The effect of succinylcholine on cat gastrocnemius muscle spindle afferents of different types

Muscle spindle and fusimotor activity in locomotion

Mammals may exhibit different forms of locomotion even within a species. A particular form of locomotion (e.g. walk, run, bound) appears to be selected by supraspinal commands, but the precise pattern, i.e. phasing of limbs and muscles, is generated within the spinal cord by so-called central pattern generators. Peripheral sense organs, particularly the muscle spindle, play a crucial role in modulating the central pattern generator output. In turn, the feedback from muscle spindles is itself modulated by static and dynamic fusimotor (gamma) neurons. The activity of muscle spindle afferents and fusimotor neurons during locomotion in the cat is reviewed here. There is evidence for some alpha-gamma co-activation during locomotion involving static gamma motoneurons. However, both static and dynamic gamma motoneurons show patterns of modulation that are distinct from alpha motoneuron activity. It has been proposed that static gamma activity may drive muscle spindle secondary endings to signal the intended movement to the central nervous system. Dynamic gamma motoneuron drive appears to prime muscle spindle primary endings to signal transitions in phase of the locomotor cycle. These findings come largely from reduced animal preparations (decerebrate) and require confirmation in freely moving intact animals.

Paraspinal Muscle Spindle Response to Intervertebral Fixation and Segmental Thrust Level During Spinal Manipulation in an Animal Model

STUDY DESIGN

In vivo cat model study.

OBJECTIVE

To determine whether intervertebral facet joint fixation and segmental thrust level alter paraspinal muscle spindle activity during simulated spinal manipulation.

SUMMARY OF BACKGROUND DATA

Intervertebral motion is commonly assessed by manual therapy practitioners during clinical evaluation and treatment. Mechanoreceptor activity elicited during spinal manipulation has been theorized as a potential mechanism of its efficacy. The degree to which intervertebral fixation and segmental thrust level alter paraspinal muscle spindle activity during high velocity low amplitude spinal manipulation (HVLA-SM) is unclear.

METHODS

Intervertebral fixation was created by inserting facet screws through the left L(5-6) and L(6-7) and left L(4-5), L(5-6), and L(6-7) facet joints of a cat spine. Changes in the mean instantaneous frequency of L6 muscle spindle discharge were determined during 5 HVLA-SM thrust durations (0-control, 75, 100, 150, 250 ms) delivered at the L4 or L6 spinous process in each of the 3 conditions within the same preparation: laminectomy-only (surgical control; n = 23), L(5-6) and L(6-7) fixations (n = 20), and L(4-5), L(5-6), and L(6-7) fixations (n = 7). Comparisons were made between thrust levels, thrust durations, and spinal joint conditions using a linear mixed model.

RESULTS

Insertion of facet screws compared with laminectomy-only significantly increased (P < 0.001) lumbar spinal stiffness during L6 HVLA-SM. Compared with laminectomy-only, both the 2 facet screw (100 ms; P < 0.05) and 3 screw conditions [75 ms and 100 ms (P < 0.001), 150 ms (P < 0.005), and 250 ms (P < 0.05)] significantly decreased L6 spindle response during the L6 HVLA-SM. HVLA-SM-delivered 2 segments rostral to the level of muscle spindle input significantly decreases spindle response compared with HVLA-SM-delivered at-level; however, nontarget HVLA-SM still elicits 60% to 80% of at-level muscle spindle response.

CONCLUSION

Intervertebral fixation decreases paraspinal muscle spindle response during L6 HVLA-SM in a cat model. Whereas HVLA-SM target accuracy maximizes spindle response, nontarget HVLA-SM still elicits substantial levels of muscle spindle activity.

LEVEL OF EVIDENCE

N/A.

Using vertebral movement and intact paraspinal muscles to determine the distribution of intrafusal fiber innervation of muscle spindle afferents in the anesthetized cat

Increasing our knowledge regarding intrafusal fiber distribution and physiology of paraspinal proprioceptors may provide key insights regarding proprioceptive deficits in trunk control associated with low back pain and lead to more effective clinical intervention. The use of vertebral movement as a means to reliably stretch paraspinal muscles would greatly facilitate physiological study of paraspinal muscle proprioceptors where muscle tendon isolation is either very difficult or impossible. The effects of succinylcholine (SCh) on 194 muscle spindle afferents from lumbar longissimus or multifidus muscles in response to computer-controlled, ramp-and-hold movements of the L(6) vertebra were investigated in anesthetized cats. Paraspinal muscles were stretched by moving the L(6) vertebra 1.5-1.7 mm in the dorsal-ventral direction. Initial frequency (IF), dynamic difference (DD), their changes (∆) following SCh injection (100-400 μg kg(-1)), and post-SCh dynamic difference (SChDD) were measured. Muscle spindle intrafusal fiber terminations were classified as primary or secondary fibers as well as bag(1) (b(1)c), bag(2) (b(2)c), b(1)b(2)c, or chain (c) fibers. Intrafusal fiber subpopulations were distinguished using logarithmic transformation of SChDD and ∆IF distributions as established by previous investigators. Increases in DD indicate strength of b(1)c influence while increases in IF indicate strength of b(2)c influence. Out of 194 afferents, 46.9 % of afferents terminated on b(2)c fibers, 46.4 % on b(1)b(2)c fibers, 1 % on b(1)c fibers, and 5.7 % terminated on c fibers. Based on these intrafusal fiber subpopulation distributions, controlled vertebral movement can effectively substitute for direct tendon stretch and allow further investigation of paraspinal proprioceptors in this anatomically complex body region.

The role of muscle spindles in the development of the monosynaptic stretch reflex

Muscle sensory axons induce the development of specialized intrafusal muscle fibers in muscle spindles during development, but the role that the intrafusal fibers may play in the development of the central projections of these Ia sensory axons is unclear. In the present study, we assessed the influence of intrafusal fibers in muscle spindles on the formation of monosynaptic connections between Ia (muscle spindle) sensory axons and motoneurons (MNs) using two transgenic strains of mice. Deletion of the ErbB2 receptor from developing myotubes disrupts the formation of intrafusal muscle fibers and causes a nearly complete absence of functional synaptic connections between Ia axons and MNs. Monosynaptic connectivity can be fully restored by postnatal administration of neurotrophin-3 (NT-3), and the synaptic connections in NT-3-treated mice are as specific as in wild-type mice. Deletion of the Egr3 transcription factor also impairs the development of intrafusal muscle fibers and disrupts synaptic connectivity between Ia axons and MNs. Postnatal injections of NT-3 restore the normal strengths and specificity of Ia-motoneuronal connections in these mice as well. Severe deficits in intrafusal fiber development, therefore, do not disrupt the establishment of normal, selective patterns of connections between Ia axons and MNs, although these connections require the presence of NT-3, normally supplied by intrafusal fibers, to be functional.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

Limb-state feedback from ensembles of simultaneously recorded dorsal root ganglion neurons

Functional electrical stimulation (FES) holds great potential for restoring motor functions after brain and spinal cord injury. Currently, most FES systems are under simple finite state control, using external sensors which tend to be bulky, uncomfortable and prone to failure. Sensory nerve signals offer an interesting alternative, with the possibility of continuous feedback control. To test feasibility, we recorded from ensembles of sensory neurons with microelectrode arrays implanted in the dorsal root ganglion (DRG) of walking cats. Limb position and velocity variables were estimated accurately (average R2 values >0.5) over a range of walking speeds (0.1-0.5 m s(-1)) using a linear combination of firing rates from 10 or more neurons. We tested the feasibility of sensory control of intraspinal FES by recording from DRG neurons during hindlimb movements evoked by intraspinal microstimulation of the lumbar spinal cord in an anesthetized cat. Although electrical stimulation generated artifacts, this problem was overcome by detecting and eliminating events that occurred synchronously across the array of microelectrodes. The sensory responses to limb movement could then be measured and decoded to generate an accurate estimate of the limb state. Multichannel afferent recordings may thus provide FES systems with the feedback needed for adaptive control and perturbation compensation, though long-term stability remains a challenge.

Spinal projection of spindle afferents of the longissimus lumborum muscles of the cat

The connections and monosynaptic projections of muscle spindle afferents of individual heads of the longissimus lumborum have been studied in cats by natural stimulation, by electrical stimulation and by spike-triggered averaging from single identified afferents. The spindle afferents were classified by sensitivity to vibration and by the effect of succinylcholine on their response to ramp-and-hold muscle stretches. Axonal conduction and synaptic effects were recorded as field potentials and focal synaptic potentials during systematic exploration of the spinal cord in segments L1 to L4 with extracellular metal microelectrodes, singly and in linear arrays. Ascending branches of afferent axons within the cord had a significantly higher mean conduction velocity (CV: 56.5 m s(-1)) than descending branches (40.8 m s(-1)). The CV of ascending branches was significantly positively correlated with a measure of the strength of intrafusal bag(2) muscle fibre contacts, but not to a measure of bag(1) contacts. Two sites of monosynaptic excitatory projection in the cord were identified, namely to the intermediate region (laminae V, VI and VII) and to ventral horn region (laminae VIII and IX). In tests of 154 single afferents, signs of central projection were detected for 60, providing 122 regions of maximum negative focal synaptic potentials (FSPs) of mean amplitude 7.51 microV. Their longitudinal spacing indicated that axons gave off descending collaterals at intervals of 1.5-3.5 mm. Based on the amplitude of FSPs, the projection of secondary afferents is stronger than that of primaries in the intermediate region and possibly also in the ventral horn region. Evidence is also presented that spindle afferent input from different heads of the longissimus converges into any given spinal segment and that input in one spinal root projects to adjacent segments. It is concluded that the organization of the longissimus monosynaptic spindle input favours relatively tonic and diffuse stretch reflexes.

Classification of muscle spindle afferents innervating the masseter muscle in rats

Taylor et al. [Taylor, A., Durbaba, R., Rodgers, J.F., 1992a. The classification of afferents from muscle spindles of the jaw-closing muscles of the cat. J Physiol 456, 609-628] developed a method to classify muscle spindle afferents using succinylcholine (Sch) and ramp and hold stretches. They demonstrated that cat jaw muscle spindle afferents show high proportion of intermediate responses to ramp and hold jaw stretch. Together with observations on the responses to Sch their data suggests that the majority of jaw muscle spindle afferents are influenced by a combination of nuclear bag(2) and nuclear chain fibres. Relatively few are influenced solely by nuclear bag(1) fibres. The purpose of this study was to categorize jaw muscle spindle afferent in rodents in response to ramp and hold stretches. Several measures were used to classify spindle afferents including (1) conduction velocity, (2) coefficient of variation (C.V.) of the interspike interval during jaw opening, and (3) the dynamic sensitivity and the initial discharge of spindle afferents before and after succinylcholine infusion (Sch, 100mg/kg, i.v.). Consistent with observations in the cat jaw muscles, the distribution of the conduction velocity and the C.V. of Vmes masseter afferents were unimodal. Therefore, these parameters were of little value in functional classification of spindle innervation. Succinylcholine injection either markedly increased the dynamic sensitivity or produced no change in Vmes afferents. Unlike cat jaw muscle spindle afferents, the effect of Sch on the initial discharge was not clearly separable from those responding or not responding to Sch. These results suggest that rat jaw muscle spindle afferents, have physiological properties that are primarily intermediate in nature and are likely to reflect a predominance of influence from nuclear bag(2) and chain fibres. However, the distinction between bag(2) and chain fibres influences is not as clearly defined in the rat compared to the cat.

Classification of longissimus lumborum muscle spindle afferents in the anaesthetized cat

Recordings have been made from 127 single muscle spindle afferents from the longissimus lumborum muscles of anaesthetized cats. They have been characterized by their responses to passive muscle stretch and the effects of succinylcholine (SCh) and by their sensitivity to vibration. The use of SCh permitted the assessment for each afferent of the influence of bag1 (b1) and bag2 (b2) intrafusal muscle fibres. From this, on the assumption that all afferents were affected by chain (c) fibres, they were classified in four groups: b1b2c (41.9%), b2c (51.4%), b1c (1.3%) and c (5.4%). All the afferents with b1 influence were able to respond one to one to vibration at frequencies above 100 Hz and were considered to belong to primary endings. On the basis of the vibration test, 64% of the b2c type afferents appeared to be primaries and 36% secondaries. Of the units classified as primaries, 41% were designated as b2c and would not therefore be able to respond to dynamic fusimotor activity. The significance of this relatively high proportion of b2c-type spindle primary afferents is discussed in relation to the specialized postural function of the back muscles.

Static and dynamic gamma-motor output to ankle flexor muscles during locomotion in the decerebrate cat

In locomotion, the flexor muscles of the leg are mainly concerned with the relatively constant task of raising the foot, whereas the extensors have the more variable task of support and propulsion at different speeds. This suggests that the way in which the fusimotor system works may differ between the two muscle groups. Observations previously made of the static and dynamic gamma-motor firing patterns in the ankle extensor medial gastrocnemius (MG) have therefore been repeated in the flexor tibialis anterior (TA). One or more single gamma-motor axons, dissected from a small filament of TA nerve, were recorded simultaneously with a number of single spindle afferents in dorsal rootlets. Cats were decerebrated and locomoted spontaneously on a treadmill. Identification of each gamma-motor axon depended on relating the changes in firing caused by midbrain stimulation to the changes in static and dynamic behaviour of the spindle afferents in response to repetitive ramp and hold stretches. Static gamma axons all showed a smooth modulation in frequency, increasing in phase with muscle shortening, superimposed on a minimum frequency of about 20-30 impulses s(-1). Dynamic gamma axons showed interrupted firing with the frequency rising abruptly from zero at the onset of shortening, and falling again to zero shortly after the onset of lengthening. The frequency during the active periods was relatively constant, even when movement amplitudes varied. The basic similarity in the static and dynamic gamma discharge patterns for the two muscles suggests that the strategy of gamma-motor control is common to both flexors and extensors. The static gamma pattern is thought to be a 'temporal template' of the expected movement, effectively expanding the dynamic response range of the spindles in active movements. The dynamic gamma pattern sensitizes the primary afferents to detect the onset of muscle lengthening and to detect departures from the intended movement trajectory.

The effect of experimental muscle pain on the amplitude and velocity sensitivity of jaw closing muscle spindle afferents

The effect of experimental muscle pain on the amplitude and velocity sensitivity of muscle spindle primary afferent neurons in the trigeminal mesencephalic nucleus (Vmes) was examined. Extracellular recordings were made from 45 neurons designated as spindle primary- or secondary-like on the basis of their response to ramp-and-hold jaw movements. Velocity sensitivity was assessed in spindle primary-like afferents by calculating the mean dynamic index of each unit in response to three different velocities of jaw opening before and after intramuscular injection with hypertonic saline (HS, 5%, 100 microl). The amplitude sensitivity of all jaw muscle spindle afferents was assessed by calculating the mean firing rate of each unit in response to three different amplitudes of jaw openings during both the open and hold phases of the movement and with best-fit lines obtained, using linear regression analysis, before and after HS injection. The variance of the two regression lines obtained for each unit before and after the injection was compared using the coincidence test, and changes in intercept and slope were determined. Seventy-five percent of the primary-like units and 80% of the secondary-like units presented with changes in static behavior after HS injection. Thirty-six percent of the primary-like units showed changes in dynamic behavior. Injection of isotonic saline (control) did not alter the responses of the spindle afferent to jaw opening. Thus, our results demonstrate that the predominant effect of noxious stimulation was a shift in the amplitude sensitivity of both spindle primary-like and secondary-like afferents and, to a lesser extent, the velocity sensitivity of the spindle primary-like unit. In accordance with earlier studies in the cat hindlimb and neck muscles, these results suggest that the activation of masseter muscle nociceptor alters spindle afferent responses to stretch acting primarily through static gamma motor neurons.

Cross-bridge mechanisms underlying the history-dependent properties of muscle spindles and stretch reflexes

The effects of prior movement on the force responses of skeletal muscle are compared with the effects of movement history on the changes in firing rate of muscle spindle receptors. Prior release results in the linearization of the mechanical properties of skeletal muscles, which can be provisionally explained by cross-bridge models of muscular contraction. The history-dependence of responses of muscle spindle receptors in unanesthetized decerebrate preparations appears to result from the kinetics of cycling and noncycling cross-bridges. The results of this comparison indicate that the integration of mechanical properties of muscle and spindle receptor promotes stiffness regulation.Key words: predictive control, muscular stiffness, muscle receptors, reflex compensation, cross-bridge cycling, nonlinear mechanical properties, feline motor control.

Direct and indirect assessment of γ-motor firing patterns

The study of the patterns of γ-motor activity which accompany natural contractions has been long and difficult, and has not as yet led to general agreement. In this review we have simplified matters by considering the case of locomotion in the cat only, and we have avoided discussion of the various hypotheses which have been advanced to provide general schemes of γ control for a wide range of movements. The development of the subject is shown to depend very much on devising ingenious methods applicable to reduced and intact animals. Direct recording from γ-motoneurones has only been possible in reduced preparations, whereas indirect assessment of γ activity from spindle afferent recordings was used in these and in intact animals. At this point in time, we still have no direct recordings from γ-motoneurones in normally behaving animals, but those obtained in decerebrate animals show distinct patterns of modulation for static and dynamic types with particular temporal relation to the stepping movements. The spindle recordings in intact animals potentially provide the most important information, and the problems of interpretation, which have previously caused difficulties, are beginning to be solved through the insights obtained from the reduced preparations.Key words: locomotion, gamma motoneurons, muscle spindles.

The influence of bag2 and chain intrafusal muscle fibers on secondary spindle afferents in the cat

Static gamma-motor activity is strongly modulated by a particular phase relationship to the cyclic movements of locomotion, and this has a profound effect on the firing patterns of muscle spindle afferents. Whilst primary afferents are affected by both static and dynamic gamma-motor output,secondary afferents are affected significantly only by the static system acting via the intrafusal bag2 and chain fibres. It is therefore important to know how fluctuating patterns of static gamma-motor activity affect secondary afferents and to relate this to the actions of bagt and chain fibres. We have studied the action of single static gamma axons on secondary afferents in cat hindlimb muscles. Various physiological methods were explored to identify which of the intrafusal muscle fibres were being activated in each case, including the use of random stimulation and ramp frequency stimulation. The effects were also recorded of I Hz sinusoidally frequency-modulated gamma-axon stimuli and the amplitude and phase of the resulting afferent modulation related to the involvement of the bag2 and chain fibres. It was found that bag2 fibres are effective in biasing the secondary discharge, but their modulating action is relatively weak and involves a marked phase lag. Chain fibres acting alone cause strong modulation with very little phase lag. Mixed bag2 and chain-fibre action is most effective in modulating afferent discharge and causes intermediate values of phase lag. The results are discussed in relation to the control of natural movements and it is concluded that an important function of the static gamma motor system is to provide a signal to sum algebraically with the length-related signal. The results do not suggest that it could also usefully control stretch sensitivity.

Toe flexor muscle spindle discharge and stretch modulation during locomotor activity in the decerebrate cat

In order to investigate the nature (i.e. static or dynamic) of fusimotor drive to the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) muscles during locomotion we recorded Ia and group II muscle spindle afferent responses to sinusoidal stretch (0.25 and 1 mm amplitude, respectively, 4-5 Hz) in a decerebrate cat preparation. FHL Ia and group II afferents generally had increased discharge rates and decreased modulation to stretch throughout the step cycle, compared to rest, suggesting raised static gamma drive at all locomotor phases. Although the modulation of Ia afferents was reduced during locomotion, most (13 of 18) showed a clear increasing trend during homonymous muscle activity (extension). This was consistent with phasic dynamic gamma drive to FHL spindles linked with alpha drive. In agreement with previous reports, FHL gave a single burst of EMG activity during the step cycle while FDL alpha drive had two components. One was related to extension while the other comprised a brief burst around the end of this phase. Typically FDL Ia and group II afferents also had elevated firing rates and reduced modulation at all locomotor phases, again implicating static gamma drive. Half the afferents (seven Ia, three group II) showed increased discharge during extension, suggesting phasic static gamma drive. There was no gamma drive associated with the late FDL alpha burst. In conclusion, the gamma drives to FHL and FDL differed during locomotion. FHL, which has the alpha drive of a classic extensor, received gamma drive that closely resembled other extensors. The gamma drive of FDL, which exhibits both extensor and flexor alpha synergies, did not match either muscle type. These observations are compatible with the view that fusimotor drive varies in different muscles during locomotion according to the prevailing sensorimotor requirements.

Sympathetic modulation of muscle spindle afferent sensitivity to stretch in rabbit jaw closing muscles

Previous reports showed that sympathetic stimulation affects the activity of muscle spindle afferents (MSAs). The aim of the present work is to study the characteristics of sympathetic modulation of MSA response to stretch: (i) on the dynamic and static components of the stretch response, and (ii) on group Ia and II MSAs to evaluate potentially different effects. In anaesthetised rabbits, the peripheral stump of the cervical sympathetic nerve (CSN) was stimulated at 10 impulses s(-1) for 45-90 s. The responses of single MSAs to trapezoidal displacement of the mandible were recorded from the mesencephalic trigeminal nucleus. The following characteristic parameters were determined from averaged trapezoidal responses: initial frequency (IF), peak frequency at the end of the ramp (PF), and static index (SI). From these, other parameters were derived: dynamic index (DI = PF - SI), dynamic difference (DD = PF - IF) and static difference (SD = SI - IF). The effects of CSN stimulation were also evaluated during changes in the state of intrafusal muscle fibre contraction induced by succinylcholine and curare. In a population of 124 MSAs, 106 units (85.4 %) were affected by sympathetic stimulation. In general, while changes in resting discharge varied among different units (Ia vs. II) and experimental conditions (curarised vs. non-curarised), ranging from enhancement to strong depression of firing, the amplitude of the response to muscle stretches consistently decreased. This was confirmed and detailed in a quantitative analysis performed on 49 muscle spindle afferents. In both the non-curarised (23 units) and curarised (26 units) condition, stimulation of the CSN reduced the response amplitude in terms of DD and SD, but hardly affected DI. The effects were equally present in both Ia and II units; they were shown to be independent from gamma drive and intrafusal muscle tone and not secondary to muscle hypoxia. Sympathetic action on the resting discharge (IF) was less consistent. In the non-curarised condition, IF decreased in most Ia units, while in II units decreases and increases occurred equally often. In the curarised condition, IF in group II units mostly increased. The results have important functional implications on the control of motor function in a state of 'high' sympathetic activity, like excessive stress, as well as in certain pathological conditions such as sympathetically maintained pain.

Modulation of primary afferent discharge by dynamic and static gamma motor axons in cat muscle spindles in relation to the intrafusal fibre types activated

1. Recordings were made from muscle spindle primary afferents from medial gastrocnemius and soleus muscles of the cat to study the modulating effects of varying gamma-motor stimulation frequency at constant muscle length. Stimulus trains had a mean frequency of 50 Hz and were sinusoidally frequency modulated at 1 Hz, with an amplitude of modulation of +/- 5 to +/- 30 Hz. 2. When dynamic gamma-axons (gamma(d)) were selected for their pure effect on bag(1) fibres, they were found to have very little modulating effect on afferent firing. 3. Static gamma-axons (gamma(s)) were tested with a random stimulus and correlation method to determine whether they acted purely on bag(2) fibres, purely on chain fibres or on both together. Pure bag(2) gamma(s)-axons had weak modulating effects with large values of phase lag. Pure chain connections were effective in modulating with very little phase lag, but their mean gain was low. Mixed bag(2) and chain axons were most effective and showed phase shifts proportional to gain. 4. The effects of muscle length changes recorded previously from locomotor movements were also tested, with and without accompanying stimulation of mixed gamma(s)-axons with pulse trains recorded from gamma(s)-axons. This gamma(s) stimulation had a powerful effect in increasing afferent discharge during muscle shortening. The difference in afferent firing between the stimulated and non-stimulated conditions accurately predicted the profile of the gamma(s) stimulation. 5. The results are discussed in relation to the ways in which the gamma-motor system may be used in natural movements.

Distinctive patterns of static and dynamic gamma motor activity during locomotion in the decerebrate cat

Simultaneous recordings were made from gamma (gamma) motor axons and from muscle spindle afferents of the medial gastrocnemius (MG) muscle during locomotion in decerebrate cats. The gamma-neurons were identified as static or dynamic (gammas or gammad) by correlating their behaviour during midbrain stimulation with changes in muscle spindle afferent responses to muscle stretch. On the basis of their behaviour during locomotion, gammas neurons could be divided into two groups. One group (type-1) showed strongly and smoothly modulated discharge increasing in parallel with the active muscle shortening in ankle extension, but with phase advance. The other group (type-2) also showed a modulated pattern, but with increased firing centred on the flexion phase. The proportions of the two were 13 type-1 and 7 type-2. The type-1 firing pattern accurately predicted the difference in firing frequency for secondary afferents obtained by subtracting from the recordings made during active movements the response of the same units to the movements repeated passively in the absence of fusimotor activity. The type-2 pattern also became consistent with the difference signal, when operated on by a phase lag appropriate to the effects of bag2 intrafusal fibres. These results suggest that there may be some degree of separate control of chain and bag2 intrafusal fibres. The discharge of gammad axons was also found to fluctuate with the locomotor cycle, with a pattern very distinct from that of the gammas records. The gammad firing frequency rose very suddenly from zero to a maximum at the onset of muscle shortening and continued into the beginning of lengthening. The term 'interrupted' discharge is suggested as a useful description. The timing of this discharge was shown to be appropriate for sensitising the primary afferents to detect the onset of stretch.

Patterns of fusimotor activity during locomotion in the decerebrate cat deduced from recordings from hindlimb muscle spindles

1. Recordings have been made from multiple single muscle spindle afferents from medial gastrocnemius (MG) and tibialis anterior (TA) muscles of one hindlimb in decerebrate cats, together with ankle rotation and EMG signals, during treadmill locomotion. Whilst the other three limbs walked freely, the experimental limb was denervated except for the nerves to MG and TA and secured so that it could rotate only at the ankle joint, without any external load. Each afferent was characterised by succinylcholine testing with regard to its intrafusal fibre contacts. Active movements were recorded and then replayed through a servo mechanism to reproduce the muscle length changes passively after using a barbiturate to suppress gamma-motor firing. 2. The difference in secondary afferent firing obtained by subtracting the discharge during passive movements from that during active movements was taken to represent the profile of static fusimotor activity. This indicated an increase before the onset of movement followed by a strongly modulated discharge in parallel with muscle shortening during locomotion. The pattern of static firing matched the pattern of unloaded muscle shortening very closely in the case of TA and with some phase advance in the case of MG. The same effects were observed in primary afferents. 3. Primary afferents with bag1 (b1) contacts in addition showed higher firing frequencies during muscle lengthening in active than in passive movements. This indicated increased dynamic fusimotor firing during active locomotion. There was no evidence as to whether this fluctuated during the movement cycles. 4. When the mean active minus passive difference profile of firing in bag2-chain (b2c) type primary afferents was subtracted from that for b1b2c afferents, the difference was dominated by a peak centred on the moment of maximum lengthening velocity (v). 5. The component of the active minus passive difference firing due to b1 fibre contacts could be modelled by f(t) = av (where a is a constant) during lengthening and by f(t) = 0.2 av during shortening. The remainder of the difference signal matched the predictions of the static fusimotor signal derived from secondary afferents. 6. The findings are discussed in relation to the concept that the modulated static fusimotor pattern may represent a 'temporal template' of the expected movement, though the relationship of the results to locomotion in the intact animal will require further investigation. The analysis of the data indicates that the combined action of muscle length changes and static and dynamic fusimotor activity to determine primary afferent firing can be understood in terms of the interaction between the b1 and b2c impulse initiation sites.

Physiological signs of the activation of bag2 and chain intrafusal muscle fibers of gastrocnemius muscle spindles in the cat

A method is described for identifying the effect of single gamma static (gamma(s)) axons on bag2 or chain intrafusal fibers using random (Poisson-distributed) stimuli. The cross-correlogram of the stimuli with the firing of spindle primary afferents took one of three forms. A large, simple, brief response was taken to indicate pure chain fiber activation and a small, prolonged response to indicate pure bag2 activation. A compound response with brief and prolonged components was taken to be a sign of mixed innervation. The correlogram components could be well fitted with lognormal curves. They could also be transformed into curves of gain as a function of frequency, which were convenient for estimating the strength of the effects. In 68 effects of gammas axons on Ia afferents, 16 were pure chain, 17 pure bag2, and 35 mixed. This distribution was significantly different (P < 0. 05) from that expected from chance nonspecific innervation of chain and bag2 fibers. Making use of the estimates of the strength of chain and bag2 effects derived from the gain curves, the classification was modified by treating mixed responses that had one effect more than five times stronger than the other as belonging to the dominant type. The distribution was then as follows: chain 16, bag2 28, and mixed 24. This differed very significantly from the prediction of chance distribution (P < 0.001). This evidence for some degree of specific innervation of chain and bag2 fibers is discussed in relation to previous work and with regard to the ways in which the two fiber types might be used in natural movements.

Models of ensemble firing of muscle spindle afferents recorded during normal locomotion in cats

1. The aim of this work was to compare the ability of several mathematical models to predict the firing characteristics of muscle spindle primary afferents recorded chronically during normal stepping in cats. 2. Ensemble firing profiles of nine hamstring spindle primary (presumed group Ia) afferents were compiled from stored data from 132 step cycles. Three sets of profiles corresponding to slow, medium and fast steps were generated by averaging groups of step cycles aligned to peak muscle length in each cycle. 3. Five models obtained from the literature were compared. Each of these models was used to predict the spindle firing profiles from the averaged muscle length signals. The models were also used in the reverse direction, namely to predict muscle length from the firing profiles. A sixth model incorporating some key aspects of the other models was also included in the comparisons. 4. Five of the models predicted spindle firing well, with root mean square (r.m.s.) errors lower than 14 % of the modulation depth of the target profiles. The key variable in achieving good predictions was muscle velocity, the best fits being obtained with power-law functions of velocity, with an exponent of 0.5 or 0.6 (i.e. spindle firing rate is approximately proportional to the square root of muscle velocity). The fits were slightly improved by adding small components of EMG signal to mimic fusimotor action linked to muscle activation. The modest relative size of EMG-linked fusimotor action may be related to the fact that hamstring muscles are not strongly recruited in stepping. 5. Length was predicted very accurately from firing profiles with the inverse of the above models, indicating that the nervous system could in principle process spindle firing in a relatively simple way to give accurate information on muscle length. 6. The responses of the models to standard ramp-and-hold displacements at 10 mm s-1 were also studied (i.e. velocities that were an order of magnitude lower than that during stepping). In these cases components of spindle primary response related to length as well as velocity were needed for good fits. Because these length-related components detracted from rather than improved predictions of the step cycle data, an attenuation of length dependence at high muscle velocities emerged as a possibility. 7. We conclude that in this study we have identified models and parameters that may be used to predict spindle afferent firing from the time course of muscle length in the cat step cycle.

Fusimotor influence on jaw muscle spindle activity during swallowing-related movements in the cat

1. The activity patterns of muscle spindle afferents in jaw-closer muscles were studied during reflex swallowing movements in anaesthetized cats. Simultaneous records were made of the electromyogram (EMG) in masseter and anterior digastric muscles and of the unloaded jaw movements. The underlying patterns of fusimotor activity were deduced by comparing afferent discharges occurring during active swallowing with those occurring when exactly the same movements were imposed passively. The interpretation of spindle behaviour was greatly facilitated by characterizing the afferents according to the evidence for their contact with the various intrafusal muscle fibres, derived from testing with succinylcholine. It was also valuable to have two different types of afferent recorded simultaneously. 2. There was clear evidence of fusimotor activity occurring during active jaw closing so as to oppose the spindle silencing. This effect was most marked in b2c-type afferents (probably secondaries) and was therefore attributed to a modulation of static fusimotor discharge approximately in parallel with alpha-activity. 3. Afferents with evidence of bag1 fibre contacts (primaries) showed much greater sensitivity to muscle lengthening during active movement than when the movement was imposed. This difference was exaggerated when anaesthesia was deepened for the passive movements. This was interpreted as evidence for a higher level of dynamic fusimotor activity maintained during active movements than at rest. 4. The results support the view that for a variety of active jaw movements, static fusimotor neurone firing is modulated roughly in parallel with alpha-activity but leading it so as to counteract spindle unloading. Dynamic fusimotor neurone firing appears to be set at a raised level during active movements. Anaesthesia appears to depress activity in the alpha-motoneurones more than in gamma-motoneurones.

EMG spike trains of succinylcholine-induced fasciculations in myalgic patients

Single spike activity from the surface electromyogram (EMG) of fasciculations induced by succinylcholine (Sch) were studied from limb muscles (biceps, triceps, anterior tibialis and gastrocnemius) in 100 female patients. About 2/3 of them (n = 72) also received nondepolarizing neuromuscular pretreatment (atracurium or vecuronium). We observed from 20% of EMG records in the myalgic (but not in the nonmyalgic) patients, sustained spike trains (mean duration 1.47 s) that resembled motor units firing at physiologically high rates (mean 21.7 spikes/s). The finding reflects Sch's distal actions at the muscle spindle. The implications for myalgia and the possible involvement of micro damage at the extrafusal muscles are discussed.

Action of cholinesters on sensory nerve endings in skin and muscle

1. This is a review of the literature on the subject of the effects of cholinesters and their agonists on sensory nerve endings. 2. The present-day view is that acetylcholine (ACh) has an excitatory action on some cutaneous receptors. Responses appear to be limited to receptors served by small myelinated and un-myelinated axons where responsiveness is multimodal; that is, the receptors are activated by noxious thermal and mechanical stimulation. 3. The possible role played by acetylcholine in sensory transduction processes is discussed, as are other explanations for the presence of nicotinic cholinergic receptors on the terminals of cutaneous receptors. 4. The excitatory action of ACh and succinylcholine (SCh) on muscle spindles is described. Two possible mechanisms are considered: a direct depolarizing action on the nerve terminals and indirect excitation, brought about by a contracture of the intrafusal fibres on which the sensory endings lie. 5. The technique of using SCh in combination with fusimotor stimulation is described. This has provided new information about the internal workings of muscle spindles. Brief mention is also made of the action of SCh on tendon organs and joint receptors. 6. It is concluded that a direct action by cholinesters is restricted to receptors served by small axons with multimodal functions. The precise role of such an action remains the subject of speculation. Possible clinical significance is discussed.

Time-dependent fusimotor effects on the discharge of cat primary muscle spindle afferents induced by a long-lasting succinylcholine infusion

The responses of 46 Ia afferents from the tibial anterior muscle of the cat to repetitive ramp-and-hold stretches were investigated under a succinylcholine (SCh) infusion of 120 micrograms.kg-1.min-1 lasting 15 to 25 min. It was possible to distinguish four consecutive phases of the effect of the SCh on the responsiveness of the Ia afferents. The first three of these four phases have already been described. We analysed in more detail the changes from Phase III to the end of Phase IV. Static fusimotor effects were dominant in the discharge patterns obtained during Phase III; dynamic fusimotor effects prevail at the end of Phase IV. Our observations were quantified by comparing the mean values of initial activity, final static value, dynamic response and slow receptor adaptation read from the discharge patterns obtained during Phase III with the mean values of the same parameters obtained from discharge patterns from the end of Phase IV: the two mean values were significantly different for each of the four measurements. This change from Phase III to the end of Phase IV is highly specific for each spindle. To demonstrate this spindle specificity, discharge patterns were selected from among those produced by each Ia afferent in Phase III and at the end of Phase IV. Each of these discharge patterns was assigned to one of six categories. Category I displays purely dynamic fusimotor effects and Category VI purely static fusimotor effects. Categories II, IV and V display combinations of static and dynamic fusimotor effects with an increasing admixture of static fusimotor effects. The spindle-specific change from Phase III to the end of Phase IV is defined in terms of the specific degree of change from a higher-number to a lower-number category in the case of each of the 46 Ia afferents. In the discussion a combination of activity by the two nuclear bag fibres of a spindle is deduced from the specific discharge pattern of each category. The conclusion from these considerations is that results obtained from the administration of SCh have to be interpreted with great caution in making any statement about the existence of a dynamic bag1 fibre in a spindle.

Segmental and supraspinal control of synaptic effectiveness of functionally identified muscle afferents in the cat

The present investigation documents the patterns of primary afferent depolarization (PAD) of single, functionally identified muscle afferents from the medial gastrocnemius nerve in the intact, anesthetized cat. Classification of the impaled muscle afferents as from muscle spindles or from tendon organs was made according to several criteria, which comprised measurement of conduction velocity and electrical threshold of the peripheral axons, and the maximal frequency followed by the afferent fibers during vibration, as well as the changes in discharge frequency during longitudinal stretch, the projection of the afferent fiber to the motor pool, and, in unparalyzed preparations, the changes in afferent activity during a muscle twitch. In confirmation of a previous study, we found that most muscle spindle afferents (46.1-66.6%, depending on the combination of criteria utilized for receptor classification) had a type A PAD pattern. That is, they were depolarized by stimulation of group I fibers of the posterior biceps and semitendinosus (PBSt) nerve, but not by stimulation of cutaneous nerves (sural and superficial peroneus) or the bulbar reticular formation (RF), which in many cases inhibited the PBSt-induced PAD. In addition, we found a significant fraction of muscle spindle primaries that were depolarized by stimulation of group I PBSt fibers and also by stimulation of the bulbar RF. Stimulation of cutaneous nerves produced PAD in 9.1-31.2% of these fibers (type B PAD pattern) and no PAD in 8.2-15.4% (type C PAD pattern). In contrast to muscle spindle afferents, only the 7.7-15.4% of fibers from tendon organs had a type A PAD pattern, 23-46.1% had a type B and 50-61.5% a type C PAD pattern. These observations suggest that the neuronal circuitry involved in the control of the synaptic effectiveness of muscle spindles and tendon organs is subjected to excitatory as well as to inhibitory influences from cutaneous and reticulospinal fibers. As shown in the accompanying paper, the balance between excitation and inhibition is not fixed, but can be changed by crushing the afferent axons in the peripheral nerve and allowing subsequent reconnection of these afferent fibers with muscle receptors.

Regularity in the generation of discharge patterns by primary and secondary muscle spindle afferents, as recorded under a ramp-and-hold stretch

The discharge frequency of primary (Ia) and secondary (II) muscle spindle afferents from the tibial anterior muscle of the cat were recorded under a ramp-and-hold stretch of the host muscle. The rate of ramp stretch and the prestretch of the muscle were varied systematically. The degree of stretch was kept constant. For a discharge pattern recorded at a ramp rate of 10 mm/s, the peak dynamic discharge, the maximum static value and the final static value were determined. These three discharge rate values were plotted against the maximum static value. In the resulting charts the II afferents presented themselves as a homogeneous group of spindle afferents, whereas the Ia fibers separated into three subgroups. The existence of three subpopulations of Ia fibers was verified by the method of Hald. Furthermore, it is shown that each subpopulation generated its discharge patterns in its own regularly systematic manner. It was concluded that, as one of the three Ia subpopulations exhibits much the same dynamic and static stretch properties as the II fibers, the encoder of this subpopulation must receive its receptor current from the sensory terminals of passive intrafusal chain fibers. The encoder of a second Ia subpopulation indicates its action potentials using the receptor current stemming from the bag1 sensory terminals, these Ia fibers eliciting a slow adaptation component of a high magnitude which is assumed to be the consequence of a high level of "creep" in the passive intrafusal bag1 fiber. The third Ia subpopulation initiates its action potential sequences by means of the receptor current stemming from the passive bag2 fiber, producing behavior patterns that lie between those of the other two Ia subpopulations.

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)

Projection of cat jaw muscle spindle afferents related to intrafusal fibre influence

1. A method of classification of muscle spindle afferents using succinylcholine (SCh) and ramp stretches has recently been described, which appears to estimate separately the strength of influence of bag1 (b1) and of bag2 (b2) intrafusal fibres. Increase in dynamic difference (delta DD) indicates b1 influence whilst increase in initial frequency (delta IF) indicates b2 influence. The significance of this classification has now been examined by correlation with the strength of synaptic projection of jaw muscle spindle afferents to the fifth motor nucleus (MotV) and the supratrigeminal region (STR) in anaesthetized cats. 2. Projection strength was estimated by computing the extracellular focal synaptic potential (FSP) from spike-triggered averages of 1024 sweeps at 100 microns intervals along tracks through STR and MotV. Trigger pulses were derived from spindle afferent cell bodies of the jaw-closer muscles recorded in the mesencephalic trigeminal nucleus, and characterized by the effect of SCh on their responses to ramp-and-hold stretches. 3. The maximum size of FSPs in tracks traversing STR and MotV ranged from 2.08 to 36.99 microV with a mean of 7.55 microV. The amplitudes were bimodally distributed into roughly equal-sized groups with high and low amplitude FSPs. 4. Mean values of delta IF were significantly greater for the group with large FSPs than for those with small FSPs. There were no significant differences in delta DD. FSP amplitude was significantly positively correlated with delta IF, but not with delta DD. 5. Spindle afferents with high values of FSP amplitude in MotV had a wide range of values of delta DD (b1b2c and b2c groups), while units with large FSPs in STR were all in the b2c category. Some evidence is presented to indicate that this reflects a preferential projection of secondary afferents to the STR. 6. For those units with projection to both STR and to MotV, there was a significant positive correlation between FSP amplitude in the two nuclei. 7. These results indicate that the extent of the b2 influence on spindle afferents predicts the central projection strength better than does the b1 influence. This finding is discussed from the viewpoint of possible developmental and functional issues.

Fusimotor effects of midbrain stimulation on jaw muscle spindles of the anaesthetized cat

The effects of electrical stimulation within the midbrain on fusimotor output to the jaw elevator muscles were studied in anaesthetized cats. Muscle spindle afferents recorded in the mesencephalic trigeminal nucleus were categorised as primary or secondary by their responses to succinylcholine during sinusoidal or ramp-and-hold stretches. Changes in their stretch responses during midbrain stimulation were then assessed by changes in bias and in dynamic sensitivity. Problems were encountered in interpreting changes in sine wave stretch responses of primary afferents, in some of which a very small change in firing pattern produced large changes in estimates of the response amplitude. Sine wave testing also sometimes over-estimated static effects and under-estimated dynamic effects relative to ramp responses. On other occasions a small amount of static fusimotor activity caused a marked increase in sine response amplitude, which could be wrongly interpreted as a dynamic effect. Consequently, ramp responses only were used for diagnosing fusimotor changes. The most effective region for producing pure dynamic fusimotor excitation was directly rostral to the red nucleus, extending dorsally and ventrally approximately in the course of the retroflex bundle. Stimulation of regions caudal and dorso-caudal to the red nucleus, previously designated as the mesencephalic area for dynamic fusimotor control of leg muscles, gave static or mixed static and dynamic effects on jaw spindles. The use of midbrain stimulation to identify fusimotor neurones of jaw muscles as static or dynamic would be most reliable with stimulation just rostral to the red nucleus and would require spindle afferent behaviour to be monitored at the same time with ramp stretches.

The classification of afferents from muscle spindles of the jaw-closing muscles of the cat

1. The effects of the muscle-depolarizing drug succinylcholine (SCh) on the stretch responses of jaw-closer muscle spindle afferents were studied in the anaesthetized cat. Using ramp and hold stretches repeated every 6 s the basic measurements made were: initial frequency (IF), peak frequency (PF) and static index (SI), the frequency 0.5 s after the end of the ramp of stretch. Derived from these were: dynamic difference (DD) = PF-IF, dynamic index (DI) = PF-SI and static difference (SD) = SI-IF. Increases in these measures caused by a single I.V. dose of SCh (200 micrograms kg-1) are symbolized by the prefix delta. 2. In a population of 234 units, delta DD and delta IF were each distributed bimodally, but were uncorrelated, thus defining four subgroups. 3. delta DD was argued to be an index of the effect of bag1 intrafusal fibre contraction and delta IF to be an index of the effect of bag2 fibre contraction. On this basis it is proposed that units can be divided into four groups according to the predominant influences of the bag1, bag2 and chain fibres as b1c (6.8%), b1b2c (22.2%), b2c (54.3%) or c (16.7%). 4. Testing with sine wave stretches at 1 Hz showed that changes in mean frequency and amplitude of response caused by SCh correlated with delta IF and delta DD respectively, but separated groups of units much less effectively than did ramp and hold testing. 5. Evidence is presented to indicate that the control value of DD in passive spindles does not relate to the potential strength of bag1 fibre effects in fully activated spindles. The bag1 fibre appears to contribute little to responses of spindle afferents in the passive state. DD is superior to DI as a measure of bag1 effects. 6. Conduction velocity was unimodally distributed in masseter spindle afferents and was not correlated with delta DD or delta IF and was therefore of no value in classifying them. 7. Neither the threshold of afferents to quick transient stretch nor the coefficient of variation of interspike intervals provided any significant additional help in classification. 8. The unexpectedly high proportion of units of b2c type is thought to include primaries lacking appreciable bag1 fibre contacts and secondaries with more or less substantial bag2 contracts.