PATTERNS OF MONOSYNAPTIC INPUT TO THE GIANT INTERNEURONS 1-3 IN THE CEREAL SYSTEM OF THE ADULT COCKROACH

1. The cerci of the cockroach Periplaneta americana bear filiform hair mechanoreceptors that are arranged in segmentally repeated rows and l ongitudinal columns. The monosynaptic connections between receptors of the same column or row and the 3 largest giant interneurons (GIs) wer e compared using the oil-gap single fibre technique. 2. For many colum ns, the synaptic efficacy of the afferents decreased gradually from th e base to the tip of the cercus, but columns with an inverted gradient or without any gradient were also observed. On the ipsilateral side ( relative to the GI axon), the inverted gradients were exclusively foun d for columns with short proximal hairs. For one column (cl) and GI3, the ipsilateral and contralateral gradients were opposite. 3. Monosyna ptic EPSPs evoked by stimulating different receptors of the same segme nt (segment 3) were of very different amplitudes, which partially acco unt for the directional sensitivity of the GIs. Differences in the loc ation, shape and size of the afferent terminals were not sufficient to explain these differences in connection strength. 4. No correlation w as found between the size of the EPSPs produced by a sensory neuron an d the length of its associated hair.