[Activity of the expiratory laryngeal motor neurons]

The activity of the whole recurrent nerve branch to expiratory laryngeal muscles, along with unitary activity of fibres from this branch were studied in cats. Conduction velocities of fibres ranged between 19.5-93 m/s. We used two types of preparations: low "encephale isole " (at C7 level) cats with concomitant light Dial anesthesia or without any anesthesia, or cats deeply anesthetized with Dial or chloralose-urethane. The vagus nerves were preserved. Under normocapnic conditions, the delay of recruitment (i.e. the lag between the interruption of phrenic nerve activity and the start of unitary expiratory recurrent nerve activity) increases as the residual phrenic nerve activity increases; in fact, the delay is nearly zero with unanesthetized or lightly anesthetized preparations (small residual phrenic activity), but can extend up to 800 ms with deeply anesthetized preparations (large and long-lasting residual activity) (fig. 4). Recurrent nerve units show a maximum firing rate at the beginning of a discharge; the rate then decreases (fig. 5). The maximum frequency is strongly reduced when residual phrenic activity is present (fig. 7). The discharge duration does not reach the expiratory pause duration. Rapidly conducting fibres are the firs ones to cease activity (fig. 5)...