Superfast vocal muscles control song production in songbirds

Abstract

Birdsong is a widely used model for vocal learning and human speech, which exhibits high temporal and acoustic diversity. Rapid acoustic modulations are thought to arise from the vocal organ, the syrinx, by passive interactions between the two independent sound generators or intrinsic nonlinear dynamics of sound generating structures. Additionally, direct neuromuscular control could produce such rapid and precisely timed acoustic features if syringeal muscles exhibit rare superfast muscle contractile kinetics. However, no direct evidence exists that avian vocal muscles can produce modulations at such high rates. Here, we show that 1) syringeal muscles are active in phase with sound modulations during song over 200 Hz, 2) direct stimulation of the muscles in situ produces sound modulations at the frequency observed during singing, and that 3) syringeal muscles produce mechanical work at the required frequencies and up to 250 Hz in vitro. The twitch kinematics of these so-called superfast muscles are the fastest measured in any vertebrate muscle. Superfast vocal muscles enable birds to directly control the generation of many observed rapid acoustic changes and to actuate the millisecond precision of neural activity into precise temporal vocal control. Furthermore, birds now join the list of vertebrate classes in which superfast muscle kinetics evolved independently for acoustic communication.

Figure 1. Song of starling exhibits fast modulations that correlate with muscle activity.

(A) Spectral derivative plot (top), oscillogram of sound and EMG activity of m. tracheobronchialis dorsalis (dTB). (B) Expanded time scale of segment i. The rate of modulation of the sound amplitude is paralleled by peaks in muscle activity (shaded areas). Positive traces; integrated values (time constant 0.2 ms), negative traces; half-wave rectified values. (C) Cross-correlations (R) of integrated sound amplitude and EMG activity signals of segments i and ii show a temporal link between EMG activity and sound modulation. The segments i and ii are indicated in (A). The distance between the local peaks in the bar diagrams equals the period of the signals. The periods are 4.6 and 6.6 ms in segment i and ii respectively, which is equivalent to repetition rates of 218 and 152 Hz. EMG; electromyogram of dTB, Sound; sound oscillogram.

Figure 2. In vitro isometric properties of syringeal muscles exhibit superfast twitch kinetics.

(A) Twitch characteristic of male and female zebra finch m. tracheobronchialis ventralis demonstrate extremely fast rise, and decay times. In zebra finches, syringeal muscle performance is sexually dimorphic. (Twitch half time 3.73±0.68 ms (male, N = 8) and 7.08±0.79 ms (female, N = 3). t-test; p<0.01) (B) Twitches of male starling m. tracheobronchialis dorsalis are completely separated at 150 Hz. Force summates at faster stimulation, and the tetanic fusion is not complete until 800 Hz. The stimulation frequencies corresponding to the curves are indicated in bold.

Figure 3. Superfast syringeal muscles produce positive work and power up to 250 Hz in vitro.

(A) Work and (C) power production of male starling m. tracheobronchialis dorsalis (dTB, N = 5). (B) Work and ((D) power production of male zebra finch m. tracheobronchialis ventralis (vTB, N = 6). Plotted values are mean±S.D. The insets show the bird species and the position of the dTB and vTB muscle on a schematic of the songbird's syrinx (modified after [46]). The arrows indicate which muscle is being tested. (E) Active workloops of starling dTB at 150, 175, 200 and 250 Hz. Dotted vertical lines indicate zero strain ( = resting length). Arrows indicate lengthening (pointing right) and shortening (pointing left) portions of the workloop.

Figure 4. Stimulation of syringeal muscles causes tracheal flow and sound modulation.

In situ muscle stimulation of syringeal muscles (A) dTB (N = 3) and (B) vTB (N = 3) modulates tracheal flow up to 250 Hz. Values are mean±S.D. The insets show the position of the dTB and vTB muscle on a schematic of the songbird's syrinx (modified after [46]). The arrows indicate which muscle is being tested. (C) Sound amplitude of vocalization during dTB stimulation at 100 Hz. The sound amplitude drops following muscle stimulation (red areas). Black trace; half-wave rectified sound signal, orange trace; integrated sound signal (time constant 2 ms).