lecture 11 ear, hearing, and equilibrium

External ear

Pinna and ear canal collect and guide sound to the eardrum

Middle ear

The ossicles transmit vibrations and increase pressure at the oval window

Auditory tube

A separate passage connects the middle ear to the nasopharynx

Round window

Membrane at the entrance of the cochlea that seals the basal scale tympani and permits cochlear ✓ fluid displacement

Sequence of generating sound waves

1. The source vibrates

2. Pressure alternates

3. Neigbouring air passes the pressure onward, carrying energy

How does sound wave pressure alternate?

Forward motion compresses the air; backward motion lets it expand.

amplitude of sound waves

refers to the size of the pressure fluctuation of the wave measured in decibels

• higher amplitude = louder sound

frequency of sound waves

refers to the number of cycles (vibrations) each second measured in hertz

• higher frequency = higher pitch

1 hertz

= 1 sound wave cycle per second

three properties of sound waves

1. frequency (pitch)

2. amplitude (loudness

• amplitude is also referred to as intensity

3. duration

pathway from the cochlea to the auditory cortex

cochlea → auditory nerve → brainstem → thalamus → auditory cortex

mechanism of perceiving pitch

the brain uses which nerve fibers are active and the timing of their signals

mechanism of perceiving loudness

stronger sounds generally increase firing rates and the number of responding nerve fibers

mechanism of perceiving duration of sound

the timing of neural activity signals when a sound starts, continues, and stops

infrasound

low frequency communication below 20 Hz

• seen in elephants and whales

human audible sound range

20-20 000 Hz in young people with healhty hearing

ultrasound

any sound above 20 000 Hz

• made by small prey and in echolocation

• can be detected by cats and dogs which catch small prey

pathway of sound transmission from the eardrum to the cochlear nerve

1. tympanic membrane: vibrated from pressure of sound

2. ossicles: vibration passes through and the stapes moves at the oval window to move cochlear fluid

3. basilar membrane: inside the cochlea, moved by fluid movement which bend stereocilia

4. bending of stereocilia opens ion channels to produce electrical signals and release neurotransmitters

5. neurotransmitter release excites sensory neurons and initiates action potentials that travel through the cochlear nerve toward the brain

how do the round oval and oval window work together?

• oval window: connected to stapes and cochlea

• round window: connected to the opposite side of the cochlea

the round window membrane flexes to accomodate fluid displaces as the stapes vibrates

hair cell activation in the cochlea

sound driven movement in the cochlea bends the sterocilia which act as sensory receptors that convert this movement into an electrical signal

three fluid filled compartments in the cochlea

• scala vestibuli: in the vestibular duct, contains perilymph

• Scala media: in the cochlear duct, contains endolymph

• Scala tympani: in the tympanic duct, contains perilymph

organ of corti

in between the scala media and basilar membrane and contains the sensory hair cells

perilymph

in the scala vestibuli and scala tympani

• is high in sodium and low in potassium

endolymph

in scala media (cochlear duct)

• is high in potassium and low in sodium

stereocilia

hair bundles arranged from short to tall in ear hair cells

structure of hair cells in the organ of corti

one row of inner hair cells and three rows of outer hair cells

• outer hair cells: the tallest stereocilia atttach to the tectorial membrane

how does hair cell mechanotransduction work?

• stereocilia are connected by protein bridges called tip-links to ensure they move together

• mechanical stress on tip links causing stereocilia to bend TOWARD the TALLER stereocilia, opens potassium channels causing depolarization

• mechanical stress on tip links causing stereocilia to bent TOWARD the SHORTER stereocilia, closes potassium channels, causing hyperpolarization

  • in the case of depolarization, calcium channels open and trigger glutamate release to activate receptors on afferent sensory neurons

pathway from hair cell to brainstem

1. inner hair cell: bends and develops a graded potential

2. depolarization of hair cell causes release of glutamate which excites an afferent neuron

3. action potential is generated in the neuron which has an axon that acts as the cochlear nerve which enters the brainstem

tonotopic frequency

different coritcol regions best respond to different frequencies to preserve frequency information

pathway from brainstem to auditory cortex

1. auditory signals pass through several relays including the cochlear nucleus in the medulla and the inferior colliculus in the brain stem

2. eventually reach the medial geniculate nucleus (MGN) in the thalamus

3. the cochlear nerve synapses onto the MGN which brings signal to the auditory cortex

three major structures of the vestibular apparatus

• semicircular canals (anterior, posterior, and lateral canals as well as ampulla and cupula)

• otolith organs (ultricle and saccule)

• two nested labyrinths (bony and membranous)

what type of acceleration is seen in the semicircular canals?

angular acceleration

what type of acceleration is seen in the otolith organs

linear acceleration and gravity

otolith organs

include the ultricle and saccule which have sensory regions called maculae to sense translational movement and head tilt

nested labyrinths in the vestibular apparatus

• membranous: contains endolymph

• bony: contains perilymph

what does the anterior (cranial) semicircular canal detect?

up and down motion (nodding yes)

• positioned at an 80 degree angle

what does the posterior (caudal) semicircular canal detect?

detects head tilts side to side (lateral tilts such as moving ear toward shoulder)

• positioned at a 50 degree angle

what does the lateral (horizontal) semicircular canal detect?

side to side head motion (shaking head no)

• positioned at a 180 degree angle

ampulla

at the ends of each semicircular canal, containing gelatinous cupula and sensory crista and working together to detect movements in different planes to provide a comprehensive sense of balance

coding for left (counterclockwise) acceleration

• turning the head counterclockwise moves endolymph clockwise

• clockwise motion causes hair cells to bend TOWARD kinocilium

• hair cells depolarize and afferent firing increases

coding for right (clockwise) acceleration

• turning the head clockwise moves endolymph counterclockwise

• counterclockwise motion causes hair bundles to bend AWAY from kinocilium

• hair cells hyperpolarize and afferent firing decreases

utricle

otolith organ that senses horizontal acceleation

saccule

otolith organ that sense vertical acceleration

difference between cristae ampullares and maculae

• cristae ampullares: in the ampullae of the semicircular ducts and detect angular acceleration

• maculae: in the otolith organs and detect linear acceleration and head tilt relative to gravity

structure of the macula

layers:

• otoconia: calcium carbonate crystals that add mass

• otolithic membrane: gelatinous later

• kinocilium protrude into membrane

• hair cells