856 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



A delta, or Group III, size (less than 4 /i in diameter). 

 Only 1.75 per cent of the myelinated fibers are in the 

 group I diameter range (i 1-22 ix). 



Conduction velocities of pyramidal tract fibers were 

 first determined by Lloyd (72) in a study which is 

 noteworthy because it is one of the few in which strict 

 attention was given to eliminating extrapyramidal 

 fibers; recordings were made in the lateral column of 

 the cord of decerebrate cats following stimulation of 

 the bulbar pyramid above a section transecting all of 

 the bulb but the pyramids. Maximal conduction 

 velocities were 60 to 65 m per sec, and although mini- 

 mal velocities were uncertain, elements conducting at 

 rates as low as 18 m per sec. were detected. Brookhart 

 & Morris (20) observed two components in the anti- 

 dromic complex recorded in the bulbar pyramids 

 following shocks to the lateral cokimn of the cord, one 

 conducting at maximal rates of 100 to 160 m per sec. 

 and the other at 45 to 55 m per sec. It seems likely 

 that the rapidly-conducting component represents 

 activity in other (perhaps reticulospinal) systems. In 

 one monkey, Patton & Amassian (80) recorded D 

 waves conducted over 14 cm from cervical to lumbar 

 cord at 52 m per sec. Bernhard et al. (10) found maxi- 

 mal velocities of 60 to 70 m per sec. in the monkey. 

 These values are consistent with the anatomically 

 determined fiber spectrum of the pyramid. 



Conduction velocities computed for the cortex-to- 

 bulb portion of the pyramid are often higher than 

 those found for the bulb-to-cord portion. Thus, Wool- 

 sey & Chang (112) estimated that antidromic im- 

 pulses traversed the distance from bulb to cortex at 

 speeds ranging from 100 down to 1.8 m per sec. 

 Patton & Amassian (80) sometimes found D-wave 

 conduction times from cortex to bulb suggesting maxi- 

 mal velocities up to 80 to 100 m per sec. Bertrand (12) 

 found D-wave velocities of 89 m per sec. from supple- 

 mentary motor cortex to cervical cord. It is uncertain 

 whether the high computed velocities in these experi- 

 ments result from inaccurate estimation of conduction 

 distance, or whether they reflect the true mean veloc- 

 ity of the upper corticospinal fibers before they have 

 become attenuated in diameter by collateral branch- 

 ing in the pons and midbrain. Single unit recording of 

 antidromic spikes in Betz cells give values closer to 

 those found for the bulb-to-cord portion of the tract. 

 Patton & Towe (unpublished observations) found 

 units conducting from 64.0 to 5.6 m per sec. (assuming 

 conduction distance of 43 mm), and Phillips (85), 

 using intracellular recording which probably prefer- 

 entially selects large cells with large axons, found 

 values from 78 to 18 m per sec. 



Bishop ('/ al. (13) recorded from the surface of the 



pyramid a bimodal compound action potential follow- 

 ing a shock to the pyramid at the level of the trapezoid 

 body. We have confirmed their findings; in our experi- 

 ments, the two deflections have velocities of 59 m per 

 sec. and 14 m per sec, and clearly represent two fiber 

 groups, for their refractory periods are independent 

 of one another and the threshold for the fast group is 

 lower than that for the slow group. Whether both 

 groups traverse the pyramids is not clear. According 

 to Bishop and his co-workers, the excised pyramidal 

 tract shows a similar compound action potential, but 

 it may be questioned whether pyramidal fibers can 

 truly be separated from lemniscal and other fibers by 

 this procedure. Following contralateral pyramidal 

 stimulation, both waves may be recorded from the 

 lateral surface of the cord as far down as the third 

 lumbar segment, and cord responses are abolished by 

 pyramidal section below the point of stimulation (49). 



AFFERENT FIBERS IN PYRAMIDS 



The bulbar pyramid has long ijeen considered a 

 purely descending bundle. This view is challenged by 

 Brodal & Walberg (17) who saw degeneration in the 

 pyramid (prepared with Marchi and Glees silver 

 stains) following lesions of the lateral or ventral spinal 

 cord and in the dorsal column nuclei of cats. Some 

 of the ascending fibers are said to arise from cord seg- 

 ments as far caudal as the fifth lumbar segment (but 

 mostly from cervical segments) and to join the pyra- 

 mids at the decussation where some cross and others 

 continue on the same side through the bulbar pyra- 

 mids, the peduncles, and the internal capsule to the 

 sensory and motor area of the cortex. .A a;reater num- 

 ber of fibers are said to arise from the dorsal column 

 nuclei (particularly the cuneate nucleus) and to follow 

 a course similar to that of the fibers arising from 

 spinal segments. Both groups of fibers give off collat- 

 erals to the pontine nuclei. A similar system of ascend- 

 ing fibers has been described in the pyramid of man 

 (78). The numbers of ascending fibers are small (only 

 about 4 per cent of the total pyramidal population). 

 Perhaps this accounts for the fact that no clear-cut 

 evidence of their presence can i)e found in- electrical 

 recording methods. The afferent acti\ily recorded by 

 Brodal & Kaada (16), and ascribed by them to as- 

 cending fibers in the pyramid, is clearly ascribable to 

 volume pickup from the medial lemniscus (see figs. 19, 

 20), as can be shown by careful histological verifica- 

 tion of recording sites and differential recordings (54, 

 82). 



