Figure 1

Schematic representation of thalamocortical and cholinergic fiber ingrowth to the auditory cortex of mice. The open circle depicits the arrival of thalamocortical afferent fibers at embryonic day (E)15-16 [1, 5], and the line extending from it represents the course of their maturation to completion at postnatal day (P)7 [6]. The closed circle represents the initial invasion to the cortex by cholinergic fibers on E18-19, and the associated line, their maturation through the first 7 weeks of life [7, 81]. The dotted lines delineate the boundaries of proposed critical periods of rapid reformation and maturation beginning at P11 [13] and extending to P21 [32], with the grey dotted line marking the end of an purposed early critical period at P13 [13]. The light-grey cloud represents the trends in M₁ expression, with a peak represents the maximal adult expression levels at ~P26 and a subsequent gradual decline throughout adult life (conservative average of data from from several articles [32, 38, 82]). The dark-grey shaded area area represents choline acetyltransferase (ChAT) expression, again with the peak of this curve being maximal expression, rising more slowly and likely sustained longer into adult life than that for M₁ (based on data published by Hohmann 1985 [81]).