A COMPARATIVE ULTRASTRUCTURAL-STUDY OF PRIMARY AFFERENTS FROM THE BRACHIAL AND CERVICAL PLEXUSES TO THE EXTERNAL CUNEATE NUCLEUS OF GERBILS

The synaptic organisation of the primary afferents from the brachial a nd cervical plexuses to the external cuneate nucleus of gerbils was co mpared following an intraneural injection of horseradish peroxidase in to the musculocutaneous, median, ulnar and radial nerves of the brachi al plexus or the main branches of the cervical plexus; 407 labelled pr imary afferent terminals from the brachial and 459 from the cervical p lexus were studied. These boutons made synaptic contacts with 586 and 633 dendritic profiles, respectively. 99.0% of the primary afferent bo utons from the brachial plexus contained clear round synaptic vesicles (R boutons); the remaining 1% of boutons contained pleomorphic synapt ic vesicles (P boutons). For boutons from the cervical plexus, 95% wer e R boutons and 5% were P boutons. The labelled R bouton profiles had a wide range of cross-sectional area from 0.4 to 13.1 mu m(2), while t he P boutonal profiles were of a small variety (range, 0.4-2.3 mu m(2) ; mean 1.5; S.D., 0.6 mu m(2)). The R boutons from the brachial plexus (mean 3.9 mu m; S.D., 2.1 mu m(2)) were generally larger than those f rom the cervical plexus (mean 3.3 mu m(2); S.D., 1.9 mu m(2)). On clos e analysis, 72.4% of R boutons from the brachial plexus were found to synapse on distal dendrites, 15.9% on secondary dendrites, 9.5% on den dritic spines and 2.2% on proximal dendrites. For R boutons from the c ervical plexus, 81.1% synapsed on distal dendrites, 12.1% on dendritic spines and 6.8% on secondary dendrites; none was observed on proximal dendrites. Such a different synaptic organisation between the two ner ve plexuses may be related to their different perceptuomotor execution s.