Ia boutons to CCN neurones and motoneurones are enriched with glutamate-like immunoreactivity

The monosynaptic connection between muscle spindle Ia afferents and motoneurones is one of the most extensively studied reflex pathways in the central nervous system. Physiological evidence has pointed to glutamate as the fast transmitter in this synaptic connection. In this study of the spinal cord segment C2, we used dual labelling with cholera toxin subunit B-horseradish peroxidase (B-HRP) and postembedding immunohistochemistry to show that group Ia primary afferent boutons synapsing on retrogradely labelled central cervical nucleus (CCN) neurones and motoneurones are enriched in glutamate-like immunoreactivity (LI). All double-labelled Ia terminals studied could be classified as S-type (CCN neurones and motoneurones) or M-type (motoneurones) boutons.