Thalamic afferents to the cortex develop from approximately 12-16 wk of gestation, reach the cortical subplate, but wait until they grow into the cortical plate (
16). At this stage, only long depolarization of the deep layers may reach the cortex (
17) (
Fig. 2).
After 24 wk, thalamocortical axons grow into the somatosensory, auditory, visual, and frontal cortices and the pathways mediating pain perception become functional around the 29-30 wk (
18). From approximately 34 wk, a synchrony of the EEG rhythm of the two hemispheres becomes detectable at the same time as long-range callosal connections, and thus the GNW circuits, are established (
18
20). From the 26th wk, pyramidal neurons in the primary visual cortex of humans develop dendritic spines (
19). At birth, the dendritic spines have not reached the adult density, but suffice for the detection of visually evoked potentials.
The connectivity of the cerebral cortex particularly in the prefrontal area, mature later than the subcortical structures. However, the fusiform area for face recognition (
21) and the left-hemispheric temporal lobe cortices for processing speech stimuli (
22) function already in the newborn. Moreover, the main fascicles of myelinated long-range connections such as the corpus callosum, cerebellar peduncles, corticospinal tract, spinothalamic tract are unambiguously identified at the age of 1-4 mo (
23). In short, the vertical brain stem, diencephalic, and thalamocortical pathways, which regulate the states of consciousness, become established before their connection with the horizontal GNW cortical circuits yielding, in the newborn, plausibly functional, though still immature, neural dispositions for access to a conscious content.
Comparison between the maturation of thalamocortical-cortical connections and somatosensory evoked potentials (SEP). In the early preterm infant (<24-25 gestational weeks), thalamic axons establish a dense synaptic network in the subplate. After approximately 25 gestational weeks thalamic fibers make synapses in the deep cortical layers. In the full-term infants, the thalamic fibers have reached their final destination in layer IV of the cortex. This is reflected by the SEP responses. In the early preterms, the evoked responses consist only of long depolarizations of the deep layers. A delayed cortical activation can be seen. When the thalamic-cortical fibers extend to the cortex, faster cortical responses are seen, paralleling the accumulation of synapses in layer IV. Reprinted from Vanhatalo
et al., Semin Fetal Neonatal Med 11:464-470, Copyright © 2006 Elsevier Ltd., with permission.