STRUCTURAL ORGANIZATION OF INTRACEREBRAL GIANT FIBER SYSTEM OF CEPHALOPODS - CHIASMA OF FIRST ORDER GIANT AXONS

The course of the two intracerebral first-order giant axons of cephalopods at their chiasma, and the fine structure of the contact area between the crossing axons are examined by light and electron microscopy in species of three taxonomic groups (Loligo vulgaris, Sepia officinalis, Illex coindeti). In addition to the well known chiasma of the adult Loligo in which the two axons are fused (Young, 1939), three other chiasma types are described. In each of them there are synapse-like contact areas that suggest a passage of impulses from one axon to the other. (1) The larval Loligo shows a chiasma with crossed axons and contralaterally descending branches; at the apposed membranes in the chiasma there are clusters of electron-transparent vesicles. (2) In the adult Sepia each crossing axon has an ipsi- and a contralaterally descending branch; the apposed membranes of the decussating axons show symmetrical synapse-like areas characterized by a monolayer of electron-transparent vesicles on each side and a regular cleft of 100 Å width. (3) In the adult Illex each axon has only an ipsilaterally descending branch and there are at the point of decussation two crossed collaterals; large masses of electron-transparent vesicles are found on each side of the apposed membranes of the collaterals and the membranes show an increased electron density. It is argued that the four chiasma types serve the same function, i.e., the establishment of functional bilaterality of the giant fiber system. By structural analogy with other, both structurally and functionally known synapses it is suggested that in the decussation of Sepia impulses pass in both ways from one axon to the other. Data of the embryological development of the giant fiber system are summarized. © 1969 Springer-Verlag.