Special Senses: The Ear

By the end of this class you should understand:

• The properties of sound waves as they relate to hearing

• The three regions of the ear and their functions

• How the inner ear creates the senses of hearing and balance

Sensory Neurons

• Recall there are five types of neurons, classified by the type of stimulus they respond to– Mechanoreceptor– Thermoreceptor– Nociceptor (Pain

receptor)– Chemoreceptor– Photoreceptor

Mechanoreceptors

• Mechanoreceptors depend on mechanically gated ion channels– When the cell is deformed,

they open and allow sodium into the cell

• There are many types of mechanoreceptors

• The type of interest today is called the hair cell

Hair Cell• Hair cells are unique in that

they do not have an axon• The “hair” of the hair cell

(also known as stereocilia) bend when there are vibrations

• The bending allows positively charged ions into the cell

• This depolarizes the cell membrane causing neurotransmitters to be released

Vibrations

• The ear is responsible for detecting two different types of movements using hair cells– Sound: vibrations of the air– Vestibular sense:

acceleration of the head

• Both of these occur in the inner ear

Parts of the Ear

Vestibular Sense

• Sense of balance or equilibrium

• Can sense both linear and angular acceleration– The vestibule detects

linear acceleration– The semicircular canals

detect angular acceleration

Acceleration Types• Linear acceleration is in a

straight line– Detectable in a car that is

braking or gunning the engine– Also tells you which way is up

• Angular acceleration is spinning or rolling– Detectable doing somersaults

or spinning in a circle• In both cases the hair cells

are stimulated by the linear or angular movements of heavy crystals called otoliths

Acceleration and Nausea

• The brain must integrate signals from the different senses– A combination of forebrain and

midbrain work• When these signals do not

match the brain struggles to integrate them– Nausea (“carsickness” and

“seasickness”) may result from feeling acceleration in the vestibular sense but not visually detecting any acceleration

Angular Acceleration• Spinning rapidly for an

extended period of time can cause the semicircular canals to build up a lot of angular momentum

• When you stop spinning you experience dizziness because the semicircular canals are still spinning– Causes the world to “spin”

because your eyes are trying to follow the movement of the canals

Nature of Sound

• The hair cells for the vestibular sense respond to the movement of heavy crystals

• The hair cells for the sense of sound respond instead to the vibrations caused by sound

• To understand this, it is important to understand the nature of sound

Pressure Waves• Sound is caused by pressure

waves moving through a medium (air, water, etc)– A single pressure wave will

only sound like a crack or boom, not a tone

– Back-and-forth differences in air pressure at a certain frequency produce a constant tone

• The faster the waves of air pressure arrive at the ear, the higher the perceived pitch

Pitch and Volume• The bigger the waves (the more

energy is carried), the higher the volume– Higher amplitude, measured in

decibels– Its all relative! The closer, the louder

the sound• The faster the waves arrive, the

higher the pitch of the sound– Higher frequency, measured in hertz– Same unit used for light frequency but

light waves have millions of times higher frequencies

Resonance• A complication is that any

object capable of producing or containing sound waves has a resonance frequency

• The best example of resonance is making waves in a bathtub slowly bigger and bigger by moving your own body in time with the waves– Sound waves can have this

happen as well!– Note the different lengths on

harp/piano strings

Resonance in the Cochlea• There is a spiral-shaped

section of the temporal bone called the cochlea– The cochlea serves as a

resonating chamber– Vibrations from the ear are

transmitted to pressure waves in the cochlea

• Much like piano strings, there is a high-frequency to low-frequency resonance of the hair cells in the cochlea

Parts of the Ear

Pressure Waves

• Pressure waves are transmitted through the outer ear– Pinna (the part of the ear you

can touch)– Auditory canal (the part of the

ear you can put a Q-tip in)– The Tympanic

membrane/eardrum forms the border between the outer and middle ear

Middle Ear

• The middle ear is a cavity with three tiny bones called ossicles that attach to the tympanic membrane at one end and the inner ear at the other– The ossicles are unique to

mammals• When air vibrations cause the

eardrum to vibrate, the three bones transmit these vibrations to the inner ear

Middle Ear Ossicles• The three ossicles are

malleus, incus and stapes– Hammer, anvil, and stirrup,

named for their shapes– Malleus is attached to the

eardrum, stapes pounds on the oval window

• The auditory tube connects this space to the nasal passage– If blocked due to illness, can

result in headaches/earaches and reduced hearing

Oval Window

• Just as the tympanic membrane forms the border between the outer and middle ear, the oval window forms the border between the middle and inner ear

• The oval window is where pressure waves enter the cochlea– The faster the vibrations transmitted

onto the eardrum, the faster the vibrations transmitted into the oval window

The Cochlea

• The cochlea is filled with a liquid called perilymph that transmits vibrations from the oval window

• The hair cells plus the membrane they attach to and the sensory neurons form the Spiral Organ, or Organ of Corti

• The axons from these sensory neurons bundle together to become the cochlear nerve

The Nerves!

• The cochlear nerve leaves the cochlea heading for the brain

• The vestibular organs produce another nerve called the vestibular nerve

• These two nerves actually join to become the vestibulocochlear nerve which is processed by the thalamus and midbrain before transmitting to the temporal lobe

That’s our show!

• Next Monday is review day! Bring your questions!

• One week from today is Exam #2!