The Telson Flexor Neuromuscular System of the Crayfish: II. Segment-Specific Differences in Connectivity Between Premotor Neurones and the Motor Giants

The telson and sixth ganglion of the crayfish contain a fast flexor system that is homologous to that found in anterior segments, but doubled (Dumont & Wine, 1986a). In this paper we document differences in connections to the motor giants (MoGs) in the telson as compared to the MoGs in the anterior five abdominal segments.

Unlike their homologues in anterior segments, the telson MoGs receive excitatory input via a trisynaptic pathway that is activated by the escape command axons, the lateral and medial giants (LGs and MGs), and includes the identified corollary discharge interneurones 12 and 13. For 13, at least, the connection to the MoGs is monosynaptic, electrical and rectifying, and is sufficiently strong that simultaneous activation of the two 13s alone fires the telson MoGs.

The trisynaptic pathway from the LGs to the telson MoGs is inhibited by central, command-derived, postsynaptic inhibition of the telson MoGs, which typically arrives earlier than the excitation. In experimental preparations, this inhibition can be partially circumvented by stimulating the LGs anywhere anterior to the third abdominal ganglion. This is possible because the polysynaptic excitatory pathway is recruited in the third ganglion, while inhibition is recruited by the LGs locally in the sixth ganglion. Hence the site of impulse initiation in the LG affects the relative timing of excitation and inhibition of the telson MoGs. This arrangement makes it possible, in principle, for the site of impulse initiation in the LG to affect the form of the resulting tailflip.

In dissected preparations, LG impulses initiated anterior to the third ganglion fired the telson MoGs in 16 out of 25 experiments, while impulses initiated posteriorly never fired the telson MoGs (nine experiments).

Behavioural studies indicate that anterior stimuli which evoke LG activity do not cause activation of the telson MoGs. We suggest that in intact animals inhibition of the telson MoGs is more effective than in physiological preparations.

As far as we can tell from available evidence, the 13 input to the telson MoG is never expressed, and therefore cannot be explained in functional terms. We suggest that the differences between the inputs to the MoGs of the telson and of the fourth and fifth ganglia is the incidental result of developmental constraints during evolution.