The cercus-to-giant interneuron system of crickets - IV. Patterns of connectivity between receptors and the medial giant interneuron

1. The pattern of connections between cereal sensory neurons and a single sensory interneuron, the medial giant interneuron (MGI), was examined using intracellular methods. 2. Two types of sound sensitive hairs are located on each cercus. Each type was examined for its ability to excite, or inhibit the MGI. 3. The effect of stimulating a large population of one hair type was examined by blocking movement of all other types, recording intracellularly from MGI, and stimulating the preparation with tones. 4. T-hairs on the ipsilateral cercus powerfully excited MGI initiating a train of action potentials. L-hairs on either cercus weakly excited MGI. T-hairs on the contralateral cercus inhibited MGI. 5. The effect of stimulating individual receptor hairs was examined by blocking the movement of all but a single hair. Intracellular recordings from MGI exhibit subthreshold synaptic potentials when L-or ipsi-T hairs are stimulated. When the hair was removed the potentials disappeared. Ipsilateral T-hairs excite MIG with a high probability (approximately 75%) while ipsilateral L-hairs excite MGI with a lower probability (approximately 50%). 6. Thus the strengths of connection between hair type and MGI can be ranked as follows: Ipsilateral T-hairs provide the strongest excitatory input and therefore determine the overall directional characteristics of MGI. Ipsilateral L-hairs provide weaker, often subthreshold excitatory inputs and contralateral L-hairs provide the weakest excitatory input to MGI. Finally the contralateral T-hairs inhibit MGI.