Neuroanatomy of the visual afferents in the horseshoe crab (Limulus polyphemus)

Abstract

The central nervous system of Limulus consists of a circumesophageal ring of fused ganglia and a paired ventral nerve cord. The anterior portion, the protocerebrum, receives sensory inputs including visual information. Three optic nerves, one each from the lateral eye, median ocellus, and ventral eye enter each side of the protocerebrum. The central connections of each optic nerve were determined by staining cut nerve trunks with cobalt chloride. The lateral optic nerve innervates the lamina, medulla, optic tract, ventral central body, and ocellar ganglion. The branching patterns of single axons, probably those of eccentric cells in the lateral eye retina, were observed. Single, large-diameter axons in the lateral optic nerve ramify at seven loci including sites in each of the structures innervated by the lateral optic nerve as a whole. The median optic nerve innervates the ocellar ganglion, central body, optic tract, and medulla. Three types of branching patterns were observed for single, large-diameter fibers in the median optic nerve. One type bypasses the ocellar ganglion and innervates the central body. A second type passes through the ocellar ganglion and optic tract without branching and innervates the posterior medulla. A third type innervates the ocellar ganglion, ventral central body, optic tract, and medulla. The ventral optic nerve is composed of large-diameter axons of ventral photoreceptors. Each axon enters the ganglion cell layer of the medulla and branches over a planar area less than 150 micrograms in diameter. We also observed that axons from mechanoreceptors on the anterior carapace innervate the posterior neuropil of the medulla, and that about 5% of the neurons in the medullar ganglion cell layer send axons to the ipsilateral circumesophageal connective.

Evolution of arthropod visual systems: Development of the eyes and central visual pathways in the horseshoe crabLimulus polyphemusLinnaeus, 1758 (Chelicerata, Xiphosura)

“…From the termination site of the median rudimentary eye projections, a tiny fiber tract, which is myosin III immunoreactive, proceeds toward the medulla in the larval brain. Comparing these findings to the adult neuroanatomy (Chamberlain and Barlow, 1980; we conclude that this fiber tract originates from a subpopulation of photoreceptors and pioneers the optic tract of the adult brain.…”

“…The fibers of the arhabdomeric cells project from the ocellar ganglia into the optic tract and continue into the medulla . The ventral photoreceptors also target the medulla (Chamberlain and Barlow, 1980;. The retinular cells of the lateral eye ommatidia project their axons by means of the lateral optic nerve to terminate in the lamina, whereas the eccentric cells of the ommatidia proceed on to the medulla and then to the ocellar ganglion by means of the optic tract.…”

Central connections of Limulus ventral photoreceptors revealed by intracellular staining

“…There are roughly 300 axons in the ventral optic nerve (Fahrenbach, 1975;Evans, Chamberlain, and Battelle, 1983). Despite suggestions to the contrary by Hanstrom (1926b) and Snodderly (1971) all of these appear t o terminate on the surface of the medullar neuropil Chamberlain and Barlow, 1980). The maximum extent of medullar surface observed in experiments where the entire ventral optic nerve is impregnated with cobalt ions coincides with that portion that is invested with the medullar ganglion cell layer.…”

“…Centrally, only the overall pattern of terminations of ventral optic nerve fibers in the protocerebrum has been described before Chamberlain and Barlow, 1980 we have injected single ventral photoreceptors with cobalt ions and stained and reconstructed their terminations in the protocerebrum.…”

Colour vision in mantis shrimps: understanding one of the most complex visual systems in the world

“…The brain centres in the eyestalk are connected with the central brain through the protocerebral tracts and optic nerves. Relatively few studies of the crustacean central brain have been undertaken (Chamberlain and Wyse, 1986, Blaustein et al, 1988, Sandeman et al, 1992, Utting et al, 2000, Sullivan and Beltz, 2001a, Harzsch and Hansson, 2008, Krieger et al, 2010, and there is still a lack of knowledge, in particular when it comes to the function of the different regions. Insects on the other hand, have been investigated in numerous studies (Strausfeld, 1976, Strausfeld, 2009b, Strausfeld, 2009a as a few examples), with recent advances in genetic work on Drosophila melanogaster providing a new method of further understanding the circuitry of the brain (Heisenberg, 2003, Olsen andWilson, 2008).…”