ASPECTS OF INHIBITORY PATHWAYS AND SYNAPSES 41 



(Fig. 10, b), when the degenerated fragments of primary sensory neurons can be 

 followed for hundreds of microns surrounding the same Clarke neuron 

 dendrite. The second type are terminal knobs of ordinary size found intact one 

 month after complete de-afferentation of the lower cord (Fig. 10, c), and 

 degenerating after lesions of the lateral funiculus or the grey matter somewhat 

 below their own level. These therefore can be considered as of intraspinal 

 origin. The third type are extremely fine fibres with strange coils and very 

 often closely attached to Clarke neurons, also persisting after complete 

 de-afferentation of the lower cord (Fig. 10, d). Details of contact with Clarke 

 neurons are beyond the power of the light microscope. Since besides the well 

 known direct synaptic action of primary, mostly muscular afferents, two types 

 of interneuronal influence impinge upon Clarke neurons, the synapses of 

 different types can be allocated to the different functions with some proba- 

 bility. The bouton-type apparently belongs to excitatory interneurons as 

 postulated by Holmquist et al. (1956) while the fine "coiled" fibre system in 

 analogy to our findings on motoneurons may be considered as the histological 

 basis of inhibitory action as first observed by Laporte et al. (1956) and shown 

 by Curtis et al. (1958) to be mediated by short interneurons. 



5. INHIBITION IN "MODEL NERVOUS SYSTEMS" 



Comparison between function and histological structure is especially 

 germane in simplified models produced by artificial recombination of tissues. 

 Completely isolated model nervous systems can easily be obtained by an 

 ingenious method described by Weiss (1950a) but not exploited so far in this 



Fig. 10. A. Diagram illustrating the tliree types of synapses identified histo- 

 logically in Clarke's column. The main synaptic system are the "giant synapses'" 

 derived from muscle afferents (M aff 1), which establish extremely long parallel 

 contacts with Clarke (CL) neuron dendrites and have very large boutons terminaux 

 occasionally. Excitatory interneurons (Ei) primarily influenced by cutaneous 

 afferents (C aff) establish synapses with Clarke neurons by means of ordinary 

 end-feet. Inhibitory interneurons (li) under synaptic influence from other 

 muscle aflerents (M aff 2) may get into synaptic contact with Clarke neurons by 

 means of a meshwork of extremely fine fibres. The microphotographs on right 

 side are giving the histological proofs of this concept, b. Clarke neuron dendrite 

 (in L3 level) accompanied for hundreds of microns by degenerated fragments of 

 the large parallel contact (giant) synapse, 5 days after transection of dorsal root 

 Z.7. Nauta method, c. Intact terminal knob on Clarke neuron (L3 level) one month 

 after complete de-afferentation of the lower cord (from Thio downwards) by 

 extirpation of all spinal ganglia in the upper and transection of dorsal roots 

 in the lower segments of this part. d. Intact meshwork (arrow) of extremely fine 

 fibres after de-afferentation of the ipsiiateral side of the cord (as in case c). 

 Bodian protargol method. 3000. 



