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Simultaneous recordings were made of the discharges of inspiratory bulbospinal neurones and phrenic or external intercostal alpha-motoneurones in the anaesthetized cat. The connexions between these neurones were studied by the construction of cross-correlation histograms from their discharges. Peaks observed in the cross-correlation histograms were divided into three groups on the basis of their time courses: narrow, medium-width and high-frequency oscillations (h.f.o.). Narrow peaks were defined as having half-widths less than 1.1 ms and medium-width peaks as having half-widths greater than this, while h.f.o. was characterized by periodic waves in the range 60-120 Hz. H.f.o. peaks were interpreted as being derived from the well known periodic synchronization of medullary inspiratory neurones in this frequency range. The time courses and latencies of the medium-width peaks could be quantitatively explained by a simple model representing excitation of the motoneurones by bulbospinal neurones whose discharges showed synchronization within +/- 1 ms of the reference spike, together with temporal dispersion in bulbospinal axons having a distribution of conduction velocities given by the measurements of this study. Such an explanation was essential for some of the medium-width peaks, whose latencies were short compared to the conduction times to the spinal cord for their own axons, but for other medium-width peaks oligosynaptic excitation of the motoneurones from the identified bulbospinal neurones was another possible explanation. The narrow peaks were of appropriate durations for monosynaptic connexions and were all at appropriate latencies (0.6-2.4 ms after the calculated arrival time of the bulbospinal impulse in the segment concerned). It is concluded from the observations of narrow peaks that monosynaptic excitation exists between inspiratory bulbospinal neurones and both phrenic and external intercostal motoneurones. However, because of the existence of presynaptic synchronization, as shown by the presence of the medium-width peaks, such a conclusion is predicated upon being able to discriminate against such an effect. The model showed that this restriction applies just as much to the measurements of excitatory post-synaptic potentials (e.p.s.p.s) by spike-triggered averaging as it does to cross-correlation measurements. We suggest that the discrimination against presynaptic synchronization here was possible only because the long conduction distance created temporal dispersion in the synchronized presynaptic impulses.

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