Plastic changes in spinal synaptic transmission following botulinum toxin A in patients with post-stroke spasticity

OBJECTIVE

The therapeutic effects of intramuscular injections of botulinum toxin-type A on spasticity can largely be explained by its blocking action at the neuromuscular junction. Botulinum toxin-type A is also thought to have a central action on the functional organization of the central nervous system. This study assessed the action of botulinum toxin-type A on spinal motor networks by investigating post-activation depression of the soleus H-reflex in post-stroke patients. Post-activation depression, a presynaptic mechanism controlling the synaptic efficacy of Ia-motoneuron transmission, is involved in the pathophysiology of spasticity.

PATIENTS

Eight patients with chronic hemiplegia post-stroke presenting with lower limb spasticity and requiring botulinum toxin-type A injection in the ankle extensor muscle.

METHODS

Post-activation depression of soleus H-reflex assessed as frequency-related depression of H-reflex was investigated before and 3, 6 and 12 weeks after botulinum toxin-type A injections in the triceps surae. Post-activation depression was quantified as the ratio between H-reflex amplitude at 0.5 and 0.1 Hz.

RESULTS

Post-activation depression of soleus H-reflex, which is reduced on the paretic leg, was affected 3 weeks after botulinum toxin-type A injection. Depending on the residual motor capacity of the post-stroke patients, post-activation depression was either restored in patients with preserved voluntary motor control or further reduced in patients with no residual voluntary control.

CONCLUSION

Botulinum toxin treatment induces synaptic plasticity at the Ia-motoneuron synapse in post-stroke paretic patients, which suggests that the effectiveness of botulinum toxin-type A in post-stroke rehabilitation might be partly due to its central effects.

Modulation of human motoneuron activity by a mental arithmetic task

This study aimed to determine whether the performance of a mental task affects motoneuron activity. To this end, the tonic discharge pattern of wrist extensor motor units was analyzed in healthy subjects while they were required to maintain a steady wrist extension force and to concurrently perform a mental arithmetic (MA) task. A shortening of the mean inter-spike interval (ISI) and a decrease in ISI variability occurred when MA task was superimposed to the motor task. Aloud and silent MA affected equally the rate and variability of motoneuron discharge. Increases in surface EMG activity and force level were consistent with the modulation of the motor unit discharge rate. Trial-by-trial analysis of the characteristics of motor unit firing revealed that performing MA increases activation of wrist extensor SMU. It is suggested that increase in muscle spindle afferent activity, resulting from fusimotor drive activation by MA, may have contributed to the increase in synaptic inputs to motoneurons during the mental task performance, likely together with enhancement in the descending drive. The finding that a mental task affects motoneuron activity could have consequences in assessment of motor disabilities and in rehabilitation in motor pathologies.

Anticipatory changes in human motoneuron discharge patterns during motor preparation

The influence of motor preparation on human motoneuron activity was studied by combining single motor unit recording techniques with reaction-time (RT) methods. The tonic activity of wrist extensor motor units associated with voluntary isometric contractions was analysed during preparation for a ballistic wrist extensor muscle contraction, using a time preparation procedure. Two durations of the preparatory period elapsing between the warning signal and the response signal were used in separate blocks of trials: a short preparatory period (1 s) allowing optimum time preparation, and a longer, non-optimum one (3 s). Changes in motoneuron tonic discharge patterns not associated with any changes in the force output were observed during the preparatory period, which suggests that these changes were subtle enough to prevent any changes in muscle contraction from occurring before the forthcoming movement. The changes observed were a lengthening of the mean interspike interval (ISI) and a decrease in the ISI variability. These data confirm that inhibitory mechanisms are activated during motor preparation and suggest that spinal inhibitory mechanisms are involved in the preparatory processes. The mechanisms possibly involved, such as presynaptic inhibition, disfacilitation processes or AHP conductance changes, are discussed. The fact that the preparation-induced effects on motoneuron activity were particularly prominent during the last part of the 3 s preparatory period suggests that they were probably related to the neural processes underlying temporal estimation. The anticipatory changes in motoneuron activity observed here during preparation for action provide evidence that central influences act on spinal motoneurons well before it is time to act.

Proprioception and myoclonus

This review focuses on sensory information originating from muscle spindles and its role in proprioception and motor control. The first part reminds of the structural and functional properties of these muscle mechanoreceptors, with arguments for an independent fusimotor command, i.e. the gamma-motoneurons, that would regulate spindle mechanical sensitivity in keeping with the requirements of ongoing motor action. The possibility that dysfunction of the fusimotor system might be responsible for clinical signs is discussed with respect to the hyperexcitability of the sensorimotor cortex that is observed in myoclonus of cortical origin. What is known about the spindle afferents projections into the spinal cord and about the dysfunction of the spinal sensorimotor networks in patients with neurological disorders, is put together in the second part. It is stressed on the significant complexity of the monosynaptic reflex in spite of its "simple" organization. The monosynaptic reflex constitutes the only possible way for testing the excitability of motoneurons and spinal networks. This method is extensively used clinically to examine changes in the nervous system with diseases. When studying changes from the norm, it is important to understand how the reflex functions in neurologically normal conditions. Different mechanisms such as pre-synaptic inhibition, post-activation depression and motoneuronal intrinsic properties are reviewed as they may induce changes in reflex amplitude and have therefore consequences for interpretation of spinal excitability.

Functional sparing of intrafusal muscle fibers in muscular dystrophies

In a previous study, we showed that patients with muscular dystrophies (MDs) perceive passive movements, experience sensations of illusory movement induced by muscle tendon vibration, and have proprioceptive-regulated sways in response to vibratory stimulation applied to the neck and ankle muscle tendons. These findings argue for preserved proprioceptive functions of muscle spindles. However, it is unclear whether the function of intrafusal muscle fibers is spared, i.e., whether they retain their ability to contract when submitted to a fusimotor drive. To answer this question, we analyzed the effects of reinforcement maneuvers (mental computation and the Jendrassik maneuver) that are known to increase muscle spindle sensitivity via fusimotor drive in healthy subjects. Nine patients with different MDs participated in the study. Reinforcement maneuvers increased both the mean amplitude of the Achilles tendon reflex (187 +/- 52.9% of the mean control amplitude) and the sensitivity of muscle spindle afferents to imposed movements of the ankle. The same reinforcement maneuvers failed to alter the amplitude of the Hoffmann reflex in the triceps surae muscle. These results suggest that the intrafusal muscle fibers preserve their contractile abilities in slowly progressive MDs. The reasons for a differential impairment of intrafusal and extrafusal muscle fibers and the clinical implications of the present results are discussed.

Changes in the Inhibitory Control Exerted by the Antagonist Ia Afferents on Human Wrist Extensor Motor Units During an Attention-Demanding Motor Task

The aim of this study was to determine the extent to which an attention-demanding visuomotor task affects the strength of the inhibitory control exerted by the wrist flexor group Ia afferents on the wrist extensor motoneurons. Effects of median nerve stimulation on the tonic activity of wrist extensor single motor units were analyzed in terms of the interspike interval (ISI) lengthening. Results show that the inhibitory effects exerted by the antagonistic group Ia afferents were significantly enhanced when the wrist extensor motoneurons were involved in an attention-demanding task. Enhanced inhibition from antagonist afferents may reflect task-related changes in the excitability of the di- and/or polysynaptic pathways mediating reciprocal inhibition due to either the action of descending inputs and/or an increase in the efficiency of the Ia inputs to the premotoneuronal inhibitory interneurons. Modulation of the inhibition exerted by proprioceptive antagonist afferents may be one of the processes which contribute to the fine adjustment of the wrist muscle force output required in fine handling tasks.

Proprioceptive control of human wrist extensor motor units during an attention-demanding task

The responsiveness of the tonically firing single motor units (SMU) to Ia afferent volleys elicited by either mechanical (T-reflex) or electrical nerve stimulation (H-reflex) was tested in the extensor carpi radialis muscle (ECR) while the subjects were maintaining a steady wrist extension force using visual feedback set either at low or high gain. The aim was to determine whether the proprioceptive control of tonic motoneuronal activity depends on the level of attentiveness required by the behavioural context. The response probability of the SMUs to tendon taps was significantly higher (p<0.0001) and that to electrical nerve stimulation was lower (p<0.001) during the more demanding task. Since these changes in SMU responsiveness were not accompanied by any differences in either the motor unit firing patterns or the mean levels of EMG muscle activity, it can be concluded that there were no attention-related changes in the net excitatory drive to the ECR motoneurons. These results are consistent with the idea that fusimotor sensitization of the muscle spindle may have occurred in the more demanding task: an increase in the mechanical sensitivity of the muscle spindles would certainly account for both the T-reflex facilitation and the H-reflex depression observed. The attention-demanding task therefore seemed to involve an independent fusimotor drive activation process. The results of this study suggest that an adaptation of the fusimotor system occurs in humans, depending on the levels of attention and accuracy required to perform the ongoing motor task, as previously reported to occur in animals.

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.

Increased muscle spindle sensitivity to movement during reinforcement manoeuvres in relaxed human subjects

1. The effects of reinforcement manoeuvres, such as mental computation and the Jendrassik manoeuvre, on muscle spindle sensitivity to passively imposed sinusoidal stretching (1.5 deg, 2 Hz) in relaxed subjects were analysed. 2. The unitary activity of 26 muscle spindle afferents (23 Ia, 3 II) originating from ankle muscles was recorded using the microneurographic method. Particular care was paid to the subjects' state of physical and mental relaxation. 3. The results showed that the activity of 54 % of the Ia afferents was modified during mental computation. The modifications took the form of either an increase in the number of spikes (mean, 26 % among 11 Ia fibres) or a shortening in the latency of the response to sinusoidal stretching (mean, 13 ms among 3 Ia fibres), or both. They were sometimes accompanied by an enhanced variability in the instantaneous discharge frequency. The three secondary endings tested exhibited no change in their sensitivity to stretch during mental computation. 4. The increased sensitivity to passive movements sometimes began as soon as the instructions were given to the subjects and sometimes increased during mental computation. In addition, the increased sensitivity either stopped after the subjects gave the right answer or continued for several minutes. 5. During the performance of a Jendrassik manoeuvre, the Ia units underwent changes similar to those described above for mental computation. 6. It was concluded that muscle spindle sensitivity to movement can be modified in relaxed human subjects. The results reinforce the idea that the fusimotor system plays a role in arousal and expectancy, and contribute to narrowing the gap between human and behaving animal data.

Comparison of the depression of H-reflexes following previous activation in upper and lower limb muscles in human subjects

When conditioning-testing (C-T) stimuli are applied to Ia afferents to elicit H-reflexes, the test reflex is abolished immediately following the conditioning reflex. As the C-T interval is increased, the test response slowly begins to recover, taking several hundred milliseconds to attain control values. The time course of this recovery is known as the H-reflex recovery curve. H-reflex recovery curves were compared using surface EMG and single motor unit activities in lower limb soleus and upper limb flexor carpi radialis (FCR) muscles in seven healthy human subjects. Under rest conditions, the recovery of H-reflexes and single motor unit activity was slow for soleus; the recovery was not complete even in 1 s. In comparison, the recovery was very fast for FCR motor units, occurring in 200-300 ms. The effects of rate of stimulation (0.1-10.0 imp/s) were also examined on the magnitude of H-reflex responses. The reflex response declined with increasing rate of stimulation, the decline being slightly greater in soleus than in FCR. When these phenomena were examined with voluntary facilitation of the spinal cord, the time of recovery shortened and the effect of stimulus rate also diminished. Changes with background facilitation were greater in FCR than in soleus. The differences between the two muscles are attributed mainly to differences in presynaptic inhibition in the two spinal segments, and/or to the differences in dynamics of the transmitter release in terminals of Ia afferents synapsing with slow soleus motoneurons and those synapsing with the fast FCR motoneurons.

Muscle spindle activity following muscle tendon vibration in man

Muscle spindle primary endings originating from the Tibialis anterior, Extensor Digitorum Longus and Lateral Peroneal muscles were recorded by the microneurographic technique. Their resting activity and stretch sensitivity after muscle tendon vibration (80 Hz, 30 s) were compared with those in the previbratory period. Most of the units (73%) exhibited a decreased spontaneous firing rate whereas a few others either conserved (13.5%) or increased (13.5%) their resting discharge after vibration. A complete recovery necessitated 40 s. The static stretch sensitivity of the units was decreased during the 3 s following vibration exposure and returned to the control level (about 14 s). The results are discussed in the light of previous psychophysiological studies reporting an altered position sense and a development of involuntary muscle contractions in postvibratory periods.

Task-dependence of muscle afferent monosynaptic inputs to human extensor carpi radialis motoneurones

The task-dependence of homonymous muscle afferent inputs was investigated in motor units of the extensor carpi radialis muscles during voluntary isometric contraction involving either the activation of agonist extensor muscles (wrist extension) or the co-activation of antagonist extensor and flexor muscles (hand clenching). The effectiveness of the muscle afferent monosynaptic inputs was tested by delivering either tendon taps or electrical stimulation to the radial nerve. In both cases, the motor unit responses, which took the form of narrow peaks in the peri-stimulus time histograms, were found to be significantly greater during hand clenching. The parallel enhancement of the responses to both mechanical and electrical stimulations observed during hand clenching could not be explained in terms of changes in the muscle spindle responsiveness. The enhancement of the motor units' responsiveness was apparent during the first 0.5 ms of the peaks in the peri-stimulus time histograms, taken to be uncontaminated by any polysynaptic components. It may therefore have reflected an increase in the amplitude of the excitatory monosynaptic potentials generated by the muscle spindle primary afferents. This is interpreted in terms of changes in the presynaptic inhibition, which might be depressed as the result of the large-scale activation of palm and finger cutaneous afferents liable to occur during hand clenching.

Peripheral proprioceptive modulation in crayfish walking leg by serotonin

In vitro serotoninergic modulation of intracellularly recorded sensory responses was examined in primary afferent terminals of a crayfish leg proprioceptor, the coxo-basal chordotonal organ (CB CO). The effects of different concentrations of serotonin (5-HT) on static and dynamic sensory responses were analysed following bath or pressure applications of the monoamine directly on the strand of the mechanoreceptor. Consequently, the reported effects result from the direct peripheral action of 5-HT on the sensory organ itself. Serotonin modulates the sensory activity by modifying the sensory discharge frequency. The firing discharge of the primary afferents is increased in a dose-dependent manner. The maximal effect is obtained with a concentration of 10(-6) M. Higher concentrations are less effective, and for 20% of the recorded cells, 10(-4) M 5-HT induces a decrease of the sensory discharge, i.e. has an inhibitory effect. Alteration in the pattern of sensory firing, resulting in bursting discharge, was observed in some units. All the recorded sensory units were responsive to the neuromodulator whatever their functional properties. The effects of 5-HT lasted as long as the amine was applied and were reversible after wash. The results suggest that 5-HT could exert a modulatory action on the proprioceptive feedback, by peripheral action on the sensory organ. The natural modalities of 5-HT action are discussed on the basis of immunohistochemistry data suggesting: (i) connections between CB CO and central serotoninergic cells, (ii) 5-HT content in sensory cells of the CB CO. Since the CB CO is involved in the control of leg movement and position, the modulation of its primary afferents might influence the organization of the locomotor pattern. The functional significance of this peripheral sensory neuromodulation was approached by the analysis of the motor reflex activity.

Serotonin and proctolin modulate the response of a stretch receptor in crayfish

The modulatory effects of proctolin and a biogenic amine (5-hydroxytryptamine; 5-HT) have been investigated on a leg mechanoreceptor in the crayfish. Single afferent sensory units were recorded extra- and intracellularly during imposed sinusoidal movement to the receptor strand; all responses were facilitated by a bath application of proctolin and 5-HT at various concentrations (10(-9)-10(-6) M).

Lack of correspondence between histochemical and structural fiber typing in antennal muscles of the rock lobster Palinurus vulgaris

1. Sarcomere lengths and fine structure were examined in three histochemical fiber types of antennal muscles of the rock lobster. 2. Sarcomere lengths are distributed over a continuum of values from 6.5 to 19 microns. 3. Although a correlation between ATPase activity and sarcomere length is demonstrated, fibers with high ATPase activity do not have the sarcomere length typical of fast contracting fibers. 4. These fibers deviated from the typical fast structure in having long sarcomeres (greater than 6.5 microns) and in having some unusual ultrastructural characteristics (absence of the H-band, presence of Z-tubules, high thin to thick ratio, 5:1) associated with other more classical features. 5. This finding demonstrates that sarcomere length measurements do not always accurately predict the physiological performance of a single muscle fiber. 6. The fiber type composition of two antagonistic antennal muscles is compared and the functional significance of the results is discussed with respect to their role in behavior.

Use of the fluorescent retrograde tracer, Granular blue, in crustacean motor system studies

Retrograde axonal transport and fluorescent retrograde labelling of neurons were investigated in a crustacean motor system. A fluorescent tracer (Granular blue) was injected into an antennal muscle of rock lobster in order to identify the motor neurons innervating it. After an appropriate post-injection survival time, the fluorescent tracer was found to have labelled motor neurons in the central ganglion. This result indicates that after uptake by the axon terminals, the fluorescent tracer was retrogradely transported through axons of motor neurons to the parent cell bodies.