THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



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FIG. 1 8. Interaction of antidromically and orthodromically evoked spike activity of Betz cells 

 in cats anesthetized with chloralose. Left. /, response of Betz cell in somatosensory area I to shock 

 to contralateral forepaw; 2-8, responses to threshold pyramidal shocks and contralateral forepaw 

 shocks delivered at different time intersals. Pyramidal shock artifact marked by white dot under 

 traces. Right. Diagram of similar data from two Betz cells showing four samples for each condi- 

 tioning-testing interval. Heavy line, antidromic spike; dotted line, expected position of blocked anti- 

 dromic spike; light line, orthodromic spike. (From Patton & Towc, unpublished observations.) 



laterals from neurons of other layers. The prominence 

 of the collateral system has led to some investigation 

 and considerable speculation concerning its function. 

 Chang (25) deduced, from analysis of antidromic 

 potentials recorded from the cortical surface in rab- 

 bits, that the collaterals are capable of re-exciting 

 Betz cells to produce repetitive discharge. Patton & 

 Towe (unpulalished observations) have been unable 

 to confirm this in cats (fig. 17). The response of Betz 

 cells to a threshold antidromic pyramidal shock was 

 invariably a single spike of short latency and was 

 capaljle of following high rates (100 to 200 per sec.) of 

 repetitive stimulation. With stronger stimulation, one 

 or more late spikes followed the early spike, and a 

 positive deflection of about 3 msec, latency was 

 grafted onto the surface cortical response. The late 

 positive surface wave was greatly attenuated, and the 

 late spikes failed at stimulus rates of 20 per sec. More- 

 over, both the late spikes and the late positive wave 

 were blocked bv an antecedent shock to the contra- 



lateral footpad (fig. 17). It thus seems likely that 

 repetitive firing to antidromic shocks occurs only 

 when the stimulus spreads to the lemniscus. 



Nor do the recurrent collaterals cause detectable 

 inhibition, as suggested by Phillips (85, 86) and Pur- 

 pura & Grundfest (87). Figure 18 shows experiments 

 in which antidromic and orthodromic volleys were 

 delivered to the cortex in combination and at various 

 conditioning-testing intervals. In no instance was 

 antidromic invasion followed by depressed excitability 

 beyond that expected from refractoriness. The func- 

 tion of recurrent axon collaterals of Betz cells remains 

 problematical. 



PYRAMIDAL FIBER SIZES AND CONDUCTION VELOCITIES 



In all species the pyramidal axons are characterized 

 by their fineness (20, 60); in man, only 61 per cent 

 are myelinated and of these, about 90 per cent are of 



