[
Worm Breeder's Gazette,
1977]
The motor nervous system of Ascaris consists of five sets of segmented neurons each containing eleven cells that make synapses onto muscle, together with six ventral interneurons that make synapses onto some of the motorneurons. The neurons can be divided into seven classes (see figure) that appear to be structurally identical to the seven classes in C. elegans, with the following equivalences . [See Figure 1] The dorsal n.m.j. cell types C, D and E receive interneuron input, but the A cell receives synapses only from ventral motorneurons. The ventral F and G cells receive input from interneurons; the B cells receive synapses only from dorsal motorneurons. All these output cells make synapses with muscle only in one nerve cord. Each segment has one copy of cell types A, D and E, and two copies of types B, C, F and G. Anatomical similarities (relative positions in nerve cords; details of the patterns of neuromuscular connectivity) are used to assign functional equivalences in the dorsal and ventral cords. [See Figure 2] The function of types of A, B, C, D and E neurons has been determined physiologically (see Walrond and Kass). The motorneuron synapses onto the inhibitory neurons provide pathways for reciprocal inhibition, between both cords, that is generated peripherally rather than centrally. [See Figure 3]
[
Chromosoma,
2006]
Sexually reproducing organisms rely on meiosis for the formation of haploid gametes. This is achieved through two consecutive rounds of cell division (meiosis I and II) after one round of DNA replication. During the meiotic divisions, chromosomes face several challenges to ultimately ensure proper chromosome segregation. Unique events unfold during meiosis I to overcome these challenges. Homologous chromosomes pair, synapse, and recombine. A remarkable feature throughout this process is the formation of an evolutionarily conserved tripartite proteinaceous structure known as the synaptonemal complex (SC). It is comprised of two lateral elements, assembled along each axis of a pair of homologous chromosomes, and a central region consisting of transverse filaments bridging the gap between lateral elements. While the presence of the SC during meiosis has been appreciated now for 50 years (Moses, Biophys Biochem Cytol 2:215-218, 1956; Fawcett, J Biophys Biochem Cytol 2:403-406, 1956), its role(s) remain a matter of intense investigation. This review concentrates on studies performed in Caenorhabditis elegans, a powerful system for investigating meiosis. Studies in this organism are contributing to the unraveling of the various processes leading to the formation of the SC and the various facets of the functions it exerts throughout meiosis.