The ear and auditory pathways Flashcards
What sort of waves are sound waves?
Transverse (compressions and rarefactions)
What is the range of frequencies that can be heard by the human ear?
20-20,000Hz
Recall the 2 main structures of the external ear
- Pinna/auricle
2. External acoustic meatus (auditory canal)
What are the 2 main functions of the auricle?
- “Catching” sound waves
2. Indicating elevation of stimulus
What are the 2 main functions of the external acoustic meatus?
- Focus sound waves
2. Increase pressure at tympanic membrane
What is the other name given to the middle ear?
Tympanic cavity
Recall the 2 principle structures of the middle ear
- Tympanic membrane
2. Auditory ossicles (malleus, incus, stapeus)
What is the middle ear continuous with?
Eustacian tube
What is the purpose of the eustacian tube?
Links middle ear and nasopharynx
What are the 2 layers of the inner ear?
Bony labyrinth
Membranous labyrinth
Recall the constituent parts of the bony labyrinth
Vestibule
Semi-circular canals
Cochlea
What separates the bony and membranous labyrinths?
Perilymph
Which auditory ossicle directly connects to the TM?
Stapeus
Describe the basic structure of the cochlea
3 chambers: scala vestibuli/ media/ tympani
Basilar membrane between SM and ST
Organ of Corti on top of BM
Recall the location and composition of the different lymphatic fluids in the inner ear
Endolymph: scala media - high K+, low Na+
Perilymph: scala vestibuli and tympani - low K+, high Na+
What is the helicotrema?
Hole that allows passage of perilymph between SV and ST
Recall the composition of the organ of corti
Inner and outer hair cells, neurons
Where is the organ of corti located?
Between the basilar membrane and beneath the tectorial membrane
Recall the differences in distribution between inner and outer hair cells
Inner hair cells found alone, outer hair cells found in groups of 3
How does the function of inner and outer hair cells differ?
Inner = send afferents to brain Outer = receive efferents
Where are stereocilia located, and what is their function?
Inner hair cells: base in perilymph, respond to endolymph movement in SM
NO CONTACT WITH TM
Recall the contacts of the hair cells with the TM
Inner hair cells = no contact
Outer hair cells = in contact
Recall 3 functions of outer hair cells
- Receive efferents from brain
- Expand/ contract to amplift vibrations (due to electromotility)
- Control inner hair cells
What is the purpose of the auditory ossicles
Amplify pressure of sound waves in tympanic cavity so that they have enough energy to produce a pressure change in the fluid-filled inner ear
Recall the 2 mechanisms by which the amplitude of sound waves is augmented in the inner ear
- Auditory ossicles - stapeus vibration against superior oval window
- TM - vibrations focused from large SA of TM to small SA of sup. OW
Where is the round window located?
Just below superior oval window
What is the function of the round window?
Pressure release - moves outwards to equalise pressure when stapeus pushes oval window into cochlear
Summarise the pathway of sound transduction
Sound wave –> movement of fibres in BM –> movement of endolymph in SM –> inner hair cell change in polarity –> endocochlear potential
Describe the relationship between hair cell movement and polarity
Upwards: K+ influx from endolymph, depolarisation
Downwards: K+ channels close, hyperpolarisation
How is a concentration gradient maintained between the endolymph and perilymph?
Stria vascularis
Recall the afferent neuronal pathway from the cochlear to the brain
Spiral ganglion Ipsilateral cochlear nuclei Superior olivary nucleus Medial genticulate nucleus Auditory cortex
Explain the process of tonotropic mapping
Fibres longer as you go down BM
At base. short + stiff fibres respond to high frequency sounds. As you go towards end, longer and looser fibres detect lower frequencies
Generates a place code depending on time it takes to reflect sound
Which muscles control ossicle movement
Tensor tympani
Stapedius
Describe the auditory reflex
Muscles contrcat to reduce ossicle movement so that large noises cause less damage
What is the difference between conductive and sensorineural deafness
Conductive: ossicle destruction prevents amplification
Sensorineural: cochlea/ cochlear nerve damage
Recall 5 causes of conductive hearing loss
Wax Otitis media Otosclerosis of ossicles Perforation of TM Congenital
