934 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



In mammals, l)ut not in tlic fros; (44), the two 

 groups of motor fibers are distinct in function as in 

 caliber. The large fibers are motor fibers to skeletal 

 muscle; the small fibers supply the intrafusal fibers 

 of muscle spindles and by their action provide 

 central control over muscle spindle afTerent re- 

 sponse, seemingly to adjust for the clifiTering circum- 

 stances of tension under which the spindle must 

 operate (44). 



SPINAL LIAISON BETWEEN AFFERENT 

 AND MOTOR PATHWAYS 



Afferent fibers enter into the spinal cord and motor 

 fibers depart therefrom. Between them they account 

 for little of the bulk of neural tissue contained within 

 the spinal cord. The vastly greater remainder con- 

 sists of interneurons. Concerning the manner in 

 which all these elements are organized, one can 

 speak for the most part only in generalities, there 

 being little known, with one notable exception, of 

 precise linkages that may be peculiar to any particu- 

 lar reflex mechanism. 



Distribution and Properties of Afferent 

 Collaterals in the Spinal Cord 



In figure 5 are to be seen the principle projections 

 in the spinal cord of the primary afferent fibers. 

 Those labeled C and c are directed into the nucleus 

 proprius of the dorsal horn and the substantia 

 gelatinosa Rolandi. These probably are concerned 

 largely with the relay of actixity to the great as- 

 cending tracts, although a distinct possibility exists 

 that the latter nucleus is concerned with the relay 

 of C fiber reflexes. The long collaterals a extend to 

 the ventral horn, giving off in passing a few colla- 

 terals h to the intermediate nucleus. In the ventral 

 horn at B these long collaterals have been shown to 

 establish synaptic connection with inotoneurons. 

 Coming from the deeper aspect of the dorsal white 

 column are dense bimdles of collaterals A directed 

 to the intermediate nucleus. It is evident that the 

 intermediate nucleus is of great importance in reflex 

 transmission, but the precise role it plays has been 

 subject to varied opinion and can be regarded as 

 controversial. 



Froin a functional point of view the central projec- 

 tions of aflferent fibers are considered to have some 

 properties that differ from those of the parent fibers 

 from which they arise. Rudin & Eisenmann (85) have 







mm 





FIG. 5. Cross section of the spinal cord to illustrate the dis- 

 tribution of primary afferent fibers throughout the gray sub- 

 stance of the spinal cord. [From Ramon y Cajal (81 ).] 



shown that the after-potential system of the fibers 

 changes aljruptly at the dorsal root-cord junction, 

 the intramedullary segment displaying a large nega- 

 tive after-potential. As soon as the afferent fibers 

 enter the spinal cord thev branch and as branching 

 takes place conduction velocitv decreases. Precise 

 measurements of intramedullary \elocities are avail- 

 able only for some of the projections in the dorsal 

 columns (66). In the much shorter collaterals into 

 the gray substance, the decrease is dramatic and 

 proijably sufficient to account for most of the formal 

 synaptic delay (56). What happens as impulses 

 penetrate into the terminal regions of the collaterals 

 is controversial. The notion that activitv in those 

 regions has an enduring qualit)- unlike that of 

 impulses in the parent fibers was brought to the fore 

 by the experiments of Barron & Matthews (6). The 

 alternative \'iew that action in the terminals is as 

 brief as peripheral axon spikes has been maintained 

 vigorously (12, 21). Lloyd c& Mclntyre (65) showed 

 that an enduring sink of current flow must exist in 

 the terminal collaterals and that it would have the 

 character of the potential change recordable in 



