FUSILLADE PATTERNS OF INSPIRATORY AND EXPIRATORY MUSCLES AND THEIR EFFECTS ON THE RESPIRATORY ACT

Abstract

Fusillade patterns of inspiratory and expiratory contractions during eupnea were compared and their mechanical effects considered. Inspiratory contractions were characterized by a progressively accelerating rate of muscle fiber twitching and by a progressive recruitment of fibers up to the end of inspiration. Expiration was initiated by a, sudden reversal of this inspiratory innervation. This mode of innervation and denervation produced a "triangular" configuration of the electrogram indicating the changing strength of muscular contraction. Expirations were of 3 types: 1, passive; 2, "rectangular"; 3, "triangular." Rectangular expiration was characterized by a uniform muscle fiber rhythm and by a uniform number of action potentials. Triangular expiration was characterized by a maximum number of potentials at the beginning of expiration progressively diminishing with time. It is suggested that "rectangular" and "triangular" expirations may be fundamentally different: that triangular expiration is a product of a direct discharge from the expiratory side of the respiratory mechanism; that rectangular expiration may be the resumption of a continuously driven tonic viscero-postural reflex temporarily inhibited during the phase of inspiration. This conception is supported by expts. on the transcostar muscle. Reflex contraction elicited by weighting the free end was rhythmically interrupted with each inspiration, giving all outward appearance of active expiratory contractions. A sporadic deep breath which enormously augments the inspiratory fusillade may have no effect upon the succeeding expiratory fusillade, though the expiratory excursion is correspondingly increased. This shows that inspiratory and expiratory innervations may be separately modified. It is proposed that proprio-ceptive reflexes may contribute towards the guidance of the respiratory act by influencing the incidence, strength and sequence of contraction of respiratory muscles. Waxing inspiration meets increasing resistance and, therefore, tends to produce a mechanically smooth inspiratory act, but the quickness of development of the counter force of the vagal proprioreceptor inhibitory impulses tends to check inspiration at slightly different levels and produce uneven filling of the lungs. The sudden liberation of expiratory forces at the beginning of expiration tends toward jerky initiation of expirations, but waning mechanical forces meeting increasing resistance tend to come into uniform equilibria at the close of expiration and produce a characteristically even emptying of the lungs. These mechanisms controlling lung volume are suggested as factors influencing the rate of breathing.