Module 8 - The Ear Flashcards
What are the two functions of the ear?
- sound detection (hearing)
2. maintenance of equilibrium (balance) and orientation
How is sound the vibration of a substance?
- the fact that you can detect tiny movements in the vocal cords of someone talking is because their vocal cords and your ears are physically connected by gas -AIR
- water and solids also conduct sounds
- it is the elastic movement of the medium that we call sound
- if we lived in a vacuum, sound would not exist
What is the nature of sound?
- sound is a pressure disturbance
- a vibrating object alternatively compresses and rarefies (pulls the molecules apart) the air - a pressure vibration
So what is hearing in one sentence?
Sounds set up VIBRATIONS in the air that beat against the EARDRUM that pushes a chain of TINY BONES that press FLUID in the internal ear against MEMBRANES that set up shearing forces that pull on TINY HAIRS that stimulate nearby NEURONS that give rise to impulses that travel to the BRAIN, which interprets them - and that is how you hear
What are the 3 parts of the human ear?
External
- pinna and external auditory meatus
- funnels sound toward the tympanic membrane
Middle
- air-filled cavity containing auditory bones
- relay vibrations to the inner ear
Inner
- fluid-filled canals: labyrinth = cochlea and semicircular
- contains the receptor organ of hearing
- contains the organ of balance and orientation
How do the external ear and middle ear function?
- airborne sound vibrates the tympanic membrane (eardrum)
- vibrations are relayed to the auditory ossicles - 3 tiny bones in the middle ear
- ossicles transfer this vibration to the inner ear via the oval window
How does the middle ear - ossicles, eustachian tube and muscles - work?
Eustachian tube: connects to the nasopharynx to equalize pressure with the atmosphere
Tensor tympani and stapedius muscles contract to protect the inner ear from loud noises
How does the inner ear work?
semicircular ducts in semicircular canals control the balance
cochlear nerve and cochlear duct in cochlea control the hearing
What is the anatomy of the cochlea?
- thhink of the cochlea as a spiraling tunnel inside the temporal bone - the boney labyrinth
- inside is a series of fluid filled membranes - the membranous labyrinth
- the cochlea contains the machinery that converts mechanical sound waves into electrical nerve impulses
What are the 3 fluid filled chambers that the cochlear contains?
the two outer canals of the membranous labyrinth are:
scala vestibuli
scala timpani
-essentially the same tube, joined at the end (helicotrema)
-contains perilymph - similar to extracellular fluid
in between lies the cochlear duct containing endolymph
-endolymph is similar to intracellular fluid, with an even higher K+ concentration compared to within most cells
What is the organ of corti?
the organ of corti in the cochlear is the place where sound is converted into nerve impulses
What is the pathway of sound waves and movement of the basilar membrane?
movement of the basilar membrane is the first step in turning mechanical sound vibrations into electrical impulses
different sound frequencies cross the basilar membrane at different locations:
- high frequency sounds displace the basilar membrane near the base
- medium frequency sounds displace the basilar membrane near the middle
- low frequency sounds displace the basilar membrane near the apex
Fibers of the basilar membrane are short and stiff near the base and get longer and floppier at the apex
How does sound vibration turn into electrical impulses?
inner hair cells - are supplied by afferent nerve fibres of the cochlear nerve. They are the sound receptors. Their “hairs” or stereocilia bend as the basilar membrane vibrates, by interacting with the tectoral membrane. This bending generates graded membrane potentials in the hair cell.
outer hair cells - are supplied by efferent nerve fibres of the cochlear nerve and attach directly to the tectoral membrane. They are motile and mechanically amplify the sound vibration
What happens when the sterocilia bend?
Bending of stereocilia open of close mechanically gated K+ channels in hair cells which varies the flow of K+ into the cell, producing graded potentials
HYPERPOLARIZATION:
K+ channel closed
–>decreased neurotransmitter release
–>decreased rate of AP in cochlear nerve
DEPOLARIZATION:
K+ channel open
–>increased neurotransmitter release
–>increased rate of AP in cochlear nerve
What is the pathway of sound waves and resonance of the basilar membrane?
- sound waves vibrate the tympanic membrane
- auditory ossicles vibrate, pressure is amplified
- pressure waves created by the stapes pushing on the oval window move through the fluid in the scala vestibuli
- sounds with frequencies bwlow hearing travel through the helicotrema and do not excite hair cells
- sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells
