Graded activation of the intrinsic laryngeal muscles for vocal fold posturing

Abstract

Previous investigations using in vivo models to study the role of intrinsic laryngeal muscles in phonation have used neuromuscular stimulation to study voice parameters. However, these studies used coarse stimulation techniques using limited levels of neuromuscular stimulation. In the current investigation, a technique for fine control of laryngeal posturing was developed using graded stimulation of the laryngeal nerves. Vocal fold strain history to graded stimulation and a methodology for establishing symmetric laryngeal activation is presented. This methodology has immediate applications for the study of laryngeal paralysis and paresis, as well as general questions of neuromuscular control of the larynx.

Figure 1

(a) Strain of the left and right vocal folds to 23 levels of graded stimulation of the left RLN and (b) 26 levels of graded stimulation of the left SLN. Reference length (L₀) was at zero stimulation amplitude (baseline). The strain values (percentage change in length) were measured at 1.0 s after the onset of the stimulation pulse train (pulse train duration 1.5 s, pulse width 0.1 ms, repetition rate 100 Hz). The figure insert shows individual changes in position of the suture landmarks at each level of graded stimulation (dots) and the overall direction of movement (arrows) plotted on the digital high-speed video frame obtained at baseline. Note the difference in trajectories between the stimulated left vocal fold and the non-stimulated right vocal fold, as well as the as the flaring movement of the left thyroid cartilage with ipsilateral SLN stimulation.

Figure 2

Symmetry of stimulation parameters was determined from analysis of left vocal fold strain (a),(d), right vocal fold strain (b),(e), and distance between vocal processes (c),(f), as a function of 11 levels of graded stimulation of the right and left RLNs (total 121 conditions). Zero SLN stimulation was applied in (a),(b),(c), and in (d),(e),(f) a low level of SLN stimulation amplitude was applied that was close to the excitation threshold (left SLN 24 μA, right SLN 34 μA). The symmetry of the isocontour lines with respect to the diagonal (straight diagonal white line) demonstrates symmetric response of the vocal folds, implying symmetric graded activation of the larynx. However, in presence of SLN activation (d)–(f) there was an abrupt transition in strain of the vocal folds when zero strain was reached. The percentage strain values at the stimulated conditions are with respect to reference length measured at zero stimulation. (Measurements for stimulated conditions taken at 404 ms after onset of stimulation pulse train; pulse train repetition rate 200 Hz; camera frame rate 500 fps with resolution 512×512 pixels.)

Figure 3

Photomontage of vocal fold postural change to symmetric graded stimulation. The experimental conditions are identical to Fig. 2 and the photomontage is taken from images along the diagonal in Fig. 2c. The medial bulging effect of the vocal folds is seen first, followed by closure of the posterior commissure.