A spiral-shaped organ of the inner ear that detects frequency variations in incoming sound waves (see figure below for the structures that make up the cochlea). With high-pitched sounds, one end of the cochlea is activated, while a low-pitched one does so for the other end. The sounds, as standing waves, displace the stereocilia on the tips of the hair cells, which are then polarised by the wave leading to vesicles in the cells releasing neurotransmitters into the auditory nerve. When the hair cells are damaged, this leads to sensorineural hearing loss. The function of the cochlear implant is to replace these hair cells.
Structures of the cochlea.
Scala media: also referred to as the cochlear duct, containing endolymph, it is the central duct of the cochlea. The cilia of the hair cells project into this spirally formed structure in the bony canal of the canal of the cochlea. It contains the organ of Corti and is separated from the scala tympani and scala vestibuli by membranes (from above by the vestibular membrane and below by the basilar membrane).
Scala tympani: the lower, perilymph-filled, bony passage of the cochlea, also referred to as the tympanic ramp. Communicates with the scala vestibuli at its apex or upper end. Essential for hearing, it is where sound waves are converted into electromechanical impulses that are sent through the cochlear nerve to the temporal lobe. If damaged, it can give rise to hearing impairment or deafness.
Scala vestibuli: also referred to as the vestibular ramp, it is the upper, perilymph-filled, bony passage of the cochlea that together with the scala tympani conducts amplified sound waves to the scala media having received sound vibrations from the stapes.
Basilar membrane: separating the fluid-filled scala media and scala tympani, it is formed by short and thin elastic fibers stretched across the cochlear duct, being closely packed in the basal region close to the stapes and becoming longer and more sparse toward the apex of the cochlea. Being under tension, the fibers vibrate like a musical instrument. In fact, it is organized tonotopically, with high frequencies at the base and low frequencies at the apex (and in this respect the musical instrument it most resembles is a xylophone). This part of the cochlea is crucially important to hearing in that it is functions as a frequency detector. In short, it where the process of determining what a sound is begins.
Cochlear nerve: see Auditory (or acoustic) nerve
Endolymph: also known as the endolymphatic or Scarpa’s space (or fluid), it is a completely unique extracellular fluid, with an ion composition not found anywhere else in the body. Its main cation (positively charged ion) is potassium, and has virtually no sodium. Due to its uniqueness, the processes by which it is maintained cannot be generalized to other fluid systems, and its fluid dynamics is still a matter of some controversy. Relative to the perilymph, the endolymph has a high positive electrical potential in the cochlea. This potential difference means that potassium ions can flow into the hair cells, thereby stimulating the hair bundle. Movement of endolymph results in vibration of the basilar membrane, which in turn causes the organ of Corti to move against the tectorial membrane (a jellylike membrane covering the organ), thus stimulating the generation of neural impulses. Fluid waves in the endolymph are not confined to scale media, but also occur in the semicircular canals (angular acceleration of the endolymph stimulates the vestibular receptors of the endolymph). Disruptions of the endolymph (e.g., through jerky movements) can generate dizziness* and motion sickness (e.g., in car passengers).
Hair cells: the sensory receptors of the auditory system (as well as the vestibular system) located in the organ of Corti. There are functionally two types of cochlear hair cells in humans: one row of inner cells and three rows of outer hair cells. The inner hair cells are the actual sensory receptors, with some 95% of auditory nerve fibers originating from them. They transform sound waves in the cochlear fluids into electrical signals relayed via the auditory nerve to the brain stem and the auditory cortex. The terminations of the outer hair cells are almost completely from efferent axons that do not send neural signals to the brain. They appear to mechanically amplify low-level sounds in the cochlea, with amplification seemingly arising from movements of their cell bodies.
Organ of Corti: found only in mammals, it is a gelatinous mass about 4 cm long. Enclosed in the cochlea and protected by the temporal none (the hardest bone in the body), it is estimated that it contains some 23,500 auditory nerve receptors, each receptor having its own hair cell. The hair cells are arranged in four rows along the basilar membrane: a single row of inner hair cells and three rows of outer hair cells, with the individual inner cells having multiple strands called stereocilia. The basilar membrane supports the organ, from which the hairs stick through the roof of the organ and embed themselves in the overhanging tectorial membrane. As the vibrating basilar membrane moves in and out, the hairs of the organ moves with it. If it moves to the left and the tectorial membrane to right, a shearing action occurs in the cochlear duct. The consequence is that the hair cells of the organ are activated to send electrochemical signals ultimately to the auditory cortex. The organ of Corti is sometimes referred to the ‘microphone of the body’ perhaps because it converts mechanical into electrical energy and conveys a coded version of the original sound, not about the fundamental frequency, but also intensity and timbre as well.
Perilymph: an extracellular fluid located in the scalae tympani and vestibuli. It is derived from blood plasma and is similar, but identical with the ionic composition of cerebrospinal fluid and the aqueous fluid of the eye. Its main cation is almost exclusively sodium, its high concentration being about 150 millequivalents per litre. Due to the fact that the perilymph and endolymph differ greatly not only biochemically, but also electrically, they are kept strictly apart in the cochlea by means of the vestibular membrane. Vibration of the stapes transmits a sound wave to the scala vestibuli, generating fluid waves in the perilymph. In turn, the waves lead to a displacement of the basilar membrane and the organ of Corti. This sequence represents the first stage in the transduction process, with the frequency induced on the basilar membrane directly related to the frequency of the sound stimulus.
Stapes (not shown in figure): a small, stirrup-shaped bone or ossicle (in fact, the smallest and lightest bone in the human body) in the middle ear (air-filled central cavity of ear behind the eardrum). It transmits sound vibrations from the incus (another small bone in the middle ear, the other being the malleus) to the oval window next to the inner ear (the oval window being a membrane-covered opening leading to the vestibule of the inner ear, which is part of the ear in the temporal bone containing the cochlea and semicircular canals).
Vestibular membrane: a thin and delicate membrane that serves the crucial task of separating the scala media from the scala vestibuli, and thus the contrasting endolymph from the perilymph. Together with the basilar membrane, it creates a cavity in the cochlea filled with endolymph that is crucial to the functioning of the organ of Corti. Also functions as a diffusion barrier allowing nutrients to travel from the perilymph to the endolymph.
* To appreciate this effect, try the following two-step maneuver:
1. Spin round rapidly 5-10 times to the right and stop. On stopping, you should feel somewhat dizzy. When feeling recovered:
2. Repeat step 1, stop, and then immediately spin the same number of times to the left and stop. On stopping, you should no longer feel dizzy. Why? Because in step 2, the endolymph moves in the opposite direction and the two motion effects cancel each other out.
See Acceleration, Auditory (or acoustic) nerve, Brain stem, Cerebrospinal fluid, Cilia, Cochlear implant, Cochlear nucleus, Cranial nerves, Extracellular matrix, Fundamental frequency (F₀), PItch, Primary sensory cortices, Timbre, Vesicles, Vestibular system