An ultrastructural study of the synaptology of gamma-motoneurones during the postnatal development in the cat

The postnatal development of cat triceps surae γ-motoneurones, retrogradely labelled with horseradish peroxidase (HRP), was studied light and electron microscopically. The mean diameter of the cell bodies of the γ-motoneurones increased by about 25% from birth to the adult stage, which was much less than the increase in cell body diameter of α-motoneurones (about 45%). Throughout development the only bouton types apposing the γ-motoneurones were the F- and S-types, with flattened and spherical synaptic vesicles, respectively. Thus, the C-, M- and T-types of boutons seen on α-motoneurones were completely lacking. The size of the boutons on the γ-motoneurones increased much less than what has been found for boutons on α-motoneurones. The number of boutons on the γ-motoneurone cell bodies seemed to decrease postnatally. This decrease was only moderate for S-type boutons but substantial for F-type boutons. In contrast, the number of boutons on the proximal dendrites appeared to increase and this was most evident for S-type boutons. The mentioned postnatal changes in synaptology were more differentiated with regard to bouton type and part of the neurones under study than what could be inferred from earlier studies on the postnatal development of α-motoneurones. These changes also occurred later than in α-motoneurones. The relative dominance of F-type boutons with probable inhibitory actions on the immature γ-motoneurone may explain the previously demonstrated poor encoding of muscle length by muscle spindles during the first postnatal weeks in the kitten.