The human ear is the organ of hearing and equilibrium. It detects and analyzes sound by the mechanism of transduction, which is the process of converting sound waves into electrochemical impulses. Audition cannot take place adequately if the anatomy is abnormal. This article will discuss the mechanisms implied in the conduction of sound waves into the ear, and its integration and transmission from the middle ear and inner ear to the brain.
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.
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.
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.
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.
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.
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