Figure 1: A middle-ear 3-D reconstruction from micro-CT imaging. The ossicles are shown in yellow, flexible ligament attachments to bony structures in blue, and the tympanic membrane and tensor-tympani muscle in red. The stapes terminates into the fluid-filled inner ear (not shown).
Within the animal kingdom, mammals are unique in having a chain of three middle-ear bones (the malleus, incus, and stapes) to transmit eardrum (tympanic membrane) vibrations to the sensory structures in the fluid-filled cochlea of the inner ear. These three ossicles are, in fact, one of the defining characteristics of mammals, and are of particular importance to paleontologists given that other well-known features of mammals (e.g., hair and milk production) do not fossilize.
In contrast to the middle ears of non-mammalian tetrapods, in which the eardrum pushes directly onto the stapes via a rodlike columella, the mammalian middle ear instead transfers eardrum motions to the stapes via a circuitous route, for which the majority of the ossicular mass is notably concentrated away from the entry axis into the cochlea (Figure 1).
What are the advantages of this peculiar middle-ear design, and why do most species of mammals retain flexible joints between the ossicles that might be expected to reduce energy transmission?
A clue may lie in the fact that terrestrial mammals can commonly hear frequencies well into the ultrasonic range and in some cases over 100 kHz (e.g. bats), while birds, reptiles, and amphibians cannot hear above 10–12 kHz, with rare exceptions.
Figure 2: Simulated motion of the eardrum and ossicular chain, based on a finite-element model of the middle ear (at 1 kHz, with motions scaled 4000x).
Could the mammalian middle ear be inherently better suited to high-frequency transmission than a columella-based structure? Could the primary advantage of its flexibility and indirect coupling to the cochlea be to protect against static pressure and impulsive stimuli? Or could the unique distribution of ossicular mass also provide a means of filtering out the din of self-generated sounds transmitted via bone conduction, which would otherwise distract from hearing external sounds critical for survival?
Our long-term goal is to understand how middle-ear characteristics affect sound transmission via air conduction from the ear canal to the cochlea, as well as via the bone-conduction pathway. We investigate possible advantages of the mammalian ossicular chain, both to further our scientific understanding and to provide a basis for improving middle-ear reconstructions, especially because the defining characteristics of the mammalian ossicular chain are absent from the columella-like prostheses used in such surgeries.
Our current central hypotheses are that the mammalian ossicular chain 1) enhances sound transmission via a lever system that makes use of rotational motions along lower-inertia axes at higher frequencies; 2) provides flexibility in the joints between bones that not only protects the cochlea, but permits complex eardrum vibrational modes to be transferred to the ossicles; and 3) reduces the amplitude of self-generated sounds entering the cochlea through the bone-conduction route. These ideas are being tested on the human, cat, and mouse permutations of the basic mammalian design, using micro-CT imaging, 3D motion measurements of the normal ossicular chain and modified versions with fused joints or a columella-style prosthesis, as well as carefully validated 3D computational models based on the tested specimens.