Hindlimb unloading affects cortical motor maps and decreases corticospinal excitability

A chronic reduction in neuromuscular activity through prolonged body immobilization of humans results in muscle atrophy and weakness as well as motor tasks performance impairment, which is correlated to a change in corticospinal excitability. In rats, hindlimb unloading (HU) is commonly used to mimic the effects of confinement to bed in patients. Several studies have reported changes in the representation of the somatosensory cortex in rodents submitted to HU or sensorimotor restriction by casting: remapping and enlargement of receptive fields, changes in the response of layer 4 neurons to peripheral stimulation. However, we have no data about motor cortical maps in rats submitted to a period of motor restriction during adulthood. Therefore, the objectives of the present study were twofold: to determine, in control rats and in rats submitted to a 14-day period of HU, the size and organization of hindlimb representation in the M1 cortex and to evaluate the overall excitability of M1 cortex by determining the stimulation thresholds. HU led to a dramatic decrease in the hindlimb representation on the M1 cortex (− 61%, p < 0.01). In addition, current thresholds for eliciting a movement were increased. The toes were less strongly affected by HU than other joint. Our main conclusion is that HU dramatically affects the organization and functioning of cortical motor maps and decreases corticospinal excitability.

► Sensory performance is altered by a period of disuse.
► We examine the effect of hindlimb unloading on the primary motor cortex.
► The hindlimb representation is reduced, and current thresholds for eliciting a movement are increased.
► The effects are different according to the joint.
► HU dramatically affects the organization and functioning of motor cortex.