BIOS1165 - Ear Anatomy

what is sound, and the 2 types of tones it has?

Sound is a series of pressure waves generated by vibrating air molecules

• Pure tones = sound wave with a single, constant frequency and a smooth sinusoidal waveform

• Complex tones = what is found in nature // any complex wave can be decomposed as a sum of sinusoidal waveforms (Fourier Analysis)

label:

what bone is the middle and inner ear housed?

the petrous temporal bone

what is the main function of the external ear?

collect sound waves = funnels them into the ear canal

what part of the ear is the auricle / pinna? what is its structure and its function?

Auricle (Pinna) > (like satellite dish > directs signal)
Structure	elastic cartilage directed slightly forward (~ 30º) attached to the facial skeleton and external auditory meatus
Functions	1) Sound collection: funnels sound waves into the ear canal 2) Sound localization: folds and ridges = location-specific reflections that help the brain determine a sound's origin (especially up vs. down)

what part of the ear is the external auditory meatus / ear canal? what is its structure and its function?

Structure	S-shaped tube (~2.5 cm long); outer 1/3 cartilage, inner 2/3 bone.
Functions	1)Protection: cerumen (earwax) protects the tympanic membrane. (Cerumen is protective and the canal is self-cleaning. Impacted cerumen is a common cause of treatable hearing loss.) 2) Natural amplification: The canal's shape and length create a natural resonance that boosts speech frequencies (~3 kHz)  Looked at using an otoscope

what part of the ear is the tympanic membrane / eardrum? what is its structure and its function?

Tympanic Membrane / eardrum = (gateway to middle ear)
Structure	slightly concave, thin (~0.1 mm thick, 8 to 10 mm ø), 3-layered membrane held in place by a cartilage ring Malleus (first middle ear bone) is attached at the center (umbo). A healthy, concave eardrum =  "cone of light" reflection. > Absence of this can indicate middle ear pathology
Function	1) sound (air pressure waves) → mechanical vibration.

what is the function of the middle ear?

Middle Ear

= amplifies sound (air field chamber)

1. Is an air filled cavity at the junction of squamous and petrous temporal bone

2. Main function: overcome the high impedance of inner ear (i.e air to fluid barrier)  by BOOSTING the signal 

what is otitis media?

1. Otitis Media: In children =  tube is shorter and more horizontal = increasing the risk of middle ear infections and fluid buildup (conductive hearing loss) = impact speech and language development.

label the middle ear:

what is the function of the eustachian tube?

Eustachian Tube: equalises pressure b/w middle ear and outside atmosphere

• Why your ear feels like its going to pop > yawning opens the Eustachian Tube

• connects middle ear to nasopharynx via Eustachian Tube

what are ossicles? their names? their functions?

Ossicles: suspended chain of 3 bones allows for large amplification of the vibrations Suspended from roof / Held in place by ligaments. Ossicles + reduced size of 2 membranes = 30-50 x increased strength of vibrations b/w tympanic membrane + oval window = easier to send vibrations in fluid (inner ear)
Malleus (hammer)	Att. to tympanic membrane by connective tissue = moves which causes other bones to move
Incus (anvil)	Bound to malleus = little movement  Has a synovial joint with stapes
Stapes (Stirrup)	Attached to oval window by annular ligament

what are 2 elements that increase sound pressure?

1. Difference in area of tympanic membrane (20x) compared to oval window = pressure concentrated in oval window = ↑ pressure (e.g thumbtack)

2. Ossicles arranged in lever action = different lever arm lengths (~ x1.3 increase pressure)

• Incudostapedial joint  = piston-like movement

label middle ear witht he muscles

what is the acoustic attenuation reflex? what are the muscles involved and their innervation? what is the function? and its limitation?

• Muscles = The stapedius (innervated by CN VII) and tensor tympani muscles (innervated by CN V3)

• Function: they contract in response to loud sounds > stiffens ossicular chain = ↓ sound transmission + protects inner ear

  • Helpful for continuous noise

• Limitation: latency (~40ms delay) = very slow reflex for nervous system = less likely to protect against sudden, loud, impulse noises (e.g gunshots)

what is the function of the inner ear?

 encodes sound

1. Function: mechanical vibrations → neural signals for brain (series of action potentials)

2. Contains heavy fluid

what is the bony labrinth?

A series of hard walled bony cavities, lies within the petrous part of the temporal bone

• lined by thin membrane that secretes a fluid called perilymph (low K+)

what is the membranous labrynith / cochlear duct? its function?

• suspended in the bony labyrinth

• contains the sensory organs of balance and hearing

• Filled with fluid endolymph (high K+)

Function

acts as a partition, separating the cochlea into three spaces:

1. Scala media = cochlear duct(Endolymph)

• Where the hair cells / sensory cells are found in the Organ of Corti

2. Scala vestibuli = space above the cochlear duct (filled with Perilymph).

3. Scala tympani = space below the cochlear duct (filled with Perilymph).

• The cochlear duct does not reach the apical end of the cochlea = creating an opening called Helicotrema, where the fluid of the scala vestibuli and scala tympani are continuous with each other.

• At the basal end of the scala vestibuli and scala tympani there are areas where the bone is absent around the cochlea, forming two windows, 

what is endocochlear potential?

Endocochlear Potential = 80mV

The difference between the two fluids (perilymph and endolymph) creates an electrical charge: the "battery" that powers the hair cells.

what is the vestibular system? its function?

= balance (from visual, proprioceptors - when eyes are closed, inner ears)

1. Vestibule

2. 3 Semicircular canals with different orientations 

3. Utricle + Saccule = tells us about linear acceleration (e.g in a lift going up) and gravity

Function: 

1) Sense of Equilibrium: body orientation and balance with respect to the placement of the head in space.

• information is used unconsciously to coordinate motor responses such as balance, posture and eye movements, though Vestibular reflexes and to the thalamus and cortex.

what is the cochear? its function?

= hearing

• mechanical signals (vibrations) → into electrical signals (neural impulses).

• Cochlea coils 2.5 times around the axis = modiolus (bony core). 

• 10mm in size, and 25mm when uncoiled.

what is scala media? scala vestibuli? scala tympani? helicotrema?

1. Scala media = cochlear duct(Endolymph)

• Where the hair cells / sensory cells are found in the Organ of Corti

2. Scala vestibuli = space above the cochlear duct (filled with Perilymph).

3. Scala tympani = space below the cochlear duct (filled with Perilymph).

• The cochlear duct does not reach the apical end of the cochlea = creating an opening called Helicotrema, where the fluid of the scala vestibuli and scala tympani are continuous with each other.

• At the basal end of the scala vestibuli and scala tympani there are areas where the bone is absent around the cochlea, forming two windows,

what is the process of auditory transduction?

1. Sound waves are directed by pinna > external auditory meatus 

2. Moves the tympanic membrane

3.  Movement of ossicles: Ossicle amplify pressure of waves = stronger energy = move oval window

4. Stapes pushes Oval window = pressure waves in perilymph fluid in inner ear

5. Moving perilymph = vestibular membrane to bulge = basilar membrane vibrates

6. Vibrating basilar membrane = hair cells move against tectorial membrane = change in membrane potential in hair cells = releases neurotransmitters to postsynaptic cells = electrical activity in CN VIII

why is scala vestibuli an important structure for transduction?

1. Scala Vestibuli is separated by:

• Reissner’s Membrane (b/w scala vestibuli and scala media)

• Basilar Membrane (b/w scala tympani and scala media)

  • wide + flexible near helicotrema // narrow + stiff near oval window

  • Each frequency activates a specific group of hair cell.

  • Cells near oval window = activated by higher frequencies (base)

  • cells near helicotrema = activated by lower frequencies (apex)

why is the organ of corti important for sound transduction? what is the function of the outer and inner hair cells?

1. Organ of Corti = where sensory hair cells (NOT neurons, but modified epithelial cells) are found

• sits ATOP the basilar membrane 

• Sensory Hair Cells:

  • They have the ability to convert stimulus → changed membrane potential → transmit signal to post synaptic cell

  • Stereocilia = tiny little hairs > embedded in the tectorial membrane (jelly like) > so when movement of basilar membrane = cilia moves too.

• 3 rows of outer hair cells = controls how much the signal is amplified

• 1 row of inner hair cells = will be stimulated > send information to brain

what is the innervation of the vestibular cochlear nerve (CNVIII)

• Delivers nerve impulse from inner ear → cochlear nuclei in brainstem → information ascends bilaterally in brain 

how are nutrients spread in the ear? what fluids are involved? where does the blood come from?

• A lot of energy required to maintain a high concentration of potassium as it has to be transported against the concentration gradient

• The inner ear's blood supply originates from the brain's circulation: linking hearing health = cardiovascular health.

• Perilymph fluid = supplies most of O2 to cochlea

  • Even 2 minutes of anoxia = stops K+ flow = malfunction of cochlea

what is conductive hearing loss? cause? treatment?

Meaning: sound waves DON’t reach inner ear

Cause: reduced transmission of air vibration to cochlea.

involve damage to external or middle ear:

• blockage of the external acoustic canal

• damage to the tympanic membrane

• fluid in the middle ear

• arthritic ossification of the ossicles

Treatment: amplification of the signal by hearing aids

wjat is sensorineural hearing loss? cause? treatment?

Cause: damage to the cochlea hair cells, vestibulocochlear nerve or auditory pathways (inner ear)

• transduction apparatus is affected

• Sound waves not processed correctly

Treatments: more complicated

what are the advantages of binaural hearing?

1. Binaural Summation: Intensity of the sound wave is increased + detects lower intensity

2. Localise sounds in space

   • Compares the information from the two ears > auditory processing regions in brain can localise signal

   • The brain devotes more attention to signals that are different in both ears

   • Uses interaural time differences = localises low-frequency sounds

   • Uses interaural intensity differences = localise high-frequency sounds

what causes monoaural hearing loss?

• nearly always due to damage to the ear or vestibulocochlear nerve 

• though selective damage to the cochlea nuclei will lead to loss of hearing predominantly from one ear