An olfactory-parotid salivary reflex in humans?

It is commonly stated that smell elicits a parotid salivary reflex in humans. However, this assumption seems to be supported only by experiments in which either a lemon or an irritating odour has been used as the primary stimulus. In this study six pleasant odours, chocolate, vanilla, peppermint, beef, tomato and lemon, were administered to ten subjects. Air was passed through a solution of the stimulant and the resultant odour was administered to the subject via a face mask. Unilateral parotid saliva was collected via a Lashley cup and salivary flow was measured using an instantaneous flowmeter sensitive to flows as low as 0.008 ml min-1. Citric acid, lemon juice, chocolate and distilled water were also delivered to the subjects at increasing concentrations. At low concentrations none of these stimuli significantly increased the parotid salivary flow above resting levels. However, an increase in salivary flow was seen when lemon juice or odourless citric acid was sniffed or delivered to the subject at high concentrations, causing irritation in the nasal cavity and/or the back of the throat. Our results suggest that there is no true olfactory-parotid salivary reflex in humans, and that acidic stimuli can cause irritation with a concomitant increase in the salivary flow.