MORPHOLOGY OF MAIN AND ACCESSORY ELECTRIC ORGANS OF NARCINE BRASILIENSIS (OLFERS) AND SOME CORRELATIONS WITH THEIR ELECTROPHYSIOLOGICAL PROPERTIES

Abstract

Gross and fine anatomy of the electric organs were studied in correlation with electrophysiological problems. Narcine has a hitherto undescribed accessory electric organ which lies between the main organ and pectoral girdle, apparently derived from a separate muscle group. Its electro-plaques are innervated on their dorsal surfaces, not as in the main organ, on the ventral. They are 20-30 u thick, while those of main organ are only 7 [mu]. Electron micrographs of latter reveal a dense system of canaliculi at uninnervated portions. Some canaliculi appear to open through surface membrane, others extend as far as fingerlike inpocketings of innervated membrane. They could be remnants of Z-line tubules that have been postulated as mediating excitation-contraction coupling in striated muscle fibers. Each electroplaque is innervated by at least 4 nerve fibers. The innervation of the organs differs somewhat from that described for Torpedo being from the facial, hypoglossal, and vagus nerves, the trigeminal nerve does not participate. A branch of the vagas supplies the geminal nerve does not participate. A branch of the vagas supplies the accessory organ. In a column of the main organ the electroplaques number about 500, and the organ discharge is about 35 volts. Thus, the contribution of each electroplaque is about 70 mV, which corresponds with electrophysiological findings, recording with microelectrodes, that these are activities devoid of spikes comprising only postsynaptic potentials. The number of cells in a column is the same in late embryos and in adult fish, but the number of columns in the latter (about 400) is considerably larger (as against 300). Presumably, therefore, electric organ tissue continues to differentiate from its precursor muscles after the fish becomes free-swimming.