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Fine structure of the axon initial segment and the axon cap of the Mauthner cell in the bullfrog tadpole.

The axonal initial segment of the Mauthner cell of tadpoles of Rana catesbeiana and its surrounding neuropil, the axon cap, were examined by electron microscopy. The initial segment is almost completely covered by numerous synaptic terminals with profuse clear spherical synaptic vesicles. These synaptic terminals are of one uniform type and are found to be extensions of preterminal swellings, which are filled with accumulated mitochondria and glycogen granules in addition to the synaptic vesicles. Some swelling were shown to have two or more terminals. The synaptic terminals as well as the preterminal swelling are occasionally linked by small synaptic junctions. The significance of this structural organization in the inhibitory control of the Mauthner cell activation was discussed.