CrashCourse:hearing

Overview

The ear enables both hearing and balance through complex structures that convert sound vibrations and head movements into electrical signals the brain interprets.

What is Sound?

• Sound consists of vibrations in air that travel as waves to the eardrum
• Frequency measures waves per unit time; higher frequency creates higher-pitched sounds
• Amplitude indicates pressure differences in sound waves; greater amplitude means louder sound
• Short, rapid waves produce high-pitched noises; longer, slower waves create low-pitched sounds
• Different vibrating objects generate differently shaped sound waves

Structure of the Ear

Sound Transmission Process

• Pinna catches sound waves and directs them through the auditory canal
• Sound waves strike the tympanic membrane, causing it to vibrate
• Vibrations pass through three tiny bones: malleus (hammer), incus (anvil), and stapes (stirrup)
• Ossicles amplify vibrations because inner ear uses fluid rather than air
• Stapes transfers vibrations to the oval window, setting inner ear fluid in motion

The Cochlea and Hearing Mechanism

• Cochlea is a snail-shaped structure containing three main chambers separated by membranes
• Basilar membrane runs through middle chamber and contains over 20,000 fibers of varying lengths
• Fibers near cochlea base are short and stiff; those at the end are longer and looser
• Different fiber sections resonate at different frequencies, like harp strings
• High-frequency sounds vibrate short fibers near the base; low-frequency sounds vibrate longer fibers at the end
• Organ of Corti sits atop basilar membrane and contains specialized sensory cells

Sound Transduction to Neural Signals

• Hair cells in organ of Corti have tiny hair-like structures that detect membrane vibrations
• When triggered, hair cells open mechanically gated sodium channels
• Sodium influx generates graded potentials that may produce action potentials
• Location of activated hair cells indicates pitch to the brain
• Louder sounds create larger graded potentials and more frequent action potentials
• Electrical impulses travel along cochlear nerve to auditory pathway and cerebral cortex
• Brain interprets signals by combining them with stored memories and experiences

Equilibrium and Balance

• Vestibular apparatus maintains balance through fluid movement responding to head motion
• Three semicircular canals sit in sagittal, frontal, and transverse planes
• Each canal detects different head rotation: side-to-side, up-and-down, and tilting movements
• Canals widen at base into utricle and saccule structures filled with hair cells
• Hair cells sense fluid motion and send action potentials along acoustic nerve
• Brain interprets hair cell location to determine movement direction
• Action potential frequency indicates speed of head acceleration

Motion Sickness

• Results from sensory conflict between different movement detection systems
• Spinning in a chair activates vestibular hair cells while body position sensors signal stillness
• On rocking boats, vestibular system detects motion but eyes see stationary surroundings
• Brain confusion from conflicting movement signals triggers nausea and vomiting

Key Terms & Definitions

• Pinna (auricle): visible part of ear made of elastic cartilage that catches sound waves
• Tympanic membrane (eardrum): translucent, cone-shaped boundary between external and middle ear
• Auditory ossicles: malleus, incus, and stapes—smallest bones in human body
• Labyrinth: complex inner ear structure with bony and membranous layers
• Basilar membrane: stiff tissue band containing fibers that vibrate at specific frequencies
• Organ of Corti: fixture containing hair cells and nerve cells for sound transduction
• Vestibular apparatus: series of sacs and canals that detect head movement