THE PYRAMIDAL TRACT! ITS EXCITATION AND FUNCTIONS 85I 



12 i6 20 24 28 32 

 MEAN INITIAL LATENCY 



16 20 24 28 32 36 40 44 

 MILLISECONDS 



FIG. 15. Firing times of orthodromically excited cortical cells of chloralose-anesthetized cats, 

 related to surface cortical potential. Left. Upper trace, reconstructed average surface cortical re- 

 sponse (somatosensory area I) to stimulating contralateral footpad; horizontal brackets are standard 

 deviations of measured points in time. Below, distribution of initial latencies of 132 cortical cells 

 responding to forepaw stimulation; numbers bracketed by arrows indicate proportions of Betz, 

 non-Betz and total populations firing first spike within the indicated time periods. Right. Upper 

 trace, surface potential as in the left part. Below, summation of firing probabilities at indicated 

 times (abscissa) for 140 cortical cells. See text. (From Patton & Towe, unpublished observations.) 



sisting of I to 1 1 spikes at frequencies up to 500 to 

 700 per sec. Although other cortical cells often fire 

 repetitively, the mean number of spikes per discharge 

 was significantly greater for 72 Betz cells {3.6) than 

 for 231 cortical cells (1.8) which did not respond to 

 antidromic pyramidal stimulation. For different Betz 

 cells, the latency to the first spike varied from 10.8 to 

 38.2 msec, and cortical delays (estimated by sub- 

 tracting latency of surface response from latency of 

 first spike) varied from 2.5 to 30.2 msec. Figure 15 

 shows the frequency distribution of initial spike 

 latencies for 74 Betz cells and 225 other cortical cells; 

 plotted on the same time base is an average surface 

 reading reconstructed by measuring the points in 

 time in 470 samples from 14 cats. (Amplitude is arbi- 

 trary but proportions are approximately correct.) 



The surface primary response in cats anesthetized 

 with chloralose consists of an initial positive wave 

 (T), with a mean latency of 7.99 ± i.ii msec, fol- 

 lowed by a larger positive-negative deflection (C), 

 with a latency of 11. 6 r ± 1.09 msec. Asphyxia or 



barbiturate anesthesia selectively attenuates or abol- 

 ishes C but leaves T virtually unaltered. It is suspected 

 (but not proved) that T reflects activity in thalaino- 

 cortical afferent systems and is thus comparable to the 

 S wave of Perl & Whitlock (84), and that C is asso- 

 ciated with postsynaptic cortical firing. Only seven of 

 310 cortical units discharged during the inscription of 

 T, and the initial discharge of these units was during 

 the latter part of T near the beginning of the C wave. 

 In any case, the beginning of T appears to be the 

 best available measure of arrival of impulses at the 

 cortex. 



The numbers jjetween the arrows represent the 

 percentage of the total cellular population which has 

 begun discharging in the time interv-al bracketed by 

 the arrows. For the Betz cells, considered alone, the 

 following data obtain : before mean peak positivity of 

 C, 44.6 per cent; between mean peak positivity and 

 peak negativity, 51.4 per cent; and later than mean 

 peak negativity, 4.0 per cent. It thus appears that, 

 although a considerable number of Betz cells are 



