What is the Significance of the Ear?

The ear is the organ that helps in hearing, which is the modification of sound waves from any source into neural signals that the brain could process. The ears aid in detecting and processing sound and help maintain an overall equilibrium of the being. Equilibrium is the sense of balance and positional awareness. Numerous sensory receptors provide information to the brain about equilibrium. However, unique receptors found in the inner ear play a very critical role in observing equilibrium. To better understand the functioning of the ear and its role in sustaining equilibrium, it is important to learn about its complex structure.

Structure of the Ear

The human ear is divided majorly into three units. They are namely the outer ear, middle ear, and inner ear. The external structure is commonly referred to as the ear, is the outer ear or the auricle. The C-shape of its structure funnels the sound waves from outside into the ear canal. At the end of the ear canal, a structure called the eardrum or tympanic membrane vibrates with the frequency of the sound waves that pass through.

Present along the length of the auditory canal is Cerumen or earwax produced by the ceruminous glands. Due to its sticky consistency, Cerumen prevents the foreign particles from entering the eardrum and inhibits bacterial growth in the tympanic membrane and the auditory canal.

Three small bones occupy the middle ear called the ossicles, whose collective function is to amplify the movements of the tympanic membrane. These bones are named the malleus, the incus, and the stapes. Here, the malleus is connected to the tympanic membrane and communicates with the incus, which communicates with the stapes. The sound waves are then transferred to the inner ear through the stapes attached at the oval window (membrane-covered opening from the middle ear to the cochlea of the inner ear).

The inner ear is completely enclosed in the temporal bone and has three distinct parts: the cochlea, the vestibule, and the semicircular canals. The inner ear converts the sound waves into neural signals, and the same is relayed to the brainstem through a set of nerve bundles, which is collectively known as the vestibulocochlear nerve.

How do Hearing and Equilibrium Work?

Hearing

The cochlea of the inner ear is almost entirely responsible for hearing. The middle ear is connected to the inner ear through the oval window, by which the vibrations from the ossicles pass into the cochlea. The cochlea consists of three chambers: the scala vestibule (upper chamber), the scala tympani (lower chamber), and the cochlear duct sandwiched between the two chambers. Found within the scala tympani and the scala vestibuli is the extracellular fluid known as perilymph. In addition, the spiral organ of the Corti that contains the sensory receptors for hearing is found throughout the cochlear duct. The organ of Corti consists of the lower basilar membrane, placed against the scala tympani and the upper tectorial membrane within the cochlear duct.

The specialized sensory receptors for hearing are hair cells with slender cilium known as stereocilia sandwiched between the basilar membrane and the tectorial membrane. The sound waves from the stapes of the middle ear travel to the cochlea through the oval window, which moves the fluids in the Scala vestibule and Scala tympani. The movement of the fluids causes the lower basilar membrane to vibrate. As a result of this movement, the stereocilia of the hair cells start to be in motion and comes in contact with the upper tectorial membrane. The movement of the stereocilia on the hair cells generates nerve impulses transmitted to the brain by the vestibulocochlear nerve.

Equilibrium

Equilibrium is the sense of balance and is achieved by the vestibule and the inner ear's semicircular canals. There are two types of equilibrium:

1. Static (gravitational) equilibrium
2. Dynamic (rotational) equilibrium

Akin to the cochlea, both the vestibule and the semicircular canals utilize the hair cells with stereocilia to interpret fluid movements resulting from changes in the head position or other stimulation.

Static equilibrium

The static equilibrium is essential to maintain balance when there is the movement of the head respective to gravity. It is achieved with the help of specialized structures known as utricle and saccule present within the vestibule. The utricle and saccule contain a set of sense organs called maculae that harbor the stereocilia and their supporting cells. These organs are present at an angle perpendicular to each other for effective response to positions in a different plane.

Example

The maculae respond to variations in position and acceleration using the tips of their stereocilia that project into a dense otolithic membrane made up of a mixture of calcium and protein known as otoliths. For example, when a human bends down to pick something up, the head moves, and gravity causes the otoliths to move. This movement within the membrane causes the stereocilia to bend, which in turn causes a neural signal to form in the vestibular nerve fiber that encapsulates them. To ensure the effectiveness of this process in any way that the head moves, bundles of stereocilia are arranged in various directions. Thus, the body's sensitivity to its position and direction of acceleration depends on the specific pattern of the stereocilial activity across the maculae.

Dynamic equilibrium

The semicircular canals are almost entirely responsible for maintaining dynamic equilibrium and are made up of three ring-like extensions that project from the vestibule. One of these rings extends in the horizontal plane, and the other two align in the vertical plane. Each semicircular canal consists of an enlarged area known as the ampulla, which is found at its base, where the ring meets with the vestibule. The ampulla contains a structure made up of hair cells, known as the crista ampullaris. The crista ampullaris is the part that responds to rotational movement. Attached to the top of the ampulla is a membrane called the cupula, into which the stereocilia of the hair cells extends.

Example

When the movement of the head is rotational, for example, like that of a ballet dancer, and the movement is parallel to the semicircular canal, the movement of the fluid in the canal is not as quick as that of the head. As a result, the cupula pushes in the opposite direction, thus averting the stereocilia and creating a nerve impulse. As the semicircular canals are present on either side of the head, an orthogonal plane is formed, which comprises the horizontal plane with the horizontal canals and the two vertical planes perpendicular to each other with an anterior canal from one side and the posterior canal on the other. On either side of the body, a difference in polarization of the hair cells occurs due to the deflection of the cupula. For example, if the head rotates to the right side, the horizontal canals are active, and the right side depolarizes. In contrast, the left side hyperpolarizes, thus signifying the direction of the movement. With the help of all six semicircular canals on each ear, the vestibular system can regulate equilibrium in any direction within the three-dimensional space.

The Inner Ear and a Musical Instrument: an Analogy

There have been numerous studies on the complex labyrinth of a structure, the inner ear, and the soothing musical instrument known as the harp. The inner ear contains the organ of Corti that houses thousands of fine hair cells whose filaments vary in length and are somewhat similar in structure to that of the harp strings. These sensory receptor cells are indeed connected at the base to the auditory nerve, as already explained.

How Significant are Hearing and Equilibrium?

It is obvious that any being, whether a mammal, amphibian, or reptile, whose hearing and equilibrium function is not being carried out properly, would have difficulty in leading a normal livelihood. Though hearing is essential, equilibrium, on the other hand, is a tad more important. As already mentioned, equilibrium is the sense of balance and awareness of a being's position. And if there is any damage or flaw in the organs responsible for the maintenance of the same, then there would be a lot of complexity for performing even basic functions such as walking or, much worse, responding to an incoming threat. Hence, the ear, like every other organ, performs some very important functions.

Pitfalls and Common Mistakes

When learning about complex biological functions of different parts of the ear or even any such organ for that matter, it is important to remember that one can easily get confused by the numerous new words. Hence, there is a chance of committing mistakes, and this can be avoided by using a technique that truly works for the student to remember the names accurately.

Context and Applications

This topic is significant in the professional exams for both Bachelors and Master courses related to biology. Some of the courses are listed below:

• Bachelor of Science in Zoology
• Bachelor of Science in Biochemistry
• Bachelor of Science in Anatomy and Physiology
• Master of Science in Anatomy and Physiology