Fig. 6

a Power spectral analyses of simultaneously recorded EKG, cranial CSF pressure, and spinal CSF pressure waves from Alligator mississippiensis. The EKG traces are rich in harmonics (upper trace); this analysis was only concerned with the fundamental frequency and first two harmonics (which occur on the far left of the upper trace). The lower panel includes the same low frequency region of the EKG analysis, overlain with the power spectral analyses of the spinal and cranial CSF pulsations (these were placed on the same Y axis). The power of the CSF pulsations clearly captures the fundamental frequency of the cardiac cycle, as well as the first and second harmonic; however, the power of the fundamental frequency is lower in the recordings from the skull compared to those from the spinal canal. b Power spectral analyses of CSF pressure traces from the same Alligator mississippiensis during a period of approximately 10 min. During this time recording were taken from the animal in a horizontal (resting) posture, it was rotated to 15° degrees head up, returned to horizontal for 3 min, then rotated to 15° head down. Only CSF pressure data from the terminal (stable) portion of each rotation was analyzed. C—heart rate. c Doppler ultrasonographic traces of the microsphere-labelled CSF in the spinal subarachnoid space of two different Alligator mississippiensis. Note that while the two traces differ in flow velocity (the Y-axis), the have a similar ~ 0.3 Hz pulsatile frequency, and a similar unidirectional flow profile. In both traces the CSF flow is indicated below the baseline of the graph (meaning the CSF was flowing toward the head); the signal above the baseline is an aliasing of the Doppler trace