On the elicitation of the diving responses in ducks

Peripheral receptors signalling in both the 3 main divisions of the trigeminal nerves and in the glossopharyngeal nerves are most likely involved in the onset of the apnea in diving ducks. Since the response also takes place when ducks are submerged in mercury, these peripheral receptors are hardly specific to water. Neither any of the 3 main groups of trigeminal, nor the glossopharyngeal receptors are alone responsible for the initiation of the apnea. Most likely the apnea takes place when the total impact on the 'respiratory centre' in the brain exceeds a certain treshold value, due to activation of a certain number of both trigeminal and glossopharyngeal peripheral receptors. However, even very local stimulation of the glottal area invariably evoked apnea, indicating a higher density of receptors there than for example on the beak or in the nares. This idea is supported by the fact that not even heavy rain evokes any response at all. It is unlikely that the cardiovascular adjustments displayed by diving ducks are directly coupled to the apnea response (i.e. to stimulation of peripheral receptors). Rather they are supposed to be components in orienting and chemoreceptor reflexes.