MECHANORECEPTION 61 



time of rotation. The amplitude of the on-wave increases with stimulus 

 intensity. It seems that the on-wave represents the summation of many 

 synchronously discharging neurons. The change in amplitude with 

 change in stimulus intensity is a reflection of the dilTcrcnccs in thres- 

 hold of the contributing units. 



The time-course of the off-wave differs slightly. If the direction of 

 rotation of the funiculus is reversed the on-wave now resembles the 

 off-wave at the end of the stimulus for the original direction of ro- 

 tation, and vice versa. Burkhardt (1960), by way of explanation, 

 suggested that some of the receptors respond to turning in one direc- 

 tion only, while others respond to turning in any direction. The 

 geometrical arrangement of sensilla is such that mechanical stress in a 

 particular direction serves as a stimulus; however, different sensilla 

 are oriented differently with respect to the axis of rotation. Directional 

 sensitivity of this sort had already been described for Locusta by 

 Uchiyama and Katsuki (1956) on the basis of action potentials 

 recorded by means of a microelectrode. There is no support for the 

 suggestion of Kuwabara (1952 b) that the Johnston's organ is stimu- 

 lated by changes in the pressure of body fluid in the pedicel caused by 

 its moment of inertia during antennal movement. 



The pattern of discharge following rhythmic stimuli of short dura- 

 tion changes as the frequency of stimulation changes. As the intervals 

 between stimuli are shortened below 20-30 msec, the off-wave of one 

 stimulus approaches more closely the on-wave of the succeeding stim- 

 ulus. When intervals are double the duration of stimuli the on- and 

 off-waves present a picture of regularity resembling the pattern 

 elicited by air-borne sound (Burkhardt and Schneider, 1957). The 

 response to sound frequencies up to 500 c/s reported by Burkhardt and 

 Schneider (1957) probably arises from rigidly synchronized discharges 

 at the beginning and end of each torsional vibration stimulus (Burk- 

 hardt, 1960). As the duration of stimulus is decreased the on- and off- 

 waves approach one another, and eventually the off-wave disappears. 

 It appears to be inhibited by the on-wave rather than to sum with 

 it. Thus, minute changes in stimulus pattern elicit marked altera- 

 tions in the spatio-temporal excitation pattern in Johnston's organ 

 (Burkhardt, 1960). 



Other patterns of excitation can also be detected in antennae, but 

 the identity of the sensilla in which they originate is not known. There 

 are a few unknown tonic receptors (Burkhardt and Schneider, 1957) 

 (also in Locusta, Uchiyama and Katsuki, 1956), by means of which a 

 constant deflexion of the antenna can be signalled. When subjected to 



