The cost of an action potential

Neuronal modules, or 'cell-assemblies'. comprising millions of mutually int erconnected cells have been postulated to form the basis of many functions of the brain, such as mood, sleep, hunger, vigilance, and more. Depending o n the extent of the module, neurocommunication in cell-assemblies might exc eed metabolic resources. A medium-size (10 000 neurons) module would requir e at least 10 J per 1 of brain, based on a calculated cost of an isolated a ction potential (AP) of 10(11)-10(12) molecules of ATP per cm(2) of cell me mbrane, with an absolute minimum of 10(6) ATP at a node of Ranvier. The fig ure matches the cost of depolarizing the unmyelinated axon of the large mon opolar cell in the blowfly retina. A circuit model of the cell membrane, ba sed on abrupt changes of Na+ and K+ conductances, is used to emulate the AP and to assess the resulting ionic unbalance. The cost of an AP is equated to the metabolic energy necessary to fuel ATP-based pumps that restore intr acellular K+. The high metabolic demand of a cell-assembly suggests that le ss expensive means of neurocommunication may be involved, such as non-synap tic diffusion neurotransmission (NDN), which would comply with a proposed l aw of conservation of space and energy in the brain. (C) 2000 Elsevier Scie nce B.V. All rights reserved.