How motoneurones control development of muscle tension

Abstract

THE sum of all synaptic currents flowing at any moment through the motoneurone membrane represents the synthesis of the commands converging on to the final common path. Transduction of such commands into muscle contraction is accomplished by the conversion of current into motoneuronal firing and, in turn, conversion of the nervous discharge into mechanical events¹. Thus, the motor unit, that is, each motoneurone with the muscle fibres it innervates, operates as the interface between the central nervous system and the environment. The motor unit response to steady inputs is well known. Constant currents are transformed into constant-rate discharges, whose frequencies are proportional, over a definite range, to the current intensities²; in isometric conditions, such constant-rate discharges build up steady and proportional levels of muscle tension^3–5. On the other hand, little is known about the motor unit response to dynamic inputs. Recent work⁵, however, has suggested that the rate of growth of the current stimulating the motoneurone may be converted into a proportional rate of development of the muscle tension. In view of this last hypothesis, we have analysed the isometric tension developed by single motor units when motoneurones were stimulated with current pulses of trapezoidal shape, that is, reaching a given steady level after rectilineal (ramp) growth of different slopes. This approach was adopted on the assumption that current injected by an intracellular electrode into a motoneurone is physically equivalent of total synaptic current acting on the cell^1,2.