Special Senses II (Hearing & Balance) PDF

Special Senses II: Hearing & Balance DEN 7101S Fiona Britton, Ph.D. Learning Objectives Describe the components & functions of the external, middle, and inner ear. Explain the roles of the tympanic membrane, the auditory ossicles (malleus, incus, and st...

Special Senses II: Hearing & Balance DEN 7101S Fiona Britton, Ph.D. Learning Objectives Describe the components & functions of the external, middle, and inner ear. Explain the roles of the tympanic membrane, the auditory ossicles (malleus, incus, and stapes) and cochlea scala vestibules in sound transmission. Describe the structure and location of hair cells in the inner ear. Describe how sound waves are transduced into nerve impulses in hair cells in the organ of Corti. Explain how pitch and loudness are coded in the auditory pathways. Compare causes of conductive and sensorineural hearing loss. Explain how hair cells in the semicircular canals detect head rotation. Explain how hair cells in the saccule & utricle detect gravity and head tilt. Describe the neural auditory and vestibular pathway from hair cell receptors to the cerebral cortex. The Ear G&H Fig 53-1.  External Ear Structures pinna (auricle) outer ear canal tympanic membrane (ear drum) Function The external ear captures sound waves and directs them toward the middle ear. The pinna acts like a directional antenna. Sound passes down the air-filled outer ear canal. Tympanic membrane vibrates with the movement of sound waves.  Middle Ear Structures Air filled cavity spanned by 3 small bones (ossicles) malleus Latin names translate to incus hammer, anvil, stirrup. stapes Function Ossicles convey sound waves across the air-filled middle ear, from the tympanic membrane to a smaller drum of the inner ear called the oval window. The malleus attached to the tympanic membrane articulates with the incus, which articulates with the stapes. The stapes is attached to the inner ear at the oval window where sound waves are transferred to the inner ear. The middle ear is connected to the nasopharynx by the auditory tube (eustachian tube) Helps equalize air pressure across the tympanic membrane.  Inner Ear Structure and Function Entirely enclosed within the temporal bone. 2 two main fluid-filled organs: Cochlea - contains receptors (hair cells) for hearing. Vestibular apparatus - contains receptors (hair cells) for equilibrium. The internal ear is called the bony labyrinth due to its complex shape. A bony channel structure encloses a membranous sac called the membranous labyrinth. The membranous labyrinth is filled with endolymph fluid. Perilymph fluid surrounds the membranous labyrinth.  Vestibulocochlear nerve (CN VIII) ▪ The cochlea & vestibular apparatus are connected to the brain stem via the vestibular cochlear nerve (CN VIII). ▪ CN VIII provides special sensory innervation for hearing & equilibrium. ▪ CN VIII has 2 branches: ▪ auditory branch to the cochlea (hearing) ▪ vestibular branch to the semicircular canal (equilibrium & balance) ▪ Injury to CN VIII can result in: ▪ deafness ▪ tinnitus (ringing in the ear) ▪ vertigo (loss of balance) What is sound? Sound is produced when something vibrates. The vibration produces a series of pressure waves = sound waves When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound.  Qualities of sound ▪ Sound waves have 4 major features: waveform, phase, amplitude, frequency ▪ Loud sound is produced when large vibrations generate large air pressure waves. ▪ Soft sound is generated by small vibrations. In humans, the amplitude of a sound correspond to loudness. ▪ High frequency sounds are generated from rapid vibrations producing closely spaced air pressure waves (wavelengths are shorter). ▪ Low frequency sounds are are generated from slow vibrations, producing longer wavelengths. In humans, the frequency of a sound roughly corresponds to pitch. The normal range of frequencies audible to humans is 20 to 20,000 Hz. A range of 200- 2000 Hz is required to understand speech. Qualities of sound The amplitude of a sound wave is directly correlated with loudness. Expressed in decibels (dB) The frequency of a sound wave is directly correlated with the pitch of a sound. Expressed in Hertz (Hz) # vibrations per second. Sound Waves What can we say about the characteristics of these sound waves? A. Is a record of a pure tone. B. Has a bigger amplitude than A & is louder than A. C. Has a similar amplitude to A, but a greater frequency, therefore its pitch is higher. D. Is a complex wave form that is regularly repeated. Such patterns are typically perceived as musical sounds. E. Waves with no regular pattern, typically perceived as noise.  Sound Transmission Sound waves travel to the ear causing the tympanic membrane to vibrate. Vibrations are transferred from malleus → incus → stapes → oval window. Sound waves are then converted into fluid motion in the inner ear.  The Cochlea & the Organ of Corti Cochlea: Spiral shaped fluid-filled structure. 3 chambers (scalae) scala vestibuli -perilymph low [K+ ] scala media -endolymph K+-rich fluid scala tympani –perilymph low [K+ ] Organ of Corti: Located in the scala media (middle chamber).  Organ of Corti The organ of Corti lies along the entire length of the basilar membrane of the cochlea. The organ of Corti contains the hair cells (receptors) for transducing sound waves into nerve impulses. Organ of Corti = organ of hearing Hair cells = auditory receptors Hair cells are located on the basilar membrane with the tips of the hair cells embedded in an overlying gel-like elastic tectorial membrane.  Organ of Corti -Hair Cells Hair cells are named for their tufts of stereocilia. Rod-shaped processes with an orderly structure. Have cores composed of parallel filaments of actin. Increase in length along a consistent axis. Note the orderly structure of the clump of stereocilia on each hair cell. Stereocilia emerge from the apical surface of hair cells. The stereocilia are embedded in the tectorial membrane. At the basal end, hair cells are in contact with the auditory (CN VIII) nerve.  Sound Transduction Hair cells are arranged in 4 rows: 3 rows of outer hair cells and 1 row of inner hair cells ~25,000 hair cells in each human cochlea. Sound waves converted into fluid motion in the inner ear causes the basilar membrane to flex. Hair cell stereocilia also bend by the shearing motion with the tectorial membrane. This lateral shearing motion pulls open mechanosensitive K+ ion channels. Receptor potentials are generated. Ultimately APs produced in the CN VIII nerve. Mechanical transduction at hair cell stereocilia ▪ Tip links are tiny thread-like connections that tie the tip of each stereocilium to a mechanically gated K+ channel on the side of the taller neighboring cilium. ▪ Tip links function like a string connected to a hinged trap door. ▪ When the cilia are bent by the shearing motion with the tectorial membrane, tip links mechanically open the K+ channels. ▪ Open K+ channels allows K + influx resulting in depolarization** **Remember: K+ is abundant in endolymph, so K+ influx is down the electrochemical gradient. Mechanical transduction at hair cell stereocilia ▪ This K+ influx and depolarization initiates the receptor potential. ▪ The depolarization opens voltage-gated Ca2+ channels ▪ In turn causes neurotransmitter release (probably glutamate), at the basal end of the hair cell and an AP is elicited in the dendrites of CN VIII. A myosin-based molecular motor in the taller stereocilia neighbor then moves the ion channel, releasing tension in the tip link and the K+ channel closes. This mechanism transduces mechanical energy into neural impulses. G&H. Fig. 53-7A  Mechanical transduction at hair cell stereocilia The more stereocilia bend in the direction of the taller stereocilia, the more K+ channels open. increased depolarization increased Ca2+ entry increased neurotransmitter release increased AP frequency in afferent neurons.  Ionic composition of Endolymph and Perilymph Mechanoelectrical transduction of sound (and motion) is highly dependent on the ionic composition of the fluids bathing the sensory hair cells. Perilymph: Low K+ in scala vestibuli & scala tympani Endolymph: High K+ in scala media Low K+ High K+ Low K+ Stereocilia of sensory hair cells protrude into the K+ - rich endolymph while the basal end of the hair cell is bathed in perilymph. The electrochemical gradient generated is necessary for K+ influx that generates the depolarizing receptor potentials. The K+ that enters hair cells via the mechanically-sensitive ion channels is recycled.  Coding Sound Intensity and Pitch A traveling wave sweeps down the basilar membrane. The idea is to displace the pliable basilar membrane. The mechanical “flexible” properties of the basilar membrane changes along its length. The basilar membrane is narrow and stiff near the oval window; wide and floppy at the apex end. Thus, areas of the basilar membrane will vibrate differently for different sound frequencies. High frequency sounds maximally displace the basilar membrane near the oval window. Low frequency sounds maximally displace at the apex. Different parts of the basilar membrane are G&H. Fig 53-5A. Amplitude pattern of vibration of the basilar membrane. Amplitude patterns for sounds for sensitive to different frequencies. different frequencies.  Coding Sound Intensity and Pitch Hz The basilar membrane codes the frequency of sounds. Loud sounds vibrate the basilar membrane more than soft sounds and produce increased AP frequency. Most sounds are complex because they contain multiple frequencies. Receptor potentials generated by an Hair cells decompose complex sound into its different frequencies. individual hair cell in response to different frequencies. Receptor potentials faithfully follow the stimulating sound frequencies  Auditory Neural Pathway Sensory hair cells Cochlear branch of auditory nerve (CN VII) Brainstem Thalamus Auditory cortex Acoustical information is processed by the brain at various levels. There are auditory association areas surrounding the auditory cortex. Wernicke's area for comprehending speech. Broca’s area for understanding language.  Sound transduction -summary Hearing is the transduction of sound waves into electrical signals. APs in the auditory nerves. Pressure changes produced by sound waves are reproduced as vibrations on the tympanic membrane and ossicles against the perilymph-filled scala vestibuli of the cochlea. These vibrations set up waves of fluid movement in the inner ear. Traveling fluid waves in the basilar membrane in the organ of Corti causes the tectorial membrane to move laterally over the hair cells. The lateral shearing motion bends the stereocilia of hair cells, pulls on the tip links, and opens mechanosensitive K + ion channels. Receptor potentials are generated due to K+ influx and subsequent APs generated in the CNVIII nerve. Hearing Loss 1. Conductive hearing loss Refers to any blockage of sound waves from reaching the hair cells. i.e. the conduction of sound is impaired. Causes: wax or foreign bodies plugging the auditory canal. otitis externa (inflammation of outer ear, “swimmer’s ear”). otitis media (inflammation of middle ear). Treated with external hearing aids. 2. Sensorineural hearing loss Occurs when hair cells or nerves are damaged. CN VIII auditory nerve is affected (tumors) Can be treated with a cochlear implant.  Hearing loss is the most common sensory defect in humans. Hearing Loss < 270 million people worldwide have moderate- profound hearing loss Major causes: ▪ Acoustical trauma extremely loud sounds (explosives, gunfire) can rupture the eardrum, tear the organ of Corti. repeated exposure to industrial noise, household machinery, music, jet engines can damage the inner ear. ▪ Infections -otitis media ▪ Ototoxin drugs: loop diuretics, some antibiotics & chemotherapy agents. These either damage the outer hair cells or the stria vascularis (K +-extruding cells that generate the endocochlear membrane potential). ▪ Presbycusis: gradual hearing loss associated with aging. maybe atherosclerotic damage to microvasculature of the inner ear. maybe genetics? -mutations in myosin of hair cells, K+ channels, Cx26 (defects prevent normal recycling of K+). Hearing loss & dentistry ▪ Maximum noise levels were between 65 and 93 dB ▪ 80% of dental students experienced noise annoyance ▪ 54% reported one of the hearing-related problems ▪ 10% claimed to have hearing loss to a certain extent. ▪ Dental professionals with service length