CHAPTER TWO
Anatomy and Physiology of the Human Ear
Richard J. Salvi, Ph.D., Edward Lobarinas, Ph.D. and Wei Sun, Ph.D.

The organ of Corti is the transducer of the cochlea. It is the structure that translates mechanical vibrations in the fluid of the inner ear to equivalent electrical impulses that can be carried up to the brain (like an old record player that converts the vibration of the stylus into an electrical signal). Figure 2-2 shows each turn of the cochlea that contains three fluid-filled compartments: scala vestibuli (top) and scala tympani (both filled with perilymph), and scala media (filled with endolymph fluid). The organ of Corti contains sensory hair cells that lay on the basilar membrane and convert sound into neural activity.

The organ of Corti extends medially from the center of the spiral toward the stria vascularis and spiral ligament on the outside wall (Figure 2-2). There are two types of the aforementioned sensory hair cells. Both contain tiny microscopic cilia that comprise the transduction channels that allow the outer hair cells (OHC) and inner hair cells (IHC) to convert mechanical energy to neural impulses. The OHCs are located toward the “outer” part of the organ of Corti, nearer the stria vascularis and the IHC, as the name suggests, are located closer to the inner portion (center) of the organ of Corti. There are three rows of OHC and only one row of IHC. The base of each OHC is supported by special (Deiter) cells while the top surface of each OHC is held in place by the reticular lamina that faces the scala media. Further from the center of the inner ear resides the single row of IHC.

On the surface of the OHC sits the tectorial membrane. This membrane is actually in contact with the OHC, but not the IHC. This is a cellular, gelatinous-like structure that is connected on the inner side of the organ of Corti only. It is free to move on the outer side and extends to the outermost row of OHC. The relative movement between the sensory cells and the tectorial membrane causes sheering (bending or deflection) of the hair bundles on OHC while fluid turbulence causes movement of bundles on the IHC. Both movements result in hair cell action potentials that send neurological impulses to the brain.