1. 1.

    The telson flexor system is homologous to the fast flexor system of anterior ganglia (Dumont & Wine, 1986a), but important differences exist in connections to the telson motor giants (MoGs) (Dumont & Wine, 1986b). In this paper, we describe additional differences in connections to the telson non-giant fast flexor (FF) motor neurones and to the telson flexor inhibitor (FI).

  2. 2.

    The telson FF motor neurones in ganglion 6 (G6) receive inputs similar to those in G4 and G5 (Miller, Hagiwara & Wine, 1985). The escape command neurones (lateral giants, LGs, and medial giants, MGs) in common provide weak disynaptic input via the telson segmental giant (SG6), and relatively strong trisynaptic input via SG2, SG3 and the corollary discharge interneurones 12 and 13. There may also be some direct input from the MGs, but it, as well as the connections from SG6, appears to vary in different preparations.

  3. 3.

    The compound PSP produced in telson FFs by a single LG or MG impulse was suprathreshold in only five of 55 experiments in isolated abdominal nerve cords, but the probability that a motor neurone would fire increased with additional giant axon impulses, showing that temporal summation of excitation outweighed the possible recruitment of inhibition. Firing probability was higher in semi-intact preparations, where at least one posterior telson FF was fired by a single LG impulse 50% of the time. As was pointed out previously (Dumont & Wine, 1986b), telson flexion would disrupt the behaviour pattern expected from LG commands.

  4. 4.

    Two pathways of feedforward inhibition were found which prevent such disruption. The sensory input that recruits the LG also recruits powerful feedforward inhibition of the telson FF motor neurones, which reduces the probability that they will be fired by the LG. The same sensory stimulus also evokes inhibition of FFs in G5, excitation of FFs in G2 and G3, and mixed excitation and inhibition of FFs in G4. In addition, the telson FIs fire at short latency during LG-mediated tailflips. This occurs because the telson FIs are excited by sensory input. In fact, the firing threshold of the telson FIs to sensory input is lower than that of the LGs, at least for electrical stimulation of nerves. When the LGs do fire, they produce additional excitation of the FIs. The telson FIs also are excited by the LGs but not by the MGs. In contrast, the anterior homologues of the telson FIs receive equivalent, delayed excitation from both MGs and LGs, and weaker sensory input, so that they tend to fire only after the peak of flexion (Wine & Mistick, 1977).

  5. 5.

    The predicted net effect of these connections is that the telson flexor muscles should not contract during naturally elicited LG tailflips, and this is consistent with observed behaviour. The results can be interpreted as providing additional examples of potentially maladaptive central connections which are not expressed in behaviour because of feedforward inhibition.

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