http://www.taser.com/research/Science/Pages/NMIScientificPrinciples.aspx
A TASER-brand device is an Electronic Control Device (ECD) that uses a technology called Neuro-Muscular Incapacitation (NMI). Its purpose is to incapacitate someone with minimal risk of serious injury. Prior non-lethal weapons function by merely causing pain or destructive injury. The intention is that the pain or the bodily injury will dissuade the subject from continuing an unwanted behavior and elicit cooperation. However, focused individuals, people under the influence of drugs, or people who are pain insensitive may not feel pain, or may be sufficiently motivated to attack or fight through pain. The proprietary Neuromuscular Incapacitation (NMI) technology in TASER devices does not rely on pain or injury for its incapacitating effect(s). Rather, the TASER device uses electrical stimuli to interfere with the signals sent by the command and control systems of the body, at the peripheral nervous system level, to impair the subject’s ability to temporarily control his own body.
Figure 10 Neurons. Fig. 3.1 of Reilly, 1998. (a) Motor (muscle) and (b) sensory neurons are responsible for sensation and movement. They operate by propagating electrical signals. The human nervous system is the command and control system of the human body. It has three primary elements: the central nervous system, the sensory nervous system and the motor nervous system.
The central nervous system includes the brain and spinal cord. This is the command center, where all decision-making processes occur. One can think of the central nervous system like the computer that controls the body, including all memory and conscious thought. Out from this central computer is a network of "wiring" that carries signals to and from the brain. This "wiring" is comprised of nerve cells, or "neurons" that function very similarly to the wiring of a computer network. In fact, neurons carry information in the form of electrical impulses to and from the brain.
The sensory nervous system includes the nerves that carry information to the brain about the state of the body and its environment. Sensory nerves in the skin communicate heat, cold, pain, touch, and other sensations. Similarly, nerves carry visual data from the eyes, audio data from the ears, and olfactory data from the nose. All of this data is transmitted in the form of electrical impulses along the neurons into the brain.
Figure 11 Sensory Receptors. Fig. 3.16 of Reilly (1998). Section of the skin showing several types of sensory receptors. Sensory receptors can include sensors for touch, heat, feel, pressure, cold, etc.
The motor nervous system includes the nerves that carry commands from the brain out to the body. These nerves are primarily involved in muscular control. Commands from the brain are transmitted as patterns of electrical impulses through the nerves into the muscles, causing the muscles to move in certain patterns caused by the pattern of stimulation from the brain.
Below is a conceptual representation illustrating the concept of operation of TASER devices. TASER devices use very short duration low energy electrical pulses that are somewhat similar to the pulses used by neurons to communicate. If you think of the nervous system as an electrical communications network, TASER devices are like remote controls that plug into that network, and temporarily take control of, or interfere with, the communication patterns between the brain and the body.
TASER Devices Stimulate the Nervous System with Pulses Similar to Those Used by Nerves to Communicate One analogy helpful in understanding TASER technology is a telephone network. If person A is talking on the phone with person B, and suddenly person C picks up another handset and begins yelling into the phone, persons A and B can no longer effectively communicate - their conversation has been interfered with. However, when person C ceases yelling and disconnects, the normal conversation can resume again. The telephone hardware is not damaged in any way by the yelling, it is just that the temporary over-stimulation of the network prevented communication on a transient and temporary basis. Similarly, TASER devices cause stimulation of the nerves that is temporary in nature with minimal risk of causing serious damage to the hardware of the communication network by the interference.
Basics of Nerve & Muscle Stimulation
As mentioned previously, the body's neurons conduct electrical stimuli to and from the brain. When a neuron is in its resting state, electrically charged ions are pumped across the cell membrane such that net positive charge collects outside the membrane and a net negative charge collects inside the membrane. In this state, the membrane serves as a charged capacitor. When the nerve cell is stimulated, channels in the membrane open up temporarily, allowing the positive ions to temporarily rush across the membrane (opposites attract). At this moment in time, the voltage potential across the membrane briefly flips polarity as the charge balance reverses. This process is called an action potential. As an action potential occurs in one section of the cell membrane, the change in the electric fields causes the adjacent section of the membrane to depolarize. The result is a chain reaction of action potentials cascading down the length of the neuron, thereby carrying an electric impulse along the neuron. One important point to understand about action potentials is that they come in only one magnitude. For each neuron, there is a threshold stimulation level. Once this threshold is attained, an action potential will occur. There are not different intensities of action potential, they are an "All-or-None" phenomenon. In other words, there is no such thing as a partial or weak action potential. Either the threshold potential is reached and an action potential occurs, or it is not reached and no action potential occurs. Each neuron can only deliver one magnitude of impulse. Whether a muscle contraction will be strong or weak is not a function of the magnitude of the impulses of the connected neurons (again, there is no difference between impulses). The difference is the pattern of impulses delivered. The section below describes the process by which these nerve impulses cause muscular contractions:
The Neuromuscular Junction
Nerve impulses (action potentials) traveling down the motor neurons of the sensory-somatic branch of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.)
The terminals of motor axons contain thousands of vesicles filled with acetylcholine (ACh). When an action potential reaches the axon terminal, hundreds of these vesicles discharge their ACh onto a specialized area of postsynaptic membrane on the fiber. This area contains a cluster of transmembrane channels that are opened by ACh and let sodium ions (Na+) diffuse in. The interior of a resting muscle fiber has a resting potential of about -95 millivolts (mV). The influx of sodium ions reduces the charge, creating an end plate potential. If the end plate potential reaches the threshold voltage (approximately -50 mV), sodium ions flow in with a rush and an action potential is created in the fiber. The action potential sweeps down the length of the fiber just as it does in an axon. No visible change occurs in the muscle fiber during (and immediately following) the action potential. This period, called the latent period, lasts from 3-10 milliseconds (ms). Before the latent period is over, the enzyme acetylcholinesterase breaks down the ACh in the neuromuscular junction (at a speed of 25,000 molecules per second) the sodium channels close, and the field is cleared for the arrival of another nerve impulse. The resting potential of the fiber is restored by an outflow of potassium ions. The brief (1-2 ms) period needed to restore the resting potential is called the refractory period.
Tetanus
The process of muscles contracting takes some 50 ms; relaxation of the fiber takes another 50 to 100 ms. Because the refractory period is so much shorter than the time needed for contraction and relaxation, the fiber can be maintained in the contracted state so long as it is stimulated frequently enough (e.g., 50 stimuli per second). Such sustained contraction is called tetanus. When shocks are given at one per second, the muscle responds with a single twitch. At five per second and 10 per second, the individual twitches begin to fuse together, a phenomenon called clonus. At 50 shocks per second, the muscle goes into the smooth, sustained contraction of tetanus. Clonus and tetanus are possible because the refractory period is much briefer than the time needed to complete a cycle of contraction and relaxation. Note that the amount of contraction is greater in clonus and tetanus than in a single twitch. As we normally use our muscles, the individual fibers go into tetanus for brief periods rather than simply undergoing single twitches.
Effects of Repeated Pulses on Muscle Tension
