Physiology, Ear

Issues of Concern

Brief Anatomical Reminder

The human ear is a rudimentary shell-like structure that lies on the lateral aspect of the head. The ear is a cartilaginous structure. For physiological study purposes, it subdivides into three fundamental substructures: the external ear, the middle ear, and the inner ear.

• The outer ear, also called auricle, is composed of cartilage and it is the part with the most contact with the external world. It has various anatomical demarcations like the helix, the antihelix, the tragus, and the antitragus and these demarcations lead to a depression called acoustic meatus. This meatus has a tube form and extends inward to end in the tympanic membrane. Two-thirds of this canal are cartilaginous, and the last third is bone, and the two external thirds have a lining with oil glands that produce cerumen to keep the canal clean from insects and other objects. At the end of the outer ear, lies the middle ear, which is limited externally by the tympanic membrane and internally by the oval window.
• The middle ear is an air-filled space. It divides into an upper and a lower chamber, the epitympanic chamber (attic) and the tympanic chamber (atrium), respectively. It is like a room because it has a rectangular-like shape. It has anatomical relations with the jugular vein, the carotid artery, the inner ear, the eustachian tube, and the mastoid. The content of this room consists of ossicles; the malleus, the incus and the stapes, namely. These bony structures are suspended by ligaments which make them suitable for transmission of vibrations into the inner ear. The vibrations that come into this part of the middle ear than get transmitted by the action of the stapes, into the inner ear.
• The inner ear is a space composed of the bony labyrinth and the membranous labyrinth, one inside the other. The bony labyrinth has a cavity filled with semicircular canals that are in charge of sensing equilibrium; this cavity is called the vestibule and is the place where the vestibular part of the VIII cranial nerve forms. The cochlea is the organ of hearing. It takes its name from the Greek language that means the shell of a snail and is the part from where the cochlear part of the VIII cranial nerve forms, thus constituting the vestibulocochlear nerve.

Sound Wave Transmission and its Physics

The hearing is the process by which sound vibrations transform from the external environment into action potentials. Vibrating objects produce sounds, like the strings of a guitar, and this vibrations pressure pulses into air molecules, better known as sound waves. So, the ear is equipped to distinguish different characteristics of sound, such as pitch and loudness; which refers to the frequency of sound waves and the perception of the intensity of sound, respectively. Frequency measurement is in hertz (Hz, cycles per second). The human ear can detect frequencies from 1000 to 4000 hertz, but a young ear can hear frequencies in the range between 20 and 20000 hertz. The intensity of sound is measured in decibels (dB); the range of human hearing on a decibel scale is from 0 to 130 dB (where the sound becomes painful). All these physical properties have to incur transformations to get into the central nervous system. The first transformation consists of the conversion of air vibrations into tympanic membrane vibrations. These vibrations then get transmitted into the middle ear and the ossicles. Then these vibrations transform into liquid vibrations in the inner ear and the cochlea, and these stimulate a region called the basilar membrane and the organ of Corti. Finally, these vibrations get transformed into nerve impulses, which travel to the nervous system.[1]

Development

The ultrastructure of the ear derives from the branchial arches. The first arch develops into the Merkel cartilage that forms the malleus head and neck, and the incus body. The second arch becomes the Reichert cartilage that forms the manubrium of the malleus and most of the stapes. The first branchial cleft forms the external auditory canal and the outer layer of the tympanic membrane.[2]

Organ Systems Involved

The system by which humans detect sounds is comprised of the external auditory canal and its microstructures (pinna, tragus, anti-tragus, namely), the tympanic membrane, the ossicles, the cochlear structure, and its contents (endolymph, organ of Corti) and the nerve fibers that give birth to the vestibular and cochlear divisions of the VIII cranial nerve.[3]

Mechanism

The outer ear serves the function of directing sound waves into the tympanic membrane. The auricle concentrates the majority of the sound waves and directs it into the funnel-shaped canal. Because the human auricle is almost immobile and not that large, it is less effective with sound gathering than the ears of other mammals. This resonance mechanism only works with short wavelength sound waves (frequencies between 2000 to 7000 Hz) thus determining the sensitivity of the human ear to some frequencies that help us distinguish vowels from consonants. Then, vibrations arrive at the tympanic membrane, triggering vibration there as well. This vibration stimulates a chain of ossicular structures that will transmit the energy to the cochlea, a spiral structure in the inner ear. In the cochlea, the energy is no longer in the form of vibrations, but in the form of hydraulic energy.[4]

Clinical Significance

Specific syndromes and disorders can affect the ear structure and dampen its function[5][6][7][8][9]:

• Structural abnormalities: Structural abnormalities like preauricular pits and tags, stenotic and atretic ear canals can be associated with hearing loss.
• Mandibulofacial dysostosis: This is an autosomal dominant condition, also known as Treacher Collins syndrome, characterized by downward slanting palpebral fissures, auricular malformations with preauricular blind fistulas, stenosis and/or atresia of the external ear canal and ossicular abnormalities.
• Oculo-auriculo-vertebral spectrum: This spectrum is characterized by abnormal development of the first two branchial arches, resulting in anotia, preauricular tags, and atretic ear canals.
• Craniofacial dysostosis: This is a rare syndrome, also known as Crouzon syndrome, that results in premature skull bone fusion, which causes the ear to be atretic or to have stenosis and low set ears[10][11][12]