The Nervous System

Chapter 11

Andy Sombke, Jörg Rosenberg & Gero Hilken

The nervous system of chilopods is divided into the central nervous system, composed of the brain and ventral nerve cord, and the peripheral nervous system with its projecting nerves. The central nervous system (brain or syncerebrum) is divided into the proto-, deuto-, and tritocerebrum. The peripheral nervous system is linked by nerves from the brain or ventral nerve cord to sense- and locomotion organs.

The protocerebrum

The protocerebrum is the largest part of the brain. It is well developed in Lithobiomorpha, Scolopendromorpha, and, especially, in Scutigeromorpha. In analogy to the Hexapoda, the posterodorsal region is called pars intercerebralis (Joly and Descamps, 1987). The protocerebrum extends laterally and includes the optic lobes. Geophilomorpha and eyeless Scolopendromorpha have reduced protocerebral lobes. The optic lobes in Scutigeromorpha (Fig. 11.3 A) comprise two neuropils: (1) the lamina, which supplies uncrossed axons to the optic tectum (Melzer et al., 1996; Strausfeld, 2005), the latter consisting of a thin plate-like neuropil extending out from the lateral edge of the protocerebrum; (2) the optic or visual tectum (formerly called medulla) is equipped with large relay neurons that send their axons into the brain. After Strausfeld (2005) the second neuropil matches the characteristics of the hexapod lobula plate. Therefore, it should be named optic tectum to avoid the assumption of a homology between the medulla of Hexapoda and malacostracan Crustacea and the second optic neuropil of Chilopoda. In Lithobius forficatus, long axons branch within the optic tectum and possess long collaterals that project to different regions of the neuropil. Large interneurons connect the optic lobes with the dorsal and ventrolateral protocerebrum. The neurons possess large neurites projecting from the optic tectum region medially and dorsally (Melzer et al., 1996). In Scutigeromorpha, the lamina has a simple organization; for each ommatidium, there are two relay neurons (monopolar cells) but no evidence of 218 A. SOMBKE, J. ROSENBERG & G. HILKEN local interneurons. The second optic neuropil is equipped with large tangential neurons that send their axons from its inner edge into the protocerebrum, where they terminate at the dendrites of descending neurons (Strausfeld, 2005). According to Strausfeld, the cell arrangements in the optic tectum are identical to the organization of neurons in optic neuropils of hexapods exemplified by the lobula plate of dipterous insects. Laterally, above the protocerebrum, a large cluster comprising many small globuli cells (characterized by a minute amount of perikaryal cytoplasm and by nuclei with condensed chromatin) provide a system of parallel fibers that provide two substantial lobate neuropils, one lateral and one close to the brain’s midline. Together these structures have been termed the mushroom bodies or corpora pedunculata or globuli (Joly and Descamps, 1987). However, homology with structures having the same name in hexapods is unclear. In Scutigeromorpha, the pedunculus is divided into two strands. In Lithobiomorpha and in Scolopendromorpha, only undivided pedunculi are present, whereas they are lacking in Geophilomorpha. The midline neuropil (or central complex) in Scolopendra spp. has been described by Loesel et al. (2002). In the investigated species, the central complex is roughly hemi- ellipsoid in shape and situated between the proximal tips of the medial lobes of the corpora pedunculata (Fig. 11.1 A, B). The central complex neuropil consists of at least three horizontal layers: a ventral, a medial, and a dorsal. In Scolopendra spp., the midline neuropil is innervated by fine fibers that are arranged in a more or less columnar manner and show an immunoreactivity against the neurotransmitter allatostatin The medial lobes of the corpora pedunculata are immunonegative. Many delineated lateral brain regions, as well as commissural tracts, are immunoreactive to allatostatine and tachy- kinine-related peptide. In addition, Bodian staining of S. polymorpha revealed a system of

Fig. 11.1 A Scolopendra polymorpha, columnar array of allatostatin-like immunoreactive fibres that innervate the unique midline neuropil from its upper surface. The arrow indicates columnar fibres. The asterisk indicates the tip of the medial lobe of the mushroom body (corpora pedunculata). B S. polymorpha, Bodian staining shows an additional system of interwoven fibres. Square brackets show corresponding layers of the midline neuropil in A and B. The open bracket indicates cluster of large cell bodies. C Dextranbiotin backfill of the left deutocerebral lobe of Scutigera coleoptrata. D Immunolocalization of synaptic proteins and phalloidin labeling in the left deutocerebral lobe of Scutigera coleoptrata (horizontal section). E 3D-reconstruction of the brain of Scutigera coleoptrata. corpus lamellosum (posterior) and olfactory neuropils (anterior). F Cross section of the head of Cryptops hortensis. All scale bars 100 μm. A and B after Loesel et al. (2002), C-E modified after Sombke et al. (2009), F original A. Sombke. Cl Corpus lamellosum; clc contralateral connection; d dorsal; Dc deutocerebrum; f frontal; fg frontal ganglion; m median; Na nervus antennalis; On olfactory neuropil; Pc protocerebrum; Tr tracheae; tr afferent tracts