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Sobell Department of Neurophysiology, Institute of Neurology, Queen Square, London, UK.

1. Synaptic actions in the thoracic spinal cord of individual expiratory bulbospinal neurones were studied in anaesthetized cats by the use of two techniques: (i) the monosynaptic connections to motoneurones were assessed by cross-correlations between the discharges of the neurones and efferent discharges in the internal intercostal nerves of several segments bilaterally; and (ii) distributions of terminal and focal synaptic potentials were measured by extracellular spike-triggered averaging in the thoracic ventral horn. 2. Monosynaptic connections were identified by both the durations and timings of observed cross-correlation peaks, taking into account accurate conduction velocity measurements derived from collision tests and from spike-triggered averaging. Discrimination was made against peaks resulting from presynaptic synchronization. 3. Monosynaptic connections to motoneurones were identified for twenty-three out of twenty-seven neurones. The connections to nerves on the side ipsilateral to the cell somata were, on average, about 36% of the strength of those on the contralateral side. The overall strength of the connections was about twice as strong as previous estimates for similar connections from inspiratory bulbospinal neurones to phrenic motoneurones. The monosynaptic pathway was calculated to be able to provide most of the depolarization for the motoneurones concerned and therefore was likely to be the main determinant of their firing patterns under the conditions of these experiments. 4. However, taking into account previous measurements it is considered possible that these connections may only involve a minority of motoneurones, perhaps only 10% of the expiratory population. Thus, in general, the control of the whole pool of expiratory motoneurones, despite the strong monosynaptic connections measured here, is suggested to be mainly dependent on spinal interneurones, as has been concluded previously for inspiratory motoneurones. 5. Spike-triggered averaging revealed that nearly all neurones gave signs of collaterals in each of the segments investigated (T7, T8 or T9), as shown by the presence of terminal potentials or focal synaptic potentials, but the projection within a given thoracic segment was non-uniform, in that large-amplitude potentials were more common in the rostral than the caudal part of the segment. This non-uniformity could be a factor involved in the apparently non-heterogeneous connections to the motoneurones.

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