Pyruvate is the electron acceptor. When lactic acid builds up in muscles you experience soreness and pain. When you stop exercising vigorously the lactic acid will diffuse from the muscles into the blood . The lactic acid will then go to the liver and be metabolized. The basic principle of brain imaging was formulated more than a century ago when it was suggested that activity of nerve cells and energy metabolism are tightly coupled.
Energy is delivered to the brain by the oxidation of glucose from the blood.
Thus the metabolic signals detected by PET bring us part way to understanding how neuronal processes such as action potentials and neurotransmitter release lead to a given brain activity. To make further progress, one has to identify and quantitate specific cellular and molecular mechanisms of nerve activity coupled to energy metabolism. New data have identified the neurotransmitter glutamate and astrocytes, a specific type of glial cell, as main players in the coupling process. Glutamate released from neurons must be removed rapidly so that the nerve can fire again. Glutamate is taken up by astrocytes that use Na ion as the driving force, resulting in tight coupling between Na and glutamate uptake.