The innervation of tracheal smooth muscle in the ferret

The innervation of tracheal smooth muscle in the ferret has been studied by monitoring pressure changes in a fluid-filled segment of trachea in pentobarbitone anaesthetized animals. Stimulation of the peripheral cut end of the vago-sympathetic trunk elicited increases in trachea segment pressure only when activity was evoked in non-myelinated efferent axons, myelinated axons were without effect. Such excitatory responses were blocked by atropine. After atropine and elevation of trachealis muscle tone with 5-hydroxytryptamine, stimulation of the vago-sympathetic trunk produced a fall in tracheal segment pressure. These inhibitory responses were completely blocked by bretylium tosylate in six animals and markedly reduced in a seventh. It is concluded that contraction of tracheal smooth muscle in the ferret is cholinergically mediated and solely dependent on activity in non-myelinated vagal efferent axons and that inhibition is mediated by noradrenaline released from sympathetic nerve endings. A non-adrenergic inhibitory system would not appear to be present in the ferret.