THE GRADATION OF THE INTENSITY OF INSPIRATORY CONTRACTIONS

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Abstract
The relatively great importance of the inspiratory act in the mechanics and in the nervous integration of breathing called for a further study of the mode of its gradation. Since inspiratory contractions produce a progressively increasing distortion of the lungs and torso the amt. of energy required to continue the act must vary with the degree of pulmonary inflation. This energy is supplied in a smoothly graded manner suitable for meeting the increasing mechanical requirements as the lungs expand. The adjustment of energy is accomplished in 2 ways, by a progressive addition of newly activated units to the initially weak contraction and by an increased frequency of twitch of all participating fibers as inspiration advances. Each newly recruited unit begins contracting at its minimum twitch frequence which accelerates as inspiration progresses. Those units recruited early in the phase of inspiration, therefore, attain the highest frequency and deliver the greatest number of twitches. The gradation of inspiratory contractions during progressive hyperpnea from O2 lack or CO2 excess is in principle the same as that employed to meet the ordinary increasing mechanical requirements of a eupneic inspiration. Increased depth of inspiration is attained by a combination of recruitment of new muscle units and an intensified activity of those units already engaged. As chemical stimulation increases, initial activity of individual muscle units moves up into the early phases of inspiration thus increasing their number of twitches and their maximum twitch frequency. The energy liberated by a single twitch of a single muscle fiber is adopted as the unit of mechanical energy of muscular contraction, and the sum total of the individual twitches of all participating fibers is regarded as the main factor determining the strength or depth of inspiration. The number of twitches and the frequency of twitch of individual muscle units are not necessarily an indication of the strength of contraction of the muscle as a whole. Central integrative mechanisms of periodicity and of gradation of the activity of the inspiratory half center are proposed.

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