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. 1983 Apr;337:373–388.1. doi: 10.1113/jphysiol.1983.sp014629

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

A H Pullen 1, T A Sears 1
PMCID: PMC1199112  PMID: 6875937

Abstract

1. Quantitative ultrastructural examinations of axon terminals synapsing with normal α-motoneurones in segment T9 of cat spinal cord provided estimates of their numbers, sizes and synaptic structure. One synapse, the C type, derived from short-axon propriospinal segmental interneurones, was studied in detail.

2. 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 spinal hemisection at T5 and T10 between 7 days and 2 years previously.

3. The proportion of C-type synapses present progessively increased from 1% in normal cats to 8·8% 200 days following hemisection, and had still attained a level of 3·1% by 2 years; these increases imply that the absolute number of C-type synapses underwent increase.

4. Mean sizes of C-type synapses increased from 4·0 μm (normal) to 5·8 μm (200 days) and retained their enlarged sizes up to 2 years (5·9 μm). Furthermore, while 84% of C-type synapses were under 6 μm in length in normal motoneurones, 48% were over 6 μm long 200 days post-operatively.

5. The unique post-synaptic structure of C-type synapses also proliferated following partial central deafferentation of the motoneurones. 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 motoneurone.

6. 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.

7. The observed greater increase in frequency of ribosomes `bound' to the rough endoplasmic reticulum, together with an over-all proliferation of this structure, specificially indicated an increased capacity for synthesis of protein for utilization in sites remote from those of synthesis (e.g. a trans-synaptic passage of protein).

8. A hypothesis is advanced on the basis of the above results relating both pre- and post-synaptic changes in structure to an increased functional activation of the segmental short-axon propriospinal interneurones forming the C-type synapses, as a compensatory response to partial central deafferentation of spinal motoneurones.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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