Hearing is a very complex procedure and involves a series of steps in between the origination of the sound source and our brain’s interpretation of it as a particular type of sound. Although this whole process is as quick as a fraction of a second, the journey of sound is quite complicated, just like any other mechanism in the human body.

You will be surprised to know that the human ear is quite well-developed at birth and even at the pre-natal stage. This explains why babies respond to sounds while in their mother’s womb. Every baby with normal and healthy hearing has the ability to hear and respond to soft and loud sounds right away. However, if your baby fails to do so, then it might be a sign that you need to be high on alert. Do consider getting your child’s hearing test done right away. If your baby is diagnosed with a hearing impairment then do consider opting for a good line of hearing loss treatment by consulting a licensed and professional Audiologist at every major or minor step that you take. Nowadays, however, every hospital performs a hearing screening of newborn babies right after birth and before you take him or her home.

One very important part of hearing are the sensory cells present inside the cochlea of the inner ear - known as the hair cells. But before we get deep into that, let’s take a brief look at the hearing mechanism once more to simplify our understanding of the hair cells. Because just like the structure and arrangement of these tiny cells, their functioning too is quite complex and requires a deeper understanding of the entire scenario.

 

The Hearing Mechanism

As already mentioned, the human hearing mechanism is a complex one. It involves a good number of phases, it also associates a number of elements and sections of the ear. So let’s see how do we actually hear.

 

The Sections of the Ear

There are three main parts of the human ear -

  • The Outer Ear - It includes the pinna, the long passage called the ear canal and the tympanic membrane, better known as the eardrum.

  • The Middle Ear - It comprises the three small ear bones - incus, malleus and stapes - together known as the ear ossicles.

  • The Inner Ear - The inner ear, which is the main component of the human hearing system, is also known as the labyrinth of the ear. Now that we know that it is solely responsible for our hearing system and the equilibrium of the body, we will head over to discuss the hair cells present inside the cochlea.

It comprises the most vital and sensitive organ - the spiral-shaped cochlea. The inner ear also includes two other important components, namely the vestibule and the semicircular canals - which together are responsible for maintaining the body balance and the body posture, thus preventing us from falling.

 

How Does the Hearing Mechanism work?

Let's break down the hearing process into simpler steps for better understanding.

  1. Sound waves enter the ear canal and strike the eardrum and make it vibrate.

  2. This vibration is then carried forward to the small ear bones or the ear ossicles.

  3. The middle ear bones amplify these sounds and send them to the cochlea of the inner ear. These vibrating waves move through the fluid present in the cochlea, thus making it move.

  4. This movement of the fluid further triggers the hair cells which are present inside the cochlea and causes them to move. These hair cells detect this movement and convert these signals into electrical signals for the auditory nerve.

  5. The auditory nerve then sends these converted electrical impulses to the brain which then interprets them as sounds. It is at this time that we actually ‘hear’.

But how well would these sounds travel from the outer ear to the inner ear and how well would these sounds be perceived by the brain depends a lot on how well your ears work. In case of any malfunctioning or disorders in your ears, you may face difficulties in hearing. In such a case, do consider getting a proper hearing loss treatment.

 

What are Hair Cells?

In all vertebrates, hair cells are sensory receptors which are not only present in the cochlea of the inner ear but are also present in the vestibular system. In this blog, however, we are concerned about the cochlear hair cells.

In mammals, the hair cells are located in the spiral organ of Corti on the basilar membrane of the cochlea. The cochlea has thousands of such tiny hair cells (we will get into the figures in the next section).

 

The Structure of the Hair Cells

Each hair cell has two parts - the body and the stereocilia present at the head of every hair cell. Stereocilia is a microscopic hair-like structure sticking out on top of every hair cell. These stereocilia are triggered by every sound vibration due to which they rock back and forth.

Stereocilia are the main components of hair cells. These are responsible for converting the sound waves into chemical signals. Whenever the hair cells move due to the cochlear fluid movement, the stereocilia bend. This bending causes the pore-like channels present at the tip of the stereocilia, to open up. This further facilitates the chemicals to rush into these hair cells, thus building up an electrical signal. These electrical impulses are then sent to the brain for further interpretation and perception of the sound. Then we recognise the sound and understand it, and immediately know how to respond to it.

Not all hair cells, however, move at the same time. Different hair cells move for different types of sounds. Hair cells located near the wide end of the cochlea detect higher-pitched sounds, such as an infant crying. Those closer to the centre of the cochlea detect lower-pitched sounds, such as the barking of a large dog. The high-frequency hair cells get damaged most easily, and hence people with hearing loss due to exposure to loud sounds often have problems hearing the high pitched sounds like crickets or birds chirping.

 

What Happens If These Hair Cells Get Damaged?

If the sounds are excessively loud, these stereocilia would bent or even break off, thus causing the death of the hair cell. A dead hair cell can never convert nor send signals to the brain, thus reducing your ability to hear well. The hair cells once broken can never grow back or be restored, and you would lose your hearing permanently in such a case. The more the number of damaged hair cells, the greater will be the intensity and degree of your hearing loss. This type of hearing impairment is known as a sensorineural hearing loss, and in such a case the best line of hearing loss treatment would be hearing aids, cochlear implants, assistive listening devices or other ear implants. Damaged hair cells might also cause tinnitus or ringing in the ears. If you are the one suffering from such a condition, do consult a skilled and professional Audiologist.

 

Types of Hair Cells

Now that we know that the primary function of the hair cells is to convert the analogue sound waves into electrical impulses so that these signals can be easily interpreted by our brain, let’s go deeper and discuss about the different types of hair cells.

In mammals. the hair cells of the cochlea can be anatomically and functionally classified into two distinct types - the inner hair cells (IHC) and the outer hair cells (OHC). In the human cochlea, there are 3,500 inner hair cells and 12,000 outer hair cells at birth.

The outer hair cells amplify the low-level sounds that enter the cochlea. The inner hair cells convert the sound vibrations travelling in the cochlear fluid into electrical signals. These vibrations are then relayed to the auditory nerve.

Damage to any of these hair cells leads to deteriorating hearing sensitivity.

 

The Difference Between the Inner and the Outer Hair Cells of the Human Cochlea

 

Inner Hair Cells

                          Outer Hair Cells

These are the main receptive cells which convert sound waves into nerve signals.

These hair cells pre-amplify the sound waves having a low amplitude.

These are 3,500 in number.

These are 12,000 in number.

These are arranged in a single row.

These are arranged in 3 rows.

These are located at the terminals of the inner hair cells.

These are located at the terminals of the outer hair cell nerves.

These are innervated by more afferent nerves than efferent nerves.

These are innervated by more efferent nerves than afferent nerves.

These have a cylindrical shape.

These are flask-shaped.

 

In a nutshell, take care of your hair cells by taking care of your ears. Protect your ears from any exposure to extremely loud sounds. Use earplugs when you are attending some loud events such as a rock show. Prevention is better than getting your ears damaged permanently and having no other option left than to go for a hearing loss treatment.

Also, make it a habit to get your hearing tested from time to time so that if you are suffering from any hearing problems (which you may not realise normally) can be diagnosed on time, and hence go for a proper hearing loss treatment on time. Hearing is a precious asset and a good hearing health paves way for a better quality of life.

Are you aware of any other facts about hair cells and their functions?

Do tell us in the comments below.