Neuromuscular mechanisms of wing beat in hummingbirds

Abstract

1. The neuromuscular mechanism of the pectoral muscle (M. pectoralis major) was analysed by recording impulse discharges and tension developments of the muscle in hummingbirds (Calypte costae, Selasphorus sasin andCalypte anna). 2. Bursts of impulse discharges were observed from the whole pectoral muscle for each wing beat during the flight movements. 3. The frequency of burst discharge i.e., that of the wing beat is fairly constant (35–45 c.p.s.) and is identical in the hovering type of flight and the fast locomotive flight. 4. The amplitude of the wing beat is variable, it is higher during fast locomotive flights and it increases during acceleration of locomotion. 5. The amplitude of wing beat is altered by increasing the number of active motor units within the muscle and by increasing the number of impulses arriving at each motor unit for each wing beat. 6. The contraction of the pectoral muscle is a “twitch” per wing beat during the hovering flight but it becomes a “tetanus” of 2–5 motor impulses of a high frequency (300–500 c.p.s.) during the locomotive flight or acceleration of locomotion. The tetanus is no longer than the twitch but only higher in amplitude. 7. The contraction time of the pectoral muscle is about 8 msec and the tetanus fusion frequency is 250–300 c.p.s. at a body temperature of 40°C. 8. Similar experiments performed with the finch (Taeniopygia castanotis) show a similar constancy of the wing beat frequency (22–25 c.p.s.), contraction time of about 14 msec (40°C) and tetanus fusion frequency of 100–150 c.p.s.