Responses to wind recorded from the cercal nerve of the cockroach Periplaneta americana - II. Directional selectivity of the sensory neurons innervating single columns of filiform hairs

1. Responses to wind puffs from different directions were recorded from the whole cercal nerve of Periplaneta americana using a suction electrode. The directional responses of specific columns of hairs (Nicklaus, 1965) were studied by covering all other columns with a sticky substance. The nerve responses were low-pass filtered and summated over 64-256 trials. 2. For a given column of hairs, the response to wind from one direction was initially depolarizing; from the opposite direction it was initially hyperpolarizing; and for either orthogonal direction there was no response. The two opposite directions which gave responses corresponded to the two directions in which deflecting the hairs required least force (Nicklaus, 1965) (Figs. 3, 4). 3. During the falling phase of a wind puff, the sensory response was in the opposite direction from that on the rising phase (Figs. 3A, 4A, B). This suggests that the direction of a hair's movement (from either a resting or a deflected starting point) rather than the momentary position of the hair determined whether the sensory cell was excited or inhibited. 4. The excitatory response peaked during the maximal wind acceleration rather than during maximal wind speed (Fig. 3A). This suggests that the sensory cells may act largely as wind acceleration detectors. 5. The best excitatory wind direction was determined for the 9 most prominent columns of hairs (Fig. 5). Judging from the wide distribution of best excitatory directions for each cercus (Fig. 6) there is no wind direction which would not excite at least one column of sensory cells on each cercus. 6. These results provide the basis for a discussion on the functional organization of the sensory hairs in terms of their responses to naturally occuring wind stimuli.