SPINAL MECHANISMS INVOLVED LN SOMATIC ACTIVITIES 



935 



FIG. 6. The 'presynaptic potential' recorded by means of an 

 extracellular microelectrode. The initial deflection can be 

 assigned to primary afferent fibers that pass by the electrode 

 location. The prolonged negative deflection that follows repre- 

 sents an enduring change in the presynaptic endings of primary 

 afferent fibers resulting from a single shock stimulus to Group I 

 afferent libers. 



motor nucleus and presented in figure 6. For a 

 variety of reasons, based on quantitative considera- 

 tions, the potential illustrated must be considered an 

 active presynaptic response rather than a passive 

 and subliminal response of the secondary neurons. 

 Suffice it to say that the properties of the primary 

 afferent fibers change as they enter the cord and 

 approach the terminal point at synapsis. 



Circiimscrihrd and Diffuse Mechanisms nf Ramon y Ca/al 



One of the great generalizations concerning the 

 functional organization of the spinal cord emerged 

 as the culmination of Ramon y Cajal's study of the 

 fine structure {8i ). It is presented in figure 7 on the 

 left of which one finds his 'circumscribed' reflex 

 mechanism in which afferent collaterals articulate 

 directly with motoneurons in a restricted region to 

 form a monosynaptic reflex arc. To the right is 

 represented the 'diffuse' reflex mechanism in which 

 interneurons are intercalated between afferent fibers 

 and motoneurons. These interneurons were regarded 

 as a means for diffusing activity over a wider field up 

 and down the spinal cord. 



Divergence and Convergence 



It is probably true that the axons of most neurons 

 in the central nervous system branch a number of 

 times to form synaptic connection with a number of 

 other neurons, as exemplified by the interneuron 

 illustrated in the diffuse mechanism of Ramon y 

 Cajal (fig. 7). This divergence is a feature in the 

 functional organization of the spinal cord. Likewise 

 one may make the generalization that many neurons 

 receive, by convergence, synaptic connections from 

 a variety of other neurons. It is immediately obvious 



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— ^ 



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— -^ ^ 



FIG. 7. Ramon y Cajal's diagram of the circumscribed reflex 

 mechanism {lefl) showing the monosynaptic connection between 

 the primary afferent libers and motoneurons, and of the diffuse 

 reflex mechanism iright] in which an interneuron is intercalated 

 between afferent fibers and motoneurons. [From Ramon y 

 Cajal (81).] 



that convergence is the necessary condition for 

 integrative action in the nervous system. This 

 truism in itself is cause for examining a little more 

 closely the generality of convergence. One will 

 grant convergence as a prime structural factor in 

 organization of the motor systems, with the moto- 

 neuron, the final common path of Sherrington (90), 

 the paradigm of convergent foci. But afferent sys- 

 tems that have been studied reveal a synaptic 

 organization rather different from that of the motor 

 systems, specifically that at the motoneuron. Output 

 to the spinocerebellar tract from Clarke's nucleus 

 is linearly related to input (66) and there is, among 

 Clarke column neurons, no subliminal fringe (77). 

 Except for the fact that output to a single synchro- 

 nized input volley is, in Clarke's column and also 

 in the dorsal column nuclei (3, 87), repetitive, and 

 for the fact that some time is lost in relay, there is 

 virtually nothing to signify that synaptic relay takes 

 place in these nuclei. Naturally none of the fore- 

 going proves aljsence of convergence; in fact, it 

 might indicate the exact antithesis. The differences 

 between certain motor and afferent relays nonethe- 

 less are worth pondering. 



