Reisaku Tanaka

Contribution of kinesthesia on human visuomotor elbow tracking movements

The role of peripheral information in smooth motor performance was analyzed in normal subjects and patients with deep sensory disturbance by the modified visually guided tracking method. The subjects task was to make the control cursor match the slowly moving target by elbow flexion movement. Performance of normal subjects was scarcely affected, but that of the patients was deteriorated by the withdrawal of the visual cue for handle movement. It is suggested that afferent information from the moving limb plays an important role in smooth and accurate tracking movements.

Reciprocal Ia inhibition in patients with spinal spasticity

Reciprocal Ia inhibition from ankle flexors to extensors was studied in five patients with spasticity due to incomplete traumatic spinal cord lesions. Nine healthy subjects were tested as controls. Excitability of the soleus motoneuron pool was estimated by H-reflex testing in the resting state. Ia inhibition was activated by conditioning stimuli to the peroneal nerve. Ia inhibition was detected in all patients tested, the amount of inhibition ranging from 8% to more than 50% of the test H-reflex size. In the control subjects only weak Ia inhibitory effects were present. These findings indicate increased excitability of the Ia inhibitory pathway to ankle extensor motoneurons in patients with spasticity due to spinal cord injury.

Reciprocal inhibition upon initiation of voluntary movement.

Phasic changes in reciprocal inhibition of the soleus motoneurons upon initiation of active ankle dorsiflexion was investigated in normal man. H-reflex and reaction time methods were combined. Two phases of reciprocal inhibition were observed. The first inhibition appeared almost simultaneously with the activity of agonist EMG but was so weak as to be cancelled with a strong test stimulus or a slight stretch of the test muscle. The second inhibition developed 100 ms after EMG onset, in the early stage of dynamic contraction, and was strong. These inhibitions increased as the voluntary effort was strengthened. The neural mechanism of these inhibitions is discussed.

Facilitation of agonist motoneurons upon initiation of rapid and slow voluntary movements in man

The time course of facilitation of the agonist motoneurons upon initiation of voluntary ankle dorsiflexion was investigated in eight healthy subjects. The H-reflex and visually guided tracking methods were used for testing the excitability of the motoneuron pool and for controlling the initiation of movement as well as speed and force. Since the onset of voluntary EMG activity (EMG reaction time: EMGvRT) was delayed and/or obscured by test H-reflexes which were evoked very close to the behavioral responses, the subject was instructed to make response movements bilaterally, and EMGvRT was measured on the side without stimulation. In every subject, the EMGvRT was invariably longer in the ramp movement than in the step movement. The onset of H-reflex facilitation prior to EMG onset, which was regarded as indicating the arrival time of the descending motor command to the motoneuron pool, always started earlier in the ramp movement than in the step movement. The difference in facilitation onset between the two tasks was smaller than that in EMGvRT. Since the amplitude of the H-reflex at the onset of the voluntary EMG was equivalent in both movements, the development of H-reflex towards the behavioral EMG onset was more gradual in the ramp movement than in the step movement. The present results demonstrate that the longer reaction time in the slow ramp task depends on 2 factors: delay in the arrival of descending facilitatory impulses to the agonist motoneuron pool, and its slow recruitment thereafter.

Reassessment of H‐reflex recovery curve using the double stimulation procedure

We conducted two types of experiments to assess the validity of the H‐reflex recovery test, using double stimulation to test soleus motoneuron pool excitability in healthy and spastic subjects. One type dealt with the mechanical effect of the conditioning H reflex on the ankle joint; the other type with the effect of change in reflex size. The mechanical effect was tested both with the ankle joint fixed (FX) and free to move (FR). Differences between FX and FR conditions commenced with relaxation of soleus muscle contraction by the conditioning H reflex. In the FR condition, abrupt facilitation occurred, and changed to marked depression. We conclude that specific facilitation and inhibition in the FR condition were secondary effects of group Ia inflows caused by the ankle extensor muscle stretching on relaxation. In some spastic patients as well as in controls, facilitation due to the mechanical effect in the FR condition was observed despite the FX condition. The effects of systematic changes on soleus H‐reflex size were investigated at conditioning–test intervals of 80 ms, so as to avoid mechanical effects. When conditioning and test reflexes were the same size, the amount of recovery increased as the H‐reflex size increased. Comparison of the relation between amount of recovery and H‐reflex size, expressed as a percentage of Mmax, showed no significant difference between the two groups. We speculate that the stronger recovery of spasticity mentioned in previous literature may have resulted from the fact that relatively greater H reflexes were tested in those studies. In conclusion, the present study indicates that double stimulation is not appropriate for assessing spinal motoneuron pool “excitability increase” in spasticity.

Discrimination of different motor units by spike-triggered averaging of surface electromyograms

An objective method was sought for the discrimination of individual motor units (MUs). Discrimination using the conventional needle electrode recording is difficult because the waveforms are easily changed by slight shifts of the electrode. A surface EMG recording was combined with the needle recording and MU activities were obtained by MU-triggered averaging of the surface EMGs. Waveforms of surface MU potentials were stable, irrelevant to changes in recording conditions of needle electrodes and thus were found useful for the purpose of discrimination. Efficacy of this method was tested in the MUs with known axonal conduction velocities.

Discharge of muscle afferents during voluntary co-contraction of antagonistic ankle muscles in man

The discharge of 38 tibialis anterior (TA) muscle spindle endings was recorded at rest and during isometric voluntary contraction of the ankle joint in eight healthy human subjects. With the ankle joint in 110 degrees plantarflexion, 24 endings (61%) were tonically active in the resting subject. During weak voluntary dorsiflexion, seven additional endings were activated, so that a total of 31 endings were active (82%). 24 of these were either newly recruited or discharged at a faster rate than at rest (average discharge rate 6.6 Hz at rest, 9.7 Hz during contraction). At matched levels of TA EMG, one ending was newly recruited and 10 were more active during co-contraction of dorsi and plantar flexors than during isolated dorsiflexion. 26 endings were equally active during the two tasks and one ending decreased its firing rate with co-contraction. Four of the 11 endings, which had a higher discharge rate during co-contraction than during dorsiflexion, discharged faster during plantarflexion than at rest although slower than during co-contraction. Plantarflexion had no effect on the discharge of three endings. The remaining four endings were not investigated during plantarflexion. It is suggested that the increased discharge rate of muscle endings during co-contraction is caused either by small changes in the length of the TA muscle or by a disproportionately high fusimotor drive during co-contraction.

Injection of horseradish peroxidase solution in calibrated volumes (nanolitre) into the spinal cord.

A method for the injection of horseradish peroxidase (HRP) into the spinal cord (SC) is described, based on the principle of fluid expansion upon application of heat. HRP solution in the tip of a glass micropipette (tip diameter 10-15 microm) was extruded when the sealed stem of the pipette, containing distilled water, was warmed. The pipette was calibrated (temperature/volume relation) before it was inserted into the SC and the injection of HRP was made according to the calibration curve. Using the pipette as an electrode it was possible to locate its tip with reference to recorded field potentials. After the injection, the volume of HRP solution actually injected was determined.

The effect of l-threo-DOPS on synaptic transmission to soleus motoneuron in normal subjects and patients with Parkinson's disease

The effect of L-threo-3,4-dihydroxyphenylserine (L-threo-DOPS) on synaptic transmission to the soleus motoneurons was studied using the H-reflex method in 12 normal subjects and in 4 patients with Parkinsons disease. A long-lasting depression of the test H-reflex, which was evoked by a conditioning group I volley in the homonymous tibial nerve with an onset latency of ca. 30 ms and a duration of more than 1 s and was regarded to represent the transmitter depletion caused by a preceding volley, diminished significantly after the oral administration of L-threo-DOPS both in the normal subjects and in the patients. On the other hand, no change was recognized in the slow depression which was caused by a group I volley of the heteronymous common peroneal nerve and was regarded as the presynaptic inhibition. The findings suggest that L-threo-DOPS influences the reduction of transmitter depletion.

Effects on EMG activities of elbow joint muscles by rapid head rotation movement in horizontal plane in man

Effects of voluntary rapid head rotation movement in horizontal plane on EMG activities of tonically contracting elbow extensor and flexor muscles were studied in five healthy subjects. Arm EMGs were averaged in relation to the EMG onset of the prime mover. M. sterno-cleidomastoideus (SCM). EMG variations could occur before SCM-EMG onset. Facilitatory and inhibitory effects were observed. They were not necessarily organized in reciprocal fashion but varied in every subject. We interpret the present findings as resulted from an incorporated part of the central motor program for head rotation which was specific to each subject.