Persistent attenuation and enhancement of the earthworm main muscle contraction generator response induced by repeated stimulation of a peripheral neuron

Abstract

Responses evoked in the earthworm, Amynthas hawayanus, main muscle contraction generator M-2 (postsynaptic mechanical-stimulus-sensitive) neuron by threshold mechanical stimuli in 2-s intertrial intervals (ITI) were used as the control or unconditioned responses (UR). Their attenuation induced by decreasing these intervals in nonassociative conditioning and their enhancement induced by associating the unconditioned stimuli (US) to a train of short (0.1 s) hyperpolarizing electrical substitutive conditioning stimuli (SCS) in the Peri-Kästchen (PK) neuron were measured in four parameters, i.e., peak numbers (N) and amplitude ( )averaged from 120 responses, sum of these amplitudes (Σ AMP ) and the highest peak amplitude (V) over a period of 4 min. Persistent attenuation similar to habituation was induced by decreasing the control ITI to 0.5 s and 2.0 s in nonassociative conditioning within less than 4 min. Dishabituation was induced by randomly pairing one of these habituated US to an electrical stimulus in the PK neuron. All four parameters of the UR were enhanced by forward (SCS-US), but not backward (US-SCS), association of the US with 25, 100 and 250-Hz trains of SCS with 40-ms interstimulus intervals (ISI) for 4 min and persisted for another 4 min after turning off the SCS.

Identification of the main generator source of longitudinal muscle contraction in the earthworm ventral nerve cord

“…Slight physiological or morphological variation may actually be the rule rather than the exception. This predetermined bias of selection was nevertheless upheld in this article because a precise quantitative measurement was a necessity for comparing the muscle contraction magnitude modifications in the two subsequent studies (1,2). But this predetermined bias in their isolation and identification imposed a limit on the comparability of the results of this article to the stimulus-response paradigm in intact preparations tested by experimental psychological methods (15,16), where not only all segmental neuronal and muscular circuits but also intersegmental circuits from the highest to the lowest centers are included.…”

“…They are the central nervous system (2,(5)(6)(7)11), the peripheral nervous system (2,7), the muscle system which transmits impulses through their nexi (5,7,19) and the spontaneous activation in a single muscle cell membrane (20). The experiments in this section were carried out to determine which of them is the "main" generator source which generates the largest percentage of the muscle contraction and to isolate it by differential sectioning ( Figure 2).…”

“…These two neural substrates may either be the known neurons identified by us (8,9) or by others (10,11), or must be identified as new neurons in these two articles. We compared the interaction between these two neural substrates (2) to the conditioning paradigm of the other animals (12)(13)(14) and to that of the intact earthworm (15,16). This comparison is expected to contribute to the knowledge of learning and memory mechanism in general.…”