Anatomy + Physiology of Ear (textbook) Flashcards
Name 3 regions of ear and their basic function
External ear (collects and directs sound waves towards middle ear), middle ear (chamber in petrous part of temporal bone, collect sound waves and transmit to inner ear), inner ear (contains sensory organs for hearing + balance)
Describe anatomical features of external ear
Auricle/ pinna surrounding external acoustic meatus (ending at tympanic membrane)
- protection + directional sensitivity
Ceruminous glands secrete wax
Hairs
- prevent entry of foreign objects + slows growth of bacteria
Describe anatomical features of middle ear
- Filled with air
- Communicates with nasopharynx through auditory tube (equalisation of pressures inside/outside tympanic membrane, can allow invasion of bacteria - otitis media) + mastoid air cells through small connections
- Auditory ossicles - malleus, incus, stapes
- act as levers, convert sound waves into mechanical movements, collects force on tympanic membrane + focuses it on oval window (opening in bone that surround inner ear) – amplification
Muscles protect tympanic membrane + ossicles from violent movements due to loud noises
- Tensor tympani (petrous part of temporal bone to handle of malleus) - contracts + stiffens tympanic membrane, limiting movement, innervated by motor fibres of mandibular branch of trigeminal nerve (V)
- Stapedius (post. wall of middle ear to stapes) - contracts + reduces movement of stapes innervated by facial nerve (VII)
Describe anatomical features of inner ear
Receptors (hair cells) lie within membranous labyrinth containing endolymph, surrounded by bony labyrinth, with perilymph between
Bony labyrinth - vestibule (utricle + saccule - membranous sacs) and 3 SC canals and cochlea
*Utricle + saccule - gravity + linear acceleration
SC canals - rotation of head
* complex of vestibule + SC canals - vestibular complex
Cochlea - contains cochlear duct (endolymph) between vestibular + tympanic duct - (perilymph) - hair cells provide sense of hearing
- fluid waves
Round window at end of tympanic duct , separating middle ear
Describe the receptors of the inner ear
- Hair cells
- Surrounded by supporting cells + structures that determine which stimulus will provide the force to deflect kinocilia + stereocilia
- Monitored by sensory afferent fibres
- Stereocilia + single kinocilium
- External force pushes processes, distortion of membrane allows influx of ions, depolarisation, AP fired along neuron
- Gravity + linear acc - vestibule
Rotation in SC canals
Sound in cochlea
Describe the features of the SC ducts
- Anterior, posterior, lateral SC ducts
- continuous with utricle
- Each duct contains a ampulla, which contains hair cells - forms raised structure called cristae
- Kinocilia + stereocilia embedded in gelatinous cupula
- When your head rotates in plane of duct, movement of endolymph along SC duct pushes cupula, distorts stereocilia
- Movement of fluid in one direction stimulates, in other direction inhibits
- Fluid stops moving, elastic nature of cupula makes it bounce back to normal
- Anterior duct - nodding yes
- Posterior duct - tilting head side to side
- Lateral duct - shaking head no
Describe features of vestibule
- Utricle + saccule connected by endolymphatic duct + endolymphatic sac that projects into subdural space to return excess fluid to circulation
- Hair cells clustered in maculae within ampulla
- Stereocilia embedded in gelatinous mass, surface of which covered by statoconia (complex as a whole called otolith)
- When head tilted, gravity pulls on statoconia to side, distorting hair cell processes, tell CNS head is not level
- When accelerating (e.g. in car), statoconia lag behind, effect similar to tilting head back
- NS uses visual cues to distinguish between tilting and linear acceleration
Describe the NS pathways for equilibrium sensations
- Hair cells monitored by sensory neurons located in adjacent vestibular ganglia
- These fibres form vestibular branch of vestibulocochlear nerve (VIII)
- Travel to vestibular nuclei between pons + medulla
Functions of vestibular nuclei:
- integrate info about balance from both sides of head
- relay info from vestibular complex to cerebellum
- relay info from vestibular complex to cerebral cortex, providing conscious awareness of head position + movement
- sending commands to motor nuclei in brain stem and spinal cord
- Reflexive motor commands issued by vestibular nuclei to motor nuclei for cranial nerves involved in eye, head + neck movements (III, IV, VI, XI)
- Descend in vestibulospinal tracts
- Automatic response of eye in response to motion directed by superior colliculus of mesencephalon - attempt to keep gaze focused on specific point in space, when body spins, eyes jump (if brain stem or inner ear damaged, this can happen when body stationary, called nystagmus)
What eye sign shows that the brain stem or inner ear is damaged?
Automatic response of eye in response to motion directed by superior colliculus of mesencephalon
- attempt to keep gaze focused on specific point in space, when body spins, eyes jump (if brain stem or inner ear damaged, this can happen when body stationary, called nystagmus)
How is frequency of a sound determined by cochlea?
Determined by which part of duct is stimulated
- Pressure waves in perilymph distort basilar membrane
- location of max distortion varies with frequency due to regional differences in flexibility of basilar membrane (high, short wavelength, vibrate membrane near oval window)
- thus, frequency info translated into info about position
How is loudness determined by cochlea?
Determined by how many hair cells are stimulated
- louder the sound, the more energy carried in sound wave, more movement at tympanic membrane + ossicles
- the louder the sound, the more the basilar membrane moves (+ stereocilia)
-
What is resonance?
With the right combination of frequency + intensity of sound, object (tympanic membrane) will vibrate at same freq. of sound
- need resonance in order to hear
Describe the features of the cochlea.
- Cochlear duct (containing endolymph) lies between vestibular duct + tympanic duct (containing perilymph)
- outer surfaces of these ducts surrounded by bony labyrinth except at oval window (base of v duct) and round window (base of t duct)
- interconnected at tip of cochlear spiral
- Hair cells in cochlear duct within Organ of Corti
- this organ sits on basilar membrane (separates cochlear + tympanic duct)
- hair cells in longitudinal rows, lack kinocilia, stereocilia in contact with tectorial membrane
-when basilar membrane bounces up + down due to pressure waves in perilymph, stereocilia pressed against tectorial membrane + distort
-
What happens if a sound is too loud?
Cause sensorineural deafness
- break stereocilia off
- protective reflex contraction of tensor tympani + stapedius may not be fast enough to prevent damage
Which drug can cause sensorineural deafness?
Aminoglycoside antibiotics (gentamicin) - diffuse into endolymph + kill hair cells
Hair cells can also be killed by infection so therefore risk vs benefit
