1. 1.

    Intracellular recordings have been made from a variety of phasic motor neurones during expression of the motor programme for backward swimming in Galathea strigosa.

  2. 2.

    Fast flexor motor neurones (FFs) are driven by a large depolarization mediated by chemical synapses and are inhibited in the interburst interval.

  3. 3.

    Fast extensor motor neurones (FEs) are driven by a barrage of unitary EPSPs during the extension phase and may receive unitary synaptic inhibition while the flexors are active.

  4. 4.

    FFs all have similar spike thresholds and fire bursts of spikes super-imposed upon the depolarized peak of the input. FEs show a gradation in spike threshold which is correlated with soma size. The largest FEs (type 2) have higher thresholds than smaller FEs (type 1), and fire fewer spikes.

  5. 5.

    The phasic inhibitor motor neurone to flexor muscles (FI) is driven by complex central pathways and fires a single spike shortly following flexion.

  6. 6.

    The extensor inhibitor (El) appears to receive the same depolarization as do the FFs, but has a low spike threshold and thus fires on the rising phase of the depolarization. Spiking in EI is terminated by unitary IPSPs which occur in phase with FF activity and which may have the same origin as the interburst inhibition seen in the FEs.

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