Structure of Ear

The human ear is a delicate sensory organ that helps us hear sound and plays a vital role in perceiving, conveying, and converting sound waves. It also maintains the balance of the body. 

1.0Human Ear Parts

The ear can be anatomically divided into three main parts:

• The External Ear
• The Middle Ear
• The Internal Ear

2.0Structure of External Ear

The outer ear, which is well-developed in mammals exclusively, consists of two main components:

• The ear pinna
• The ear canal

• The pinna, which humans cannot voluntarily move, is composed of yellow elastic cartilage and contributes to the anatomy.
• The pinna gathers vibrations in the air that create sound, and it plays a crucial role in auditory perception.
• The external auditory canal extends inward, leading to the tympanic membrane, the eardrum.
• After the ear canal lies a taut, thin, obliquely positioned membrane known as the tympanic or eardrum, this membrane serves as a barrier, separating the ear canal from the middle ear.
• Inside the external auditory canal, there are sebaceous glands called ceruminous glands. These glands produce cerumen, commonly known as earwax, which moistens the eardrum and provides protection.
• The eardrum, also known as the tympanic membrane, is a component of the external ear. It consists of connective tissue, which is covered by skin externally and lined with a mucous membrane internally.

3.0Structure of Middle Ear

• The middle ear houses three intricately small interconnected bones: the hammer (malleus), anvil (incus), and stirrup (stapes).
• The hammer bone touches one end of the eardrum while its other end connects to the anvil bone.
• The anvil, in turn, connects to the stirrup, and the free end rests against the membrane covering the oval window of the inner ear.
• Additionally, the middle ear contains air, and a narrow tube called the Eustachian tube extends from its lower part to the throat.
• This tube serves to equalize the air pressure inside the middle ear with that outside of it.

4.0Structure of Internal Ear

The inner ear, known as the labyrinth, comprises two components: the bony labyrinth and the membranous labyrinth, filled with fluid.

• The bony labyrinth comprises a network of channels housing the membranous labyrinth, enveloped by perilymph fluid.
• The membranous labyrinth contains endolymph fluid.

Function of the inner ear

• Hearing - Cochlea
• Balance - Vestibular apparatus

Cochlea

• It serves as the sensory organ for hearing.
• Positioned beneath the vestibular apparatus, it is a spiral-shaped structure.
• The cross-section of the cochlea reveals three canals-

1. Scala vestibuli: Located dorsally and filled with perilymph.
2. Scala media: Positioned between the scala vestibuli and scala tympani, it contains endolymph.
3. Scala tympani: Found ventrally below the scala media, it contains perilymph.

• Reissner's membrane forms the roof of the scala media, separating it from the scala vestibuli.
• The basilar membrane, located on the floor of the scala media, separates it from the scala tympani.
• At the cochlear base, the scala vestibuli terminates at the oval window, while the scala tympani ends at the round window, which opens into the middle ear.

Internal Structure of Cochlea & Cochlear Canal

• The Organ of Corti, located on the basilar membrane within the inner ear, contains hair cells tasked with auditory reception.
• These hair cells are arranged in rows along the internal aspect of the Organ of Corti.
• At the basal end of each hair cell, afferent nerve fibres forming the cochlear nerve branch are close.
• Each hair cell extends numerous processes known as stereocilia from its apical portion.
• A delicate elastic tectorial membrane is positioned above the rows of hair cells within the Organ of Corti.
• The primary attribution for hearing lies with the Organ of Corti.

Vestibular Apparatus

• Serving as the sensory hub for balance, the vestibular apparatus is positioned above the cochlea and comprises:
• Three semicircular canals (SCC)
• One utricle
• One saccule
• Each semicircular canal is positioned in a distinct plane perpendicular to the others.
• The base of each canal expands into a swollen structure known as the ampulla, housing hair cells atop a projecting ridge called the crista ampullary (crista).

Internal Structure of Ampulla

• Within this ampulla, a ridge-like projection called the acoustic ridge is formed by internal cuboidal epithelium.
• The acoustic ridge consists of immovable stereocilia located at the free edges of sensory cells.
• Additionally, a single movable cilium known as the kinocilium is present.
• The stereocilia of the ridge are bound together, forming a cupula-like structure.

Internal Structure of Macula

• The utricle and saccule each contain a raised ridge known as the macula.
• Within the utricle and saccule are specific structures called maculae, where numerous calcium carbonate particles, known as otoconia, are found in the endolymph.
• These sensory cells within the inner ear are connected to small nerves.
• These nerves converge to form the vestibular nerve, an auditory nerve branch.
• The crista and macula serve as specialized receptors within the vestibular apparatus, responsible for maintaining body balance and posture.
• The crista responds to movement, while the macula regulates equilibrium in static positions.

Also Read: Structure of the Eye

5.0Mechanism of Hearing

Working of external ear

• The pinna of the outer ear collects sound waves emitted by a source. These waves travel through the ear canal and reach the eardrum. Sound waves comprise areas of high pressure (compressions) and low pressure (rarefactions). When a compression reaches the eardrum, it pushes inward, increasing the pressure. Conversely, when a rarefaction reaches the eardrum, the pressure decreases, causing it to move outward. Consequently, as sound waves interact with the eardrum, they vibrate back and forth rapidly.

Working of middle ear

• The process begins with the eardrum vibrating, which in turn sets off a chain reaction involving a series of small bones: the hammer, anvil, and stirrup. These bones transmit the vibrations successively, culminating in the stirrup striking the membrane of the oval window, which then transfers the vibrations to the liquid within the cochlea. As a result, the liquid inside the cochlea begins to vibrate, stimulating the nerve cells within it. These neurons produce electrical signals that are subsequently conveyed to the brain by the auditory nerve. The brain interprets these impulses as sound, allowing us to perceive it.
• In essence, the hammer, anvil, and stirrup function as a system of levers, amplifying the initial vibrations from the eardrum by more than 20 times before transmitting them to the inner ear. This amplification is crucial because the nerve cells in the cochlea only respond to strong vibrations within the liquid medium.

Working of inner ear

• The organ of hearing in the inner ear is the cochlea, composed of 3 fluid-filled chambers. The central chamber is the cochlear duct, where mechanical vibrations are transformed into nerve impulses. 
• Four rows of hair cells within the cochlear duct are supported on the basilar membrane. The movement back and forth of the stapes pushes on the fluid in the cochlear duct, causing the basilar membrane and hair cells to move up and down. These movements bend the cilia of hair cells, opening the mechanically gated potassium channels on their surface. 
• An influx of potassium depolarizes the cells, stimulating them to send nerve impulses to the cochlear nerve and onto the brain.

6.0Functions of Ear

The ear serves several essential functions

• Hearing: The ear's main role involves sensing sound waves and transforming them into electrical impulses that the brain interprets as sound.
• Balance and equilibrium: The inner ear contains structures like the semicircular canals and vestibule, which help maintain balance and equilibrium by detecting changes in head position and movement.
• Protection: The ear canal and its wax provide a natural barrier against foreign objects, dust, and bacteria, helping to protect the delicate structures of the ear.
• Thermoregulation: The blood vessels in the ear help regulate body temperature by expanding or constricting to release or conserve heat.