Feedback loop hypothesis to explain hematopoietic cycling. NE is postulated to inhibit further differentiation by a myeloblast (red line). Blue sine wave denotes neutrophil count oscillations. In this model, NE is produced by the terminally differentiating cohort of neutrophils and ultimately feeds back to inhibit further production of neutrophils, which results in loss of the inhibitory cycle—at least for a while, until production of neutrophils resume, followed again by the inhibitory action of NE in a cyclic manner.