858 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



The course of the pyramidal tract in the spinal cord 

 has been traced repeatedly with degeneration tech- 

 niques (6, 27, 39, 40, 44-46, 60, 65, 89, 90, 93, 96). 

 The major bundle is crossed and occupies the lateral 

 column, but a variable number of uncrossed fibers in 

 the lateral and ventral columns have been described. 

 D waves following stimulation of the precentral gyrus 

 in monkey are recorded only in the contralateral 

 lateral column. (Surface electrodes on either side of 

 the cord record a triphasic deflection larger on the 

 contralateral than on the ipsilateral side. That the 

 ipsilateral recording results froin volume pickup from 

 the opposite side is indicated by the fact that penetra- 

 tion converts the contralateral response to a pure posi- 

 tive deflection, but the ipsilateral responses remain 

 triphasic regardless of depth of recording. ) Bertrand 

 (12) found prominent ipsilateral responses following 

 stimulation of the supplementary motor area. 



The spinal extent of the tract is variable in different 

 species; in the cat, monkey and man, it reaches the 

 lumbar segments. However, the distribution to differ- 

 ent segments is unequal; in man, 50 per cent of the 

 fibers terminate in the cervical region, 20 per cent in 

 the thoracic and 30 per cent in the lumbrosacral seg- 

 ments (107). Uncrossed fibers appear to be destined 

 largely for cervical segments. 



SPINAL MECHANISM OF PYRAMIDAL TRACT 



The connection between pyramidal tract fibers and 

 motoneurons in the spinal cord has been investigated 

 repeatedly with a variety of anatomical techniques. 

 In the cat it appears that no fibers terminate directly 

 on anterior horn cells." After lesions of the sigmoid, 

 the coronal or the anterior ectosylvian gyri, pre- 

 terminal degeneration was found largely in the base of 

 the contralateral dorsal horn with a few degenerating 

 terminals in the intermediate gray. No degeneration 

 was found in the ventral horn. 



These anatomical findings are amply confirmed by 

 electrophysiological studies. Lloyd (72) found that a 

 volley initiated in the cat bulbar pyramid (after sec- 

 tion of other descending and ascending tracts) 

 reached the lumbar cord after about 4.5 msec, and 

 because of temporal dispersion, persisted for 10 msec, 

 or more. The earliest postsynaptic activity was in the 

 external basilar region and occurred very shortly after 



" The authors arc indebted to W. VV. Chambers and C. N. 

 Liu for permission to use material from a manuscript in prepara- 

 tion describing studies with Nauta stain of degeneration in the 

 cat spinal cord following cortical lesions. 



arrival of the pyramidal volley, although exact 

 measurements were made impossible by the pro- 

 longed asynchronous discharge of the pyramidal 

 collaterals which intermingle with the external basilar 

 interneurons. A single pyramidal volley is capable of 

 eliciting a discharge in these elements lasting 20 to 

 30 msec. ; whereas four shocks at brief intervals in- 

 crease the discharge duration to some 40 msec. It 

 thus appears that the cells of the external basilar 

 region constitute the major direct target of pyramidal 

 endings. 



Cells in two other regions of the cord fire only later; 

 these are the solitary cells of the dorsal horn (latency, 

 9 to 10 msec.) and the cells of the intermediate gray 

 substance (latency, 12 to 20 msec). Neither of these 

 groups responds to single pyramidal volleys but re- 

 quires usually three or more before beginning to dis- 

 charge. It seems likely that most of the presynaptic 

 drive to these elements is relayed through the external 

 basilar interneurons, although the possibility of sparse 

 direct connections with pyramidal endings cannot be 

 excluded. 



The influence of pyramidal volleys on motoneurons 

 is most conveniently studied by measuring the facilita- 

 tion of segmental monosynaptic discharges following 

 pyramidal conditioning. Such facilitation is detectable 

 only after three or more pyramidal volleys, delivered 

 at brief intervals, and has a latency of 1 2 to 20 msec, 

 measured from the first pyramidal shock. Since moto- 

 neuron facilitation closely parallels in time the dis- 

 charge of interneurons in the intermediate gray sub- 

 stance, it seems proiaable that the two events are 

 causally related and that most pyramidal impulses 

 are relayed through external basilar and intermediate 

 interneurons before reaching the motoneuron. This 

 conclusion is further supported by the fact that pyra- 

 midal volleys regularly facilitate three-neuron-arc dis- 

 charges (which can be facilitated at the interneuronal 

 as well as at the motoneuronal level) soine 3 msec, 

 earlier than the monosynaptic discharge (which, of 

 course, can be facilitated only at the motoneuronal 

 level). The sequence of events can thus be summarized 

 as follows. The pyramidal volley reaches the lumbar 

 cord about 4.5 msec, after bull^ar pyramidal stimula- 

 tion. External basilar cell discharge apparently re- 

 quires a summation period of about 4.5 msec, since 

 facilitation of trisynaptic arcs is not detected until 9 

 msec, after conditioning. After a summation time of 

 another 3.0 msec, solitary cells and intermediate 

 neurons begin to discharge, leading to motoneuronal 

 facilitation at about 12.0 msec. The relationships are 

 shown diagrammatically in figure 21. 



