Cochlear Nerve and Central Auditory Pathways of an Ear

The ear is the organ that allows mammals to hear. In mammals, the ear is divided into three sections: the inner ear, the middle ear, and the outer ear. The pinna and ear canal make up the outer ear. Since the outer ear is the only visible part of the ear in numerous animals, the term "ear" is frequently used to refer to only that visible part of the ear. The tympanic cavity and the three ossicles make up the middle ear. 

The semicircular canals that permit balance and eye tracking when moving; the utricle and saccule that permit balance while stationary; and the cochlea that permits hearing, are all found in the bone labyrinth. The ears of vertebrates are symmetrically situated on either side of the skull, which helps with sound localization.

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The ear is formed by the first pharyngeal pouch and six little swellings termed otic placodes which are formed in the early embryo stage and are made of ectoderm. Disease, infection, and severe damage to the ear can all influence it. Tinnitus, hearing loss, and balance disorders like vertigo can be caused by ear diseases. However, some of these disorders can also be caused due to the injury to the brain or neuronal pathways that lead from the ear. For thousands of years, the ear has been adorned with earrings and other jewellery in many ethnic cultures, and it has been vulnerable to surgical and cosmetic changes.

Structure

The outer ear, middle ear, and inner ear are the three sections of the human ear. The eardrum separates the outer ear's ear canal from the middle ear's air-filled tympanic chamber. The middle ear includes three basic tiny bones (ossicles) engaged in sound transmission and is linked to the throat through the pharyngeal aperture of the Eustachian tube at the nasopharynx. The otolith organs—the utricle and saccule— and also the vestibular system's semicircular canals and the auditory system's cochlea—are all placed in the inner ear.

Outer Ear

The fleshy visible pinna (also known as the auricle), the ear canal, and the external layer of the eardrum make up the outer ear (also termed as the tympanic membrane).

The pinna is made up of the helix, which curves outward, and the antihelix, which curves inward, and emerges into the ear canal. The tragus, like the confronting antitragus, protrudes and partially obscures the ear canal. The concha is the hollow portion in front of the ear canal. The ear canal is around 1 inch long (2.5 cm). The canal is enclosed by cartilage in the first section and bone in the second section around the eardrum. The auditory bulla is a bony structure created from the tympanic section of the temporal bone. Sebaceous and Ceruminous glands create protective ear wax on the skin around the ear canal. The ear canal terminates at the eardrum's exterior surface.

Middle Ear

Between the inner ear and the outer ear lies the middle ear. The three ossicles and their attached ligaments, the auditory tube, and the round and oval windows are all contained within the tympanic cavity, which is an air-filled hollow. The ossicles are a group of three small bones that collect, amplify, and transfer sound from the eardrum to the inner ear. The malleus (hammer), incus (anvil), and stapes (stirrup) are the ossicles. The stapes is the smallest identified bone in our body. The pharyngeal aperture of the Eustachian tube links the middle ear to the upper neck at the nasopharynx.

Inner Ear

The bony labyrinth, a complicated hollow within the temporal bone, houses the inner ear. The utricle and saccule are two tiny fluid-filled recesses in the vestibule, which is recognized as a central area. The semicircular canals and the cochlea are connected by these recesses. The dynamic balance is maintained by three semicircular canals that are positioned at right angles to one another. The hearing sense is provided by the cochlea, a coiled shell-shaped organ. The membranous labyrinth is made up of such structures.

Functions

Hearing

Sound waves go from the outer ear to the middle ear, where they are modulated, and then are conveyed to the inner ear's vestibulocochlear nerve. This nerve provides signals to the brain's temporal lobe, which are translated into sound.

When the sound goes via the outer ear, it hits the eardrum, causing it to vibrate. The sound is transmitted through the three ossicle bones to a second window (the oval window) that guards the fluid-filled inner ear. The pinna of the outer ear aids in the focus of sound, which causes an influence on the eardrum. The malleus is positioned on the membrane and absorbs vibrations. This vibration is conveyed to the oval window via the stapes and incus. The tensor tympani and stapedius, two tiny muscles, may aid in noise modulation. To attenuate excessive vibrations, the two muscles tighten instinctively. The endolymph in the vestibule and the cochlea vibrates when the oval window vibrates.

Balance

The ear's primary job is to maintain equilibrium, whether moving or stationary. Static balance, which enables a person to sense the impacts of gravity, and dynamic balance, which enables a person to detect acceleration, are two different types of balance that the ear facilitates.

Two ventricles, the utricle, and the saccule establish static equilibrium. Delicate filaments line the walls of such ventricles, and the cells are coated in a fine gelatinous covering. Each cell contains 50–70 tiny filaments and the kinocilium, which is the largest filament. Otoliths, small calcium carbonate crystals, are found within the gelatinous layer. These otoliths shift location when an individual moves. This shift in filament locations exposes ion channels within cell membranes, resulting in depolarization and an action potential which is sent to the brain via the vestibulocochlear nerve.

Cochlear Nerve And Central Auditory Pathways

Auditory Nerve Fibres

The cochlear nerve, which innervates the organ of hearing, and the vestibular nerve, which innervates the organs of balance, are morphologically and functionally separate sections of the vestibulocochlear nerve. The spiral ganglion in the cochlea's modiolus is where the cochlear nerve's fibres begin. As they have 2 pairs of processes, or fibres, which extend from opposite sides of the cell body, spiral ganglion neurons are termed bipolar cells. The cochlear nerve is made up of longer core fibres, also known as primary auditory fibres, and shorter peripheral fibres that stretch to the roots of the inner and outer hair cells. They run in a radial pattern from the spiral ganglion to the habenula perforata, a sequence of minute openings under the inner hair cells. These lose their myelin sheaths during this point and reach the Corti organ as thin unmyelinated fibres. Only around 30,000 such fibres exist, and the majority of them—roughly 95%—innervate the inner hair cells.

The rest travel through the Corti tunnel to innervate the external hair cells. The cochlear nerve trunk is made up of the elongated central processes of bipolar cochlear neurons that are coiled together like rope cables. These principal auditory fibres exit the modiolus via the internal meatus, or pathway, and reach the medulla oblongata, a component of the brainstem.

Auditory Pathways

Ascending Pathways

From the medulla to the cerebral cortex, the central auditory circuits run. They are composed of a succession of nuclei linked by axonal fibre tracts. This intricate network of nerve cells aids in the processing and relaying of auditory information transmitted as nerve impulses to the brain's greatest cerebral levels in the cortex. Different features of the auditory stimuli are communicated across separate parallel pathways to a certain level. 

Other sensory systems use this kind of transmission to allow the central nervous system to examine diverse features of a single auditory stimulus, with certain information processed at low levels as well as other information processed at greater tiers. Information on the pitch, volume, and localization of sounds is assessed at lower tiers of the channel, and appropriate responses, like eye and head motions, intra-aural muscle contractions, or whole-body movements, are begun.

Descending Pathways

A pathway descending from the brain to the cochlear nuclei runs parallel to the pathway ascending from the cochlear nuclei to the brain. Certain fibres in both pathways stay on the very same part of the brain, whereas others traverse the midline to the opposite side. A "spur" line rising from the dorsal cochlear nucleus to the cerebellum as well as another descending from the inferior colliculus to the cerebellum has also been discovered. 

The importance of these brain connections is unknown, though they may predate the cerebral cortex's evolutionary origins. In particular, the descending fibres might be thought of as performing an inhibitory function via "negative feedback." They could also decide that ascending impulses should be stopped and which should be permitted to flow to the brain's higher centres.