NEURONAL INTEGRATIVE MECHANISMS 9 



former at high average frequency and the latter at low frequency of firing. 

 But at any given average frequency there are types of neurons which are 

 markedly regular and others which are markedly irregular in successive 

 intervals. This suggests intracellular variables of importance in effective 

 magnitude, such as fluctuating spike threshold, fluctuating amplitude of 

 sinusoidal potential or of rate of rise of generator potential, fluctuating 

 area of nonpropagated or decrementally propagated activity, and the pres- 

 ence of multiple loci of origin of spontaneous subthreshold activity 

 (Wiersma, 1952a). 



Besides these short-term changes in the frequency of firing there are in 

 some cases long-term changes in average frequency on the scale of minutes. 

 We will consider their significance below. 



Eyzaguirre and Kuffler (1955) have just described a puzzling case of 

 repetitive firing in the cell body of a neuron which has recently received an 

 antidromic spike. Whatever the explanation — and these authors propose a 

 tenable one based on difl^ering local delays or partial blocks in the several 

 dendrites — it is germane as an indication of the degrees of freedom present, 

 and our purpose here is only to review the available ways in which the 

 neuron can exhibit the several variables whose interaction could accom- 

 plish integration. In our present state of knowledge this means we have to 

 include some observations whose "explanation" is less obvious than that 

 of others. Eyzaguirre and Kufiler's interpretation of the intracellular after- 

 discharge in the stretch receptor cell of the dorsal muscle sense organ of 

 crayfish may be correct ; but Bullock and Turner (1950) reported a similar 

 phenomenon in the giant fiber of the earthworm, so the phenomenon does 

 not depend on the particular anatomy in the crayfish receptor. In the earth- 

 worm, a spike initiated at stimulating electrodes and conducted a long dis- 

 tance down the giant fiber arrived at a locus of partial or complete block 

 ( anode of a polarizing circuit) where it hesitated before proceeding or died 

 out ; after five or more milliseconds a burst of several impulses at high fre- 

 quency originated at that locus or immediately adjacent to it. 



One of the consequences of spontaneity may be sensitivity to zveak elec- 

 tric fields. At any rate one can control the frequency of discharge of spon- 

 taneous integrating centers by passing a fraction of a microampere through 

 a mass of tissue of a few ten-thousands of an ohm resistance, where the volt- 

 age drop along the length of a single cell must be a fraction of a millivolt 

 (Bullock, Burr, and Nims, 1943, on Limulus ; Maynard, 1956b, and Terzu- 

 olo and Bullock, 1956, on lobster cardiac ganglion ; Hagiwara, Oomura, and 

 Takagi, unpub., on citrated squid axon) . The voltage drop across the mem- 

 brane must be still smaller. Either an excitable mechanism not familiar to us 

 is operating or the curve of membrane potential against firing interval is 

 exceedingly steep, which means the threshold is very critical and constant. 



