Membranes are the working surfaces of every cell, carrying out the essential functions of cellular communication and hormonal signal transduction. Nerve cells, in particular, depend on healthy membrane function for normal neurotransmitter metabolism and nerve signal transmission. Choline, as a component of phosphatidylcholine, is used for synthesis and maintenance of normal cell membranes. An extract of the Chinese club moss, Huperzia serrata, huperzine A has been used in Chinese medicine for centuries. It is an effective inhibitor of acetylcholinesterase, the enzyme that degrades acetylcholine, the neurotransmitter important for normal memory and learning function. Huperzine A may help maintain cognitive function that has been degraded by a reduction in the brain's functional levels of the neurotransmitter acetylcholine. Acetyl-L-carnitine contributes its acetyl group to the production of acetylcholine, the primary neurotransmitter for memory and thought. The enzyme that makes acetylcholine from acetyl groups and choline is choline acetyl transferase. The activity of this important enzyme has a tendency to decline with age, causing low acetylcholine levels which in turn are thought to contribute to the impairment of brain function that is associated with aging. Research has also found that acetyl-L-carnitine is active in optimizing the functioning of cerebral blood flow, as well as of nerve cell membranes.