Special Senses Lecture 2

Hearing and Equilibrium

Introduction

The senses of hearing and equilibrium are essential for perceiving sound and maintaining balance. Both are mediated by specialized structures within the ear, which convert mechanical stimuli into neural signals for interpretation by the brain.

• Hearing is the response to vibrating air molecules.

• Equilibrium refers to the sense of motion, body orientation, and balance.

• Both senses reside in the inner ear, which contains fluid-filled passages and sensory receptor cells.

• Movement of fluid within the ear stimulates sensory cells, generating action potentials that are interpreted by the nervous system.

The Nature of Sound

Definition and Mechanism

Sound is produced by the vibration of molecules, typically in air, which is detected by the ear.

• Sound: Any audible vibration of molecules.

• A vibrating object pushes on air molecules, which then push on other air molecules, creating a wave.

• When these waves reach the eardrum, they cause it to vibrate, initiating the process of hearing.

Anatomy of the Ear

Overview of Ear Sections

The ear is divided into three main sections, each with distinct roles in the process of hearing and equilibrium.

• External (Outer) Ear: Collects and channels sound waves.

• Middle Ear: Transmits and amplifies sound vibrations.

• Inner Ear: Converts vibrations into nerve signals and houses equilibrium organs.

External Ear

Structures and Functions

• Auricle (Pinna): The visible part of the ear that collects sound waves and directs them into the ear canal.

• External Auditory (Acoustic) Canal: A tube that channels sound waves toward the tympanic membrane.

• Tympanic Membrane (Eardrum): A thin, semitransparent sheet that vibrates in response to sound and separates the external ear from the middle ear.

Ceruminous Glands

• Located along the external acoustic canal.

• Secrete cerumen (earwax), which lubricates and waterproofs the canal and membrane, keeps foreign objects out, and slows microbial growth.

Middle Ear

Structures and Functions

• Also called the tympanic cavity.

• Communicates with the nasopharynx via the auditory (eustachian) tube, which equalizes air pressure on both sides of the tympanic membrane.

• Encloses and protects the three auditory ossicles:

Ossicle	Common Name	Function
Malleus	Hammer	Receives vibrations from the tympanic membrane
Incus	Anvil	Transmits vibrations from malleus to stapes
Stapes	Stirrup	Transfers vibrations to the oval window of the inner ear

Auditory (Eustachian) Tube

• Connects the middle ear cavity to the nasopharynx.

• Equalizes air pressure; opens during swallowing or yawning.

• Allows infections to spread from the throat to the middle ear, especially in children.

Vibration of Tympanic Membrane

• Converts sound waves into mechanical movements.

• Auditory ossicles amplify and conduct vibrations to the inner ear at the oval window.

Middle-Ear Infection (Otitis Media)

• Common in children due to short, horizontal auditory tube.

• Symptoms: Fluid accumulation, pressure, pain, impaired hearing; can lead to meningitis or ossicle fusion.

• Tympanostomy: Surgical procedure to drain fluid and relieve pressure.

Inner Ear

Major Parts and Their Roles

• Bony Labyrinth: Complex system of interconnected cavities in the temporal bone, subdivided into:

Region	Function
Vestibule	Equilibrium (gravity and linear acceleration)
Semicircular Canals	Equilibrium (rotational movement)
Cochlea	Hearing

• Membranous Labyrinth: Lies within the bony labyrinth, filled with endolymph and lined by epithelium.

• Contains:

  • Utricle and Saccule (vestibule)

  • Membranous semicircular ducts (semicircular canals)

  • Cochlear duct (cochlea)

Fluid Compartments

• Endolymph: Fills the membranous labyrinth; similar to intracellular fluid.

• Perilymph: Fills the space between the bony and membranous labyrinths; similar to cerebrospinal fluid.

Specialized Sensory Areas of the Inner Ear

Overview

• Specialized sensory areas project into the wall of the membranous labyrinth.

• Composed of:

  • Maculae of Utricle and Saccule: Detect gravity and linear acceleration.

  • Cristae ampullaris: Detect rotational movement.

  • Organ of Corti: Detect sound vibrations.

Vestibule: Saccule and Utricle

• Encloses saccule and utricle.

• Receptors (hair cells) provide sensations of gravity and linear acceleration.

• Contains a gelatinous otolithic membrane weighted with otoliths (protein-calcium carbonate granules) to enhance sensitivity to gravity and motion.

Semicircular Canals: Cristae Ampullaris

• Contain semicircular ducts at right angles to each other.

• Receptors (hair cells) are stimulated by rotation of the head.

• Gelatinous structure called cupula bends in response to fluid movement, stimulating hair cells.

Cochlea: Cochlear Duct and Organ of Corti

• Cochlear duct is the elongated portion of the membranous labyrinth within the cochlea.

• Contains the Organ of Corti, the primary sensory organ for hearing.

• Organ of Corti consists of:

  • Receptor cells (hair cells) resting on the basilar membrane

  • Supporting cells

  • Tectorial membrane overlays the hair cells; bending of stereocilia against this membrane generates receptor potentials.

Membranes of the Cochlea

• Vestibular membrane: Separates cochlear duct from vestibular duct.

• Basilar membrane: Separates cochlear duct from tympanic duct.

• Tectorial membrane: Stereocilia of hair cells are in contact with this membrane.

Summary Table: Major Structures and Functions of the Ear

Section	Main Structures	Function
External Ear	Auricle, External Auditory Canal, Tympanic Membrane	Collects and channels sound waves
Middle Ear	Malleus, Incus, Stapes, Auditory Tube	Amplifies and transmits vibrations; equalizes pressure
Inner Ear	Vestibule, Semicircular Canals, Cochlea	Equilibrium and hearing; converts mechanical stimuli to nerve signals

Key Equations and Concepts

• Sound Wave Transmission: $v = f \lambda$ Where $v$ is the speed of sound, $f$ is frequency, and $\lambda$ is wavelength.

• Pressure Transmission: $P = F/A$ Where $P$ is pressure, $F$ is force, and $A$ is area.

Clinical Application

• Otitis Media: Middle ear infection, common in children, can impair hearing and lead to complications.

• Tympanostomy: Surgical intervention to drain fluid and relieve pressure in the middle ear.

Summary of Sensory Functions

• Gravity and Acceleration: Detected by hair cells in the vestibule (utricle and saccule).

• Rotation: Detected by hair cells in the semicircular canals.

• Sound: Detected by hair cells in the cochlea (Organ of Corti).

Additional info: The notes have been expanded with definitions, tables, and equations for clarity and completeness.