Intracellular labeling of neurons in the medial accessory olive of the cat: III. Ultrastructure of axon hillock and initial segment and their GABAergic innervation.

The gamma-aminobutyric acid (GABA) synaptic input of identified axons in the cat inferior olive was studied by use of combination of intracellular labeling with horseradish peroxidase and postembedding gold-immunocytochemistry. With this technique olivary cells were physiologically identified and light microscopically reconstructed, and the horseradish peroxidase reaction product and the immunogold labeling were subsequently simultaneously visualized for electron microscopic investigation with the use of serial ultrathin sections. The axons of cell type I (characterized by dendrites which radiate away from the cell body) originated from the soma, whereas those of type II neurons (characterized by dendritic trees which curve back towards the soma) were derived from a primary dendrite. The axons of olivary neurons stand out by the length of their axon hillock (up to 21 microns) and initial segment (up to 40 microns). The hillock forms various spiny appendages which were located within glomeruli together with dendritic spines of other olivary neurons. Axonal spines of type II neurons were more numerous and complex looking than those of type I. The axonal spines, the shaft of the axon hillock, and the transition between the hillock and initial segment were primarily innervated by GABAergic terminals (65%) but non-GABAergic terminals (35%) were present as well. The terminals apposed to the axons of type I neurons contacted mainly the axonal shafts, whereas most of the terminals adjacent to the axons of type II neurons established synaptic contacts with the axonal spines. The initial segments were largely devoid of synaptic input. Distally, the initial segment acquired a myelin sheath.