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Partial coherence analysis was used to evaluate the extent to which coherence between resting muscle sympathetic activity (MSA) in different pairs of limbs in humans is explained by the common baroreceptor input and by other noncardiac-related factors. Multiunit MSA in two or three nerves, arterial blood pressure, and electrocardiogram were recorded simultaneously. Correlated MSA consisted of a sharp periodic component at the heart rate and a wideband component of relatively low power distributed between 0 and 2–2.5 Hz. Quantitative analysis revealed stronger coupling between MSAs in close limbs than in distant limbs (peak coherence leg-leg, 0.94 ± 0.03; arm-leg, 0.76 ± 0.11). Furthermore, the wideband component, unaffected by partialization with circulatory signals, was significantly stronger between leg-leg (0.67 ± 0.10) than between arm-leg pairs (0.29 ± 0.10), i.e., noncardiac-related components explained 71% of leg-leg and 38% of arm-leg coherences at the frequency of the heart. Our results indicate that nonuniform relationship exists between resting sympathetic outflow to muscles in close and distant extremities which is, however, partially masked by the effect of the common rhythmic baroreceptor input.

Autospectral and coherence analyses were used to study the frequency-domain relationships between frontal-parietal cortical activity [electroencephalogram (EEG)] and the discharges of the interior cardiac and renal sympathetic nerves of baroreceptor-denervated and vagotomized cats anesthetized with either alpha-chloralose or pentobarbital sodium. Delta slow-wave activity in the EEG was correlated to sympathetic nerve discharge (SND) as shown by sharp peaks between 0.5 and 4 Hz in the coherence function. The relationship was stronger in chloralose- than in pentobarbital-anesthetized cats. Coherence of the two signals could be attributed to descending influences of forebrain delta slow-wave generators on sympathetic circuits, since midbrain transection preferentially reduced the power in SND at frequencies that cohered to the EEG before transection. In contrast, the power in the EEG was not reduced by midbrain transection. The relationship between cortical delta slow-wave activity and SND was stronger during than between cortical spindlelike events that lasted 1-3 s and recurred once every 5-10 s. These events were similar to cortical spindles observed during the early stages of sleep and under light barbiturate anesthesia. These observations raise the possibility that the influences of forebrain delta slow-wave generators on SND are gated by thalamic mechanisms normally involved in the sleep-wake cycle.

Analyses in the frequency and time domains were used to study the relationships between the discharges of single brain stem neurons and postganglionic sympathetic nerves in baroreceptor-innervated and -denervated cats anesthetized with 5,5-diallylbarbiturate-urethan. Spike-triggered averaging was used initially to identify single neurons with sympathetic nerve-related activity in the medullary lateral tegmental field, rostral ventrolateral medulla, and medullary raphe. The discharges of such neurons were correlated to the 2- to 6-Hz rhythm in sympathetic nerve discharge (SND). Frequency-domain analysis revealed that the relationship between medullary unit activity and the sympathetic nerve rhythm was not fixed from cycle to cycle. First, the coherence values relating the activity of these neurons to SND were closer to zero than to unity in most cases. Second, whereas most of the power in the autospectra of SND was contained between 2 and 6 Hz, that in the autospectra of medullary unit activity was more evenly distributed over a much wider frequency band. These and other observations indicate that the 2- to 6-Hz rhythm is an emergent property of a network of brain stem neurons whose discharges are probabilistically rather than strictly related to the phases of the population rhythm.

The present study was designed to determine the extent to which the brain stem neural networks, normally capable of synchronizing the sympathetic nerve discharge (SND) into 2- to 6- and 10-Hz rhythmic fluctuations, contribute to the control of autonomic reactions during brain hypoxia and/or hypercapnia. Vertebral, cardiac, and renal nerve discharges were recorded electrophysiologically in 34 anesthetized, curarized, and artificially ventilated cats. The sympathetic nerve responses to cerebral ischemia (elicited by reducing the blood supply to the brain), intracranial pressure elevation (Cushing reaction), and systemic asphyxia were tested with special focus on the rhythmic structure of the SND. It has been found that there are two phases of SND changes during cerebral ischemia differing mainly in the frequency content of the signals and less in the compound action potential amplitude. During the first phase the rhythmic generators controlling the tonic sympathetic outflow are more strongly activated, which is reflected in a stronger, more regular, and more widespread manifestation of these rhythms on the efferent neurograms. After some time the normal SND structure abruptly changes to a desynchronized activity with loss of the three main sympathetic rhythms and responsiveness to baroreceptor reflex activation. The same stereotyped changes can be observed regardless of the way in which the brain hypoxia and/or hypercapnia has been produced. Nor does the denervation of peripheral baro- and chemoreceptors substantially alter the general pattern of the responses.