PHYSIOLOGY OF CHEMORECEPTION 229 



gration with other activity in the central nervous system, which undoubt- 

 edly influences patterns of efferent potentials controlling swimming, orien- 

 tation, and other behavior. However, analysis of these neural integrative 

 mechanisms is still in its infancy. 



The beginnings of electrophysiological approaches to the chemical senses 

 of aquatic vertebrates are noted in G. H. Parker's (1922) classic book, 

 Smell, Taste, and Allied Senses in the Vertebrates. Parker reported that 

 a weak electrical stimulus seemed ". . . in every way to duplicate the stimu- 

 lus normal for the organ of taste" in certain fishes. In fact, if the currents 

 led into aquaria through water-filled glass tubes were reduced to less than 

 a microampere, catfish (Ictalurus) would approach the open ends of the 

 tubes and ". . . nibble at the current as though it were a bait." Although at 

 that time Parker was actively investigating the responses to chemicals by 

 sharks, he apparently did not extend his observations on electrical stimula- 

 tion to any of the elasmobranchs. 



During the 1930s, the first neurophysiologies recordings were made 

 from chemosensory systems of fishes. Nerve discharges from gustatory 

 fibers of the facial nerve, innervating the barbels of catfish, were recorded 

 by Hoagland (1933). Shortly thereafter, Adrian and Ludwig (1938) achieved 

 the first recordings from the olfactory nerve of the same species. These 

 early studies hinted at some properties of the chemosensory systems, and 

 experimental studies of them, that are still noteworthy. The effective chemi- 

 cal stimuli were found to initiate impulses (action potentials) of lower ampli- 

 tude than those from mechanoreceptors— an observation frequently con- 

 firmed in subsequent studies, and one that is obviously important for 

 experimental design and the interpretation of results. 



In this period, when investigators sought to apply the new electrophysio- 

 logical techniques to a wide variety of aquatic animals, it seems significant 

 that the first notable successes were achieved with experiments on fresh- 

 water rather than marine species. Undoubtedly the lower conductivity of 

 freshwater, which lessened the problems of short circuiting between elec- 

 trodes, influenced the ease with which results could be obtained using the 

 recording techniques then available. It is noteworthy also that none of the 

 early techniques recorded electrical events in actual chemoreceptor cells, 

 but only in sensory nerve fibers supplying the receptor cells. 



The 1950s and 1960s were a period of rapid advances in neurophysio- 

 logical analysis of chemosensory systems. Hodgson, Lettvin, and Roeder 

 (1955) used the special anatomical advantages of insects to obtain the first 

 records of electrical activity in primary chemoreceptor cells. Rapid strides 

 were made in experiments on the chemical senses of insects and mammals, 

 summarized in the international symposia on olfaction and taste, a series 

 of conferences that still continues (e.g., Zotterman 1963, Hayashi 1967, 

 Schneider 1972). 



Despite new findings from studies on insects and mammals, which had 

 important implications for concepts of sensory physiology as a whole 

 (Beidler 1970, Hodgson 1965), the experimentation on groups other than 

 mammals and insects was sparse; among aquatic animals it was limited to 



