DIAPHRAGMATIC MUSCLE TONE

It is generally believed that there is a scarcity of muscle spindles in the diaphragm and that there is no tonic activity at end expiration. A number of muscle spindles were found in the newborn human diaphragm, concentrated in the region of the central tendon. Tonic activity was detected by decreasing it (by rapid eye movement (REM) sleep or anesthesia) or by increasing it (with abdominal loading). During REM sleep in 5 infants and 5 adults, using subcostal electrodes a marked fall in tonic activity (P < 0.001) was compared to non-REM or quiet sleep. A reduction in diaphragmatic tonic activity was also observed with halothane anesthesia (P < 0.001). With esophageal electrodes in adult subjects, there was a rise in tonic diaphragmatic activity proportional to the amount of abdominal load (P < 0.001). There apparently are muscle spindles in the human diaphragm and tonic activity at end expiration.