122 



PHYSIOLOGICAL TRIGGERS 



Fig. I. Mechano-transducer response in crayfish stretch receiptors. /) Stretch of the terminal 

 dendrites of a slowly adapting receptor causes a maintained depolarization of 6-7 mv at the 

 cell body. 2) A rapidly adapting ceil. The depolarization falls off during a maintained stretch, 

 j) At first, slight stretch caused 5 spike discharges in a slowly adapting receptor cell. In- 

 creasing the stretch raised the frequency of the discharges to about 12-14/sec., maintained 

 during the stimulus. The depolarization level at which firing of the cell occurred was constant 

 (broken line). 4) The rapidly adapting receptor cell stopped discharging after 3 spikes. A 

 stronger stretch then caused a new burst at higher frequency. The depolarization then de- 

 creased and the responses ceased. An additional stretch again called forth a new burst, at high 

 frequency, but again not maintained. The 'adaptation' is associated with decrease of the 

 depolarization level. 5) Increasing stretch of a slowly adapting cell raised the de])oiarization 

 level and increased the discharge frequency (.1, B), but the spike decreased and the res])onsive 

 membrane became inexcitable at a depolarization of about 35 mv (C). Recovery shows 

 hysteresis {D, E, F). Time in all records, i sec. (composed from ref. 68). 



