Rostral ventrolateral medullary and caudal medullary raphe neurons with activity correlated to the 10-Hz rhythm in sympathetic nerve discharge

Abstract

1. The current study is the first to identify medullary neurons whose naturally occurring discharges were correlated to the 10-Hz rhythm in sympathetic nerve discharge (SND). Spike-triggered averaging showed that 44 of 164 rostral ventrolateral medullary (RVLM) and 44 of 174 caudal medullary raphe neurons had activity correlated to the 10-Hz rhythm in inferior cardiac postganglionic SND of 23 baroreceptor-denervated, decerebrate cats. 2. When the frequency of the rhythm in SND was decreased by lowering body temperature, the discharges of the 10 neurons tested (6 RVLM and 4 raphe) remained locked to the peak of the next 10-Hz sympathetic nerve slow wave rather than to the peak of the preceding slow wave. This observation supports the contention that the 10-Hz rhythm in basal SND was generated in the brain stem rather than in the spinal cord. 3. Frequency-domain analysis was used to characterize further the relationship between the 10-Hz rhythm in SND and the discharges of 30 RVLM and 24 raphe neurons. The autospectra of the discharges of eight RVLM and four raphe neurons contained a sharp peak near 10 Hz, although the mean firing rates of these neurons were lower than the frequency of the rhythm in SND. Coherence values as high as 0.76 characterized the relationship between the discharges of these "rhythmically firing neurons" and the 10-Hz rhythm in SND. A coherence value of 1.0 indicates a perfect correlation. The autospectra of the discharges of the 22 RVLM and 20 raphe neurons did not contain a peak near 10 Hz. The mean firing rates and coherence values relating the discharges of these "nonrhythmically firing neurons" and the 10-Hz rhythm in SND were significantly lower than those for the rhythmically firing neurons. Because the frequency of the population rhythm recorded from the inferior cardiac nerve was higher than the firing rates of individual medullary neurons, the 10-Hz rhythm in SND appears to be an emergent property of a network of neurons whose discharges are probabilistically related to the population rhythm. 4. In addition to the peak near 10-Hz, the autospectrum of SND often contained considerable power at frequencies < 6 Hz. This component of SND is called the 2- to 6-Hz rhythm.(ABSTRACT TRUNCATED AT 400 WORDS)