SUMMARY1. The hypothesis is advanced that the joint occurrence of unitary excitatory post-synaptic potentials (e.p.s.p.s) evoked in motoneurones by branches of common stem pre-synaptic fibres causes short-term synchronization of their discharge during the rising phases of the unitary e.p.s.p.s.2. This hypothesis was tested using the pre-and post-stimulus time (PPST) histogram to detect synchronized firing among groups of intercostal motoneurones discharging in response to their natural synaptic drives.3. Motor nerve action potentials were recorded monophasically from nerve filaments of the external intercostal muscles of anaesthetized, paralysed cats maintained on artificial ventilation.4. Computer methods were used to measure peak spike amplitude, spike interval and filament identification for simultaneous recordings from four filaments. The spike amplitude histograms were derived for each filament and groups of spikes were selected for analysis.5. With spikes of one group designated as 'stimuli' (occurring at zero time) and those of a second as 'response' the PPST histogram was computed with different time bin widths.6. With bin widths of 100 and 10 msec the central respiratory periodicity was apparent in the PPST histogram. With 1D0 msec bins the PPST histogram showed a narrow central peak extending to + 3 0 msec at its base. This 'short-term synchronization' supports the hypothesis of joint firing due to common presynaptic connectivity.7. It was shown that detection of short-term synchronization was critically dependent on a sufficient quantity of data but that provided a simple criterion of adequate counts per bin in the PPST histogram was met, short-term synchronization could be detected between intercostal motoneurones of the same and adjacent segments.

1. Voltage‐clamp studies were carried out on single rabbit myelinated nerve fibres at 14 degrees C with the method of Dodge & Frankenhaeuser (1958). 2. A method was developed to allow the ionic currents through the modal membrane to be calibrated exactly under voltage‐clamp conditions by measuring the resistance of the internode through which the current was injected. 3. The ionic currents in a rabbit node of Ranvier can be resolved into two components, a sodium current and a leak current. Potassium current is almost entirely absent. 4. The sodium currents in rabbit nodes were fitted by the Hodgkin‐Huxley model using m2h kinetics. The kinetics of sodium currents in a rabbit node differ from that in a frog node under similar experimental conditions in two respects: (a) inactivation is faster, tau h for rabbit being 2‐3 times smaller around ‐50 mV; (b) the P(Na) (E) curve for mammal is shifted 10‐15 mV in the hyperpolarizing direction. 5. From the kinetics of sodium current, the non‐propagating rabbit action potential was reconstructed at 14 degrees C. The transient inward sodium current is responsible for the fast initial depolarization phase of the action potential, while the repolarizing phase is accounted for by leak alone. The computed shape of the action potential was in good agreement with the experimentally obtained action potential. 6. At 14 degrees C, frog and rabbit nodes with similar diameters have similar measured gNa values.

SUMMARY a 1. The three different types of synchronization of intercostal motoneurones which were described in the preceding paper (Kirkwood, Sears, Tuck & Westgaard, 1982) were studied for motoneurones of the same segment or for different segmental separations (up to five segments) and for motoneurones on opposite sides of the same segment.2. The strength ofsynchronization declined with segmental separation for all three categories, although the rate of decline was more variable for broad-peak synchronization than for the two other types. Short-term synchronization was undetectable for separations greater than three or four segments but clear peaks were still visible in the cross correlation histograms for high-frequency oscillation (h.f.o.) or broad-peak synchronization at a segmental separation of five. Synchronization between motoneurones on opposite sides of the cord was generally weak although less so for broad-peak or h.f.o. components.3. The decline in strength ofshort-term synchronization with segmental separation could not be explained by temporal dispersion of impulses in presynaptic axons. 4. A time shift was observed in the position of the cross-correlation histogram peak which was dependent on segmental separation and equivalent to a mean descending conduction velocity of 28 m/s in the assumed common input. This figure is similar to the mean conduction velocity of bulbospinal respiratory neurones derived from published values.5. We conclude that the short-term synchronization in these preparations is generated by the bulbospinal respiratory neurones and that the majority of their axons do not branch to make strong synaptic connexions to motoneurones over more than three to four segments.6. Interpretations ofthe different distributions ofthe other types ofsynchronization are discussed.

1. The volume and time components of individual breaths have been investigated under 'steady-state' conditions during air-breathing in fifteen subjects and, in a further six subjects, also during the addition of 1-5 and 3% CO2 to the inspired gas. 2. A computer-assisted method has been used to derive from the air flow record the individual breath values of tidal volume (VT), inspiratory duration (TI), expiratory duration (TE) and cycle duration (TC = TI + TE) for a sequence of breaths in the various steady-state conditions. 3. When the subjects were breathing room air, for breath sequences of over 200 breaths (n = 228-365), mean values of TI (TI) mostly lay between 1 and 2-5 sec, TE between 2-0 and 3-5 sec and VT between 0-4 and 0-91. The distributions of VT, TI and TE were in general unimodal. 4. Significant negative correlations between VT and F, and positive correlations between VT and TC, have been confirmed. 5. In all fifteen subjects, a highly significant positive correlation existed between VT and TI (mean r = +0-704), which was stronger than that between VT and TC (mean r = +0-533). Weaker positive correlations were demonstrated between VT and TE, and between TI and TE (mean r = +0-359 and +0-381 respectively). 6. The intercept of the regression of VT on TI passed close to the origin, typically slightly positive on the VT axis (mean = +0-0991.). This, coupled with the strong positive correlation between VT and TI, indicates that the mean inspiratory flow rate (VI = VT/TI) for each breath is held relatively constant from breath to breath. 7. In the six subjects studied during CO2 inhalation, the mean % contributions of VT and F to the increase in ventilation associated with breathing 3% CO2 were 75 and 25% respectively. 8. At the three different levels of ventilation, neither VT, TI nor TE showed a wholly consistent trend, although VT tended to increase, TE to decrease and TI to be unchanged. In contrast, the average values of VI (VI) consistently increased as the chemical stimulus was raised. 9. It is concluded that the previously observed tendency for ventilation to be held constant from breath to breath during steady-state breathing depends predominantly on the tendency for VI to be held constant. Close restraints are evidently not imposed on the individual values of VT and TI under these conditions. The neural mechanism generating breathing appears to control ventilation principally by regulating the rate of inspiratory air flow and secondarily TE.

No abstract