Barrel Cortex

Analogous to the visual system, neurotrophins play an important role in the development of the barrel cortex. This distinct structure in the somatosensory cortex receives sensory information derived from the whiskers of rodents. Incoming sensory information passed to the thalamus originate from axons of the ventral posterior medial (VPM) nucleus, which then segregate into distinct rings termed barrels. Each barrel consists of a set of neurons that receives sensory information from a single whisker. In TrkB mutant mice that lack the kinase domain receptor but express the truncated receptor, a developmental delay in barrel formation was observed109. It was later identified that BDNF and not NT-4 was the endogenous ligand for TrkB involved in thalamocortical axon segregation and barrel formation110.

BDNF also plays an important role in synaptogenesis occurring during adulthood. In mature animals, altered sensory experience results in robust physiological changes in the barrel cortex111, which occur in parallel with structural changes112,113. By using two-photon microscopy, Trachtenberg and colleagues demonstrated that formation of synapses is influenced by dendritic spine growth and retraction as a result of altered sensory input. Remarkably, stimulation of a single whisker results in an increase in BDNF gene expression in the barrel corresponding to that whisker114. The same whisker stimulation induces the formation of inhibitory synapses112, raising the possibility that the synaptogenesis observed in the barrel cortex is mediated by activity-dependent expression of BDNF. Indeed, structural plasticity that normally occurs in wild-type animals does not occur in mice heterozygous for BDNF115.

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