Fig. 4. Transmission electron microscopy of the peripheral portion of the anterior cardiac nerve (acn). (A) A low magnification micrograph of the periphery of the acn. An epineurium (ep) separates the nerve from the hemolymph space. Directly under the epineurium lie numerous nerve terminals, three of which are indicated with asterisks. Glial protuberances (g) are also present in the periphery of the acn. (B) A high magnification micrograph of the periphery of the acn. The epineurium is composed of a moderately dense, amorphous material that is fenestrated with minute open spaces (three of the larger fenestrations are indicated with black arrowheads). All or portions of five nerve terminals (asterisks) are present below the epineurium in this image. These terminals contain numerous dense-core vesicles (DCVs) and often mitochondria (m) and a small number of electron-lucent vesicles (ELVs). Morphological correlates of vesicle secretion, including vesicles docked to the plasma membrane and omega ()-figures, are common on the nerve terminals. In this micrograph four -figures are clearly visible. A and B are taken from the same preparation. Scale bars, 1 µm (A); 500 nm (B).