Patterns of membrane potential changes in vocal cord tensor motoneurons

Abstract

Problem: The larynx plays critical roles in respiration, vocalization, and several airway defensive reflexes. Functional roles and neural acts of the vocal cord tensor in these various behaviors are not well known. Membrane potential changes in cricothyroid muscle motoneurons (CTMs) are monitored during these behaviors in cats to reveal the nature of central drives to CTMs. Methods: Intracellular recordings of CTMs were performed in 11 decerebrated and paralyzed cats during fictive respiration, vocalization, coughing, and swallowing. CTMs were identified by antidromic activation from the superior laryngeal nerve (SLN). Fictive behaviors were induced by electrical stimulation of either the pontine call site (for vocalization) or the SLN contralateral to the recording site (for coughing and swallowing). Intracellular application of continuous negative currents was attempted to confirm the presence of inhibitory postsynaptic potentials. Results: All CTMs recorded depolarized during the inspiratory phase of breathing; one half depolarized during the expiratory phase as well. During vocalization, CTMs rapidly depolarized at the onset of the vocal phase and retained strong depolarization throughout the vocal phase. During coughing, CTMs depolarized during the inspiratory phase, repolarized prior to the onset of the abdominal nerve burst, which was confirmed to exist inhibitory postsynaptic potentials, and then depolarized during the abdominal burst. During swallowing, most CTMs exhibited a hyperpolarization, sometimes followed by a weak depolarization, whereas some CTMs exhibited a slight depolarization. Conclusion: The principal role of the vocal cord tensor is vocalization rather than airway defensive reflexes. CTM activities are primarily generated by excitatory inputs during breathing and vocalization, while inhibitory inputs play an important role to control CTM activities during coughing and swallowing. Significance: This study demonstrates that, among several critical functions in the larynx, the vocal cord tensor plays a crucial role in vocalization but a minor role in coughing and swallowing, which is different from other laryngeal muscles. Support: None reported.