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 and cervical plexuses to the external cuneate nucleus of gerbils was compared following an intraneural injection of horseradish peroxidase into the musculocutaneous, median, ulnar and radial nerves of the brachial plexus or the main branches of the cervical plexus; 407 labelled primary afferent terminals from the brachial and 459 from the cervical plexus were studied. These boutons made synaptic contacts with 586 and 633 dendritic profiles, respectively. 99.0% of the primary afferent boutons from the brachial plexus contained clear round synaptic vesicles (R boutons); the remaining 1% of boutons contained pleomorphic synaptic vesicles (P boutons). For boutons from the cervical plexus, 95% were 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 microns 2, while the P boutonal profiles were of a small variety (range, 0.4-2.3 microns 2; mean, 1.5; S.D., 0.6 micron 2). The R boutons from the brachial plexus (mean, 3.9 microns; S.D., 2.1 microns 2) were generally larger than those from the cervical plexus (mean, 3.3 microns 2; S.D., 1.9 microns 2). On close 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 dendritic spines and 2.2% on proximal dendrites. For R boutons from the cervical 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 nerve plexuses may be related to their different perceptuomotor executions.