Feedforward Mechanisms Adjust Cerebral Blood Flow During Anticipated Postural Changes

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Head‐up tilt can result in a decrease in venous return to the heart and a decline in cerebral perfusion. Although feedback mechanisms such as vestibulosympathetic and baroreceptor reflexes compensate for orthostatic hypotension, we postulated that feedforward mechanisms also adjust cerebral blood flow (CBF) during anticipated postural changes. To test this hypothesis, cats were instrumented to record blood flow from the carotid arteries (cerebral blood flow, CBF) and acclimated for restraint on a tilt table. We trained the animals to associate 60° head‐up tilt (HUT) with a visual cue, which was provided 10 sec before tilt onset. Within 10 days following pairing of the light cue and 60° HUT, there was a significant attenuation (p<0.05, ANOVA) in lability of CBF, which changed <5% at the onset of the tilt. After animals were well‐trained, a false cue (light cue before 20° HUT or a light cue but no tilt) was provided during 10% of trials. Such false cues did not result in a significant change in CBF. These data support the hypothesis that anticipation of movement acts to stabilize CBF and decreases the likelihood of orthostatic hypotension. However, since anticipation of movement without a postural change did not affect brain perfusion, we concluded that feedback signals can override inappropriate feedforward signals. These data suggest that feedback and feedforward signals are integrated by the nervous system to provide for homeostasis during postural alterations.