Somato-sensory paths to the second cortical projection area of the group I muscle afferents.

1. Cats anaesthetized with chloralose and paralysed with Flaxedil were used. The projections of muscle, joint and skin afferents to the cortical fold hidden in the anterior suprasylvian sulcus were investigated with micro-electrode recording techniques.2. Electrical stimulation of Group I muscle afferents from the contralateral forelimb evoked a negative focal potential (latency 5 msec) in a locus of 1-2 mm diameter found in the lower bank of the fold. In one experiment a response to Group I muscle afferents from the contralateral hind limb was observed. The Group I potentials disappeared after sectioning of the dorsal columns at C3.3. Groups II and III muscle afferents, low threshold skin afferents and joint afferents also evoked potentials in the Group I locus. It was concluded that the joint afferents originated mainly in the Ruffini endings of the joint capsule.4. Groups II and III muscle afferents, low threshold skin and low threshold joint afferents projected to the upper bank of the suprasylvian fold. A certain somatotopic arrangement was observed.5. The possibility of connexions between the cortex of the anterior suprasylvian fold and the primary somato-sensory projection areas was discussed, as well as the organization of the loci in the fold in terms of cell colonies with different properties.