The Chemistry of Hearing

The ear is among of the five sense organs, and functions for the perception of sound – or hearing. Hearing happens in three (3) sections of the ear: the outer, middle and inner ear. The outer ear consists of the pinna (auricle), the ear canal and the surface of the ear drum. Ossicles and the tympanic membrane comprise the middle ear, while the cochlea, vestibule and the semi-circular canals compose the inner ear. The outer ear is characterized by a cup-shaped pinna which captures sound. The pinna diverts sound into the ear canal, towards the surface of the eardrum.

Vibrations occur as sounds arrive at the eardrum. The middle ear, composed of the ossicles: hammer, anvil and stirrup, vibrate to transmit sound into the inner ear, which is fluid-filled, to be converted into nerve signals that can be interpreted by the brain. The objective of this paper is to relate the mechanism of hearing to the principles of chemistry. Chemistry, as a science, deals with matter, its structure, composition and transformations. Hearing begins with the generation of sound. Sound is a form of energy that travels through matter.

There are three states of matter: solid, liquid and gas. Each state differs in the distance of its molecules from one another. Solid molecules are compact compared to their gaseous counterpart, which are spread very widely. The speed of sound depends on the kind of medium it was propagated with. As Russell (2007) points out, “liquid and solids conduct the vibrations of sound energy much better than gasses. There is less space between the molecules so it is easier for them to bump into each other. ” Henderson (2007) states that sound waves are pressure waves.

As sound is integrated with a particle of its medium, the particle is temporarily displaced. Its displacement causes the displacement of another particle, which eventually results into a massive displacement of particles. This causes a compression, or a high pressure region. Theoretically, a force is responsible to restore the displaced particles into its former resting position, creating a low pressure region or rarefaction. The combination of compression and rarefaction makes sound waves into pressure waves.

The displacement of particles is summarized in the kinetic-molecular theory of gases (Hardy, 1998). Gases are minute particles spread at large distances, traveling at high speeds and causes pressure as it collides with a surface. The outer ear is designed to capture sound. In its cup-shaped structure, the pinna is able to localize sound. It amplifies sound and enhances hearing sensitivity. Moreover, the pinna leads the sound waves into the ear canal which acts as a resonator enhancing sounds into a 2-5 kilohertz range (Nave, 2006).

As the sound waves travel through the auditory canal, its compression and rarefaction causes the vibrations of the eardrums that stimulate vibrations in the tympanic membrane. As sound approaches the middle ear, the tympanic membrane vibrations stimulate the action of the ossicles: hammer, anvil and the stirrup. This step is critical in converting the sound into mechanical vibration, since the ossicles are basically three tiny bones. The mechanical vibration caused by the ossicles cause a movement within the fluid-filled cochlea. The cochlea, a part of the inner ear, initiates the transformation of sound vibrations into nerve signals.

This happens as the movement of the cochlear fluid ignites receptors in the organ of Corti, stimulating the spiral ganglion conveying information to the eighth cranial nerve to the brain (Ear, 2007). This is the process of mechano-electrical transduction. Wada (2000) explains that the receptors in the organ of Corti – in the form of hair cells contain stereocilia. The stereocilia deflects, in response to the basilar membrane vibrations, causing an opening ion channel in its apical region and an influx of ions into the hair cells.

This leads produces action potentials in the cranial nerve, responsible for the discharge of impulses to the auditory nerve fibers (collectively the eighth cranial nerve). The action potential happens when the exchange of ions in the stereocilia begins. The opening of an ion channel pertains to depolarization, where the sodium (Na+) ions enter the hair cells due to the greater concentration of Na+ ions and a positive voltage on the outside of the axon (one end of the nerve fiber).

When the Na+ channels in the stereocilia close, the K+ channels open to release K+ ions which is attributed to the greater concentration of K+ and reversed voltage levels on the interiors of the hair cells (Krantz, 1997). Notice that as Na+ ions invades the axon, the K+ ions pulls itself out. This is a simple rule: Like charges repel. This falls under electrochemistry. The main idea that surrounds the action potential of the auditory nerve fibers is the movement of ions which involves the loss or gain of electrons (Electrochemistry, 2007).

All in all, the mechanism of hearing is not merely a monotony of biology or anatomy. As a science, biology is a holistic study which also involves chemistry and its principles about matter. Beginning with the states of matter, the transmission of sound was thoroughly discussed. The kinetic-molecular theory of gases explained the behavior of gases – the matter medium of sound waves. Finally, electrochemistry explains the transformation of mechanical vibration into impulses.

The few principles listed in this essay are only a speck of the many relations of chemistry to biology, and vice-versa. This paper aims at viewing science as a body of knowledge that isn’t specialized, but is integrated to each of its disciplines – whether that may be biology or chemistry. References: (“Ear”, 2007; , “Electrochemistry”, 2007; Hardy, 1998; Henderson, 2006; Krantz, 1997; Nave, 2006; Russell, 2007; Wada, 2000) Ear [Electronic (2007). Version]. Retrieved April 26, 2007 from http://en. wikipedia. org/wiki/Ear.

Electrochemistry [Electronic (2007). Version]. Retrieved April 26, 2007 from http://ro. zrsss. si/badoko/electrochemistry. htm. Hardy, J. (1998). The Kinetic-Molecular Theory [Electronic Version]. Retrieved April 26, 2007 from http://ull. chemistry. uakron. edu/genchem/. Henderson, T. (2006). Soundwaves and the Eardrum [Electronic Version]. Retrieved April 26, 2007 from http://www. glenbrook. k12. il. us/gbssci/phys/mmedia/waves/edl. html. Krantz, J. (1997). The Action Potential [Electronic Version]. Retrieved April 26, 2007 from http://psych.hanover. edu/KRANTZ/neural/actionpotential. html. Nave, C. (2006).

The Ear and Hearing [Electronic Version]. Retrieved April 26, 2007 from http://hyperphysics. phy-astr. gsu. edu/hbase/sound/ear. html. Russell, M. (2007). Light and Sound Energy [Electronic Version]. Retrieved April 26, 2007 from http://www. utmsi. utexas. edu/staff/dunton/GK12/lessons/LightandSoundEnergy1. doc. Wada. (2000). The Inner Ear [Electronic Version]. Retrieved April 26, 2007 from http://www. wadalab. mech. tohoku. ac. jp/inner_ear-e. html.