PHYSIOLOGY OF CHEMORECEPTION 



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be obtained under similar test conditions; this is a less successful recording 

 technique, however, since the swimming sharks often dislodged the im- 

 planted electrodes by wiping their snouts against the walls of the observa- 

 tion tank during the initial acclimatization period. Figure 6 shows an in- 

 stance in which pronounced rhythms were obtained from the bulb, and 

 electrodes implanted near the tail recorded muscle potentials, providing 

 another measure of the shark's swimming activity. It is important to note 

 several other differences between the records shown in Figures 5 and 6. 

 The polygraph speed in Figure 6 is slower, the horizontal calibration repre- 

 senting 3 s; this slower speed of the traces compresses the potentials so that 

 they more nearly resemble (but are definitely not) afferent spike potentials. 

 Also, the amplification of the record from the bulb has been increased, with 

 the vertical calibration in this case being 20 uV. The record shown in Fig- 

 ure 6 starts when a few low-level potentials were first recorded from the 

 olfactory bulb, at the onset of chemical stimulation. An extra gill beat 

 and large increase in bulb activity follow, coinciding with stronger swim- 

 ming movements, as indicated by the muscle potentials from the tail region. 



While the overall reactions of the nurse and lemon sharks are similar, a 

 comparison of the records in Figures 5 and 6 does reveal differences. Lemon 

 sharks are characteristically (but not invariably) in motion at all times. Slow 

 swimming with open mouth is a widespread method of irrigating the gills 

 in sharks, contrasting with the pumping of the gills when nurse sharks are 

 immobile. The amplification of the tail muscle potentials in Figure 6 has 

 been decreased to a level where the slow minimal contractions necessary 

 for base level swimming by the lemon shark do not produce significant 

 fluctuations in the record. Stronger swimming, characteristic of positive 

 responses to chemical stimuli, produces obvious deflections in the lower 

 trace. 



A second difference between the nurse and lemon sharks is seen in the 

 medullar centers controlling gill beats. In both species, the extra beat at 

 the onset of stimulation is seen. However, the lemon shark, while it may 

 temporarily prolong a few gill beats (see Figure 6), does not stop its gill 



.0001 M GLYCINE 



Figure 6 Reaction of a free-swimming lemon shark to stimulation with glycine. See 

 explanation in text. 



