Structure, mechanism of hearing and other functional activities of human ear


The ear is an important sense organ of the human body. This article deals with the detailed structure of the human ear and the mechanism of its functioning. It also deals with the additional functions of balance and equilibrium carried out by ear with examples. Along with this resource, some frequently asked questions in exams are also included.

Introduction

The human ear is the most specialized organ meant for the perception of sound and for the maintenance of body balance. Hence they are known as stat-acoustic organs. The ear has parts like the external ear, middle ear, and inner ear. Each of these parts will do some specific functions like receiving sound waves, amplifying, and converting sound energy into a nerve impulse.

Structure of human ear

Human ear can be divisible into three distinct regions-the the outer external ears, the middle ear, and the inner ear.

Structure of Human ear
Anatomy of Human ear (Courtesy:-wikipedia.org)

External ear

It consists of the outer projecting part ear pinna (auricle), auditory canal (auditory meatus) and the tympanum (eardrum). The ear pinnae are made up of skin and cartilage and are arranged in an oblique fashion on the lateral surface of the head. The function of ear pinna is to collect and direct sound waves to the middle ear through the auditory canal. The pinna and auditory canal help us to locate the source of the sound.

The auditory canal is a one-inch tube that connects external ear pinna with the eardrum. On the inner surface of the auditory canal, few hairs and numerous ceruminous glands are present. These glands are formed from sebaceous glands that got modified and specialized into ceruminous glands. These specialized glands secrete cerumen (ear wax or glue) and that helps to prevent any foreign particles to move into the ear.

Towards the end of the auditory canal, there presents a thin membranous tympanum or eardrum. The tympanum separates the middle ear from the external ear. When sound waves strike the tympanum, it vibrates and changes sound waves into mechanical vibrations. These sound vibrations move into the middle ear. In order to hear the sound, the eardrum has to move only by 0.00000001 mm. when the sound waves strike the tympanic membrane.

Middle ear

It is an air-filled cavity and is embedded in the cranial bone. It is also known as the tympanic cavity. The tympanic cavity includes three tiny small bones called ear ossicles arranged in a lever like fashion. The three ear ossicles are Malleus (hammer), Incus (anvil), and Stapes (stirrup) and are arranged in the same order articulating with each other. The beginning part of the hammer bone is placed over the eardrum and the end part of the stapes is placed over the oval window of the inner ear. The ear ossicles are the smallest bones present in the human body. The function of ear ossicles is to amplify the sound vibration and transmit them into the inner ear through an oval window. The sound vibrations received from the eardrum are magnified by 20-25 times by the lever like the action of the ear ossicles

The middle ear or tympanic cavity is connected to the throat or pharynx by a tube-like structure called the eustachian canal. This eustachian tube helps to equalize the air pressure on either side of the eardrum allowing the eardrum and ear ossicles to vibrate freely. Suppose if air pressure becomes more in the middle ear than the external ear, some air from the middle ear escapes your throat while you yawn, and thus the air pressure external ear and middle ear get balanced. Similarly, if the air pressure in the middle ear becomes less than the external ear, some air from the throat enters the middle ear through the Eustachian canal and thus air pressure on either side of the eardrum gets balanced. If air pressure is not balanced on either side of the eardrum, the eardrum gets damaged.

Inner ear or labyrinth

It is a fluid-filled chamber that contains a complex system of canals and cavities. The inner ear is composed of the bony labyrinth and the membranous labyrinth. The bony labyrinth is filled with a fluid called perilymph and the membranous labyrinth is filled with endolymph. The membranous labyrinth is concerned with the hearing and balance of the body and thus it is known as state acoustic organ. The middle ear connected to the inner ear through the oval window (fenestra ovalis) and round window (fenestra rotunda). The oval window transmits the sound vibrations from the middle ear to the inner ear through this membrane. The round window membrane present below the oval window will help to relieve the excessive pressure caused by sound vibration in the inner ear.

Cochlea internal Structure
The internal structure of Cochlea (Courtesy:-Wikipedia.org)

The inner ear has two main parts-the hearing machine cochlea and the body balancing organ semi-circular canals. The cochlea is a two and a half turns coiled snail shell-like structure. The inner winding cavity of the cochlear canal is divided into three cavities by Reissner's membrane and basilar membrane. The three cavities of the cochlea are scala vestibuli, scala media, and scala tympani. The vestibular and tympanic canals are filled with liquid perilymph while the scala media canal is filled with liquid endolymph. On the basilar membrane of the scala media cavity, sensory hair cells are present and that are in contact with the tectorial membrane present on the other side of this cavity. The sensory cells, the basilar membrane, and the tectorial membrane together form the hearing organ called organ of corti. The nerve fibers from sensory hair cells together form a branch of the cochlear nerve that joins the main auditory nerve. Thus the sensory cells of the organ of Corti transmit the sound impulses from the ear to the brain through the auditory nerve.

Semi-circular canals

Attached to cochlea is a vestibular apparatus consisting of three semi-circular canals. These three semi-circular canals are arranged in three different planes and are arranged at right angles to each other. Of these three canals one is arranged in a horizontal plane and the other two are arranged in a vertical plane. One end of these canals becomes swollen to form an ampulla which contains sensory cells for dynamic balance of the body while the body is in motion. The semi-circular canals are filled with liquid endolymph. Sensory hair cells present in ampullae of Semi-circular canals send the impulses of body motion through the auditory nerve. The balance brought about while the body is in motion by ampullae of semi-circular canals is known as dynamic balance. For example, if you are rotating around yourself continuously, the endolymph present in the three canals rotates around the tubes and press against the sensory hair cells present in ampullae. This information about your body's rotation is sent through the impulses passed from sensory cells of ampullae through the auditory nerve to the brain. Now the brain sends an order to bring your body to normal condition and thus dynamic equilibrium is brought out. The dizziness caused immediately after stopping your rotation is due to the endolymph liquid still continues to rotate even after you are in still condition and false information sent to the brain is the cause of this situation.

Structure of Semi-circular canals
Structure of Semi-circular canals (Courtesy:-wikimedia.commons)

A stem-like structure formed by the union of ampullae of semi-circular canals contains two parts-a utriculi and a sacculus. These parts also contain sensory hair cells in them in patches. Chalky granules called otoliths present in the fluid of endolymph in this stem press on hair cells which send messages about the position of the body to the brain through the auditory nerve. Suppose if you keep your head facing sky without any movement for a long time, the otoliths present in endolymph press the sensory cells and which in turn send messages about the static position of your head to the brain. Then the brain sends an order to bring back your head to normal position. Such an equilibrium brought about by utriculus and sacculus of semi-circular canals with respect to the position of the body or gravity is known as Static balance.

Mechanism of hearing

The following events occur in the process of hearing.
  • Sound waves through external ear travel through the auditory canal and strike the eardrum and make it vibrate freely. Now the sound waves change into mechanical vibrations at eardrum.
  • Now the sound vibrations from eardrum get transmitted through the three small ear ossicles and passed into the inner ear through the membrane of the oval window. Due to the lever like the action of ear ossicles, the sound vibrations get amplified 20-25 times as they pass into inner ear.
  • The sound vibrations which entered into cochlear canals sets vibrations in the endolymph fluid contained in it.
  • The vibrating movements of the endolymph fluid stimulate the sensory hair cells of the organ of Corti in scala media and the impulses from sensory cells are then transmitted to the brain through auditory nerve.
  • The brain interprets the impulses received from the ear in different ways and thus perception of sound can be felt. Hearing is controlled by the auditory area of the temporal lobe of cerebral cortex.


The human ear can perceive sound vibrations between 20 to 20,000-hertz frequencies. The most clearly heard sounds fall in frequencies between 100 to 4000 hertz. But the dogs have the ability to hear 40 hertz to 60,000 hertz.

Frequently asked questions:-


  1. What do you mean by stat-acoustic organ?
    Ans. The ear is a stat-acoustic organ as it helps in both hearing and balance of the body.
  2. What is the functional role of the tympanum in-ear?
    Ans. Tympanum converts the sound waves into mechanical vibrations and transmits them through ear ossicles into inner ear.
  3. What are ear ossicles?
    Ans. Ear ossicles are three small pieces of bones arranged in a lever like fashion and located in the middle ear. The names of these three ear ossicles are Malleus, Incus, Stapes. Their function is to amplify sound vibrations received from tympanum and transmit them into inner ear.
  4. what is the eustachian tube? State its functional role.
    Ans. The eustachian tube is a tube-like structure that connects the middle ear to the throat. Its functional role is to balance the air pressure on either side of the eardrum and help the eardrum as well as ear ossicles to vibrate freely.
  5. Name the audio receptor region present in cochlea responsible for hearing.
    Ans. Organ of corti
  6. Name the two parts responsible for static balance in semi-circular canals.
    Ans. Utriculus and Sacculus.
  7. Name the parts of external ear.
    Ans. Ear pinna, Auditory canal and Tympanum.
  8. State the functions of semi-circular canals.
    Ans. It helps in maintaining dynamic equilibrium and staic equilibrium of the body.

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