SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 



947 



hibited; they conclude from this that the reflex action 

 represented is nociceptive. Pain being heavily repre- 

 sented in the C fiber bands in mammals (28), it is a 

 fair assumption that the reflex in mammals too is 

 nociceptive in nature. 



SPINAL ORGANIZATION FOR REFLEX CONTROL 

 OF MIDLINE STRUCTURES 



Most of that which is known of spinal refle.x organi- 

 zation has to do with those regions innervating the 

 limbs. At the caudal end of the spinal cord, specifically 

 the last sacral and caudal segments, reflex control is 

 concerned with mid-line structures conspicuous 

 among which, in the cat, is the tail. The terminal 

 segments of the spinal cord have a structure differing 

 from that characteristic of the enlargements (81 ), the 

 principle distinguishing features being the presence of 

 a large dorsal mid-line nucleus, or 'broadening of the 

 dorsal gray commissure' (83) and the decussation of 

 primary afferent fibers (97). 



The terminal segments display special organization 

 from a functional point of view to match the structural 

 specialization (98). Some aspects of this are illustrated 

 in figures 26 and 27. Records A, B and C of figure 26 

 show the ipsilateral result, recorded on the third sacral 

 ventral root, of stimulating the third sacral dorsal 

 root. At the weakest strength A a monosynaptic post- 

 synaptic potential is recorded. Stronger stimulation, 

 still at Group I strength, brings out a monosynaptic 

 reflex discharge B. Still stronger stimulation evokes, 

 in addition to the monosynaptic reflex which in this 

 experiment is further augmented, a prominent di- 

 synaptic reflex C which is nearly as synchronous in 

 character as is the monosynaptic reflex. Only at this 

 last strength of stimulation does any change occur in 

 consequence of stimulating the contralateral root. It 

 consists of a postsynaptic potential of latency 0.8 to 

 i.o msec, longer than that recorded following ipsi- 

 lateral stimulation D, from which fact a disynaptic 

 pathway is postulated. Conjoint stimulation of con- 

 tralateral dorsal root at the strength employed for 

 record D and of ipsilateral dorsal root at a strength 

 that evokes a monosynaptic refle.x record E, the stim- 

 uli being synchronous, produces the result recorded in 

 record F. The monosynaptic reflex usually is inhiijited, 

 and by a disynaptic convergence a large disynaptic 

 reflex is realized. There is, therefore, a convergence 

 upon the motoneurons of ipsilateral excitatory and 

 contralateral inhibitory connections from Group I 

 primary afferent fibers. Also there is an internuncial 

 nucleus that receives excitatorv connections from 



FIG. 26. Ipsilateral 

 and contralateral post- 

 synaptic potentials and 

 reflex discharges in the 

 third sacral segment. 

 Complete description in 

 text. [From Wilson & 

 Lloyd (98).] 



primary aff'erent fibers of both sides and which in 

 turn relays to the motoneurons. There is as yet no in- 

 formation as to the location of this internuncial nu- 

 cleus, but it is not unlikely that the dorsal mid-line 

 nucleus, peculiar to the region, is responsible for the 

 powerful bilateral disynaptic relay which likewise is 

 peculiar to the region. 



The third sacral segment provides a unique svstem 

 for determining unequivocally the nature of the direct 

 inhibitory pathway between primary aflferent fibers 

 and motoneurons. This type of connection was origi- 

 nally described (53, 57) as being monosynaptic, a view 

 that has been attacked vigorously (22) and the pres- 

 ence of an 'inhibitory interneuron' postulated in 

 order to simplify the nature of a chemical hypothesis 

 of excitation and inhibition. Conclusion as to the 

 presence or absence of this postulated interneuron 

 depends upon interpretation of the time relations 

 between conditioning inhibitory activity and testing 

 excitatory activity, or upon latencies for production 

 in motoneurons of excitatory and inhibitory post- 

 synaptic potentials. It is evident that most of the sys- 

 tems for study of inhibitory action in inonosynaptic 

 reflexes are not satisfactory, with uncertainties as to 

 longitudinal conduction in the cord and, indeed, as 

 to what to measure being difficulties. The bilateral 

 system of the third sacral segment obviates all these 

 difficulties for one can dispense with all questions of 

 conduction in the pathways to the motoneurons. To 

 achieve this, it is necessary only to observe the mono- 

 synaptic reflexes of both sides simultaneously and to 

 vary the relation between the afferent volleys in such 



