Synchronization of ventral-group, bulbospinal inspiratory neurons in the decerebrate rat |
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Authors: | G-F Tian J Duffin |
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Institution: | (1) Department of Anaesthesia, University of Toronto, Ontario, Canada, CA;(2) Department of Physiology, University of Toronto, Toronto, Ontario, Canada M5S 1A8 Tel.: +1-416-978-6379, Fax: +1-416-978-4940, e-mail: j.duffin@utoronto.ca, CA |
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Abstract: | We examined the synaptic connections between ventral-group, bulbospinal inspiratory neurons in 27 vagotomized, paralyzed,
ventilated, and decerebrated rats using cross-correlation and spike-triggered averaging of intracellular potentials. The neurons
were recorded in the medulla about the level of the obex and identified by their inspiratory firing pattern and antidromic
activation from the spinal cord at C7. Whole C5 phrenic nerve recordings were made using bipolar electrodes from the central
cut ends of the nerve. Most (108/137, 79%) inspiratory neurons discharged only during inspiration but some (29/137, 21%) also
discharged during early expiration. Their intracellular membrane potentials displayed a pattern of depolarization during inspiration,
repolarization during early expiration, and hyperpolarization during late expiration. Intracellular chloride iontophoresis
changed the inspiratory membrane potential trajectories from augmenting to decrementing in 11 of 19 neurons tested (58%),
and demonstrated the presence of both early-decrementing and late-augmenting waves of inhibitory postsynaptic potentials during
expiration in 11 of 19 neurons tested (58%). Cross-correlation histograms were computed between pairs of extracellularly recorded
neurons to detect short time scale synchronizations indicative of synaptic connections (26 ipsilateral; 23 contralateral).
While none of the cross-correlation histograms for contralateral pairs showed peaks, most (23, 88%) of those for ipsilateral
pairs showed peaks (mean half-amplitude width ± SD = 1.3 ± 0.4 ms) at time zero suggestive of common activation. Some of the
latter (6, 23%) showed troughs superimposed on the central peaks (mean half-amplitude width ± SD = 0.9 ± 0.2 ms) at short
latencies (mean latency ± SD = 1.8 ± 1.9 ms) suggestive of inhibition; others (8, 31%) had asymmetrical central peaks and
two had bilateral peaks suggesting more complex interconnections. Averages of intracellular membrane potentials of inspiratory
neurons (n = 24), triggered by action potentials of a nearby extracellularly recorded inspiratory neuron, were computed to detect synchronized
postsynaptic potentials. Over half (16, 67%) showed postsynaptic potentials (mean amplitude ± SD = 201 ± 176 μV; mean half-amplitude
width ± SD = 2.3 ± 0.8 ms) confirming the cross-correlation findings of common excitation. We conclude that in decerebrated
rats, ventral-group inspiratory neurons projecting to the C7 spinal segment share powerful, ipsilaterally distributed excitatory
inputs which enhance their synchronous activity during inspiration. They also receive inhibition during inspiration and early-decrementing
and late-augmenting inhibitory inputs during expiration.
Received: 25 October 1996 / Accepted: 21 March 1997 |
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Keywords: | Respiration Ventral-group bulbospinal inspiratory neurons Cross-correlation Spike-triggered averaging Decerebrate rat |
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