Chelicerate Skeletal Neuromuscular Systems

In the past decade several laboratories began investigating the neuromuscular physiology of the arthropod subphylum, Chelicerata. This paper concerning chelicerate skeletal neuromuscular systems concentrates on three major areas: fine structural features of the skeletal muscles; fine structure of synaptic areas; and innervation patterns and neuromuscular physiology. For all chelicerates investigated, the fine structural features of the muscle fibers appear to be similar with the exception that the sarcomere lengths fall into two general size categories (ca. 4 μ. and 7 μ.) . Two types of synaptic areas are found in the chelicerates. In horseshoe crabs the synaptic region is formed in a large evagination of the sarcoplasm, and evaginations from more than one muscle fiber may combine into one synaptic region. In the arachnids the synaptic area is formed in a slight furrow in the muscle fiber or along the surface of the fiber. Studies of innervalion patterns reveal that horseshoe crabs have a much greater number of motor axons innervating the skeletal musculature than do other chelicerates. The motor axons appear to be similar in function in horseshoe crabs, but in scorpions there are at least two functional types. In only one investigation has peripheral inhibition been demonstrated in Chelicerata.