Difference Between Inner and Outer Hair Cells

The main difference between inner and outer hair cells is that the inner hair cells convert sound vibrations from the fluid in the cochlea into electrical signals that are then transmitted via the auditory nerve to the brain whereas the outer hair cells amplify low-level sounds that enter into the fluids of the cochlea mechanically.

Inner and outer hair cells are the receptive cells found in the inner ear. They are organized in two rows along the cochlear duct. The hairs of these cells project towards the inside of the cochlear duct. Generally, outer hair cells are more numerous than inner hair cells in humans.

Key Areas Covered

1. What are Inner Hair Cells
     – Definition, Facts, Function
2. What are Outer Hair Cells
     – Definition, Facts, Function
3. What are the Similarities Between Inner and Outer Hair Cells
     – Outline of Common Features
4. What is the Difference Between Inner and Outer Hair Cells
     – Comparison of Key Differences

Key Terms

Electromotility, Inner Hair Cells, Kinocilium, Outer Hair Cells, Prestin, Stereocilia

What are Inner Hair Cells

Inner hair cells are the main type of receptive cells found in the cochlea and they are responsible for converting sound waves into nerve impulses. Generally, around 3,500 inner hair cells are present in the human cochlea at birth. The sound vibrations come through the fluid of cochlea. When converted, the nerve impulses are transmitted through the auditory nerve to the auditory brainstem and to the auditory cortex.

Figure 1: Steriocilia and Kinocilium

At the apical surface of the inner hair cells, both steriocilia and one longer steriocilia called the kinocilium are present. There are approximately 100 tiny stereocilia. The kinocilium is present at one side of the inner hair cell. Both steriocilia and kinocilium face the tectorial membranes of the cochlea. The adjacent steriocilia are connected to each other by protein filaments at the apex of the steriocilia. The associated ion channels of these filaments open in response to tension. While beating the steriocilia along with the fluid of the cochlea, the deformation of the steriocilia towards the kinocilium generates a tension on the filaments. This opens the ion channels, entering the calcium ions into the cell in order to depolarize it. Once depolarized, the hair cell releases a neurotransmitter, probably glutamate from its base into the afferent fibers of the auditory vestibular nerve. Kinocilium is also responsible for the identification of head movement apart from converting sound waves into nerve impulses. Therefore, inner hair cells also serve as a vestibular organ.

What are Outer Hair Cells

Outer hair cells are the other type of hair cells present in cochlea. They are arranged in the organ of corti in three rows. The sensitivity of hearing in mammals is similar to that of other vertebrates without the functioning of the outer hair cells. The sensitivity is around 50 dB. Therefore, the main function of the outer hair cells is to pre-amplify the sound waves with a low amplitude. Outer hair cells increase the sensitivity from about 200 kHz in marine mammals.

Figure 2: Organ of Corti

The amplification of the low-amplitude sound waves is by the electromotility. Prestin is the trasnmembrane protein specifically expressed in the lateral membrane of the outer hair cells. It contracts and elongates according to the depolarization and the hyperpolarization of the outer hair cells. This mechanical response is due to the voltage-dependent conformational changes of the prestin. The system is called the cochlear amplifier.

Similarities Between Inner and Outer Hair Cells

• Inner and outer hair cells are two types of receptive cells found in the inner hair.
• They occur in the organ of corti, which is situated on the basilar membrane in one of the three compartments of the cochlea.
• They are organized along the cochlear duct.
• Steriocilia cover the apex of the cells. A kinocilium occurs in one side of the apex.
• Steriocilia and kinocilium are collectively called hairs.
• The hairs of them project towards the tectorial membranes of the cochlea.
• The resting membrane potential of the hair cells is -65 mV.
• When depolarized, calcium ions enter into these cells, which results in the release of glutamate at the peripheral terminals of fibers of the vestibulocochlear nerve.

Difference Between Inner and Outer Hair Cells

Definition

Inner hair cells refer to the receptive cells in the cochlea, converting the sound waves into a nerve signal while outer hair cells refer to the receptive cells in the cochlea, which mechanically pre-amplify the low-level sound by the movement of the hair bundles.

Number

Inner hair cells are low in number and approximately 3500 inner hair cells are present in human cochlea. There are more outer hair cells; approximately 12,000 cells are present.

Number of Rows

Inner hair cells are arranged into a single row while outer hair cells are arranged into three rows.

Location

Inner hair cells are located at the terminals of the inner hair cells while outer hair cell nerves are located at the terminals of the outer hair cell nerves.

Function

Inner hair cells convert sound vibrations from the fluid in the cochlea into electrical signals while outer hair cells mechanically amplify low-level sounds that enter into the fluids of the cochlea.

Innervation

Prestin

Inner hair cells do not express prestin while outer hair cells express prestin in high levels as it is the motor protein that aids in the electromotility process.

Conclusion

Inner hair cells are the major type of receptive cells found in the organ of corti, which are responsible for both auditory and the vestibular sensitivity. The outer hair cells are receptive to the sound waves with low amplitude and they mechanically amplify the signal, allowing mammals to hear sounds with a higher sensitivity than other vertebrates. The main difference between inner and outer hair cells is mainly their function.

Reference:

1. “Chapter 7D – Auditory System.” REVIEW OF CLINICAL AND FUNCTIONAL NEUROSCIENCE – SWENSON, Available Here   
2. Frolenkov, Gregory I. “Regulation of Electromotility in the Cochlear Outer Hair Cell.” The Journal of Physiology, Blackwell Science Inc, 1 Oct. 2006, Available Here

Image Courtesy:

1. “Haircell frog sacculus” By A. James Hudspeth, M.D., Ph.D. – Personal communications (CC BY-SA 3.0) via Commons Wikimedia 
2. “Organ of corti” By Madhero88 – Own work (CC BY-SA 3.0) via Commons Wikimedia