Special Senses: Hearing (NEURO 4) - PDF

Pag. 1 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing NEURO #5 - 22.10.21 NEURANAT 5 - SPECIAL SENSES: HEARING The acoustic apparatus is dedicated to initially collect the vibrations in the air. These vibrating molecules in the air could be perceived as “so...

Pag. 1 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing NEURO #5 - 22.10.21 NEURANAT 5 - SPECIAL SENSES: HEARING The acoustic apparatus is dedicated to initially collect the vibrations in the air. These vibrating molecules in the air could be perceived as “sounds''. The vibrations are characterized by waves, which are collected, converged and conveyed into a specific system to be analyzed and recognized. 1. External ear The external ear's shape is correlated to its role: convexities and concavities are present and its entrance is protected from the entrance of internal and external pathogens into the canal by the tragus. The skeletal portion is made up of cartilage, whereas the bony portion consists of temporal squama. In the external ear the vibrations have to be localized and collected. It is the most lateral and external portion of the ear, consisting of: - the auricle: it is responsible for the collection of the sound. It is the most lateral and visible part and its skeleton is made of elastic cartilages that create an extraflexion in the wall. - External acoustic meatus: from the auricle the sound is conveyed into an opening called external acoustic meatus: it allows the sound to enter in a duct called external auditory canal, which is not completely horizontal. The sounds can be conveyed from lateral to medial, conducting the vibrations into the tympanic membrane - the tympanic membrane: separates the external ear from the middle ear and transmits sound wave vibrations to the ossicles  Pag. 2 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing 2. Middle ear The middle ear or tympanic cavity is a second pathway which functions in the transmission and transduction of tympanic membrane vibrations to the inner ear (from an air to a fluid medium). The receptors that are responsible for the conversion of mechanical waves into electrochemical signals are inside these liquids. Since the liquids have an impedance the inertia of the liquid surrounding the acoustic receptors has to be overcome, and hence their impedance. In order to do this the vibrations collected at the beginning have to be amplified. It consists in an irregular-shaped air-filled chamber, embedded in the petrous portion of the temporal bone. Into the lateral half of the pyramid there is the tympanic cavity, while into the medial half there is the labyrinth. The tympanic cavity has 2 openings, one on the lateral side, where there is the continuation of the temporal bone with the external auditory canal: auricular cartilage + tympanic cavity = auditory canal; while the other opening is medially, where there is the continuation with the eustachian tube into the rinopharynx. It contains different elements: 1. Three small articulating bones: the auditory ossicles: a. the malleus b. the incus c. the stapes 2. Two miniature skeletal muscles: a. the tensor tympani muscle, which attaches to the malleus b. the stapedius muscle, which attaches to the stapes 3. Two membrane-covered foramina in the bone: a. the oval window b. the round window 4. Two additional openings:  Pag. 3 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing a. the Eustachian (pharyngeal, auditory) tube, which permits communication between the middle ear (tympanic cavity) and nasopharynx b. the communication to the mastoid air cells 5. The chorda tympani nerve: a branch of the facial nerve (CN VII), which passes through but has no function in the middle ear. a. The eustachian tube/canal It represents the anterior continuation of tympanic tube (posteriorly continues inside the mastoid process). The eustachian tube is made by bone at first, but as we move antero-medially and inferiorly it continues with cartilage and fibers, until it opens into the rinopharynx, allowing pressure to equalize between the middle ear and throat. It serves to equalize the atmospheric pressure between the middle ear and outer ear. Since the wall of the Eustachian tube is normally collapsed, pressure differences can be relieved by the action of swallowing, chewing, yawning, or coughing, which open the Eustachian tube, allowing equalization of pressure on the two sides of the tympanic membrane. The Eusctachian tube also allows any fluid accumulation in the middle ear to drain into the nasopharynx. In the lateral wall of the pharynx, we can see the torus, a cartilage prominence of the eustachian tube The view of tympanic cavity inside the skull can be seen in pink in the image on L. The Eustachian tube’s beginning is very close to the carotid canal. The wall is very thin (about 1 mm) and there is the possibility to enter to the carotid artery. Posterior to the pharynx there is the opening of the Eustachian tube. b. Muscles Two muscles may stiffen the movement of the ossicles. - The stapedius muscle, the smallest skeletal muscle in the body, connects to the stapes and is controlled by the facial nerve; - the tensor tympani muscle connects to the base of the malleus and is under the control of the medial pterygoid nerve which is a branch of the mandibular nerve of the trigeminal nerve.  Pag. 4 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear and therefore protecting it against high frequencies. This is called the acoustic reflex. c. Ossicles On the medial side of the tympanic membrane there is the attachment of bones, the ossicle, very small bones responsible for the amplification of the vibrations collected and transmitted to the tympanic membrane. They are connected by synovial joints and enclosed by a capsule with ligaments that allows movements. On the medial side of the tympanic membrane/eardrum the first ossicle (malleus/hammer) collects the vibrations and transmits them to the next ossicle. The head of this ossicle moves superiorly, entering into the epitympanic cavity, where it articulates (diarthrosis) with the incus. In between the longer process of incus and malleus there is a passage called chorda tympani, a branch of the facial nerve that passes inside the tympanic cavity close to the tympanic membrane and eventually reaching the infratemporal fossa. The incus has the shape of a premolar tooth, with the short process located inside the epitympanic cavity, while the long process articulates with the stapes. The stapes collects the vibration and, through its base, transfers them to the medial wall of the tympanic cavity, where there is the oval window of the cochlea. In each of the joint there is the possibility to transmit and to control the transmission: the control of transmission is done by the attachment of the skeletal muscles at the level of malleus and stapes; they have somatic motor fibers which are responsible for the contraction of these muscles, possibly leading to a reduction or complete block of the transmission. Arriving at the lateral wall of the inner ear (which is represented by a portion of a temporal bone), there are two fenestrations: the oval window and the round window. The oval window is closed by a membrane but it is also the point of arrival of basal stapes, so that the vibrations can be transmitted. At this point there is a transition in the medium from air to fluid. On the medial wall of the tympanic cavity there is an inferior window called the round window, but it has no bone against. Amplification of the vibrations is possible thanks to the bigger area of the tympanic membrane in respect with the membrane of the oval window. This amplification is needed in order to overcome the inertia of the liquid inside the middle ear where there are the receptors able to transduce the signals  Pag. 5 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing 3. Inner ear The medial most part of the ear is the inner ear, located in the petrous portion of the temporal bone. The inner ear encloses the cochlea, which contains the organ of Corti (the receptor for hearing), associated with the mechano-electrical transduction, and the vestibular apparatus, which contains the receptors associated with maintenance of equilibrium. It consists of 2 main labyrinths: the bony (osseous) labyrinth and the membranous labyrinth that form an interconnected system of channels filled with fluid. a. Bony labyrinth The bony labyrinth is formed by 3 portions: the vestibule, the semicircular canals and the cochlea. In the lateral wall of the labyrinth are present the oval window superiorly and the round window inferiorly, opening in the vestibule.  Pag. 6 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing The vestibule continues anteriorly as a canal that will coil anteriorly and medially to create the cochlea. The canal enlarges from anterior to posterior and, at the end of the enlargement (posteriorly to the vestibule), are attached 3 thin canals with a ring shape: - superior, - posterior - lateral semicircular canals. They are all oriented in 3 different planes, perpendicular to each other. The lateral canal has 2 independent openings into the vestibule, whereas the superior and posterior canals have each one independent attachment and one common attachment, since they fuse together at the level of the crus commune. Each semicircular canal has an enlarged portion called ampulla. Inside, they present receptors for the vestibulsystem (not acoustic!), but they present a connection with the acoustic system thanks to the presence of the membranous system located inside the bony one and filled in by a liquid which has a flux all around. The bony labyrinth hosts inside 2 canals containing fluids: the scala vestibuli and scala tympani. These canals start at the level of the vestibule and are in continuity with the windows: the scala vestibuli with the oval window, while the scala tympani with the round window. Therefore, the canal starting at the vestibule is dived into 2 floors, one superior and one inferior, by a bony horizontal lamina, the spiral lamina, which is present only along the medial wall of the vestibule (as it goes laterally it ends) and continues also anteriorly in the spiraling part of the cochlea. 1) Cochlea The cochlear canal is divided by the lamina. The end of the canal is called helicotrema. The cochlea’s interior is divided into three parallel, fluid-filled, scalae (compartments) by two membranes: the vestibular (Reissner’s) membrane (floor of the cochlear duct) and the basilar membrane (roof of the cochlear duct). The scalae are the (bony) scala vestibuli, the (membranous) scala media (cochlear duct), which is wedged between the vestibular and the basilar membranes (as a blind-ending duct ending at the apex of the cochlea), and the (bony) scala tympani. The scala tympani and vestibuli are filled with perilymph, a fluid similar to the cerebrospinal fluid that has a high concentration of sodium compared to potassium levels. The scala media (cochlear duct) is filled with the endolymph, which is similar to extracellular fluid and has an even higher concentration of sodium compared to potassium.  Pag. 7 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing The scala vestibuli is in direct communication with the vestibule of the vestibular apparatus. The scala vestibuli and scala tympani, both filled with perilymph, are joined at the helicotrema, a small connecting aperture located at the apex of the cochlea, which permits perilymph from the scala vestibuli to flow into the scala tympani. These canals are connected to the subarachnoid space via the endolymphatic duct, and the perilymph flows into the subarachnoid space thus contributing to the cerebrospinal fluid. The inner surface of the bony labyrinth bordering the scala media is the stria vascularis, which produces endolymph.  Pag. 8 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing 2) Organ of Corti Bubble-like hollow areas dispersed within the modiolus house groups of nerve cell bodies of the bipolar sensory neurons that collectively form the cochlear (spiral) ganglion. The peripheral processes of these bipolar neurons terminate in the basal aspect of the hair cells of the organ of Corti, the cochlear receptor for hearing, whereas the central processes of these neurons form the root of the cochlear nerve. The receptors associated with the sense of hearing form a structure, the (spiral) organ of Corti, which rests like a carpet along the floor of the cochlear duct on the basilar membrane. The organ of Corti consists of supporting epithelial cells and neuroepithelial receptor “hair cells”. Each hair cell displays numerous stereocilia (long microvilli) and a single kinocilium projecting from its apical cell surface. The free ends of the stereocilia project into the tectorial membrane, an acellular, gelatinous substance, joined to the osseous spiral lamina (a bony shelf protruding from the modiolus). The hair cells are are organized into 2 types: - The inner hair cells: they form only one row on the medial side of the tunnel, which is formed by supportive cells called inner rods. They extend obliquely to reach the opposite side, where there are the outer rods. Inner hair cells can synapse with many fibers that enter the spiral lamina by passing through the foramina nervina. - Outer hair cells: they form many rows and are located on the opposite side of the inner ones, just after the outer rods. o In between them there are many supportive cells like Deiters, Hensen and Claudius cells. o Some of outer hair cells can also synapse with fibers going into the spiral lamina. o They form many pillars (the inner and outer rods), creating galleries or tunnels through which the flux of the endolymph passes. 3) Deflection of stereocilia The spiral lamina functions as attachment for the tectrial membrane, which curves forming a roof over the stereocilia of the hair cells: the stereocilia are therefore blocked. When soundwaves are transmitted to the perilymph, there is the vibration of the liquid inside the 2 scalae, causing a vibration of the basilar membrane as well. When the basilar membrane moves, the stereocilia deflect, opening the canals for ions flow in order to have transduction (via depolarization of the receptors)  Pag. 9 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing The outer hair cells are more than 3 times the inner ones. However the very discriminative receptors are the inner, while the outer participate in the creation of the sound, mainly in its amplification. Indeed they participate in the adjustment of the movement of the floor creating in a certain way more deflection of the inner ones. 4) Modulation of the auditory signal Hair cells have different roles: - Inner hair cells (3.000): sound discrimination. One inner cell can synapse with many fibers of the acoustic nerve - Outer hair cells (13.000): amplification. They can increase the movement of the basilar membrane in order to have a bigger deflection of the stereocilia of inner hair cells. Through its association with the tectorial membrane, the motion of the basilar membrane can enhance vibrations in the cochlea. Many outer cells can synapse with only one fiber of the acoustic nerve. Syntonization on specific frequencies: by increasing the rigidity of the basilar membrane, the deflection of stereocilia reduces in order to protect the receptors. 5) Features of the basilar membrane The rigidity of the basilar membrane depends on its distance from the oval window, so its movements will be different throughout its length, since they are related to the pitch frequency. - If there are low frequency sounds, the the apex (helicotrema) of the membrane will move the most, while if the frequency is higher, the base will move the most.  Pag. 10 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing Therefore we can detect different frequencies depending on which portion of the organ of Corti will be activated. Base: narrow, thick, rigid Apex: large, thin, flexible Intensity: function of the number of recruited fibers of the cochlear nerve. The intensity depends on how many fibers are activated by hair cells, more fibers, more intense. Sensitivity: 20-20.000 Hz (wider in children) The intensity of the sound is recognized based on how many fibers are activated. The frequency is recognized based on the level of the depolarized receptors and the intensity based on how many neurons are activated. This is linked to a tonotopism inside all the pathways till the cortex. Noise: irregular frequencies Musical sound: mathematic combination of harmonic frequencies b. Membranous labyrinth Inside the bony labyrinth, which is excavated, there is a membraneous portion. 2 vesicles, the saccule and the utricle are present inside the excavated portion of the vestibule and from the saccule, thanks to the canalis reuniens, there is the continuation of the membranous portion anteriorly, inside the cochlear spiral.  Pag. 11 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing The saccule is connected to the utricle by 2 small canals, the saccular and the utricular branches, which continue into a bigger canal, the endolymphatic duct, that ends with an enlargement called the sac. In this portion there is the reabsorption of the endolymph. The semicircular canals are all attached with the utricle via 3 openings corresponding with the ampullae of the bony part. Since the membranous parts of the labyrinth are all connected with each other, the endolymph can travel freely in the various portions. In between the bone and the membranous labyrinth there is a liquid, the perilymph, while inside the membranous labyrinth there is the endolymph. 4. Acoustic chain 5. Pathway of acoustic fibers From the foramina of the spiral membrane, acoustic fibers (CN VIII) will reach the 1st order neuron inside the spiral ganglion, located inside the bony portion of the cochlear canal.  Pag. 12 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing From spiral ganglion the axons (of 2nd order neurones), forming the cochlear nerve, enter into the brainstem, where they terminate in the dorsal and ventral cochlear nuclei. Second order fibers arising from the cochlear nuclei either - terminate ipsilaterally from the anteroventral cochlear nucleus to the medial and lateral superior olivary nuclei - decussate forming three different separate pathways: o dorsal, o intermediate, o ventral acoustic striae. The tracts ascend partially contralaterally and partially ipsilaterally, at the end it is a bilateral conformation. There can be decussations at different levels, but the majority of contralateral fibers decussate at the level of superior olivary nucleus. Most of the decussating fibers arising from the anteroventral cochlear nucleus form a readily evident group of fibers, the trapezoid body, also known as the ventral acoustic striae in the ventral pontine tegmentum. Ascending pathway created at this level will arrive to mediate geniculate body of the thalamus and then go into the sublentiform limb of internal capsule and reach auditory cortex in the temporal lobe. These tracts are referred as lateral lemniscus. NB the spiral ganglion of Corti collects all the fibers coming from the foramina. It is located inside a cave cone of bone, with its apex at the level of the helicotrema and its base at the level of the internal acoustic meatus. There is the turning of the canal perpendicularly around an axis along a cave cone canal of the bony portion, this is called modioulu, apex to olicotrim and base on anterior acoustic meatus; since the cave is  Pag. 13 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing open we can reach from internal acoustic meatus the posterior cranial fossa and then arrive to the brainstem. There are some obligated stations at the level of the tracts: - the superior olivary nucleus, - inferior colliculus of lamina quadrigemina - in thalamus, the medial geniculate body. - From there the fibers project to the temporal lobe of the telencephalon. From the ventral or dorsal cochlear nuclei the fibers ascend creating two possible tracts which can ascend either ipsilaterally or contralaterally. If they decussate in a contralateral pathway, the synapse is at the level of superior olivary nucleus and then they ascend with the lateral lemniscus to the inferior colliculus, then to medial geniculate body and to the acoustic cortex. From the ventral cochlear nucleus fibers decussate and part of the fibers remain ipsilateral, whereas the dorsal can have both ipsilateral or contralateral fibers. The dorsal cochlear nerve presents monoauricolar info, whereas the ventral cochlear nerve presents biauricular info. The target cortex is found in the Broadman areas 41 and 42, it is mainly close to the sylvian fissure and it is called the primary acoustic area as the superior temporal gyrus. The collaterals given by the cochlear nuclei are important to create reflexes in order, for example, to protect the receptors. The tensor tympani, since it si innervated by the trigeminal nerve, is controlled also with this reflex.  Pag. 14 a 14 International Medical School –NEUROAN #5 – prof.Dellavia – Special senses: Hearing From these last nuclei, fibers can continue ipsilaterally or decussate to ascend in the opposite lateral lemniscus. - The first portion of fibers which partially decussate form the ventral acoustic striae, from the ventral nucleus; - another stria located more dorsally is the dorsal stria, which derives from the dorsal nucleus. - From the ventral nucleus it’s possible to find also an intermediate stria. The dorsal acoustic stria can enter in the medial longitudinal fascicle which can connect many nuclei of the brainstem and it is useful for some reflexes because it involves vestibular and oculomotor nuclei. At all levels descending pathways from the auditory cortex accompany the ascending ones as a feedback mechanism useful to suppress some signals and enhance others. The medial longitudinal fascicle collects both ascending and descending pathway, it is found at the level of posterior commissure in the brain and from there the nuclei descend and connect vestibular, cochlear nuclei to the 4th, 6th cranial nerves nuclei (nuclei controlling the extraocular muscles), with collaterals connecting the trigeminal system. It is possible to have a bundle in efferent system (Rasmussen’s fasciculus) which arise from the superior olivary nucleus and it goes to the organ of Corti inhibiting the ciliate cells in order to adjust the cochlear response to acoustic stimuli at the level of the basilar membrane (it has protective and adjustive functions).