ASPECTS OF INHIBITORY PATHWAYS AND SYNAPSES 33 



hypothesis, it should be lacking in preparations with a haphazard arrangement 

 of nervous elements. If on the contrary inhibition were brought about by 

 chemical transmission from certain neurons with a specific metabolic set-up, 

 inhibitory activity could be expected to persist in such models. 



1. DIRECT INHIBITION OF MOTONEURONS BY 

 PRIMARY AFFERENTS 



The existence of a direct synaptic contact between primary sensory neurons 

 and motoneurons has been known from the early classical descriptions of 

 spinal cord structure. Their existence could be ascertained by secondary 

 degeneration of synapses. The fact postulated by physiological evidence, that 

 only muscle spindle afferents have direct monosynaptic contact with moto- 

 neurons, could be verified histologically in the trigeminal system, where the 

 perikaria of muscular afferents are situated outside the Gasserian ganglion 

 within the mesencephalic tract of the trigeminus (Szentagothai, 1948). 

 Earlier it has generally been supposed that the annulospiral afferents of 

 muscle may have inhibitory endings on motoneurons (Lloyd, 1946; Laporte 

 and Lloyd, 1952; Brock et al., 1952). More recent investigations with intra- 

 cellular recording, howe\er, have led to the assumption that an inhibitory 

 interneuron — probably situated in the intermediate nucleus of Cajal — must 

 be intercalated in the pathway (Eccles et al., 1954). Degeneration of synapses 

 after transection of lower lumbar dorsal roots has given conflicting results 

 (Schimert, 1939; Sprague, 1956; Szentagothai, 1958) probably because of 

 relatively long descending excitatory collaterals of la afferents (Eccles et al., 

 1957). On the whole, however, histological evidence is against assumption of 

 direct inhibitory collaterals to motoneurons of the lumbosacral enlargement 

 of the cord (Szentagothai, 1958). 



More recently Lloyd and Wilson (1959) claim direct inhibitory connexions 

 of primary afferents to contralateral motoneurons in S3. This has been 

 investigated in our laboratory after transection of the dorsal root S3. Direct 

 dorsal root collaterals crossing the midline are rather rare in other segments 

 (Schimert, 1939), their number is considerable, however, in the small sacral 

 segments. They cross in both commissures, most of them in the middle 

 bundle (faisceau moyen of Cajal) of the dorsal grey commissure, but a fair 

 number is seen also in the anterior white commissure. Most of these collaterals 

 end with terminal knobs on smaller medial cells of the ventral horn. Very few 

 fragments of extremely fine collaterals can be traced into the region of contra- 

 lateral motoneurons, but no degenerating terminal knobs have been found 

 in immediate neighbourhood of motoneurons (Figs. 3 and 4). The difference 

 between the ipsilateral and contralateral side is spectacular (Figs. 1-3). In the 

 S2 segment there are a few direct sensory neuron collaterals to be traced to 



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