Biological Batteries
As you read this, your body is crackling with electricity. The 639 muscles in your body are in a state of sustained partial contraction, giving your body a kind of constant background 'static'. Your body contains three basic types of muscle: striated, smooth, and cardiac. It is electrical changes within striated ('voluntary') muscle that gives you the ability to move through and manipulate your external environment. Biochemically, it makes no difference whether you're built like Arnold Schwarzenegger or Danny DeVito, for all striated muscles work in precisely the same way.
A muscle cell at rest is characterized by a charge separation — called 'polarization' — across its membrane. A relative excess of sodium ions outside the membrane results in a net positive charge, while an excess of potassium ions inside results in a net negative charge. Whenever you choose to move, your central nervous system sends volleys of nerve impulses to the relevant muscle group. A flood of the neurotransmitter acetylcholine from motor neurons stimulates the breakdown of energy-storing molecules, either ATP in an oxygen-rich environment or glycogen in an oxygen-poor one. This breakdown powers a localized reversal in the distribution of sodium and potassium ions across the muscle cell membrane. This activates yet another system, causing the release of calcium ions from a system of storage membranes, which — in turn — induces the uncovering of active sites on ratchet-like proteins called 'actin filaments'. Waves of this localized charge reversal — called 'depolarization' — flow from one end of the muscle fibre to the other, stimulating contractile proteins called 'myosin fibrils' to contract and slide over the actin filaments. The myosin fibrils become short and thick, catching on molecular 'hooks' on the thin actin filaments, holding the contracted muscle taught. At the end of muscle contraction, release of a compound called creatine phosphate causes the actin hooks to relax their grip on the myosin fibrils, calcium ions are actively pumped back into their storage membranes, and the cell returns to its polarized state.
This elegant biochemical choreography holds true for all vertebrates. As a result, all backboned animals produce weak electrical currents. But some fishes have 're-wired' regions of their nerves and muscles in such a way that enables them to generate very powerful electric charges. These so-called 'electrogenic fishes' vary widely in generating capacity, form, and ecology, but are remarkably similar in their electricity producing mechanism. With the exception of the freshwater apteronotid knifefishes (in which the electric organs are derived from modified nerve fibres), the electric organs of all electrogenic fishes are modified from striated muscle fibres, consisting of stacks of flattened cells innervated on one side. This serial arrangement sums the small electric potentials arising from membrane depolarizations, thus giving rise to much higher external potentials. These 'supercharged' electrical potentials are put to uses other than locomotion. Weakly electrogenic fishes that live in very turbid water have been shown to use distortions in their electromagnetic field to navigate around obstacles and detect other organisms, including potential mates (males and females of the same species have slightly different 'signature crackles'). Strongly electrogenic fishes use their shocking abilities to stun prey or deter predators. In a few of these fishes, the electric organs can generate very large potentials — 50 to 500 volts — sufficient to modify the hairstyle or be downright dangerous to humans.
