Trophism between C-type axon terminals and thoracic motoneurones in the cat

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Abstract
Quantitative ultrastructural examinations of axon terminals synapsing with normal .alpha.-motoneurons in segment T9 [thoracic] of cat spinal cord provided estimates of their numbers, sizes and synaptic structure. One synapse, the C type, derived form short-axon propriospinal segmental interneurons, was studied in detail. The numbers, sizes and post-synaptic structure of normal C-type synapses at T9 were compared with similar estimates from material provided by cats subjected to partial central deafferentation by double pinal hemisection at T5 and T10 between 7 days and 2 yr previously. The proportion of C-type synapses present progressively increased form 1% in normal cats to 8.8% 200 days following hemisection, and had still attained a level of 3.1% by 2 yr; these increases imply that the absolute number of C-type synapses underwent increase. Mean sizes of C-type synapses increased from 4.0 .mu.m (normal) to 5.8 .mu.m (200 days) and retained their enlarged sizes up to 2 yr (5.9 .mu.m). While 84% of C-type synapses were < 6 .mu. in length in normal motoneurons, 48% were > 6 .mu.m long 200 days post-operatively. The unique post-synaptic structure of C-type synapses also proliferated following partial central deafferentation of the motoneurons. The elongated cistern, increased numbers and individual lengths of lamellae of the associated underlying rough endoplasmic reticulum indicated a trophic interaction between the presynaptic C terminal and its post-synaptic motoneuron. Counts of ribosomes bound to lamellae of the subsynaptic rough endoplasmic reticulum, and of the lamellae-associated polyribosomes interposed between individual lamellae for normal and 200 day post-operative C-type synapses indicated an over-all post-operative increase in capacity for local subsynaptic protein synthesis topographically directed towards this type of axon terminal. The observed greater increase in frequency of ribosomes bound to the rough endoplasmic reticulum, together with an over-all proliferation of this structure, specifically indicated in increased capacity for synthesis of protein for utilization in sites remote from those of synthesis (e.g. a trans-synaptic passage of protein). A hypothesis is advanced on the basis of these results relating both pre- and post-synaptic changes in structure to an increased functional activation of the segmental short-axon propriospinal interneurons forming the C-type synapses, as a compensatory response to partial central deafferentation of spinal motoneurons.