Effects of dorsal column stimulation on primate spinothalamic tract neurons

The effect of dorsal column stimulation on spinothalamic tract cells was investigated in anesthetized monkeys. The dorsal column stimuli were applied at midthoracic or at cervical levels of the cord, while the responses of spinothalamic tract cells of the lumbosacral enlargement were examined. A dorsal column volley depressed the activity of spinothalamic tract cells for about 150 ms. A similar depression was observed whether the spinothalamic tract cell was classified as hair activated, low, or high threshold, based on its response properties to cutaneous stimulation. The hair-activated and low-threshold spinothalamic tract cells were initially excited by the dorsal column volley, but often it was possible to demonstrate that a depression could be produced by stimuli which were too weak to cause excitation of these cells. Depression was produced both of the responses of spinothalamic tract cells to electrical stimulation of peripheral nerves and to mechanical stimulation of cutaneous nociceptors. A similar depression was produced by electrical stimulation of large afferents in peripheral nerves. The pathway mediating the depression of spinothalamic tract cells was shown to involve antidromic invasion of collaterals of dorsal column fibers. The best points for stimulation of the cord to produce a depression were over the ipsilateral dorsal column. A lesion interrupting the dorsal column eliminated the depression of cells below the lesion, whereas a lesion of much of the lateral column had no effect. The mechanism of the depression is likely to be complex. Apart from interactions at an interneuronal level, dorsal column volleys can be presumed to collide with sensory input from afferents which project up the dorsal column; collision would interfere chiefly with the responses of hair-activated and low-threshold spinothalamic tract cells. In addition, dorsal column volleys were shown to evoke inhibitory postsynaptic potentials in some spinothalamic tract neurons, and they also produced primary afferent depolarization, at least of large cutaneous afferemts. The excitation of hair-activated and low-threshold spinothalamic tract cells argues against their participation in signaling pain, since dorsal column stimulation in humans does not produce pain at stimulus intensities and frequencies which should activate such neurons. Alternatively, an ascending volley in the dorsal column or in other pathways may interfere with pain transmission in the brain.