Auditory system

The auditory system is the sensory system for the sense of hearing.

Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came.

The sound waves enter the ear canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 kHz. At the far end of the ear canal is the eardrum (tympanum, or tympanic membrane), which marks the beginning of the middle ear.

Middle ear
Sound waves traveling through the ear canal will hit the tympanum, or eardrum. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval window. Higher pressure is necessary because the inner ear beyond the oval window contains fluid rather than air. The sound is not amplified uniformly across the ossicular chain. The auditory reflex of the middle ear muscles helps protect the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the cochlea.

Inner ear
The inner ear consists of the cochlea and several non-auditory structures. The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. Strikingly, one section, called the cochlear duct or scala media, contains an extracellular fluid similar in composition to endolymph, which is usually found inside of cells.

The organ of Corti forms a ribbon of sensory epithelium which runs lengthwise down the entire cochlea. The hair cells of the organ of Corti transform the fluid waves into nerve signals. The journey of a billion nerves begins with this first step; from here further processing leads to a panoply of auditory reactions and sensations.

Hair cell
Hair cells are columnar cells, each with a bundle of 100-200 specialized cilia at the top, for which they are named. These cilia are the mechanosensors for hearing. Lightly resting atop the longest cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia and allowing electric current into the hair cell (2).

Hair cells, like the photoreceptors of the eye, show a graded response, instead of the spikes typical of other neurons. These graded potentials are not bound by the “all or none” properties of an action potential.

At this point, one may ask how such a wiggle of a hair bundle triggers a difference in membrane potential. The current model is that cilia are attached to one another by “tip links”, structures which link the tips of one cilium to another. Stretching and compressing the tip links may open an ion channel and produce the receptor potential in the hair cell.

Neuron to hair cell relationship
There are far fewer hair cells than afferent nerve fibers in the cochlea. The nerve that innervates the cochlea is the vestibulocochlear nerve, or cranial nerve number VIII.

Neuronal dendrites innervate cochlear hair cells. The neurotransmitter itself is thought to be glutamate. At the presynaptic juncture, there is a distinct “presynaptic dense body” or ribbon. This dense body is surrounded by synaptic vesicles and is thought to aid in the fast release of neurotransmitter.

Efferent projections from the brain to the cochlea also play a role in the perception of sound. Efferent synapses occur on outer hair cells and on afferent dendrites under inner hair cells.

Central auditory system
This sound information, now re-encoded, travels down the auditory nerve, through parts of the brainstem (for example, the cochlear nucleus and inferior colliculus), further processed at each waypoint. The information eventually reaches the thalamus, and from there it is relayed to the cortex. In the human brain, the primary auditory cortex is located in the temporal lobe.

Associated anatomical structures include:

Cochlear nucleus
The cochlear nucleus is the first site of the neuronal processing of the newly converted “digital” data from the inner ear. This region is anatomically and physiologically spit into two regions, the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus (VCN).

Trapezoid body
The Trapezoid body is a bundle of decussating fibers in the ventral pons that carry information used for binaural computations in the brainstem.

Superior olivary complex
The superior olivary complex is located in the pons, and receives projections predominantly from the anteroventral cochlear nucleus, although the posteroventral nucleus projects there as well, via the ventral acoustic stria.

Inferior colliculi
The IC are located just below the visual processing centers known as the superior colliculi. The central nucleus of the IC is a nearly obligatory relay in the ascending auditory system, and most likely acts to integrate information (specifically regarding sound source localization from the SOC and dorsal cochlear nucleus) before sending it to the thalamus and cortex.

Medial Geniculate Nucleus
The Medial Geniculate Nucleus is part of the thalamic relay system.

Primary auditory cortex
Primary Auditory Cortex is the first region of cerebral cortex to receive auditory input.

Perception of sound is associated with the right posterior superior temporal gyrus (STG). The superior temporal gyrus contains several important structures of the brain, including Brodmann areas 41 and 42, marking the location of the primary auditory cortex, the cortical region responsible for the sensation of basic characteristics of sound such as pitch and rhythm.

The auditory association area is located within the temporal lobe of the brain, in an area called the Wernicke's area, or area 22. This area, near the lateral cerebral sulcus, is an important region for the processing of acoustic signals so that they can be distinguished as speech, music, or noise.