[Central interactions between laryngeal motoneurones (author's transl)]

Activities of single efferent fibres which innervate the laryngeal muscles in adult cats were studied during repetitive stimulation of the recurrent laryngeal nerve or the vagus nerve. The orthodromic response (primary response) can be followed by another response (secondary response) which has a latence consistant with the hypothesis of a central origin. This secondary response has an irregular incidence of occurrence, about 5.6 time for 100 stimulations. This secondary response: 1. Is present for 90% of the tested inspiratory or expiratory laryngeal fibres; 2. Depends on a central origin because it no longer occurs when the cervical vagus nerve is cut rostral to the site of stimulation; 3. Is not the consequence of the stimulation of afferent fibres because it is not altered by destruction of the centripetal vagal pathway (Fig. 4); 4. Can be obtained by the stimulation of other branches than those containing the recorded fibre (Fig. 2), so it is not necessarily the consequence of the antidromic invasion of this fibre; 5. Depends on a cholinergic synapse because it is more frequently observed after intra-arterial injection of acetylcholine (Fig. 6) and disappears after injection of Mecamylamine (an antinicotinic drug) (Fig. 7); 6. Persists after injection of strychnine and becomes more frequently observed; hence it does not seem to be associated with a disinhibitory mechanism; 7. Depends on a phenomenon of convergence because it occurred more frequently when the amplitude of stimulation was increased (Fig. 3); 8. Has a central delay which seems not to be consistent with an electrotonic origin; 9. Can be recorded from an inspiratory fibre when stimulating the remaining inspiratory or expiratory fibres. A secondary response can also be recorded from an expiratory fibre by stimulating the same or other expiratory fibres, but not after excitation of inspiratory fibres (Fig. 5). These results suggest: (1) a functional organisation into the pool of laryngeal motoneurones; (2) the possible existence of intrabulbar axonal collaterals in that pool.