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Ear, Page 1 Lecture Outline Ear - Hearing perspective Dr. Darren Hoffmann Lecture Objectives: After this lecture, you should be able to: -Describe the surface anatomy of the external ear in anatomical language -Recognize key anatomy in an otoscopic view of the tympanic membrane -Describe the anatomy and function of the ossicles and their associated muscles -Relate the anatomical structures of the middle ear to the anterior, posterior, lateral or medial walls -Explain the anatomy of middle ear infection and which regions have potential to spread to ear -Describe the anatomical structures of the inner ear -Discriminate between endolymph and perilymph in terms of their origin, composition and reabsorption mechanisms -Identify the structures of the Cochlea and Vestibular system histologically -Explain how hair cells function to transform fluid movement into electrical activity -Discriminate the location of cochlear activation for different frequencies of sound -Relate the hair cells of the cochlea to the hair cells of the vestibular system -Contrast the vestibular structures of macula and crista terminalis Let’s look at the following regions: Hoffmann – Ear, Page 1 Lecture Outline: C1. External Ear Function: Amplification of Sound waves Parts Auricle Visible part of external ear (pinna) Helix – large outer rim Tragus – tab anterior to external auditory meatus External auditory meatus Auditory Canal/External Auditory Meatus Leads from Auricle to Tympanic membrane Starts cartilaginous, becomes bony as it enters petrous part of temporal bone Earwax (Cerumen) Complex mixture of Sebaceous and Apocrine secretions Contains: Oils Dead skin cells (keratin) Cholesterol Helps clean the ear canal Conveyor Belt Concept Movement of the TMJ massages the auditory canal and moves wax and infectious material out Lubricates the ear canal (prevents dryness and cracking) May have some antimicrobial effects Slightly acidic pH (pH ~6) Tympanic Membrane Lateral surface of Tympanic membrane is end of external ear Visible in otoscopic exam Semitransparent Malleus and Chorda tympani are visible through membrane Tympanic Membrane Histology Thin membrane is actually a combination of all three germ layers! External ear surface: Surface ectoderm Continuous with auditory canal Middle ear surface: Pharyngeal endoderm Continuous with lining of middle ear Thin layer of Mesoderm in between Forms fibrous connective tissue structure of the membrane Hoffmann – Ear, Page 2 C2 Middle Ear Function: Transmits sound waves as bony vibration Contents Medial (internal) surface of Tympanic membrane Ossicles (Malleus, Incus, Stapes) Ossicle-associated muscles Tensor Tympani and Stapedius Pharyngotympanic tube Borders Lateral – Tympanic membrane Medial – Inner ear Posterior – Mastoid cavity Anterior – Pharyngotympanic tube Ossicles and Their Muscles Malleus Contacts Tympanic membrane Incus Stapes Contacts oval window Tensor Tympani Supplied by V3 Dampens loud noises *Sounds of mastication Enters the tympanic cavity from the anterior Stapedius Supplied by CN VII Dampens loud noises *Sounds of your own voice Enters the tympanic cavity from the posterior Hoffmann – Ear, Page 3 Mastoid Cavity Hollow spaces within the Mastoid process Connected to the middle ear through posterior wall Function: Amplifies vibration to assist in hearing Can be used to test for conductive hearing loss Pharyngotympanic (Eustachian) Tube Exits tympanic cavity Anterioinferiorly Function: Permits flow of fluid out of middle ear Permits equilibration of middle ear air pressure during elevation change Site of attachment for Tensor and Levator Veli Palatini (palate muscles) Tube is statically closed unless Palatini muscles contract (during swallowing/yawning) Starts as a horizontal tube during development, gets more vertical as skull grows Otitis Media (Middle Ear Infection) Typically follows an Upper Respiratory Infection Occasionally follows odontogenic infection Common in children Shorter and more horizontal pharyngotympanic tubes Bacteria/Viruses can get into ear more easily Treated with Tympanostomy tubes Fluid drainage Antibiotics (depending on the infection type) Hoffmann – Ear, Page 4 C3. Inner Ear Function: Translate vibrational energy into electrical energy Location: Deep in Petrous part of temporal bone Contents: Bony labyrinth for Cochlea (Hearing) Semicircular canals (Balance) Membranous duct system Neurovascular supply to Internal ear Nerve Supply: CN VIII Cochlear nerve: To Cochlea for hearing Vestibular nerve: To Semicircular Canals for balance Blood Supply: Labyrinthine Artery (Branch of Basilar a.) Membranous Ducts inside bony labyrinth Vestibule – Contains Utricle and Saccule Semicircular Canals – Contains Semicircular ducts Cochlea – Cochlear ducts Ducts are filled with Endolymph Spaces around ducts are filled with Perilymph Both fluids are returned to the circulation through assigned ducts (endolymphatic and perilymphatic ducts) Cochlear Duct Scala Media Contains Endolymph Site of hearing cells (Organ of Corti) Scala Vestibuli Contains Perilymph Site of first wave of fluid vibration Scala Tympani Contains Perilymph Site of wave dispersal after sound activation Hoffmann – Ear, Page 5 C4. The Cochlea Cochlea makes 2.5 turns around a central bony core (Modiolus – bony center) A section through cochlea shows multiple cross-sections of the spiraling duct Ganglion for the special sensory nerve fibers is located in the Modiolus (G in figure at right) Endolymph vs. Perilymph Perilymph Fills bony labyrinth space Similar to extracellular fluid Dumped into CSF via Perilymphatic duct Endolymph Fills membranous ducts Highly unique fluid Very high in K+ and + charged amino acids Synthesized by Stria Vascularis of Scala Media Volume/Pressure of endolymph seems to be regulated through Endolymphatic duct/sac Stria Vascularis Highly unique epithelium on outer wall of Scala Media Synthesizes Endolymph (responsible for high K+ concentration) The ONLY known vascularized epithelium! Hoffmann – Ear, Page 6 Organ of Corti Hair Cells Specialized cells which sense changes in fluid movement Each cell has many stereocilia (non-motile) Each cell has one kinocilium (longest, motile) Movement of the cilia by fluid movement triggers influx of K+ ions Triggers release of neurotransmitters at the base of the hair cells This electrical gradient one of the largest in the human body (~150 mV between endolymph and hair cells) This means these cells are extremely fast to trigger an action potential Organ of Corti Construction Hair cells’ stereocilia are embedded in a gelatinous membrane (Tectorial membrane) 2 types of Hair cells with COMPLETELY different jobs -Inner Hair Cells Sensory cells of the Organ of Corti Trigger action potential in sensory neurons of CN VIII -Outer Hair Cells MOTILE cells that vibrate when Inner Hair Cells are activated Amplifies specific sounds in the cochlea (quiet sounds over loud ones) “Cochlear amplifier” Vibration of these cells actually makes noise that is detectable by a small microphone in the ear (part of current hearing testing) Hoffmann – Ear, Page 7 Histology of Organ of Corti in Cochlear Duct Section Tectorial membrane Inner Hair (sensory) Cell Outer Hair (sensory) Cell Phalangeal Cells (Inner and Outer) Basilar Membrane Site-specific localization of sound on Cochlea Pitches are “heard” in different regions of the cochlea Highest pitches (frequencies) Base of cochlear spiral Lowest pitches (frequencies) Apex of cochlear spiral Range of audible pitches is ~11 octaves Different frequencies also map to different areas of the brain! Big Picture of Hearing in Sequence 1. Sound in through External Acoustic Meatus 2. Tympanic Membrane vibration 3. Ossicle vibration (10X Amplifier) 4. Oval Window 5. Perilymph vibration into Scala vestibuli 6. Vibration through vestibular membrane 7. Scala Media vibration 8. Activation of Inner Hair Cells and Outer Hair cells 9. Receptor potential – release of neurotransmitters 10. Activation of primary sensory neurons of CN VIII 11. Cell bodies in the spiral ganglion 12. CN VIII travels through Internal Acoustic Meatus to brain Hoffmann – Ear, Page 8 C5. The Vestibular System Similar to Cochlear system, but different Hair cells located in two places Utricle/Saccule Semicircular ducts Hair cells stimulated by fluid movement Movement of the head stimulates fluid movement in the membranous duct system Utricle: Oriented in the horizontal plane Senses acceleration changes in linear horizontal movements Saccule: Oriented in the vertical plane Senses acceleration changes in linear vertical movements Semicircular ducts: 3 ducts oriented at right angles for each possible plane of angular (turning) movement Senses changes in angular velocity (each semicircular duct senses one plane) Sensory tissues: Utricle/Saccule – The Macula Hair Cells Glycoprotein gel layer layered on top with Otoliths Semicircular Ducts – The Crista Ampullaris Hair cells Glycoprotein gel cap (cupula) with NO Otoliths Hoffmann – Ear, Page 9 .
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