In the last blog, we discussed about what are the General and the Special senses and saw in details the special sense, Taste. We shall see Vision and olfaction in the coming posts.

In this post, let’s understand about the other special sense: Hearing.

The special organ dedicated for the purpose of hearing is the ear. Each individual has a pair of ears, each situated on the either side of the head.

Parts of the ears:
The ear is divided into three different parts
the external ear, the middle ear and the inner ear (see Fig 1).

Parts of ears
Fig 1: Different parts of ear.

1. The external ear:
The external ear includes the pinna, the concha, the ear (or auditory) canal and the ear drum (fig 2).

External ear
Fig 2: Different parts of the external ear.

The pinna diffracts and focuses the sound waves towards the concha. The concha and the ear canal acts as the resonator, and resonates the sound waves. The resonating sound waves eventually travel to the ear drum and causes it to vibrate.

Hence, the external ear acts like an antenna and collects the sound waves and transmits it to the middle ear.

2. The middle ear:
Interior to the external ear is the middle ear, which is made up of three small bones (or ossicles) namely malleus, incus and stapes in the sequence of most exterior to interior part.

The malleus is located between the eardrum and the incus. The incus is in turn attached to the stapes. The foot plate of the stapes is further attached to the oval window of the cochlea of inner ear (see fig 3).

Middle ear
Fig 3 : Middle ear: malleus, incus and stapes.

When the sound waves collected cause the ear drum to vibrate, the malleus also experiences the vibration, passing it on to the incus and eventually to the stapes. The foot plate of the stapes in turn applies the mechanical pressure on the oval window of the inner ear. Hence the sound waves collected from the external ear is now transmitted to the internal ear.

3. The inner ear:
The hearing portion of the inner ear is called the cochlea. Cochlea literally means snail in Greek, so named because of its appearance (see fig 4).

Inner ear (The Biotech Notes)
Fig 4: The inner ear.

Internally, the cochlea is divided into three sections: scala vestibuli, middle cochlear duct and scala tymphani. Though separated by cochleae duct, scala vestibuli and scala tymphani are in continuous with each other at the helicotrema, the final tip of the cochlea (fig 5). Scala vestibuli and scala tymphani are filled with perilymph. Cochlear duct is filled with fluid called endolymph.

The oval window is at one end of the cochlea and the opening named the round window is at the other end. The vibration travels from the oval window and is later absorbed and dampened by the round window (fig 5).

Cochlea (The Biotech Notes)
Fig 5: The structure of cochlea.

In the cross section of the cochlea (fig 6), it can be observed that the scala tymphani is lined with basiliar membrane, over which is organ of Corti, finally layered with tectorial membrane within cochlear duct. Within the organ of Corti there are hair cells of two different types: inner hair cells and outer hair cells. The hair cells have Stereocilia or the bundle of hair.

Organ of corti ( The Biotech Notes)
Fig 6: The cross-section of cochlea and the organ of Corti.

When the sound waves are transmitted to the perilymph by the vibration of oval window, the basilar membrane (located over scala tympani) vibrates ( Fig 7b 1). Due to this vibration, the stereocilia of the outer hair cells bend and cause excitation of the outer hair cells, this in turn leads to the excitation of the inner hair cells, which release glutamate, a neurotransmitter (Fig 7b 2). The inner hair cell synapses with the vestibulocochlear nerve (Fig 7b 3).

The vibration caused by the sound wave finally reaches the round window and is dampened (as seen in Fig 5).

Excitation of hair cells (The Biotech Notes)
Fig 7: The excitation of hair cells and sending the information to the brain.

The vestibulocochlear nerve (auditory vestibular nerve) also known as the eighth cranial nerve transmits sound as well as the equilibrium (balance) information from the inner ear to the auditory cortex in the brain (Fig 8).

Fig 8: The Auditory System

Auditory cortex processes acoustic signals so they can be interpreted as speech, music or other sounds.

Please watch this amazing video by Brandon Pletsch about the process of hearing.

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Also read other posts by The Biotech Notes:


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Sources of images

Fig 1, 2 & 6- Raven et al (2002). Biology (6th ed). McGraw-Hill, Boston

Fig 3- modification from Rolvien et al (2018) Early bone tissue aging in human auditory ossicles is accompanied by excessive hypermineralization, osteocyte death and micropetrosis. Scientific Reports, 8, Article: 1920

Fig 4- Hussain et al (2017) Hearing impairments, presbycusis and the possible therapeutic interventions. Biomedical Research and Therapy, 4(4), 1228-1245.

Fig 5: Loftus et al (2010) Chapter 2 – Clinical Neuroanatomy. Neurology Secrets (Fifth Edition), 18-54.

Fig 7: Fettiplace & Hackney (2006) The sensory and motor roles of auditory hair cells. Nature Reviews Neuroscience, 7, 19–29.

Fig 8: Waldman (2009) Chapter 9 – The Vestibulocochlear Nerve—Cranial Nerve VIII. Pain Review, 22-25.