THE PHYSIOLOGY OF NERVOUS SYSTEMS OF INVERTEBRATE ANIMALS

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Abstract
The comparative physiology of nervous systems gives less information for tracing phylogenetic relations than does the comparative physiology of many other systems. This is because (1) the basic nature of nervous conduction is a cellular problem, and (2) most of the general nervous properties evolved very early. In the coelenterate nerve net, for example, there is already a tendency for increasing speed by through-fiber tracts; there is facilitation in such high degree that it determines a sort of polarity. Inhibition in the sense of an inhibitory state and central antagonism with reciprocal innervation of muscles are present in annelids and, assuming phylogenetic bifurcation below annelids to the two main lines of descent, inhibition may well be found among flatworms. The use of a thick myelin sheath and nodes is a vertebrate development; syncytial giant fibers probably arose twice[long dash]those with contributing cells scattered along the nervous system in the annelid-crustacean line, and those with contributing cells grouped together in the cephalopod molluscs. Unicellular giant fibers are large neurones and grade into ordinary ones; such enlargement of neurones has occurred many times. It is not clear why peripheral conducting systems or subepidermal networks have been replaced so completely by local reflex centers. It is quite possible that the subepidermal plexus in the earthworm, in molluscan feet (but not in echinoderms) is even now vestigial. The closely interconnected evolution of central nervous and neuromuscular mechanisms is impressive. In the coelenterate nerve net system there is little distinction as to whether facilitation is neuro-neural or neuro-muscular. Some of the so-called peripheral conduction in worms may be muscular. In the arthropods, the neuromuscular junction with multiple innervation, facilitation, inhibition becomes more important than the central nervous system in grading motion. Passing toward the most successful or ''higher'' invertebrates, an increasing headwardness in the nervous system is observed. The sensory basis for this is clear, the integrative basis less so. Learning in the sense of a reversal of a stereotyped response is probably a property of all animals whether they have a nervous system or not, as Jennings has pointed out. Many of the alterations in feeding reactions obtained in coelenterates are undoubtedly sensory adaptation phenomena. In the earthworm, conditioning was carried out by the chain of ventral ganglia. Only in the verticalis complex of cephalopods, in the corpora pedunculata of certain insects (and possibly in those of lower arthropods and poly-chaete worms) is there anything comparable to the association areas of a vertebrate brain. Technical difficulties of small size and fragility have prevented the study that these structures suggest. Not enough work has been done with pharmacologically active agents in invertebrates to draw conclusions regarding chemical mediators. It is likely that there are cholinergic nerves beginning with flatworms and possibly with coelenterates. Invertebrate nerves offer many useful preparations for the cellular physiologist. A few examples are: giant fibers for studying membrane phenomena, nerve fibers which remain depolarized for mins. (Crustacea), neuro-muscular junctions in which facilitation persists for many secs. (coelenterates, annelids), inhibitory nerve fibers that can be isolated from excitatory fibers, and muscle fibers which receive from 2-5 nerve axons each of which elicits a different response.

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