Auditory & Vestibular Lecture Notes (Oct 4, 2024) PDF

OCT 1172Y INTRODUCTION TO AUDITORY SYSTEMS AND VESTIBULAR SYSTEMS OCTOBER 4TH , 2024 Mayleen Torres, OT Reg.(Ont.), MScOT, 2024© AUDITORY SYSTEM OVERVIEW W hat is the Auditory System? Sound Wave Mechanical Ear Structures → Where receptors are located...

OCT 1172Y INTRODUCTION TO AUDITORY SYSTEMS AND VESTIBULAR SYSTEMS OCTOBER 4TH , 2024 Mayleen Torres, OT Reg.(Ont.), MScOT, 2024© AUDITORY SYSTEM OVERVIEW W hat is the Auditory System? Sound Wave Mechanical Ear Structures → Where receptors are located and sensory stimulus is received Neurological Pathway → Where nerve signals synapse in CNS and interpreted  WHAT IS THE AUDITORY SYSTEM? HOW SOUND IS PROCESSED: → PERCEIVED (Ear collects sound) → PROCESSED (Mechanical Vibration of inner structures) → RELAYED to higher centres (Transduction of Nerve impulses) → REASSEMBLED for interpretation https://www.soundwavehearing.ca/b/what-part-of-the-brain-processes-what-you-hear (Cortex of brain)  WHAT IS SOUND? Vibration of air molecules from a source (i.e. voice, object) Sound Wave = Alternating or fluctuating pressures (High and Low) of air molecules 2 TYPES OF SOUND WE INTERPRET: 1. FREQUENCY (Hertz, Hz) = Pitch - How many sound waves (#) in a period of time (cycle) - Higher Frequency → Higher Pitch Ex. Human Range (20 – 20,000 Hz) Ex. Animal Range (45,000 – 123,000 Hz) https://www.sciencelearn.org.nz/images/605-graphs-of-sound-waves 2. AMPLITUDE (Decibels, dB) = Volume - Higher Amplitude → Higher Volume  MECHANICAL STRUCTURES OF THE AUDITORY SYSTEM  3 MAIN STRUCTURES OF AUDITORY SYSTEM 1) OUTER EAR PINNA/AURICLE EXTERNAL AUDITORY MEATUS (AUDITORY CANAL) TYMPANIC MEMBRANE 2) MIDDLE EAR OSSICLES: MALLEUS, INCUS, STAPES OVAL WINDOW PHARYNGOTYMPANIC (EUSTACHIAN) TUBE 3) INNER EAR COCHLEA https://www.youtube.com/watch?v=53fW3RUgShc&t=50s → ORGAN OF CORTI  AUDITORY SYSTEM: OUTER EAR Function: to collect and funnel sound waves onto eardrum → PINNA / AURICLE Additional function: sound localization → EXTERNAL AUDITORY MEATUS canal to further funnel sound waves/vibrations onto Tympanic Membrane → TYMPANIC MEMBRANE -Eardrum -Sound vibration first stop -Vibration of Tympanic Membrane to Middle Ear (ossicles)  AUDITORY SYSTEM: MIDDLE EAR Function: Amplifies mechanical vibrations (sound energy) from eardrum (Tympanic Membrane) to inner ear TYMPANIC CAVITY: Enclosed, air-filled space OSSICLES: 3 small bones Function: mechanical vibration from Tympanic Membrane to Oval Window i) MALLEUS (Hammer) ii) INCUS (Anvil) iii) STAPES (Stirrup) → OVAL WINDOW - smaller membrane than Tympanic Membrane - amplifies the vibration to push fluid in Inner Ear https://en.wikipedia.org/wiki/Incudomalleolar_joint#/media/File:Illu_au - stronger vibration required going from air-filled ditory_ossicles-en.svg space (Middle Ear) to fluid-filled space (Inner Ear)  AUDITORY SYSTEM: MIDDLE EAR PHARYNGOTYMPANIC (EUSTACHIAN) TUBE Connects middle ear to nasopharynx ; regulates pressure within the middle ear. 2 Muscles that dampen ossicle movement when they contract are: i) TENSOR TYMPANI (CN V) ii) STAPEDIUS MUSCLE (CN VII) Why is this important? Protective measure for the auditory nerve (by reducing vibration of ossicles) Equalizes pressure on either side of Tympanic Membrane Ex. Pop ears on airplane; yawning Selectively filters low-frequency background noises https://texassinusandsnoring.com/eustachian-tube/ Ex. Trying to have a conversation at a loud party IMPORTANT STRUCTURES ▪ Vestibular and auditory structures are embedded in a bony labyrinth within the petrous part of the temporal bone of the skull ▪ Inner Ear structures on the right and left side are oriented opposite to each other (mirror orientation)  INNER EAR STRUCTURES Function: Houses structures for Auditory and Vestibular organs where nerve impulses occur. Neurological signals are initially detected within here before travelling to end destination (Cerebral Cortex). *Enclosed, fluid-filled space *Location: Petrous portion of Temporal Lobe https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  INNER EAR: PERILYMPH & ENDOLYMPH PERILYMPH -fluid in space between Bony Labyrinth and Membranous Labyrinth -low K+ ; high Na+ -composition: similar to CSV + extracellular fluid ENDOLYMPH -fluid within Membranous Labyrinth -high K+; low Na+ -composition: similar to intracellular fluid -critical role in signal transduction https://www.youtube.com/watch?v=53fW3RUgShc&t=50s INNER EAR: COCHLEA COCHLEA: Snail shell shaped bony structure 3 Chambers within Cochlea: →VESTIBULAR DUCT (SCALA VESTIBULI) → **COCHLEAR DUCT (between Vestibular & Tympanic Duct) → TYMPANIC DUCT (SCALA TYMPANI)  INNER EAR: COCHLEAR DUCT ORGAN OF CORTI: Location of specialized cells and sensory receptors for auditory function Function: the organ where the transduction of sound TECTORIAL MEMBRANE waves (vibration) into action potential (nerve impulse) occurs -Description: Gelatinous, extends over hair cells, stationary IMPORTANT STRUCTURES WITHIN ORGAN OF CORTI: - When Basilar Membrane vibrates, it creates a → BASILAR MEMBRANE friction/shearing force against the Tectorial -Organi of Corti attached here Membrane causing the Stereocilia of the Outer Hair -Flexible floor of Cochlear Duct Cells to move. → HAIR CELL + SUPPORTING HAIR CELL - Note: Inner Hair Cells do not come into contact with -Receptor Cells the Tectorial Membrane; they only move (are → STEREOCILIA + KINOCILIA activated) through fluid movement in the Cochlear Duct (Endolymph) -Mechanoreceptors (respond to vibration of sound wave) → TECTORIAL MEMBRANE -Gelatinous, extends over hair cells, stationary INNER EAR: TRANSMISSION OF SOUND As vibrations travel through the tympanic duct > Middle Ear > Inner Ear → it results in Basilar Membrane vibrations When Basilar Membrane vibrates, it creates a friction/shearing force against the Tectorial Membrane causing the Stereocilia of the Outer Hair Cells to move. Displacement of STEREOCILIA Towards tallest hair fibre = depolarization (action potential fires) Away from tallest hair fibre = hyperpolarization (action potential does not fire) Note: Inner Hair Cells do not come into contact with the Tectorial Membrane; they only move (are activated) through fluid movement in the Cochlear Duct (Endolymph) INNER EAR: TRANSMISSION OF SOUND WHAT HAPPENS WHEN STEREOCILIA (Hair Cells) ARE STIMULATED? 1. Movement of Stapes pushes fluid (Endolymph) in Cochlear Duct 2. Causes Basilar Membrane to move up and down 3. Hair cells (Stereocilia) bend via shearing force with Tectorial Membrane INNER EAR: TRANSMISSION OF SOUND (continued) WHAT HAPPENS WHEN STEREOCILIA ARE STIMULATED? 4. Opens K+ (Potassium) channels via TIP LINKS (links top of cilia together) 5. K+ depolarizes cells (which creates nerve impulses/action potential) 6. Nerve impulse travel along Cochlear Nerve 7. Impulses continue to travel up to Brain/CNS  INNER EAR: SOUND INTERPRETATION →FREQUENCY High Pitch: Base of Basilar Membrane (stiffer and shorter) Low Pitch: Apex (end) of Basilar Membrane = HELICOTREMA (flexible/loose and longer) Ex. Piano strings *Vibration can only travel so far across the Basilar Membrane depending on the Frequency →AMPLITUDE Loud Sounds: causes a collection of more hair cells to move (therefore generates a greater nerve signal to the brain) Nerve fibres are attached at the bottom of HOW SOUND IS REPRESENTED ON THE BASILAR MEMBRANE the hair cells that join to create the Cochlear > Different sections are stimulated depending on the sound Nerve wave/vibration received  NEUROLOGICAL PATHWAY OF THE AUDITORY SYSTEM  BASIC OVERVIEW OF ASCENDING AUDITORY PATHWAY Sound Wave Mechanical Ear Structures Neurological Process: Brainstem (synapse) Thalamus (synapse) Cerebrum – Primary Cortex (sound processed and interpreted here) Terminology Review: TRANSDUCTION: Conversion of energy into another form IPSILATERAL: Same side CONTRALATERAL: Opposing side AUDITORY NERVE FIBRES AND THEIR SYNAPSES: COCHLEAR NERVE COCHLEAR NERVE of CN VIII: → SPIRAL GANGLION: Outside of Cochlear Duct (Within Cochlea) -Collection of cell bodies from axons connected to Outer and Inner hair cells -it relays auditory information from sensory hair cells to auditory system/circuits INNER HAIR CELLS: -Majority of axons from Inner hair cells (the primary sensory cells of cochlea; responsible for relaying sound information to the brain) OUTER HAIR CELLS: -primarily act as mechanical amplifiers -Outer Hair Cells allows inner ear to focus on relevant sounds while outer hair cells filter out background noises INTERNAL ACOUSTIC MEATUS -Passage through which axons from Spiral Ganglion and Vestibular Ganglion exit from  BRAINSTEM REVIEW 1 → Location where CN VIII enters brainstem: 2 Pontomedullary Junction (between Pons and Medulla) 3 3 3 https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  AUDITORY NERVE FIBRES AND THEIR SYNAPSES Afferent Pathway to Brain ▹ Cochlear nerve joins with vestibular nerve > CN VIII ▹ Neurons from the cochlear division of CNVIII synapse in the cochlear nuclei (Ventral and Dorsal aspect) ▹ Majority of second-order neurons decussate to the contralateral side and travel to the inferior colliculus (PRIMARY PATHWAY) ▹ Some neurons remain on Ipsilateral side (SECONDARY PATHWAY) ▹ Third-order neurons travel to the medial geniculate nucleus of the thalamus, and subsequently to the primary auditory cortex ▹ Actual pathway is more complex https://www.youtube.com/watch?v=53fW3RUgShc&t=50s AUDITORY NERVE FIBRES AND THEIR SYNAPSES From Organ of Corti (within COCHLEA): SPIRAL GANGLION NEURONS that lead to form COCHLEAR NERVE CN VIII (Eighth Nerve) → Cochlear Division COCHLEAR NUCLEUS → receives coded signals from Cochlea SUPERIOR OLIVARY COMPLEX → collection brainstem nuclei LATERAL LEMNISCUS → intermediary of axons carrying auditory information INFERIOR COLLICULUS → signal integration: frequency recognition; pitch discrimination MEDIAL GENICULATE NUCLEUS → major relay station for auditory pathway to cerebral cortex AUDITORY CORTEX → Consciously perceives sound MNEMONIC: “ECOLI MA” VESTIBULAR SYSTEM  OVERVIEW What is the Vestibular System? Mechanical Ear Structures -Where receptors located and sensory stimulus is received Neurological Pathway (complex) -Where nerve signals synapse in CNS and interpreted https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  WHAT IS THE VESTIBULAR SYSTEM? SENSORY SYSTEM that detects the following: i) Motion ii) Head position iii) Spatial orientation How it’s involved in our MOTOR FUNCTIONS: i) Balance (maintains) ii) Head & Body movement (keeps it stable) https://www.youtube.com/watch?v=53fW3RUgShc&t=50s iii) Posture (maintains) MECHANICAL STRUCTURES OF THE VESTIBULAR SYSTEM  REVIEW INNER EAR: BONY LABYRINTH https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  REVIEW INNER EAR: MEMBRANOUS LABYRINTH https://www.youtube.com/watch?v=53fW3RUgShc&t=50s REVIEW INNER EAR: PERILYMPH & ENDOLYMPH https://www.youtube.com/watch?v=53fW3RUgShc&t=50s VESTIBULAR SYSTEM: IMPORTANT STRUCTURES 3 bony semicircular canals contain membranous semicircular ducts (containing endolymph): ▹ Anterior ▹ Posterior ▹ Horizontal ** Perceive rotational movement Vestibule ▹ Saccule ▹ Utricle ** Perceive linear movement  RECEPTOR ORGANS: SEMICIRCULAR CANAL AMPULLA -enlargement at the base of the semicircular ducts that contain the following receptor organs → CRISTA AMPULLARIS (CRISTA) -Hair cells and Supporting Hair Cells embedded here → STEREOCILIA/KINOCILIA -Hair cell bundles that shift to create nerve impulse → CUPULA -Gelatinous material surrounding Cilia and Crista Ampullaris  RECEPTOR ORGANS: VESTIBULE VESTIBULE: 2 OTOLITH ORGANS containing MACULA -2 enlarged sacs (endolymph filled) of the membranous labyrinth -contain the Otolith organ -each contain MACULA where receptor hair cells are located UTRICLE -connected to the ampullae of the semi- circular ducts -Utricular Macula is on floor of Utricle (horizontal plane) -detects Horizontal Linear Movement (forward and backward acceleration) SACCULE -Saccular Macula is on medial wall of Saccule (vertical plane) -detects Vertical Linear Movement https://www.britannica.com/science/semicircular-canal (gravitational forces) HOW THE RECEPTORS FUNCTION IN VESTIBULAR SYSTEM: SEMICIRCULAR CANAL HAIR CELL FUNCTIONS WITH SPECIFIC HEAD MOVEMENTS -A nerve impulse (Action Potential) is generated when the Cupula is shifted and causes the Cilia to be displaced Primary role of Semicircular Canals: to detect rotational acceleration and balance within a directional plane 3 Planes of the Semicircular Canal and the movements they detect: ANTERIOR -detects head tilt RIGHT/LEFT (Coronal) POSTERIOR -detects head nod UP/DOWN (Sagittal) HORIZONTAL or LATERAL -detects head turn R SIDE/L SIDE (Transverse) https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  VESTIBULAR SYSTEM: ROTATIONAL ACCELLERATION + BALANCE Complimentary Pairing of Semicircular Canals ▹ Semicircular canals on the right and left side are oriented opposite to each other (mirror orientation) ▹ Depolarization of hair cells on one side results in reciprocal hyperpolarization on the contralateral side.  VESTIBULAR SYSTEM: ROTATIONAL ACCELLERATION +BALANCE  HOW THE RECEPTORS FUNCTION IN VESTIBULAR SYSTEM: VESTIBULE https://www.youtube.com/watch?v=53fW3RUgShc&t=50s Within Utricle and Saccule: Primary role of Otolith Organs: to detect Linear -hair cells of Maculae are embedded in gelatinous Acceleration and Gravity mass (OTOLITH MEMBRANE) -it’s outer layer is covered in Calcium Carbonate This is detected when the Otolith Membrane is Crystals called OTOCONIA / OTOLITHS shifted  VESTIBULAR SYSTEM: LINEAR ACCELLERATION +BALANCE NEURALOGICAL PATHWAY OF THE VESTIBULAR SYSTEM  VESTIBULAR SYSTEM PATHWAYS Text box FOR INTEREST ONLY https://www.youtube.com/watch?v=53fW3RUgShc&t=50s  VESTIBULAR SYSTEM PATHWAYS Peripheral to CNS (outside Inner Ear structure): Vestibular Ganglion → Vestibular Nerve - Vestibular Ganglion is a collection of cell bodies from axons connected to receptor cells within Ampulla, Utricle and Saccule https://www.youtube.com/watch?v=53fW3RUgShc&t=50s - it relays information from vestibular sensory hair cells to areas within CNS  VESTIBULAR SYSTEM PATHWAYS https://www.youtube.com/watch?v=53fW3RUgShc&t=50s Peripheral to CNS (outside Inner Ear structure): Within CNS (Brainstem): INTERNAL ACOUSTIC MEATUS (small - Nerve impulses are relayed in the VESTIBULAR bony canal through which CN VIII passes) NUCLEAR COMPLEX (First Synapse)  VESTIBULAR SYSTEM PATHWAYS: VESTIBULAR NUCLEAR COMPLEX VESTIBULAR NUCLEAR COMPLEX: -location: between pons and medulla, adjacent to inferior cerebellar peduncle and cochlear nuclei 4 Vestibular Nuclei: MEDIAL LATERAL INFERIOR SUPERIOR → Vestibular nuclei are integration centres that have multiple functions: i) it receives afferents from inner ear ii) Projects to visual system (extraocular nuclei), cerebellum, spinal cord and is integrated in a feedback loop. Also projects https://www.youtube.com/watch?v=53fW3RUgShc&t=50s to thalamus.  VESTIBULAR SYSTEM PATHWAYS: VESTIBULAR NUCLEAR COMPLEX https://www.youtube.com/watch?v=53fW3RUgShc&t=50s VESTIBULAR SYSTEM PATHWAYS  VESTIBULAR SYSTEM: REFLEX PATHWAYS 3 major reflex pathways that vestibular nuclei complex participate in and what is ii) Vestibulo-cervical Reflex their importance: -postural adjustments of the head in response to rotational movements (detected in semicircular canals) i) Vestibulo-ocular Reflex -see MEDIAL VESTIBULAR NUCLEUS GROUP -adjusts eye movements to head movements -stabilizes images on retina even though iii) Vestibulo-spinal Reflex head is moving -postural stability of the body -without this reflex, images would be -also contribute to muscle tone blurred with any head movement -see both MEDIAL VESTIBULAR NUCLEUS AND -ex. Eyes turn in opposite direction of LATERAL VESTIBULAR NUCLEUS GROUP head FOR INTEREST ONLY  VESTIBULAR SYSTEM PATHWAYS: REFLEX PATHWAY Vestibular Ocular Reflex (VOR) Connections in ascending tract: Abducens (CN VI) Trochlear (CN IV) Oculomotor (CNIII) FOR INTEREST ONLY  VESTIBULAR SYSTEM PATHWAYS FOR INTEREST ONLY  REFERENCES: 1) Krebs, C., Weinberg, J., & Akesson, E. and Dilli, E. (2018). Neuroscience (2nd ed). New York: Lippincott, Williams & Wilkins. (KWA2) 2) Agur, A.M. R., & Dalley, A. F. (2022). Moore's Essential Clinical Anatomy (7th ed.). Wolters Kluwer Health. (ECA7) ?? QUESTIONS ??