Evolution of the arthropod neuromuscular system. 2. Inhibitory innervation of the walking legs of a scorpion: Vaejovis spinigerus (Wood, 1863), Vaejovidae, Scorpiones, Arachnida

Inhibitory motoneurons which supply the leg musculature are identified and characterized in the scorpion, Vaejovis spinigerus (Wood, 1863) (Vaejovidae, Scorpiones, Arachnida). (1) Successive intracellular muscle fiber recordings from antagonists, and correlation of the monitored inhibitory postsynaptic potentials with spikes in motor nerves, suggest supply of the scorpion leg musculature by common inhibitory motoneurons. (2) Anti-GABA immunohistochemistry is combined with transmission electron microscopy to estimate the number of inhibitory motor axons present in the main leg nerve. The number of immunoreactive axons decreases toward more distal leg segments, from 14 to 18 in the basis to 6-8 in the tibia. No immunoreactive axons are detected beyond the tibia. (3) The distribution of putative inhibitory neurons in the subesophageal ganglion mass is determined by anti-GABA immunohistochemistry, revealing notable similarities to the situation in pterygote insects. This provides a framework for the characterization of the inhibitory motoneurons. (4) Backfills from leg nerves are combined with anti-GABA immunocytochemistry to identify inhibitory motoneurons in the central nervous system. Putative inhibitory motoneurons occur in three clusters per hemi-segment. Two clusters are located near the posterior edge of the neuromere, one lateral, the other more medial, and both contain ca. 8-10 cell bodies. The third cluster consists of two somata located contralaterally, just off the ganglion midline.