The physiological mechanism of catalepsy in the stick insectCarausius morosus Br.

Abstract

1. The phenomenon of catalepsy in the stick insect,C. morosus, during thanatosis (death feigning) was investigated quantitatively by applying controlled flexions of known amplitude, rise-time, and hold-time to the femorotibial joint of the prothoracic leg and observing the behaviour of the leg on release. 2. After release the leg always returned towards its fully extended resting position. An initial rapid extension (the fast phase), was followed by a slower movement (the slow phase), which often took several minutes. In some preparations the leg returned completely to its fully extended position (non-cataleptic). In others there was a maintained displacement of the leg after bending (partial catalepsy). 3. This misalignment between the initial extended position of the leg and its final position was proportional to the angle through which it had been bent. Rate of bending and time held bent had no effect on the final position of the leg. However the amplitude of the fast phase of the return curve was greater with slow rates of bend and with short times held bent. 4. Repeated bending caused a decline in the rate and extent of return and hence an increase in the misalignment between initial and final leg positions. 5. Denervation of the flexor tibiae had no effect on the return curve or on the final resting position of the leg, whereas extensor tibiae denervation resulted in the leg assuming a 90 ° flexed position, as did total denervation or CO2 anaesthesia. 6. Ablation of the femoral chordotonal organ, which causes considerable modification of locomotory behaviour, also had complex effects on the characteristics of the return curve following imposed tibial flexion. 7. Recordings of motor output to the extensor tibiae muscle indicated that imposed leg flexion was accompanied by a reflex increase of slow extensor excitor (unit 1) output and recruitment of a larger fast extensor unit (unit 2) during bending. Peak frequency and steady state (held) frequency of unit 1 in the flexed position were related to the angle of bend and to the rise time (inversely related to the rate of bend). This anomalous velocity response is an unusual feature of the reflex. 8. The behaviour of the leg after release from bending and the shape of the return curve could be accounted for by the operation of normal reflex pathways involving the extensor tibiae and the femoral chordotonal organ. The misalignment between the initial and final positions of the leg in insects showing partial catalepsy was found to be due to a maintained depression of unit 1 frequency after a bend/hold/ release cycle. This frequency depression was also found in non-cataleptic insects where no maintained misalignment was observed. 9. The relationship between leg position and extensor tension was found to be non-linear, and large increases in extensor tension are required to maintain the leg at femorotibial angles greater than 160 °. 10. The overall frequency of unit 1 output was found to be greater in non-cataleptic insects than in those showing partial catalepsy. This suggests that in non-cataleptic insects the maintained frequency depression after imposed flexion is not sufficient to depress the overall frequency below the critical level required to maintain the fully extended position, whereas in insects showing partial catalepsy reduction of the already lower extensor unit 1 frequency results in depression below this critical frequency and hence in maintained leg displacement. 11. It is suggested that the instances of “catalepsy” reported by previous authors were the result of repeated bending and hence cumulative depression of extensor unit 1 frequency.