Auditory and Vestibular Systems

Anatomy and Physiology II

BIO 232

Structure of the Ear

Middle & Inner Ear

The Three Ossicles

Inner Ear

• Labyrinth - bony labyrinth surrounds the membranous labyrinth (sacs & ducts)

• Perilymth - surrounds the membranous labyrinth - similar to cerebrospinal fluid

• Endolymph - fills the interior of the sacs and ducts - similar to K⁺ rich extracelullar fluid

Inner Ear

• Vestibule - central egg shaped cavity of the bony labyrinth - posterior to the cochlea & anterior to the semicircular canals

• Saccule & Utricle are found here

• Semicircular canals - 2/3 circles - swelling called the ampulla (receptors are found here)

• Cochlea - Latin for “snail” -

Inner Ear

Sound

• Sound is transmitted as a longitudinal wave in air.

• Series of condensations and rarefactions

Sound Pressure Level

• Correlates with loudness

• SPL - ratio of pressure amplitudes

• 1dB= 20log P_t/P_r

• P_r = SPL required to make 300Hz audible

^• 2.0X 10^-4 dynes/cm²

Intensities of Common Sounds

• 160dB = Jet plane or rock concert

• 140dB = Pain

• 120dB = Discomfort

• 100dB = Subway

• 80dB = Traffic

• 50-65dB = Conversation level

• 20dB = Whisper

• 10dB = Rustle of leaves in a gentle breeze

• 0dB = Not absence of but pressure equal to reference sound

Sound Intensity Comparison

• P_t = 10x (log =1) greater that P_r= 20dB

• P_t = 100x (log =2) greater that P_r= 40dB

• P_t = 1000x (log =3) greater that P_r= 60dB

Human Hearing

• Dynamic Range

• 0 -120dB

• Faintest sound one-millionth the amplitude of the loudest sound - processed without discomfort

Terminology

• Frequency - rate of periodic pressure fluctuations

• Herz (Hz)=cycles/second

• Tone - sound of one frequency

• Ordinary sounds more complex - many different frequencies

Analysis of Sound

• Begins when air borne sounds strike tympanic membrane

• Begins to vibrate

• Resultant motion transferred to the oval window of the cochlea by lever system

Analysis of Sound

• Movement of the stapes causes wavelike motion of the fluid in the scalae (canals) of the cochlea

• Pressure wave exits at the round window

• Fluid motion caused by the pressure wave in the cochlea affects the Organ of Corti which rests atop the basilar membrane

• This is where transduction takes place

Analysis of Sound

• Components of Transduction

• Hair cells (inner & outer)

• Basilar membrane

• Tectorial membrane

Hair Cells

Response of the Basilar Membrane

• Traveling wave starting at the oval window

Analysis of Sound

• Transduction mechanism

• As basilar membrane moves a net shearing force is produced upon the microvilli of the hair cells

• This occurs because the tectorial membrane dos not move to the same degree as the basilar membrane

• This shearing force leads to a potential change which results in AP’s in the 8th cranial nerves which synapse on the hair cells

Stereocilia and Their Movement

Place Theory of Hearing

• For every frequency there is particular region along the length of the basilar membrane where there is peak motion.

• The base processing higher frequencies

• The apex processing lower frequencies

8th Cranial Nerve Afferents

• 1. The 8th nerve fibers are spontaneously active.

• 2. Each 8th nerve fiber is most sensitive to a particular frequency called it’s characteristic frequency.

Auditory Pathway

Role of Vestibular system

• Keep body balanced

• Coordinate head & body movements

• Most remarkably - enables the eyes to remain fixed on a point in space when the head is turning

Static Equilbrium

• Utricle and saccule

• Thicken spots - maculae

• Supporting cells and vestibular hair cells

• Covered with a gelatinous mass - otolithic membrane

• Embedded in the membrane are crystals of calcium carbonate - otoliths

Stimulation of the Utricle

• If head tilted or undergoes linear acceleration

• Otoliths displace the gelatinous mass

• This displacement deflects the hairs of the macula

Stimulation of Semicircular Canals

• When the head undergoes angular acceleration the viscous fluid in the semicircular canals lags behind, due to inertia and pushes on the cupula

• Cupula distortion

• Receptor potential in the hair cells of the crest

• Excitation carried into CNS via bipolar neurons of the vestibular ganglion

Stimulation of Semicircular Canals

• Hairs cells are arranged in an orderly fashion in the ampullae

• Horizontal duct - all face toward the utricle

• Thus rotation in one direction excites all the hair cells while in the opposite direction hyperpolarizes them

• Thus a single duct or canal can detect rotation in either direction

Stimulation of Semicircular Cells