Responses of mesothoracic motor neurons to giant interneuron stimulation in the cockroach

1. During wind-evoked turning in the cockroach the mesothoracic (T2) legs tend to move in antiphase with the ipsilateral metathoracic (T3) legs, although this relationship is not absolute. 2. Intracellular stimulation of giant interneurons coupled with extracellular records from leg nerves 6Br4 and 5r1 indicate some substantial differences between the responses in T2 and those in T3. 3. In T2 there is a predominance of depressor outputs for all dGIs, whereas, in T3 the biases between dGIs are more balanced. Thus, taking the responses of all dGIs into consideration, levator and depressor motor neurons should be equally excited in T3 but not in T2. 4. Also, in T2 there is more variability in the responses than in T3. Although most motor responses in T2 involve initial depressor activity, some cases were found for each dGI in which levator activity consistently preceded depressor activation and some cases where both sets of motor neurons responded essentially at the same time. 5. These data suggest that, in response to GI activity, more factors may be involved in determining the direction of leg movement in T2 than in T3.