The brain is the most complex and highly of semi-permeable excitable membranes which specialised of all mammalian organs. Under­ can be caused, rapidly and transiently, to under­ standing the complexity of its function remains go changes in permeability to small chemical man's greatest challenge. The functional unit is molecules and to cations. The highly specialised the neurone, or excitable nerve cell, making ana­ nature of the constituent cells, with their unique tomical and chemical connections with other function and specificity, is c10sely related to units in the system. Many of the essential bio­ the structure of the whole tissue. The underlying chemical connections of the nerve cell are de­ chemical processes cannot be discussed or seen pendent upon special morphological features: in perspective without constant awareness of synaptic contact is mediated by chemical mol­ related aspects of physiology and morphology. ecules, 'neuro-transmitters' which ensure the The hrain is structurally extraordinarily com­ continued propagation of electrical impulses plex in its distinct anatomical regions, each of through sequential units of the system. Also which is heterogeneous in the types and struc­ c10sely related to the morphology of the ner­ tures of the constituent cells. vous system is the chemical energy expended in One aspect of the biochemical function of maintaining distribution gradients of cations the brain can be seen in its efficient production across cellular membranes. Chemical neuro­ of the energy required to support the unique pro­ transmission results in an alteration in cation cesses referred to above.