Neural responses of goldfish lateral line fibres to vortex-ring stimuli

Boris Chagnaud

Institut für Zoologie, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany

Fish use the mechanosensory lateral line to detect weak water motions. The smallest unit of the lateral line are the neuromasts, sensory structures located either freestanding on the skin (superficial neuromast) or in lateral line canals (canal neuromast). Among natural lateral line stimuli are the water motions in the vortex trail of a swimming fish or the vortex motions downstream of objects situated in water currents (Bleckmann 1994). Fish can detect such vortex rings (Tou 1991) and might take advantage of them to optimise their swimming behaviour in streams or in fish-schools (Liao et al. 2003). Here we present the responses of 69 posterior lateral line nerve fibres of the goldfish, Carassius auratus (N = 18), to vortex ring stimuli for the first time. We stimulated immobilized fish with artificial vortex-rings travelling along the side of the fish. The propagation velocity and the duration of the generated vortex rings could be regulated with a computer-controlled valve-bank. In parallel to the physiological data acquisition, we visualised the water motions using the particle-image velocimetry (PIV) method. Particle movements at a distance of approximately 3mm from the side of the fish were monitored. Based on the discharge patterns in response to a standardised vortex-ring (150ms opening of the valves, 5cm distance to the neuromast recorded from), the following responses were found: (i) I-afferents (n = 24) responding with a transient decrease in discharge rate followed by an long-lasting increase in discharge rate and (ii), E-afferents (n = 34) responding with the an transient decrease followed by a long-lasting increase in discharge rate. The initial and transient responses were always reproducible from trial to trial while in the majority of units the later increase in discharge rate was usually less defined. Using the PIV-method we show, that the two different initial responses (E or I-responses) as well as the early components of the less defined responses can be explained by the flow-patterns close to the neuromasts innervated by the recorded fibres. In 10 fibres that showed reproducible and transient responses following the initial response we show that the transition between these responses is correlated with reversals of the flow-direction close to the innervated neuromast. These transitions of the flow-direction are characteristic for the passing of the vortex ring. Thus the neuromasts are capable of encoding the local flow-direction. In summary we present a novel approach using a natural-like stimulus that is suitable for comparing the actual water movements with neural responses. This technique will be applied to gain further insight in the processing of sensory information in the ascending lateral line pathway.