, and scaling of response amplitudes in proportion to inputs. In the final comprehensive network, motor units are driven by the combined activity of cortical, rubral, spinal and afferent units during step tracking and perturbations.The model displayed many emergent properties corresponding to physiological characteristics. The resulting neural network provides a working model of premotoneuronal circuitry and elucidates the neural mechanisms controlling motoneuron activity. It also predicts several features to be experimentally tested, for example the consequences of eliminating inhibitory connections in cortex and red nucleus. It also reveals that co-contraction can be achieved by simultaneous activation of the flexor and extensor circuits without invoking features specific to co-contraction.     

The influence of increased muscle spindle sensitivity on Achilles tendon jerk and H-reflex in relaxed human subjects

Whether the fusimotor system contributes to reflex gain changes during reinforcement maneuvers is re-examined in the light of new data. Recently, from direct recordings of spindle afferent activity originating from ankle flexor muscles, we showed that mental computation increased the muscle spindle mechanical sensitivity in completely relaxed human subjects without concomitant alpha-motoneuron activation, providing evidence for selective fusimotor drive activation. In the present study, the effects of mental computation were investigated on monosynaptic reflexes elicited in non-contracting soleus muscle either by direct nerve stimulation (Hoffmann reflex, H) or by tendon tap (Tendinous reflex, T). The aim was to relate the time course of the changes in reflex size to the increase in spindle sensitivity during mental task in order to explore whether fusimotor activation can influence the size of the monosynaptic reflex. The results show changes in reflex amplitude that parallel the increase in muscle spindle sensitivity. When T-reflex is consistently facilitated during mental effort, the H-reflex is either depressed or facilitated, depending on the subjects. These findings suggest that the increased activity in muscle spindle primary endings may account for mental computation-induced changes in both tendon jerk and H-reflex. The facilitation of T-reflex is attributed to the enhanced spindle mechanical sensitivity and the inhibition of H-reflex is attributed to post-activation depression following the increased Ia ongoing discharge. This study supports the view that the fusimotor sensitization of muscle spindles is responsible for changes in both the mechanically and electrically elicited reflexes. It is concluded that the fusimotor drive contributed to adjustment of the size of tendon jerk and H-reflex during mental effort. The possibility that a mental computation task may also operate by reducing the level of presynaptic inhibition is discussed on the basis of H-reflex facilitation.     

Fusimotor reflexes influencing secondary muscle spindle afferents from flexor and extensor muscles in the hind limb of the cat.

The purpose of this study was to investigate secondary muscle spindle afferents from the triceps-plantaris (GS) and posterior biceps and semitendinosus (PBSt) muscles with respect to their fusimotor reflex control from different types of peripheral nerves and receptors. The activity of single secondary muscle spindle afferents was recorded from dissected and cut dorsal root filaments in alpha-chloralose anaesthetized cats. Both single spindle afferents and sets of simultaneously recorded units (2-3) were investigated. The modulation and mean rate of firing of the afferent response to sinusoidal stretching of the GS and PBSts muscle were determined. Control measurements were performed in the absence of any reflex stimulation, while test measurements were made during reflex stimulation. The reflex stimuli consisted of manually performed movements of the contralateral hind limb, muscle stretches, ligament tractions and electrical stimulations of cutaneous afferents. Altogether 21 secondary spindle afferents were investigated and 20 different reflex stimuli were employed. The general responsiveness (i.e. number of significant reflex effects/number of control-test series) was 52.4%, but a considerable variation between different stimuli was found, with the highest (89.9%) for contralateral whole limb extension and the lowest (25.0%) for stretch of the contralateral GS muscle. The size of the response to a given stimulus varied considerably between different afferents, and, in the same afferent, different reflex stimuli produced effects of varying size. Most responses were characterized by an increase in mean rate of discharge combined with a decrease in modulation, indicative of static fusimotor drive (Cussons et al., 1977). Since the secondary muscle spindle afferents are part of a positive feedback loop, projecting back to both static and dynamic fusimotor neurones (Appelberg Et al., 1892 a, 1983 b; Appelberg et al., 1986), it is suggested that the activity in the loop may work like an amplified which, during some circumstances, enhance the effect of other reflex inputs to the system (Johansson et al., 1991 b).     

Heterogeneity of spindle units in the cat tenuissimus muscle

Three tandem spindles and their nerve supplies, reconstructed by light microscopy of serial transverse sections of the cat tenuissimus muscle, were compared to single spindle units. Each tandem spindle consisted of one large unit containing a dynamic bag1, a static bag2, and several static chain fibers (b1b2c unit) linked by the bag2 fiber to a small unit containing only a bag2 and chain fibers (b2c unit). Most features of primary afferents, secondary afferents, and motor neurons were qualitatively and quantitatively similar in both single and tandem b1b2c units. However, b1b2c units of tandem spindles had a lower density of skeletofusimotor innervation than did single b1b2c spindles. The b2c spindle units differed greatly from single or tandem b1b2c units. The b2c spindle units had fewer intrafusal fibers and incoming axons than either the tandem or single b1b2c units. The motor innervation of b2c units was typified by nonselective gamma axons that coinnervated both bag2 and chain fibers, in contrast to the regular occurrence of both selective and nonselective motor axons in b1b2c spindle units. The afferent located at the equator of b2c units differed in size, branching pattern, and intrafusal distribution of its ending from both the primary and secondary sensory axons of b1b2c units and, therefore, might represent a third category of spindle afferent. Thus, cat tenuissimus muscles contain three types of spindle units that differ in the number and organization of muscular and neural elements. These differences in structure and neural organization among tenuissimus spindle units may be a source for generation of different sensory signals in response to common mechanical or fusimotor stimuli.     

Fusimotor reflex profiles of individual triceps surae primary muscle spindle afferents assessed with multi-afferent recording technique.

The experiments were performed on 21 cats anaesthetized with alpha-chloralose. The aim of the study was to investigate sets of simultaneously recorded spindle afferents (2-4 in each set) from the triceps surae muscle (GS) with respect to the pattern of fusimotor reflex effects evoked by different types of ipsi- and contralateral reflex stimulation. The afferents' responses to sinusoidal stretching of the GS muscle were determined and the fusimotor reflex effects were assessed by comparing the afferent responses (i.e. the mean rate of firing and the depth of modulation) elicited during reflex stimulation with those evoked in absence of any reflex stimulus. Natural of electrical activations of ipsi- and contralateral muscle, skin and joint receptor afferents were used as reflex stimuli. The spindle afferents were influenced by several modalities and from wide areas, with a majority responding to both ipsi- and contralateral stimuli. A particular reflex stimulus often caused different effects on different afferents, and the various reflex stimuli seldom gave similar effects on a particular afferent. Multivariate analysis revealed that the variation in response profiles among simultaneously recorded afferents were as great as between afferents recorded on different occasions. This suggests that the individualized response prifiles, observed in earlier investigations, represent a very diversified reflex control of the spindle primary afferents, and are not a reflection of changes in the setting of the spinal interneuronal network, occurring during the time interval between the recordings of different units. Also, there was no relation between the conduction velocity of the afferents and the reflex profiles of the afferents, but non-linear relations were found between effects elicited by different types of stimuli. Indications were also found that it may be possible to separate the population of GS muscle spindles into subgroups, according to the fusimotor effects exhibited by activation of various categories of ipsi- and contralateral receptor afferents. It is concluded that one possible way of making the very complex reflex system controlling the muscle spindles intelligible may be a combination of multiple simultaneous recordings of spindle afferents and multivariate analysis.     

Effects of acetylcholine and succinylcholine on isolated frog neuromuscular spindle]

The mode of action of acetylcholine (ACh) and succinylcholine (SCh) on the isolated frog's muscle spindle has been studied. Receptor afferent nervous supply was maintained; the appropriate spinal roots were dissected for stimulating motor axons and recording from sensory fibres. Excitatory effects on the afferent activity, when the receptor was held still and during stretching, were found with ACh or SCh concentrations of 10(-8) to 10(-3); 10(-6) g/ml being usually effective. These effects are similar to those obtained by stimulating fusimotor nerve fibres. The contractile activity of intrafusal muscle fibres which occurred during these effects was observed. Seldom, and only for high concentrations of ACh and SCh, a decrease in afferent activity following the excitatory effects was found. Tubocurarine chloride (10(-5)-10(04) g/ml) in the bath prevented both motor fibres and drugs effects. Sometimes slight transient excitation occurred at very high concentrations of the two tested substances; however, this effect was prevented by stronger curarization. The observed blocking effects were always reversed by removing tubocurarine from the bath. No more excitatory effects by motor fibres stimulation and by ACh and SCh action could be found after destruction of intrafusal muscle fibres, by pinching them as close as possible to the ends of the spindle. It is suggested that ACh and SCh act indirectly by causing mechanical changes in intrafusal muscle fibres, and that a direct action on sensory nerve endings, if any, cannot, by itself, increase the afferent activity of the receptor.     

Modulation of human muscle spindle discharge by arterial pulsations--functional effects and consequences

Arterial pulsations are known to modulate muscle spindle firing; however, the physiological significance of such synchronised modulation has not been investigated. Unitary recordings were made from 75 human muscle spindle afferents innervating the pretibial muscles. The modulation of muscle spindle discharge by arterial pulsations was evaluated by R-wave triggered averaging and power spectral analysis. We describe various effects arterial pulsations may have on muscle spindle afferent discharge. Afferents could be "driven" by arterial pulsations, e.g., showing no other spontaneous activity than spikes generated with cardiac rhythmicity. Among afferents showing ongoing discharge that was not primarily related to cardiac rhythmicity we illustrate several mechanisms by which individual spikes may become phase-locked. However, in the majority of afferents the discharge rate was modulated by the pulse wave without spikes being phase locked. Then we assessed whether these influences changed in two physiological conditions in which a sustained increase in muscle sympathetic nerve activity was observed without activation of fusimotor neurones: a maximal inspiratory breath-hold, which causes a fall in systolic pressure, and acute muscle pain, which causes an increase in systolic pressure. The majority of primary muscle spindle afferents displayed pulse-wave modulation, but neither apnoea nor pain had any significant effect on the strength of this modulation, suggesting that the physiological noise injected by the arterial pulsations is robust and relatively insensitive to fluctuations in blood pressure. Within the afferent population there was a similar number of muscle spindles that were inhibited and that were excited by the arterial pulse wave, indicating that after signal integration at the population level, arterial pulsations of opposite polarity would cancel each other out. We speculate that with close-to-threshold stimuli the arterial pulsations may serve as an endogenous noise source that may synchronise the sporadic discharge within the afferent population and thus facilitate the detection of weak stimuli.     

Proprioception during voluntary movement

In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha-gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of proprioceptive behaviour, corroborated in separate laboratories. (i) Mean Ia firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both Ia and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles. (iii) During powerful shortening contractions performed against significant loads, Ia firing is often appreciable, suggesting that there is at least some underlying alpha-gamma coactivation. (iv) During fast imposed muscle stretches, Ia afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5-1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of gamma- and alpha-motoneuron function of antagonistic muscles during cold tremor in cats

Activity of flexor and extensor γ- and α-motoneurons during cold tremor was investigated in anesthetized cats. General hypothermia and local cold stimulation or electrical stimulation of the dorsomedial region of the posterior hypothalamus were shown to induce primary activation of flexor and simultaneous inhibition of activity of extensor γ-motoneurons, followed by activation of flexor α-motoneurons (extensor muscles are not involved in temperature regulating activity). Electrical stimulation of the medial preoptic region during cold tremor led to primary inhibition of flexor α-motoneuron activity. It is concluded from these results that the development of temperature-regulating muscular activity is preceded by activation of γ-motoneurons. The use of cold tremor induced in anesthetized cats as a natural model with which to study the role of the fusimotor system in regulation of function of motor nuclei during postural activity is argued.     

Proprioceptive input from the jaw muscles and its influence on lapping, chewing, and posture

Experiments are described which define criteria for identifying fusimotor axons discharging in filaments of the masseter nerve in lightly anaesthetized cats. During reflex movements of the jaw two patterns of discharge were observed in different fusimotor fibres. One type, called "sustained," fired at a fairly constant increased rate. The other, called "modulated," fired at high frequencies during the extrafusal muscle contraction. Evidence from spindle primary and secondary recordings in similar experiments strongly suggests that the "sustained" type were dynamic fusimotor fibres and the "modulated" type were static fusimotor fibres. New spindle recordings in normal unanaesthetized cats indicate that the modulated pattern of static fusimotor discharge also occurs in these conditions. Its effect is to reduce the tendency for spindle afferents to become silent during muscle shortening. A proposal is made that the static fusimotor discharge in cyclic movements may represent a temporal "template" for the intended movement as directed by the central pattern generator.     

Effect of motor stimulation and stretching on afferent activity of the neuromuscular spindle isolated from the frog]

The arrangement of muscle spindles in m. ext. long. dig. IV has been examined by microdissection. It is confirmed that spindle systems generally appear to consist of individual receptors. Stimulation effects of fast motor fibres (conduction velocities greater than 12 m/sec) on the spindles of the same muscle were studied. Receptors were isolated with their nerves and the appropriate spinal roots, the latter ones were used for stimulating efferent fibres and recording sensory discharges. Single shocks to the ventral root filaments caused afferent responses ranging from a single action potential to a train of impulses. During repetitive stimulation (train of stimuli at frequency of 10 to 150/sec) a marked increase in afferent activity was found. Afferent activity could be driven by the frequency of stimuli ("driving") and the stimulus/action potentials ratio varied from 1:1 to 1:3 or more. The rate of sensory discharge depended on the frequency of stimuli: the maximum effect, was attained at 30 to 50 stimuli/sec and, in the most responsive receptors, up to 80 stimuli/sec. Slight increases of the initial lengths of the receptors caused facilitation of sensory responses to motor stimulation. Moreover, impairing effects, which appear during sustained or high-frequency stimulation, possibly related to fatigue in intrafusal neuromuscular transmission, could be relieved by increasing the initial length. The repetitive stimulation of fast fusimotor fibres increased both dynamic and static responses and also raised the afferent activity after a period of stretching, when usually a depression occurs; these effects varied according to the preparation, its initial tension and the frequency of stimulation. The main feature of the examined motor fibres, when stimulated, is the constant excitatory action on muscle spindle static response. Results are discussed. It is suggested that the different characteristics of intrafusal muscle fibres, the receptor initial tension and the frequency of motor units discharges, may together affect muscle spindles static or dynamic performance.     

Characterization of muscle spindle afferents in the adult mouse using an in vitro muscle-nerve preparation

We utilized an in vitro adult mouse extensor digitorum longus (EDL) nerve-attached preparation to characterize the responses of muscle spindle afferents to ramp-and-hold stretch and sinusoidal vibratory stimuli. Responses were measured at both room (24°C) and muscle body temperature (34°C). Muscle spindle afferent static firing frequencies increased linearly in response to increasing stretch lengths to accurately encode the magnitude of muscle stretch (tested at 2.5%, 5% and 7.5% of resting length [Lo]). Peak firing frequency increased with ramp speeds (20% Lo/sec, 40% Lo/sec, and 60% Lo/sec). As a population, muscle spindle afferents could entrain 1:1 to sinusoidal vibrations throughout the frequency (10-100 Hz) and amplitude ranges tested (5-100 μm). Most units preferentially entrained to vibration frequencies close to their baseline steady-state firing frequencies. Cooling the muscle to 24°C decreased baseline firing frequency and units correspondingly entrained to slower frequency vibrations. The ramp component of stretch generated dynamic firing responses. These responses and related measures of dynamic sensitivity were not able to categorize units as primary (group Ia) or secondary (group II) even when tested with more extreme length changes (10% Lo). We conclude that the population of spindle afferents combines to encode stretch in a smoothly graded manner over the physiological range of lengths and speeds tested. Overall, spindle afferent response properties were comparable to those seen in other species, supporting subsequent use of the mouse genetic model system for studies on spindle function and dysfunction in an isolated muscle-nerve preparation.     

Interaction of the Eighth Component of Guinea Pig Complement (C8) with the Membrane-Bound C5b-7 Complex

The eighth component (C8) of guinea pig complement consists of three polypeptide chains, the α-, β-, and γ-chains with M.W. of 60,000, 60,000, and 24,000, respectively. The α- and γ-chains are bound by a disulfide bond(s) forming an α-γ subunit, which is linked noncovalently to the β-chain. The α-γ subunit and the β-chain were separated and purified from C8 by treatment with sodium dodecyl sulfate (SDS) and gel chromatography on Sephacryl S-300 in the presence of SDS. After removal of SDS, neither α-γ nor β showed the hemolytic activity of C8 when assayed independently, but showed significant activity in combination, indicating reconstitution of active C8. The recovery of hemolytic activity was 3.48%. When α-γ and β were incubated successively with EAC-7 with intervening washing, reconstitution of active C8 on the cells was insignificant, irrespective of the order of the reactions. α-γ and β did not bind to EAC-7 when added separately, but after recombination 7% of α-γ and 9% of β bound to EAC-7 when EAC-7 was in excess. These results indicate that the binding site of guinea pig C8 to the membrane-bound C5b-7 complex does not exist on either α-γ or β only but stretches over both or is formed on one subunit after recombination of the subunits.     

An In Vitro Adult Mouse Muscle-nerve Preparation for Studying the Firing Properties of Muscle Afferents

Muscle sensory neurons innervating muscle spindles and Golgi tendon organs encode length and force changes essential to proprioception. Additional afferent fibers monitor other characteristics of the muscle environment, including metabolite buildup, temperature, and nociceptive stimuli. Overall, abnormal activation of sensory neurons can lead to movement disorders or chronic pain syndromes. We describe the isolation of the extensor digitorum longus (EDL) muscle and nerve for in vitro study of stretch-evoked afferent responses in the adult mouse. Sensory activity is recorded from the nerve with a suction electrode and individual afferents can be analyzed using spike sorting software. In vitro preparations allow for well controlled studies on sensory afferents without the potential confounds of anesthesia or altered muscle perfusion. Here we describe a protocol to identify and test the response of muscle spindle afferents to stretch. Importantly, this preparation also supports the study of other subtypes of muscle afferents, response properties following drug application and the incorporation of powerful genetic approaches and disease models in mice.     

Muscle spindle formation and differentiation in regenerating rat muscle grafts

Muscle spindle development and function are dependent upon sensory innervation. During muscle regeneration, both neural and muscular components of spindles degenerate and it is not known whether reinnervation of a regenerating muscle results in reestablishment of proper neuromuscular relationships within spindles or whether sensory neurons may exert an influence upon differentiation of these spindles. Muscle spindle regeneration was studied in bupivacaine-treated grafts of rat extensor digitorum longus (EDL) muscles. Three types of EDL graft were performed in order to manipulate the extent to which regenerating spindles might be reinnervated: (1) grafts reinnervated following severance of their nerve supply (standard grafts); (2) grafts in which intact nerve sheaths appear to facilitate reinnervation (nerveintact grafts); and (3) grafts in which reinnervation was prevented (nonreinnervated grafts). Complete degeneration of muscle fibers occurred in all grafts prior to regeneration. Initial formation of spindles in regenerating EDL grafts is independent of innervation; intrafusal muscle fibers degenerate and regenerate within spindle capsules that remain intact and viable. The extent of spindle differentiation was evaluated in each type of graft using criteria that included nucleation and ATPase activity, both of which have been shown to be regulated by sensory innervation, as well as the number of muscle fibers/spindle and morphology of spindle capsules.While most spindles contained normal numbers of muscle fibers, most of these fibers were morphologically and histochemically abnormal. Alterations of ATPase activity occurred in all spindles, but were least severe in nerve-intact grafts. While fully differentiated nuclear bag and chain fibers were not observed in regenerated spindles, large, vesicular nuclei, similar to those of normal intrafusal fibers, were present in a small number of spindles in nerve-intact grafts. Sensory nerve terminations were observed only in those spindles that also contained the distinctive nuclei. This study suggests that a specific neurotrophic influence is necessary for regeneration of normal intrafusal muscle fibers and that this influence corresponds to the properly timed sensory neuron-muscle interaction which directs muscle spindle embryogenesis. However, the infrequent occurrence of characteristics unique to intrafusal muscle fibers indicates that reinnervation of regenerating muscle grafts by sensory neurons is inadequate and/or faulty.     

Ia presynaptic inhibition in human wrist extensor muscles: effects of motor task and cutaneous afferent activity.

The task-dependence of the presynaptic inhibition of the muscle spindle primary afferents in human forearm muscles was studied, focusing in particular on the modulation associated with the co-contraction of antagonist muscles and the activation of cutaneous afferents. The changes known to affect the motoneuron proprioceptive assistance during antagonist muscle co-activation in human leg and arm muscles were compared. The evidence available so far that these changes might reflect changes in the presynaptic inhibition of the muscle spindle afferent is briefly reviewed. The possible reasons for changes in presynaptic inhibition during the antagonist muscle co-contraction are discussed. Some new experiments on the wrist extensor muscles are briefly described. The results showed that the changes in the Ia presynaptic inhibition occurring during the co-contraction of the wrist flexor and extensor muscles while the hand cutaneous receptors were being activated (the subject's hand was clenched around a manipulandum) could be mimicked by contracting the wrist extensor muscles alone while applying extraneous stimulation to the hand cutaneous receptors. It is concluded that besides the possible contribution of inputs generated by the co-contraction of antagonist muscles and by supraspinal pathways, cutaneous inputs may play a major role in modulating the proprioceptive assistance during manipulatory movements.