THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



843 



D and I, the conclusion is inescapable that the units 

 firing during the I \va\e outnumber those active dur- 

 ing the D wave. Therefore, at least some of these 

 indirectly-excited cells must have escaped direct exci- 

 tation. The same thing is shown even more clearly in 

 figure 4, where the responses to stimulation of a focus 

 in area 4 are compared with those evoked by stimula- 

 tion of a point rostral to area 4. The latter are char- 

 acterized by a small, late (4 msec. ) D wave, followed 

 by a series of I waves each of which has a far greater 

 amplitude and area than the D wave. It must thus be 

 concluded that the cortical interneuron system diffuses 

 excitation through the cortex and excites some Betz 

 cells situated too far from the stimulating electrodes 

 to be directly excited. The significance of this for 

 cortical mapping is discussed below. 



To answer the rest of the question, i.e. do cortical 

 interneurons cause repetitive firing of some cells 

 directly fired, requires single unit recording from 

 pyramidal axons or Betz cells. Occlusive interaction 

 (with 50 per cent reduction) of a test D wave timed to 

 fall during a conditioning I discharge can be demon- 

 strated (80, fig. 7), but this evidence is not conclusive 

 because distinction between occlusion and inhibition 

 is uncertain. Figure 5.4 shows the response of a single 

 pyramidal axon to a weak cortical shock; two spikes 

 occurred, the first having a latency of about 3 msec. 

 The lower trace (B) shows that at a stimulus repeti- 

 tion rate of 430 per sec. the unit failed to fire, suggest- 

 ing that it was indirectly excited. In C, the stimulus 

 strength was increased (thus enlarging the effective 

 area of the stimulus). The unit then fired four spikes, 

 the first with a latency of about 1.2 msec. Trace D 

 shows that this early spike followed a stimulus of 430 

 per sec, and hence the unit must have been directly 

 excited. Brookhart (19) found that pyramidal axons 

 stimulated in the cord, even with strong shocks of 2 

 msec, duration, do not fire repetitively, a fact suggest- 

 ing that the last three spikes of trace C reflect synaptic 

 excitation of a cell previously fired directly by the 

 stimulus. Recording with intracellular electrodes, 

 Phillips (86) also noted direct and relayed firing of 

 the same Betz cell to long (10 msec.) cortical shocks. 

 Thus, cortical interneurons cause repetitive firing of 

 Betz cells near the stimulating electrodes and initiate 

 delayed firing of cells outside the directly effective orb 

 of the electrodes. 



The interval between the D wave and the first I 

 wave should provide an estimate of synaptic delay in 

 the cortex. Indeed, the experimental arrangement is 

 comparable to that employed bvLorente deNo (73) to 

 determine synaptic delay in the oculomotor nucleus. 





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FIG. 5. Pyramidal axon .spikes in a cat anesthetized with 

 chloralose and given tubocurarine. Recording electrode in 

 lateral column between Ci and Cj. .4. Response to weak contra- 

 lateral precruciate stimulus. B. .Sweep taken during stimulus 

 train at 430 per sec. and same strength as in A. C. Response to 

 stronger stimulus than in A; note addition of early (2 msec.) 

 spike. D. Sweep during stimulus train at 430 per sec. ; strength 

 as in C; short-latency spike follows stimulus rate. [From Patton 

 & Amassian (80).] 



In preparations in which D and I waves are recog- 

 nizably distinct (figs. 3, 4), the time interval is usually 

 of the order of 2.0 to 2.5 msec. As an estimate of 

 synaptic delay, this interval is about double that 

 given by other sources. One possible interpretation of 

 I wave periodicity is based upon two assumptions. 

 First, that Golgi type II cells are responsible for syn- 

 aptic excitation of Betz cells. Second, that longitudi- 

 nally orientated neurons are inore easily excited by 

 an electric stimulus applied to the cortical surface 

 than are the compact field Golgi type II cells. An 

 electrical stimulus to the cortex would then excite 

 Betz cells and other longitudinally oriented cells 

 directly. The latter would subsequently excite Betz 

 cells through the intermediary Golgi type II cells. 

 The over-all delay for the indirect excitation of Betz 

 cells following electrical stimulation would thus be 

 two synaptic delays or a multiple thereof. In the 

 inonkey, the rather regular recurrence of I waves, 

 again at intervals of 2.0 to 2.5 insec, suggests periodic 

 bombardment of the Betz cells through chains of 

 neurons with fixed temporal characteristics. Often, 

 the I waves increase progressively in ainplitude (figs. 

 3, 4B) perhaps reflecting avalanche conduction in the 

 longer chains. In the cat anesthetized with chloralose, 

 however, the I activity often i^egins on the tail of the 

 D wave and persists for as long as 15 to 20 msec, as a 

 slowly waning discharge without clear maxima, a 

 configuration suggesting an asynchronous discharge 

 through chains with varying temporal characteristics 

 (80).^ 



