User:ThatSpiderByte/Nervous system of spiders

Central nervous system
The central nervous system (CNS) of spiders is located entirely within the prosoma, and is relatively dense compared with the CNS of other arthropods. It is composed of two compact ganglia, separated by the oesophagus into the supraoesophageal ganglion, which lies above the oesophagus, and the suboesophageal ganglion, which lies below the oesophagus. In young spiders, the CNS takes up almost half the volume of the prosoma, but this reduces to 5-10% in mature individuals. Structurally, the CNS can be divided into the cortex, an outer layer of neurons, and the neuropil, a dense network of nerve fibres where the majority of synaptic connections occur.

Supraoesophageal ganglion
The supraoesophageal ganglion is located above the oesophagus. It has only visual and association centres, and receives only the optic nerves. Each optic nerve enters at specific areas of the optic lobe, at the first optic neuropil (ON1) of its respective eye, passes through the second optic neuropil (ON2), and terminates at the third optic neuropil (ON3). The arrangement of the optic neuropils in the brain corresponds to the arrangement of the eyes on the carapace, and there is no overlap between the optic neuropils of different eyes. The optic pathways of the principal and secondary eyes differ both in structure and function; the pathways of the principal eyes are adapted to perceive form and texture, whereas those of the secondary eyes are specialised for detection of horizontal motion.

Suboesophageal ganglion
The suboesophageal ganglion is located below the oesophagus. It is mainly made up of the fused ganglia of the ten appendages, connected by many interneurons. Some of the interneurons are simple and limited to a small area, whereas others (plurisegmental interneurons) are more complex, and connect to many or all of the ganglia in the suboesophageal ganglion. Around half of the plurisegmental interneurons serve to combine sensory information from tactile hairs, trichobothria and slit sensilla, while the other half process information from the other sensory organs.