The detection of monosynaptic connexions from inspiratory bulbospinal neurones to inspiratory motoneurones in the cat.

Abstract

SUMMARY1. Simultaneous recordings were made of the discharges of inspiratory bulbospinal neurones and phrenic or external intercostal z-motoneurones in the anaesthetized cat.The connexions between these neurones were studied by the construction of cross-correlation histograms from their discharges.2. 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.3. H.f.o. peaks were interpreted as being derived from the well known periodic synchronization of medullary inspiratory neurones in this frequency range.4. The time courses and latencies ofthe 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.5. The narrow peaks were of appropriate durations for monosynaptic connexions and were all at appropriate latencies (0-62-4 ms after the calculated arrival time of the bulbospinal impulse in the segment concerned).6. 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 2 P HY 368 J. G. McF. DAVIES, P. A. KIRKWOOD AND T. A. SEARS 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.

Medullary respiratory neurones and control of laryngeal motoneurones during fictive eupnoea and cough in the cat

“…The biphasic feature represents 'motoneurone' action potentials. The presence of an offset positive feature in the rectified but not the unrectified signal suggested that the trigger neurone was (1) a premotor neurone that projected to the monitored motoneurone pool, or (2) a neurone that was tightly synchronized with premotor neurones by functionally antecedent shared inputs (Davies et al 1985;Botteron & Cheney, 1989;Maier et al 1998;Perlmutter et al 1998). The presence of an offset trough in a rectified average was consistent with functional inhibition, i.e.…”

“…The evidence for neuronal interactions is based upon measured time-locked fluctuations in the recruitment and firing rates of motoneurones due to direct, indirect or parallel paths. Advantages and limitations of these methods have been considered elsewhere (Cohen et al 1974;Davies et al 1985;Botteron & Cheney, 1989;Maier et al 1998;Perlmutter et al 1998). An additional limitation to feature interpretation had to be considered in this study.…”

“…The primary peak in each average reflects the joint parallel influences of the particular trigger neurone and other 'observed' and 'unobserved' premotor neurones that projected to the monitored motoneurone pool. The peaks could also reflect indirect actions; with the synchrony of the trigger neurones a consequence of functionally antecedent inputs shared with the motoneurones (Davies et al 1985;Botteron & Cheney, 1989;Maier et al 1998;.…”

Organization of synaptic transmission in the mammalian solitary complex, studied in vitro.

“…In addition, the ventrolateral n.t.s. has been found in cats, to be essential in both reflex and central control of the respiratory rhythm (Von Euler, Hayward, Martilla & Wyman, 1973;Cohen, 1979;Richter, 1982; Morin-Surun, Champagnat, Boudinot & Denavit-Saubie, 1984;Davies, Kirkwood & Sears, 1985).…”

Cross-correlation analysis of neuronal activities.

“…Three types of synchronization-short-term, broad-peak, and high-frequency oscillation were found in the cross-correlograms between the efferent spikes [43,45]. Crosscorrelograms between spikes of inspiratory bulbospinal neurons and phrenic or external intercostal motoneurons revealed peaks which could be divided into three groups on the basis of the time course: narrow, medium-width, and high-frequency oscillation [19,20]. Intercostal muscle activity during purring of the cat was also analyzed [44].…”