Unit 9 - Hearing Anatomy Flashcards
Auditory system
Exteroceptive system concerned with perception of sound
Vestibular system
Proprioceptive
Concerned with maintenance of equilibrium
Where does the vestibulocochlear nerve (CN VIII) emerge
SENSORY
Emerges from brainstem at cerebellopontine angle - junction between cerebellum, pons and medulla
Lateral to facial nerve
Skull opening that transmits the vestibulocochlear nerve
Internal auditory meatus
Structures of ear
Separation between external and middle ear
Tympanic membrane (ear drum)
Contents and boundaries of middle ear
Air filled space
Bounded by tympanic membrane and inner ear
Contents of inner ear/lebyrinth (in petrous portion of temporal bone)
Cochlea
Semicircular canals
Vestibule (utricule & saccule)
Transmission through outer ear
Air pressure fluctuations move tympanic membrane (ear drum) back and forth
Attached to tensor tympani muscle - which it pulls inwards
Cochlea in inner ear conducts sound through fluid instead of air
Before sound passes into inner ear it must be amplified - OSSICLES
ossicle bones
How do the ossicles amplify sound
Stapes rests against cochlea - through oval window
Air pressure pushes on tympanic membrane - vibrates - ossicles move - stapes pushes against cochlea and displaces fluid within SCC and in cochlea to move fibres on hair cells
Tensor tympani muscle attached to malleus
stapedius attached to stapes
Tensor tympani muscle attaches to
Malleus
Stapedius attaches to
Stapes
Receptors of auditory system
Function
Where are they found
Responsible for converting mechanical energy (fluid displacement) into electrochemical energy - travels along cochlear nerve
Hair cells enclosed within inner ear in tubular system are filled with fluid
Auditory hair cells in spiral organ of corti in cochlea
What is the cochlea spiral shell composed of
3 fluid filled spaces
Scala vestibuli
Scala tympani
Cochlear duct
Scala vestibuli & scala tympani
Partially enclosed in bone
Bony labyrinth - contains perilymph
continuous with each other
Cochlear duct
Part of membranous labyrinth (contains endolymph) suspended between bony labyrinth
membranous labyrinth also includes
utricle
saccule
semicircular canals
Outer spirals =
pitch of noise
= base
Low pitch of noise
Centre spirals =
pitch of noise
Apex
High pitch of noise
Where are the hair cells located
Basilar membrane (blue)
Oval vs round window
Relationship between movement of stapes and round window
Oval window - opens into scala vestibuli
- Stapes footplate occupies oval window
Round window - opens into scala tympani
- flexible tympanic membrane at round window
As the stapes moves inward, the round window moves outward
What happens when vibrations reach cochlear duct
Cochlear duct and basilar membrane are set in motion - hair cells are activated (mechanoreceptor cilia)
Categories of auditory hair cells
Organ of Corti - 2 hair cell types
Flask shaped inner hair cells
Rectangular shaped outer hair cells
Where are stereocilia found
What is the kinocilium in contact with
At apical end of each hair cell
Kinocilium located at tallest row of stereocilia
Tips of longest stereocilia in contact with overlying membrane
Basilar membrane moved by fluid movement - stereocilia bend and it changes MP of hair cells
Tonotopic representation of hair cells
Higher freq sounds activate hair cells near oval window
Lower freq sounds activate hair cells near apex of cochlea
Auditory info tonotopically organised in auditory pathway
Spiral shaped cochlea
Hair cells are innervated by primary sensory auditory neurons in spiral ganglion
Cell bodies of sensory neurons at spiral ganglion - axons to cochlear nerve
At each of the spirals is the cochlear duct and within that the basilar membrane (with hair cells) - sensory neurons head towards ganglia (collection of cell bodies in same discrete location) - axons converge at the cochlear nerve
Pathway of vestibulocochlear nerve (CN VIII)
Ascending auditory pathways
Cochlear nuclei located near inferior cerebellar peduncle
Send fibres medially along trapezoid body to superior olivary nucleus
Fibres ascend bilaterally (mostly on contralateral side) through pons and midbrain
Fibres ascend to nucleus of inferior colliculus or are relayed to thalamus
Eventually terminate in primary auditory cortex
How do fibres for auditory pathway travel after the thalamus
Fibres pass from thalamus to auditory cortex via auditory radiations
Primary auditory cortex (area 41 & 42) on upper medial surface of superior temporal gyrus (Heschl’s transverse gyri)
Recognition and interpretation of sound on basis of past experience takes place in secondary auditory area
(17 mins - come back to it)
Pathway of descending auditory pathways
Fibres originate in auditory cortex and nuclei in auditory pathway
Efferent fibres descend to nerve cells at different levels of auditory pathway and hair cells of organ of corti
Nerve fibres act as feedback mechanism to inhibit reception of sound
Auditory sharpening, suppressing some signals and enhancing others
Function of vestibular system
Maintain equilibrium
Direct gaze of eyes
Preserve constant plane of vision (lead position) by modifying muscle tone
Truncal ataxia nerve damage
Flocculonodular nerve
Structure of cochlea
Arrangement of semicircular canals
Function
Anterior, Posterior and Lateral
Detect angular acceleration around 3 orthogonal axes
Change in motion of head (kinetic) - SCC
Change in position of head with respect to gravity (static) - utricle & saccule (vestibule)
What contributes to the feeling weightlessness
Canaliths
Move away from hair cells to feel weightless
What does each inner ear of vestibular system contain
Utricle
Saccule
3 semicircular ducts
Contents of utricle and saccule
Function
Contain maculae
Cristae - in ampulla of each semicircular duct (head movement)
Head position
What is found in the vestibule
Hair cells
Where are hair cells found
Macula of utricle
Macula of saccule
Cristae ampullares of semicircular canals
Contents of membranous labyrinths of semicircular canals, utricle and saccule
What is found in each crista
Filled with endolymph
Hair cells - stereocilia (75-100) and kinocilium (1) of each hair cell in each crista
How are hair cells depolarised/hyperpolarised
Each vestibular hair cell is depolarised when sterocilia bend in direction of kinocilium
Hair cell hyperpolarised when deflected in opposite direction
Vertigo
Imbalance input from right and left sides
Layout of macula of saccule & utricle
What amplifies the force of deflection
Hair cells embedded in gelatinous matrix
Layers of crystals (otoliths) amplify force caused by deflection of stereocilia
Difference between otoliths and cristae
Cristae - semicircular canals
Otoliths - in macula of saccule and utricle
Utricle & saccule function
Changes in position of head with respect to gravity (static)
Detect linear acceleration
Info in relation to sense of gravity
Semicircular canals function
Changes in motion of head (kinetic)
Detect angular acceleration around 3 orthogonal axes
Where do axons of hair cells travel to
Ganglia (vestibular)
Vestibular pathway
Fibres enter anterior surface of brainstem
Nerve fibres divide into short ascending & long descending fibres
TRAVEL TO:
- cerebellum
- vestibulospinal tract (head & neck - medial and lateral)
- vestibulo-ocular reflexes (info in relation to movement of eye - 3rd 4th and 6th - connection to vestibulocochlear nerve)
- cerebral cortex (specifically primary sensory cortex - behind face, feeling of vertigo)
