| Abstract: | A two-stage defense response was induced in the freshwater snailPlanorbarius corneus by stimulating the head. It consisted of the shell being rapidly lowered over the head and foot followed by the snail gradually withdrawing into its shell. These movements are performed by contracting the columellar muscle. Motoneurons of the columellar muscle were identified in the cerebral, parietal, and pedal ganglia. Stimulating the lip nerve was found to induce 2-stage excitation in motoneurons (responsible for the 2-stage muscular contraction) in preparations of central nervous system with the columellar muscle attached. The same 2-stage motoneuronal excitation can also occur spontaneously. This implies that defense reaction in the cell is at least partially a "fixed action" underlying a central mechanism or program and triggered by afferent stimuli. Activation of the central mechanism of defense response can also induce depolarization in certain columellar muscle motoneurons. This points to the existence of a feedback between neurons of the central mechanism and motoneurons.Institute of Research on Transmission of Information, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov State University, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 6, pp. 786–795, November–December, 1990. |
