Structure of the Ear

Structure of the inner ear

The inner ear is located near the middle ear, and under the temporal bone of the brain that protects structures and nerves of the ear.

The inner ear consists of two different labyrinths or networks of passages, the membranous labyrinth, and the bony labyrinth. The first labyrinth is contained within the latter. The bony labyrinth is a hollow cavity located at the temporal bone of the skull with multiple passages that consist of three main parts, the central chamber called the vestibular system, the three semicircular canals, and the spiral-coiled cochlea.

Structure of the inner ear. commons.wikimedia.org

Various key parts in the ear work together in order to collect sensory information, convert that information into signals that are identifiable by the brain, and are used for sound identification and balance. These are listed below.

Cochlea structure

The cochlea is made of a bone that is structured similar to a snail, about 9mm x 5mm. It is divided into two compartments by a membrane. These chambers are filled with fluid, which vibrates when sound passes through. Due to the vibrations, the small hairs called stereocilia that are located in that area also vibrate, sending electrical impulses or sound signals to the brain.

Semicircular canals structure

The semicircular canals are located above the cochlea and are connected by the vestibular system. There are three such canals, which help the brain to identify the direction of motion of the head. Like the cochlea, these canals are also filled with fluid and tiny hairs that sense the motion of the fluid. These include the horizontal, superior, and posterior semicircular canals. The horizontal canal detects angular acceleration of the head when it turns horizontally, while the superior and posterior canals detect vertical head movements.

Vestibular system structure

The vestibular system is the central part of the bony labyrinth. It is separated from the middle part of the ear and communicates with the cochlea and semicircular canals. There are three main compartments in the vestibular systems - the cochlear duct, semicircular ducts, and utricle and saccule.

Cochlear duct

The cochlear duct is located inside the bony labyrinth in the cochlea, but it is considered to be a part of the vestibular system as it is a major functioning part of the vestibule. It is comprised of two canals, the scala vestibule and the scala tympani, located above and below the duct respectively. They are separated from the main duct by membranes.

The membrane between the two canals, also known as the basilar membrane, is the area where the primary hearing organ, Corti, is located. The upper membrane is called Reisner's membrane; its purpose is to control the flow of fluid from the duct to the scala vestibule.

Semicircular ducts

The semicircular ducts stimulate the receptor cells that translate their movement into nerve impulses, that the brain perceives as sound. Sensory receptors in the semicircular ducts detect this change in speed and direction and send signals to the brain to help maintain physical balance.

Utricle and saccule

The utricle and saccule are composed of sacks and are within the membranous labyrinth (balancing system). Their purpose is to detect vertical and horizontal movement by the use of small stones and a viscous fluid that stimulates hair cells to detect motion and orientation.

The function of the inner ear

The purpose of the inner ear is to sense and process information about sound and balance and send signals to the brain. Each part of the inner ear has a specific function.

Cochlea

The cochlea is filled with fluid and is comprised of several layers surrounding the main organ, also known as Corti, which is lined with thousands of tiny hairs called cilia. Sound is turned into different pitches based on the sound's locations along the length of the cochlea. Hence, the cochlea is responsible for hearing. The pressure patterns collected from the outer ear are converted into electrochemical impulses from the motion and vibration of the cilia due to the sound waves that travel through the fluid in the cochlea.

Semicircular canals

The semicircular canals are a part of the vestibulae. They are a three-loop type system that uses the motion of fluid to send signals to the brain. The first canal can sense vertical movement, while the second canal senses horizontal movements, and the third canal is used to send the information of motion as electrical signals to the brain when the head is tilted. Each canal is filled with fluid and cilia. When the fluid shifts as the head tilts from side to side, the cilia send signals to the brain through the seventh cranial nerve which carries information about both balance and hearing. Hence, the semicircular canals are responsible for balance.

Vestibule

The vestibule is able to detect changes in altitude, rotation, and linear movement. It can sense when the body is standing, upside down, or rotating. Each type of movement is sensed from a specific structure. The semicircular canal detects rotational motion while the saccule and utricle sense vertical and horizontal motion.

The balance system

The balance system sends continuous electrical impulses to the brain. Tilting or moving the head causes the fluid in the semi-circular canals to shift which affects the electrical impulses to the brain accordingly. The brain uses the signals to make any adjustments necessary for the body to maintain balance.

The middle and outer ear

The outer part of the ear is responsible for collecting external sound waves into the ear canal. Sound is then amplified in the middle part of the ear due to vibrations. The vibrations create waves in the fluid in the inner ear, which stimulate tiny hairs located in the cochlea to vibrate, sending an electrical impulse in the eighth cranial nerve to the brain. The brain is then able to identify the position and the motion of the head. The function of each part and its structure is summarised below.