Complex synapses in the medial cuneate nucleus of the cat.

An electronmicroscopical study using an analysis of thin sections and experimental degeneration of the medial cuneate nucleus of the cat has revealed five types of axon terminals: (1) Relatively large axon terminals characterized by a population of large (45-60 nm) spheroid vesicles which are the terminals of the spinal afferents: (2) Small axon terminals - probably of local interneuronal source - containing characteristic flattened vesicles: (3) Small axon terminals, mostly of local source (probably relay cell axon collaterals or possibly interneurons) containing small spheroid vesicles. A few axon terminals with similar small spheroid vesicles are of (contralateral) cortical origin from the somatosensory area; (4) Medium size axon terminals containing pleomorphic (ovoid to round) vesicles and being of local source (probably from local or neighbouring interneurons); (5) Occasionally axon terminals containing large dense-core vesicles (100-120 nm) were also observed. While types (1), (2), (3) and (4) are presynaptic to dendrites and (less frequently) to cell bodies, type (2), (3) and (4) were observed to be presynaptic also (axo-axonally) to type (1). Section series revealed that such axo-axonal synapses are in fact always arranged in "triadic combination" having the general formula of: 1 leads to dendrite (or soma), 2 (or 3, or 4) leads to 1, and 2 (or 3 or 4) leads to dendrite (or soma); the arrow indicating the morphological polarity of the synapse. -Presynaptic dendrites forming dendritic synapses and occasionally engaged in reciprocal synapses were also found, but in view of their rarity are probably only of secondary significance.