ASPECTS OF INHIBITORY PATHWAYS AND SYNAPSES 35 



2. THE HISTOLOGICAL BASIS OF RENSHAW INHIBITION 



Initial motor axon collaterals in the spinal cord have been described by 



several authors in the last century. Nothing has been known until recently on 



their synaptic contacts. The findings of Eccles et al. (1954) seemed to offer an 



excellent opportunity for histological control and identification of inhibitory 



synapses, by the method of persisting elements. The results having been 



published (Szentagothai, 1958) ; a diagram suffices to summarize them (Fig. 7). 



The ventral quadrant of the spinal cord has been isolated at L? by sphtting 



the cord longitudinally for about 1 cm, crushing half of the cord at the upper 



and lower end of the incision, and finally removing the dorsal quadrant of the 



isolated half. The completely isolated ventral quadrant of the cord receives 



its blood supply from the vessels of the preserved anterior root. After two 



months only a small marginal part of the ventral horn remains intact, because 



of the destruction of the deep anterior fissure arteries, which supply the medial 



and central parts of the ventral column (Fig. 7, b). In this part of the grey 



matter only motoneurons and the supposed Renshaw cells are preserved, so 



that any intact synapse found must either belong to motoneuron initial 



collaterals or Renshaw cell axons. Not a single terminal knob has been found 



on several hundred well preserved motoneurons investigated. That terminal 



knobs of motoneurons may be well preserved in isolated spinal cord segments, 



emerges from other experiments in which the whole segment has been isolated 



by crushing the sacral part of the cord at two levels 5 mm apart and transecting 



the dorsal roots entering the isolated segment. In these preparations intact 



terminal knobs are seen frequently on motoneurons. Well preserved terminal 



knobs are found, however, in our isolated ventral horn preparations on smaller 



cells generally situated on the anterior border of the ventral horn (Fig. 7, c 



and d), where the motor axons begin to gather into bundles. These observations 



substantiate the suggestion of Eccles et al. (1954a) concerning the pathway 



of recurrent inhibition. Their evidence for the excitatory nature of these 



synapses of initial motoneuron collaterals on Renshaw cells, gives weight to 



the argument that terminal knobs are generally excitatory. We of course 



looked most thoroughly for intact synapses on motoneurons, since these 



would have to carry inhibition from Renshaw cells. Unfortunately we could 



find nothing apart from an extremely fine meshwork of nerve fibers, being 



apparently in close contact with motoneurons (Fig. 6). It could of course be 



argued that these are regenerative phenomena evoked by the degeneration 



of the majority of fibres enveloping the motoneuron. Considering the very 



fine cahbre of the fibres this explanation is not very probable; the abortive 



regeneration in the CNS always produces rather coarse fibres of varicose 



character.* Thus our results are not conclusive concerning the inhibitory 



* In Golgi preparations of isolated slabs of cerebral and cerebellar cortex, where arbori- 

 zation patterns of neurons are so characteristic and well known, no significant regeneration 

 has been observed (Szentagothai, unpublished). 



