The tectorial membrane is a gelatinous, ribbon-like structure located within the cochlea of the inner ear. It extends along the entire length of this snail-shaped organ, running parallel to the basilar membrane. This membrane plays an important role in the sense of hearing by facilitating the conversion of sound vibrations into the electrical signals that the brain interprets as sound.
Structure and Composition
The tectorial membrane is a highly hydrated extracellular matrix, largely composed of water (about 97% of its weight). The remaining dry weight consists of a complex mix of components, including collagen, non-collagenous glycoproteins, and proteoglycans. Collagen, specifically types II, IX, and XI, makes up approximately 50% of the dry weight, providing structural integrity.
Non-collagenous glycoproteins, such as α-tectorin and β-tectorin, contribute significantly, accounting for about 25% of the dry mass. These proteins are important as they form the striated sheet matrix, a framework that organizes the collagen fibers within the membrane. The membrane also contains glycosaminoglycans like uronic acid and keratan sulfate, and another glycoprotein called otogelin.
Role in Sound Transduction
The primary function of the tectorial membrane is sound transduction, converting mechanical sound vibrations into electrical signals. When sound waves enter the ear, they cause the basilar membrane, located beneath the tectorial membrane, to vibrate. This vibration generates a shearing motion between the basilar membrane and the tectorial membrane.
This shearing motion causes the stereocilia, tiny, hair-like projections extending from the sensory hair cells in the organ of Corti, to bend. The outer hair cell stereocilia are directly attached to the tectorial membrane, while inner hair cell stereocilia extend into small indentations on its lower surface. This mechanical bending of the stereocilia opens ion channels on the hair cells, allowing potassium ions to flow in.
The influx of potassium ions depolarizes the hair cell membrane, triggering the release of neurotransmitters. These neurotransmitters then excite associated nerve fibers, generating electrical signals. These electrical signals are transmitted along the auditory nerve to the brain, where they are interpreted as sound. The intensity of the sound is encoded by the number of hair cells stimulated and the rate of action potentials generated.
How It Refines Hearing
The tectorial membrane possesses complex mechanical properties that allow for precise and sensitive hearing. It exhibits anisotropy, meaning its mechanical properties vary depending on the direction of force applied, due to the organized arrangement of its collagen fibrils. This results in higher stiffness in the direction of the collagen fibers compared to other directions.
The membrane also displays stiffness gradients along its length, which are important for distinguishing different sound frequencies. It can also support and propagate traveling waves at frequencies relevant to sound. This ability suggests its involvement in the longitudinal transmission of energy within the cochlea, contributing to cochlear sensitivity and frequency selectivity. These properties enable the mammalian ear to detect faint sounds and precisely differentiate between various pitches.
Consequences of Dysfunction
Dysfunction or damage to the tectorial membrane can lead to various forms of hearing impairment. Genetic mutations in the proteins that make up the tectorial membrane are recognized causes of inherited hearing loss. For instance, mutations in the TECTA gene, which codes for α-tectorin, can result in nonsyndromic hearing loss.
Over 40 different TECTA gene mutations have been identified, leading to conditions like DFNA8/12 and DFNB21. These mutations can alter the structure of the tectorial membrane, preventing the proper conversion of sound into nerve impulses. The severity and characteristics of the hearing loss can depend on the specific location of the mutation within the α-tectorin protein, with some mutations affecting mid-frequency hearing and others impacting high-frequency sounds.