Hartmans Methods

To test the responses of the tricholith/positional interneuron system, a male Arenivaga cockroach was fastened ventral-side down to a motor-driven tilt table in the horizontal plane with pins and wax. The ventral nerve cord (VNC) was exposed by removing the wings and the cuticle on the back between terga 1 to 4. The internal organs were removed to expose the nerve cord between abdominal ganglia 1 and 2. A buffered insect saline solution was used to keep the preparation moist and neurons alive. Electrical recordings were made from the left and right VNC connectives using two suction electrodes attached to the tilt table. A variable-speed dc motor was run to generate various rates of tilt and maximum tilt angles ($). Tilt was measured electronically using an LVDT sensor coupled to the platform. By rotating the preparation and electrodes on the tilt table, Walthall and Hartman (1981) were able to subject the insect to various degrees of roll and pitch.

Tilt, using aerodynamic nomenclature, can be characterized in terms of pitch and roll (Figure 2.6-2). Pure pitch is when the animal is oriented along the 0° to 180° axis. (The platform always pivots about the 90° to 270° axis.) Pitch up is when the animal is facing 0° and its head is tipped up. The amount of pitch up can vary between 0° and 90°. Thus, a pure pitch up of 30° can be written in terms of two angular coordinates, (. = 0°, $ = +30°): the rst number is the angle the animal is facing, and the second the maximum amount of displacement. If the head is pitched down 30°, then (0°, -30°). Pure roll is when the animal is rotated around its longitudinal axis. Roll up right 45° is when the right side of the animal goes up and the left side down; it is written (90°, 45°). Roll down right is (90°, -45°). As it turns out, because the cerci and the rows of tricholiths project at approximately 135° and 225° in the horizontal plane, maximum sensitivity of the four VNC positional interneurons to roll and pitch occurs when the animal has both roll and pitch at a tilt so that the . is at 45° or 315° (Figure 2.6-3).

FIGURE 2.6-2 Dorsal view of an Arenivaga on a horizontal tilt platform, showing tilt directional conventions.

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