Lesson12 The inner ear

The inner ear consists of two parts:

1. The cochlea- responsible for hearing

2. The semicircular canals- three tiny, fluid filled tubes in the inner ear that help maintain balance

The base of the cochlea is located nearest to the oval window and is often referred to as the basal turn. The apex is the other end of the coil and is referred to as the apical turn.

There are two membranes that divide the cochlea into three fluid filled chambers :

1. Scala Vestíbuli

2. Scala media

3. Scala tympani

The dividing membranes are the basilar membrane and Reissners membrane.

Reissner’s membrane separates the Scala media from the Scala vestibuli. The basilar membrane separates the Scala media from the Scala tympani. The Scala vestibuli and the Scala tympani meet at the apex of the cochlea (Helicotrema) and share a fluid called perilymph.

The organ of Corti is the sensory receptor/end organ of hearing and is located in the Scala media. It consists of rows of cells called hair cells

The oval window is one of two openings at the base of the cochlea that enables the transfer of energy between the middle and inner ears.

Transfer of energy:

1. Acoustical energy: sound vibrations strike the eardrum

2. Mechanical energy: the auditory ossicles vibrate and the footplate of the stapes moves at the oval window

3. Hydraulic energy: the pressure wave is propagated throughout the cochlea

4. Electrochemical energy: nerve impulses are created by hair cell stimulation

Frequencies range of normal human hearing extends from 20 to 20000 Hz. The structure of the basilar membrane determines where frequencies are recognized within the cochlea.

The basal turn of the cochlea responds to high frequencies. The apical turn responds to low frequencies. This is the foundation of the Place Theory of Hearing.