The ultrastructure of group Ia afferent fiber synapses in the lumbosacral spinal cord of the cat.

Ia synapses in laminae VI and IX of the cat's spinal cord were examined in the electron microscope following iontophoretic injection of horseradish peroxidase (HRP) into single, identified, Ia afferent fibers from gastrocnemius muscles. Ia boutons contacting motoneuron dendrites in lamina IX contained spherical synaptic vesicles and generally contacted only one postsynaptic profile. The Ia boutons were often postsynaptic to smaller P-type axonal terminals. Consequently Ia boutons may be classified as S-boutons with axo-axonic contacts.