The brain of the horseshoe crab (Limulus polyphemus). III. Cellular and synaptic organization of the corpora pedunculata.

The large, hemispherical mass of the Limulus corpora pedunculata consists of two highly branched lobes, each connected to the protocerebrum by a narrow stalk. About 10(4) afferent fibers enter through the stalks and make diverse, profuse, and often reciprocal contacts with several million Kenyon (intrinsic) cells and one another. The Kenyon cell axonal arborizations converge on a few hundred efferent dendrites. The afferent fiber types can be classified into five types. Type A forms the club-shaped core of glomeruli and circumglomerular annuli, and contains small flat vesicles, suggesting an inhibitory function. Type B terminates with bushy endings in glomeruli and is presynaptic to both Kenyon cells and to Type A terminals. It has clear round vesicles and is the presumptive excitatory input. Type C terminates on other afferents, in glomeruli, and rarely on Kenyon cell bodies, contains angular (neurosecretory) granules and is postulated to impart circadian rhythm. Type D terminates on Kenyon cell somata and the initial neurite segment (but not in glomeruli), and contains dense-cored vesicles. Type E terminates in peduncles on other afferents and Kenyon cell telodendria. It contains dense vesicles. The C, D, and E afferents have reciprocal synaptic connections with Kenyon cell axon terminals. Glomeruli thus receive three different inputs of presumptive inhibitory (A), excitatory (B), and neuromodulatory nature (C). Kenyon cells, increasing in number up to about 1 x 10(8) in the adult, show minor variations in their dendritic pattern and have only one rare variant cell type. Interactions between them occur primarily at their axonal boutons as they crowd around efferent fibers. The latter have large receptive fields, some of their large somata are located within the confines of the corpora pedunculata, and they receive input almost only from Kenyon cells. Numerical and directional details of the circuitry in the corpora pedunculata have been extracted by a combination of light and electron microscopy, serial sectioning, silver staining, and stereology. The corpora pedunculata appear to process primarily the voluminous chemosensory input from the appendages, an assumption that is supported by the major connections of the organ.