Motor output characterizing thanatosis in the cricket Gryllus bimaculatus

The cricket Gryllus bimaculatus displays a sudden rigid immobility (thanatosis) when voluntary leg movements are forcibly restrained. The tibial joints in all legs are stiffly immobilized for several minutes. The flexed-leg posture typical of thanatosis is maintained by the flexor tibiae muscle. To characterize thanatosis at the motor output level, the mechanical and physiological properties of the metathoracic tibia muscle were investigated. The accessory flexor muscle, especially well-developed in the cricket, acts to stably maintain the tibial flexion driven by the main flexor muscle. Extracellular recordings from the flexor muscle of tethered crickets revealed that activity of intermediate- and fast-excitatory units was almost completely suppressed, while slow-excitatory units persisted in firing tonically during thanatosis. The firing rate of slow-excitatory units progressively increased as the tibia flexed, but remained less than the firing rate seen in the quiescent state. Common inhibitory motor neurones that fire sporadically in the quiescent state were suppressed during thanatosis, especially in the beginning, and showed a large excitation immediately after arousal. These findings suggest that the entire motor neuronal pool is held under active suppression during thanatosis, and that flexor muscle rigidity is maintained by a weak discharge of the slow exciters together with suppression of the inhibitors.