Reflex conditioning: a new strategy for improving motor function after spinal cord injury

Abstract

Spinal reflex conditioning changes reflex size, induces spinal cord plasticity, and modifies locomotion. Appropriate reflex conditioning can improve walking in rats after spinal cord injury (SCI). Reflex conditioning offers a new therapeutic strategy for restoring function in people with SCI. This approach can address the specific deficits of individuals with SCI by targeting specific reflex pathways for increased or decreased responsiveness. In addition, once clinically significant regeneration can be achieved, reflex conditioning could provide a means of reeducating the newly (and probably imperfectly) reconnected spinal cord.

Figure 1

A: Soleus H-conditioning protocol in the rat. Soleus EMG is monitored continuously in a rat with chronic EMG electrodes and a tibial nerve cuff. When EMG absolute value is in a defined range for several seconds, nerve cuff stimulation elicits a threshold M response (a direct muscle response and an H-reflex. For the first 10–20 days, the rat is exposed to the control mode, in which the H-reflex is simply measured. For the next 40–50 days, it is exposed to the HRup or HRdown mode, in which a food-pellet reward occurs if the H-reflex is above (HRup) or below (HRdown) a criterion. Background EMG and M response are constant throughout. B: Results. Top: Average daily H-reflexes (±SEM) from HRup (▲) and HRdown rats (▼) under control mode (days −10 to 0) and HRup or HRdown mode (days 0–50). In HRup rats, the H-reflex rises gradually to about 175% of control, while in HRdown rats it falls gradually to about 60%. Bottom: Average poststimulus EMG for representative days from an HRup (left) and an HRdown (right) rat under the control mode (solid) and near the end of conditioning (dotted). The H-reflex is much larger after up-conditioning and much smaller after down-conditioning. Background EMG (EMG at 0 time) and M responses are unchanged. C: From left to right: up (▲) and down (▼) conditioning of triceps surae H-reflex in monkeys, biceps brachii SSR in monkeys, soleus and gastrocnemius H-reflex in mice, and biceps brachii SSR in humans. Time courses and magnitudes of change are similar to those in the rat.

Figure 2

Spinal cord plasticity induced by H-reflex conditioning. A: Motoneurons have more positive firing thresholds and tend to have smaller Ia EPSPs after H-reflex down-conditioning. As a result, they are less likely to fire in response to nerve stimulation. B: Schematic showing that down-conditioned monkeys have bigger F terminals than up-conditioned monkeys, and that their F terminals have more active zones. C: Soleus motoneurons of successfully down-conditioned rats (DS) have more detectable GAD67-labeled terminals, higher density of GAD immunoreactivity, and larger GAD-terminal coverage of soma than those of naive control rats (NC) or unsuccessful down-conditioned rats (DF).

Figure 3

Average (∀SEM) final H-reflex size (average for final 10 days as % of initial size) for intact rats, CST-transected rats, and contralateral SMC-ablated rats after continued-control, HRup, or HRdown mode exposure. Continued control-mode exposure has no significant effect in any group. In intact rats, the HRup and HRdown modes have mode-appropriate effects. In CST rats, the HRup and HRdown modes have no significant effect. In cSMC rats, the HRup mode has no significant effect, while the HRdown mode actually increases H-reflex size. Asterisks indicate significant differences from initial size (***P < 0.001; **P < 0.005 by paired t-test).

Figure 4

Summary of current understanding of spinal and supraspinal plasticity associated with H-reflex conditioning. The shaded ovals indicate the sites of definite (red) or probable (pink) spinal or supraspinal plasticity associated with H-reflex conditioning. Abbreviations: MN, motoneuron; CST, main corticospinal tract; IN, spinal interneuron; and GABA IN, GABAergic interneuron. Open synaptic terminals are excitatory, solid ones are inhibitory, half-open ones could be either, and the subdivided one is a cluster of C terminals. Dashed pathways imply the possibility of intervening spinal interneurons. The monosynaptic and probably oligosynaptic H-reflex pathway from Ia and Ib inputs to the motoneuron is shown. The sites of definite or probable plasticity include: the motoneuron membrane (firing threshold and axonal conduction velocity); motor unit properties; GABAergic terminals and C terminals on the motoneuron; the Ia afferent synaptic connection; interneurons and their terminals conveying oligosynaptic group I inhibition or excitation to the motoneuron; and sensorimotor cortex. The essential roles of the corticospinal tract (originating in sensorimotor cortex) and of cerebellar output to cortex are indicated. The spinal cord plasticity that is directly responsible for H-reflex conditioning appears to be induced and maintained by cortical plasticity that itself depends for its long-term survival on the cerebellum.

Figure 5

A: (Top) A rat walks on a treadmill during EMG data collection (top); (Bottom) concurrent right (bottom upper trace) and left (bottom lower trace) soleus EMG activity. B: The relationship between right soleus bursts and the right stance phases of locomotion. The arrows indicate the onsets of the right soleus bursts. The vertical dashed lines indicate the onsets of the right stance phase of locomotion, and the horizontal lines indicate the duration of the right stance phase. The onsets and duration of the right soleus burst are closely related to the onsets and duration of the right stance phase of locomotion.

Figure 6

A: Effects of conditioning on the conditioning H-reflexes and the locomotor H-reflexes. The average (∀SEM) final values of conditioning, stance, and swing H-reflexes from successful HRdown and HRup rats are shown. The conditioning and locomotor H-reflexes are similarly decreased in HRdown rats and similarly increased in HRup rats. See for detail. B: Soleus H-reflex conditioning affects soleus activity during locomotion in normal rats. Average right soleus locomotor bursts before (solid) and after (dotted) conditioning from a down-conditioned (left) and an up-conditioned (right) rats. After conditioning, the soleus burst is smaller in the down-conditioned rat and larger in the up-conditioned rat. See for detail.

Figure 7

A: Rectified right (upper trace) and left (lower trace) soleus EMG bursts during treadmill walking from a normal rat. Filled circles indicate onsets of right soleus bursts (RBOs) and dashed vertical lines are midpoints between onsets of right soleus bursts. Open circles mark onsets of left soleus bursts (LBOs). LBOs occur on the midpoints between RBOs. Thus, the times from RBO to LBO and from LBO to RBO are equal and the gait is symmetrical. B: Average right (dashed line) and left (solid line) soleus bursts are similar in shape and duration.

Figure 8

Right and left soleus bursts (rectified EMG) from a rat with a right lateral column injury for the first (i.e., before up-conditioning) treadmill session and the second (i.e., after up-conditioning) session. (Horizontal scale bar: 0.5 sec; vertical scale bar: 100 and 150 :V for the right and left bursts, respectively.) Each RBO (●) or LBO (○) is marked. The short vertical dashed lines mark the midpoints between RBOs (i.e., the midpoints of the step-cycles), which is the time when LBOs should occur (as in normal rats). Prior to H-reflex up-conditioning, LBO occurs too early; after up-conditioning, it occurs on time.