An ultrastructural study of cat lumbosacral gamma-motoneurons after retrograde labelling with horseradish peroxidase.

Twelve retrogradely horseradish peroxidase (HRP)-labelled triceps surae motoneurons of gamma size (mean cell body diameter less than 38 micron) were studied ultrastructurally. The contours of the cell bodies, as observed in the transverse midnucleolus plane, were elongated to rounded. The axons identified all originated from the cell body. The mean diameter of the stem dendrites was 4.5 micron. A substantial part of the cell membrane was covered by glial extensions. The boutons and synaptic contacts apposing the gamma-motoneurons could be classified into two categories on the basis of the type of synaptic vesicles: S-type boutons with spherical synaptic vesicles and F-type boutons with flattened vesicles. In each neuron, the values for mean length and mean area of apposition, percentage synaptic covering, and packing density of S-type, F-type, and S+F-type boutons were estimated on the cell body and in two dendritic compartments. In comparison with alpha-motoneurons and Renshaw cells, the cell bodies of the gamma-motoneurons were covered by smaller and strikingly fewer boutons of both the S- and F-types. The values for percentage synaptic covering and packing density of boutons on the proximal dendrites were also lower for gamma-motoneurons than for both alpha-motoneurons and Renshaw cells, although the differences were less pronounced than on the cell body. No boutons of the C-, M-, and T-types described for alpha-motoneurons were found on the gamma-motoneurons.