Receptors and giant interneurons signaling gravity orientation information in the cockroachArenivaga

Summary

• 1. The burrowing cockroachArenivaga is apprised of spatial position by two parallel rows of pendulous sensilla located on the ventral surface of each cercus (Hartman et al. 1979).

• 2. Electrophysiological recordings made during controlled displacements from primary orientation indicate that these receptors provide afference to two giant interneurons in each connective of the ventral nerve cord.

• 3. When the insect is maintained at primary orientation, except for sporadic neural activity evoked by air movements of the trichobothria, there is little neural activity. However, upon roll to the left two interneurons in the right connective produce action potentials (Fig. 3C). Upon roll to the right, two interneurons in the left connective fire. As the angle of roll is increased, the frequency of firing of action potentials increases. And, if the insect is maintained at a particular angle of roll, the appropriate interneurons fire tonically. Upon return to primary orientation, the interneurons cease firing.

• 4. If instead the cockroach is pitched from primary orientation, these same interneurons respond but in a different combination (Fig. 3A). Pitch forward causes the smaller interneuron of each connective to fire while pitch backward evokes neural activity from the larger interneuron of each connective. The frequency of firing increases with the angle of pitch, and when a pitch angle is maintained, the interneurons fire tonically. The interneurons are silenced upon return to primary orientation.

• 5. Rotation from primary orientation about other compass points permits determination of the receptive fields for these interneurons. Each interneuron is maximally responsive to rotations 45 ° from the longitudinal axis of the body with the receptive field approximately equivalent to one quadrant. The receptive fields overlap at the most stable (90–270 °) and least stable (0–180 °) axes of the insect (Fig. 6).

• 6. Extirpation experiments reveal that each row of receptors drives a particular interneuron (Fig. 5). Those in the medial row provide input to a large interneuron in the ipsilateral connective; the lateral row drives a smaller interneuron in the contralateral connective (Fig. 10).

• 7. These findings support the theory that cockroach cerci and cereal receptors function as equilibrium organs.