ROLE OF SYRINGEAL MUSCLES IN GATING AIR-FLOW AND SOUND PRODUCTION IN SINGING BROWN THRASHERS

1. The role of syringeal muscles in song production, particularly in r egulating airflow through the syrinx, was studied in singing brown thr ashers (Toxostoma rufum). In nine individuals, muscle activity was rec orded electromyographically together with bilateral syringeal airflow, subsyringeal air sac pressure, and vocal output. 2. Dorsal muscles, m . syringealis dorsalis (dS) and m. tracheobronchialis dorsalis (dTB), are consistently activated during ipsilateral closing of the syrinx or increasing syringeal resistance, suggesting that their main role is a dduction. This interpretation is supported by the motor patterns accom panying syllables with rapid oscillations in the rate of airflow. Burs ts of electrical activity (2-10 ms) in dorsal muscles are precisely sy nchronized with decreasing airflow. 3. Electrical activity in m. trach eobronchialis ventralis (vTB) and m. tracheolateralis (TL) is associat ed with active abduction. An important contribution of vTB is to open the syringeal lumen for short inspirations in between syllables. In sy llables with oscillatory flow modulations, vTB bursts show variable al ignment with the phase of increasing flow. From this and activity duri ng other syllables, it appears that, during phonation, vTB activity fi ne tunes the syringeal configuration, which is set by action of the do rsal muscles into a partially constricted state. 4. Activity in the ve ntral portion of TL, an extrinsic muscle, is strikingly similar to tha t of vTB, an intrinsic muscle, suggesting that the two muscles have a similar functional role. This supports the notion that intrinsic syrin geal muscles of songbirds evolved from extrinsic muscles of nonpasseri nes. 5. M. syringealis ventralis (vS) does not appear to contribute di rectly to gating of airflow. Its activity is not consistently correlat ed with active changes in syringeal resistance. 6. Activity in m. ster notrachealis (ST) is most prominent during rapid changes in the rate o f airflow or when switching between expiratory and inspiratory flow, s uggesting a role in stabilizing the syringeal framework.