Fast rhythms are present in neural discharges of many motor systems. In the decerebrate cat, high-frequency oscillations (HFO, range 50–100 Hz) are ubiquitous in the discharges of Imotoneurons (phrenic, recurrent laryngeal, external intercostal) and of medullary Ineurons. The amplitude and frequency of HFO are greater when respiratory drive is increased. Coherence spectral analysis shows that the various discharges (population and unit) are significantly correlated at the HFO frequency. This indicates that the common rhythm arises in the brainstem and is transmitted to cranial motoneurons via propriobulbar projections and to spinal motoneurons via bulbospinal projections.
HFO phase relations of medullary I neuron discharges
Cross correlation analysis was used to measure the phase lag between unit and phrenic discharges. The distributions of unit-phrenic lag times were distinctly different between Iunit types classified by overall discharge pattern. The I-augmenting (ramp) units showed a higher proportion of positive unit-phrenic lags (0.0 to 8.8 ms), whereas the I-decrementing and I-plateau units showed a higher proportion of negative lags (-7.6 to 0.0 ms), ie, peak unit HFO lagged peak phrenic HFO. Laryngeal Imotoneurons fell into the latter group. The existence of lag distributions that distinguished populations of Ineurons suggests that the HFO arises from feedback loops between different populations.
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Intracellular recordings from medullary E neurons
For both augmenting Eneurons and decrementing laryngeal Eneurons, the hyperpolar-ization during the Iphase had superimposed HFOs, which presumably are based on rhythmic IPSP inputs from I neurons.
HFOs in rat
In adult rat in vivo preparations, HFOs were present in Idischarges, but these were in a higher frequency range (100–150 Hz) than those in the cat (50–100 Hz).
Expiratory HFOs (EHFOs)
In some cat preparations during spontaneous respiration, strong oscillations (range 24–74 Hz) were observed in recurrent laryngeal (RL) E nerve and unit discharges. These EHFOs were highly coherent between unit and nerve discharges and between bilateral nerve discharges. Similar oscillations were consistently obtained during fictive vocalization (FV) elicited by midbrain periaqueductal gray electrical stimulation, which produces a large increase of RLE discharge. These were intrinsic rhythms (range 50–70 Hz), since the spectral frequency was different from the stimulation frequency. Thus, when the RLE network is subjected to strong excitation, the neuronal interactions result in generation of strong coherent rhythms in the population.
Abdominal E discharges
During FV, there was a large increase of lumbar (L1, L2) nerve discharges. However, spectral analysis revealed that no intrinsic rhythm was generated; the only rhythm observed was locked to the stimulus frequency. Thus the medullary premotor system cannot readily generate correlated rhythmic activity.
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Synchronization mechanisms
The observations suggest that HFO (of both I and E types) is an emergent property of the respective networks. Interspike interval distributions indicate that modal interval durations may occur in a 1:1, 2:1, or 1:2 relation to the HFO period. The heterogeneity of phase lags for different populations suggests that the oscillation may arise from feedback loops in the brainstem.
Acknowledgement
This research was supported by N.I.H. Grant HL-27300.