850 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



guish ipsilateral from contralateral responses. All of 

 these properties suggest that the uncrossed afferent 

 pathway to the Betz cells is less direct (i.e. entails a 

 greater number of synapses) than the crossed pathway. 



Conditioning-testing procedures suggest overlap 

 between callosal and thalamocortical pathways to the 

 Betz cells. The pyramidal response to contralateral 

 forepaw stimulation is depressed for 400 to 500 msec, 

 following a discharge elicited by a callosal volley 

 evoked by a single shock to the opposite hemisphere, 

 and a similar interaction occurs when the forepaw 

 shock is used to condition a discharge evoked by 

 callosal stimulation. Similarly, corticofugal discharges 

 elicited by ipsilateral and contralateral forepaw stimu- 

 lation show depressive interaction (sometimes fol- 

 lowed by supernormality) with conditioning-testing 

 shock intervals up to 200 msec. Forepaw-hindpaw 

 (contralateral) tests reveal less interactive depression. 

 Whether the interaction is occlusive or inhibitory has 

 not been determined (2). 



In addition to the primary afferent and callosal 

 pathways, the thalamocortical pathway responsiijle 

 for the augmenting response (31) can elicit a pyrami- 

 dal discharge which begins during the positive phase 

 of the augmenting wave seen with bulbar recording 

 (21, 79). Stimulation of the pontine reticular forma- 

 tion blocks both the positive component of the aug- 

 menting response and the pyramidal discharge (79) 

 Pyramidal discharge is also associated with the cortical 

 spontaneous spindle bursts (14) of barbiturate-anes- 

 thetized or sleeping animals (i, 21, 109). In contrast, 

 Brookhart & Zanchetti (2 1 ) did not find any pyrami- 

 dal discharge or change in pyramidal excitability 

 during elicitation of the recruiting response (30) by 

 low-frequency repetitive stimulation of diffusely pro- 

 jecting thalamic nuclei. These authors ascribe Arduini 

 & Whitlock's contrary findings (5) to contamination 

 of the recruiting response with an augmenting re- 

 sponse. On the basis of other evidence, Towe (98) has 

 suggested that Betz cells discharge during cortical 

 primary, repetitive and augmenting responses, but 

 not in secondary or recruiting responses. 



TIMING OF BETZ CELL DISCH.^RGE 



Simultaneously recording the pyramidal discharge 

 and the surface cortical response following a shock 

 to a peripheral nerve provides data for estimating the 

 cortical delay in Betz cell excitation. Such an experi- 

 ment in a cat under chloralose-decamethonium anes- 

 thesia yielded the records in figure 14. The upper 



FIG. 14. Cortical delay of py- 

 ramidal discharge in a cat (chlor- 

 alose-tubocurarine anesthesia), 

 .^bove are shown responses 

 recorded in the bulbar pyramid 

 {upper) and on the surface of 

 somatosensory area I (lower) fol- 

 lowing shock to contralateral 

 ulnar nerve. .Middle and tower 

 traces show D-I complexes re- 

 corded in the pyramid following 

 a single shock to area I. Middle 

 trace, 10 msec, time scale; lower 

 trace, I msec, time scale. 



trace shows the pyramidal response (at the level of 

 the trapezoid body) to stimulation of the contralateral 

 ulnar nerve; the lower trace shows the cortical re- 

 sponse simultaneously recorded in somatosensory area 

 I. The pyramidal discharge began 4.6 msec, after the 

 beginning of the cortical response and during its 

 positive phase (4). The middle and bottom records 

 are slow and fast sweeps of the pyramidal response to 

 cortical stimulation, made to determine cortex-to-bulb 

 conduction time, which proved to be 0.52 msec. The 

 cortical delay in Betz cell excitation was thus about 

 4.1 msec. In other experiments, values ranging from 

 4.0 to 7.5 msec, were found. Such calculations are 

 obviously only approximate and err in the direction 

 of overestimation of the delay because a minimal 

 conduction time (D wave latency) from cortex to bulb 

 is subtracted. Errors due to conduction time can be 

 eliminated by simultaneously recording the spike 

 activity of individual cortical cells and the surface 

 response. Betz cells can be identified by their ability 

 to follow antidromic pyramidal stimulation at repeti- 

 tion rates of 100 per sec. or greater. (Spikes ortho- 

 dromically excited by current spread to medial 

 lemniscus do not follow repetitive stimulation at rates 

 exceeding 10 to 20 per sec.) 



The Betz cell in somatosensory area I character- 

 istically responds to a single orthodromic volley from 

 the contralateral forepaw with a repetitive burst con- 



