Cough is elicited by the stimulation of receptor areas supplied by the vagus nerve in the laryngeal area – laryngeal cough, and in the area of tracheobronchial tree – tracheobronchial cough [1]. The superior laryngeal nerve (SLN) innervates the larynx and plays a significant role in initiating of swallowing reflex [2].

Peripheral afferent inputs from the respiratory system in the – cervical vagus and superior laryngeal nerves were able to suppress the reflex of cough and swallow (pharyngeal stage).

The study was performed on 24 non-paralyzed anaesthetized, spontaneously breathing cats (3.56 ± 0.18 kg) under pentobarbiton anaesthesia. The conduction primarily in myelinated nerve fibres (vagus nerve and SLN) was reduced by cooling the nerves below 6°C. In the experiments, the vagus nerve was cooled in 13 animals, in 8 cats SLN was tested, in 5 cats the control cooling of the sympathetic part of vagosympathetic trunk was performed. 

The tracheobronchial cough was elicited by mechanical stimulation of the tracheobronchial mucosa (10-second stimulus) and the swallowing reflex was elicited by 3 ml of water instilled into oropharynx. The electrical activity of diaphragm, abdominal, styloglossus, cricopharyngeus and thyroarytenoid muscles was recorded using fine wire hook electrodes.

The results showed that compared to control, unilateral vagus cooling reduced the number of coughs as well as the intensity of inspiratory and expiratory cough efforts (p<0.01). Usually the vagus nerve section on one side dominated the suppression of cough. Vagus cooling also resulted in prolonged inspiratory and expiratory phase, the total and active portion of the cycle, and the distance between diaphragm and abdominal maxima during cough. Lower respiratory rate has been observed as well.

During unilateral cooling of SLN there was no effect on coughing, however during bilateral blockade of SLN there was reduction of cough expiratory efforts (p<0.05). SLN cooling also reduced the swallowing reflex. In all our experiments no appreciable changes in mean arterial blood pressures and heart rate were seen.

The evaluation of changes in the cough and swallow reflexes during the cold-induced blockade of reflex-related afferent pathways may contribute to understanding of modulation and suppression of studied reflexes. This can lead to procedures that could elucidate and improve some serious neuromuscular problems such as dysphagia and dystussia in patients. Our results are consistent with crucial role of vagal afferents in production and modulation of cough, with significant influence of SLN related feedback in modulation of studied responses, and with critical role of vagal afferent drive in control cough excitability as well as in shaping of cough motor pattern. 

This work was supported by the VEGA 1/0126/12. The project is co-financed from EU sources – ERDF – European Regional Developmental Fund. This work was supported by the Slovak Research and Development Agency under the contract No. APVV-0189-1 (prof. MUDr. Ján Jakuš, DrSc.).

[1] Jakus, Tomori, Stransky, Neuronal Determinants of Breathing Coughing and Related Motor Behaviours: Basics of Nervous Control and Reflex Mechanisms. Martin, Wist, 2004.
[2] Takahashi, K, Shingai T, Saito I, Yamamura K, Yamada Y, Kitagawa J., Facilitation of the swallowing reflex with bilateral afferent input from the superior laryngeal nerve. Neurosci Lett. 2014; 562:50-53.