Response of cricothyroid muscles to frequency-modulated sounds in FM bats, Myotis lucifugus

Abstract

BATS of the species Myotis lucifugus use short frequency-modulated (FM) ultrasonic pulses for echolocation. The duration, repetition rate and frequency of these pulses vary from time to time depending upon whether the animal is searching for, approaching or actually capturing its prey. The bat emits 2–5 ms pulses at a rate of 10–20 s⁻¹ with the frequency of each pulse sweeping approximately one octave from 80 to 40 kHz during the searching and cruising flight; it produces 0.3–1.0 ms pulses at a rate of 150–250 s⁻¹ with a downward-sweep FM from 40–20 kHz immediately before avoidance of obstacles or capture of a prey^1,2. Among the laryngeal muscles of the bat, the cricothyroid muscle (CTM) in particular, is highly developed and is essential in producing the intense orientation pulses^1,3. The CTM not only discharges action potentials before the vocalisation^3–5, but also during the acoustic stimulus^6,7. The latter phenomenon is called the acoustic laryngeal muscle reflex and might serve as a negative feedback to stabilise the performance of the vocalisation system⁶. Comparative studies of the properties of the middle ear and laryngeal muscles revealed significant differences between these two muscles (refs 6 and 7 and P.H.-S.J., J.O., and N. Suga, in preparation). We found that CTM fibres were more sensitive to 4 ms FM signals sweeping downward with a range of 10 or 20 kHz across their best frequencies (BFs) than to pure tones.