Effects of short-term arm immobilization on motor skill acquisition.

Learning to sequence movements is necessary for skillful interaction with the environment. Neuroplasticity, particularly long-term potentiation (LTP), within sensorimotor networks underlies the acquisition of motor skill. Short-term immobilization of the arm, even less than 12 hours, can reduce corticospinal excitability and increase the capacity for LTP-like plasticity within the contralateral primary motor cortex. However, it is still unclear whether short-term immobilization influences motor skill acquisition. The current study aimed to evaluate the effect of short-term arm immobilization on implicit, sequence-specific motor skill acquisition using a modified Serial Reaction Time Task (SRTT). Twenty young, neurotypical adults underwent a single SRTT training session after six hours of immobilization of the non-dominant arm or an equivalent period of no immobilization. Our results demonstrated that participants improved SRTT performance overall after training, but there was no evidence of an effect of immobilization prior to task training on performance improvement. Further, improvements on the SRTT were not sequence-specific. Taken together, motor skill acquisition for sequential, individuated finger movements improved following training but the effect of six hours of immobilization was difficult to discern.

Short-term arm immobilization modulates excitability of inhibitory circuits within, and between, primary motor cortices.

Previous research has suggested that short-term immobilization of the arm may be a low-cost, non-invasive strategy to enhance the capacity for long-term potentiation (LTP)-like plasticity in primary motor cortex (M1). Short-term immobilization reduces corticospinal excitability (CSE) in the contralateral M1, and interhemispheric inhibition (IHI) from ipsi- onto contralateral M1 is increased. However, it is unclear whether reduced CSE and increased IHI are associated with changes in intracortical inhibition, which has been shown to be important for regulating neuroplasticity in M1. The current study used transcranial magnetic stimulation to evaluate the effects of short-term (6 h) arm immobilization on CSE, IHI, and intracortical inhibition measured bilaterally in 43 neurotypical young adults (23 immobilized). We replicated previous findings demonstrating that immobilization decreased CSE in, and increased IHI onto, the immobilized hemisphere, but a significant change in intracortical inhibition was not observed at the group level. Across individuals, decreased CSE was associated with a decreased short-interval intracortical inhibition, an index of GABAA -ergic inhibition, within the immobilized hemisphere only in the immobilization group. Previous research has demonstrated that decreases in GABAA -ergic inhibition are necessary for the induction of LTP-like plasticity in M1; therefore, decreased intracortical inhibition after short-term arm immobilization may provide a novel mechanism to enhance the capacity for LTP-like plasticity within M1 and may be a potential target for strategies to augment plasticity capacity to enhance motor learning in health and disease.

Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke.

Integration of sensory and motor information is one-step, among others, that underlies the successful production of goal-directed hand movements necessary for interacting with our environment. Disruption of sensorimotor integration is prevalent in many neurologic disorders, including stroke. In most stroke survivors, persistent paresis of the hand reduces function and overall quality of life. Current rehabilitative methods are based on neuroplastic principles to promote motor learning that focuses on regaining motor function lost due to paresis, but the sensory contributions to motor control and learning are often overlooked and currently understudied. There is a need to evaluate and understand the contribution of both sensory and motor function in the rehabilitation of skilled hand movements after stroke. Here, we will highlight the importance of integration of sensory and motor information to produce skilled hand movements in healthy individuals and individuals after stroke. We will then discuss how compromised sensorimotor integration influences relearning of skilled hand movements after stroke. Finally, we will propose an approach to target sensorimotor integration through manipulation of sensory input and motor output that may have therapeutic implications.