How do electric eels generate a voltage, and why don't they get shocked?

Angel Caputi, senior scientist and head of the department of integrative and computational neurosciences at the Clemente Estable Institute for Biological Research in Montevideo, Uruguay, provides this answer:

The electric eel generates large electric currents by way of a highly specialized nervous system that synchronizes the activity of disk-shaped, electricity-producing cells packed into a specialized electric organ. The nervous system uses a command nucleus that tells the electric organ to fire. A complex array of nerves activates the thousands of cells at once.

Each electrogenic cell carries a negative charge of a little less than 100 microvolts on its outside compared with its inside. When the command signal arrives, the nerve terminal emits a minute puff of acetylcholine, a neurotransmitter. This release creates a transient path with low electrical resistance connecting the inside and the outside of one side of the cell. Thus, each cell behaves like a battery, with the activated side carrying a negative charge and the opposite side a positive one.

Because the cells are equally oriented inside the electric organ like a series of batteries piled into a flashlight, the current generated by an activated cell “shocks” any inactive neighbor into action, setting off a cascade that runs its course in just two milliseconds or so. If the eel lived in air, the resulting current could be as high as one ampere, turning the creature's body into the equivalent of a 500-volt battery. But water provides additional outlets, diminishing the current.

One explanation for why eels can shock other animals without zapping themselves could be that the severity of an electric shock depends on the amount and duration of the current flowing through any given area of the body. For purposes of comparison, an eel's body has roughly the same dimensions as an adult man's arm. To cause muscles in an arm to spasm, 200 milliamps of current must flow into them for 50 milliseconds. An eel generates much less energy than that with its two-millisecond current flow. Also, a large part of the current dissipates into the water through the eel's skin. The current discharged into the smaller bodies of prey is much stronger proportionally. For example, prey one tenth the eel's length is about one one-thousandth the animal's volume. Therefore, small animals close by get shocked, rather than the discharging eel.

What causes stuttering, and is there a cure? —Z. SCHWARTZ, VINELAND, N.J.

J. Scott Yaruss, associate professor in communication science and disorders at the University of Pittsburgh School of Health and Rehabilitation Sciences and co-director of the Stuttering Center of Western Pennsylvania, explains:

The classification of stuttering encompasses a number of communication disorders. Neurogenic stuttering and psychogenic stuttering, as their names imply, have a specific known cause—either a flaw in the makeup of the brain or a profound psychological challenge. Developmental stuttering, the most common version of this disorder, typically starts between the ages of two and a half and four. In addition to disruptions in their speech, people who stutter often experience negative emotional, cognitive or behavioral reactions that can further affect their ability to communicate.

Current theories suggest that stuttering arises from a combination of several genetic and environmental influences. Some of the elements now being examined are motor skills, language skills and temperament.

There is no known cure for stuttering, although many treatment approaches have proved successful for reducing its effects. Options include training to change speech patterns, counseling by speech-language pathologists to minimize negative reactions, pharmaceutical interventions and electronic devices that may enhance fluency. Self-help and support groups also play a prominent role in speech improvements for many people.