Sources of variation in the output of locust spiracular motoneurones receiving common synaptic driving

1. The closer muscles of the left and the right spiracles of a thoracic segment are both innervated by two motoneurones, which spike in a variety of patterns during expiration. This paper seeks to explain the origin of these patterns. 2. No direct coupling between the two motoneurones is revealed. In an isolated thoracic ganglion both motoneurones spike at different frequencies with no tendency for their spikes to become synchronized. 3. The two closer motoneurones in one segment receive common, patterned depolarizing synaptic potentials during expiration caused by interneurones relaying information from the metathoracic ganglion. 4. The closer motoneurones of all the thoracic segments receive the same pattern of synaptic potentials from these interneurones. Despite this, the spiracles of one segment may remain shut while those on other segments continue to open and close rhythmically. 5. The interplay between common synaptic driving, the threshold of the motoneurones for spike initiation, and the tendency for a motoneurone to spike at a particular frequency even in the absence of interneuronal driving, explains the various patterns of spikes during expiration. Common synaptic driving imposes the same basic pattern of commands on all the motoneurones, but the individual motoneurones determine the final pattern of motor spikes. 6. To be effective in producing a patterned output, an input pattern must operate within narrow limits on either side of the threshold of the motoneurone. If the depolarization is too large, a high frequency of unpatterned spikes will result; if too small, then either there will be no output or a low frequency of spikes will result whose patterning will be affected by other inputs.