Module 8: Hearing Anatomy & Physiology Study Notes

Module 8: Hearing Anatomy & Physiology

Introduction

  • Course Instructor: Dr. Brianna Williams

  • Textbook: Anatomy & Physiology for Speech, Language, and Hearing, Seventh Edition by J. Anthony Seikel, David G. Drumright, Daniel J. Hudock (Copyright © 2025 by Plural Publishing, Inc. All rights reserved)

Key Concepts

  • Anatomy

  • Outer Ear

  • Middle Ear

  • Inner Ear

  • Auditory Physiology

  • Impedance Matching

  • Vestibular System

  • Mechanical Events

  • Electrical Events

  • Auditory Pathway

What is Hearing?

  • Transduction of Sound:

    • Conversion from acoustical energy to mechanical energy.

    • Further conversion of mechanical energy into electrochemical energy.

  • Transmission and Interpretation:

    • Conveyance and understanding of signals produced electrochemically.

Simple Overview of Hearing Process

  • Outer ear collects and shapes sound waves.

  • Middle ear provides an impedance match between air's low impedance and the high impedance of endolymph.

  • Inner ear converts sound in fluid to an electrical code.

  • Eighth cranial nerve (VIII nerve) and brainstem pathways transmit sounds and perform rudimentary processing.

  • The cortex plays a role in speech interpretation and language processing.

Anatomy of the Ear

Anatomy of the Outer Ear

  • Pinna (Auricle):

    • Includes features: Helix, antihelix, concha, scaphoid fossa, tragus, antitragus, lobule.

  • External Auditory Meatus:

    • Dimensions: 7 mm wide, 2.5 cm long.

    • Structure:

    • Outer 1/3: Cartilage

    • Inner 2/3: Osseous (bony).

    • Isthmus: A point of constriction.

    • Courses downward and medially to the tympanic membrane (TM).

  • Tympanic Membrane:

    • Slightly concave when viewed from the external auditory meatus.

    • Umbo is the most depressed part.

    • Mainly composed of fibrous tissue, including three layers:

    • Outer Layer: Continuous with the EAM.

    • Intermediate Layer: Fibrous.

    • Inner Layer: Continuous with the mucosa of the middle ear.

Anatomy of the Middle Ear

  • Ossicles:

    • Comprise three bones: Malleus, incus, stapes.

  • Muscles:

    • Stapedius Muscle:

    • Function: Dampens sound vibrations to protect inner ear from loud noises (acoustic reflex).

    • Tensor Tympani Muscle:

    • Function: Pulls malleus anteriorly and medially, tensing the TM, reducing movement (part of acoustic reflex).

  • Walls of the Middle Ear:

    • Medial, anterior, posterior, floor.

Tympanic Muscles

  • Acoustic Reflex:

    • Mechanism that reduces the range of movement of ossicles.

    • Serves as a protective measure against loud noises, limiting transmission to the inner ear and protecting the cochlea from trauma.

Landmarks of the Middle Ear

  • Medial Wall:

    • Oval Window: Opening where the stapes fits; transmits sound into the inner ear.

    • Round Window: Below the oval window; covered by the secondary tympanic membrane.

    • Promontory: Basal turn of the cochlea.

    • Prominence of lateral semicircular canal and facial nerve visible.

  • Anterior Wall:

    • Entrance of the auditory tube.

  • Posterior Wall:

    • Prominence of the stapedial pyramid visible.

  • Lateral Wall:

    • Tympanic membrane present.

Anatomy of the Inner Ear

  • Structures include the vestibule, semicircular canals, and cochlea.

Osseous Labyrinth

  • Consists of:

    • Vestibular region

    • Semicircular canals

    • Cochlear canal

Cochlear Canal

  • Appearance resembles a coiled snail shell, wrapping around itself 2 and 5/8 times from base to apex.

  • The core consists of the modiolus, a bone that allows nerve fibers and blood vessels to pass.

Bony Labyrinth of Cochlea

  • Divided into two incomplete fluid-filled chambers (scala vestibuli and scala tympani) separated by the osseous spiral lamina.

Semicircular Canals

  • House sense organs for body movement.

  • Consist of three canals:

    • Anterior vertical: Detects lateral rotational movements.

    • Posterior vertical: Detects forward and backward head movements.

    • Lateral: Detects rotational movement.

Utricle and Saccule

  • Otolithic organs located in the vestibule.

  • Detect linear acceleration during head tilting.

Cochlear Structures

Scala Media

  • Tube placed between scala tympani and scala vestibuli.

  • Contains: Reissner's membrane, stria vascularis, spiral limbus, hair cells (inner and outer), pillars of Corti, basilar membrane.

Basilar Membrane

  • Forms the floor of scala media; critical in sound perception.

Organ of Corti

  • Contains sensory hair cells organized in four rows.

    • Outer Hair Cells (OHC): 3 rows amplify sound vibrations.

    • Inner Hair Cells (IHC): 1 row acts as primary sensory receptors.

  • Hair Cells Generally: Cilia connected by links, with longer stereocilia at the apex.

Hair Cell Innervation

  • Hair cells receive afferent (incoming) and efferent (outgoing) innervation.

  • Afferent innervation:

    • Inner hair cells: Connected to 10 VIII nerve fibers (many-to-one).

    • Outer hair cells: Share innervation with 10 other outer hair cells (one-to-many).

  • Efferent fibers:

    • Olivocochlear Bundle (OCB): Originates in the superior olivary complex and projects to the cochlea, providing inhibitory functions.

Auditory Physiology

Outer Ear Physiology

  • Functions mainly to funnel acoustic energy into the external auditory meatus, boosting resonances between 1500 and 8000 Hz.

Middle Ear Physiology

  • Acts as an impedance matcher, converting air sound waves to a form suitable for fluid motion.

    • Mechanisms include:

    • Area ratio (TM vs. oval window): TM is 17 times larger than the oval window, resulting in a +25 dB gain.

    • Lever action of ossicles adds +2 dB.

    • Buckling of the TM adds +4 to 6 dB.

  • Total Middle Ear Gain: 33 dB.

Vestibular System

  • Function: Balances body and maintains spatial orientation.

  • Mechanisms:

    • Semicircular canals detect rotation.

    • Utricle detects horizontal acceleration.

    • Saccule detects vertical acceleration.

Cochlea Function

  • Spectral and temporal analysis of sound; separates acoustic signals into frequency and timing components.

  • Movement of the stapes causes fluid pressure waves in the cochlea, resulting in the movement of the basilar membrane.

Hair Cell Excitation

  • Outer Hair Cells respond to low-level stimuli; more sensitive and crucial in coding intensity below ~40 dB SPL.

  • Inner Hair Cells rely on the traveling wave for excitation.

Electrical Events in Cochlea

Key Roles of the Cochlea

  1. Spectrum analyzer (separation of sound frequencies).

  2. Transducer (conversion of mechanical energy to electrochemical energy).

Hair Cell Activation

  • When the basilar membrane moves, hair cell activation causes electrical potential changes, initiating neural signaling.

  • Ionic Activity: K+ ions flow through hair cells, recycling by the stria vascularis creates a high positive endocochlear potential essential for hearing.

Resting Potentials in Cochlea

  • Scala Vestibuli: +5 mV slightly positive.

  • Scala Media: +80 mV strongly positive (endocochlear potential).

  • Creates a substantial 150 mV driving force, akin to a charged battery, vital for hearing capabilities.

Potentials Generated by Hair Cell Stimulation

  • Stimulation generates various electrical potentials; not all are equally vital for auditory processing.

Types of Auditory Nerve Fibers

  • High Spontaneous Fibers: Detect soft sounds at low intensities with random firing.

  • Low Spontaneous Rate Fibers: Respond primarily to louder sounds and demonstrate precision at high intensities.

Auditory Pathway Responses

  • Cochlear Nucleus: First processing center, maintaining tonotopic organization and analyzing acoustic signal.

  • Superior Olivary Complex: Processes binaural interaction, crucial for sound localization (high-frequency and low-frequency processing).

  • Inferior Colliculus: Major auditory integration site; accommodates localization processing.

  • Medial Geniculate Body: Final sensory way station in the brainstem.

  • Cerebral Cortex: Processes sounds across various cortical levels: Core (AI), belt, and parabelt areas distinguish complexity, integration of sounds, and language processing.

Conclusion

  • Understanding the anatomy and physiology of hearing is fundamental for grasping auditory processing and the role of hearing in speech and language.