Quantitative synaptology of functionally different types of cat medial gastrocnemius alpha-motoneurons

The aim of this ultrastructural investigation was to study quantitatively the synaptology of the cell bodies and dendrites of cat medial gastrocnemius (MG) alpha-motoneurons of functionally different types. In electrophysiologically classified and intracellularly HRP-labelled MG alpha-motoneurons of the FF (fast twitch, fatigable), FR (fast twitch, fatigue resistant) and S (slow twitch, very fatigue resistant) types, the synaptic covering of the soma as well as that of dendritic segments located within 100 microns and at 300, 700, and 1,000 microns distance, respectively from the soma, was analyzed. The synaptic boutons were classified into the L-(apposition length > 4 microns) and S-types (< 4 microns) with spherical synaptic vesicles, and the F-type with flat or pleomorphic synaptic vesicles. The length of apposition towards the motoneuron membrane was measured for each bouton profile. Approximately 1,000 boutons contacted the soma and a similar number of boutons contacted the proximal dendrites within 50 microns from the soma. The number of dendritic boutons was larger at the 300 microns distance than at the 100 and 700 microns distances. The three types of motoneurons showed similar values for percentage synaptic covering and synaptic packing density in the proximal dendrites, while in the most distal dendritic regions the S motoneurons had more than 50% higher values for percentage covering, packing density and total number of boutons. The S motoneurons also exhibited a larger preponderance of F-type boutons on the soma. The ratio between the F- and S-types of boutons decreased somatofugally along the dendrites in the type FF and FR motoneurons, while in the S motoneurons it remained fairly constant.