Premium
Efferent axons in the avian auditory nerve
Author(s) -
Köppl Christine
Publication year - 2001
Publication title -
european journal of neuroscience
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.346
H-Index - 206
eISSN - 1460-9568
pISSN - 0953-816X
DOI - 10.1046/j.0953-816x.2001.01567.x
Subject(s) - efferent , anatomy , biology , basilar membrane , hair cell , tonotopy , population , efferent neuron , vestibular nerve , vestibular system , barn owl , sensory system , neuroscience , cochlear duct , cochlea , afferent , medicine , paleontology , environmental health , predation
The sensory hair cells of the inner ear receive both afferent and efferent innervation. The efferent supply to the auditory organ has evolved in birds and mammals into a separate complex system, with several types of neurons of largely unknown function. In this study, the efferent axons in four different species of birds (chicken, starling, barn owl and emu) were examined anatomically. Total numbers of efferents supplying the cochlear duct (auditory basilar papilla and the vestibular lagenar macula) were determined; separate estimates of the efferents to the lagenar macula only were also derived and subtracted. The numbers for auditory efferents thus varied between 120 (chicken) and 1068 (barn owl). Considering the much larger numbers of hair cells in the basilar papilla, each efferent is predicted to branch extensively. However, pronounced species‐specific differences as well as regional differences along the tonotopic gradient of the basilar papilla were documented. Myelinated and unmyelinated axons were found, with mean diameters of about 1 µm and about 0.5 µm, respectively. This suggests two basic populations of efferents, however, they did not appear to be distinguished sharply. Evidence is presented that some efferents lose their myelination at the transition from central oligodendrocyte to peripheral Schwann cell myelin. Finally, a comparison of the four bird species evaluated suggests that the efferent population with smaller, unmyelinated axons is the phylogenetically more primitive one. A new population probably arose in parallel with the evolution and differentiation of the specialized hair‐cell type it innervates, the short hair cell.