The auditory system is the sensory system for the sense of hearing. See also Sense A sensory system is a part of the Nervous system responsible for processing sensory information
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Ear
Outer ear
The folds of cartilage surrounding the ear canal are called the pinna. The ear is the sense organ that detects Sounds The Vertebrate ear shows a common biology from Fish to Humans with variations The outer ear is the external portion of the Ear, which consists of the pinna, Concha, and Auditory meatus. The pinna ( Latin for Feather) is the visible part of the Ear that resides outside of the head (this may also be referred to as the auricle or 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 ( external auditory meatus, external acoustic meatus) is a tube running from the Outer ear to the Middle ear. The ear canal amplifies sounds that are between 3 and 12 kHz. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. At the far end of the ear canal is the eardrum (or tympanic membrane), which marks the beginning of the middle ear. The tympanic membrane (also tympanum or myrinx is a thin membrane that separates the External ear from the Middle ear. The middle ear is the portion of the ear internal to the Eardrum, and external to the Oval window of the Cochlea.
Middle ear
Sound waves traveling through the ear canal will hit the tympanum, or eardrum. The middle ear is the portion of the ear internal to the Eardrum, and external to the Oval window of the Cochlea. The tympanic membrane (also tympanum or myrinx is a thin membrane that separates the External ear from the Middle ear. 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). The malleus or hammer is a hammer-shaped small Bone or ossicle of the Middle ear which connects with the Incus and is attached For the record label see Incus Records. The incus or anvil is the Anvil -shaped small Bone or ossicle The stapes or stirrup is the stirrup-shaped small Bone or ossicle in the Middle ear which attaches the Incus to the Fenestra ovalis 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 (or elliptical) window. The ossicles (also called auditory ossicles) are the three smallest Bones in the human body The oval window (or vestibular window) is a membrane-covered opening which leads from the Middle ear to the vestibule of the inner Ear. 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 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
| Cochlea | |
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| Diagrammatic longitudinal section of the cochlea. Scala media is labeled as ductus cochlearis at right. The cochlear duct (or scala media) is an Endolymph filled cavity inside the Cochlea, located in between the Scala tympani and the Scala |
The inner ear consists of the cochlea and several non-auditory structures. The inner Ear is the bony labyrinth, a system of passages comprising two main functional parts the organ of hearing or Cochlea The cochlea is the auditory portion of the Inner ear. Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. The basilar membrane within the Cochlea of the Inner ear is a stiff structural element that separates two liquid-filled tubes that run along the coil of the cochlea 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 cochlear duct (or scala media) is an Endolymph filled cavity inside the Cochlea, located in between the Scala tympani and the Scala Endolymph is the fluid contained in the Membranous labyrinth of the Inner ear. 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 organ of Corti (or spiral organ) is the organ in the Inner ear of Mammals that contains auditory sensory cells or " Hair cells " 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. Hair cells are the Sensory receptors of both the Auditory system and the Vestibular system in all Vertebrates. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately 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. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal 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.
Neurons
Afferent neurons innervate cochlear inner hair cells, at synapses where the neurotransmitter glutamate communicates signals from the hair cells to the dendrites of the primary auditory neurons. Hair cells are the Sensory receptors of both the Auditory system and the Vestibular system in all Vertebrates. Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its
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 nerve to form the vestibulocochlear nerve, or cranial nerve number VIII. The cochlear nerve (also auditory nerve or VIIIth nerve) is part of the Vestibulocochlear nerve, (or 8th Cranial nerve) that is found in higher The vestibular nerve is one of the two branches of the Vestibulocochlear nerve (the Cochlear nerve being the other The vestibulocochlear nerve (also known as the auditory or acoustic nerve) is the eighth of twelve Cranial nerves, and is responsible for transmitting sound [1]
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 parts of the brainstem (for example, the cochlear nucleus and inferior colliculus), further processed at each waypoint. The vestibulocochlear nerve (also known as the auditory or acoustic nerve) is the eighth of twelve Cranial nerves, and is responsible for transmitting sound The brain stem (or brainstem) is the lower part of the Brain, adjoining and structurally continuous with the Spinal cord. The cochlear nuclei consist of (a the Dorsal cochlear nucleus, corresponding to the Tuberculum acusticum on the dorso-lateral surface of The inferior colliculi ( Latin, lower hills) together with the superior colliculi form the eminences of the Corpora quadrigemina, and also part The information eventually reaches the thalamus, and from there it is relayed to the cortex. The thalamus (from Greek θάλαμος = room chamber, IPA= /ˈθæləməs/ is a pair and symmetric part of the brain In the human brain, the primary auditory cortex is located in the temporal lobe. The human brain controls the Central nervous system (CNS by way of the Cranial nerves and Spinal cord, the Peripheral nervous system (PNS The primary auditory cortex is the region of the Brain that is responsible for processing of auditory ( Sound) information The temporal lobes are parts of the cerebrum that are involved in speech, Memory, and Hearing.
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. The cochlear nuclei consist of (a the Dorsal cochlear nucleus, corresponding to the Tuberculum acusticum on the dorso-lateral surface of The cochlear nuclei consist of (a the Dorsal cochlear nucleus, corresponding to the Tuberculum acusticum on the dorso-lateral surface of This region is anatomically and physiologically split into two regions, the dorsal cochlear nucleus (DCN), and ventral cochlear nucleus (VCN). The dorsal cochlear nucleus (DCN also known as the "tuberculum acousticum" is a cortex-like structure on the dorso-lateral surface of the brainstem The ventral cochlear nucleus (or anterior, or accessory) placed between the two divisions of the Cochlear nerve, is on the ventral aspect of the
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. The trapezoid body is part of the Acoustic pathway. It is a bundle of fibers and cells in the Pontine tegmentum. The trapezoid body is part of the Acoustic pathway. It is a bundle of fibers and cells in the Pontine tegmentum.
Superior olivary complex
The superior olivary complex is 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. The pons (sometimes pons Varolii after Costanzo Varolio) is a structure located on the Brain stem. Within the superior olivary complex lays the lateral superior olive (LSO) and the medial superior olive (MSO). The former is important in detecting interaural intensity differences while the latter is important in distinguishing interaural time difference.
Lateral lemniscus
The lateral lemniscus is a tract of axons in the brainstem that carries information about sound from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus of the midbrain. The lateral lemniscus is a tract of Axons in the Brainstem that carries information about sound from the Cochlear nucleus to various brainstem nuclei and An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell The brain stem (or brainstem) is the lower part of the Brain, adjoining and structurally continuous with the Spinal cord. The cochlear nuclei consist of (a the Dorsal cochlear nucleus, corresponding to the Tuberculum acusticum on the dorso-lateral surface of The inferior colliculi ( Latin, lower hills) together with the superior colliculi form the eminences of the Corpora quadrigemina, and also part In biological anatomy the mesencephalon (or midbrain) comprises the Tectum (or corpora quadrigemini Tegmentum, the ventricular mesocoelia (or "iter"
Inferior colliculi
The IC are located just below the visual processing centers known as the superior colliculi. The inferior colliculi ( Latin, lower hills) together with the superior colliculi form the eminences of the Corpora quadrigemina, and also part The inferior colliculi ( Latin, lower hills) together with the superior colliculi form the eminences of the Corpora quadrigemina, and also part 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 complex and dorsal cochlear nucleus) before sending it to the thalamus and cortex. The dorsal cochlear nucleus (DCN also known as the "tuberculum acousticum" is a cortex-like structure on the dorso-lateral surface of the brainstem
Medial Geniculate Nucleus
The Medial Geniculate Nucleus is part of the thalamic relay system. The Medial Geniculate Nucleus or Medial Geniculate Body (MGB is part of the auditory Thalamus and represents the thalamic relay between the Inferior colliculus The Medial Geniculate Nucleus or Medial Geniculate Body (MGB is part of the auditory Thalamus and represents the thalamic relay between the Inferior colliculus
Primary auditory cortex
Primary Auditory Cortex is the first region of cerebral cortex to receive auditory input. The primary auditory cortex is the region of the Brain that is responsible for processing of auditory ( Sound) information The primary auditory cortex is the region of the Brain that is responsible for processing of auditory ( Sound) information The cerebral cortex is a structure within the Brain that plays a key role in Memory, Attention, perceptual Awareness, Thought,
Perception of sound is associated with the right posterior superior temporal gyrus (STG). The superior temporal gyrus is one of three (sometimes two gyri in the Temporal lobe of the human Brain. 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. A Brodmann area is a region of the cortex defined based on its Cytoarchitecture, or organization of cells The primary auditory cortex is the region of the Brain that is responsible for processing of auditory ( Sound) information
The auditory association area is located within the temporal lobe of the brain, in an area called the Wernicke's area, or area 22. The temporal lobes are parts of the cerebrum that are involved in speech, Memory, and Hearing. Wernicke's area is a part of the Human cerebrum that forms part of the cortex, on the posterior section of the Superior temporal gyrus, encircling the 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. The lateral sulcus (also called Sylvian fissure or lateral fissure) is one of the most prominent structures of the human Brain.
Bibliography
Kandel, et al Principles of Neuroscience. Fourth ed. pp 591-624. Copyright 2000, by McGraw-Hill Co.
See also
- Music and the brain
- Noise health effects
- Sound
- Tinnitus
- Auditory Brainstem Response (ABR) and ABR audiometry test for newborn hearing
References
External links
- Promenade 'round the cochlea
- Washington University Neuroscience Tutorial - Auditory system
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