While it is not fully understood to what extent neural innervation regulates the function of the specific cell types present in the liver, progress has been made in the characterization of the intrahepatic neuroanatomy in humans and other species. These studies have indicated that there is considerable variation in the distribution of intrahepatic nervous fibers between different mammalian species. This can be studied by specific staining with markers for different classes of efferent nerves. In most species, a number of markers for sympathetic and parasympathetic nerves have been observed surrounding the hepatic artery, portal vein, and bile ducts, indicating general innervation of these structures by a variety of neuron subtypes.