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Matthias Niemeier

Chapter 11 The auditory and vestibular systems Introduction Sense of hearing: Audition. Allows us to detect and locate sound, as well as perceive and interpret its nuances. Mechanism of audition: 1. Translating sounds in our environment into meaningful neural signals 2. Transformations are carried out in stages rather than all at once 3. Within the inner ear, neural responses are generated by auditory receptors from the mechanical energy in sound 4. At subsequent stages in the brain stem and thalamus, signals from the receptors are integrated before they ultimately reach the auditory cortex Sense of balance: Regulated by the vestibular system, it is a personal and internalized process. The vestibular system informs our nervous system where our head and body are and how they are moving. This information is used without conscious effort to control muscular contractions that will put our body where we want it to be, to reorient ourselves when something pushes us aside, and to move our eyes so that our visual world stays fixed on our retinas even when our head is bouncing around. Mechanism of the vestibular system: 1. Translating the movements of our head into a sense of where we are 2. Transformations are carried out in stages rather than altogether. 3. Within the inner ear, neural responses are generated by vestibular receptors form the tilts and rotations of the head 4. At subsequent stages in the brain stem and thalamus, signals from the receptors are integrated before they ultimately reach the vestibular cortex The nature of sound Sounds: Sounds are audible variations in air pressure. 1. When an object moves toward a patch of air, it compresses the air, increasing the density of the molecules. 2. When an object moves away from a patch of air, the air is rarefied (made less dense). Frequency: Frequency of a sound is the number of compressed or rarefied patches of air that pass by our ears each second. Sound frequency is expressed in hertz (Hz); the number of cycles per second 1. Because sound waves all propagate at the same speed, high frequency sound waves have more compressed and rarefied regions packed in to the same space than low-frequency waves Cycle: One cycle of sound is the distance between successive compressed patches Pitch: The sensation of a frequency is commonly referred to as the pitch of a sound. 1. A high pitch sound corresponds to a high frequency 2. A low pitch sound corresponds to a low frequency Intensity: The difference in pressure between compressed and rarefied patches of air. Sound intensity determines the loudness we perceive; loud sounds have higher intensity. The structure of the auditory system Pinna: The visible portion of the ear made up of cartiledge covered by skin, forming a sort of funnel. 1. Helps collect sounds from a wide area 2. Makes us more sensitive from sounds coming from ahead than from behind 3. Convolutions in the pinna play a role in localizing sounds Auditory canal: The entrance to the internal ear. It extends about 1 inch into the ear before it ends at the tympanic membrane. Tympanic membrane: Also known as the eardrum. It is a thin, cone shaped membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air to the ossicles inside the middle ear, and then to the oval window in the fluid filled cochlea. It ultimately converts and amplifies vibration in air to vibration in fluid. Ossicles: Connected to the medial surface of the tympanic membrane. It is located in a small air filled chamber and transfers movements of the tympanic membrane into movements of a second membrane covering a hole in the bone of the skull called the oval window. Cochlea: Located behind the oval window. It contains the apparatus for transforming the physical motion of the oval window membrane into a neuronal response. The basic auditory pathway: 1. Sound wave moves the tympanic membrane 2. Tympanic membrane moves the ossicles 3. Ossicles move the membrane at the oval window 4. Motion at the oval window moves fluid in the cochlea 5. Movement of fluid in the cochlea causes a response in sensory neurons The ear: Has three main divisions: 1. Outer ear a. Pinna b. Tympanic membrane 2. Middle ear a. Tympanic membrane b. Ossicles 3. Inner ear a. Apparatus medial to the oval window Generation of neural response in the inner ear: 1. Results in the transfer and processing of the signal by a series of nuclei in the brain stem. 2. Output from these nuclei is sent to a relay in the thalamus; the medial geniculate nucleus (MGN). 3. The MGN projects to primary auditory cortex, or A1, located in the temporal lobe. The middle ear Components of the middle ear: 1. Tympanic membrane a. Conical in shape with the point of the cone extending into the cavity of the middle ear 2. Three ossicles a. Malleus i. Attached to the tympanic membrane ii. Forms a rigid connection with the incus b. Incus i. Forms a flexible connection with the stapes c. Stapes i. The flat bottom portion of the stapes called the footplate moves in and out like a piston at the oval window 1. Thus transmitting sound vibrations to the fluids of the cochlea in the inner ear 3. Two tiny muscles that attach to the ossicles 4. Eustachian tube a. Passage that connects the air in the middle ear with the air in the nasal cavities b. This tube is usually open and closed by a valve i. Opening this valve relieves pressure build up associated with changes in air pressure 1. E.g. high altitudes in airplane; Sound force amplification by the ossicles: 1. Sound waves move the tympanic membrane 2. Ossicles move another membrane at the oval window a. Ossicles provide the necessary amplification in pressure to influence the fluid filled membrane of the oval window b. Amplification is necessary because fluid resists sound changes more than air 3. Pressure on a membrane is defined as the force pushing it divided by its surface area. a. The middle ear uses two mechanisms to amplify sound i. The pressure at the oval window will become greater than the pressure at the tympanic membrane if: 1. The force on the oval window membrane is greater than that on the tympanic membrane a. The force is greater because the ossicles act like levers b. Sound causes large movements of the tympanic membrane which are transformed into smaller but stronger vibrations of the oval window 2. The surface area of the oval window is smaller than the area of the tympanic membrane a. The surface area of the oval window is much smaller than that of the tympanic membrane The attenuation reflex: Two muscles attached to the ossicles have a significant effect on sound transmission to the inner ear: 1. The tensor tympani muscle: a. Anchored to bone in the cavity of the middle ear at one end b. Attached to the malleus at the other end 2. The stapedius muscle: a. Extends from a fixed anchor of bone b. Attaches to the stapes When these muscles contract: 1. The chain of ossicles become much more rigid 2. Sound conduction to the inner ear is greatly diminished a. The attenuation reflex: The onset of a loud sound triggers a neural response that causes these muscles to contract i. Sound attenuation is much greater at low frequencies than at high frequencies Importance of the attenuation reflex: 1. Adapts the ear to continuous sound at high intensities a. Loud sounds that would otherwise saturate the response of the receptors in the inner ear could be reduced to a level below saturation by the attenuation reflex 2. Protects the inner ear from loud sounds that would otherwise damage it a. However, the reflex has a delay of 50-100 ms from the time the sound reaches the ear, so the reflex will not protect you from a sudden explosive sound 3. Tends to make high frequency sounds easier to discern in an environment with a lot of low frequency noise a. This capability enables us to understand speech more easily in a noisy environment 4. May also be activated when we speak, so we don’t hear our own voices as loudly as we otherwise would The inner ear The inner ear consists of: 1. Cochlea a. Involved with audition 2. Labyrinth a. Involved with the vestibular system b. Helps maintain body equilibrium i. Discussed later Anatomy of the cochlea: 1. Has a spiral shape resembling a snail’s shell 2. The structure of the cochlea is similar to a drinking straw wrapped two and a half times around the sharpened tip of a pencil 3. The hollow tube of the cochlea has walls made of bone 4. The central pillar of the cochlea is a conical bony structure called the modiolus 5. At the base of the cochlea are two membrane covered holes: a. The oval window b. The round window 6. The tube is divided into three fluid filled chambers: a. Scala vestibuli b. Scala media c. Scala tympani 7. Reissner’s membrane separates the scala vestibuli from the scala media 8. The basilar membrane separates the scala tympani from the scala media 9. Sitting on the basilar membrane is the organ of corti 10. The organ of corti contains auditory receptor neurons 11. The tectorial membrane hangs over the organ of corti 12. At the apex of the cochlea: a. The scala media is closed off b. The scala tympani becomes continuous with the scala vesibuli at a hole in the membranes called the helicotrema 13. At the base of the cochlea: a. The scala vestibuli meets the oval window b. The scala tympani meets the round window 14. The fluid in the scala vestibuli and scala tympani is called perilymph a. Perilymph has an ionic content similar to that of cerebrospinal fluid 15. The scala media is filled with endolymph a. Endolymph has an ionic content similar to that of intracellular fluid 16. The difference in ion content between perilymph and endolymph is generated by active transport processes taking place at the stria vascularis 17. The stria vascularis a. Is the endothelium lining one wall of the scala media b. Reabsorbs sodium and secretes potassium against their concentration gradients 18. Because of the ionic concentration differences and the permeability of reissner’s membrane, the endolymph has an electrical potential that is about 80 mV more positive than that of the perilymph; this is called the endocochlear potential 19. Endocochlear potential is important in enchancing auditory transduction Physiology of the cochlea: Ossicles work like tiny pistons: 1. Inward motion at the oval window pushes perilymph into the scala vestibuli a. If the membrane inside the cochlea were completely rigid, then the increase in fluid pressure at the oval window would reach up the scala vestibuli through the helicotrema and back down the scala tympani to the round window. 2. Because the fluid pressure has nowhere else to escape, the membrane at the round window would bulge out in response to the movement of the membrane at the oval window a. Any motion at the oval window must be complemented by a motion at the round window 3. Some structures inside the cochlea are not rigid; a. The basilar membrane is flexible and bends in response to sound The response of the basilar membrane to sound: The basilar membrane has two structural properties that determine the way it responds to sound: 1. The membrane is wider at the apex than at the base by a factor of 5 2. The stiffness of the membrane decreases from base to apex a. The base being about 100 times stiffer When sound pushes the footplate of the stapes at the oval window: 1. Perilymph is displaced within the scala vestibuli 2. Endolymph is displaced within the scala media because reissner’s membrane is very flexible When sound pulls the footplate of the stapes at the oval window: 1. Reverses the pressure gradient Movement of the endolymph: 2. Makes the basilar membrane bend near its base a. Starting a wave that travels up the basilar membrane towards the apex i. The distance the wave travels up the basilar membrane depends on the frequency of the sound 1. If frequency is high, the stiffer base of the membrane will vibrate more, dissipating most of the energy (it will not propagate very far) 2. If frequency is low, the stiffer base of the membrane will vibrate less, allowing the wave to travel further before the energy is dissipated The response of the basilar membrane: 1. Establishes a place code in which different locations of membrane are maximally deformed at different sound frequencies a. The differences in the traveling waves produced by different sound frequencies are responsible for the neural coding of pitch The organ of corti and associated structures: The auditory receptor cells which convert mechanical energy into a change in membrane polarization are located in the organ of corti, which consists of: 1. Hair cells 2. Rods of corti 3. Various supporting cells Hair cells: 1. AKA auditory receptor cells. 2. Each hair cell has about 100 hairy looking stereocilia extending from its top. 3. The critical event in the transduction of sound into a neural signal is the bending of these cilia. 4. Hair cells are sandwiched between the basilar membrane and a thin sheet of tissue called the reticular lamina. 5. Hair cells between the modiolus and rods of corti are called inner hair cells a. About 3500 form a single row 6. Hair cells farther out than the rods of corti are called outer hair cells a. About 15000-20000 arranged in three rows 7. The stereocilia at the tops of the hair cells extend above the reticular lamina into the endolymph a. Their tips end either i. In the gelatinous substance of the tectorial membrane (outer hair cells) ii. Just below the tectorial membrane (inner hair cells) 8. Hair cells form synapses on neurons whose cell bodies are located in the spiral ganglion within the modiolus a. Spiral ganglion cells are bipolar with neurites extending to the bases and sides of the hair cells where they receive synaptic input b. Axons from the spiral ganglion cells enter the auditory nerve; a branch of the auditory- vestibular nerve VIII which projects to the cochlear nuclei in the medulla c. It is possible to treat certain forms of deafness by using electronic devices to bypass the middle ear and the hair cells and activate the auditory nerve axons directly The rods of corti: Span the basilar membrane and reticular lamina and provide structural support Membranes: 1. Basilar membrane a. At the base of the organ of corti 2. Reticular lamina a. In the middle of the organ of corti b. Holds onto the hair cells 3. Tectorial membrane a. Forms a roof over the organ of corti Transduction by hair cells: When the basilar membrane moves in response to a motion at the stapes, the entire foundation supporting the hair cells moves because the basilar membrane, rods of corti, reticular lamina and hair cells are all rigidly connected. These structures move as a unit, pivoting up toward the tectorial membrane or away from it: 1. When the basilar membrane moves up, the reticular lamina moves up and in towards the modiolus. 2. When the basilar membrane moves down, the reticular lamina moves down and away from the modiolus. When the reticular lamina moves inward or outward relative to the modiolus, it also moves in or out with respect to the tectorial membrane. Because the tectorial membrane holds the tips of the outer hair cell stereocilia, the lateral motion of the reticular lamina relative to the tectorial membrane bends the stereocilia on outer hair cells one way or the other. The tips of stereocilia from inner hair cells are also bent, probably because they are pushed by moving endolymph. Actin filaments: 1. Aligned actin filaments make stereocilia rigid rods and they bend only at the base where they attach to the top of the hair cell. 2. Cross link filaments make the stereocilia stick to one another so that the cilia all move as a unit The bending of stereocilia produced by the upward motion of the basilar membrane: 1. At rest, the hair cells are held between the reticular lamina and the basilar membrane and the tips of the outer hair cell stereocilia are attached to the tectorial membrane 2. When sound causes the basilar membrane to deflect upward, the reticular lamina moves up and inward toward the modiolus, causing the stereocilia to bend outward. Transduction mechanism of hair cells: When a sound wave causes the stereocilia to bend back and forth, the hair cell generates a resting potential that alternately hyperpolarizes and depolarizes from the resting potential of -70 mV 1. When stereocilia bend in one direction, the hair cell depolarizes 2. When stereocilia bend in the other direction, the hair cell hyperpolarizes TRPA1 channels: Special type of cation channels located on the tips of the stereocilia: 1. TRPA1 channels are a member of the transient receptor potential family of ion channels 2. It is likely that TRPA1 channels are induced to open and close by the bending of stereocilia a. Thereby generating changes in the hair cell receptor potential 3. Each channel is connected by an elastic filament called a tip link to the wall of the adjacent cilium. 4. When the cilia are straight, the tension on the tip link holds the channel in a partially opened state, allowing a small leak of K+ from the endolymph into the hair cell 5. Displacement of the cilia in one direction increases tension on the tip link, increasing the inward K+ current 6. Displacement in the opposite direction relieves tension on the tip link, allowing the channel to close completely, preventing inward K+ movement 7. Entry of K+ into the hair cell causes a depolarization, which in turn activates voltage gated calcium channels a. Depolarization due to the unsually high K+ concentration in the endolymph b. Depolarization also due to the 80 mV endocochlear potential, which help create a 125 mV gradient across the stereocilia membranes 8. The entry of Ca2+ triggers the release of neurotransmitter glutamate which activates the spiral ganglion fibers lying postsynaptic to the hair cell Innervation of hair cells: The auditory nerve consists of the axons of neurons whose cell bodies are located in the spiral ganglion. Thus, the spiral ganglion neurons provide all the auditory information sent to the brain: 1. The number of neurons in the spiral ganglion is about 35000-50000 2. Inner hair cells are outnumbered by outer hair cells by a factor of 3:1 3. More than 95% of the spiral ganglion neurons communicate with the relatively small number of inner hair cells a. One ganglion fiber receives input from only one inner hair cell b. One inner hair cell feeds about 10 spiral ganglion neurites 4. Less than 5% of the spiral ganglion neurons communicate with the more numerous outer hair cells a. One spiral ganglion fiber synapses with numerous outer hair cells Amplification by outer hair cells: Outer hair cells seem to play a role in sound transduction: 1. Outer hair cells seem to act like tiny motors that amplify the movement of the basilar membrane during low intensity sound stimuli a. Outer hair cells on the basilar membrane are referred to as the cochlear amplifier b. The keys to this function are motor proteins found in the membranes of outer hair cells i. Motor proteins can change the length of outer hair cells 1. The hair cells’ motor is driven by the receptor potential but does not use ATP as an energy source 2. It is extremely fast as it keeps up with the movements produced by high frequency sounds 3. The hair cell’s motor may be a protein called prestin a. Prestin is tightly packed into the membranes of the outer hair cells and is required for outer hair cells to move in response to sound b. Without prestin, animals are nearly deaf c. Outer hair cells respond to sound with both a receptor potential and a change in length d. Because the outer hair cells are attached to the basilar membrane and the reticular lamina, when motor proteins change the length of the hair cell, the basilar membrane is pulled toward or pushed away from the reticular lamina and the tectorial membrane i. The outer hair cells actively change the physical relationship between the cochlear membranes ii. When the outer hair cells amplify the response of the basilar membrane, the stereocilia on the inner hair cells will bend more and the increased transduction process in the inner hair cells will produce a greater response in the auditory nerve In this way, outer hair cells contribute significantly to the output of the cochlea. Descending input from the brain to the cochlea can regulate auditory sensitivity by modifying the effect of outer hair cells on the response of inner hair cells via neurons outside the cochlea from the release of acetylcholine. Central auditory processes The anatomy of auditory pathways (basic): 1. Afferents from the spiral ganglion enter the brainstem in the auditory vestibular nerve. 2. At the level of the medulla, the axons innervate the dorsal cochlear nucleus and ventral cochlear nucleus ipsilateral to the cochlea where the axons originated a. Each axon branches so that it synapses on neurons in both cochlear nuclei One particularly important pathway from cochlear nuclei to auditory cortex: 1. Cells in the ventral cochlear nucleus send out axons that project to the superior olive (superior olivary nucleus) on both sides of the brain stem 2. Axons of the olivary neurons ascend in the lateral lemniscus (collection of axons) and innervate the inferior colliculus of the midbrain Many efferents of the dorsal cochlear nucleus follow a route similar to the pathway from the ventral cochlear nucleus (but the dorsal path bypasses the superior olive) Although there are different pathways from cochlear nuclei to inferior colliculus, all ascending auditory pathways converge onto the inferior colliculus: 1. The neurons in the inferior colliculus send out axons to the medial geniculate nucleus (MGN) of the thalamus 2. The MGN projects to the auditory cortex Important points to note: 1. Projections and brain stem nuclei other than the ones described also contribute to the auditory pathways; a. The inferior colliculus sends axons not only to the MGN but also to: i. The superior colliculus where the integration of auditory and visual information occurs ii. The cerebellum b. There is extensive feedback in the auditory pathways c. Each cochlear nucleus receives input from just the one ear on the ipsilateral side i. All other auditory nuclei in the brainstem receive input from both ears 1. Thus the only way brainstem damage can produce deafness in one ear is if a cochlear nucleus (or auditory nerve) on one side is destroyed Response properties of neurons in the auditory pathway: 1. Most spiral ganglion cells receive input from a single inner hair cell at a particular location on the basilar membrane a. They fire action potentials only in response to sound within a limited frequency range i. Hair cells are excited by deformations of the basilar membrane 1. Each portion of the membrane is maximally sensitive to a particular range of frequencies Characteristic frequency: The frequency at which a neuron is most response to sound and less responsive at neighboring frequencies. Ascending the auditory pathway: Response properties of the cells become more diverse and complex 1. Some cells in the cochlear nuclei are especially sensitiv
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