Tail-flick related activity in medullospinal neurons

Using the classification system of Fields et al. 131 neurons in the rostral ventromedial medulla (RVM) of lightly anesthetized rats were divided into 3 groups according to their response during tail-flick (TF) testing: those with an abrupt increase in activity prior to TF (on-cells); those with a sudden pause in activity prior to TF (off-cells); those with no change in activity prior to TF (neutral cells). Collision testing was performed using a cervical spinal cord stimulating electrode to determine whether these neurons projected to the cord. Conduction velocities were determined for all cord-projecting neurons. All 3 cell types projected to the cord and approximately 38% of cord-projecting neurons were flick-related (off-or on-cells). All projecting neurons were within or immediately adjacent to the nucleus raphe magnus. The mean conduction velocity of on-cell axons (17.7 m/s) was significantly greater than that of off-cell axons (10.7 m/s) and neutral cell axons (12.4 m/s). Conduction velocities for all cells were within the range for myelinated axons. These findings support the hypothesis that off-and on-cells in the RVM play a significant role in pain modulation at the spinal cord level.