Tail nerve electrical stimulation (TANES):
Physical therapy has become a commonly used strategy for functional recovery from spinal cord injury, and electrical stimulation is one of the most widely applied methods [18
]. Recently we developed a technique, TANES-dependent physical therapy, which can directly trigger the activation of the central pattern generator (CPG) below the lesion level through the tail nerves, resulting in temporary body weight-supported plantar stepping with characteristic features of active, alternative movement of left-right hind limbs and coordination of front-hind limbs in rats with chronically contused spinal cord [60
]. This is similar to an early study in which the spinally transected rats were induced to exhibit a strong tendency to step with alternating limbs during tail pinch [61
]. The TANES may also have an effect analogous with that of epidural stimulation which enabled the SCI patient to achieve full weight-bear standing and stepping with assistance during stimulation [59
]. The noninvasive TANES seems to show more beneficial effects on locomotor training over other approaches such as treadmill training and functional electrical stimulation [62
] (Figure 4).
Effect of TANES on functional recovery:
In the recent studies, a number of promising outcomes have been reported, which include restoration of connections, remyelination, revascularization and sparing of tissue from secondary damage. However, the desired final outcome is recovery of function, especially for the chronic spinal cord injury; so far neither approach has led to a completely effective repair strategy [69
]. We used TANES as an open field locomotor training method to promote the functional recovery in rats with chronic contusion injury. Six weeks after contusion injury, rats received glial scar ablation and LP/OEC transplantation followed by TANES-induced open field motor training. The electrical stimulation was produced by using a physical therapy
instrument (Type J18A1, China; it has been demonstrated no risk and no side effect in the application in clinic and home). The open field motor training lasted 20 min per session, 5 sessions a week, for a total of 16 weeks.
With TANES-induced open field locomotor training, the locomotor function and spinal cord conduction of rats was significantly improved compared with the control animals, assessed by using Basso, Beattie, and Bresnahan (BBB) open field locomotor rating scale (BBB score: 11.46 ± 0.47 vs 9.38 ± 0.32, p<0.01, n=12), horizontal ladder rung walking test (p<0.01, n=12), qualitative kinematic analysis, and electrophysiological tests (somatosensory evoked potentials, p<0.05, n=12; Hoffman reflex, p<0.05, n=12) in our lab. In addition, we also found that the TANES-induced open field locomotor training promoted axonal regeneration including those located in brain stem associated with locomotor control, such as red nuclei, lateral vestibular nuclei, reticular formation, raphe nuclei, and locus coerulei, verified by retrograde tracing. We have noted that the supraspinal axonal regeneration may have a positive relationship with BBB scores (r=0.89, p<0.001, n=23) [43
] (Figure 5).
In addition, transection at T8 was performed 22 weeks after injury. There was a marked decrease in BBB score after transection, but 2 weeks later, the BBB score in the trained rats (6.83 ± 0.17) is still higher than that of control groups (2.83 ± 1.59, p<0.05) [43
may be the key for functional recovery. Plasticity is the interface between physical and neural activity. Spontaneous injury-induced plasticity anatomically includes regenerative sprouting from damaged and intact neurons, synaptogenesis and synaptic remodeling, and functionally includes changes in neuronal excitability and inhibition, conduction velocity and synaptic efficacy [16
]. Distributed plasticity underlies recovery of foot kinematics by generating new patterns of muscle activity that are motor equivalents of the normal ones [77
]. Physical exercise has profound effects on cellular and molecular function [78
]. A recent study has shown that the quality of hind-limb stepping in rat with spinal cord transection is dependent on the amount of treadmill training that is imposed after transection surgery [81
], this may be related to the formation of plasticity, and this intrinsic plasticity can allow functional recovery in the absence of significant regeneration [23
]. In our previous study [42
], we have found transplantation of LP/OEC to the injury site following scar ablation did not directly result in markedly functional recovery although robust axonal regeneration were found at the injury site, implying that it may need plasticity triggered by physical therapy.
Activity-dependent plasticity occurs in the spinal cord throughout life. Driven by input from the periphery and the brain, this plasticity plays an important role in the acquisition and maintenance of motor skills and in the effects of SCI and other CNS disorders [82
]. After a complete loss of supraspinal control by cord transection, a higher degree of BBB score still remained in trained rats when compared to untrained rats. This may be related to the activity-dependent plasticity promoted by TANES, which results in the maintenance of motor skills, although there is no more supraspinal input or control. We consider this maintenance of motor skills as evidence of transformation of temporary ability to walk into permanent ability. The activity-dependent plasticity may be the morphological basis of this transformation [43
Role of CPG in functional recovery:
CPG is a local neural system (network) in animal and human spinal cord, mediating coordinated activity between groups of agonist and antagonist limb muscles on opposite sides [63
]. The CPG can generate patterns of rhythmic activity for locomotion even in the absence of external feedback or supraspinal control, but normally it is modulated by supraspinal and peripheral inputs [84
]. Evidence from animal experiments showed that CPG may be located in rat lumbar (L) segments L1-2, and controlled by the descending locomotor command from cortex and brainstem. In turn, the CPG produce rhythmic output, through interneurons, onto rhythmic elements distributed throughout the lumbar enlargement [86
]. The CPG also receives afferent feedback originating from the muscles via proprioceptive afferents (groups Ia, Ib and II) as well as cutaneous afferents (Aα, β) [88
]. In our studies, since contusion injury fell on thoracic (T) segment T10, the CPG located in L1-2 and the connection between CPG and motor neurons in lumbar segments should remain largely intact, in despite of that the descending locomotor command pathways, at least mostly, from the cortex and brainstem have been cut off. In addition to spinal cord reflexes, which by definition are single-phased motor response to sensory input, the spinal cord is able to generate complex, rhythmic behaviors in the absence of both supraspinal and movement-related (proprioceptive) information [64
It has been reported that simulating the rat tail, which induces tonic afferent input, can increase the strength and rate of locomotion produced by CPG, and make the pattern more consistent [89
]. Recently, electrically epidural stimulation of L2 segment was shown to induce bilateral stepping patterns with body weight support on a treadmill in adult rats with spinal cord transection on T7-9 [90
], suggesting that the CPG without supraspinal information but activated by electrical stimulation, is able to produce and send signals to the leg muscles. We believe that rats in our studies functioned following the same mechanisms. When CPG was activated by TANES via both cutaneous and proprioceptive afferents through the tail nerves, in turn, the CPG send signals to the motor neurons at the anterior horn, leading to the alternative contract of hind-limb muscles and the coordination of front- and hind-limbs. However, after neurons at L1-2 were destroyed by transection of spinal cord, there was no more movement of hind limbs during the electrical stimulation through the tail nerve, negatively indicating the important role of CPG in locomotor function [60
Effect of neural re-growth on functional recovery
: Our studies showed that 2 or 3 weeks after applying the TANES-induced open field locomotor training, rats either with or without cell transplantation started to show their functional improvement, indicating that this fast improvement of neurological outcome may be directly associated with the activity-dependent plasticity promoted by physical therapy, but not with the axonal regeneration. However, 7 or 8 weeks later, the BBB scores of rats without cell transplantation, i.e. without significant regeneration of axons, stay on a plateau without further raising till the end of experiment (22 weeks after initial contusion injury) , while rats with scar ablation and cell transplantation continue to gain higher BBB scores. This fact implies that without obvious tissue repair and axonal regeneration as an anatomic foundation, the role of physical therapy is limited [43
]. This point of view has been supported by a study of cat spinal cord injury [91
], and some of cases in the clinic trial recently [92
]. It is likely that only being based on the tissue repair and axonal regeneration, the effect of physical therapy on the functional recovery be reliable and permanent, therefore a strategy which not only promotes tissue repair and axonal regeneration, but also induces the spinal plasticity will be considered as an effective, perfect strategy for the final functional recovery from spinal cord injury, especially chronic spinal cord injury.