Acquisition of a Simple Motor Skill: Task-Dependent Adaptation and Long-Term Changes in the Human Soleus Stretch Reflex

Changing the H-reflex through operant conditioning leads to CNS multi-site plasticity and can affect previously learned skills. In order to further understand the mechanisms of this plasticity, we operantly conditioned the initial (M1) component of the soleus stretch reflex. Unlike the H-reflex, the stretch reflex is affected by fusimotor control, comprises several bursts of activity resulting from temporally dispersed afferent inputs, and may activate spinal motoneurons via several different spinal and supraspinal pathways. Neurologically normal participants completed six baseline sessions and 24 operant conditioning sessions in which they were encouraged to increase (M1up) or decrease (M1down) M1 size. Five of eight M1up participants significantly increased M1; the final M1 size of those 5 participants was 143±15% (mean±SE) of the baseline value. All eight M1down participants significantly decreased M1; their final M1 size was 62±6% of baseline. Similar to the previous H-reflex conditioning studies, conditioned reflex change consisted of within-session task-dependent adaptation and across-session long-term change. Task-dependent adaptation was evident in conditioning session 1 with M1up and by session 4 with M1down. Long-term change was evident by session 10 with M1up and session 16 with M1down. Task-dependent adaptation was greater with M1up than with the previous H-reflex up-conditioning. This may reflect adaptive changes in the muscle spindle sensitivity, which affects the stretch reflex but not the H-reflex. Because the stretch reflex is related to motor function more directly than the H-reflex, M1 conditioning may provide a valuable tool for exploring the functional impact of reflex conditioning and its potential therapeutic applications.