Lecture 16: Sounds and Language

What does the outer ear (pinna + canal) do?

Gathers sound waves from the environment and funnels them toward the tympanic membrane

Difference between wind and sound?

WIND = air molecules move through space; SOUND = air molecules vibrate but return to original positions

What is sound (perceptual definition)?

Sensation produced when the tympanic membrane is stimulated by vibrating air molecules

What are sound waves in physical terms?

Alternating cycles of compressed and rarefied air propagating through a medium

What is the tympanic membrane?

Eardrum; membrane that vibrates in response to air pressure waves

What is the threshold of hearing (TOH)?

Lowest sound level detectable ~50% of the time by a normal human (0 dB reference)

What does 0 dB represent?

Threshold of hearing

Approximate loudness of whisper? ~20 dB Normal conversation loudness? ~60 dB Street traffic loudness? ~70 dB iPod max volume? ~100 dB Rock concert front row? ~110 dB Threshold of pain? ~130 dB Military jet takeoff? ~140 dB Eardrum perforation level? ~160 dB What does sound frequency determine?

Pitch (low frequency = low pitch, high frequency = high pitch)

Low frequency wave has what property?

Longer period → lower pitch

High frequency wave has what property?

Shorter period → higher pitch

What is the middle ear function?

Transfers vibration from tympanic membrane to inner ear via ossicles

What are the 3 ossicles?

Malleus, Incus, Stapes

Malleus function?

Attached to tympanic membrane; transfers vibrations

Incus function?

Acts as lever between malleus and stapes

Stapes function?

Taps oval window to transmit vibration into cochlea

What is the oval window?

Membrane receiving stapes vibration; entry point to cochlear fluid

What is the round window?

Pressure release valve for cochlear fluid movement

What is tympanic reflex?

Muscle reflex (tensor tympani + stapedius) that dampens loud sounds

Function of tympanic reflex?

Protect ear and adjust hearing gain for loud noises

What is the inner ear bony labyrinth?

Fluid-filled structure containing hearing and balance organs

What fluid fills inner ear labyrinth?

Endolymph

Two parts of inner ear labyrinth?

Semicircular canals (balance) and cochlea (hearing)

Function of semicircular canals?

Sense head rotation and angular acceleration

Function of cochlea?

Sense sound vibrations

Shape of cochlea?

Coiled snail-like structure

Three cochlear chambers?

Scala vestibuli, Scala media, Scala tympani

Basilar membrane location?

Between scala media and scala tympani

Function of basilar membrane?

Vibrates in response to sound waves in cochlear fluid

What is the organ of Corti?

Hearing organ sitting on basilar membrane containing hair cells

What are hair cells?

Mechanoreceptors that convert mechanical vibration into neural signals

Do hair cells fire action potentials?

No — they release neurotransmitter instead

What are stereocilia?

Hair-like projections on hair cells that detect mechanical bending

What are tip links?

Protein strands connecting stereocilia that gate ion channels

What happens when stereocilia bend toward kinocilium?

K+ channels open → depolarization of hair cell

Why does K+ influx depolarize hair cells?

K+ concentration is higher outside stereocilia (unusual gradient)

What happens when stereocilia bend away?

K+ channels close → hyperpolarization

What neurotransmitter do inner hair cells release?

Glutamate

What is function of inner hair cells?

Main sensory receptors for sound

Function of outer hair cells?

Amplify vibration of basilar membrane

What is tonotopy?

Mapping of sound frequency along basilar membrane

Where are high frequencies detected?

Base of cochlea

Where are low frequencies detected?

Apex of cochlea

First neural firing in auditory pathway?

Spiral ganglion neurons

Pathway: spiral ganglion → ?

Cochlear nucleus

Cochlear nucleus projects to?

Superior olive

Superior olive function?

Binaural processing (sound localization)

Why is superior olive important?

Compares timing and intensity between ears

Interaural time difference helps detect?

Left vs right sound location

Interaural intensity difference helps detect?

Direction based on loudness differences

Cochlear nucleus inputs are?

Bilateral to superior olive

Superior olive → next structure?

Inferior colliculus

Inferior colliculus function?

Integrates sound localization + frequency maps

Inferior colliculus also contains?

Spatiotopic maps of sound source location

Inferior colliculus → ?

Medial geniculate nucleus (thalamus)

MGN function?

Relay auditory information to cortex

MGN → ?

Auditory cortex

Where is auditory cortex located?

Temporal lobe (lateral sulcus region)

What is auditory cortex responsible for?

Processing sound and speech

What is vestibular system function?

Detect head motion and spatial orientation

Utricle detects what motion?

Horizontal linear acceleration + head tilt

Saccule detects what motion?

Vertical acceleration

Semicircular canals detect?

Rotational (angular) acceleration

What are otoliths? Calcium carbonate crystals in utricle/saccule that detect gravity/acceleration

What is cupula?

Gelatinous structure in semicircular canals that bends with fluid movement

What causes hair cell activation in semicircular canals?

Fluid movement bends cupula → stereocilia deflection

Vestibular nerve increases firing when?

Head rotates toward its side

What is vestibulo-ocular reflex (VOR)?

Stabilizes gaze during head movement

VOR function outcome?

Keeps eyes fixed on target while head moves

Key VOR pathway starts at?

Vestibular nerve (Scarpa’s ganglion)

VOR motor output includes CN?

CN III (oculomotor) and CN VI (abducens)

Medial rectus muscle does what?

Pulls eye inward (adduction)

Lateral rectus muscle does what?

Pulls eye outward (abduction)

What is key idea of LH vs VMH appetite control?

LH = hunger (orexigenic), VMH = satiety (anorexigenic)

Lesion of VMH causes?

Increased appetite and higher body weight

Lesion of LH causes?

Decreased appetite and lower body weight

LH role in sleep system?

Contains orexin neurons that promote wakefulness

VLPO function?

Inhibits arousal systems → promotes sleep

Neurotransmitters released by VLPO?

GABA + galanin

What is flip-flop sleep switch?

Mutual inhibition between LH (wake) and VLPO (sleep)

What causes narcolepsy (core idea)?

Loss of orexin signaling prevents stabilization of wake state