Evoked potentials in the primary and secondary olfactory projection areas of the forebrain in elasmobranchia

Summary

• 1. The pattern of potentials evoked by electrical stimulation of the olfactory mucosa in the olfactory bulb of the shark Scylliorhinus was similar as in other vertebrate groups. The evoked potentials consisted of the incoming volley and two components of the main wave, which were negative at the surface of the central part of the bulb containing the glomerula. Outside of this glomerular area the first component of the main wave was positive (Fig. 1B). In depth recordings the main wave changed polarity in the mitral cell layer (Kg. 3). The latency of the incoming volley corresponded to a conduction velocity of 0.1–0.2 m/sec in the fila olfactoria. These findings can be understood in terms of the anatomical structure of the bulb and are compatible with the theoretical considerations by Rall et al. (1968).

• 2. Secondary olfactory potentials could be evoked in the forebrain after electrical stimulation of the nasal mucosa and the olfactory bulb in Scylliorhinus (Fig. 1E, S) and after electrical stimulation of the olfactory tract in Torpedo (Fig. 5B, S). Their latencies corresponded to a conduction velocity of 1–2 m/sec in the fibres of the olfactory tract. The potentials consisted of a surface negative wave restricted to a small lateral area of the ipsilateral forebrain (Fig. 5). The preceding incoming volley of the lateral olfactory tract was seen only in Torpedo after stimulation of the olfactory tract when it was not masqued by the simultaneous spread potential from the olfactory bulb.

• 3. Olfactory potentials of higher order could be evoked outside the secondary olfactory area of the forebrain (Figs. 1E,t; 5B, 5–10). The negative wave elicited about 20 msec after the secondary olfactory potential by bulbar stimulation appeared to be a tertiary olfactory potential; it showed complete fatigue with very low stimulus frequencies and was restricted to the caudal part of the ipsilateral forebrain. Late potentials with latencies up to 800 msec and regular EEG-synchronous afterpotentials were observed but not studied in detail.

• 4. The present findings are consistent with the view that the olfactory bulb has a very uniform and conservative structure and neuronal circuitry in all vertebrate groups. The secondary olfactory area of the forebrain hemispheres is much more restricted than concluded from older anatomical work. This result, supported by new anatomical work of Ebbesson et al. (1970), shows the existence of a comparable gross structure of the forebrain in elasmobranchs and higher vertebrates.