FUNCTIONAL ORGANIZATION OF THE GIANT FIBER SYSTEM OF LUMBRICUS

Abstract

A consistent pattern of 2 giant action potential spikes may be recorded from any part of the nerve cord or from the skin of intact specimens of Lumbricus terrestris in response to single shock stimulation. The giant spikes come from the giant nerve fibers. The median fiber conducts upwards of 45 m./sec, the laterals 1/2-1/3 as fast. The rate increases as the animal is stretched but not proportionately, elapsed time also increases. There is no significant time available to ascribe to synaptic delay at each of the segmental septa. The fibers and the septa are unpolarized and normal responses in life involve conduction in both directions. Only 1 of the 2 spike forms is elicited by mechanical stimuli at any one place[long dash]the median fiber anterior to approx. segment 40 the laterals posterior to this point. Sensory cells adequate to fire the giant fibers must be confined to these levels but whichever fiber fires, it conducts both forward and backward from the point of stimulation. The apparent polarization found by earlier workers is explained on this basis. The septa are regarded as macrosynapses which support the theory that polarization and delay are not inherent physiologic properties of the synapse but the result of a particular anatomic arrangement. The pattern of normal volleys of impulses in the giant fibers is described and histologic predictions as to their connections made.