The response to professor K.S. Turker on his "an opinion on the 'delayed spikes' in human motoneurons".

This is the response to Prof. Turker on his note "challenging" our hypothesis assuming the possibility of two modes of spiking (direct and delayed) evoked by Ia-afferent electrical stimulation in human firing motoneurones. Besides, we touch on some common questions about the most relevant, functionally significant, estimations of motoneuronal firing behaviour in human motor control. In particular, in this context, the question arises: what is more important estimation, motoneurone spiking, itself, or a value of the corresponding interspike interval of the firing motoneurone?

[Discharge frequency and excitability of human firing motoneurone].

The excitability of firing motoneurones activated by voluntary contraction of the flexor carpi ulnaris or tibialis anterior was tested by single excitatory Ia afferent volleys. In order to estimate the stimulation effects, the peri-stimulus time histograms of single motoneurones were plotted, and the firing indices were calculated. It was shown that the firing-frequency effect was absent within the range of 4-14 imp/s during testing by low-intensity excitatory volley. At higher intensity of afferent volley, the excitability increased at a low firing rate. It is suggested that the characteristics of the interspike-interval excitability trajectories underlie these relations. These findings made it possible to explicate some conflicting literature data which were usually reported without taking the afferent volley intensity effect into account. The mechanisms controlling the firing motoneurone excitability and possible trajectories of interspike-interval membrane potential in human motoneurones are discussed.

Analysis of firing behaviour of human motoneurones within 'subprimary range'.

Firing behaviour of human motoneurones within a low range of frequencies was studied during voluntary muscle contraction. It was found that, in contrast to the higher 'primary range', both excitability and inhibitibility of these motoneurones were significantly higher. As to their minimal firing rates, no correlation between them and the reciprocal values of afterhyperpolarization (AHP) duration was found. This suggests that AHP can hardly be regarded as the main factor controlling the behaviour of human motoneurones within the low-frequency range of firing and that this range (termed here 'subprimary range') should be kept apart from the 'primary range'.

After-potentials and control of repetitive firing in human motoneurones.

Characteristics of motoneurone after-potentials in man were derived from the recovery curve of motoneurone excitability after a single discharge evoked by threshold stimulation of Ia afferents or by gentle voluntary muscle contraction. The motoneurone excitability was estimated by the firing index of a single motor unit whose potentials were recorded by needle electrodes. The soleus (a slow muscle) and the flexor carpi ulnaris (a fast muscle) were investigated. The duration of motoneurone after-hyperpolarization of the soleus evaluated by this method ranged between 145 and 255 msec; for the flexor carpi ulnaris it was 55-150 msec. In some motoneurones of the fast muscle, an early short-lasting recovery of excitability (within 5-20 msec after a discharge) was revealed. It was accounted for by delayed depolarization of the motoneurone. The relationship between after-potentials and the characteristics of repetitive firing of motoneurones activated by weak voluntary muscle contraction was analysed. It was observed that the motoneurones with early excitability recovery were capable of firing double discharges with a 5-15 msec interspike interval. It was found also that the minimal firing rate of motoneurones (up to 3.1-5.2 imp/sec in the soleus and 3.8-9.0 imp/sec in the flexor carpi ulnaris) was not correlated with the after-hyperpolarization duration. This differs from the results obtained for cat's motoneurones under intracellular stimulation. The findings suggest that after-hyperpolarization is not the only leading mechanism controlling the low firing rate of motoneurones under conditions of their natural activity in man.

Repetitive doublets of human motoneurones: analysis of interspike intervals and recruitment pattern.

In order to study the probable mechanisms of repetitive doublets in human motoneurones, the firing patterns of single motor units (MUs) of the trapezius were analysed during a weak voluntary muscle contraction. The mean frequencies of MUs were 9.4-21.7 imp/sec (the mean interspike interval ranged from 46.0 to 106.7 msec). Repetitive doublets (up to 28 in succession) were recorded in 21 out of 120 MUs, mostly at the onset of a slow recruitment. These were followed by single discharges. Intradoublet intervals ranged between 2.5 and 20.0 msec. A significant difference between single spike firing and doublet firing was revealed by plotting interspike interval histograms, showing that two distinct mechanisms were involved. The analysis of interspike interval successions belonging to several MUs firing simultaneously showed that one of the MUs could start with doublets while the others went on firing single spikes with the regular mean frequency and interspike interval scatter. The results lead us to suggest that the intrinsic properties of motoneurones can be regarded as the main factor in the origin of repetitive doublets. It seems that a descending synaptic drive also contributes to the control of double firing since in a number of cases no doublets were produced at the beginning of MU activity. The findings are discussed with regard to the problems of regulating repetitive firing of human motoneurones by after-potentials. Steady delayed depolarization is assumed to be a possible mechanism of repetitive doublets.

Testing excitability of human motoneurones capable of firing double discharges.

Single motor unit (MU) action potentials were recorded from the flexor carpi ulnaris during its weak voluntary contraction in 6 healthy subjects. At low frequencies of motoneuronal firing (the average interspike interval ranged from 70 +/- 8 to 200 +/- 22 msec), double discharges with interspike intervals from 5 to 20 msec were seen from about one-fourth of all MUs (62 out of 249). Monosynaptic testing of the excitability of firing motoneurones was carried out to study the probable mechanism of doublets. By stimulating the ulnar nerve against the background of firing MUs an H-reflex was elicited. The excitability of the motoneurones was evaluated by a firing index whose changes within an interspike interval were analysed (the step width was 10 msec). The testing of 47 MUs has shown that motoneurones capable of firing double discharges during voluntary muscle contraction have a higher excitability at the beginning of an interspike interval (the first 20 msec) as compared to those incapable of firing double discharges. Our data confirm that the leading part in the formation of double discharges is played by delayed depolarization occurring in a firing motoneurone after its successive discharge. The correlation between the characteristics of doublets and the properties of delayed depolarization are discussed. It is proposed that motoneurones with a delayed depolarization possess certain transduction properties as they produce a maximal instantaneous increase in discharge frequency on a minimal increase in synaptic input. This study has revealed a phase of increased excitability in the motoneurone axon during the first 15 msec after a regular rhythmic discharge has been propagated.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitability of firing motoneurones tested by Ia afferent volleys in human triceps surae.

The excitability of a firing human motoneurone was tested by Ia afferent excitatory volleys. An H reflex was evoked by stimulation of the tibial nerve while the subjects maintained the background rhythmic firing of single MU of the soleus and of the medialis gastrocnemius during weak voluntary contraction. The mean background firing rate of MUs ranged from 3.6 to 9.4 imp/sec. In order to estimate the 'effectiveness' of an afferent volley, PSTHs were constructed. The firing index was used as a measure of motoneurone excitability. It has been shown that the 'effectiveness' of an afferent volley decreases with an increase in background firing rate of a MU. Such a dependence was deduced from changes in current motoneurone excitability within an interspike interval. The results indicate that during natural muscle contraction at a low firing rate of a motoneurone, the discharge rate is one of the factors affecting the excitability of a firing motoneurone and, hence, its input-output relations. The method of testing changes in human motoneurone excitability within an interspike interval may prove useful in studies on interaction of volleys arriving at a motoneurone via different excitatory inputs.

Recurrent inhibition of firing motoneurones in man.

The effects of antidromic stimulation of large motoneurones on firing small motoneurones of soleus muscle have been studied. Against the background of rhythmic firing of small motor units (MUs) activated during weak voluntary muscle contraction, thick efferents of the tibialis posterior nerve were selectively stimulated and an M response was evoked in which small MUs were not involved. This provided a means of avoiding antidromic stimulation of the motoneurone under study and, thus, analysing the effect of stimulation without its summation with after-hyperpolarization. The background firing rate of MUs was 4.5-9.2/sec. PSTHs revealed a distinct inhibitory effect with a latency of 35-40 msec (slightly exceeding the latency of monosynaptic reflex) and duration 5-30 msec. It was concluded that the short-latency inhibition could be identified as recurrent inhibition. The effectiveness of recurrent inhibition evaluated by the lengthening of the interspike interval was shown to depend on the arrival time of the volley in the interval and on the background firing rate of the motoneurone. When the inhibitory volley arrived at the beginning of the interspike interval it was ineffective. This indicates that in the investigated range of firing rates the motoneurone is unable to exert an inhibitory effect on its own firing via recurrent collaterals. The inhibitory volley became highly effective at the end of an interspike interval, when the membrane potential approached threshold. The lengthening of interspike interval was more marked at a lower firing rate of the motoneurone. An increase in the background firing rate reduced the extent of recurrent inhibition (at a rate above 10/sec up to its complete ineffectiveness).(ABSTRACT TRUNCATED AT 250 WORDS)

[Testing the excitability of a spiking human motor neuron during the interspike interval]. / Testirovanie v mezhimpul'snom intervale vozbudimosti impul'siruiushchego motoneirona u cheloveka.

Excitability of voluntary activated human triceps surae motor units was tested by monosynaptic excitatory Ia afferent volley evoking weak H-reflex. Firing index was used as a quantitative measure of motoneuron excitability. During transition of a low firing rate of a motoneuron (up to 6 imp/s) to moderate one (6-8 imp/s) the firing index fell; with a further increase in the rate (up to 12 imp/s) it practically did not change. Dependence of the firing index on the rate is based on changes in the motoneuron excitability within the interspike interval. The results indicate that at low firing rates of a motoneuron which are characteristic of its natural activation, the firing rate is one of the factors influencing the excitability of the firing motoneuron and hence its input-output relations.

[Correlation of the discharge frequency of a firing motor neuron and the efficiency of an excitatory volley of Ia-afferents in man]. / Sootnoshenie chastoty razriada impul'siruiushchego motoneirona i éffektivnosti vozbuzhdaiushchei posylki Ia-afferentov u cheloveka.

Single motor unit potentials of the flexor carpi ulnaris muscle were recorded during voluntary contraction and H-reflex was evoked. The afferent volley effectiveness was estimated by the firing index of a motoneuron. The background firing rate of motor units ranged from 3.3 to 16.0 imp/s. Afferent volley effectiveness depended on the background firing rate of fast muscle units under study like slow soleus muscle units investigated earlier. The effectiveness of afferent volley decreased with the motor units firing rate growth. At all range of firing rate the efficiency of afferent volley was higher for motor units of fast muscle as compared to slow muscle. It is suggested that the motoneuron excitability changes within the interspike interval underlay the relationship between the motoneuron firing rate and effectiveness of the excitatory afferent volley.

[Detection and analysis of recurrent inhibition of spiking human motor neurons]. / Vyiavlenie i analiz vozvratnogo tormozheniia impul'siruiushchikh motoneironov u cheloveka.

Effect of electrical stimulation of n. tibialis efferents on m. soleus motor units was studied during weak voluntary muscle contraction. Peristimulus histograms of low-threshold units (which were not stimulated) revealed a strong inhibitory effect with latency 35-40 ms and duration 10-20 ms. The results showed that this inhibitory effect represents recurrent inhibition whose peculiarities were analyzed. The dependence between the efficiency of the inhibitory volley and the moment within an interspike interval at which the volley arrived was found. Lengthening of the interval was observed only when the inhibitory drive occurred close to the end of the interval.

Reflex effects of muscle afferents on antagonist studied on single firing motor units in man.

The effects of electrical stimulation of mixed nerves and of the evoked tendon reflex on single firing MUs of antagonists were investigated. The following muscles were studied: m. biceps fem., m. tibialis ant. and m. triceps surae. Under a weak voluntary contraction MU potentials were picked up. Post-stimulus histograms of MU potentials were plotted and the duration of interspike intervals in the post-stimulus period was compared with that in the prestimulus period. Under electrical stimulation of the nerve (Ia afferents) a distinct reciprocal inhibition was observed on MUs of all the 3 investigated muscles. In some cases a weak excitatory drive could be revealed along with the reciprocal inhibition. The tendon (knee) reflex was accompanied by a distinct excitatory effect on MUs of the antagonist. The dependence between the efficiency of the inhibitory volley and the moment within an interspike interval at which the volley arrived was found. The lengthening of an interval was observed only when an inhibitory drive occurred close to its end. The peculiarities of reciprocal inhibition in the firing motoneurone, as well as the possible mechanism of the excitatory effect on the antagonist accompanying the tendon reflex, are discussed.

[The "tuning" mechanism of the segmental apparatus before voluntary movement]. / O mekhanizme "nastroiki" segmentarnogo apparata pered pered proizvol'nym dvizheniem.

The probability of motor unit discharges and changes in the inter-spike interval duration before the voluntary contraction were studied on m. soleus of human beings. A strong motoneuron facilitation was revealed 15-20 ms before the burst of potentials in EMG and only in some experiments a weak facilitation took place earlier (30-50 ms before the burst). So, "tuning" (H-reflex increasing) 60 ms before the voluntary contraction may be regarded as a result of H-reflex presynaptic disinhibition.

Influence of a tendon reflex on the motor neurons of antagonistic muscles in man. / Vliianie sukhozhil'nogo refleksa na motoneirony myshtsy-antagonista u cheloveka

The knee reflex was evoked during rhythmic activity of relatively low-threshold motor units of m. biceps fem. activated by weak voluntary contraction. Along with a reflectory response from m. rectus fem. the discharges from impulsating units of m. biceps fem. synchronous to the response were observed, as well as the recruitment of new units. Post-stimulation histograms and statistical analysis of interspike intervals of m. biceps fem. units revealed in most units a noticeable excitatory effect syndhronous to a reflectory response from m. rectus fem. This may be accounted fro by the presence of excitatory inputs of Ia afferents on motoneurons of the antagonist. Some delayed inhibitory effects on the antagonist motoneurons were found to follow their synchronous excitation. Apparently, they are analogous to the "silent period" of the antagonist motoneurons during the evoked tendon reflex.