The tympanic membrane: highly developed smooth muscle arrays in the annulus fibrosus of mustached bats.

The annulus fibrosus tympanicus is the thickened peripheral rim of the pars tensa of the tympanic membrane. It is an area into which the connective tissue matrix of the membrane extends to attach to the tympanic bone (ring). Light microscopy, SEM, TEM and confocal microscopy were used to study the area in mustached bats Pteronotus p. parnellii, P. p. portoricensis and P. quadridens. In cross sections of the annulus morphologically distinct apical and basal or myovascular zones could be recognized. The apical zone had a collagenous matrix that was continuous with the pars tensa. The myovascular zone contained an extensive network of thin walled endothelial tubes and a well-developed array of radially arranged smooth muscle cells that filled the interval between the vessels. The muscle tissue occupied the interval between the densely collagenous "apical zone" and the bony tympanic ring and was closely associated with bundles of unmyelinated nerve fibers. The structure and arrangement of the tissue suggests a highly developed specialization for the tonic control of tension of the pars tensa and a system that potentially regulates sound transmission to the middle and inner ear.