Insect filiform sensilla are simple sensory organs. They consist of a thin cuticular hair and a single sensory cell. Despite the simplicity they can encode a surprising amount of information, such as stimulus intensity, velocity and direction, and are therefore an interesting case study of mechanosensory systems. The hair of the sensillum is a lever that transforms movement into force that acts upon the sensory dendrite. The sensory cell encodes the time course of the force, first into a receptor potential and then into a spike train. During this process mechanical components of the sensory apparatus and the sensory adaptation play a crucial role. In the first part of this poster we show that in firebug filiform sensilla the adaptation has multiple sources, and in the second part we explore the basis of their directional sensitivity. The firebug filiform sensilla are classified into three types. Types T1 and T2 have a phasic-tonic response and T3 a phasic response. During response to a ramp stimulus all units exhibit a complex sensory adaptation that decreases spike frequency already during the dynamic part of the stimulus. During the static part it has at least two phases (t1 [asymp] 10 ms and t2 [asymp] 1 s), which differ between the types. A negative correlation between neighboring interspike-intervals during resting activity indicates that the adaptation takes place on the level of the spike generator. To test this hypothesis we modeled the response of type T1 receptor with a mathematical model of sensory adaptation (Benda, 2003) that assumes adaptation on the level of the spike generator. The simulation was successful for slow stimuli, however, it failed at higher velocities. Another mechanism, activating at higher velocities and adapting the signal prior to the generator, can therefore be anticipated. Out of all three types only T1 is directionally sensitive. We investigated the stimulus transmitting apparatus of types T1 and T3 with standard methods of light and electron microscopy to link its structure to sensilla directionality. Our preliminary results show that the dendrite is on one side attached to a fibrous lamina, which appears to be the main stimulus transmitting component. However, only the dendrite of type T1 is pronouncedly bent in the excitatory direction. This distinction between the types could explain the directional sensitivity as well as the differences in their excitatory dynamics.

Benda J., Herz A. 2003. A universal model for spike-frequency adaptation. Neural Comput, 15:2523-2564.