Peripheral Auditory System
Outer Ear
The pinna are the parts of the outer ear that appear as folds of cartilage. They surround the ear canal and function as sound wave reflectors and attenuators when the waves hit them. The pinna helps the brain identify the direction from where the sounds originated. From the pinna, the sound waves enter a tube-like structure called auditory canal. This canal serves as a sound amplifier. The sound waves travel through the canal and reach the tympanic membrane (eardrum), the canal’s end.
The external ear’s three main components are the auricle, external auditory meatus, and tympanic membrane. The auricle (pinna) is a funnel-like plate of elastic cartilage sandwiched between two layers of skin. Modified apocrine sweat glands in the skin, the ceruminous glands, secrete a waxy cerumen. The auricle collects and focuses sound waves toward the tubelike external auditory meatus. This canal is surrounded by elastic cartilage along its outer third and by bone along its inner two-thirds. Sounds gathered by the auricle are carried inward by the meatus to vibrate the tympanic membrane (eardrum) covering its internal orifice. The membrane’s three layers are the outer epidermis, middle dense connective tissue, and inner cuboidal epithelium.
Middle Ear
As the sound waves hit the eardrum, the sensory information goes into an air-filled cavity through lever-teletype bones called ossicles. The three ossicles include the hammer (malleus), anvil (incus), and stirrup (stapes). These delicate bones convert the sound vibrations made when the sound waves hit the ear \drum into sound vibrations of higher pressure. These transformed vibrations (still in wave form) enter the oval window.
Inner Ear
Beyond the oval window is the inner ear. This segment of the ear is filled with liquid rather than air, that is why there is a need of conversion of low pressure sound vibrations to higher pressure ones in the middle ear. The main structure in the inner ear is called the cochlea, where the sensory info in wave form is transformed into the neural form. The cochlear duct contains the organ of Corti. This organ is comprised of inner hair cells that turn the vibrations into electric neural signals. Each hair innervates many auditory nerve fibers, and these fibers form the auditory nerve. The auditory nerve (for hearing) combines with the vestibular nerve (for balance), forming cranial nerve VIII or the vestibulocochlear nerve.
+++++++++++++
The cochlea, within the inner ear, is the specialized organ that registers and transduces sound waves. It lies within the cochlear duct, a portion of the membranous labyrinth within the temporal bone of the skull base.
The human ear. The cochlea has been turned slightly, and the middle ear muscles have been omitted to make the relationship clear.
Sound waves converge through the pinna and outer ear canal to strike the tympanic membrane.
Schematic view of the ear. As sound waves hit the tympanic membrane, the position of the ossicles (which move as shown in blue and black) changes.
The vibrations of this membrane are transmitted by way of three ossicles (malleus, incus, and stapes) in the middle ear to the oval window, where the sound waves are transmitted to the cochlear duct.
Two small muscles can affect the strength of the auditory signal: the tensor tympani, which attaches to the eardrum, and the stapedius muscle, which attaches to the stapes. These muscles may dampen the signal; they also help prevent damage to the ear from very loud noises.
The inner ear contains the organ of Corti within the cochlear duct. As a result of movement of the stapes and tympanic membrane, a traveling wave is set up in the perilymph within the scala vestibuli of the cochlea. The traveling waves propagate along the cochlea; high-frequency sound stimuli elicit waves that reach their maximum near the base of the cochlea (ie, near the oval window). Low-frequency sounds elicit waves that reach their peak, in contrast, near the apex of the cochlea (ie, close to the round window). Thus, sounds of different frequencies tend to excite hair cells in different parts of the cochlea, which is tonotopically organized.
Cross section through one turn of the cochlea.
The human cochlea contains more than 15,000 hair cells. These specialized receptor cells transduce mechanical (auditory) stimuli into electrical signals.
The traveling waves within the perilymph stimulate the organ of Corti through the vibrations of the tectorial membrane against the kinocilia of the hair cells. The mechanical distortions of the kinocilium of each hair cell are transformed into depolarizations, which open calcium channels within the hair cells. These channels are clustered close to synaptic zones. Influx of calcium, after opening of these channels, evokes release of neurotransmitter, which elicits a depolarization in peripheral branches of neurons of the cochlear ganglion. As a result, action potentials are produced that are transmitted to the brain along axons that run within the cochlear nerve.
Structure of hair cell. (Reproduced with permission from Hudspeth AJ: The hair cells of the inner ear. They are exquisitely sensitive transducers that in human beings mediate the senses of hearing and balance. A tiny force applied to the top of the cell produces an electrical signal at the bottom, Sci Am Jan;248(1):54–64, 1983.)
+++++++++++++++++
The axons that carry auditory information centrally within the cochlear nerve originate from bipolar nerve cells in the spiral (or cochlear) ganglion, which innervate the cochlear organ of Corti. Central branches of these neurons course in the cochlear portion of nerve VIII (which also carries vestibular fibers). These auditory axons terminate in the ventral and dorsal cochlear nuclei in the brain stem where they synapse. Neurons in these nuclei send both crossed and uncrossed axons rostrally
(Fig 16–5; see also Chapter 7).
The vestibulocochlear nerve.
Thus, second-order fibers ascend from the cochlear nuclei on both sides; the crossing fibers pass through the trapezoid body, and some of them synapse in the superior olivary nuclei. The ascending fibers course in the lateral lemnisci within the brain stem, which travel rostrally toward the inferior colliculus and then project to the medial geniculate body. Because some ascending axons cross and others do not cross at each of these sites, the inferior colliculi and medial geniculate bodies each receive impulses derived from both ears (Fig 16–6).
Diagram of main auditory pathways superimposed on a dorsal view of the brain stem.
From the medial geniculate body (the thalamic auditory relay), third-order fibers project to the primary auditory cortex in the upper and middle parts of the superior temporal gyri (area 41; see Figs 10–11 and 16–6).
Auditory signals are thus carried from the inner ear to the brain by a polysynaptic pathway, unique in that it consists of both uncrossed and crossed components, including the following structures:
++
Cochlear hair cells → Bipolar cells of cochlear ganglion → Cochlear (VIII) nerve → Cochlear nuclei → Decussation of some fibers in trapezoid body → Superior olivary nuclei → Lateral lemnisci → Inferior colliculi → Medial geniculate bodies → Primary auditory cortex.
++
Reflex connections pass to eye muscle nuclei and other motor nuclei of the cranial and spinal nerves via the tectobulbar and tectospinal tracts. These connections are activated by strong, sudden sounds; the result is reflex turning of the eyes and head toward the site of the sound. In the lower pons, the superior olivary nuclei receive input from both ascending pathways. Efferent fibers from these nuclei course along the cochlear nerve back to the organ of Corti. The function of this olivocochlear bundle is to modulate the sensitivity of the cochlear organ.
++
Tonotopia (precise localization of high-frequency to low-frequency sound-wave transmission) exists along the entire pathway from cochlea to auditory cortex.
++++++++++++++++
Central Auditory System
Once the sound waves are turned into neural signals, they travel through cranial nerve VIII, reaching different anatomical structures where the neural information is further processed. The cochlear nucleus is the first site of neural processing, followed by the superior olivary complex located in the pons, and then processed in the inferior colliculus at the midbrain. The neural information ends up at the relay center of the brain, called the thalamus. The info is then passed to the primary auditory cortex of the brain, situated in the temporal lobe.
Primary Auditory Cortex
The primary auditory cortex receives auditory information from the thalamus. The left posterior superior temporal gyrus is responsible for the perception of sound, and in itthe primary auditory cortex is the region where the attributes of sound (pitch, rhythm, frequency, etc.) are processed.
++++++++++++++++++
The cochlea, within the inner ear, is the specialized organ that registers and transduces sound waves.
It lies within the cochlear duct, a portion of the membranous labyrinth within the temporal bone of the skull base.
The human ear. The cochlea has been turned slightly, and the middle ear muscles have been omitted to make the relationship clear.
Sound waves converge through the pinna and outer ear canal to strike the tympanic membrane.
Schematic view of the ear. As sound waves hit the tympanic membrane, the position of the ossicles (which move as shown in blue and black) changes.
The vibrations of this membrane are transmitted by way of three ossicles (malleus, incus, and stapes) in the middle ear to the oval window, where the sound waves are transmitted to the cochlear duct.
Two small muscles can affect the strength of the auditory signal: the tensor tympani, which attaches to the eardrum, and the stapedius muscle, which attaches to the stapes. These muscles may dampen the signal; they also help prevent damage to the ear from very loud noises.
The inner ear contains the organ of Corti within the cochlear duct. As a result of movement of the stapes and tympanic membrane, a traveling wave is set up in the perilymph within the scala vestibuli of the cochlea. The traveling waves propagate along the cochlea; high-frequency sound stimuli elicit waves that reach their maximum near the base of the cochlea (ie, near the oval window). Low-frequency sounds elicit waves that reach their peak, in contrast, near the apex of the cochlea (ie, close to the round window). Thus, sounds of different frequencies tend to excite hair cells in different parts of the cochlea, which is tonotopically organized.
Cross section through one turn of the cochlea.
The human cochlea contains more than 15,000 hair cells. These specialized receptor cells transduce mechanical (auditory) stimuli into electrical signals.
The traveling waves within the perilymph stimulate the organ of Corti through the vibrations of the tectorial membrane against the kinocilia of the hair cells. The mechanical distortions of the kinocilium of each hair cell are transformed into depolarizations, which open calcium channels within the hair cells. These channels are clustered close to synaptic zones. Influx of calcium, after opening of these channels, evokes release of neurotransmitter, which elicits a depolarization in peripheral branches of neurons of the cochlear ganglion. As a result, action potentials are produced that are transmitted to the brain along axons that run within the cochlear nerve.
Structure of hair cell. (Reproduced with permission from Hudspeth AJ: The hair cells of the inner ear. They are exquisitely sensitive transducers that in human beings mediate the senses of hearing and balance. A tiny force applied to the top of the cell produces an electrical signal at the bottom, Sci Am Jan;248(1):54–64, 1983.)