Structure of Human Ear

Introduction

The ear is a vital organ that helps in hearing and, in mammals, it also helps in maintaining body balance using the vestibular system. In mammals, the ear is mainly defined as having three parts—the outer ear, the middle ear, and the inner ear. The outer ear comprises the pinna and the ear canal. As the outer ear is the only visible region of the ear in most animals, the word “ear” often means the external part only. The middle ear comprises the tympanic cavity and the three ear ossicles. The inner ear is deeply set in the bony labyrinth, and comprises structures that are key to several senses: the semicircular canals, is the one that helps in balancing and eye-tracking while moving; the utricle and saccule, are the ones that function to balance while stationary; and the cochlea, is the only organ that helps in hearing. The vertebrate ears are placed symmetrically on either side of the head, it is an arrangement that functions in sound localisation. The ear originates from the first pharyngeal pouch and some six small swellings that are located in the early embryo known as the otic placodes that originate from the ectoderm layer. The ears can be affected by diseases, such as infection and traumatic damage. Diseases of the ear can cause hearing loss, tinnitus, and balance disorders like vertigo, however many of these conditions may also be affected by damage to the brain or neural pathways resulting from the ear.

 Structure of Ear

The human ear comprises three parts: the outer ear, middle ear, and inner ear. The ear canal of the outer ear is separated from the air-filled tympanic cavity of the middle ear via the eardrum. The middle ear is composed of three small bones namely: the ossicles—it is mainly involved in the transmission of sound and is connected to the throat at the nasopharynx, by the pharyngeal opening of the Eustachian tube. The inner ear is composed of the otolith organs—the utricle and saccule—and the semicircular canals belong to the vestibular system, along with the cochlea of the auditory system.

Outer Ear

The outer ear generally represents the external portion of the ear and it comprises the fleshy visible pinna (also known as the auricle), the ear canal, and an outer layer of the eardrum (also known as the tympanic membrane).

The hollow portion in front of the ear canal is known as the concha. The ear canal elongates for about 1 inch. The cartilage surrounds the first part of the canal, whereas the second part present near the eardrum is surrounded by bone. This bony portion is called the auditory bulla and is made by the tympanic part of the temporal bone. The skin covering the ear canal is composed of ceruminous and sebaceous glands that helps in the production of protective ear wax. The ear canal extends up to the external surface of the eardrum. The pinna is composed of a single layer of elastic cartilage with a complex relief on its inner surface and a fairly smooth composition on its posterior surface. 

Middle Ear

The middle ear lies in between the outer ear and the inner ear. It is composed of an air-filled cavity known as the tympanic cavity and involves the three ossicles and their attaching ligaments; the auditory tube; and the round and oval windows. The ossicles represent three smaller bones that together function to receive, amplify, and transmit the sound from the eardrum towards the inner ear. The ossicles are as follows- the malleus (hammer), incus (anvil), and the stapes (stirrup). The stapes are the smallest bone in the body. The middle ear also helps in connecting with the upper throat at the nasopharynx by the pharyngeal opening of the Eustachian tube.

The three ossicles help in transmitting sounds from the outer ear up to the inner ear. The malleus is the one that receives vibrations due to the sound pressure on the eardrum, where it is situated at its longest part (the manubrium or handle) via a ligament. It transfers the vibrations to the incus, which in turn transmits the vibrations to the smaller stapes bone. The wide base of the stapes lies on the oval window. Once the stapes vibrate, these vibrations are transmitted through the oval window, causing the movement of fluid inside the cochlea.

The round window helps in allowing the fluid present within the inner ear to move. When the stapes pushes the secondary tympanic membrane, fluid present in the inner ear moves and it further pushes the membrane of the round window out via a corresponding amount into the middle ear. The ossicles help in amplifying the sound waves by nearly 15–20 times.

Inner Ear

The inner ear is situated inside the temporal bone in a complex cavity known as the bony labyrinth. A central area called the vestibule is composed of two small fluid-filled cavities, the utricle, and saccule. These help to connect the semicircular canals and the cochlea. There are mainly three semicircular canals that are angled at right angles to one another; these are responsible for the dynamic balance of the body. The cochlea represents a spiral shell-shaped organ that is responsible for the sense of hearing. These structures together make up the membranous labyrinth.

The bony labyrinth represents the bony compartment that comprises the membranous labyrinth, situated within the temporal bone. The inner ear structurally starts at the oval window, which receives vibrations from the incus of the middle ear. Vibrations are transferred to the inner ear into a fluid known as the endolymph, which helps in filling the membranous labyrinth. The endolymph is located in two vestibules, the utricle, and saccule, and then transmits to the cochlea, which is a spiral-shaped structure. The cochlea is composed of three fluid-filled spaces known as the vestibular duct, the cochlear duct, and the tympanic duct. Hair cells are responsible for transduction—changing the mechanical stimuli into electrical stimuli are present in the organ of Corti in the cochlea.

However, the ear is a relatively small structure, it is acted by five cranial nerves:

• Trigeminal nerve (fifth cranial nerve);

• The facial nerve (seventh cranial nerve);

• Vestibulocochlear nerve (eighth cranial nerve);

• The glossopharyngeal nerve (ninth cranial nerve);

• Vagus nerve (tenth cranial nerve).

Functions of Ear

Hearing

The eardrum vibrates as and when the sound waves enter the auditory canal. The ossicles of the three tiny bones (including the stapes, the smallest bone in the body), pass the vibrations to the oval window, which is a membrane located at the entrance of the inner ear.

Balance

The Eustachian tube and the vestibular complex are the crucial parts of the ear that are mainly responsible for the balance of the body. It also helps in equalizing air pressure in the middle ear and maintains the balance. The vestibular complex is composed of receptors that maintain body balance.

Conclusion

The ear is small in size, it is essential for hearing and balance, and problems with the ear may be linked to any other medical conditions. Here we come to an end of this topic, hope you were able to grasp a clear concept of the structure and function of the ear.