Structure and Function of Inner Ear

Introduction:

Our ears are divided into three parts that are outer, middle, and inner ears. The inner ear is located in the petrous region of the temporal bone (lateral skull bone). The inner ear comprises two structures that are the bony labyrinth and the membranous labyrinth. The middle ear is connected to the inner ear by a bony labyrinth with two windows which are oval and round windows. The inner ear contains the semicircular canals and the cochlea (snail-shaped organ). The cochlea is a snail-shaped spiral structure present in the cochlear duct and is filled with fluid. The cochlea plays an important role in the hearing mechanism.

What Are the Parts of the Inner Ear?

The ear consists of the following:

• Labyrinth: It is a fluid-filled sac present in the inner ear. Labyrinths are of two types that are bony and membranous labyrinths.

• Cochlea: It is a spiral or snail-shaped structure which is filled with fluid. The cochlea consists of thousands of sensory cochlear hair cells, which help to convert the mechanical vibrations into electrical signals and further transfer those electrical impulses to the auditory nerve (the eight cranial nerves).

• Vestibular Apparatus: It is a part of the inner ear that helps in maintaining balance and position. The vestibular apparatus consists of the following:

1. Semicircular Ear Canals: There are three semicircular canals that are anterior, posterior, and lateral. These semicircular canals are also filled with fluid and help to detect motion changes. It is connected to the utricles by an ampulla.

2. Saccules: These are the sac-like structures present in the inner ear, which consist of otoliths (calcium-carbonate crystals). It helps to sense the vertical motions of the head.

3. Utricles: It is a sac-like organ present in the inner ear. It contains otoliths (calcium-carbonate crystals). It helps to sense the side-to-side, forward, and backward motions of the head.

What Is the Nerve Supply to the Inner Ear?

The nerve supply to the inner ear is:

Auditory Nerve: It is the eighth cranial nerve, also known as the vestibulocochlear nerve. These nerves help to transmit electrical impulses to the brain for the perception of both hearing and balance.

What Is the Blood Supply to the Inner Ear?

Arterial Supply

The inner ear is supplied by the labyrinthine artery, also called the internal auditory artery. The labyrinthine artery has three other branches:

• The vestibulocochlear artery.

• Anterior vestibular artery.

• Cochlear artery.

Venous Supply:

The vein supplying the inner ear are:

• Anterior and Posterior Vestibular Veins- These drain blood from the vestibule.

• Anterior and Posterior Modiolar Vein - It drains blood from the cochlea.

• Inferior Cochlear Vein: It further drains into the inferior petrosal sinus.

What Are the Functions of the Inner Ear?

The inner ear has two vital functions that are:

• It helps in hearing.

• It helps in maintaining the balance of the body.

How Does the Inner Ear Help in the Hearing Mechanism?

The inner ear consists of the cochlea, which is filled with fluid. The sound waves from the outer ear canal approach the eardrum. The vibration of the eardrum results in the transfer of sound waves from the outer ear to the middle ear. The tiny bony ossicles in the middle ear, which are the malleus, incus, and stapes, connect to the inner ear by an oval window and help in coupling the sound waves with the fluid waves of the inner ear. As the sound waves reach the cochlea, the tiny cochlear hair cells that are sensory get bent, converting the mechanical vibrations into electrical signals. These electrical signals reach the auditory nerve (the eighth cranial nerve) or hearing nerve, which further transmits these signals to the brain for the perception of sound, thus helping with the hearing mechanism.

How Does the Inner Ear Help With the Balance Mechanism?

The vestibular apparatus within the inner ear plays an important role in the balance and coordination mechanism of the body. The semicircular canals, utricles, and saccules are parts of the vestibular apparatus. These semicircular canals are filled with fluid (endolymph) and help to detect motion changes. These motion changes within the endolymph are converted into electrical signals that are transferred to the eighth cranial nerve (auditory nerve), which further transmits it to the brain for the perception of the balance and position of the body.

What Are the Diseases Associated With the Inner Ear?

Various diseases associated with the inner ear are:

• Labyrinthitis: Any infection of the inner ear can result in swelling of the labyrinth (labyrinthitis).

• Meniere's Disease: It is a disease-causing improper fluid drainage or blockage of the inner ear resulting in dizziness, hearing, and balance loss.

• Vertigo: Any infection in the vestibular parts of the inner ear can result in the spinning sensation of the head or dizzy spells, which are called vertigo.

• Auditory Neuropathy: Any bacterial or viral infections affecting the inner ear, like cytomegalovirus or rubella virus, can result in infection and swelling of the auditory or hearing nerve, which is called auditory neuropathy.

• Vestibular Schwannoma: It is also known as acoustic neuroma. It is a non-cancerous tumor affecting the Schwann cells, which line the auditory or hearing nerve resulting in hearing loss and vertigo.

What Are the Signs and Symptoms Associated With Inner Ear Diseases?

The signs and symptoms associated with inner ear disorders are:

• Sensorineural hearing loss (hearing loss due to involvement of the auditory nerve).

• Conductive hearing loss (hearing loss due to a defect in the mechanical transmission of sound waves).

• Vertigo (spinning sensation in the head).

• Tinnitus (ringing sensation in the inner ear).

• Loss of balance.

• Pressure changes in the inner ears.

• Drainage from the ears.

• Disorientation.

• Frequent headaches.

• Nystagmus (involuntary eye movements).

• Blurred vision.

• Nausea and vomiting.

Conclusion:

Our inner ear has complex structures but has a vital role in hearing and balance mechanisms. Any symptoms associated with inner ear infections, like dizziness, ear fullness, ear pain, discharge from the ears, or tinnitus, should be reported to the ENT (ear, nose, and throat) specialist immediately for prompt diagnosis and management to avoid the complications associated with the diseases like permanent hearing loss.