Comparative analyse of single motor unit pattern in healthy subjects and patients with neuromuscular disorders.

OBJECTIVE: The motor unit (MU) spike trains in human muscle contractions are an object to new mathematical processing. The aim is to identify the interspike interval relations characterizing normal and changed physiological states (neuro-muscular disorders). METHODS: MU activities in healthy subjects and patients with Schwartz-Jampel syndrome, neuromiotonia and Parkinson disease were investigated. Motor unit action potentials (MUAPs) were recorded by surface multielectrode with small leading-off area without provoking a burst of activity, as usually was observed during the needle electromyography study in patients with Schwartz-Jampel syndrome and neuromiotonia. RESULTS: Different single motor unit activity in healthy subjects and patients was observed. Discharge pattern of patients was basically changed. Disturbance of different parts of the motor system leads to the changes of temporal order of interspike interval. This permit us to suppose that the pattern of repetative neuronal discharges suggests an influence at a higher level than the muscle in all investigated patients but the reason of multiple discharges with an interimpulse interval of 2 to 10 ms probably originated in the muscle membrane. CONCLUSIONS: The comparative analysis of MU patterns in healthy subjects and patients with neuro-muscular disorders can help us to disclose inapparent connections between cortical and spinal level of motor control.

Conduction velocity of antigravity muscle action potentials.

The conduction velocity of the impulses along the muscle fibers is one of the parameters of the extraterritorial potentials of the motor units allowing for the evaluation of the functional state of the muscles. There are no data about the conduction velocities of antigravity muscleaction potentials. In this paper we offer a method for measuring conduction velocity of potentials of single MUs and the averaged potentials of the interference electromiogram (IEMG) lead-off by surface electrodes from mm. sternocleidomastoideus, trapezius, deltoideus (caput laterale) and vastus medialis. The measured mean values of the conduction velocity of antigravity muscles potentials can be used for testing the functional state of the muscles.

Time relations between the impulses of two motor units from symmetrical muscles upon voluntary control.

The activity of two motor units (MUs) belonging to two symmetrical muscles of both hands in six well trained subjects was investigated. Bipolar selective surface electrodes were used to lead off the electrical activity of mm interossei dig. dors. I or mm opponentes pollices. The "Medelec" electromyograph insured the visual and auditory feed-back from the firing MUs. The continuous or intermittent activity of both MUs was controlled voluntarily and simultaneously by the subjects. The probability of appearance of an impulse (the first or the last impulse in an intermittent train) from one unit when the other was firing continuously was studied. Alternation of the probability at 20 ms steps was found. At the same time the rhythm of the continuously firing MU was slightly affected. The data suggest a joint supraspinal influence or segmental contralateral outflow from the symmetrical motor pool. The hypothesis of the alphamotoneuron firing as a random process could not be used to explain the cases of joint activity.

Motor unit potentials at high muscle activity recorded by selective electrodes.

The selectivity of different kinds of electrodes for recording of single motor unit potentials was theoretically evaluated. A new type of selective "branched", superficial and subcutaneous wire electrodes was described. The discharge of single alpha motoneurons during movements against elastic resistance was investigated studying motor unit potentials in m. biceps brachii and m.interosseus dorsalis l. Depending on the velocity of the movement, the following pattern of discharge was found: 1) At slow velocity the frequency of discharge increased and new motor units were recruited. 2) At higher velocity the frequency of discharge was constant. The muscle force increased only when new motor units were recruited. 3) At very quick ballistic movements the frequency of discharge decreased during the movement.

The reaction time of single motor units in the human muscle.

The reaction time for activating a single motor unit (MU) with one impulse only, upon visual and auditory feed-back was studied in six healthy subjects. Use was made of MUs from m.abductor dig.V. The subjects were trained until they achieved more than 50% correct performances. One hundred responses were investigated in each series of experiments. The stimulus to which the subjects should have to respond was a beam presented on the screen of a "Disa" 14A30 electromyograph. Simultaneously with the beam the stimulator switched on a digital chronometer which was switched off by the MU impulse. Another two series of experiments were also performed in which the subjects responded with a train of impulses from the MU or with a burst of impulses from the whole muscle. It was been found that the mean reaction time for one impulse from one MU is longer (300 ms), for a train of impulses from one MU it is shorter (260 ms) and for a burst of impulses it is the shortest (200 ms). The histogram of distribution in the first series of experiments deviated from the normal distribution and showed a second maximum at 120 ms after the first maximum. The latter did not differ from the maxima in the distributions of the second and third series of experiments--about 200 ms after the visual stimulus presentation. The difference in the mean reaction time for the three series of experiments is due to the differences in the motor tasks connected with different velocity of increase of the muscle tension, as increasing velocity decreases the MU recruitment thresholds. The separate MUs might be activated in the same short periods as the whole muscle but they showed some specificities in the time distribution of the responses which might be due to some discrete mechanisms of the motor control system.

Voluntary control of the rate of increase of the isometric tension of human muscles.

Experiments were made with eight subjects to study the possibilities for voluntary control of the rate of increase of isometric muscle tension in m. biceps brachii and m. interosseus dig. dors. 1. In four consecutive experiments the voluntary effort was increased until maximum muscle tension was achieved. Each subsequent experiment was at a higher velocity. All subjects were able to rank the velocities. Repetition of performances with respect to the velocity values was low. A considerable difference existed between the group of chosen velocities which were very similar in the different subjects, and the maximum possible velocities which differed for each subject. Individually different was also the maximum muscle tension which in its turn was in a limited dependence on the rate of increase of the tension. No stable relations were established among the velocities, which could correspond to a standard description elaborated in the system for motor control.

Firing frequency of single motor units upon voluntary control of the isometric tension in human muscles.

Selective surface electrodes were used to study 120 motor units (MUs) from m. biceps brachii of six subjects in experiments for grading the rates of increase of isometric muscle tension until its maximum value. Correlation dependences were sought between the rate of increase of the tension, the force of maximum tension and the initial firing frequency of the activated MUs. A positive correlation was established between velocity and force in a narrow range of the average velocities. The initial firing frequency of the MUs was in positive correlation relation with the rate of increase of the tension, though only according to summated data. This connection may not be manifested for the single MUs. The pattern of the activity of the single MUs is considered to be only statistically connected with the activity parameters of the entire muscle.

Discharge frequency of the motor units in human muscles under different stationary isometric tension, investigated by a new method.

A new method has been worked out for selective recording of the impulses of individual MU under high muscular efforts. It is suggested to use the EMG derivatives (the differentiated EMG-DEMG and the double differentiated EMG-DDEMG and possibly the higher derivatives). It was demonstrated that at different stationary isometric tensions the dependence between the discharge frequency and the tension may be linear or non-linear, with a bigger growth at the small efforts and a smaller growth at the big efforts. The inverse non-linearity--sharp rise of the frequency at the high efforts--was not established. Conformation was found for the possibility of distinguishing between two types of MU according to the type of this dependence.

Motor units activity of dog postural muscles in standing position.

Discharges of separate motor units (MU) of dog muscles are studied in standing position and under conditions of passive stretch and shortening of the muscles. In standing position the average discharge frequency in the fore leg extensors is found to be of the order of 25-30 imp/s. A doublet pattern of firing and recruitment and derecruitment of single MUs over different periods of time is observed. The transient processes of the instantaneous discharge frequency of single MUs are observed upon passive stretch and shortening of the muscles. Different types of transient processes are described, both during recruitment and derecruitment in standing-position, and during the passive changes in muscle length. A dependence is found of the transient processes in the velocity of the passive changes in length and the probably physiological mechanisms of these phenomena are discussed.

Significance of different feedbacks for the transient processes of the discharges of single motor units during step-like voluntary movements.

Studies are made of the transient processes of the discharges of single tonic low-threshold and phasic high-threshold motor units (MUs) during step-like voluntary movements performed in case of visual feedback by the result of the movement and in case of artificial auditory feedback by the sound of the MU impulses, as well as in cases of absence of exteroceptive feedback. The studies comprise isometric and isotonic flexion and extension in the elbow joint, the MUs studied being from m. biceps brachii. The previously observed differences of the transient processes in tonic and phasic MUs are found to exist not only during movements taking place under visual control but also under other conditions studied. Under conditions of articicial auditory feedback the subjects may or may not use this feedback for control of the motor response.

Interactions between unloading reflex and voluntary muscle activity under different instructions.

Changes in the instantaneous firing frequency of a separate motor unit (MU) in the unloading reflex and voluntary movement according to instruction, triggered by external mechanical disturbance in the position of the arm, were studied. The activity of m. biceps brachii and m. pronator teres was recorded using surface bipolar electrodes with small area and small interelectrode distance. The correct position of the electrode with respect to the MU was controlled vectorelectromyographically. The probabilities of the appearance of an impulse in successive 10 ms intervals before and after the unloading were determined. The effect of the voluntary movement performed by the subject according to instruction on the reflex response is demonstrated, as well as the opposite effect, i.e. of the type of the external disturbing action on the voluntary movement. Certain problems related to the transcortical reflex response are discussed. It is assumed that some maxima in the summated electromyogram (EMG) which correspond in time to transcortical reflex influences are not due to a higher excitability of the alpha-motoneurones, but they are obtained by grouping of the impulses of many activated tonic MUs whose activity is recorded by recording the summated EMG.