Synapses and ephapses in the spiral ganglion.

Spiral ganglion cells and their nerve fibers in Rosenthal's canal of the macaque monkey were studied in serial sections by electron microscopy. There were four types of neurons. In type I myelinated neurons, the first segments of the peripheral processes established direct contact with each other demonstrating asymmetric densities at opposing junctional membranes. The first segments were also in direct contact with the peripheral processes of type II neurons (ephaptic contact). In type II unmyelinated neurons the perikarya and the peripheral and central processes showed synapses, presumably with the efferent fibers. The processes of type II neurons came in direct contact with each other and also with the adrenergic type fibers. In the third type of neuron (IIA), which showed thinly or partly myelinated perikarya with unmyelinated axons, the peripheral and central processes showed synapses similar to those shown in type II neurons. The peripheral process formed a varicosity and made presynaptic contacts (dendrodendritic synapses) on the peripheral processes of type II and IIA neurons. In the fourth type, the perikaryon was unmyelinated but the axons were myelinated. All cytological features and synapses on the perikaryon were similar to those of type II neurons. These findings suggest that nerve fibers of different types communicate with each other, and when sensory cells degenerate, the surviving neurons which have lost their connections to these sensory cells, may still send electrical signals to the cochlear nuclei through the synapses and ephapses of their neurites in Rosenthal's canal.