Die funktionelle Organisation der motorischen Einheiten des Zwerchfells

Summary

The object of the present investigation—carried out with electrophysiological methods—was to analyse the general principles underlying the cooperative action of the motor units of the diaphragm during quiet and forcible breathing in the rabbit. The experiments were designed to determine the relation between the inspiratory discharge patterns of the motor units and a) the time interval between the corresponding nerve and muscle impulses, b) the spatial arrangement of the motor subunits within the diaphragmatic sector studied (sternal part), and c) the tension developed by the inspiratory contractions of this strip under auxotonic conditions.

Up to the level of the muscle, the order of motor unit recuitment remained practically unchanged during consecutive periods of inspiration. The onset of firing from one unit to the next was, however, subject to variations, thus, progressive shortening of the intervals preceding discharge onset occurred as respiration deepened. During forcible breathing the average discharge rates showed a tendency to increase progressively parallel with unit recruitment. Regularity in the discharge patterns of the motor units was closely related to the structural and mechanical properties of the latter, freshly recruited units being differentiated from those previously activated by a) higher conduction velocity in the associated nerve and muscle fibres, b) augmented field size and amplitude of subunit action currents, c) diminished contraction and relaxation times, and d) increased tetanic tension.

These results indicate that the stability in order of motor unit recruitment ensures adequate timing of activity at the level of muscle during inspiratory adjustment of the diaphragm to changing respiratory conditions.