Nov. 5, 1999 PROVIDENCE, R.I. — Like the steady synchronized blink of a string of holiday lights, certain types of nerve cells in the cerebral cortex communicate with each other through electrical connections, forming a new type of brain circuitry described in the current Nature.
Until now, scientists thought nerve cells in the cerebral cortex, the sinuous bumps on top of the brain, communicated only through chemical signals.
The cerebral cortex contains two types of nerve cells – excitatory or inhibitory. Each neuron – a nerve cell in the brain – communicates with other neurons through chemical connections that fire off a tiny bit of chemical that either inhibits or excites the next neuron. These connections between neurons are called synapses.
While studying the chemical synaptic connections in the cerebral cortex of rats, Brown University researchers found that two separate types of inhibitory neurons were also using electrical synaptic connections to communicate, but only within their specific groups.
The cerebral cortex is the biggest part of the brain. This large and complicated neural circuit is involved in most of the brain’s highest functions, such as memory, language and sight. Within each type of excitatory or inhibitory cell, circuitry keeps neurons interconnected and communicating to keep overall brain activity in balance. Too much excitation and too little inhibition, for example, may lead to seizures. The opposite may lead to a loss of consciousness, coma or death.
The presence of electrical synapses in the cerebral cortex allows each network of inhibitory neurons to fire in a highly coordinated and direct way, as if there were a wire directly connecting the cells, said Barry Connors, professor of neuroscience and senior author of the study. “We think the inhibitory cells are coordinating their activity through the electrical synapses,” he said. The result is synchrony similar to the steady blinking of Christmas lights.
One of the two circuits, dubbed LTS neurons, may be involved in preventing runaway excitation among nerve cells in the cerebral cortex, Connors said. The electrical synapses may allow these neurons to generate activity over a large area of the brain, he said.
“It appears this one group is especially suited to regulating cortical function,” he said. “Most of the time it is not doing anything. But it becomes active when the brain’s activity increases to a high level. This network of inhibitory neurons may act like the governor on the engine of the cortex, keeping excitability from running away and becoming an epileptic seizure.”
Some scientists have suggested that inhibitory neurons generate the brain’s electrical rhythms. These rhythms offer clues to the brain’s state. Rhythms are smaller and faster when one is awake and slower and larger during sleep. LTS neurons may be the rhythms’ source.
“As we continue this research, we do suspect that this group of inhibitory cells may be the ‘pacemaker’ for generating some of the brain’s rhythmic electrical activity, the kind measured by an EEG,” Connors said.
The other electrical network of inhibitory neurons described in the study, called FS neurons, seems to be more directly involved in the processing of sensory information, he said.
Connors and colleagues study epilepsy, an illness often controlled by drugs that steady the brain’s chemical signals to keep cellular networks in balance. Discovery of electrical interconnections among cells in the cerebral cortex may one day provide another pathway for the treatment of brain-based illnesses.
The study’s lead author is Jay Gibson, postdoctoral fellow. The other author is graduate student Michael Beierlein. The National Institutes of Health funded the research.
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