The functional organization of invertebrate ganglia.

Abstract

ing ability of sensory units.-An important generalization about the function of sensory systems which emerges from the studies of Wiersma and others is that sensory interneurons operate so as to abstract certain fea­ tures of the sensory input. The abstracting ability of some interneurons may be related to their particular geometry. For example, in the crayfish there is an interneuron that responds to any tactile stimulus applied to the left side of the body, independent of the particular position (308) . This unit does not respond to stimulation from any place else on the body, and consequently it is abstracting the quality of leftness. Interestingly, this type of interneu­ ron consists of a single long process that can receive synaptic input in each segmental ganglion through which it runs. It can initiate spikes at any one of multiple trigger zones (122, 146, 147) . This type of arrangement permits the interneuron to receive input from any combination of areas of the body INVERTEBRATE GANGLIA-FUNCTIONAL ORGANIZATION 217 surface. An additional consequence of this structural organization of cray­ fish interneurons is that activation of an interneuron elicits impulses that both ascend and descend the fiber. Furthermore, impulses initiated from dif­ ferent segments can collide and cancel one another. A given stimulus can therefore produce a complex pattern of spikes. The possible functional sig­ nificance of these complex spike patterns has not been determined. A common way by which the nervous system determines the abstracting property of sensory neurons is by means of inhibitory gating. A neuron re­ sponds to some stimulus, but is inhibited from responding by some other event. Thus the sensory response to a particular stimulus becomes contin­ gent upon the absence of some event. In the previously discussed crayfish movement fibers and space-constant fibers, the response of the unit to a vi­ sual stimulus was contingent upon the absence of eye movement or certain proprioceptive input. Another example of inhibitory gating is seen in the two L-fibers in the auditory system of an insect (269) . The L-fiber responds to sound stimulation of the ipsilateral tympanic organ. However, the contra­ lateral tympanic organ exerts a powerful inhibitory effect. This system ac­ centuates slight differences in the degree of stimulation between the left and right tympanic organs, and consequently is ideally suited to signal direc­ tional information about the sound source. We have cited a number of ex­ amples in invertebrate sensory systems where individual neurons respond to certain abstract qualities of the sensory world, much as they do in the verte­ brate eNS ( 108, 1 18, 179) . We should emphasize, however, that each cell is only part of a more complicated sensory process and that to fully under­ stand the function of a sensory neuron it is necessary to understand its rela­ tion to other neurons, and its relation to a final behavioral output ( 1 15, 1 18) . This type of analysis may be possible in invertebrates because of the relatively few units composing their nervous system.