Auditory Cortex 35 

Neurons originating in the MGB radiate to the auditory areas of the cerebral cortex completing the ascending auditory system. The cerebral cortex consists of six horizontal cell layers (I molecular; II external granular; III external pyramidal; IV internal granular; V internal pyramidal; VI multiform) which can be distinguished by the cell type, density, and arrangement (Carpenter & Sutin, 1983). Cells responsive to acoustic stimuli exist in all cell layers, except layer I. Sensory inputs first activate neurons in layer IV, which propagate the excitement up to layers II and III, and from there down to layers V and VI.

Brodmann areas 41 and 42 mark the location of the primary auditory cortex which is the cortical region responsible for the sensation of sound. The primary area is buried in the floor of the lateral sulcus (i.e., sylvian fissure). The principle auditory receptive area (41) is located in the middle part of the anterior transverse gyrus (i.e., Heschl’s). The remaining parts of Heschl’s gyrus and adjacent portions of the sylvian fissure, compose area 42, which is largely an auditory association area. Each receptive area is thought to process different aspects of the acoustic stimuli (e.g., processing complex sounds and discrimination of timing and temporal patterns). As in the brainstem, distinct tonotopic organization exists in area 41 with high frequencies rostral and with low frequency arranged caudally (Merzenich & Brugge, 1973).

Shortly after neurulation is complete in early embryonic life, the dorsolateral walls of the cerebral vesicles start to form the layers of the cortex by the third week of fetal development. At about the 12th week GA neurons migrate out of the epithelium to form the cortical plate. At this same time, the first stages of the development of the sylvian fissure are recognized (Cant, 1998; Streeter, 1912). The six distinct cortical layers within the cerebral cortex are identified by the 14th week GA (Krmpotic-Nemanic, Kostovic, Nemanic, & Kelovic, 1979; also Bayer, et al., 1995, who estimates neurogenesis is complete in the 16th week GA), and are differentiated by the 24th week GA (Cant, 1998; Streeter, 1912). By the 18th week GA synaptic vesicles are present above and below the cortical plate (Molliver, Kostovic, & van der Loss, 1973), and synaptic terminals with vesicles are present by the 23rd week GA (Krmpotic-Nemanic et al., 1979). By the 27th week GA development of the temporal lobe has begun and just 10 weeks later by the 37th week GA the transverse temporal gyrus and superior temporal gyrus has become apparent (Moore & Guan, 2001).

Axons in cortical layer I mature from the 28th week GA to the fourth postnatal month. Subsequently, axons begin to radiate into the deeper cortical layers--IV, V and VI--of the cortex up to one year of age. By two years of age the laminar organization of the cortex is adult-like and maturation continues in the deep cortical layers until age five. At six years of age mature axons begin to appear in the superficial cortical layers--II and III--and their density is equivalent to that of young adults by 12 years of age (Moore & Guan, 2001; Moore, 2002). Myelination of the associated auditory areas in the cortex matches the protracted cycle of myelination in the reticular formation (Yakovlev & Lecours, 1967).

In summary, the CAP is a collection of nuclei, interconnecting fiber tracts, relay and processing centers that analyze and transmits auditory signals from the ear to the primary

37

auditory cortex. To relate in brief, the functional bases of the CAP begins with at CN which receives, transforms and relays auditory information to the SOC. The SOC then receives and integrates this information arising from both the ipsilateral and contralateral CN. The LL then carries both afferent and efferent auditory information to the IC. The IC then serves as an obligatory relay center for all auditory fibers. The MGB then sends auditory attention information and multimodal arousal information to the auditory cortex, where it is then processed. In addition, the tonotopic organization established in the cochlea is preserved at all of these levels of the auditory system.

The nuclei of the CAP develop concurrently and are established and functional early in embryonic life, well before cochlear function is demonstrated (Rubel, 1978). From about the third to the eighth week GA, the generation of nuclei across the CAP appears to develop as a unit, with the exception of the IC and the auditory cortex, which continues until about the 17th week GA. The myelination of neurons across the CAP also appears to develop as a unit, from about the 26th week GA until about one year postnatal age, with the exception of the MGB, the reticular formation and the auditory cortex; the MGB demonstrates a condensed cycle whereas the reticular formation and the auditory cortex demonstrate a protracted cycle of myelination (cf. Table 2).

Chapter subsections 3.2 and 3.3 offered a complete overview on the development of the auditory system from the ME to the auditory cortex. The complexities of these systems as they mature differentially affect AEP response characteristics. Subsection 3.4 will detail these changes as transient and steady-state AEPs develop.

Table 2 Embryonic Development of the Central Auditory System

GA CN SOC LL IC MGB ARAS AI

3rd Layers of the cortex start _formation.

4th

Interstitial nuclei originate until the 7th _week Neurons of the ARAS generate until the 7th week. 6th Neurons of the IC generate until the 17th _week Neurons of the MGB develop. Late in the week, the MGB is displaced and begins to migrate until the 8th _week. 7th Neurons form the _CN. Neurons of the LL _generate.

8th

Differentiation of first and second- order neurons occurs. Nuclei of the SOC are apparent. 12th

Cortical plate forms. First stages of the sylvian fissure are recognized.

14th

Six distinct cortical layers are identified. 16th Cochlear nerve fibers innervate the CN of the brainstem. 18th

Synaptic vesicles are present above and below the cortical plate. 21st Cell differentiation begins. 38

39

Table 2 continued.

23rd

Synaptic terminals with vesicles are present above and below the cortical plate.

24th Cortical layers are _{differentiated.}

26th Myelin is _visualized. Myelin is _visualized. Myelin is _visualized. Myelin is _visualized.

28th Myelination begins and is complete by the fourth postnatal month.

Axons in cortical layer I start to mature until the fourth

postnatal month. Axons begin to radiate into layers IV, V and VI up to one year of age, and continue to mature until age five.

29th

Myelin increases until term; approximates an adult one year postnatal age

Myelin increases until term; approximates an adult one year postnatal age.

Myelin increases until term; approximates an adult one year postnatal age

Myelin increases until term; approximates an adult one year postnatal age 30th Morphology is similar to that of an adult. Additional growth occurs before the 7th _{postnatal week.}

40th

The MGB is seen in its final shape and location.

Myelination begins one year postnatal age and extends until puberty.

Axons continue to mature in the remaining cortical layers until 12 years of age. Myelination begins one year postnatal age and extends until puberty.