Impulse pattern generation in a crayfish abdominal postural motoneuron

Summary

• 1. The largest crayfish tonic flexor motoneuron, f6, exhibits a patterned discharge in the form of repetitive bursts. The mechanism of burst production was examined using intracellular and extracellular recording and stimulation of identified tonic flexor motoneurons in the third or fourth abdominal ganglia.

• 2. Depolarizing currents (over a restricted range) injected into the somata or neuropilar processes of f6 result in the clustered discharge characteristic of ‘command’ or sensory induced activation (Fig. 4).

• 3. Both driven and spontaneous bursts in the motoneuron are associated with unpatterned synaptic activity (Fig. 5, 6, 7).

• 4. The frequency-current (f-I) relationship reveals no increase in slope or average sensitivity over the range of injected currents which give rise to bursting (Fig. 5).

• 5. Impulses generated during a burst, as seen in neuropilar impalements, are associated with a depolarizing afterpotential (DAP). The DAP appears to arise from a passive invasion of dendritic membrane by the preceding impulse (Figs. 7, 8).

• 6. The time course of the DAP is closely paralleled by an increase in excitability of the spike initiation zone following a singlet action potential. The excitability cycle following a rhythmic impulse is a good predictor of the ‘preferred’ intradoublet (intraburst) intervals (Fig. 9A-C).

• 7. Each burst terminates with a prominent postburst hyperpolarization (PBH) (Fig. 10).

It is concluded that burst formation is endogenous to the motoneuron andnot strongly dependent on patterned presynaptic input.