Cochlea

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Named after the Latin word for snail shell, the cochlea is a coiled, tapered tube inside the mammalian inner ear, responsible for transmitting sound to the Organ of Corti, the sensory organ of hearing.

Contents

Anatomy

The cochlea consists of three fluid-filled chambers - scala tympani and scala vestibuli(both of which contain perilymph), and scala media (which contains endolymph). The scala tympani and the scala vestibuli are contiguous with each other, merging at the tip of the "snail's shell" - the helicotrema. The stapes transmits vibrations to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph in scala vestibuli. This in turn vibrates the endolymph in scala media, which causes movement of the basilar membrane between scala media and scala tympani. When the basilar membrane moves, it does so relative to the tectorial membrane, a noncellular structure composed of collagen and specialised proteins. In between the basilar membrane and the tectorial membrane lies the Organ of Corti containing the sensory hair cells that convert vibration into electrical potentials. The hair cells are arranged in four rows along the entire length of the Organ of Corti, three rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). Only the IHCs are sensory cells, ie. those which convert motion to neural signals. The outer hair cells instead receive neural inputs and convert them to motion, elongating the cells with each impulse. This is the basis for the OHCs role as part of a "mechanical amplifier". More information can be found on the hair cells page.

Sensory transduction

The tectorial membrane acts as an anchorage point for the tips of the hair cell stereocillia, and as the basilar membrane moves upwards causes a sideways motion of these "hairs". In IHCs this opens mechanically gated ion channels and allowing the influx of potassium ions from the potassium rich endolymph, causing the membrane potential of the hair cells to rise. The inner hair cells are connected to the auditory nerve via the spiral ganglion, a structure with connections to both the outer hair cells and the brain. The vestibulocochlear nerve (cranial nerve VIII also known as the auditory nerve) transmits sound data to the cochlear nucleus within the brainstem as action potentials.

Comparative physiology

Because the properties of sound waves are universal - a 100Hz tone has the same properties whether it is being received by a bat cochlea or a human cochlea - so the dimensions of the cochlea remain relatively constant across mammalian species. Unlike an eye, making it larger will not increase sensitivity. Instead, mammals have evolved a wide variety of external ear structures or Pinna which vary in size and shape according to the requirements of the species. In birds the organ containing the sensory cells is still called the cochlea, although it is not coiled up like a snail. Instead it forms a blind ended tube sometimes called the cochlear duct.

Frequency Discrimination in the Cochlea

As we have seen, the cochlea is a sound sensitive organ, but it is also specialised to convey information on pitch, or the frequency of sounds.

Georg von B髩sy won the Nobel Prize for Physiology or Medicine in 1961 for his work elucidating the workings of the cochlea.

See also: Cochlear implant

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Sensory system - Auditory system Edit (http://en.wikipedia.org/w/wiki.phtml?title=Template:Auditory_system&action=edit)
Pinna - Ear canal - Eardrum - Ossicles - Cochlea - Basilar membrane - Organ of Corti - Hair cells - Auditory nerve - Primary auditory cortex
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