Fig. 1

Location and morphology of the third pair of large paired neurons (LPN3s). a Survey view of the head of a ca. 12-day B. floridae larva. The LPN3s are the largest of a group of interneurons that together form the primary motor center. Other anterior landmarks: frontal eye (fe), infundibular cells (inf), lamellar body (lam) and tegmental neuropile (tn). Scale bar 50 μm. b 3D reconstruction of the two LPN3s (red and green) from serial TEM of a 12-day larva, showing the nerve cord in outline, anterior to the left, with dendrites projecting forward and axons behind. c 3D reconstruction, as above, with the LPN1s and 2s added (magenta and light blue), dendrites not shown. d Vesicular glutamate transporter (VGLUT) mRNA expression in a 4-day larva, a previously unpublished image using the same riboprobe as in [8], showing increased cell numbers in the LPN cluster (arrow). This indicates that LPN1s, 2s and 3s share the same transmitter, i.e., that all three pairs are glutamatergic. Other features: frontal eye pigment spot (arrowhead), cerebral vesicle (cv), mouth (m) and notochord (not). Scale bar 20 μm. e TEM image of a JR junction, in this case a double junction (asterisks) between a DC motor axon (a) and an IPN cell body, upper right. Double arrows mark ER cisternae, with matching cisternae on the asterisk side; see [10] for further examples and details. Scale bar 0.5 μm. f Dorsal view of the two LPN3s in (b) in matching colors; section numbers for the series are on the right, marked at 50-section intervals. Arrowheads indicate the reciprocal synapses between cells, and small arrows the JR junctions they form with the adjacent cell bodies of the first pair of DC motoneurons; see [10, 13] for details. Scale 50 sections = 3.4 μm