Cochlea - Comparative Physiology

Comparative Physiology

The coiled form of cochlea is unique to mammals. In birds and in other non-mammalian vertebrates, the compartment containing the sensory cells for hearing is occasionally also called "cochlea," despite not being coiled up. Instead, it forms a blind-ended tube, also called the cochlear duct. This difference apparently evolved in parallel with the differences in frequency range of hearing and in frequency resolution between mammals and non-mammalian vertebrates. Most bird species do not hear above 4–5 kHz, the currently known maximum being ~ 11 kHz in the barn owl. Some marine mammals hear up to 200 kHz. The superior frequency resolution in mammals is due to their unique mechanism of pre-amplification of sound by active cell-body vibrations of outer hair cells. A long coiled compartment, rather than a short and straight one, provides more space for mapping pitches, so is therefore better adapted to the highly derived functions in mammalian hearing.

As the study of the cochlea should fundamentally be focused at the level of hair cells, it is important to note the anatomical and physiological differences between the hair cells of various species. In birds, for instance, instead of outer and inner hair cells, there are tall and short hair cells. There are several similarities of note in regard to this comparative data. For one, the tall hair cell is very similar in function to that of the inner hair cell, and the short hair cell is very similar in function to that of the outer hair cell. One unavoidable difference, however, is that while all hair cells are attached to a tectorial membrane in birds, only the outer hair cells are attached to the tectorial membrane in mammals.