Lectures 17 and 18: Auditory System

Auricle perichondrium

Auricle elastic cartilage

Auricle

stratified squamous epithelium (keratinized)

hair follicles

elastic cartilage (with elastic fibers)

External acoustic meatus

1: Ceruminous glands

2: Hair follicles

external acoustic meatus

derives from pharyngeal cleft 1

stratified squamous epithelium

hair follicles

ceruminous glands (modified apocrine sweat glands)

elastic cartilage, temporal bone

Eustachian tube goblet cells

Eustachian tube

derives from pharyngeal pouch 1

pseudostratified respiratory epithelium

numerous goblet cells

temporal bone, fibrocartilage

Tympanic Membrane and middle ear ossicles

cuticular layer

fibrous layer

mucous layer

cartilage

compact bone

tympanic membrane layers

cuticular layer of stratified squamous epithelium (ectoderm)

ectoderm that derives from pharyngeal cleft 1

fibrous layer with collagen II and III (mesoderm)

mucous layer of cuboidal cells (endoderm)

endoderm that derives from pharyngeal pouch 1

middle ear ossicles

compact bone formed by endochondral ossification

malleus, incus, stapes articulate with synovial joints

Otosclerosis

Tissue overgrowth and fixation of stapes to oval window (can be inherited)

tympanic membrane

air vibrations through external acoustic meatus oscillate tympanic membrane

during otoscopic exam a cone of light radiates anteroinferiorly from umbo

cone of light is reflection of otoscope illuminator

otitis media (inflammation/infection of middle ear) can scar middle ear ossicles, rupture tympanic membrane, or cause facial nerve palsy and diminished taste

internal ear

spiral lamina spirals around bony modiolus and houses spiral ganglion

membranous cochlea

scala vestibuli – oval window

scala tympani – round window

scala media (cochlear duct)

organ of Corti is sensory organ of auditory system internal ear

spiral ganglion

housed in spiral lamina

collection of neuronal cell bodies of neurons with axons in CN VIII

dendrites of these neurons synapse in organ of Corti

Ear Development

otic placode invaginates to form otic vesicle

otic placode forms otic pit

otic pit forms otic vesicle

statoacoustic ganglion forms from cells of otic vesicle wall and migrating neural crest, and later splits into vestibular and cochlear (spiral) ganglia

otic vesicle forms membranous labyrinth

dorsal component forms utricle, semicircular canals, and endolymphatic duct

ventral component forms saccule and cochlear duct

membranous labyrinth development

cochlear duct surrounded by mesenchyme

mesenchyme differentiates into cartilage by 10 weeks

vacuoles appear in cartilage

membranes separate cochlear duct from scala vestibuli and scala tympani

organ of corti development

by 5 months neuroepithelial cells and tectorial membrane form in cochlear duct

spiral tunnels become tunnel of Corti

middle ear ossicles development

malleus and incus derive from first pharyngeal arch

stapes derives from second pharyngeal arch

mesenchymal condensation by 7 weeks

cartilaginous precursors in mesenchyme remain embedded until eighth month

tympanic membrane development

cuticular layer is ectodermal epithelium from pharyngeal cleft 1

mucous layer is endodermal epithelium from pharyngeal pouch 1

sound frequency localization

frequency of sound determines frequency of oval window displacement and frequency of traveling wave in basilar membrane

each traveling wave maximally activates basilar membrane in specific location in cochlea

high frequency tones activate near base of cochlea, low frequency near apex of cochlea

retinoic acid establishes BMP7 gradient along developing cochlea

BMP7 higher at apex of cochlea promoting development of low frequency sensing hair cells

this tonotopic representation along the cochlea is conserved through the auditory pathway

interaural time difference

for localizing low-frequency sounds

interaural intensity difference

for localizing high-frequency sounds

endolymph and perilymph

stria vascularis is responsible for maintaining the ionic composition of endolymph through selective secretion and absorption of ions

endolymph of scala media is similar to intracellular fluid, has high concentration of K+ for depolarization

perilymph of scala vestibuli and scala tympani is similar to cerebrospinal fluid, has high concentration of Na+ for neuron viability; also located in tunnel of Corti (between inner and outer hair cells) and basal end of hair cells

tight junctions

at apical end of hair cells with phalangeal cells reticular laminae of phalangeal cells separate endolymph from perilymph

Tight junctions

Tectorial membrane

organ of Corti includes

tectorial membrane

supporting cells including phalangeal cells

inner hair cells (single row)

tunnel of Corti

outer hair cells (typically 3-4 rows)

cochlear afferent neurons (type I and II)

olivocochlear efferent neurons

basilar membrane

cochlear afferent neurons

cell bodies in spiral ganglion

innervate hair cells

unmyelinated in organ of Corti

inner hair cell

sensory receptors for sound stimuli

outer hair cells

alter mechanics of basilar membrane by changing length when stimulated

increases sensitivity of cochlear afferent neurons conveying information from inner hair cells

mechanoelectrical transduction mechanism

displacement of basilar membrane relative to tectorial membrane

displacement of stereocilia of hair cells toward tallest stereocilium

mechanically-gated potassium channels open in stereocilia

increased potassium flow into hair cell from endolymph

depolarization of hair cell

voltage-gated calcium channels open at base of hair cell

increased calcium flow into hair cell

increased synaptic vesicle fusion

increased neurotransmitter (glutamate) release

glutamate binds receptors of cochlear afferent neurons

these first order neurons depolarize and increase firing rate

type I cochlear afferent neurons

bipolar

90-95% of neurons in spiral ganglion

synapse with 1-2 inner hair cells each

as many as 20 may converge on 1 inner hair cell

inner hair cells are receptors for sound stimuli

respond to narrow frequency range (high precision)

type II cochlear afferent neurons

bipolar

5-10% of neurons in spiral ganglion

synapse with 10+ outer hair cells each

outer hair cells alter mechanics of basilar membrane to improve

type I cochlear afferent neuron sensitivity

respond to wide frequency range (low precision)

Frequency

coded in the cochlear nerve by the position of the cochlear afferent neurons in the cochlea

with high frequency near the base and low frequency near the apex of the cochlea

intensity

coded in the cochlear nerve by both the discharge rate of the cochlear afferent neurons and the recruitment of additional cochlear afferent neurons as stimulus intensity increases

characteristic frequency

for a type I cochlear afferent neuron the frequency with the lowest threshold intensity

Vestibulocochlear nerve

cochlear nuclei

first order neurons

cell bodies in spiral ganglion

axons in vestibulocochlear nerve

synapse in ipsilateral cochlear nuclei

second order neurons

cell bodies in cochlear nuclei

axons in several pontine structures including trapezoid and lateral lemniscus

information ultimately reaches inferior colliculus

inferior colliculus neurons

cell bodies in inferior colliculus

axons in brachium of inferior colliculus

synapse in MGN of thalamus

medial geniculate nucleus (MGN) divisions

anterior division: involved in complex feature detection (not just simple tones)

posterior division: conveys information about moving or novel stimuli that direct auditory attention

medial geniculate nucleus (MGN) supplied by

thalamogeniculate artery of PCA (posterior thalamus)

sublenticular limb

MGN axons project to primary auditory cortex as sublenticular limb then auditory radiation

sublenticular limb supplied by lenticulostriate arteries

connections between thalamus and cortex

anterior transverse temporal gyrus of Heschl

Primary auditory cortex

primary auditory cortex

Brodmann area 41

anterior transverse temporal gyrus of Heschl

(1-3 gyri of Heschl, 2 is most common)

heterotypical granular cortex

receives axons from anterior division of MGN

contributes to complex feature detection

posterior transverse temporal gyrus of Heschl

secondary auditory cortex

secondary auditory cortex

Brodmann area 42

posterior transverse temporal gyrus of Heschl

cortex is less granular than primary auditory cortex

receives axons from posterior division of MGN

contributes to auditory attention to moving or novel stimuli

auditory association cortex

Part of Wernicke’s area

auditory association cortex

Brodmann area 22

superior temporal gyrus

part of Wernicke’s area

also receives visual and other sensory information

damage affects language comprehension for speech (but not fluency)

part of Wernicke’s area

Broadman area 39,40

Brodmann area 39,40

angular and supramarginal gyri of parietal lobe

part of Wernicke’s area

damage affects language comprehension for reading and writing (but not fluency)

Broca’s area

Broca’s area

Brodmann area 44,45

inferior frontal gyrus

arcuate fasciculus projects from Wernicke’s area to Broca’s area

damage affects language fluency (but not comprehension)

internal ear, cochlear nuclei supplied by

AICA (labyrinthine)

inferior colliculus

quadrigeminal

MGN

thalamogeniculate

primary auditory cortex, auditory

radiation

middle cerebral (inferior trunk)

internal capsule (including sublenticular limb)

MCA lenticulostriate arteries

primary somatomotor cortex, primary somatosensory cortex, and frontal eye field

MCA superior trunk

inferior temporal optic radiation (including Meyer’s loop), primary auditory cortex, and auditory radiation

MCA inferior trunk

primary somatomotor cortex, primary somatosensory cortex, inferior temporal optic radiation (including Meyer’s loop), frontal eye field, primary auditory cortex, and auditory radiation

MCA distal stem

primary somatomotor cortex, primary somatosensory cortex, internal capsule, complete optic radiation, frontal eye field, primary auditory cortex, and auditory radiation

MCA proximal stem

descending auditory pathways

monaural information

routed to the contralateral side

binaural information

handled by central pathways that compare the differences between sounds at both ears

basilar membrane in cochlea tonotopic organization

primary auditory cortex tonotopic organization

superior olivary nuclei

superior olivary nuclei

receives bilateral auditory input

important for localizing sounds in space

bilateralism

orientation of head, eyes, and body to novel sound

for acoustic startle reflex

no voluntary aspects

afferent limb is lateral lemniscus, inferior colliculus, MGN, secondary auditory cortex, superior colliculus

efferent targets are reticulospinal tracts (UMNs), spinal cord (LMNs), facial motor nucleus (LMNs)

middle ear reflex

dampen movement of middle ear bones to compensate for impedance difference between air and fluid

afferent limb: cochlear nuclei neurons, superior olivary nuclei neurons

efferent limb: facial motor nucleus neurons, trigeminal motor nucleus neurons

efferent targets: stapedius muscle, tensor tympani muscle

occurs as sound intensity increases beyond 85 dB

stapedius muscle is attached to stapes, is innervated by CN VII, and diminishes sound intensity when contracted

tensor tympani muscle is attached to malleus, is innervated by CN V, and contracts sometimes when swallowing (it does not contract in response to sound in humans)

in humans the middle ear reflex is mediated exclusively by the stapedius muscle