Ears are one of the five sensory organs of our body. Other than hearing, the main function is to maintain the balance of the body. The hair cells present in the inner ear of mammals help in sensing the position of the body, in accordance with gravity and maintain the equilibrium.
The ear has three main sections: the outer ear, middle ear and inner ear.
Anatomy of Ear
- Pinna is the outermost part, has very fine hairs and glands. The glands secrete wax. It protects foreign organisms and dust from entering
- External auditory canal or meatus is connected to pinna at the outer side and extends till tympanic membrane or eardrum. They also have wax glands
- Tympanic membrane or eardrum is made up of connective tissue. Skin covers the outer portion and from inside, it is covered by mucous membrane.
Pinna receives the sound in the form of vibration. The sound waves reach and vibrate the eardrum through the external auditory canal.
- It contains a chain of three tiny bones malleus, incus and stapes, present in the same order
- Malleus is a hammer-shaped bone, attached to the tympanic membrane
- Incus is an anvil-shaped bone, present between the malleus and stapes
- Stapes is the smallest bone of the body. It is stirrup-shaped and attached to the oval window of the cochlea
- The eustachian tube is the connection between the middle ear and the pharynx. It equalises pressure between the middle ear and outer atmosphere
The middle ear amplifies the sound waves and transmits to the inner ear.
- The inner ear is called the labyrinth. It is composed of a group of interconnected canals and sacs
- The membranous labyrinth is present inside the bony labyrinth and surrounded by a fluid known as perilymph
- The endolymph is filled within the membranous labyrinth
- Auditory receptors are located in the cochlea and vestibular apparatus maintains the body balance
- Cochlea (Auditory organ)
- The Cochlea is a coiled portion of the membranous labyrinth, which looks like a snail.
- The cochlea is made up of three canals, upper vestibular canal or scala vestibuli, middle cochlear duct or scala media and the lower tympanic canal or scala tympani, which are separated by thin membranes
- The scala vestibuli is filled with the perilymph and terminates at the oval window
- The scala tympani is also filled with the perilymph and ends at the opening in the middle ear, i.e. round window
- The Reissner’s membrane separates scala media and scala vestibuli
- The scala media is filled with endolymph and contains the auditory organ, the organ of Corti
- Each organ of Corti contains ~18000 hair cells. Hair cells are present in the basilar membrane, which separates scala media from scala tympani
- Stereocilia project from the hair cells and extend till the cochlear duct. There is another membrane called the tectorial membrane present above hair cells
- Hair cells present in the cochlea detect pressure waves, there are sensory receptors (afferent nerves) present at the base of hair cells that send signals to the brain
- Vestibular apparatus maintains the equilibrium and is present above the cochlea. It is present in the membranous labyrinth. It has two sac-like chambers called saccule and utricle and three semicircular canals
- Saccule and utricle have macula, which is a projecting ridge
- Macula has hair cells, which are sensory. Stereocilia protrude out from the hair cells
- Stereocilia are covered by ampullary cupula, which is gelatinous and otoliths are embedded in it
- Otoliths are calcium ear stones, which press stereocilia against the gravity and play an important role in spatial orientation
- Each semicircular canal is filled with endolymph and present at the right angle to each other and connects to the utricle. The base of canals is swollen and known as the ampulla
- Crista ampullaris is present in each of the ampulla and responsible for sensing angular rotation. It has hair cells
- There are no otoliths present in cristae like maculae of saccule and utricle and stereocilia of hair cells are stimulated by the movement of endolymph in the canals
Physiology of Ear
Ears perform two main functions, hearing and equilibrium maintenance.
- The organ of Corti (Cochlea) is responsible for hearing function
- Maculae (Saccule and Utricle) are responsible for static equilibrium
- Cristae (semicircular canals) are responsible for dynamic equilibrium
Mechanism of Hearing
- The pinna receives the sound waves and it reaches the tympanic membrane through the meatus
- The eardrum vibrates and these vibrations get transmitted to the three ossicles present in the middle ear
- Malleus, incus and stapes amplify the sound waves
- These vibrations then reach the perilymph (scala vestibuli) through the oval window
- Then the pressure waves get transferred to the endolymph of scala media and reach basilar membrane and then to perilymph of scala tympani. This movement of fluid is facilitated by the round window present at the end of scala tympani
- The basilar membrane movement causes rubbing of stereocilia against the tectorial membrane
- Stereocilia are bent resulting in the opening of ion channels in the plasma membrane of hair cells. Glutamate, a neurotransmitter, is released due to Ca++ ion movement inside the cell
- These neurotransmitters bind to the receptors of afferent neurons, which synapse with hair cells causing depolarization of neurons. A nerve impulse is generated and transmitted to the auditory cortex of the brain through auditory nerve (cranial nerve VIII)
- The brain analyses the impulses and we hear the sound. The brain not only recognises the sound but also judges the direction, loudness and pitch of the sound
To summarize the process of hearing below is the sequence of events:
Mechanism of Maintaining Equilibrium
We all know that hearing ability is due to ears. Other than hearing, ears are also responsible for maintaining equilibrium.
The vestibular apparatus is the main organ for maintaining equilibrium.
Static equilibrium is maintained by macula of saccule and utricle. Otoliths press against stereocilia due to gravitational pull and stimulate the initiation of a nerve impulse. When the head is tilted or moves in a straight line with increasing speed, otoliths press on stereocilia of different cells. The brain interprets the nerve impulses resulting in the awareness of body position with respect to ground, irrespective of the head position.
Utricle responds to the vertical movement and Saccule responds to the sideways movement of the head.
Dynamic equilibrium is detected by cristae of semicircular canals