Nociceptive inputs to C3, a motoneuron of the tentacle withdrawal reflex in Helix aspersa

Abstract

The tentacle withdrawal reflex of snails is perhaps the fastest, most sensitive reflex in the animals' repertoire. We have investigated the sensory inputs to a major motoneuron (C3) mediating the reflex. The cell C3 is sensitive to both chemical and mechanical stimulation, but there is little or no discrimination of quality in chemical stimuli. Small increments in the concentration of chemical stimuli produce large changes in neuronal responses. When chemicals are applied to the afferent nerve, the effects are comparable to those caused by applications to the olfactory epithelium, suggesting that the transducing elements are unspecialized. The afferent pathway is independent of the procerebrum, which is the primary olfactory lobe. Two excitatory synaptic inputs are identified, both of which originate in the tentacle, propagate centrally and synapse directly onto C3. A small, low threshold input is assigned to dendritic sites distant from the soma. A larger, higher threshold input is assigned to proximal dendritic sites. The latter input is largely responsible for the strong activation of C3 following noxious stimulation of the tentacle. The sensory inputs to C3 have properties similar to those of fibres in the nasal branch of the vertebrate trigeminal nerve.