Anatomy and ultrastructure of the axons and terminals of neurons R3‐R14 in <i>Aplysia</i>

Abstract

Abstract Using light and electron microscopy and autoradiography, we have traced the axons of neurons R3‐R14 in the parietovisceral ganglion (PVG) of Aplysia to terminal fields associated with vascular tissue. The axons are identified by their large size (15‐30 μm diameter), extensive glial infolding, characteristic dense core vesicles (DCV; ∼180 nm diameter), and specific, rapid uptake of 3 H‐glycine. Each neuron in this homogeneous group sends an axon via the branchial nerve to the pericardial region surrounding the junction of the efferent gill vein and the heart. R14 also sends axons to major arteries near the PVG. The R3‐R14 axons branch extensively; we estimate that there are at least several hundred per cell. Branches along axons in the branchial nerve exit the nerve, subdivide, and end blindly in the sheath which is bathed by hemolymph. Similar blind endings from R3‐R14 occur in the sheath of the PVG (Coggeshall, '67). Axonal branches in the pericardial region and the special R14 axons in the arterial walls from both varicose endings near and terminals in contact with vascular smooth muscle. All R3‐R14 endings are free of glia, packed with DCV, show occasional omega‐shaped profiles and rapidly take up 3 H‐glycine. R3‐R14 manufacture specific low molecular weight peptides (Gainer and Wollberg, '74), and both the cell bodies (Iliffe et al., '77) and the terminals contain unusually high concentrations of glycine. The presence of peptides as putative neurohormones and sheath endings (neurohormonal release areas) are consistent with R3‐R14 being neurosecretory (Coggeshall et al., '66). While glycine could not be a circulating hormone due to its high circulating levels (Iliffe et al., '77), glycine could act as a local chemical messenger between R3‐R14 and smooth muscle. The terminal morphology of R3‐R14 is consistent with these neurons having both synaptic‐type and neurosecretory‐type functions.