The Morphology of Auditory Neurons and Auditory Localization

Abstract

The mammalian auditory system of the brain stem contains a number of classes of nerve cells the morphology of which may be relevant to the binaural localization of sounds. The axons of the acoustic nerve give rise to large synaptic endings (bulbs of Held) on the cell bodies of two types o[ nerve cells (types c and g) of the cochlear nucleus. The size of these endings suggests that they dominate the firing patterns of the postsynaptic cells, preserving in them the temporal characteristics of the activity in the acoustic nerve fibers. The medial superior olivary (mso) nucleus contains horizontally oriented bipolar nerve cells whose lateral dendrites probably receive synaptic endings from contralateral type c cells. Their bilateral innervation from cells that are synaptically dominated by the acoustic nerve fibers has suggested that the cells of the mso are of significance in the location of sounds. The medial nucleus of the trapezoid body contains nerve cells each of which is innervated by a single massive synaptic ending (chalice of Held) from large axons of the contralateral type g cells. This would appear to be a rapidly conducting system in which temporal characteristics of acoustic nerve activity may be preserved. The number of nerve cells in the major nuclei of the superior olivary complex varies across mammalian species. The mso varies from 0 (bats and dolphin) to 4200 (cat) nerve cells. The absence of this nucleus in the echolocating bats and dolphin shows that this is not an essential structure for sound localization in mammals. The lateral superior olivary nucleus is well developed in bats (4300) and dolphin and is also present in all species we have examined. This suggests that this nucleus is involved in auditory localization and complex auditory discriminations. The medial nucleus of the trapezoid body is well developed (2100–5800 nerve cells) in all species examined.