Intra- and extraganglionic nerve endings formed by neurosecretory cells of the cerebral ganglion of the earthworm (Lumbricus terrestris L.).

In the cerebral ( = supraesophageal, suprapharyngeal) ganglion of the earthworm, a number of neurosecretory Gomori-positive perikarya are bipolar; others are unipolar, or multipolar. Some of the neurosecretory cell processes project centrally into a fibrous zone; peripheral processes enter small nerves which leave the dorsocaudal aspect of the ganglion. In the central fibrous zone, the neurosecretory fibers form varicose Gomoripositive terminals. Here, also zinc-iodine-osmium (ZIO)-positive fibers and monoamine fluorescent fibers are found. With the electron microscope, nerve terminals containing synaptic vesicles and either large neurosecretory "peptidergic" granular vesicles (diameter more than 1500 A), or smaller granular vesicles (diameter about 1300 A, or 900 A) are observed. These axon endings mainly form axo-dendritic synapses. "Peptidergic" profiles are both pre- and postsynaptic. Some of the extraganglionic "peptideric" fibers appear to terminate around vessels, but most of them form terminals on the visceral muscle cells which surround the ganglion. We think that the central neurosecretory processes communicate with the fibers of the synaptic zone of the ganglion. The peripheral neurosecretory "peptidergic" fibers are supposed to form a primitive neurohemal area and/or to function as vasomotor nerves. The fibers innervating the visceral muscle cells may represent vegetative nerves.