Ascending spinal axons that signal the position of the hindlimbs under static conditions: location and receptor input.

Psychophysical experiments have shown that signals from slowly adapting subcutaneous receptors are used to sense limb position under static conditions (i.e., when the joints are stationary). The ascending collaterals of the slowly adapting primary sensory neurons supplying the deep tissues of the hindlimb do not project to the brain via the fasciculus gracilis. In experiments on cats, we have found a population of axons in the lateral fasciculus that signal the position of the ipsilateral hindlimb with a slowly adapting discharge. In the lower thoracic cord these fibers lie between the spinocervical tract and the ventral roots. Although plentiful in the lower thoracic cord, they are sparse or absent below L3. In addition, a few position signaling axons with crossed input were found in the ventral part of the lateral white matter and in the ventral columns. Since the clinical evidence suggests that the spinal pathway for position sense is uncrossed, we propose that information used for conscious judgments of limb position when the joints are stationary initially ascends via the dorsal columns and then relays to the lateral fasciculus on the same side. These slowly adapting signals also may be used to judge limb position when the joints are moving. To determine whether this slowly adapting discharge originates from muscle or joint receptors, the tendons crossing the ankle joint were exposed but left in continuity and then pulled on while the joint was stationary. In this way individual lateral fascicular axons that signaled ankle flexion, extension, abduction or adduction could be shown to receive a strong excitatory input from muscle receptors. After the muscle tendons crossing the ankle joint were cut, tract fibers signaling ankle flexion, extension, abduction or adduction could no longer be found in this portion of the spinal white matter. Axons signaling clockwise or counterclockwise twist of the ankle were reduced in number but a few were still present. These results suggest that muscle receptors provide the predominant signal used to sense ankle flexion, extension, abduction and adduction and that receptors in articular tissues may signal ankle twist.