Avian Chemoreception: An Electrophysiological Approach
The first detailed physiological evidence for olfactory and trigeminal chemoreception in an avian species is provided by a series of investigations in the chicken (Gallus domesticus). Initial work indicated that the activity of avian olfactory bulb neurones closely resembles that of other vertebrates, exhibiting variable spontaneous temporal firing patterns with mean firing rates between those reported for mammals and reptiles. Application of odors directly to the olfactory epithelium showed that like mammals, avian olfactory bulb neurones respond in the form of inhibition and excitation with accompanying changes in temporal firing pattern. When exposed to a range of concentrations of a single odor, all responsive neurones exhibited an ability to discriminate small step-changes in concentration producing clear stimulus response relationships. Avian trigeminal chemoreception was also investigated by examining the responses of single mucosal receptors in the nasal cavity and palate. Slowly and rapidly adapting nasal mechanoreceptors were identified, some of which exhibited chemical sensitivity when exposed to ammonia gas, acetic acid vapor or carbon dioxide. These results demonstrate that polymodal nociceptors are present in avian nasal mucosa and represent the first attempt in any species to quantify the responses of single trigeminal receptors to a range of concentrations of noxious airborne chemicals. Collectively, the findings demonstrate how an electrophysiological approach can improve our understanding of the underlying sensory physiology relating to avian perception of the chemical environment.
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