The outer spiral network and its innervation by the olivocochlear system

Outer hair cells (OHCs) are key elements of the mammalian hearing system, which amplify sound-evoked signals transmitted into the inner ear. OHCs are innervated by peripheral projections of olivocochlear (OC) and type-II spiral ganglion neurons. Type-II neurons innervate up to 100 OHCs, but their function is unknown. It has been suggested that type-II neurons interconnect neighboring OHCs through reciprocal synapses, which are bi-directional (afferent and efferent) synaptic interactions (Nadol, 1981). Since reciprocal synapses on the OHCs have been shown to be prevalent only in aged primates, they were thought to be a pathological finding. In addition to their interactions with OHCs, type-II neurons are also innervated by OC neurons. Synapses between OC and type-II neurons (OC/type-II synapses) have been described (Smith and Rasmussen, 1963), but these interactions have not been characterized in detail. Serial and semi-serial section transmission electron microscopy were used to study the synaptic interactions of type-II neurons with OHCs and OC neurons in a young human and in adult cats. A high prevalence of nerve terminals with reciprocal synapses was observed in the young human and in adult cats. These reciprocal terminals were processes of type-IIs, and not of OC neurons. Reciprocal type-II terminals were found in all frequency regions studied in cats, but were most prevalent below 4,000Hz. All the type-II fibers traced to more than one OHC in an adult cat had reciprocal interactions with OHCs. Type-II fibers/terminals were heavily innervated by OC neurons, which preferentially targeted terminals with reciprocal synapses that were predominantly afferent in an adult cat.

Department dc:contributor.department

Harvard University--MIT Division of Health Sciences and Technology.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• Thiers, Fabio Albuquerque

Advisor dc:contributor.advisor

• Joseph B. Nadol, Jr.

• Harvard University--MIT Division of Health Sciences and Technology.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/43871

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/43871