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Trophism between C‐type axon terminals and thoracic motoneurones in the cat
Author(s) -
Pullen A. H.,
Sears T. A.
Publication year - 1983
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1983.sp014629
Subject(s) - axon , endoplasmic reticulum , synapse , cats , ultrastructure , anatomy , biology , neuroscience , spinal cord , axon terminal , chemistry , biophysics , microbiology and biotechnology , medicine
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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