BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 477 



quency. Furthermore, the cells di.scharge their impulses in no fixed 

 temporal relation to the incoming, excitatory impulses. In short, the 

 cells which are activated through tiie pre-synaptic endings initiate im- 

 pulses at a Irequency that depends upcjn the characteristics of each cell, 

 as well as upon the frequency and tlie number of pre-synaptic im- 



FiGURE 18. Impulses discharged by a few sympathetic ganglion cells in response to stimu- 

 lation of the preganglionic nerve at a frequency of 50 per second. The ganglion was perfused 

 with a modified Ringer's solution containing various amounts of calcium chloride: 



Uppermost record, 4.4 mM ; middle record, 2.2 mM ; bottom record, 1.1 mM. The middle rec- 

 ord represents the normal level of calcium. 



,Time in 0.1 seconds. 



pulses.^' If these cellular characteristics are modified by any means 

 (by nerve impulses or by chemical agents), the rhythmic processes are 

 altered, and this modifies the frequency of their action. 



Such a modification of the rhythmic response of a nerve cell to neural 

 activation can be accomplished, as would be expected, by varying the 

 concentrations of calcium ions in the synaptic regions (figure 18). If 

 activity is excited in a ganglion cell by trains of pre-synaptic impulses, 

 the frequency is decreased by raising the concentration ojf calcium in 

 the perfusion fluid. Alternatively, the frequency of the impulses dis-- 



