Projection of Ia and Ib afferents from forelimb muscles to the C3-C4 segments of the cat spinal cord.

Group Ia muscle spindle afferents were activated separately by small stretches applied to the tendons of antibrachial muscles in the forelimb in the cat. Group Ib tendon organ afferents were stimulated electrically after a selective increase of the threshold of the Ia afferents. Recordings of focal synaptic potentials were made in the C3-C4 segments in the medial part of the base of the dorsal horn. It has been found that both Ia and Ib afferents have monosynaptic connections with neurones in this medial region. Quantitatively, these two groups of afferents produced focal synaptic potentials of approximately the same size. The connections may be to inhibitory interneurones projecting to the C3-C4 propriospinal neurones, which are known to receive disynaptic IPSPs from group I muscle afferents.