1.4. The inner ear
The inner ear comprises three main parts: the cochlea, the vestibule and the semi-circular canals
The vestibule and the semi-circular canals form the vestibular system that regulates balance. This section will not be explaining the vestibular system. Below, the parts of the inner ear that relate to the hearing system, ie the cochlea, are explained.
The cochlea is a canal, coiled spirally like a snail shell. The cochlea has 2¾ turns in humans. The length is approximately 35 mm and the cross-sectional area averages 3 mm. The turns taper from a wide end to a narrower base. The base is associated with picking up high-pitched sounds and the apex low-pitched sounds.
The cochlea is divided longitudinally into three compartments by two dividing membranes: the Reissner’s membrane, which is the ‘upper wall’, and the basilar membrane which is the ‘lower wall’. The central compartment is filled with a fluid referred to as endolymph. The outer two compartments are filled with a fluid referred to as perilymph.
Within the central area of the cochlea on the basilar membrane, sits the organ of Corti. The organ of Corti extends the entire length of the cochlea, and within, there are tiny cells called ‘hair cells’ on either side of the organ of Corti’s tunnel. On top of the hair cells are stereocilia, which are delicate, hair-like projections that react to cochlear fluid movement.
The hair cells are arranged in rows in the longitudinal direction of the cochlea. On the outside, there are three rows of ‘outer hair cells’ (OHC) in humans. On the inside is another row of ‘inner hair cells’ (IHC). The hairs of the OHC form the characteristic W-shape, while those of the IHC form a straight row. In total, there are 12,000 to 19,000 OHC and 3,000 to 4,000 IHC in a cochlea.
The function of the organ of Corti is to convert the sound vibrations into electrical signals that form the basis of the neural information for the higher centres of hearing. The mechanical energy, supplied from the middle ear, is converted by the movement of the stapes into a form of a pressure wave in fluid (hydraulic energy). It is transported sequentially through the perilymph and causes a transverse travelling wave in the endolymph along the basilar membrane. The place at which the deflection is maximum depends on the frequency(ies) of the sound presented. This is a direct consequence of the change in the mechanical properties of the basilar membrane during upward spiralling.
The following video provides a brief and clear visual explanation of the inner ear:
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1.2 The outer ear
1.3 The middle ear