Activation of dorsal horn cells by ventral root stimulation in the cat

Abstract

Responses of dorsal horn cells to ventral root stimulation were determined for the L7 and S1 levels of the spinal cord of 14 anesthetized cats. Forty-six dorsal horn cells were found that were excited by stimulation of the distal stump of the cut ventral root. For maximum excitation it was necessary to use a train of stimuli. For the 34 dorsal horn cells whose peripheral receptive-field properties could be characterized, 14 were wide dynamic range cells and 19 were high-threshold cells. The other cell responded exclusively to stimulation of deep tissue. None of the cells responded exclusively to innocuous stimuli, and all responded more vigorously to noxious than to innocuous stimuli. Some cells also responded to noxious heat applied to the skin of the receptive field. Locations of 10 of the activated dorsal horn cells were identified. They were distributed throughout the dorsal horn, but most were found in laminae V and VI. In four animals, both the proximal and distal stumps of the cut S1 ventral root were stimulated while searching for dorsal horn cells. Ten dorsal horn cells were found that were excited by stimulation of the distal stump of the ventral root. No cells were found that responded to proximal stump stimulation. To prevent current spread by stimulation of the ventral root, an extra ground electrode was placed distal to the stimulating electrodes. When the ground electrode was removed, distinctive signs of current spread appeared in that a cord dorsum potential could be recorded and the dorsal horn neuronal responses changed. Dorsal horn neurons could also be excited by nonelectrical stimuli such as crushing the ventral root. If the ventral root was crushed distal to the stimulating electrodes, however, the initially excited cell could no longer be activated by ventral root stimulation. Activation of dorsal horn cells by stimulation of the distal stump of a cut ventral root was abolished when the dorsal root of the same segment was sectioned. Conduction velocities of the fibers in the ventral root that excited dorsal horn cells ranged between 0.25 and 1.78 m/s with a mean of 0.91 +/- 0.47 (SD) m/s. These results show that there are unmyelinated afferent fibers in the ventral root that enter the spinal cord through the dorsal root and excite dorsal horn cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of renal sympathetic activity by static contraction: evidence for mechanoreceptor-induced reflexes from skeletal muscle.

“…Furthermore, the reflex response was caused by muscle contraction per se and not by direct electrical stimulation of afferents in the L 7 and S, ventral roots because the current intensity applied to the ventral roots in our experiments (i.e., 2.5 times motor threshold) was far too low to stimulate ventral root afferents 12 and to fire the dorsal horn cells that receive synaptic input from these afferents. 13 Chung et al 12 have demonstrated that the minimum current intensity needed to elicit reflex autonomic responses from electrical stimulation of ventral roots in paralyzed cats is 200 times motor threshold, a level of stimulation that is 80 times greater than that which we used to elicit muscle contraction.…”

A quantitative study of the central projection patterns of unmyelinated ventral root afferents in the cat.

“…This includes branches or loops of dorsal root afferents and pial afferents. Activation of this fibre group could elicit the phenomenon of recurrent sensitivity (Frykholm et al 1953;Chung et al 1985Chung et al , 1986. The results of the present study indicate that they are a small minority of the ventral root afferents and their relative contribution are discussed below.…”

“…It is unclear whether the same applies for thin myelinated afferent fibres which are also present in the ventral root (see Results; Coggeshall et al 1974;Clifton et al 1976;Floyd et al 1976). On one hand, physiological studies have repeatedly failed to elicit pseudoaffective responses or to activate secondary spinal interneurones by stimulation of the central stump of the cut ventral root (Chung et al 1985(Chung et al , 1986. This shows that very few afferents enter the spinal cord through the ventral root or that these afferents do not have a prominent central effect.…”

Differences in Waveform Characteristics Have No Effect on the Anti-Hyperalgesia Produced by Transcutaneous Electrical Nerve Stimulation (TENS) in Rats With Joint Inflammation

“…In animals without tissue injury, high-frequency TENS produces analgesia as measured by the tail flick test 31 and reduces dorsal horn neuron responses to noxious stimuli. 6,9 In an animal model of nerve injury, high-frequency TENS reduces hyperalgesia when applied ipsilaterally on the skin of L1-L6 segments if the treatment was started within the first day after surgery but not if given after development of hyperalgesia. 29 Both high-and low-frequency TENS reduce mechanical and heat hyperalgesia in animals with joint inflammation, muscle inflammation, or cutaneous inflammation.…”