Auditory brain stem response generation by parallel pathways: differential maturation of axonal conduction time and synaptic transmission.

In attempting to correlate developmental anatomical data with electrophysiological data on maturation of the auditory brain stem response (ABR), a model of ABR generation was necessary to match neuroanatomical structures to ABR components. This model has been developed by reviewing quantitative studies of human brain stem nuclei, results of intrasurgical recordings, studies of correlation of pathology with ABR waveform alterations, and findings from direct stimulation of the human cochlear nuclei through a brain stem implant device. Based on this material, it was assumed that waves I and II are generated peripherally in the auditory nerve and that waves III, IV, and V are generated centrally, i.e., by brain stem structures. It was further assumed that wave III is generated by axons emerging from the cochlear nuclei in the ventral acoustic stria and that waves IV and V reflect activity in parallel subpopulations of these ascending axons at a higher brain stem level. Beyond the cochlear nucleus, the largest component of the brain stem auditory pathway consists of axons projecting without interruption from the cochlear nuclei to the contralateral lateral lemniscus and inferior colliculus. In the proposed model of ABR generation, the III-IV interwave interval is assumed to reflect only axonal conduction in this asynaptic pathway. Electrophysiological data from infants indicate that the III-IV interwave interval becomes adult-like by the time of term birth. The second largest component of the brain stem auditory pathway is the bilateral projection through the medial olivary nucleus. The model assumes that activity in this monosynaptic pathway, consisting of axonal conduction time plus one synaptic delay, is reflected in the III-V interwave interval. If both of the preceding assumptions are true, the IV-V interwave interval represents the difference between the two pathways, i.e., the time of transmission across one synapse. The electrophysiological ABR data indicates that the IV-V interval does not mature until one year of age. It is also possible to apply this model to the peripherally generated portion of the ABR. The I-II interwave interval, assumed to solely represent conduction in VIIIth nerve axons, is adult-like before the time of term birth. The II-III interval, presumed to contain a synapse in the cochlear nuclear complex, does not reach an adult level until between 1 and 2 yr postnatal age.