THE PYRAMIDAL TRACT: ITS EXCITATION AND FUNCTIONS 



859 



FIG. 21. Diagiam of spinal connections of pyramidal tract 

 fibers in the cat. E, small cells of the external basilar region ; /, 

 intermediate gray nucleus of Ramon y Cajal; />, other neurons 

 of the intermediate region; M.N., motoneurons; P, pyramidal 

 tract; P,A., primary afferent collaterals; S, solitary cells of the 

 dorsal horn; VR., ventral root; 2, 3, and 3a, terminal collaterals 

 of the primary afferent system. [From Lloyd (72).] 



It is of interest that, in the cat, the influence of 

 pyramidal conditioning on motoneuronal discharge 

 was invariably facilitatory. In a few instances inhibi- 

 tion of tonic discharges in interneurons was observed, 

 but the over-all effect on ventral root discharges was 

 facilitatory. It is likely that interneuronal inhibition 

 reflects reciprocal innervation; to determine the effect 

 of such reciprocal innervation of interneurons on 

 motoneuronal excitability would require separate 

 recordings from motor nerves supplying antagonistic 

 muscles rather than ventral root recording which 

 samples motoneurons without regard to peripheral 

 destination. Such experiments have not been per- 

 formed. 



In the monkey, where the pyramidal tract is more 

 highly developed than it is in the cat, there appear to 

 be some direct connections between pyramidal axons 

 and motoneurons. Hoff & Hoff (46) found degenerat- 

 ing boutons on motoneurons in monkeys surviving 

 precentral lesions. Figure 22 shows the distribution of 



FIG. 22. Degeneration in cervical spinal cord of monkey 

 following lesion of arm motor area. Photograph at upper right 

 shows lesion as revealed at autopsy i o days after operation. 

 Diagrams on left show distribution of degenerating terminals at 

 indicated spinal lesels. In lateral columns density of degenera- 

 tion is indicated by density of shading. Photomicrograph of 

 Nauta-stained section from region A in Cj is shown in .4 on 

 right. Note absence of degeneration. B shows a comparable 

 section from the ventral horn contralateral to the lesion (region 

 B in Cs). Degenerating terminals are abundant, and many 

 appear to end on motoneurons (note cell on extreme right). 

 (Courtesy of Orville Smith and June DeVito.) 



degeneration (as revealed by the Nauta technique) 

 in the monkey cervical spinal cord following a lesion 

 of the arm motor area. Degenerating terminals on and 

 around motoneurons at Cs are shown clearly in the 

 photomicrograph on the lower right (B). In agree- 

 ment with this anatomical evidence, Bernhard and 

 others (8, 9, 11, 19) found facilitation of monosynaptic 

 discharges occurred after arrival of a single cortically- 

 induced pyramidal volley with a latency too Ijrief to 

 permit interneuronal relay. However, monosynaptic 

 discharge of motoneurons occurred only after repeti- 

 tive cortical stimulation, and during prolonged repeti- 



