Mapping of spinal projection of primary nociceptive neurones in the rat.

The FRAP (fluoride-resistant acid phosphatase) reaction was used to draw up the map of the somatotopic organization of the primary nociceptive neurones of the forelimb in the rat spinal cord. The disappearance of FRAP activity from the central terminals of primary sensory neurones in the substantia gelatinosa was studied after transection of the brachial plexus and the median nerve. A map of the central terminals of FRAP-reactive primary sensory neurones was drawn up by the recording of enzyme depletion. After complete transection of the brachial plexus, partial depletion of FRAP was seen in the lower cervical and upper thoracic cord; the degree of FRAP disappearance was striking in segments C5, C6 and C7, and slight between segments C5 and C6. After transection of the median nerve, the FRAP reaction was observed to disappear completely in the medial portion, but remained over approximately one-quarter of the width in the middle of the Rolando substance. The enzyme activity remained in a narrow strip about 2 segments long. The maps resulting from microscopic and geometric analysis of serial sections showed that the terminals of FRAP-reactive primary nociceptive neurones of the forelimb projected mostly to the medial portion of lamina II in the rat spinal cord, from segment C4 to T3. The findings are discussed in relation to the somatotopic organization of primary afferents.