Beyond brain circuitry.
The intricate maze of connections among nerve cells in the brain, although awe-inspiring, does not provide the whole story behind animal behavior. Recent evidence indicates that a hardwired electrical circuit is not an adequate model for the brain. Characteristics invariant in an electrical circuit are flexible in nervous systems, so that fixed connections can vary their output during an animal's performance of different behaviors. The best-described nerve cell circuits are those of invertebrates, whose relatively few nerve cells can be identified. The specific connections among nerve cells have been determined in a variety of invertebrates. Now novel principles are emerging from what the scientists call "the second generation' of study of small neuronal circuits.
"Cells can dramatically change their own properties,' says Peter A. Getting of the University of lowa in Lowa City. "They show personal bias.' Getting studies a circuit used in walking and swimming by a sea slug called Tritonia. The nerve cells he has examined form an anatomically fixed circuit, but at one synapse in that circuit the same neurotransmitter can have opposite effects. Normally, when the animal is "cruising' along the sea bottom, this neurotransmitter, serotonin, stimulates the next cell in the circuit. But in a situation where the animal needs to make a rapid escape, the serotonin causes inhibition. Getting and his collegues have demonstrated that input from other cells reorganizes the interactions within the network and alters its function. He suggests that interactions among nerve cells in the spinal cord of vertebrates might also have variable patterns of "who talks to whom.'
A nerve cell circuit that excites stomach muscles and generates motor patterns in crabs provides another example of variability in the functioning of neural circuitry. Eve Marder of Brandeis University in Waltham, Mass., reports that the output of this circuit is influenced by many different neurotransmitters and other substances, called neuromodulators, present in the group of nerve cells. These chemicals can change the strength, frequency or timing of nerve cell activity. Marder says, "Each substance reconfigures the circuit.'
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|Author:||Miller, Julie Ann|
|Date:||Nov 9, 1985|
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