Beyond brain circuitry.Beyond brain circuitryThe 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 (programming) invariant - A rule, such as the ordering of an ordered list or heap, that applies throughout the life of a data structure or procedure. Each change to the data structure must maintain the correctness of the 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 sea slug, name for a marine gastropod mollusk that lacks a shell as an adult and is usually brightly colored. Sea slugs, or nudibranchs, are distributed throughout the world, with the greatest numbers and the largest kinds found in tropical waters. called Tritonia. The nerve cells he has examined form an anatomically fixed circuit, but at one synapse synapse (sĭn`ăps), junction between various signal-transmitter cells, either between two neurons or between a neuron and a muscle or gland. A nerve impulse reaches the synapse through the axon, or transmitting end, of a nerve cell, or neuron. in that circuit the same neurotransmitter neurotransmitter, chemical that transmits information across the junction (synapse) that separates one nerve cell (neuron) from another nerve cell or a muscle. Neurotransmitters are stored in the nerve cell's bulbous end (axon). 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 spinal cord, the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column. 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 Brandeis University, at Waltham, Mass.; coeducational; chartered and opened 1948. Although Brandeis was founded by members of the American Jewish community, the university operates as an independent, nonsectarian institution. in Waltham, Mass., reports that the output of this circuit is influenced by many different neurotransmitters Neurotransmitters Chemicals within the nervous system that transmit information from or between nerve cells. Mentioned in: Bulimia Nervosa, Impotence, Pain, Withdrawal Syndromes 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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