Auditory System

As we move through the environment we are continually receiving auditory information. Whether it is our own breathing, the pounding of waves, the playing of instruments, or the conversations whirling around us, the auditory system constantly detects the small, rapid fluctuations in air pressure that we call sound. The range of the human auditory system is vast, detecting changes in air pressure that occur as often as 20,000 times per second or as rarely as twenty times per second, and detecting changes in pressure that are as small as those caused by rising one floor in a building.

Different sounds are composed of different frequencies. A "pure" tone is a single frequency. A "complex" sound is more than one frequency. A human voice is a rich combination of frequencies. For voices and musical instruments, the frequencies are evenly spaced forming "harmonics". The spacing, or fundamental frequency, determines the pitch while the temporal qualities refer to changes in the sound, from soft to loud, or a sound that starts high and drops in pitch.

Fluctuations in air pressure, called sound, are funneled by the outer ear, or pinna, into the ear canal, also called the external auditory meatus (Figure 1). The shape of the outer ear and the ear canal filters the sound, amplifying some frequencies and diminishing others. At the end of the ear canal, the sound causes movement of the eardrum, which is connected to three small bones: the malleus, the incus, and the stapes. These bones conduct the movement of the eardrum to something called the oval window, and, in the process, amplify the movement. The stapes is the smallest bone in the body, and it fits into the oval window of the cochlea (Figure 2).

Hearing loss that occurs prior to the oval window is called a "conductive hearing loss". In other words, there is a problem conducting the sound to the cochlea. Doctors can test for conductive losses using something called "bone conduction". When a vibrating object, such as a tuning fork, is touched to the head, vibrations conduct directly to the cochlea through the bone. A person with only a conductive hearing loss will be able detect these sounds as well as someone with normal hearing.

The cochlea is a bony spiral that makes roughly 2.5 revolutions. Uncoiled, the cochlea is divided along its length into three fluid-filled compartments: the scala vestibuli, the scala media, and the scala tympani. Reissner's membrane divides the scala vestibuli from the scala media and the basilar membrane divides the scala media from the scala tympani. The scala tympani and the scala vestibuli compartments are continuous at the apex (or tip of the spiral) through the helicotrema. The oval window is at the base of the cochlea in the scala vestibuli. The round window is at the base of the cochlea in the scala tympani. Since fluids are basically incompressible, in order for the stapes to push the oval window in, and move the fluid inside the cochlea, the round window must bulge out. As the stapes moves back and forth, responding to fluctuations in air pressure, liquid inside the cochlea is also moved back and forth. Hair cells imbedded in the basilar membrane extend their cilia into the liquid in the scala media (Figure 3). Any movement of the stapes causes movement of the basilar membrane and the cilia causing the hair cell to depolarize, initiating (or firing) an action potential (or spike) that moves up the auditory pathway via individual fibers of the auditory nerve.

A hearing problem within the cochlea is called a "sensorineural loss". A sensorineural loss occurs when the mechanical energy is correctly conducted to the oval window of the cochlea, but the hair cells within the cochlea are damaged and not capable of converting the mechanical energy to a signal carried by the nerves.

The hair cells and the basilar membrane develop such that high frequencies most strongly stimulate the hair cells closest to the oval window, while low frequencies most strongly stimulate the hair cells furthest from the oval window. To accomplish this, the basilar membrane is narrow and tight near the base, and wide and loose near the apex. In addition, hair cells are slightly longer at the apex than at the base. Thus, complex sounds are divided into the frequencies of which they are composed; auditory nerve fibers at one end of the cochlea carry information about high frequencies while auditory nerve fibers from the other end of the cochlea carry information about low frequencies. The hair cells are arranged so that each octave, or doubling in frequency, covers roughly the same distance along the basilar membrane (Figure 4).

Auditory nerve fibers synapse with neurons in the ipsilateral cochlear nucleus. From the cochlear nucleus, the pathway goes through the trapezoidal body and crosses to the contralateral side to synapse within the superior olivary nuclei. The superior olivary nuclei, including the lateral superior olivary nucleus and medial superior olivary nucleus are the first nuclei to receive substantial input from both the ipsi and the contralateral sides. Using input from both ears, it is one of the key nuclei for localizing sound. Continuing up the auditory pathway, some fibers continue up to the inferior colliculus while others synapse at the lateral lemniscal nuclei before crossing to the other side and continuing up to the inferior colliculus. From the inferior colliculus, the pathway either crosses to the contralateral inferior colliculus or continues on to the medial geniculate body (on the ventral posterior portion of the thalamus). From the medial geniculate body, signals continue up the auditory pathway to the auditory cortex (Figure 5).

There are many auditory areas within the cortex. The primary auditory cortex has been the most thoroughly studied. Other areas include the secondary auditory cortex, the posterior auditory field, and the anterior auditory field. In addition, Wernicke's area (area 22) is associated with the interpretation of language. Broca's area - area 44 in the left hemisphere - is associated with the production of language in most people.

Many of the subcortical nuclei (including the cochlear nucleus and the inferior colliculus) are groups of nuclei, distinguishable by their different cell types, the different response characteristics of the cells, and the different functional organizations of the cells. Because of redundancy within the numerous pathways and crossings, injury to an individual pathway is often difficult to detect. (Compare this to the visual pathway where the location of the injury can be determined by the specific visual field that is damaged.)

In addition to the ascending pathway (from the cochlea to the cortex), there is a descending pathway (from the cortex to the cochlea). Many of these descending fibers end up synapsing to the outer hair cells as well as to afferent fibers from the inner hair cells. Little is known about this pathway other than that it aids in the detection of sounds in a noisy background.

As a finely developed system, evolved to detect small rapid changes in pressure, the auditory system is a masterpiece. Through a combination of mechanical tricks and physiological processing, the process of hearing allows humans and animals to interact with a complex environment, and to communicate with one-another.