Effects of limb exercise after spinal cord injury on motor neuron dendrite structure

Abstract

An integration center subserving locomotor leg movements resides in the upper lumbar spinal cord. If this neuronal network is preserved after a spinal cord injury, it is possible to stimulate this circuitry to initiate and promote walking. The several effective approaches (electrical stimulation, pharmacologic agents, physical therapy training programs) may all share a common modus operandi of altering synaptic activity within segmental spinal cord. To understand the neural substrate for the use-dependent behavioral improvement, we studied the dendritic architecture of spinal motor neurons. In the first experiment, we compared three groups of animals: animals with an intact spinal cord, animals that had a complete spinal cord transection (SCT) and animals with SCT who engaged in a daily exercise program of actively moving paralyzed hindlimbs through the motions of walking. When compared with animals with an intact spinal cord, the motor neurons from animals with SCT displayed marked atrophy, with loss of dendritic membrane and elimination of branching throughout the visible tree within transverse tissue slices. None of these regressive changes were found in the motor neurons from SCT animals that underwent exercise. In a second experiment, we inquired whether exercise of animals with an intact spinal cord influenced dendrite structure. Increased exercise had very modest effects on dendrite morphology, indicating an upper limit of use-dependent dendrite growth. Our findings suggest that the dendritic tree of motor neurons deprived of descending influences is rapidly pruned, and this finding is not observed in motor neurons after SCT if hindlimbs are exercised. The functional benefits of exercise after SCT injury may be subserved, in part, by stabilizing or remodeling the dendritic tree of motor neurons below the injury site.

Training locomotor networks

“…Given that the morphology of a motoneuron has a predominant effect on its electrophysiological properties, it is interesting that motoneuron structure is affected by training after spinal transection (Gazula et al, 2004). The size of the dendritic tree of motoneurons following spinal cord transection decreases, but these morphological features can be returned to normal values by training on a cycle.…”

Effects of 12 days of artificial rearing on morphology of hypoglossal motoneurons innervating tongue retrusors in rat

“…However, we cannot discount the possible alterations in the rest of the dendritic tree. Gazula et al (2004) found significant changes within the dendritic tree, including the number of primary dendrites 5 days after spinal cord transection in the rat. However, the spinal cord transection group that performed exercise was not different from the control group (no injury).…”

“…In addition, the use of artificial rearing ("pup in a cup" model) from postnatal days 4 to 14 has demonstrated altered contractile properties of the tongue retrusor musculature (Kinirons et al, 2003). The dendritic pattern, including the number of primary dendrites, is altered within 5 days of spinal cord transection in young rats (Gazula et al, 2004). However, the dendritic pattern is no different from a control group when the rats with spinal cord transection performed routine passive hindlimb exercise.…”

Transcriptional and Epigenetic Regulation in Injury-Mediated Neuronal Dendritic Plasticity

“…In addition, neurons that do not ultimately die may undergo extensive morphological alterations including dendritic and somatic atrophy [13][14][15][16], as well as synaptic and dendritic remodeling [17]. By 1 week after thoracic spinal cord injury (SCI), neurons in the cord show atrophic dendritic arbors in addition to reduced soma size [18].…”