**6. Conclusion**

reflects a steady increase in primates corresponding to the capability of low-frequency

The well-developed human medial olivary nucleus seems to be the basis for extraction of interaural time and phase differences, whereas the smaller human lateral olivary nucleus probably functions in the analysis of interaural differences in frequency and intensity. The lateral and medial nuclei together form the basis for localization of a sound stimulus and enable

The myelinated medial fibres, which innervate outer hair cells, project ipsi- and contralateral and the unmyelinated lateral fibres, which innervate the dendrites of afferent nerve fibres, project mainly ipsilateral [66]. There exists distinct but complex geometrical and functional

Neurotransmission of the efferent system takes place by inhibitory and excitatory transmitters reflecting fine regulation. The numerous neurotransmitters provide for the auditory system a wide operating range to enhance or depress environmental stimuli and can be co-localized as well. The neurotransmitter of the medial olivocochlear fibres includes ACh (acetylcholine), GABA (gamma aminobutyric acid), CGRP (calcitonin gene-related peptide), ATP (adenosine triphosphate), enkephalins and NO (nitric oxide) [68, 69]. The transmitter of the lateral efferent system includes ACh, GABA, CGRP, dopamine, serotonin and opioids such as dynorphin or

**Figure 4.** Course of the medial and lateral efferent systems. (**A**) The auditory brainstem section. Sound representations from the ear ascend to the olivary complex via the ventral afferent pathway and project back to the ear via dorsal crossed and uncrossed medial and lateral efferent fibres. (**B**) Cross-sectional view of the inner ear. The major ascending afferent pathway arises from inner hair cells. Descending olivocochlear projections terminate on inner and outer hair cells (with permission from Ref. [78], © 1990, Elsevier; and Ref. [79], Copyright © 2004 American Medical Association.

alignment of the efferent fibres, their connections and neurotransmitters [67].

us to function in a three-dimensional auditory world [64, 65].

hearing [63].

216 Advances in Clinical Audiology

enkephalin [70–72].

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The cochlea is a unique sense organ with regard to its qualities in continuous stimulus perception and stimulus discrimination capacity. This corresponds to the finely aligned hair cell receptors, their surrounding supporting cells and structures embedded in unique fluid spaces, and to further afferent and efferent neural processing.




**Table 1.** Comparison of the perception qualities of the human sense organs.



already the first step of central processing of **form and motion**;


molecules/ml;







simulate one of these four qualities;

function of outer hair cells and the stapedius reflex);



opening of Na+

vision theory);

*Olfactory system*

*Gustatory system*

off-center (light-off) neurons;

nm;

**Perception range**

218 Advances in Clinical Audiology

**Discrimination capacity**

**Further qualities**

**Perception characteristics**

**Perception range**

**Discrimination capacity**

**Further qualities**

**Perception characteristics**

**Perception range**

**Discrimination capacity**

channels;


than in the periphery; sharpest vision decrease to the periphery of the retina;





















**Table 2.** Comparison of the structural characteristics of the human sense organs.
