Muscle Spindle Afferents Research Articles

Proprioceptive control of multijoint movement: bimanual circle drawing.

Proprioception is used by the central nervous system (CNS) in the control of the spatial and temporal characteristics of single joint and multiple joint movement. The present study addressed the role of proprioception in the control of bilateral cyclical movements of the limbs. Normal blindfolded human subjects drew circles simultaneously and symmetrically with the two arms (16 cm diameter, 1 /s) upon two digitizing tablets. In selected trials, vibration (60-70 Hz) was applied to the tendon of the biceps and/or anterior deltoid muscles of the dominant arm to distort the proprioceptive information from muscle spindle afferents. One goal of this study was to identify whether tendon vibration influenced the spatial characteristics of circles drawn by the vibrated, dominant arm and the non-vibrated, non-dominant arm. A second goal was to determine the effect of vibration on the temporal coupling between the two arms during circle drawing. The results revealed that tendon vibration affected the spatial characteristics of circles drawn by the vibrated arm in a manner similar to that previously found for unilateral circle drawing. During bimanual circle drawing, vibration had only a minimal effect on the spatial characteristics of the non-vibrated, non-dominant arm. Temporal interlimb coupling was quantified by the relative phasing between the arms. Without tendon vibration, the dominant arm led the non-dominant arm. Vibration of the dominant arm increased the average phase lead. In a first control experiment, vibration of the non-dominant arm decreased the phase lead of the dominant arm, or even reversed it to a non-dominant arm phase lead. In a second control experiment, the subjects performed the bimanual circle-drawing task with vision of only the vibrated arm, in which case there was no spatial distortion of the circles drawn by the vibrated arm, but the phase relation between the two arms was still shifted as if vision were completely unavailable. It was concluded that, in bimanual movements such as these, the spatial and temporal characteristics of movement are controlled independently. Whereas the spatial characteristics of hand movement seem to be controlled unilaterally, the temporal characteristics of interlimb coupling appear to be controlled by proprioceptive information from both limbs, possibly by a proprioceptive triggering mechanism.

Muscle spindle afferent input to motoneurons in human masseter.

The H-reflex response in large and small single motor units in human deep anterior masseter was studied to investigate the distribution of muscle spindle afferents onto masseter motoneurons. We found that only the larger units displayed H-reflex responses. This indicates preferential distribution of muscle spindle input onto large motoneurons or a skewed distribution of tonic presynaptic inhibitory mechanisms.

Axoaxonic synapses on terminals of group II muscle spindle afferent axons in the spinal cord of the cat.

The purpose of the present study was to determine if terminals of identified group II muscle spindle afferents participate in axoaxonic synaptic arrangements and, if so, to investigate the transmitter content of presynaptic terminals in these arrangements. Group II muscle afferents supplying the gastrocnemius-soleus or semitendinosus muscles were identified in adult cats and stained intra-axonally with horseradish peroxidase. In total, three group II axons were labelled and processed for combined light and electron microscopy. Group II axons gave rise to collaterals which characteristically descended through the superficial dorsal horn and formed relatively sparse terminal arborizations in the dorsal horn (laminae IV and V) and more profuse arbors in the intermediate grey matter (laminae VI-VII). Forty boutons were examined through series of ultrathin sections and all but four were postsynaptic to other axon terminals. Occasionally, more than one axon was presynaptic to a single group II terminal. Immunogold studies showed that all axons in presynaptic apposition to group II boutons contained gamma-aminobutyric acid (GABA) and also that glycine was colocalized in the majority of these axons. This evidence suggests that transmission from group II muscle afferents is under strong presynaptic inhibitory control and that it is mainly the subgroup of GABAergic interneurons with colocalized glycine which mediate this inhibition. Seventeen group II boutons were components of synaptic triads where the presynaptic axoaxonic bouton formed a synapse with the same dendrite as the group II axon. Therefore, a proportion of the interneurons which form axoaxonic synapses with group II axons are also likely to have postsynaptic inhibitory actions on target neurons of group II afferents.

Changes in reflex responses of the masseter and digastric muscles during sleep in freely behaving rabbits.

The aim of this study was to clarify the modulation of motoneuron excitability in masticatory muscles during sleep. For this purpose, changes in the reflex responses of the masseter and digastric muscles between sleep and wakefulness were studied in freely behaving rabbits. Stimulation of the jaw closing muscle spindle afferents induced the masseteric monosynaptic reflex (MMR). During quiet sleep (QS), which gradually replaced quiet wakefulness, the mean MMR amplitude showed no change. During active sleep (AS). MMR amplitudes were markedly reduced, but they were confounded by the occasional occurrence of facilitation in the amplitude. The facilitatory reflexes were often related to rapid eye movements (REMs). However, the excitatory input probably did not originate from the same region as the REM generator, since the REM and the large MMR did not always occur simultaneously. On the other hand, jaw opening reflexes remained inhibited. The results indicated that there is mainly a difference in the excitability between the two groups of motoneurons during AS; masseter motoneuron activity was inhibited but occasionally facilitated by excitatory inputs occurring in association with REMs, however, the digastric motoneuron activity was remained inhibited. The excitatory inputs may induce dysfunctional muscle contraction of the jaw closing muscles as seen in bruxism.

Special sensory ataxia in Miller Fisher syndrome detected by postural body sway analysis.

To investigate whether ataxia in Miller Fisher syndrome (MFS) is caused by loss of proprioception or cerebellar dysfunction, we studied the power spectrum peak of the body sway frequency in 10 MFS patients, and compared the results with those of patients with cerebellar or sensory ataxia. The cerebellar patients had a peak at 2.4 Hz, whereas sensory ataxia patients had a 1-Hz peak. Nine of the MFS patients had a distinct 1-Hz peak. Clinical sensory loss or abnormal sensory nerve potentials were present in only 3 patients, whereas soleus H-reflexes were absent in all the MFS patients. MFS patients have dysfunction of the proprioceptive afferent system, and the special sensory ataxia may be caused by the selective involvement of muscle spindle afferents.

The response of muscle spindle primary afferents to simultaneously presented sinusoidal and ramp-and-hold stretches

Fifteen primary (Ia) muscle spindle afferents from the tibial anterior muscle of the cat were subjected to a ramp-and-hold stretch (stretch rate 10 mm/s, stretch amplitude 8.5 or 7 mm) of the muscle, upon which was superimposed a sinusoidal stretch (10 Hz) of five different amplitudes (50, 100, 500, 1000 and 2000 μm peak to peak). The response of the Ia afferents to the sinusoidally overlaid ramp-and-hold stretch was subjected to computer analysis, by means of which the response to the superimposed sinusoids and the response to the underlying ramp-and-hold stretch were each obtained separately. Four basic discharge frequencies were determined from the response to the underlying ramp-and-hold stretch (obtained after elimination of the response to the concomitant sinusoidal stretch). The evaluation yielded the result that the concomitant sinusoidal stretch affected the response to the underlying ramp-and-hold stretch: the level of discharge of the Ia afferent was significantly increased and the dynamic and static indices were significantly diminished. These two effects intensified with increasing amplitude of the concomitant sinusoids. From among the oscillatory responses of the Ia afferent to the concomitant sinusoidal stretch (obtained after elimination of the response to the underlying ramp-and-hold stretch) the amplitudes of four selected responses were determined. These four oscillatory responses occurred during four time spans during which the four basic discharge frequencies were read. Evaluation showed that the response to the underlying ramp-and-hold stretch affected the modulation depth of the oscillatory responses, which decreased during the ramp and increased again during the plateau. The absolute amount of the change was independent of the amplitude of the sinusoids. The effect of the sinusoids on the response to the underlying ramp-and-hold stretch and that of the underlying ramp-and-hold stretch on the response to the sinusoidal stretches are interpreted in terms of ion currents occurring at the Ia sensory terminals. It is assumed that the probability of the stretch activated (SA) channels opening is increased by a stretching of the sensory terminals. A depolarizing Na + and Ca ++ inward current due to the activation of the SA channels in its turn activates a Ca ++-activated and/or a potential-dependent K + outward current which has a repolarizing effect. Ideas are put forward in the discussion as to how the inward and outward currents that are sinusoid-related and those that are ramp-and-hold-stretch related interact.

Chapter 13 A Positive Feedback Circuit Involving Muscle Spindle Secondaries and Gamma Motoneurons in the Cat

Publisher Summary Many negative feedback neuronal circuits are known in the mammalian nervous system. Negative feedback is represented, for example, by recurrent inhibition of motoneurons through their axon collaterals and Renshaw cells, by recurrent inhibition of several other neurons, or by autogenetic inhibition of motoneurons following contractions of the muscles that they innervate. Examples of a clear-cut positive feedback are more difficult to find; in a recent review on proprioceptive feedback and movement regulation, positive feedback was mentioned only once and only in the context of force feedback. An effective positive feedback would require the connection between muscle spindle afferents and γ-motoneurons to be as direct as possible, preferably to be monosynaptic. Previous studies led to the conclusion that group II afferents excite γ-motoneurons only through di-, tri-, or even polysynaptic pathways because of the rather long latencies of their actions. Latencies of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) evoked in α-motoneurons by electrical stimulation of group II afferents in muscle nerves were similarly classified as polysynaptically evoked. A number of subsequent observations have, however, shown that at least some synaptic actions of group II muscle afferents on α-motoneurons might be evoked di-synaptically or even mono-synaptically.

Chapter 9 Selectivity of Presynaptic Inhibition: a Mechanism for Independent Control of Information Flow through Individual Collaterals of Single Muscle Spindle Afferents

This chapter discusses the observations on the concept of the intraspinal arborizations of afferent fibers that are not obligatory routes for the conduction of action potentials but are instead dynamic systems that can be modified by central mechanisms to restrict information to selected neuronal targets. The differential inhibition of primary afferent depolarization (PAD) is envisaged as a means by which different spinal postsynaptic targets coupled by sensory input from a common source could be decoupled by central control mechanisms. This could be of relevance for motor performance and sensory discrimination. The extent to which these changes are reflected at the population level depends on the degree of synchronization of the PAD-mediating interneurons, a feature that can also be centrally controlled. A more functional approach to the understanding of the central mechanisms involved in the pre-synaptic control of information flow in sensory fibers will require the use of noninvasive methods in behaving animals performing specific motor tasks.

Dynamic response of human muscle spindle afferents to stretch during voluntary contraction.

1. The response of twenty-eight human muscle spindle afferents from m. extensor carpi radialis brevis to large amplitude ramp stretch and release at the wrist joint was recorded. The dynamic index was calculated as the difference in firing rate between that just before the end of stretch and that during the subsequent static phase of stretch. The value during steady voluntary contraction was compared with that during relaxation. 2. In twenty-three primary afferents, the dynamic index increased in eleven and decreased in twelve afferents with a range of -8 to +25 impulses s-1. In five secondary afferents the change was less than 2 impulses s-1. 3. The primary afferents abruptly stopped firing when the stretch was released in the relaxed muscle. This cessation was prevented during contraction in seventeen primary afferents. 4. The results suggest the presence of dynamic and static fusimotor actions on the human muscle spindles during voluntary contraction.

Neuronal basis of afferent-evoked enhancement of locomotor activity.

Activation of ankle extensor group Ia muscle spindle or Ib tendon organ afferents during locomotion can prolong and enhance hindlimb extensor motoneuron activity. A growing body of evidence suggests that these group I evoked reflexes not only compensate for a changing environment but also help shape extensor activity during normal, unperturbed locomotion. In this paper we review four mechanisms that underlie the group I evoked enhancement of ipsilateral extensor activity during locomotion. The first three are pre-motoneuronal mechanisms that are part of group I reflex pathway reorganization during locomotion. They are (1) a suppression of group I evoked nonreciprocal inhibition, (2) a release from inhibition of excitatory interneurons in disynaptic pathways from group I afferents to extensor motoneurons, and (3) longer latency excitation evoked through extensor portions of the locomotor circuitry. The fourth factor contributing to group I evoked increases in motoneuron activity during locomotion is the increase in motoneuron excitability produced by postsynaptic changes in motoneuron membrane conductances. Most results to be discussed were obtained during locomotion in decerebrate cats in which fictive locomotion was evoked by stimulation of the midbrain following neuromuscular blockade.

Local control of information flow in segmental and ascending collaterals of single afferents.

In the vertebrate spinal cord, the activation of GABA(gamma-amino-butyric acid)-releasing interneurons that synapse with intraspinal terminals of sensory fibres leading into the central nervous system (afferent fibres) produces primary afferent depolarization and presynaptic inhibition. It is not known to what extent these presynaptic mechanisms allow a selective control of information transmitted through specific sets of intraspinal branches of individual afferents. Here we study the local nature of the presynaptic control by measuring primary afferent depolarization simultaneously in two intraspinal collaterals of the same muscle spindle afferent. One of these collaterals ends at the L6-L7 segmental level in the intermediate nucleus, and the other ascends to segment L3 within Clarke's column, the site of origin of spinocerebellar neurons. Our results indicate that there are central mechanisms that are able to affect independently the synaptic effectiveness of segmental and ascending collaterals of individual muscle spindle afferents. Focal control of presynaptic inhibition thus allows the intraspinal branches of afferent fibres to function as a dynamic assembly that can be fractionated to convey information to selected neuronal targets. This may be a mechanism by which different spinal postsynaptic targets that are coupled by sensory input from a common source could be uncoupled.

Bradykinin and muscle stretch alter contralateral cat neck muscle spindle output

The objective of this study was to investigate the activity of primary and secondary muscle spindle afferents (MSAs) in neck muscles, when the contralateral splenius or trapezius muscles were (1) exposed to i.m. injection of bradykinin (BK) (6-86 μg/ml), (2) stretched, (3) stretched during exposure to BK or (4) stretched after exposure to BK. It was found that injection of BK, muscle stretch and the combination of the two stimuli significantly excited primary and secondary MSAs via reflex effects onto static fusimotor neurones. BK, alone, and in combination with muscle stretch, induced more frequent, potent and long lasting effects as compared to muscle stretch. The effects of muscle stretch was significantly increased after exposure of BK, indicating that stretch-sensitive nerve-endings within neck muscles are sensitised by BK. The results are discussed in relation to the disturbances in motor co-ordination and proprioception found in patients suffering from chronic muscle pain in the cervical region.

New observations on coupling between group II muscle afferents and feline gamma-motoneurones.

1. Extra- or intracellular recordings were made from seventy-six gamma-motoneurones of hindlimb muscles in chloralose anaesthetized cats to re-assess the coupling between secondary muscle spindle afferents (group II muscle afferents) and these neurones. The latencies of a number of responses evoked by group II muscle afferents in gamma-motoneurones were shorter than minimal latencies of responses induced disynaptically in other spinal neurones. These latencies are therefore compatible with monosynaptic coupling between muscle spindle secondaries and gamma-motoneurones. 2. Responses fulfilling criteria for monosynaptically evoked responses were seen in about one third of gamma-motoneurones with input from the group II muscle afferents tested (in 6 of 18 motoneurones recorded intracellularly and in 26 of 74 motoneurones recorded extracellularly). They were usually evoked from only one of the stimulated nerves, stimulation of group II afferents of other nerves being followed by responses at longer latencies. 3. Most gamma-motoneurones were excited by group II afferents from several muscles, both flexors and extensors. However, a comparison of group II input to gamma-motoneurones innervating medial gastrocnemius and four other hindlimb muscles revealed differences in both incidence and sources. 4. This study extends results of previous studies by providing evidence that some synaptic actions of group II afferents, including afferents from the same muscle, are evoked monosynaptically, and may assist in sustaining the activation of gamma-motoneurones by positive feedback.

Coupling between single muscle spindle afferent and EMG in human wrist extensor muscles: physiological evidence of skeletofusimotor (beta) innervation

Objectives: This study was conducted to find physiological evidence of skeletofusimotor innervation in man. Methods: Discharges of 38 single muscle spindle afferents from m. extensor carpi radialis were recorded from 9 subjects during steady isometric contractions of wrist extension. Correlation between these afferents and rectified surface EMG was investigated by estimating cumulant density function. Results: In the cumulant density estimate between spindle afferent and EMG, a positive EMG peak was obtained prior to afferent firing between −30 and −10 ms in 15 afferents (39%). Conclusions: The present finding indicates coupling between spindle afferents and extrafusal activity and suggests the widespread skeletofusimotor innervation in human muscle spindle.

Oro-facial muscles: internal structure, function and ageing.

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to be much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II ('fast-twitch') fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I ('slow twitch') and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.

Ago-antagonist muscle spindle inputs contribute together to joint movement coding in man

The proprioceptive feedback associated with the performance of even quite simple movements is always generated by the whole set of muscles subjected to mechanical deformation (lengthening, shortening, contraction, etc.) during that particular movement. The question was addressed here as to how muscle spindle feedbacks arising from agonist and antagonist muscles may contribute to the coding of movement parameters such as the direction and velocity. For this purpose, the activity of single muscle spindle afferents located in the lateral peroneal nerve was analysed using the microneurographic technique, in human subjects performing repetitive voluntary movements, i.e., plantar/dorsal flexions of the ankle, at three different velocities (3, 4.5 and 6°/s). The data obtained suggest that in humans, the direction of a slow movement may be specified on the basis of the spindle discharge rate, which is greater in the stretched than in the shortened muscle, and that the velocity of this movement might be correlated with the difference between the spindle activity occurring in the agonist and antagonist muscles. These neurophysiological data are in agreement with the results of previous psychophysical studies showing for example that a sensation of illusory movement can be elicited only when there exists an imbalance between the agonist versus antagonist vibration-induced Ia inputs. In addition, the greater the difference between the vibration frequencies applied to the two antagonist muscles, the higher the perceived movement velocity was found to be. All in all, joint movement perception seems to result from the co-processing by the central nervous system of the multiple spindle feedbacks originating from the whole set of muscles involved in the performance of a movement.

Sensory axons are guided by local cues in the developing dorsal spinal cord.

During development, different classes of sensory neurons establish distinctive central projections within the spinal cord. Muscle spindle afferents (Ia fibers) grow ventrally through the dorsal horn to the ventral cord, whereas cutaneous sensory collaterals remain confined to the dorsal horn. We have studied the nature of the cues used by Ia fibers in establishing their characteristic projections within the dorsal horn. An organotypic culture preparation of embryonic chicken spinal cord and sensory ganglia was used to test the influence of ventral spinal cord and local cues within the dorsal spinal cord on the growing Ia afferents. When the ventral half of the spinal cord was replaced with an inverted duplicate dorsal half, Ia fibers entering through the dorsal columns still grew ventrally within the host dorsal horn. After the fibers entered the duplicate dorsal half, they continued growing in the same direction. With respect to the duplicate dorsal tissue, this was in an opposite, ventral-to-dorsal, direction. In both cases, however, Ia collaterals remained confined to the medial dorsal laminae. Restriction to these laminae was maintained even when the fibers had to change their direction of growth to stay within them. These results show that cues from the ventral cord are not required for the development of correct Ia projections within the dorsal horn. Local, rather than long-range directional, cues appear to determine the pattern of these projections. When the ventral half of the spinal cord was left intact but sensory axons were forced to enter the dorsal gray matter growing rostrally or caudally, their collateral axons grew in random directions, further showing the absence of directional cues even when the ventral cord was present. Taken together, these observations suggest that Ia fibers are guided by local positional cues that keep them confined to the medial gray matter within the dorsal horn, but their direction of growth is determined primarily by their orientation and position as they enter the dorsal gray matter.

Central distribution of synaptic contacts of primary and secondary jaw muscle spindle afferents in the trigeminal motor nucleus of the cat

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The present study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological characteristics of the physiologically classified afferents at the light and electron microscopic levels through the use of the intra-axonal horseradish peroxidase (HRP) injection technique. The effects of SCh on stretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty-eight and 61 afferents were classified as group Ia and group II afferents, respectively. The central projections of 17 intra-axonally stained afferents (10 group Ia and 7 group II afferents) were examined. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II afferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, the proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II afferents. The ultrastructure of serially sectioned afferent boutons (63 group Ia and 72 group II boutons) also was examined. The boutons from the two groups were distributed widely from the soma to small-diameter dendrites, but the frequency of synaptic contacts on proximal dendrites was higher in group Ia than group II afferents. The present study provides evidence that the two groups of jaw muscle spindle afferents differ in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons. J. Comp. Neurol. 391:50–63, 1998. © 1998 Wiley-Liss, Inc.

Central distribution of synaptic contacts of primary and secondary jaw muscle spindle afferents in the trigeminal motor nucleus of the cat

Little is known about the differences of the terminations of group Ia and group II afferents within the brainstem or spinal cord. The present study was performed to classify cat jaw muscle spindle afferents by the use of succinylcholine (SCh) and to examine the morphological characteristics of the physiologically classified afferents at the light and electron microscopic levels through the use of the intra-axonal horseradish peroxidase (HRP) injection technique. The effects of SCh on stretch responses of 119 jaw muscle spindle afferents from the masseter were examined. The SCh converted the single skew distribution of the values for dynamic index (DI) into a bimodal one. Fifty-eight and 61 afferents were classified as group Ia and group II afferents, respectively. The central projections of 17 intra-axonally stained afferents (10 group Ia and 7 group II afferents) were examined. The spindle afferents terminated mainly in the supratrigeminal nucleus (Vsup), region h, and the dorsolateral subdivision of trigeminal motor nucleus (Vmo.dl) but differed in the pattern of projections of group Ia and group II afferents. The proportion of group Ia afferent terminals was higher in Vmo.dl but lower in Vsup than that of group II afferents. In Vmo.dl, the proportion of group Ia afferent terminals was higher in the central region but lower in the more outer regions than that of group II afferents. The ultrastructure of serially sectioned afferent boutons (63 group Ia and 72 group II boutons) also was examined. The boutons from the two groups were distributed widely from the soma to small-diameter dendrites, but the frequency of synaptic contacts on proximal dendrites was higher in group Ia than group II afferents. The present study provides evidence that the two groups of jaw muscle spindle afferents differ in their central projection and the spatial distribution of their synaptic contacts on Vmo.dl neurons.

The information transmitted by ensembles of primary spindle afferents is diminished when ketamine is used as a pre-anaesthetic

The effect of pre-anaesthetic ketamine on ensemble coding of different stimuli consisting of muscle stretches of various amplitudes was studied for ensembles of simultaneously recorded primary muscle spindle afferents (MSAs). The experiments were conducted on 8 α-chloralose anaesthetised cats. Three of the cats received a pre-anaesthetic dose of ketamine (25 mg/kg) injected subcutaneously (ketamine group), while the remaining five animals did not (non-ketamine group). Data for ensemble coding were collected both before and after cutting the ventral root. A method based on principal component analysis and algorithms was used to quantify stimulus discrimination and an ANOVA tested differences between groups as well as differences due to ventral root cutting. When the fusimotor supply was intact, a general trend of an increase in the ability to discriminate stimuli with increasing ensemble size was observed for both groups, however, this trend was significantly greater for the non-ketamine group as compared to the ketamine group. When the ventral root was cut, the discrimination pattern for the non-ketamine group decreased significantly (as compared to before ventral root cutting), however, no change occurred for the ketamine group. Consequently, no difference in discrimination pattern was detected between groups after ventral root cutting. The reduction in information transmitted by ensembles of primary MSAs when ketamine is used as a pre-anaesthetic may suggest that ketamine elicits an adverse affect on the fusimotor system.