Adaptive plasticity in primate spinal stretch reflex: persistence

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316

Activity-dependent plasticity occurs throughout the CNS. However, investigations of skill acquisition usually focus on cortex. To expand the focus, we analyzed in humans the development of operantly conditioned H-reflex change, a simple motor skill that develops gradually and involves plasticity in both the brain and the spinal cord. Each person completed 6 baseline and 24 conditioning sessions over 10 weeks. In each conditioning session, the soleus H-reflex was measured while the subject was or was not asked to increase (HRup subjects) or decrease (HRdown subjects) it. When the subject was asked to change H-reflex size, immediate visual feedback indicated whether a size criterion had been satisfied. Over the 24 conditioning sessions, H-reflex size gradually increased in six of eight HRup subjects and decreased in eight of nine HRdown subjects, resulting in final sizes of 140 Ϯ 12 and 69 Ϯ 6% of baseline size, respectively. The final H-reflex change was the sum of within-session (i.e., task-dependent) adaptation and across-session (i.e., long-term) change. Taskdependent adaptation appeared within four to six sessions and persisted thereafter, averaging ϩ13% in HRup subjects and Ϫ15% in HRdown subjects. In contrast, long-term change began after 10 sessions and increased gradually thereafter, reaching ϩ27% in HRup subjects and Ϫ16% in HRdown subjects. Thus, the acquisition of H-reflex conditioning consists of two phenomena, task-dependent adaptation and long-term change, that together constitute the new motor skill. In combination with previous data, this new finding further elucidates the interaction of plasticity in brain and spinal cord that underlies the acquisition and maintenance of motor skills.

Section: Discussionsupporting

confidence: 92%

Acquisition of a Simple Motor Skill: Task-Dependent Adaptation Plus Long-Term Change in the Human Soleus H-Reflex

Thompson¹,

Chen²,

Wolpaw³

2009

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“…This persistence of the effects of conditioning on the H-reflex is consistent with the slow time course of H-reflex conditioning [both during its initial development and during its reversal by exposure to the opposite mode (Wolpaw et al, 1986;]. The time course implies that the motoneuron plasticity and other spinal cord plasticity that underlies the H-reflex change develops gradually over days and weeks and thus would not be expected to simply disappear when the rat begins to walk and reappear when it stops.…”

Section: The Effects Of Locomotion On H-reflex Conditioningsupporting

confidence: 80%

The Interaction of a New Motor Skill and an Old One: H-Reflex Conditioning and Locomotion in Rats

Chen¹,

Chen²,

Jakeman³

et al. 2005

New and old motor skills can interfere with each other or interact in other ways. Because each skill entails a distributed pattern of activity-dependent plasticity, investigation of their interactions is facilitated by simple models. In a well characterized model of simple learning, rats and monkeys gradually change the size of the H-reflex, the electrical analog of the spinal stretch reflex. This study evaluates in normal rats the interactions of this new skill of H-reflex conditioning with the old well established skill of overground locomotion.In rats in which the soleus H-reflex elicited in the conditioning protocol (i.e., the conditioning H-reflex) had been decreased by down-conditioning, the H-reflexes elicited during the stance and swing phases of locomotion (i.e., the locomotor H-reflexes) were also smaller. Similarly, in rats in which the conditioning H-reflex had been increased by up-conditioning, the locomotor H-reflexes were also larger.Soleus H-reflex conditioning did not affect the duration, length, or right/left symmetry of the step cycle. However, the conditioned change in the stance H-reflex was positively correlated with change in the amplitude of the soleus locomotor burst, and the correlation was consistent with current estimates of the contribution of primary afferent input to the burst.Although H-reflex conditioning and locomotion did not interfere with each other, H-reflex conditioning did affect how locomotion was produced: it changed soleus burst amplitude and may have induced compensatory changes in the activity of other muscles. These results illustrate and clarify the subtlety and complexity of skill interactions. They also suggest that H-reflex conditioning might be used to improve the abnormal locomotion produced by spinal cord injury or other disorders of supraspinal control.Key words: H-reflex conditioning; spinal cord plasticity; motor control; learning; memory consolidation; locomotion; rehabilitation IntroductionThe nervous system maintains a broad repertoire of adaptive behaviors acquired through practice, commonly referred to as skills (Compact Oxford English Dictionary, 1993). New skills may interfere, or interact in other ways, with old ones. These interactions are often addressed in terms of unitary concepts of memory consolidation, reactivation, and interference (Shadmehr and Holcomb, 1997;Krakauer et al., 1999;Goedert and Willingham, 2002;Wigmore et al., 2002;Walker et al., 2003;Caithness et al., 2004). However, it is now clear that even the simplest skills involve complex distributed patterns of activitydependent plasticity (Wolpaw and Lee, 1989;Carrier et al., 1997;Cohen et al., 1997;Lieb and Frost, 1997;Thompson et al., 1997;Whelan and Pearson, 1997;Lisberger, 1998;Garcia et al., 1999;Medina et al., 2000Medina et al., , 2002Hansel et al., 2001;King et al., 2001;Wolpaw and Tennissen, 2001;Carey and Lisberger, 2002;van Alphen and De Zeeuw, 2002;Wolpaw, 2002;Blazquez et al., 2003). A new skill may involve different kinds of plasticity that occur at different sites at dif...

“…In humans with partial spinal cord injuries, the effects of down-conditioning are still evident 4 months later (Segal and Wolf, 1994). In normal monkeys, the effects of SSR up-conditioning decline with a halflife of ϳ17 d, whereas the effects of down-conditioning appear to last considerably longer (Wolpaw et al, 1986). It is possible that the functional effects of H-reflex conditioning might require periodic reinforcement by reexposure to the conditioning protocol.…”

Section: Discussionmentioning

confidence: 99%

Operant Conditioning of H-Reflex Can Correct a Locomotor Abnormality after Spinal Cord Injury in Rats

Chen¹,

Chen²,

Jakeman³

et al. 2006

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This study asked whether operant conditioning of the H-reflex can modify locomotion in spinal cord-injured rats. Midthoracic transection of the right lateral column of the spinal cord produced a persistent asymmetry in the muscle activity underlying treadmill locomotion. The rats were then either exposed or not exposed to an H-reflex up-conditioning protocol that greatly increased right soleus motoneuron response to primary afferent input, and locomotion was reevaluated. H-reflex up-conditioning increased the right soleus burst and corrected the locomotor asymmetry. In contrast, the locomotor asymmetry persisted in the control rats. These results suggest that appropriately selected reflex conditioning protocols might improve function in people with partial spinal cord injuries. Such protocols might be especially useful when significant regeneration becomes possible and precise methods for reeducating the regenerated spinal cord neurons and synapses are needed for restoring effective function.