Within the primary olfactory neuropil of the crayfish brain--the paired
olfactory lobes (OLs)--it is thought that axon terminals of individual, class-specific ORNs from all of the antennular sensory ganglia are segregated each within their own, uniquely identifiable glomerulus, where synaptic connections are made with local interneurons, such as the Type I studied here, and with projection neurons to higher brain centers.
Lateral view of the brain: A= cerebrum, B= optic lobe, C= cerebellum, D= medulla oblongata, E= spinal cord, 1=wulst, 2= vallecula, 3= olfactory nerve, 4=
olfactory lobe, 5= orbital print, 6= lateral part of transverse fissure, 7= optic tract, 8= optic chiasm, 9= optic nerve, 10= infundibulum, 11= hypothysis,12= midbrain, 13= pons, 14= trigeminal nerve, 15= cerebellar fissures, 16= cerebellar lobe.
Abbreviations: AGB, antennular grooming behavior; ASW, artificial seawater; DW, distilled water; LAN, lateral antennular neuropil; L-glu, L-glutamate; OL,
olfactory lobe; SE, squid extract.
Removal of the aesthetasc-bearing portion of the lateral antennule flagellum causes the crayfish to lose as much of its
olfactory lobe as if the entire appendage were removed, suggesting that aesthetascs are the primary olfactory sensilla (Mellon, 2000; Horner et al., 2008).
After the general scooping, the portion of the
olfactory lobe left in the depression rostral to the wing of the presphenoid was also scooped out, as was the hypophysis from the sella turcica and any other part of the brain that was attached to the meninges.
In all decapods, ORN axons run within the antennular nerve and terminate within a glomerulus of the ipsilateral
olfactory lobe (Sandeman and Denberg, 1976; Sandeman and Luff, 1973; Mellon et al., 1989; Mellon and Munger, 1990; Mellon and Alones, 1993; Schmidt and Ache, 1992; Schmidt et al., 1992).
In decapods, as in insects, this pathway is composed of two elements: bilateral glomerular neuropils of the deutocerebrum, where olfactory afferents transmit information onto local interneurons and projection neurons (antennal lobes in insects;
olfactory lobes in decapods); and bilateral second-order neuropils of the protocerebrum, to which the projection neurons ascend (mushroom bodies in insects; hemiellipsoid bodies in decapods) (e.g., Boeckh et al., 1984; Blaustein et al., 1988; Mellon et al., 1992).