Neurophysiological mechanisms underlying habituation of the tentacle retraction reflex in the land slug Ariolimax

Habituation of the tentacle retraction reflex was studied at the following response levels: (1) Muscle tension elicited in the tentacle retractor muscle by repeated stimulation of a cerebral nerve (at 60-sec intervals) declined in parallel with evoked activity of the largest unit in the tentacle retractor nerve. (2) The largest unit in the tentacle retractor nerve (L₄) showed spontaneous recovery and dishabituation. The rate of response decrement was inversely related to the strength of stimulus, and an optimal interstimulus interval ca. 60 s was found. Retention of habituation for 24 h was exhibited. (3) The major retractor motoneurons (L₂, L₃, L₄) all showed habituation, dishabituation, and spontaneous recovery. The decline of L₄ activity was parallelled by a decline in muscle response. (4) Compound EPSPs elicited in the retractor motoneurons by stimulation of sensory pathways showed habituation and dishabituation. (5) Unitary EPSPs elicited by stimulation of cerebral nerves and connectives with minimal stimulus strengths also showed habituation and were unaffected by spontaneously occurring EPSPs. Dishabituation by another pathway was also shown. (6) Depolarization of L₄ by a constant current produced spike trains of constant firing rate and evoked a constant level of muscle tension in repeated trials, suggesting the absence of habituation in a peripheral nerve net or at the neuromuscular junction.