The ear is the vibrating organ of hearing. It has a standard mammal structure, and is composed of three main parts. It has a sound-collecting outer ear structure, a sound-transmitting middle ear, and a sensory inner ear (see below).
The structure of the ear, [Panagi] StudySmarter (from an image from commons.wikimedia.org)
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.
There are also two nerves that are the final parts of the inner ear:
- The seventh cranial nerve is also known as the facial nerve, as it controls facial muscle movement. It provides signals to the brain from the tympanic membrane. One section of the nerve controls the muscle, and the other controls sensory signals associated with hearing.
- The eighth cranial nerve, also known as the vestibulocochlear nerve, combines vestibular and cochlear sensory nerve fibres. The vestibular nerve fibres provide information to the brain concerning balance, while the cochlear nerve fibres convey sound.
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.
Structure of the middle ear
The middle ear is the part that connects the inner and outer parts of the ear. It consists of the hammer, the anvil, the stapes, the round window, and the Eustachian tube. The Eustachian tube is also known as the auditory tube. The hammer, anvil, and stapes are three bones inside the middle ear that sound passes through to reach the round window.
The round window is a membrane that covers the cochlea. Its function is to amplify the sound waves before they enter the inner ear. Sound waves are amplified due to the difference in the size of the eardrum and the round window. The pressure suddenly increases as the sound waves enter a relatively small space. The Eustachian tube’s function is to equalise the air pressure on both sides of the eardrum to avoid pressure buildup. The tube opens during swallowing which equalises the air pressure inside and outside the ear.
Structure of the outer ear
The outer ear is the external part of the ear; its function is to collect sound waves and to direct them into the ear. Important parts of the outer ear are the pinna, the ear canal, and the eardrum.
The pinna is the visible part of the ear. It acts as a funnel to direct sound further inside the ear without loss. The ear canal is located right after the pinna to transmit the sound to the eardrum. The latter is the key part of the outer ear. It is a very sensitive part of the ear that vibrates due to the pressure of sound waves.
Structure of the Ear - Key takeaways
- The ear is responsible for hearing and balance.
- The ear is comprised of three main parts, the outer, middle, and inner ear.
- The inner part is responsible for converting sensory info to electric signals.
- The middle part is responsible for the amplifying of sound.
- The outer part is responsible for collecting sound waves.
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