Mechanism of Hearing - Lecture Notes PDF

Text. Important Lecture Formulas No.8 Numbers َْ ْ َّ ‫( َو َج َع َل َل ُك ُم‬ Doctor notes ‫الس ْم َع َواْلب َص َار‬ َ ُ ُ ْ َ ْ ُ َّ َ َ َ َ ْ...

Text. Important Lecture Formulas No.8 Numbers َْ ْ َّ ‫( َو َج َع َل َل ُك ُم‬ Doctor notes ‫الس ْم َع َواْلب َص َار‬ َ ُ ُ ْ َ ْ ُ َّ َ َ َ َ ْ َ ْ َ Extra notes and explanation ‫واْلف ِئدة لعلكم تشكرون) صدق هللا العم م‬ 1  Mechanism of Hearing Objectives: 1-Describe sound characteristics and explain the difference between discrimination of loudness & pitch (tone). 2- Describe the steps involved in transmission of sound waves into neuronal activity in the inner ear. 3- Differentiate between the functions of the inner and outer hair cells. 4- Appreciate that deafness can be caused by defects in either conduction or neural processing of sound waves. 2 Characteristics of sound Sound is: a mechanical wave (travelling vibration of air). Or it is a vibration that propagates as an audible wave of pressure, through a transmission medium such as gas, liquid or solid. Sound waves are alternating regions of compression and rarefaction (expansion) of air molecules by vibrating body. Characteristics of Sound: 3  ONLY IN MALES’ SLIDES Cont. In human physiology and psychology, sound is the reception of such waves and their perception by the brain Hearing: Hearing is the ability to perceive sound by detecting vibrations through the ear. Humans have a narrow range of hearing compared to other species 20 Hz - 20,000 Hz. 4 Doctors’ notes When fork vibrates inward and outward, the air molecules surrounding it compress and rarefact. This change in air molecules pressure produces sound waves. Measurements of these waves are: 1. frequency ‫ التردد‬or tone: (number of cycles per second) and it is the reason why letters have different sounds (letter G has more cycles than letter C) and humans can hear frequencies between 20-20000 per second while animals can hear more and less and thus sometimes we see birds hiding before storms start. 2. amplitude/loudness/intensity: (how loud the voice is ex: raising the volume of TV). 3. timbre(quality): if we have pure tone of one sound or overtones of more than one tone overlapping. Ex: when someone has flu, vocal cord is covered by mucous and thus each point produces a different tone in the same time and they overlap thus producing a low quality voice ‫بحة‬ 5  Extra Relative magnitude of common sounds Loudness in Comparison to Faintest Audible Sound Sound Decibels (dB) (Hearing Threshold =0.0002 db) Rustle of leaves 10 dB 10 times louder Ticking of Watch 20 dB 100 times louder Whispering 30 dB 1000 times louder Normal Conversation 60 dB 1 million times louder Food Blender,Lawn 90 dB 1 billion times louder Mower, Hair Drier Ambulance Siren 120 dB 1 trillion louder Takeoff of Jet Plane 150 dB 1 quadrillion times Louder Hearing sensitivity is 15-20 dB less in absence of ossicular system and tympanic membrane 6  ONLY IN FEMALES’ SLIDES Anatomy of the ear The ear consists of External ear Middle ear Inner ear Pinna Air filled cavity - Cochlea: is a snail like, coiled fluid-filled tubular system laying deep in the temporal bone. Contains the hearing sensory organ External canal Ossicles (bones) (organ of corti). - Bony labyrinth. - Membranous labyrinth. Tympanic Membrane Stapes (with its foot sitting Malleus* Incus on the oval window of For the eardrum to move freely, the inner the resting air pressure on both ear) sides of eardrum must be equal. Muscles (funnel shaped, pointing inward) tensor tympan stapeduis 7 * Malleus is connected to the tympanic membrane by its lower end and to incus bone by its lower end. Wax has a bactericidal agent.  ** 1. dust stick to wax covering the external canal. Functions of ear 2. 3. hair filtrating the air. warming of air (low temperature damages cochlea). Functions of Ear: Hearing (parts involved → External ear, Middle ear, Internal ear). Equilibrium (parts involved → Internal ear). The ear consists of External ear Middle ear Inner ear 1. Sound localisation (front, back, 1. Ossicles amplify vibrations of 1. transduction: convert sound high, low): Pinna provides clues tympanic membrane to oval window. waves (mechanical) into nerve about location of sound.(Alter 2. This is needed for movement of impulses. amplitude) sound waves in the fluid of the 2. Transmission: sound auditory 2. Sound collection: act as funnel* inner ear. signals to the CNS. to collect sound, Gathers and 3. protection from constant loud Inner ear: focuses sound energy on noise, but not sudden noise, Cochlea (hearing function). tympanic membrane (ear latency of 40-80 msec. semicircular canals (hearing and drum). 4. magnifying effect. balance). 3. Protection**. + Wax. 8 * Ear drum is not flat instead its funnel shape collects the waves on one central point of the tympanic membrane. Functions of ear: Middle ear It is a space between tympanic membrane and the inner ear (opens via eustachian tube into nasopharynx). Other function of the tube: draining accumulated fluids inside cochlea into nasopharynx, ex: when fluid accumulate due to infection Ossicles: Manbrium of the malleus attached to → the back of the tympanic membrane. And its short process attached to→ the incus. The incus then articulates with →the head of the stapes. And stapes’s foot plate attached to → the oval window. Muscles: They help us to reduce and minimize the sound sound. Muscles contract reflexly in response to constant loud sound (over 70db). Contraction of the tensor tympani pulls the manubruim & makes the tympanic m. Tens. Thus decreasing the vibration. Contraction of the stapeduis pull the foot plate outward so thatvibration are reduced. Protection from constant loud noise, but not sudden noise, latency of 40-80 msec. 9 Cont. Transmission of sound through the middle ear: sound waves vibrate the tympanic membrane → Tympanic membrane moves the handle of malleus → Incus moves → Stapes move in & out of the oval window → pressure transmitted through cochlea → cause stimulation of hair cells in the organ of corti → which will stimulate the auditory nerve. Middle ear magnifying effect: ONLY IN FEMALES’ SLIDES The force from a large surface area (drum/Tympanic membrane) is concentrated to a small (oval window) at a ratio of 17:1 Lever action of ossicles: increase the force of movement 1:3 times. The total increase is 17 × 1.3 = 22 times. Two mechanisms of magnifying sound: 1- The size of tympanic membrane is 17 times bigger than the membrane covering the oval window so the waves are magnified (concentrated) 17 times on the oval window (pats ‫ الخفافيش‬has very large tympanic membranes thus they can hear more efficiently). 2- the size difference between the 3 ossicles is 1.3 (and they work like gears ‫ أتراس الساعة‬each bone makes the next one moves more). Guyton corner: The ossicular lever system does not increase the movement distance of stapes, as is commonly believed. Instead, the system actually reduces the distance but increases the force of movement about 1.3 times. 10 Cochlea The cochlea is a system of 3 coiled tubes (divided by the basilar membrane & the reissners membrane) through its length filled with fluid: A. Scala vestibule. B. Scala media (cochlear duct): 1. Vestibular membrane: separates scala media from scala vestibule (very thin). 2. Basilar membrane: separates scala media from scala tympani. 3. Tectorial membrane: attached to the sterocelia of hair cells. 4. Organ of corti (hearing sense organ) with hairs of cells (stereocilia) C. Scala tympani. Organ of corti: Tube Na K Located (resting) on the basilar m. Scala Vestibuli * high low Contain inner & outer hair cells Scala Tympani * high low Extend from base to apex Scala Media ** low high 11 * Similar to extra cellular fluid. ** Similar to intracellular fluid.  ONLY IN MALES’ SLIDES Cochlea 12 Cochlea: hair cells Steroclia extend from the top Arrangement: Three rows of outer hair cells (attached to the reticular lamina or tectorial membrne) One row of inner hair cells (not attached to tectorial membrane) Functions: Function of hair cells inner hair cells outer hair cells Striocellia not embedded in tectorial membrane, but bent by Large number, but stimulate only small fraction of nerve ONLY IN FEMALES’ fluid movement under the tectorial membrane. fibres in the cochlear nerve. SLIDES They are primary receptors for sound, transducing fluid If damaged, significant loss of hearing (they control the movement in cochlea into action potential in the auditory sensitivity of inner hair cells to particular sound frequency). nerve. 13 Doctors’ explanations of previous slide We hear through inner hair cells… outer hair cells control the ability of inner hair cells to hear; that is by a two way mechanism between the outer hair cells and the brain: First from outer hair cells → to nerve fibers → to brain. Then from brain to basilar membrane (to make basilar membrane more tense or more loose similar to musical َ instruments) ‫شبيه بأوتار االالت الموسيقية‬ So outer hair cells controls the reflex between the brain and basilar membrane. When basilar membrane is tense the inner hair cells will be in touch with outer hair cells… but when the basilar membrane is loose inner hair cells will be a bit away of outer hair cells downward ‫هب سلز‬ ‫ممبين اإلنر ر‬ ‫يرخ البازيالر ر‬ ‫لما ي‬ ‫ تكون نازلة لتحت‬and thus when tectorial membrane moves up and down the inner hair cells won’t be as sensitive to outer hair cells as before. So this mechanism determine inner hair cells sensitivity to different frequencies..‫مرخ‬ ‫مشابهة لآلالت الموسيقية الوترية لما يكون الوتر مشدود يكون الصوت مختلف عن لما يكون الوتر ي‬ 14 Receptors & endocochlear potentials 1. Sound transmission into the inner ear → cause upper & lower movements of the reticular membrane (tectorial membrane). 2. produce bending of steriocillia of the hair cells → alternatively open & close cation channels at the tip of the steriocillia. 3. (inward current) → depolarization. 4. (outward current) → hyperpolarization. 5. the net results is depolarization. 6. Production of cells receptors potentials → release of neurotransmitter → production of action potentials. Inner hair cells communicate via a chemical synapse (Glutamate) with the terminals (dendrites) of spiral ganglion neurons. These 1st order (type 1) neurons are bipolar. The collection of their cell bodies form the spiral ganglion. Their axons (central) (form the auditory nerve; cranial nerve VIII) make their way and synapse on the cochlear nucleus in the medulla. 15 The central auditory pathway This pathway begins in the organ of corti: 1. Spiral ganglion neurons (Cochlea). 2. Cochlear nerve (VIII). 3. Cochlear nuclei (Medulla). 4. Superior olivary complex (Pons) (bilateral). 5. Lateral lemniscus. 6. Inferior colliculus (Midbrain). 7. Medial geniculate nucleus (Thalamus). End in the primary auditory cortex (are 41& 42, superior temporal gyrus in the temporal lobe of the brain). Fibres end in the auditory area, where it is heard, then interpretation occurs in the auditory association areas (wernikes area)*.If wermikes area damaged will hear the sound but we can not interpret the meaning of sound. There is a bilateral cortical connection of auditory area. Thus damage to one side only slightly reduces hearing. There are 4 relay stations in CNS for sound signals. 16 * Wernikes area is in posterior end of superior temporal gyrus, It is next to primary auditory area so we can hear by the primary area but understand what’s heard by Wernikes area.  ONLY IN MALES’ SLIDES Cont. 17  ONLY IN MALES’ SLIDES The central auditory pathway Organ of Corti: Located within the cochlea. Hearing receptors. hair cells on the basilar membrane. Gel-like tectorial membrane is capable of bending hair cells. Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe. 18  ONLY IN MALES’ SLIDES Events involved in activating hair cells This Slide Is Very Important 19 Masking effect Presence of one sound decreases an individual's ability to hear other sounds. This phenomenon is known as masking. Presence of background noise affect the ability to hear another sound, due to some receptors are in refractory period. Masking is more clear if two sound are having the same frequencies. Noise pollution: ONLY IN MALES’ SLIDES Noise pollution is an environmental hazard Exposure to sound intensity above 80dB may damage outer hair cells ONLY IN FEMALES’ SLIDES Sound localization: Differences in the time arrival of the sound wave at the ears (time-lag). Differences in the loudness. 20  Conduction of sound Cont. wave Extra Conduction of sound wave Air conduction Bone conduction Normal situation of Sound cause hearing, sound travel vibration of skull in air causes vibration bones directly of Tympanic m., transmitting the transmitted by sound vibration to ossicles to the oval the cochlea window. Ex: when placing tuning fork on the head or mastoid process. Tonotopic mapping of frequency in the basilar membrane 21 Typical hearing disorders Deafness Perceptive Sensorineural Conductive hearing loss (nerve) Inadequate transmission of sound through external or middle ear due Both air and bone conduction is disturbed. to: Hearing loss caused by disruption anywhere in pathway from hair cells to Blocked auditory canal (wax, fluid). the auditory cortex / congenital or damage to cochlea or auditory nerve pathway due to: Repeated infection Loss of hair cells (explosion, chronic loud noise). Perforated drum. Damage to vestibulocochlear nerve (VIII). Restriction of ossicular movements (e.G. By fibrosis or calicification) / destruction of ossicles, repeated infection may Damage to nuclei / tracts to the cortex. lead to this fibrosis and adhesions. Toxins (antibiotics, gentamycine), Damages cochlea if used for more than 2-3 weeks. Osteosclerosis (pathological fixation of stapes on the oval window). All sound frequencies are equally affected. Inflammation. Bone conduction is better than air conduction, because air Vascular. conduction is disturbed. Tumour. Normally: air conduction is better than bone conduction. Both air and bone conduction are affected. 22 * Neuronal presbycusis: degenerative age related process occurs as hair cells wear out with use (loss of ~ 40% of hair cells by age 65) * Cochlear implants have become available (do not restore normal hearing!) Hearing tests Hearing tests 1. rinne’s test 2. Weber’s tes 3. Audiometer The base of a vibrating tuning fork is A vibrating tuning fork is placed on FEMALES’ SLIDES placed on mastoid process until the the middle of the head. sound is not heard. The patient answers where the ONLY IN Then the prongs of the fork held in sound is coming from: the left ear, the air near the ear. right ear, or both. Air phone connected to electronic Normal subject continue to hear Weber test results: device emitting tones of low & high near ear (positive test). Normal hearing will indicate sound in frequencies. If not reveres the test (if heard near both ears. For assessment of degree of deafness. the mastoid process, negative test) Conductive loss: sound travels towards the poor ear (lateralization to bad ear). Nerve loss: sound travels towards the good ear 23  ONLY IN MALES’ SLIDES Hearing tests 24  Thank you!.‫ اعمل و أنت تعلم أن هللا ال يضيع أجر من أحسن عمال‬،‫ اعمل لتمسح دمعة‬،‫اعمل لترسم بسمة‬ The Physiology 436 Team: Team Leaders: Lulwah Alshiha Females Members: Males Members: Ghada Almazrou Talal alenezi Laila Mathkour Dania Alkelabi Mohammad Alayed Ghada Alskait Zeena Alkaff Contact us: References: Females and Males slides. Guyton and Hall Textbook of Medical Physiology (Thirteenth Edition.) 25