Figure 5
Illustration of the interactions and processes underlying typical or atypical sensorimotor development in rats. (A) Schematic representation of the putative interactions between the different variables assessed (locomotion, musculoskeletal tissues, Hoffman-reflex, somatosensory and motor representations of the hind limb and excitation-inhibition neurotransmission in the sensorimotor cortex) that may occur during typical sensorimotor development in control rats. Daily activities provide early typical sensorimotor experience that allow muscle synergies and typical gait to interplay harmoniously. Normal movements during typical gait produce typical sensorimotor feedback to the immature brain, contributing to the development and refinement of connections, maps and functions, e.g., normal motor command to muscles that in turn contribute to muscle synergies and typical movements in a virtuous self-perpetuating cycle. Excitation-inhibition balance also participates in the refinement of neural maps and connections. (B) Schematic illustration of the deleterious impact of postnatal sensorimotor restriction (SMR) on neuromuscular interplay. Movement restriction during postnatal hind limb immobilization provides early atypical sensorimotor experience that leads to gait disorders, musculoskeletal pathologies and hyperreflexia, as signs of spasticity (see23). In turn, gait disorders and spastic muscles provide atypical somatosensory feedback/reafference to the immature sensorimotor circuitry, mainly the spinal network and the primary somatosensory (S1) and motor (M1) cortices that intercommunicate. Such atypical somatosensory feedback likely degrades the functional organization of the somatosensory and motor maps and spinal network, thus producing abnormal motor commands that finally aggravate gait disorders, spasticity and musculoskeletal abnormalities into interrelated self-perpetuating cycles. Excitation-inhibition imbalances that tip the balance to hyperexcitability in sensorimotor hind limb cortices have more impact on the reorganization of somatosensory maps than on motor maps. Hyperexcitability in the lumbar spinal cord has been shown previously as an enduring hyperreflexia in adult SMR rats (see23).
