When you pull the trigger of a Taser gun, a blast of compressed nitrogen launches its two barbed darts at 55 meters per second, less than a fifth the speed of a bullet from a typical pistol. Each projectile, which weighs 1.6 grams, has a 9-millimeter-long tip to penetrate clothing and the insulating outer layer of skin. Two whisper-thin wires trail behind for up to 9 meters, forming an electrical connection to the gun.
Because the barbs get stuck in clothing and fail to reach the skin about 30 percent of the time, the gun is designed to generate a brief arcing pulse, which ionizes the intervening air to establish a conductive path for the electricity. The arcing phase has an open-circuit peak voltage of 50,000 volts; that is, the voltage is 50 kilovolts only until the arc appears or until the barbs make contact with conductive flesh, which in the worst conditions offers around 400 ohms of resistance.
The target’s body is never exposed to the 50 kV. The X26 — the model commonly used by police departments — delivers a peak voltage of 1200 V to the body. Once the barbs establish a circuit, the gun generates a series of 100-microsecond pulses at a rate of 19 per second. Each pulse carries 100 microcoulombs of charge, so the average current is 1.9 milliamperes. To force the muscles to contract without risking electrocution, the signal was designed to exploit the difference between heart muscle and skeletal muscle.
Skeletal muscle constitutes 40 percent of a typical person’s mass and is responsible for making your biceps flex, your fingers type, and your eyelids wink. It’s organized into bundles of single-cell fibers that stretch from tendons attached to your skeleton. When your brain orders a muscle to flex, an electrical impulse shoots down a motor nerve to its termination at the midpoint of a muscle fiber. There the electrical signal changes into a chemical one, and the nerve ending sprays a molecular transmitter, acetylcholine, onto the muscle. In the milliseconds before enzymes have a chance to chew it up, some of the acetylcholine binds with receptors, called gated-ion channels, on the surface of the muscle cell. When acetylcholine sticks to them, they open, allowing the sodium ions in the surrounding salty fluid to rush in.
The movement of those ions raises the cell’s internal voltage, opening nearby ion channels that are triggered by voltage instead of by acetylcholine. As a result, a wave of voltage rolls outward along the fiber toward both ends of the muscle, moving as fast as 5 meters per second. As the voltage pulse spreads, it kick-starts the molecular machinery that contracts the muscle fiber.
By directly jolting the motor nerves with electricity, a Taser can stimulate the muscle and get the same effect.
The force with which a skeletal muscle contracts depends on the frequency at which its nerve fires. The amount of contraction elicited is proportional to the stimulation rate, up to about 70 pulses per second. At that point, called tetanus, contractions can be dangerously strong. (The same thing happens in the disease tetanus, whose primary symptom, caused by the presence of a neurotoxin, is prolonged contraction of skeletal fibers.) The Taser, with its 19 pulses per second, operates far enough from the tetanus region so that the muscles contract continuously but without causing any major damage.
Heart muscle has a somewhat different physical and electrical structure. Instead of one long cell forming a fiber that stretches from tendon to tendon, heart muscle is composed of interconnected fibers made up of many cells. The cell-to-cell connections have a low resistance, so if an electrical impulse causes one heart cell to contract, its neighbors will quickly follow suit. With the help of some specialized conduction tissue, this arrangement makes the four chambers of the heart beat in harmony and pump blood efficiently. A big jolt of current at the right frequency can turn the coordinated pump into a quivering mass of muscle. That’s just what electrocution does: the burst of electricity causes the heart’s electrical activity to become chaotic, and it stops pumping adequately — a situation known as ventricular fibrillation.
The Taser takes advantage of two natural protections against electrocution that arise from the difference between skeletal and cardiac muscle. The first — anatomy — is so obvious that it is typically overlooked. The skeletal muscles are on the outer shell of the body; the heart is nestled farther inside. In your upper body, the skeletal muscles are arranged in bands surrounding your rib cage. Because of skeletal muscle fibers’ natural inclination to conduct low-frequency electricity along their length, a larger current injected into such a muscle tends to follow the grain around the chest rather than the smaller current that penetrates toward the heart.
The second protection results from the different timing requirements of the nerves that trigger muscle contractions and the heart’s intrinsic electronics. To lock up skeletal muscle without causing ventricular fibrillation, an electronic waveform has to have a specific configuration of pulse length and current.
The key metric that electrophysiologists use to describe the relationship between the effect of pulse length and current is chronaxie, a concept similar to what we engineers call the system time constant. Electrophysiologists figure out a nerve’s chronaxie by first finding the minimal amount of current that triggers a nerve cell using a long pulse. In successive tests, the pulse is shortened. A briefer pulse of the same current is less likely to trigger the nerve, so to get the attached muscle to contract, you have to up the amperage. The chronaxie is defined as the minimum stimulus length to trigger a cell at twice the current determined from that first very long pulse. Shorten the pulse below the chronaxie and it will take more current to have any effect. So the Taser should be designed to deliver pulses of a length just short of the chronaxie of skeletal muscle nerves but far shorter than the chronaxie of heart muscle nerves.
And that’s the case. To see just how different skeletal and heart muscles are, let’s look at what it takes to seriously upset a heart’s rhythm. Basically, there are two ways: by using a relatively high average current, or by zapping it with a small number of extremely high-current pulses.
In terms of average current, the 1.9 mA mentioned earlier is about 1 percent of what’s needed to cause the heart of the typical male to fibrillate. So the Taser’s average current is far from the danger zone for healthy human hearts.
As far as single-pulse current goes, the Taser is again in the clear. The heart’s chronaxie is about 3 milliseconds — that’s 30 times as long as the chronaxie of skeletal muscle nerves and the pulse lengths of a Taser. The single-pulse current required to electrocute someone by directly pulsing the most sensitive part of the heartbeat using 3-ms pulses is about 3 A. Because a Taser’s 100-ms pulses are such a small fraction of the heart’s chronaxie, it would take significantly higher current — on the order of 90 A — to electrocute someone using a Taser.
When you factor in that the Taser barbs are likely to land in current-shunting skeletal muscle not near the heart, you wind up with a pretty large margin of safety.
