230 CHEMICAL SENSES 



teleost fishes (Bardach et al. 1967). It was not until the mid-1960s that 

 electrophysiological techniques were applied to studies of the chemical 

 senses of elasmobranchs, when Gilbert, Hodgson, and Mathewson (1964) 

 recorded changes in electroencephalograms following chemical stimulation 

 of sharks. 



BASIC THEORY 



In considering the prospects and limitations of electrophysiological analyses 

 of the chemical senses of elasmobranchs, it is appropriate to consider first 

 the types of electrical potentials that may be recorded by various methods. 

 An evaluation of these possibilities will also help explain why electrical 

 recording methods were applied relatively late to chemoreceptors. 



The small sizes and relatively inaccessible locations of most primary 

 chemoreceptor cells have posed serious problems for practical experimen- 

 talists. The small size of the cells means that the electrical potentials they 

 generate are also small, often below the amplitude of the electronic noise 

 levels of available amplifying systems. Penetrating the cells by micro- 

 electrodes, an often useful method of improving the signal-to-noise ratio 

 in neurological studies, is typically ruled out by either small size or in- 

 accessibility of the primary chemoreceptor cells. When those difficulties 

 can be overcome, however, the electrical phenomena associated with 

 chemosensory functions have proved less diverse than the anatomical de- 

 tails of the sense organs and their associated structures. For the latter 

 reason, it seems justified to draw on comparative data to provide useful 

 theory about those points of elasmobranch chemoreception for which data 

 on elasmobranchs remain inadequate. Data from teleost fishes, other verte- 

 brates, and even insects can be helpful in this regard and will be referred to, 

 as appropriate, throughout this review. 



Generator Potentials 



The electrical symptoms closest to initial stimulation of a receptor cell are 

 the generator potentials. They are graded, sustained electrical potentials, 

 arising at primary sites of stimulation and preceding the afferent impulses 

 (Figure 1A). 



Granit (1955) developed much of the early theory of generator poten- 

 tials and has provided important reviews of their significance. Because of 

 their closeness to initial transduction processes of stimulation, physiologists 

 concerned with basic mechanisms of stimulation prefer to record generator 

 potentials wherever possible. Although this has been done in certain experi- 

 mental preparations of insect and mammalian chemoreceptors, it has not 

 yet been achieved with elasmobranchs. When this level of analysis is reached 

 with elasmobranchs, it will be necessary to bear in mind an assortment of 

 constraints on the definition of true generator potentials. These are beyond 

 the scope of the present discussion but have been reviewed by Hodgson 

 (1965). 



