ARTHROPOD NERVOUS SYSTEMS: A REVIEW OF THEIR STRUCTURE AND FUNCTION

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Abstract
The immediate purpose of this review is to bring together such literature on arthropod nervous systems, and their physiology, as might facilitate studies on the mode of action of insecticides classed as neurotoxins. . . . The central nervous system consists of a ''brain'' formed by the fusion of paired ga.nglia, followed by two ventral gan-glionic chains with various degrees of lateral and longitudinal fusion. Sensory neurons have their cell bodies located peripherally. Numerous association neurons are found in the ganglia along with the motor cell bodies which are relatively few in number. Recent studies with the polarizing microscope have shown that arthropod nerve, like that of the vertebrates, is surrounded by a lipoprotein sheath. The carbohydrate reserves of crustacean nerve are much greater than those of vertebrate nerve, as are its O2 consumption and heat production. It is also more readily fatigued than vertebrate nerve. Evidently the metabolism of crustacean nerve is less efficient than that of the vertebrates. Crustacean nerve usually responds to a single stimulus, unless of very short duration, with a series of impulses. This may be due to its accommodation time being slower than that of vertebrate nerve. Isolated arthropod ventral nerve cords show a persistent rhythmic discharge. Apparently certain cells in the central nervous system discharge without sensory stimulation. Each of the axons innervating a crustacean muscle produces a different type of response. In a triply-innervated muscle, one axon produces a fast contraction, another a slow contraction, while the third inhibits the response to the other two. Since each axon innervates all the fibers of the muscle, the entire muscle acts as a single motor unit. The tension developed is independent of the intensity of the stimulus, and is governed entirely by its frequency and duration. Each muscle fiber receives many branches from each of the axons innervating it. The excitation process is a local one, and is not conducted over the fiber. Hence, crustacean muscle shows a graded rather than an all-or-none response. A selected series of drugs (nicotine, curare, muscarine, pilocarpine, acetylcholine, eserine, atropine, adrenalin, yohimbine, veratrine, piperidinomethylbenzedi-oxane (933F), pyrethrum), whose action on arthropod nervous systems has been studied, is considered. So far as information is available their actions on the heart, central nervous system and peripheral nerves are reviewed. It is difficult in the case of the arthropod heart to determine the exact locus of action of a given drug since the possible loci are several: on cell bodies of pace-maker neurons (when present); on axons of these neurons; at their endings; and directly on the heart muscle. In the intact heart, possessing extrinsic regulatory nerves, the situation is even more complex. . . Although arthropod nervous systems contain large quantities of acetylcholine and cholinesterase it has been difficult, by means of pharmacologic studies, to demonstrate precise roles for acetylcholine. Many drugs do not have comparable actions in arthropods and vertebrates. For example, neither nicotine nor curare block transmission from nerve to skeletal muscle in arthropods but both substances have a central action. A variety of unrelated substances, including veratrine, 933 F, eserine and pyrethrum, when applied to peripheral crustacean nerve, produce multiple discharge to single weak stimuli.