The Depolarizing Nature of the Trigger 35 



completely dark-adapted. The response ceased immediately upon 

 re-illumination of the preparation, and the nerve remained 

 electrically silent until the photic energy was removed. At this 

 point, a transient impulse discharge several times greater than 

 that which occurs in the dark-adapted preparation was recorded. 

 Similar behavior has more recently been shown to occur in single 

 photoreceptor neurons of the surf clam, Spisula.^^ In the latter 

 preparation, an excitatory component also is believed to contribute 

 to the electrical activity of the same cell during illumination, 

 although the gross response of the neuron is similar to that of the 

 nerve supplying the distal retinal layer in the eye of the scallop. 



Fig. 13. Extracellular record- 

 ings from a sugar-sensitive 

 neuron in a blowfly chemo- 

 receptor. (A) Depolarizing 

 receptor potential and a train 

 of action potentials evoked by 

 a solution containing 0-25 m 

 sucrose. (B) Response to 0-25 

 M sucrose and 0-05 m calcium 

 chloride. (C) A hyperpolarizing 

 receptor potential in response to 

 a solution containing 0-05 M 

 calcium chloride. 



Note the burst of impulses following removal of the stimulus. Cali- 

 bration markers indicate i millivolt and 1/60 second. (From Morita and 

 Yamashita*', Fig. 2.) 



In both instances histological evidence was presented which 

 suggested that the electrical records might represent activity 

 occurring in primary sensory neurons. If this is the case, it would 

 be justifiable to ascribe the cessation of impulse activity during 

 illumination to a hyperpolarizing receptor potential. This 

 potential would be capable of inhibiting ongoing impulse activity 

 arising in the neuron in response to intrinsic or other extrinsic 

 influences. This is the simplest interpretation that can be accorded 

 the data. This explanation is, however, supported by experi- 

 mental observations made on a more complex visual system — that 

 of the beetle, Dytiscus — by Bernhard and his colleagues, ^^ who 

 found that anodal (hyperpolarizing)curreiita_were effective in 



