The auditory system is the
sensory systemfor the sense of hearing.
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
auditory 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(or tympanic membrane), which marks the beginning of the 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 ossiclesact 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 (or elliptical) window. Higher pressure is necessary because the inner ear beyond the oval window contains liquid rather than air. The sound is not amplified uniformly across the ossicular chain. The auditory reflexof 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.
Name = Cochlea
Caption = Diagrammatic longitudinal section of the cochlea.
Scala mediais labeled as "ductus cochlearis" at right.
Width = 350px
The inner ear consists of the
cochleaand several non-auditory structures. The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membraneseparating 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 is located at this duct, and transforms mechanical waves to electric signals in neurons.
Organ of Corti
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 cells are columnar cells, each with a bundle of 100-200 specialized
ciliaat 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.
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. Recently it has been shown that cdh23 and pchh15 are the adhesion molecules associated with these tip links. It is thought that a calcium driven motor causes a shortening of these links to regenerate tensions. This regeneration of tension allows for apprehension of prolonged auditory stimulation.
Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter
glutamatecommunicates signals from the hair cells to the dendrites of the primary auditory neurons.
There are far fewer inner hair cells in the cochlea than afferent nerve fibers. The neural dendrites belong to neurons of the
auditory nerve, which in turn joins the vestibular nerveto form the vestibulocochlear nerve, or cranial nerve number VIII. [ [http://www.meddean.luc.edu/lumen/meded/GrossAnatomy/h_n/cn/cn1/cn8.htm Meddean - CN VIII. Vestibulocochlear Nerve] ]
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 (towards the brain) dendrites under inner hair cells.
Central auditory system
This sound information, now re-encoded, travels down the
vestibulocochlear nerve, through intermediate stations such as the cochlear nucleiand superior olivary complexof the brainstemand the inferior colliculusof the midbrain, being 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 cortexis located in the temporal lobe.
Associated anatomical structures include:
cochlear nucleusis the first site of the neuronal processing of the newly converted “digital” data from the inner ear. This region is anatomically and physiologically split into two regions, the dorsal cochlear nucleus(DCN), and ventral cochlear nucleus(VCN).
Trapezoid bodyis a bundle of decussating fibers in the ventral pons that carry information used for binaural computations in the brainstem.
Superior olivary complex
superior olivary complexis located in the pons, and receives projections predominantly from the ventral cochlear nucleus, although the posterior cochlear nucleus projects there as well, via the ventral acoustic stria. Within the superior olivary complexlies the lateral superior olive (LSO) and the medial superior olive (MSO). The former is important in detecting interaural level differences while the latter is important in distinguishing interaural time difference.
The lateral lemniscus is a tract of
axons in the brainstemthat carries information about sound from the cochlear nucleusto various brainstem nuclei and ultimately the contralateral inferior colliculusof the midbrain.
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
superior olivary complexand dorsal cochlear nucleus) before sending it to the thalamus and cortex.
Medial Geniculate Nucleus
Medial Geniculate Nucleusis part of the thalamic relay system.
Primary auditory cortex
Primary Auditory Cortexis the first region of cerebral cortexto 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 areas41 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 lobeof 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.
Kandel, et al Principles of Neuroscience. Fourth ed. pp 591-624. Copyright 2000, by McGraw-Hill Co.
Music and the brain
Noise health effects
Auditory Brainstem Response(ABR) and ABR audiometry test for newborn hearing
* [http://www.iurc.montp.inserm.fr/cric/audition/english/start_gb.htm Promenade 'round the cochlea]
* Washington University Neuroscience Tutorial - [http://thalamus.wustl.edu/course/audvest.html Auditory system]
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