The cortical representation of sensory inputs arising from bone.

In the present study, we show that sensory information from bone reaches the discriminative areas of the somatosensory cortices by electrically stimulating the nerve to the cat humerus and recording evoked potentials on the surface of the primary (SI) and secondary (SII) somatosensory cortex. The SI focus was located over the rostral part of the postcruciate cortex, caudal to the lateral aspect of the cruciate sulcus. The SII focus was identified on the anterior ectosylvian gyrus, lateral to the suprasylvian sulcus. These foci were located adjacent to, or within areas that responded to stimulation of the median, ulnar and/or musculocutaneous nerves. The latency (6-11 ms) to onset of cortical responses in SI and SII were indistinguishable (unpaired t-test; P>0.05), and were consistent with activation of A delta fibers in the peripheral nerve. The amplitudes of the cortical responses were graded as a function of stimulus intensity, and may reflect a mechanism for intensity coding. We did not observe long latency cortical responses (50-300 ms) that would be consistent with C fiber activation in the peripheral nerve, and provide evidence that this may be attributable to inhibition of cortical responsiveness following the initial A delta response. Our finding of discrete, short latency evoked potentials (presumably of A delta origin) in the primary and secondary somatosensory cortices, following stimulation of a nerve innervating bone, may reflect a mechanism for the discriminative component of bone pain.