232 CHEMICAL SENSES 



potentials as a possible signal of significance to the central nervous system 

 (CNS) (Hodgson 1965). Similar kinds of spontaneous activity occur in the 

 ampullary electroreceptors of elasmobranchs, and are discussed elsewhere 

 in this volume by Bennett and Clusin (1978). 



Experimental preparations of single chemosensory afferent nerve fibers, 

 or "few-fiber" preparations (which permit recognition of impulses from 

 different cells on the basis of their different impulse voltages) have been 

 achieved in a number of fishes (e.g., Bardach et al. 1967). They have been 

 the object of investigation in several elasmobranchs. Tester (1975) reported 

 that action potentials had been recorded from the olfactory tracts of 

 hammerhead sharks (Sphyrna) during chemical stimulation. However, a 

 later reevaluation of the records led to the conclusion that the impulse 

 patterns actually had no correlations with amounts or types of chemical 

 stimulation (Tester, personal communication). Attempts to obtain record- 

 ings from few-fiber preparations of the olfactory tract of various sharks 

 and rays have also failed to yield reproducible results (Hodgson and 

 Mathewson, unpublished). 



Olfactory Bulb and CNS Recordings 



Because the central neuropil is a complex mixture of very fine nerve proc- 

 esses, the electrophysiological methods usually applied to single cell units 

 can rarely be used in the central nervous system (CNS). Multi-unit recordings, 

 used with CNS preparations, include recordings of evoked potentials, syn- 

 chronized responses, and rhythmic electroencephalogram (EEG) patterns. 

 It was the latter method that first demonstrated electrical correlates in the 

 elasmobranch nervous system during stimulation by chemicals (Gilbert, 

 Hodgson, and Mathewson 1964). An example of such an EEG pattern is 

 diagramed in Figure 1C. Eventually, this technique was refined so that 

 chronically implanted electrodes could record EEGs in swimming sharks, 

 which made possible the correlation of brain potentials with behavioral re- 

 sponses during controlled stimulation by chemicals (Hodgson, Mathewson, 

 and Gilbert 1967). In addition to the changes in rhythmic potentials in the 

 forebrain during chemical stimulation, there are changes in the medulla 

 associated with gill movements during chemical stimulation. In general, the 

 EEG responses of sharks and rays appear similar to those of teleost fishes. 

 The larger sizes of some of the shark brains studied may be an advantage 

 over some of the teleost preparations used in experiments, and this may 

 explain some earlier reports of failures to detect forebrain responses in 

 teleosts during chemical stimulation e.g. Adrian and Ludwig 1938). 



Evoked potentials, following electrical stimulation of the olfactory 

 mucosa, have been recorded from various parts of the olfactory bulb and 

 forebrain in the shark Scyliorhinus and in Torpedo (Bruckmoser and 

 Dieringer 1973). The evoked potential studies have been focused mainly 

 on the structure and circuitry of the olfactory bulbs and the secondary 

 olfactory areas of the forebrain hemispheres. Indications are that the 



