Characteristics and Distribution of Spinal Focal Synaptic Potentials Generated by Group II Muscle Afferents

The location of the segmental relays of group II afferents in the spinal cord was investigated by recording extracellular “focal synaptic potentials” (FSP), generated by group II afferents in the gastrocnemius‐soleus nerve. Three types of group II FSPs were differentiated by their location and latency. (1) The “dorsal group II FSP” was located in the dorsal horn dorsolateral to the intermediate nucleus. Its short latency from the incoming group II volley (0.6‐1.0 ins; led to the conclusion that at least the onset is monosynaptically generated. (2) The “ventral group II FSP” was found between the intermediate nucleus and the motor nucleus. This type of FSP had a longer central latency ranging from 1.2‐2.5 ms, which made it difficult to decide whether the linkage was monosynaptic or disynaptic. However, when considering other results (Fu and Schomburg 1974) it seems probable that the ventral FSPs in the low range of central latencies were generated monosynaptically. (3) The “late group II FSP” had a long and varying central latency and was found over a wider region although less regularly than the other FSPs. Presumably the late FSP represents the postsynaptic effect in second and third order neurones. It seems likely that at least the main part of the investigated dorsal and ventral group II FSPs was generated by secondary afferents from muscle spindles.