Neurobiology

Abstract

The generalized arthropod central nervous system (CNS) typically consists of a cerebral (supraesophageal) ganglion or brain which lies above the esophagus, a circumesophageal nerve ring (connective), a subesophageal ganglion or mass, and a ventral nerve cord giving rise to ganglia in each somite. A stomatogastric nervous system (SNS) supplies nerves to the alimentary canal and usually consists of two, three or four paired nerves from the area of the circumesophageal nerve ring with varying levels of fusion to form the so-called median nerves. There has been a tendency toward a centralization of the nervous system in many extant arthropod groups resulting in some modification of the primitive architectural design (Gupta, 1987). In the chelicerates, for example, the ganglia have moved anteriorly to form a generalized nerve mass in the prosomal region (Figure 3 – 1). In general, most of the arachnid neural tissues come together forming a prosomal (cephalothoracic) nerve mass around the esophagus which gives rise to nerves innervating various regions of the body. Spiders, for example, possess a highly condensed CNS with only two compact ganglia (cerebral and subesophageal), both located in the prosoma. The ventral nerve cord is absent. Fusion of the subesophageal ganglion is characteristic of arachnids. The cerebral ganglion innervates the chelicerae (cheliceral ganglion) and also contains the so-called association centers which are involved in the expression of many complex behaviors.