COCHLEA & ITS FUNCTIONS

ANMOL JILANI

PHYSIOLOGICAL ANATOMY OF COCHLEA

Cochlea is a system of coiled tubes.

It consists of: Three tubes: 1-Scala vestibuli. 2-Scala media. 3-Scala tympani.

MEMBRANES

Two membranes:1-Reissners (vestibular

membrane).2-Basilar membrane

FLUID PRESENT IN COCHLEA

o Perilymph: Present in scala vestibuli

and scala tympani. Almost identical to CSF.o Endolymph: Present in scala media.

BASILAR MEMBRANE AND RESONANCE IN COCHLEA

Contains 20,000 to 30,000 basillar fibers.

These fibers are stiff,elastic,reed-like structures that are fixed at their basal ends in the center of cochlea (modiolus)but not fixed at their distal ends.

BASILAR FIBERS

Length:The lenghth of the basilar fibers increase

progressively from the base of cochlea to its apex.

Diameter:decreases from base to apex of cochlea.

Frequency: Base :high frequency. Apex :low frequency.

TRANSMISSION OF SOUND WAVES IN THE COCHLEA-THE TRAVELING WAVES(PLACE PRINCIPLE)

If the foot of stapes moves inward.

The round window bulge outward instantaneously because the cochlea is bounded on the sides by bony walls.

Therefore,the initial effect is to cause the basilar membrane at the very base of the cochlea to bulge in the direction of the round window.

CONTINUE…

however,the elastic tension that is built up in the basilar fibers as they bend toward the round window initiates a wave that travels along the basilar membrane toward the helicotreama.

PATTERN OF VIBRATION OF BASILAR MEMBRANE FOR DIFFERENT SOUND FREQUENCIES

High frquency sound wave. Medium frequency sound wave. Very low frequency sound wave.

ORGAN OF CORTI Lies on the surface of bassilar

membrane. Contain mechanically sensitive hair cells

which are receptive end-organs and generate nerve impulses in response to sound vibrations.

SENSORY RECEPTOR IN ORGAN OF CORTI

Inner hair cell. Outer hair cell. Depolarization. Hyperpolarization

.

HAIR CELL RECEPTOR POTENTIAL

Stereocilia Direction toward the longer one Rapid K influx to tips of the

stereocilia Depolarization of cell membrane Basilar fiber to scala vestibuli ………

depolarization Endocochlear Potential

Perilymph (Na, K )EndoLymph(K, Na ) +80mv diff. all the time

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