Impact of thalamocortical input on barrel cortex development

Abstract

The development of cortical maps requires the balanced interaction between genetically determined programs and input/activity-dependent signals generated spontaneously or triggered from the environment. The somatosensory pathway of mice provides an excellent scenario to study cortical map development because of its highly organized cytoarchitecture, known as the barrel field. This precise organization makes evident even small alterations in the cortical map layout. In this review, we will specially focus on the thalamic factors that control barrel field development. We will summarize the role of thalamic input integration and identity, neurotransmission and spontaneous activity in cortical map formation and early cross-modal plasticity.

Keywords: calcium waves; cortical maps; plasticity; somatosensory system; spontaneous activity; thalamus.

Figure 1. Increasing severity in barrel map deficits from thalamic to cortical manipulations.

(A) Schematic representation of severity levels in defective phenotypes for three different components of barrel map formation: TCAs clustering (top), alignment to barrel walls of cell bodies from L4 spiny stellate cells (middle), and polarization of their dendritic arbors (bottom). Severity is divided in four categories: no phenotype (circle), mild (one cross), moderate (two crosses), and severe (three crosses). (B) Summary of thalamic and cortical tissue-specific manipulation and their consequences in barrel map formation using the severity code described in A.

Figure 2. Thalamic calcium waves communicate sensory modalities and regulate S1 barrels formation.

(A) Embryonic Ca²⁺ waves propagate among principal thalamic sensory nuclei and reach the neocortex through thalamocortical axons at E16.5 in mice. (B) Bilateral embryonic enucleation at E14.5 triggers more Ca²⁺ waves in VPM and LGN, eventually leading to an increment in barrel size and posteromedial barrel subfield (PMBSF) area. (C) Silencing Ca²⁺ waves in the MGv increases Ca²⁺ waves frequency in the VPM. This change precedes barrel and PMBSF area enlargement.