Connections of neurons in the lumbar ventral horn of spinal cord are altered after long-standing limb loss in a macaque monkey

Abstract

Explanations for the massive reorganization in primary motor cortex, M1, after limb amputation typically focus on processes that occur in cortex. Few have investigated whether changes in more peripheral parts of the pathway might also play a role in the reorganization. In the present study, we examined the integrity and connectivity of the spinal cord motoneurons in a macaque monkey (Macaca mulatta) that lost a hindlimb as a result of accidental injury more than 3.5 years earlier. To label motoneurons, multiple small injections of a neuroanatomical tracer were placed in the muscles of the hip just adjacent to the stump of the amputated leg, and in matched locations in the opposite side for control purposes. Injections of a second tracer were made in the intact foot. In the ventral horn that related to the intact hindlimb, motoneurons labeled by the hip injections were concentrated rostral and ventromedial to those labeled by the foot injections. Hip injections on the side of the amputation labeled neurons that were located well beyond the normal territory for motoneurons related to the hip and into the zone normally occupied by neurons projecting to the foot. Labeled motoneurons innervating the intact limb were significantly larger than neurons on the side of the amputation (x = 2410 and 2061 µm², respectively). The findings suggest that many neurons survived the long-standing amputation, and made new connections with remaining intact muscles. These new patterns of connectivity likely contribute to the reorganization of motor cortex in amputees, and perhaps to abnormal behaviors like those reported by human amputees.

• Motoneuron shrinkage
• neuroanatomical tracers
• plasticity