Figure 1.

Time courses of glycogen synthase (GS) and glycogen phosphorylase (GP) reaction rates. Brain stimulation produces a rapid activation of GP because of increased energy demand, which effect glucosyl equivalents concentration in astrocytes (inset). The GP-catalyzed mobilization of glycogen decreases during the late phase of the stimulation period, and readily returns to basal level after the end of stimulation. The GS-catalyzed incorporation of glucose into glycogen is delayed and much slower than glycogen breakdown; however, it remains significantly elevated during the poststimulus period. Note that, although the resting activity of both GS and GP is very small, substantial synthesis and degradation of glycogen occur simultaneously during activation. The simulated neuron/astrocyte activation ratio is 1.5:1.