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Journal of Experimental Biology 76,63-84 (1978)
Published by Company of Biologists 1978

Multiple Sites of Spike Initiation in a Bifurcating Locust Neurone

W. J. HEITLER ¹ and COREY S. GOODMAN ²

¹ Department of Zoology, University of California, Berkeley, California 94720, U.S.A.; Zoology Department, University of California, Davis, California 95616
² Department of Zoology, University of California, Berkeley, California 94720, U.S.A.; Biology Department, University of California, San Diego, La Jolla, California 93093

Recordings were made from the metathoracic dorsal unpaired median neurone to the extensor tibiae muscle (DUMETi) in the locust. This is a bifurcating neurone with axons exiting both sideS of the ganglion, whose soma can support a full action potential. Four different spike types were recorded in the soma, each of which we associate with a different region of the neurone. These were (1) a soma (S) spike of 70-90 mV, (2) a neurite (N) spike of 20-40 mV, occurring between the axon hillock and axon branch point, (3) and (4) axon (A) spikes of 8-15 mV, occurring distal to the branch point on the left and right axons. Each of these regions must therefore have its own spike initiation site.

At spike frequencies greater than about 10 Hz at room temperature or 1-5 Hz at 32 °C (the preferred environmental temperature of the locust) the S-spike may fail, revealing A-spikes, or more rarely N-spikes. A-spikes usually consist of two more-or-less separate components, A_l and A_r, which can be correlated with action potentials in the left and right axon branches by recording spikes extracellularly in the peripheral nerves on each side. Occasionally single component A-spikes occur when an action potential is initiated in only one axon, and fails to propagate across the branch point to the contralateral axon. Thus, action potentials may occur independently in the branches of this bifurcating neurone. After unilateral axotomy only S-spikes and N-spikes are recorded, indicating that action potentials no longer fail to propagate across the branch point. Anatomical asymmetries in the axon branches of DUMETi have been correlated with physiological asymmetries recorded in the soma of the same neurone.

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