Functional plasticity in the respiratory drive to thoracic motoneurons in the segment above a chronic lateral spinal cord lesion.

A previous neurophysiological investigation demonstrated an increase in functional projections of expiratory bulbospinal neurons (EBSNs) in the segment above a chronic lateral thoracic spinal cord lesion that severed their axons. We have now investigated how this plasticity might be manifested in thoracic motoneurons by measuring their respiratory drive and the connections to them from individual EBSNs. In anesthetized cats, simultaneous recordings were made intracellularly from motoneurons in the segment above a left-side chronic (16 wk) lesion of the spinal cord in the rostral part of T8, T9, or T10 and extracellularly from EBSNs in the right caudal medulla, antidromically excited from just above the lesion but not from below. Spike-triggered averaging was used to measure the connections between pairs of EBSNs and motoneurons. Connections were found to have a very similar distribution to normal and were, if anything (nonsignificantly), weaker than normal, being present for 42/158 pairs, vs. 55/154 pairs in controls. The expiratory drive in expiratory motoneurons appeared stronger than in controls but again not significantly so. Thus we conclude that new connections made by the EBSNs following these lesions were made to neurons other than α-motoneurons. However, a previously unidentified form of functional plasticity was seen in that there was a significant increase in the excitation of motoneurons during postinspiration, being manifest either in increased incidence of expiratory decrementing respiratory drive potentials or in an increased amplitude of the postinspiratory depolarizing phase in inspiratory motoneurons. We suggest that this component arose from spinal cord interneurons.

The respiratory drive to thoracic motoneurones in the cat and its relation to the connections from expiratory bulbospinal neurones.

The descending control of respiratory-related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike-triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5-T9 of the spinal cord of anaesthetized cats. Spike-triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 microV. The overall strength of the connection supports previous measurements made by cross-correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike-triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

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.

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.

Variability in the amplitude of skeletal muscle responses to magnetic stimulation of the motor cortex in man.

We have investigated variability in the amplitude of compound motor evoked potentials (cMEPs) in right and left thenar and wrist extensor muscles in response to synchronous bilateral transcranial magnetic stimulation (TMS) of the motor cortices using two figure-of-eight stimulating coils. Trials of 50 stimuli revealed a wide range of variability in cMEP amplitudes in relaxed muscles (coefficient of variation, range 0.22-1.12). The amplitudes of the cMEPs in one muscle correlated positively with those in the others. The r2 values (mean +/- SEM) were 0.27 +/- 0.06 for muscles on the same side of the body and 0.19 +/- 0.04 for muscles on opposite sides. Employing the ECG to trigger TMS, clamping the coil relative to the head or altering the orientation of the coil all failed to affect the variability of cMEPs. We conclude that fluctuations in excitability of the corticospinal pathway give rise to the variability in the response to TMS, that they are wide-ranging with respect to the muscles affected, and include a bilateral component. We argue that the variability reveals fluctuations in excitability mainly at the cortical rather than the spinal level. We suggest that measures of variability might provide an indication of cortical activity in conditions where consciousness and voluntary movement are compromised.