Functional properties of neurons in the cat gracile nucleus that project to the dorsal accessory olive

This study evaluated the functional properties of neurons in the gracile nucleus that project to the dorsal accessory portion of the inferior olive (DAO) and compared these with properties of other efferents from the dorsal column nuclei (DCN). Projection neurons were identified in anesthetized cats by microstimulation within DAO. They were further tested to insure that they could not be antidromically activated by stimulation in the medial lemniscus. Forty percent of the DAO projection neurons failed to respond to any form of natural stimulation. Of the 60% that did, most: 1) required stimuli of intensities greater than those needed to activate peripheral mechanoreceptors but less than those needed to excite peripheral nociceptors; 2) had receptive fields on the contralateral hindlimb or tail; and 3) could not follow peripheral stimuli presented at rates greater than 20 Hz. The conduction velocities of DAO projection neurons averaged only 4 m/s. In contrast, most of the neurons with axons in the medial lemniscus responded to very light cutaneous stimuli and followed peripheral stimuli presented at rates up to 100 Hz. Their average conduction velocity was 14 m/s. The low conduction velocity of the DAO projection neurons is unusual for DCN but is consistent with published descriptions of DAO. The limited responsiveness of the DAO projection neurons is, however, not uncharacteristic of DCN but places this projection at one end of the spectrum of DCN efferents, with the diencephalic projection at the other end. It is proposed that the DCN provide a filtered (and possibly movement-modulated) signal to the inferior olive for use in the control of movement.