Should One Hand (Paw) Really Not Know What the Other Is Doing? Focus on “Reducing Contralateral SI Activity Reveals Hindlimb Receptive Fields in the SI Forelimb-Stump Representation of Neonatally Amputated Rats”

Cortical plasticity induced by different degrees of peripheral nerve injuries: a rat functional magnetic resonance imaging study under 9.4 Tesla

BACKGROUND

Major peripheral nerve injuries not only result in local deficits but may also cause distal atrophy of target muscles or permanent loss of sensation. Likewise, these injuries have been shown to instigate long-lasting central cortical reorganization.

METHODS

Cortical plasticity changes induced after various types of major peripheral nerve injury using an electrical stimulation technique to the rat upper extremity and functional magnetic resonance imaging (fMRI) were examined. Studies were completed out immediately after injury (acute stage) and at two weeks (subacute stage) to evaluate time affect on plasticity.

RESULTS

After right-side median nerve transection, cortical representation of activation of the right-side ulnar nerve expanded intra-hemispherically into the cortical region that had been occupied by the median nerve representation After unilateral transection of both median and ulnar nerves, cortical representation of activation of the radial nerve on the same side of the body also demonstrated intra-hemispheric expansion. However, simultaneous electrical stimulation of the contralateral uninjured median and ulnar nerves resulted in a representation that had expanded both intra- and inter-hemispherically into the cortical region previously occupied by the two transected nerve representations.

CONCLUSIONS

After major peripheral nerve injury, an adjacent nerve, with similar function to the injured nerve, may become significantly over-activated in the cortex when stimulated. This results in intra-hemispheric cortical expansion as the only component of cortical plasticity. When all nerves responsible for a certain function are injured, the same nerves on the contralateral side of the body are affected and become significantly over-activated during a task. Both intra- and inter-hemispheric cortical expansion exist, while the latter dominates cortical plasticity.

Dynamic representations of the somatosensory cortex

Neural representation of somatosensory events undergoes major transformation in the primary somatosensory cortex (SI) from its original, more or less isomorphic, form found at the level of peripheral receptors. A large body of SI optical imaging, neural recording and psychophysical studies suggests that SI representation of stimuli encountered in everyday life is a product of dynamic processes that involve competitive interactions at multiple levels of cortical organization. Such interactions take place among neighboring neurons, among local groups of minicolumns, among neighboring macrocolumns, between SI and SII, between Pacinian and non-Pacinian channels, and bilaterally between homotopic somatosensory regions of the opposite hemispheres. Together these interactions sharpen SI response to suprathreshold and time-extended tactile stimuli by funneling the initially widespread stimulus-triggered activity in SI into the local group of macrocolumns most directly driven by the stimulus. Those macrocolumns in turn fractionate into stimulus-specific patterns of differentially active minicolumns. Thus SI dynamically shapes its representation of a tactile stimulus by selecting among all of its neurons initially activated by the stimulus a subset of neurons with receptive-field and feature-tuning properties closely matching those of the stimulus. Through this stimulus-directed dynamical selection process, which operates on a scale of hundreds of milliseconds, SI achieves a more faithful representation of stimulus properties, which is reflected in improved performance on tactile perceptual tasks.

Ipsilateral whiskers suppress experience-dependent plasticity in the barrel cortex

Each cerebral hemisphere processes sensory input from both sides of the body, but the impact of this convergence on shaping and modifying receptive field properties remains controversial. Here we investigated the effect of chronic deprivation of ipsilateral sensory whiskers on receptive field plasticity in primary somatosensory cortex. In the absence of ipsilateral whiskers, cortical receptive fields were significantly larger than control after 1 week. Removal of all but a single whisker from one side of the face [single-whisker experience (SWE)] has been shown to result in the expansion of the cortical area responding to the spared whisker. We compared the effects of SWE in the presence (SWE-unilateral) and absence (SWE-bilateral) of ipsilateral whiskers. SWE-bilateral deprivation results in a significant increase in neuronal responses to spared whisker stimulation both in its cognate barrel column and in adjacent, surrounding barrel columns compared with control and SWE-unilateral deprived animals. Surround receptive fields in deprived columns were maintained in SWE-bilateral treated animals but depressed in SWE-unilateral animals. The increase in spared whisker responses was progressive with longer deprivation periods. These data show that ipsilateral whiskers can constrain receptive field size in the barrel cortex.