Motor threshold as indicator of premotor and motor cortex excitability.

UNLABELLED: The premotor cortex is a second generator of motorics, involved in mass inborn movements performance, and in pathology--in genesis of spasticity and other motor disorders. As far as the repetitive transcranial magnetic stimulation (TMS) is expected to be a therapeutic tool in some movement disorders, the investigation of premotor cortex response to TMS seems to be an important first step. The goal of our work was to picture the difference in motor responses of premotor and primary motor areas to TMS, by means of motor threshold (MT), and to give a simple and easy testing method, which may be of use before trying therapeutic TMS in some motor disorders. It is based on the motor threshold values for arms and legs motor responses, as a primary motor and premotor cortex excitabity indicator. Only a transcranial magnetic stimulator is necessary for the investigation. A MagPro stimulator (Medtronic, Denmark) with an original C125 coil have been used. The hand and finger contraction motor threshold by TMS at C(z) was measured. After that the stimulation intensity (combined with facilitation), necessary for gaining muscle contraction in every arm and leg, contra- and ipsilaterally, by TMS at C(z), C3, C4, CF1 and CF2 (left and right premotor zones) was tried. The responses have been assessed visually. The results showed a bilateral arm and leg motor response to unilateral TMS of premotor area. The stimulation intensity necessary to evoke contraction in leg musculature was significant lower at premotor area than this at C(z). On the contrary the TMS at C3 and C4 resulted always only in a contra-lateral arm and sometimes leg contraction. The visual assessment of contra- and ipsilateraly leg and arm muscles participation allowed this to be scanned all over the body. CONCLUSION: The two motor generators (premotor and primary motor) show different behavior by TMS. The proposed method of motor threshold comparison in TMS of both motor and premotor cortex, necessary to gain motor responses in four limbs may be useful as an easy, fast and noninvasive functional test.

Facilitation of skilled finger movements by repetitive peripheral magnetic stimulation (RPMS) - a new approach in central paresis.

A new therapeutic method for the rehabilitation of central paresis of the upper extremity, especially of fine skilled finger movements, is presented. The therapeutic concept is the activation of reorganization processes in the CNS. These processes are elicited by the induction of proprioceptive input to the CNS which corresponds physiologically to the lost input during active movements. The input is generated by repetitive peripheral magnetic stimulation (RPMS) at the innervation zone of the paretic muscles. The stimulation leads to a motion of the activated muscles. The proprioceptive input is generated by two mechanisms: adequately by activation of mechanoreceptors of the stimulated muscles during the induced contractions and relaxations and inadequately by direct activation of the involved sensorimotor afferents. The method has been applied to 52 patients suffering from spastic paresis of the upper extremity. A simple clinical quantification using the Ashworth scale revealed that spasticity could be remarkably (1-2 points) reduced already by one session of RPMS lasting 15 minutes. In order to get an objective insight into the improvement of active motor performances, a neurophysiological investigation of active finger extensions was performed in eight patients suffering from a central hemiparesis. Following RPMS of the paretic finger extensors, the patients could perform rapid finger extensions with larger displacement and velocity at diminished amounts of EMG activity.

Anticipatory reduction of the muscle activity associated with self-triggered electrical stimulation of mixed nerves and mecanical taps on muscle tendons.

Brief hammer taps self-imposed on the Achilles tendon or self-triggered electrical single-pulse stimulation delivered to the tibial nerve in the middle of the popliteal fossa are associated with an anticipatory reduction in the muscle activity (RMA) of the reflex-bearing soleus. The anticipatory RMA starts shortly prior tothe onset of the perturbation and lasts until the consequent T- or H-responses. When the amplitudes of the control T- and H-reflexes in a resting state are the same and equal to 50% of Hmax, then the anticipatory RMA patterns revealed with mechanical and electrical stimulation are similar. By selective skin stimulation in the medial region of the popliteal fossa (1.5-2.0 cm apart from the tibial nerve) with the same stimulus strength, the anticipatory RMA is significantly reduced. It is apparent that the main afferent sources for the elaboration of anticipatory RMA are the I afferents (presumably Ia afferents) and partly low threshold fast conducting cutaneous afferents.

First steps in functional magnetic stimulation (FMS)-movements of forearm and fingers induced by closed-loop controlled FMS.

For rehabilitation of cerebral paresis, e.g. following stroke, we developed a Method that helps to relearn lost movements of paretic arms, hands and fingers. The concept is to restore the lost proprioceptive afferent inflow to the CNS to facilitate reorganization by neuromodulation. Continuous movements are induced using closed-loop position controlled functional magnetic stimulation (FMS). In order to induce a equivalent proprioceptive afferent pattern to the lost pattern, the controller has to elicit movements of single fingers using a physiological stimulation frequencies in the range of 15 to 40 Hz. It has to integrate the remaining voluntary motor performance of the paretic extremity while stimulating. The presented controller is able to induce smooth movements and lifts the finger into the target position within two seconds. Following the stimulation of the plegic finger extensors the patients could perform rapid finger extension movements with larger displacement amplitude and velocity at rather diminished amounts of activity (EMG).

Repetitive peripheral magnetic stimulation alleviates tactile extinction.

Despite its frequency in right brain damaged patients crucial mechanisms of tactile extinction are still obscure and treatments are unavailable. Recent PET observations suggest a hypometabolism in the primary and secondary somatosensory cortex of the lesioned hemisphere in patients with tactile extinction. Functional and morphological investigations have shown that the sensorimotor cortex has a remarkable capability of reorganization when the sensory inflow is changed. Repetitive peripheral magnetic stimulation (RPMS) applied in patients suffering from central paresis alleviates sensorimotor as well as cognitive deficits by the induction of proprioceptive inflow, thereby activating plasticity in the CNS. Based on the observation of reduced metabolic activity in patients suffering from tactile extinction we applied RPMS to explore the effects of peripheral sensory stimulation on tactile extinction. Fourteen right-hemisphere lesioned patients with tactile extinction were randomly allocated to an experimental and a control group. The experimental group received one single RPMS treatment of the left forearm as well as a condition of attentional cueing known to improve visual extinction. The control group, with comparable tactile extinction scores, neither received RPMS nor verbal cueing, but was tested twice to evaluate possible learning or test repetition effects. In the experimental group RPMS led to a significant reduction of left-sided extinctions in the recognition of different tactual surfaces, but had no effect on ipsilesional errors. In contrast, attentional cueing had no significant effect on left-sided extinction errors but unexpectedly increased right-hand extinction errors slightly but significantly. The control group showed stable extinction scores of the left- and right-hand stimulus across two measurements, thus ruling out learning or test repetition effects. These results show that sensory inflow is an important modulatory factor in tactile extinction. Furthermore, multiple RPMS may prove a promising way for the rehabilitation of patients with this disorder.

Facilitation of motor evoked potentials by postcontraction response (Kohnstamm phenomenon).

We have applied repeated transcranial magnetic stimuli during the involuntary postcontraction muscle activity (Kohnstamm phenomenon) or during a tonic vibration reflex, both presumably arising from subcortical levels. The motor evoked potentials (MEPs) were compared with the MEPs evoked during a comparable voluntary contraction (cortical origin). The MEP amplitudes from the deltoid muscle appeared linearly related to the mean amplitude of the smoothed rectified background EMG preceding the stimulus. No differences in the facilitatory effect between voluntary and involuntary preinnervation manoeuvres were found. If we accept the hypothesis of a subcortical origin of the involuntary muscle activity in the Kohnstamm phenomenon, the similar facilitatory effect of involuntary and voluntary background EMG supports a predominantly spinal localisation of the facilitatory mechanism in this proximal muscle both during involuntary and during voluntary activity, at least under the present conditions of rather low stimulus strengths. In about 20-30% of all the trials an extra facilitatory effect on the MEP amplitude was observed during the shortening contraction compared to an MEP elicited during the lengthening contraction, in spite of a similar background EMG. This extra facilitatory effect of the shortening contraction was observed during involuntary and voluntary preactivation, suggesting an elevated excitatory state at the spinal level.

The boundary effect in magnetic stimulation. Analysis at the peripheral nerve.

The optimal stimulus position for a figure-8-shaped coil for magnetic stimulation of the ulnar nerve at the wrist was not coincident with the optimal electrical stimulus point but was shifted 18.3 mm to the ulnar side (P < 0.01). For the median nerve the optimal stimulus site was 9.6 mm radial to the optimal position for electrical stimulation (P < 0.05). This shift of the stimulus point for magnetic stimulation is significantly smaller after interposition of a homogenous electrically conducting medium between coil and arm but not changed after interposition of distilled water. This so-called boundary effect is therefore due to the different conductivities of the medium interposed between coil and nerve. It may also distort precise localisation of other excitable structures such as cranial nerves, nerve roots and cortical areas by means of magnetic stimuli. The amplitudes of the compound muscle action potentials elicited with identical magnetic stimulus strength were larger after the interposition of isotonic solution between coil and skin but not after interposition of distilled water. Consideration of the boundary effect provided an improved response amplitude to magnetic stimulation, but this could not adequately compensate for its poor localisation compared to electrical stimulation.

Self-generated rapid taps directed to the opposite forearm in man: anticipatory reduction in the muscle activity of the target arm.

A brief hammer tap applied passively (by the experimenter) to the forearm elicits a short-latency reflex response in the forearm flexors and extensors. When the same tap is performed actively (by the subject) using the opposite forearm, the reflex response is preceded by a short-lasting anticipatory reduction in the muscle activity appearing around the impact. This anticipatory reduction is interpreted as an alternative mode of feedforward motor control associated with damping of kinetic impulses generated within the bimanual system.

Artifact reduction in magnetic stimulation.

A new biopotential amplifier is described in which the stimulus artifact from recordings of electrically or magnetically evoked biopotentials is minimized by a special active filter. The amplifier is tested in different experiments and the recordings are compared to standard EMG recordings.

Tremor and skeletal muscle tone.

Stereotactic interventions at thalamic/subthalamic levels for parkinsonian tremor give the chance to investigate the influence of the supraspinal proprioceptive loops on the tonic component of motor tasks. Studies demonstrate not only a normalization of the late stretch reflex component and a decrease of the pre-reflex stiffness in muscles contralateral to the side of the lesion, but also a reduction of low threshold motor unit firing frequency in the elbow flexors during isometric contractions. These data indicate a shift from a more tonic to a more phasic mode of innervation, which we interpret as the cause of hypotonia.

[Control of isometric muscle contraction in muscle hypotonia of central origin: EMG mapping analysis]. / Kontrolle der isometrischen Muskelkontraktion bei muskulärer Hypotonie zentralen Ursprungs: EMG-Mapping-Analyse.

Electromyographic and mechanographic investigations in patients with muscular hypotonia, which is, for instance, a side-effect after stereotactic treatment of tremor syndromes, permit the presumption that in this sensomotor open-loop situation the decreased muscular resistance to stretching during isometric contraction (initial stiffness) is caused by changes of muscular innervation pattern. Probably, the innervation pattern during tonic activity is changed by a shift of a more tonic motoneurone behaviour to motoneurone activities with predominantly phasic characteristics. In 17 controls and 4 patients with muscular hypotonia caused by stereotactic lesions of VIM area (treatment of tremor syndromes) the EMG of right and left side forearm flexors (especially the activity of the M. biceps brachii) was investigated by a sophisticated, topographically oriented 16-channel-surface-EMG-technique ("EMG-Mapping") during slight isometric contraction. EMG-Maps of forearm flexors (especially of M. biceps brachii) in patients with centrally evoked muscular hypotonia demonstrate that in these open-loop conditions the motor control is changed. For this the reason could be a shift of the activated motor units from a predominantly static to a more phasic functional behaviour. The latest results on muscular activation processes in cats support this presumption.

Motor cortex transcranial magnetic stimulation topography assessed by distribution and size of evoked potentials.

25 volunteers aged 18 to 70 were examined. Bilateral EMG was performed of m. m. biceps brachii et flexores digitorum profundus using surface pick-up electrodes. The evoked potentials were recorded on Nicolet Pathfinder II. Magstim 200 Novametrix was used for stimulation with A and B sides of a coil with mean diameter of 9 cm. The examinees were sitting on chairs, their forearms flexed voluntarily to approximately 90 degrees, resisting a standing pressure of 5 Nm exerted by the torque motor. Three groups were examined. In the first, we recorded the amplitude, the latencies and spreading of potentials in the muscles on the left and right stimulating with the A and B sides and positioning the coil on the vertex (Cz), 2 and 4 cm forwards and backwards. Moving the coil away from Cz brought about reduction of the amplitudes of the evoked potentials or resulted in the disappearance of potentials. This was better seen in backward shift of the coil. The latencies did not change. The responses were bilateral in 10 out of 11 subjects the amplitudes being higher on the side corresponding to the destination of stimulation. In the second group we wanted to see what the effect of the shift of stimulation away from the meddle line would be. C-P1, C-P2, Cz1 and Cz2 are not suitable sites for obtaining potentials. Stimulation at C-F1 and C-F2 yielded potentials which, contrary to the expectation, evoked ipsilateral responses. In the third group stimulation was combined with focal sensor influence. At the beginning it was a focused attention which, however, was difficult to define.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the response to magnetic and electrical stimulation of the motor cortex following muscle stretch in man.

1. The effect of muscle stretch on the EMG response from the stretched muscle to transcranial magnetic stimulation of the motor cortex was studied in eight subjects. Muscle stretch was produced by increasing the torque of a motor acting through a lever which was held at constant position by a flexion force of the index and middle fingers. EMG responses were recorded from fine-wire electrodes inserted into flexor digitorum profundus muscle in the forearm. They consisted of a spinal latency component and a long-latency component which could in some subjects be separated into an early and a late phase. 2. In four subjects, four intervals between the stretch and the cortical stimulus were explored using three intensities of cortical stimulation. At all three intensities, when the magnetic cortical stimulus was timed to produce an EMG response in the period of the later part of the long-latency stretch reflex the response was larger than when it was timed to produce a response in the period of the short-latency spinal reflex or when superimposed on the tonic muscle activity used to resist the standing torque of the motor. 3. When the intensity of magnetic cortical stimulation was reduced so that it was just below threshold to produce an EMG response in the short-latency reflex period or on the background tonic EMG activity, it still was capable of producing a response when superimposed on the long-latency stretch reflex. 4. In four subjects the time course of this effect was studied in more detail using only one intensity of magnetic cortical stimulus set to be just above threshold to produce a response in tonically active muscles. The time course of the facilitatory effect was similar to the time course of the later part of the long-latency stretch reflex. From these data it was not possible to determine whether the early part of the long-latency stretch reflex also was accompanied by the facilitatory effect since this component was present in only one of the four subjects. 5. The facilitatory effect persisted after the ulnar and median nerves were totally blocked at the wrist by injections of local anaesthetic. This suggests that inputs from muscle receptors of the stretched muscle contribute to the effect.4=

[Electrophysiologic diagnosis of peripheral nerve lesions]. / Elektrophysiologische Diagnostik bei peripheren Nervenläsionen.

In the last two decades the surgical treatment of peripheral nerve lesions has improved considerably, mostly because of the introduction of the microsurgical technique. It was not until the introduction of microsurgery that the surgeon was able to achieve restoration at the level of the nerve fasciculi. The simultaneous use of electrophysiological techniques can improve the results of the reconstruction and especially the extent of functional outcome of nerve restoration. Electrophysiological methods allow detailed and objective assessment of the site, extent, and cause of the lesion and are therefore crucial for the decision upon the appropriate therapeutic procedure and for the success of the operation. The aim of the review is the demonstration of: 1. Today's understanding of peripheral nerve function/ 2. The neurophysiological mechanism of denervation and renervation/ 3. The different electrophysiological methods/ 4. The indications and 5. Application of these techniques in the context of reconstructive surgery.

Discharge pattern of tonically activated motor units during unloading.

In order to analyse the EMG pattern during unloading of brachial biceps muscle, the interference EMG and single motor unit activity were investigated. The measurements were done on seven healthy subjects with two types of unloading techniques: a) active unloading, when the subjects resisted against an external load (10, 20, 30 and 40 N) which is suddenly released, and b) passive unloading, performed by low inertia torque motors with independently adjustable background extension and suddenly applied flexion torques. Following active unloading the silent period duration, the amplitude of the rebound and its segmentation into consecutive bursts is changing with initial load, whereas the silent period latency remains constant. Following passive unloading the acceleration influences predominantly the amplitude of the rebound, without changing its latency and silent period duration. The initial voluntary activity influences both silent period duration and rebound parameters (latency, amplitude and duration).

Amyotrophic lateral sclerosis: macro-EMG and twitch forces of single motor units.

Macro-EMG potentials (MEP)s and twitch contractions (spike-triggered-averaging) of single motor units (MUs) have been recorded in the first dorsal interosseus muscle (FDI) of 10 control subjects and 20 patients with amyotrophic lateral sclerosis (ALS). MUs over the full range of voluntary recruitment thresholds were studied. Patients with slightly affected FDIs (5) mainly showed MUs with enlarged MEPs and increased twitch forces. In contrast, the patients with more severely affected FDIs (15) revealed decreased twitch forces, especially in the MUs with higher thresholds. The corresponding MEPs could be enlarged as well as normal. It appears that MU sprouting and the resulting increase of twitch force can compensate for the loss of motoneuron in early stages of ALS. In more advanced stages, however, a decline of the force of the surviving MUs, especially of those with higher thresholds, seems to contribute to the progressive muscle weakness, in addition to the corticospinal degeneration and the reduction in the number of motoneurons.

Responses of human muscle spindle afferents during isotonic position holding and active movements.

Discharge characteristics of muscle spindle primary afferents from human finger flexors were investigated during load-bearing isotonic position holding, and intentional shortening or lengthening contractions. During position holding, units mostly showed a negative relationship between muscle length and impulse rate, particularly when the load was small. When the load was increased, Ia-discharge rates increased more in the elongated than in the shortened muscle, and the inverse position response became less pronounced. During active lengthening contractions, a stretch response pattern was sometimes absent or could be abolished by increasing the load.