Critical period for the whisker-barrel system

Abstract

A remarkable feature of the sensory maps in the neocortex is the patterning of pre- and post-synaptic elements which replicate a specific feature of the sensory periphery. Ocular dominance columns in both the feline and the primate visual cortex and whisker-specific neural modules, the “barrels” in the rodent somatosensory cortex are perhaps the most notable examples. In common laboratory rodents, such as the mouse and the rat, thalamocortical axon arbors form synaptic terminal aggregations, around which layer IV stellate neurons arrange their cell bodies to form “barrels,” and they orient their dendritic trees to embrace the presynaptic afferents. The spatial arrangement of barrels replicates the patterned distribution of whiskers on the snout (Woolsey and Van der Loos, 1970). Barrel patterns are established during the first few days after birth and depend on the information flow from the sensory periphery and the subcortical somatosensory structures (see reviews O’Leary et al., 1994; Woolsey, 1990). The instructive role of the sensory periphery in shaping neural patterns has been shown by whisker follicle or infraorbital nerve lesions in neonatal rodents, and in mice bred for aberrant numbers of whiskers (Ohsaki et al., 2002; Ohsaki and Nakamura, 2006; O’Leary et al., 1994; Welker and Van der Loos, 1986; Woolsey, 1990). When the sensory periphery is damaged during the first few days after birth, thalamocortical terminals fail to develop their normal patterning. Consequently, layer IV stellate cells do not form barrels (O’Leary et al., 1994; Woolsey, 1990). In newborn rats and mice, if one row of whisker follicles is cauterized, the area of cortex devoted to the representation of those follicles fuse and shrink, and then the neighboring barrels expand (Belford and Killackey, 1980; Van der Loos and Woolsey, 1973). A developmental time window exists during which these structural alterations take place and it ends by postnatal day (P) 4; as a consequence, lesions after this period do not lead to major structural alterations in the barrel patterning of the cortex or whisker-specific neural patterns in subcortical structures (Belford and Killackey, 1980; Datwani et al., 2002b; Durham and Woolsey, 1984; Rebsam et al., 2005; Van der Loos and Woolsey, 1973; Woolsey and Wann, 1976). In the extant literature, this phenomenon is referred to as the critical period for structural plasticity along the rodent trigeminal pathway (reviewed in Erzurumlu, 2009).