Action Potential Initiation and Conduction in Axons

Transmission of electrical signals in biological systems operates under constraints imposed by thematerials used to build the conducting pathways. The conduction medium in nerves is a membrane-enclosed dilute salt solution rather than a metal wire, for example, as in macroscopic physical systems. Axons have been compared for decades to undersea cables that are surrounded by saltwater.However, the axon relies on a salt solution both inside and outside the axon. Since the resistivity of mammalian physiological saline is about 1 ohm-meter and that of copper is about 1.7! 10"8 ohm-meter, the ease of current flow in 1mmof an axon (a typical length constant in a large-diameter axon) is equivalent to that of a wire that is 170km long. The difficulty of passive propagation of signals in an axon is compounded by two factors. First, the signal is carried by cell membranes that have a capacitance (#1mF cm"2) that needs to be charged to propagate a voltage change. Second, the membrane is inherently leaky due to a requirement for ion channels that are open to set the negative resting membrane potential. Thus, passive conduction of a signal in neurons is effective over distances of only a fewmillimeters. For this reason, it does not matter whether you are a worm or a whale: transmission of signals in axons requires a booster mechanism to replenish the decrementing electrical signal. The analogy between undersea cables and axons has other similarities and divergences. Undersea cables use repeater or booster stations that amplify the analog signal and send it on. The nervous system also uses a booster system but transforms the analog signal into a digital one (i.e., the continuously varying voltage signal becomes either ‘on’ or ‘off’). This encoding is a trade-off that neurons accept for the sake of longdistance communication. The amplitude of the graded signal is transformed into the timing and frequency of action potentials.