Bidirectional axonal plasticity during reorganization of adult rat primary somatosensory cortex

Cortical Reorganization of Sensorimotor Systems and the Role of Intracortical Circuits After Spinal Cord Injury

The plasticity of sensorimotor systems in mammals underlies the capacity for motor learning as well as the ability to relearn following injury. Spinal cord injury, which both deprives afferent input and interrupts efferent output, results in a disruption of cortical somatotopy. While changes in corticospinal axons proximal to the lesion are proposed to support the reorganization of cortical motor maps after spinal cord injury, intracortical horizontal connections are also likely to be critical substrates for rehabilitation-mediated recovery. Intrinsic connections have been shown to dictate the reorganization of cortical maps that occurs in response to skilled motor learning as well as after peripheral injury. Cortical networks incorporate changes in motor and sensory circuits at subcortical or spinal levels to induce map remodeling in the neocortex. This review focuses on the reorganization of cortical networks observed after injury and posits a role of intracortical circuits in recovery.

A complementary role of intracortical inhibition in age-related tactile degradation and its remodelling in humans

Many attempts are currently underway to restore age-related degraded perception, however, the link between restored perception and remodeled brain function remains elusive. To understand remodeling of age-related cortical reorganization we combined functional magnetic resonance imaging (fMRI) with assessments of tactile acuity, perceptual learning, and computational modeling. We show that aging leads to tactile degradation parallel to enhanced activity in somatosensory cortex. Using a neural field model we reconciled the empirical age-effects by weakening of cortical lateral inhibition. Using perceptual learning, we were able to partially restore tactile acuity, which however was not accompanied by the expected attenuation of cortical activity, but by a further enhancement. The neural field model reproduced these learning effects solely through a weakening of the amplitude of inhibition. These findings suggest that the restoration of age-related degraded tactile acuity on the cortical level is not achieved by re-strengthening lateral inhibition but by further weakening intracortical inhibition.

Diverse excitatory and inhibitory synaptic plasticity outcomes in complex horizontal circuits near a functional border of adult neocortex

The primary somatosensory cortex (SI) is topographically organized into a map of the body. This organization is dynamic, undergoing experience-dependent modifications throughout life. It has been hypothesized that excitatory and inhibitory synaptic plasticity of horizontal intracortical connections contributes to functional reorganization. However, very little is known about synaptic plasticity of these connections; particularly the characteristics of inhibitory synaptic plasticity, its relationship to excitatory synaptic plasticity, and their relationship to the functional organization of the cortex. To investigate this, we located the border between the forepaw and lower jaw representation of SI in vivo, and used whole cell-patch electrophysiology to record post-synaptic excitatory and inhibitory currents in complex horizontal connections in vitro. Connections that remained within the representation (continuous) and those that crossed from one representation to another (discontinuous) were stimulated differentially, allowing us to examine differences associated with the border. To induce synaptic plasticity, tetanic stimulation was applied to either continuous or discontinuous pathways. Tetanic stimulation induced diverse forms of excitatory and inhibitory synaptic plasticity, with LTP dominating for excitation and LTD dominating for inhibition. The border did not restrict plasticity in either case. In contrast, tetanization elicited LTP of monosynaptic inhibitory responses in continuous, but not discontinuous connections. These results demonstrate that continuous and discontinuous pathways are capable of diverse synaptic plasticity responses that are differentially inducible. Furthermore, continuous connections can undergo monosynaptic inhibitory LTP, independent of excitatory drive onto interneurons. Thus, coordinated excitatory and inhibitory synaptic plasticity of horizontal connections are capable of contributing to functional reorganization.