Synchronous oscillations of brain activity have been described almost ubiquitously in the mammalian brain and the frequency of these span a wide range of time scales. Synchrony on a millisecond time scale, which generates 2–100 Hz oscillations, has been implicated in many functions, such as memory (Kahana,2006
) and sensory processing (Stopfer et al.,1997
), sleep (Steriade et al.,1993
) and movement (Baker and Baker,2003
Inspiratory motoneurons within the brainstem respiratory network are synchronized on long and short time scales to generate inspiratory rhythm and inspiratory-phase short time scale synchronous oscillations, respectively. On a long time scale (more than hundreds of milliseconds), PreBötzinger Complex (PreBötC) neurons located in the medulla generate inspiratory rhythm (Smith et al.,1991
; Koshiya and Guyenet,1996
; Pierrefiche et al.,1998
; Johnson et al.,2001
) that is characterized by regularly recurring bursts of action potentials, or inspiratory bursts, each of which are followed by a period of quiescence. Inspiratory rhythm results in contractions in inspiratory muscles and is critical for respiration. Inspiratory neurons are further synchronized on a short time scale (tens of milliseconds) to generate synchronous oscillations in motoneuron firing during an inspiratory burst (Cohen et al.,1997
; Funk and Parkis,2002
). Inspiratory-phase short time scale synchronous oscillations are characterized by clusters of action potentials that occur at a regular interval and are separated by periods of little or no spike firing.
The function of inspiratory-phase synchronous oscillations in the brainstem has been investigated, although to a lesser extent relative to oscillations observed in other brain regions (Seager et al.,2002
). Short time scale synchronous oscillations are not required for respiratory rhythm generation, but they are important in shaping the pattern of inspiratory discharge. At the motoneuron level, oscillations increase the input-output efficiency of inspiratory motoneurons as well as the precision of spike timing (Parkis et al.,2003
). Furthermore, synchronous firing of inspiratory motoneurons may also serve to increase muscle force output (Baker et al.,1999
Since Cohen initiated an extensive study of these oscillations in 1973 (Cohen,1973
), the origin of short time scale synchronous oscillations has been the subject of great interest. These oscillations have been categorized into high frequency and medium frequency oscillations (HFOs and MFOs, respectively). Although HFOs and MFOs typically occur at high and low frequencies, respectively, the frequency ranges in which these oscillations occur are not exclusively their defining characteristic. Instead, the characteristics that distinguish HFOs from MFOs reflect the different regions of the inspiratory network in which they are thought to be generated.
Short time scale synchronous oscillations recorded from different motor pools or from the left and right phrenic nerves are considered HFOs if they are temporally coincident and occur at the same frequency. In order for oscillations recorded from different motor pools or from the left and right phrenic nerves to be synchronized in this way, they must be generated by a common input source, such as the inspiratory pattern generator (Cohen et al.,1997
), that transmits inspiratory activity to multiple motor pools. Within the slice, this common input source corresponds to the PreBötC, which generates and transmits inspiratory rhythm to hypoglossal motoneurons (HMs) (Smith et al.,1991
In contrast, if short time scale synchronous oscillations recorded from different motor pools or from the left and right phrenic nerves are not temporally correlated and do not occur at the same frequency they are considered MFOs. Given that MFOs exhibited by different motor outputs are asynchronous, they are likely generated within or immediately upstream of the motor pool (Cohen et al.,1987
; Christakos et al.,1991
; Cohen et al.,1997
). Thus, within the slice preparation, MFOs would likely be generated by upstream premotoneurons that send projections to the XII nucleus or within the XII nucleus itself.
Although the regions of the inspiratory network in which short time scale synchronous oscillations (MFOs and HFOs) may be generated have been proposed (Christakos et al.,1991
; Cohen et al.,1997
), only one lesion study in the cat brainstem has addressed where oscillations may be generated (Richardson and Mitchell,1982
). In the present work, we have taken advantage of the simplified respiratory network within the rhythmically active medullary slice preparation to determine where in the slice synchronous oscillations may be produced. Within the rhythmic slice, oscillations may be produced in the PreBötC, which generates inspiratory rhythm, by subsequent premotoneurons that project to HMs or within the XII motor nucleus itself. We tested these candidate regions using a combination of experimental and data analysis techniques.
To experimentally test where in the slice short time scale synchronous oscillations are generated, we unilaterally excited either the PreBötC or the XII nucleus and monitored changes in the power spectral density whose shape is dependent on the degree to which synchronous oscillations occur in inspiratory-phase XII nerve activity. From intracellular recordings of rhythmically active phrenic motoneurons, we know that inspiratory motoneurons receive inspiratory-phase oscillatory synaptic inputs that increase spike firing probability and that action potentials occur at the peaks of the oscillatory inputs (Parkis et al.,2003
). In the present work, we have increased the excitability of neurons in the PreBötC or the XII nucleus. If oscillations are generated in the PreBötC, PreBötC neurons should receive oscillatory synaptic inputs from other PreBötC neurons. Unilaterally exciting PreBötC neurons should increase the probability that these neurons will fire action potentials at the peaks of the oscillatory synaptic inputs thereby increasing oscillation power. If instead oscillations are generated in the XII nucleus or immediately upstream in the premotor area, HMs should receive oscillatory synaptic inputs from neurons within the XII nucleus or from premotoneurons. Thus if oscillations have a motor or premotor origin, unilaterally exciting either the XII nucleus, and not the PreBötC, should increase the probability that HMs will fire at the peaks of the oscillatory synaptic inputs and thereby this should increase oscillation power.
We also applied crosscorrelation and coherence analyses to determine whether short time scale synchronous oscillations recorded from the left and right XII rootlets are temporally coincident and occur at the same frequency, respectively. If bilaterally recorded oscillations are temporally coincident and occur at the same frequency, then the oscillations would likely be generated by the PreBötC since this structure serves as the common input source within the slice. In contrast, if bilaterally recorded oscillations are not temporally coincident and do not occur at the same frequency, they are likely to be generated individually within the XII nucleus or by premotoneurons that project to HMs.