Audition - Part 1

What are the main structures for the pathway from the peripheral to the central nervous system? (There are 7 structures covered)

External ear, middle ear, basilar membrane, receptor cells, auditory nerve, brainstem, cortex

What is the function of the external ear?

Gather/reflect and conduct sound, protect eardrum

What is the function of the middle ear?

Conducting/modifying sound, protect cochlea from loud sound

What is the function of the basilar membrane?

Filtering (frequency analysis)

What is the function of the receptor cells?

Transduction

What is the function of the auditory nerve?

Transmission (signal coding)

What is the function of the brainstem?

Complex coding, feature abstraction

What is the function of the cortex?

Feature abstraction, perception

What is the nature of the signal at each step?

External ear: Mechanical vibration

Middle ear: Mechanical vibration

Basilar Membrane: Mechanical hydro-dynamics

Receptor cells: Mechanical to electrochemical

Auditory nerve: Electrochemical

Brainstem: Electrochemical

Cortex: Electrochemical

Describe the change that occurs in the way sound travels once it goes from the external ear to the inner ear (Hint: impedance)

Sound is passing through fluid in the inner ear. Compared to air, there is much greater impedance in fluid (mechanical hydrodynamics). There is therefore a loss of energy (33 dB) from the impedance mismatch. In the middle ear, the sound is amplified to accommodate for this.

What is transduction in general and what is it specific to the auditory system?

In general, transduction is the conversion of mechanical signal to electrical signal. In the auditory system, it is when the receptor cells convert the incoming mechanical vibration to electrochemical signal via the MET channel at the top of the hair cells.

A) What is included in the peripheral auditory system and what is included in the central? (For now, you do not have to memorize). What is at the border?

PAS: Outer ear, middle ear, cochlea

Border: Auditory nerve

CAS: Brainstem, midbrain, thalamus, cortex

What are the shapes of the external, middle and inner ear?

External ear: S-shape

Middle ear: Cube

Inner ear: Snail shape

What is a unique feature of the auditory peripheral organ?

Structure features are closely associated with function.

What do the external, middle and inner ear contain?

External: Hair/wax

Middle: Bone chain, ligaments and muscles

Inner ear: Basilar membrane, hair cells, cochlear fluid, Organ of Corti, etc.

What are the 4 main functions of the external ear canal (briefly)?

Gathering/conducting sound, signal processing (via ear canal resonance), providing cues for sound localization, protecting eardrum against foreign bodies

Where does the ear lobe block more sound from (ie. which direction)?

From the back

What are examples of foreign bodies that the external ear canal provides protection from?

Glass from a car accident. Cotton from a Q-tip, tape from a hearing aid self-repair.

In general, what are the three main middle ear functions?

Conducting sound from eardrum to inner ear via bone chain, impedance match by amplification, protection against loud sounds

What is impedance matching? (Why does it occur and what is the ME's role)

It occurs due to the change in media through which the sound is traveling between the external ear (air) and inner ear (fluid). There is a greater impedance in fluid, which causes an energy loss of 33 dB. The role of the ME is to compensate for this loss via amplification.

How many dB is lost in this match?

33 dB

What is the difference in what the middle ear is protecting compared to the external ear canal?

The middle ear is protecting the inner ear from damage by loud sound via the ME acoustic reflex. The external ear canal is protecting the tympanic membrane from physical damage by foreign bodies.

Briefly describe (based on what was said in lecture) what the eustachian tube is balancing? What happens if it is unbalanced?

The eustachian tube is balancing air pressure between the middle ear and the outside world. If it is unbalanced, i.e., if negative pressure develops in the middle ear space, it could cause the TM to retract and the possibility of fluid developing behind the TM. This is what typically causes ear infections.

What are the 4 functions of the cochlea (for hearing) briefly/point form?

Converting acoustic signal to biochemical-electrical signals

Frequency processing

Encoding sound features to action potentials in the auditory nerve

Feedback control

What occurs in encoding: what is the signal being processed from and to?

Note: transmission and encoding are very closely related but they're not exactly the same.

Encoding: the sound features are being converted to something that can be "understood" and processed by the auditory nerve (i.e., action potentials).

Transmission: The receptor potential is converted to the action potential.

What is the difference between an action potential and a receptor potential?

The main difference between an action potential and a receptor potential is that the action potential carries that signal (communicates) up to the brain however the receptor potential is limited to the sensory cells (in this case, the hair cells).

What are the three steps of signal processing in an auditory peripheral organ?

Conduction, transduction, transmission

What are the steps in conduction?

External sound is collected by the earlobe. It then travels along the outer ear, middle ear (it is modified here), and inner ear (via basilar membrane vibration) to stimulate the hair cells.

What are the steps in transduction?

Mechanical signal is converted to electrical signal (receptor potential) at the top of the hair cells (via the MET chanel).

What are the steps in transmission? What kind of process is this?

The receptor potential is converted to action potentials across the synapses between the hair cells and the spiral ganglion neurons (more on this later). This is an electrochemical process.

What are the 4 major jobs of the central auditory system?

Encoding of sound features

Transmission

Signal processing

Decoding

What are the three major differences the central auditory system has compared to the peripheral system?

Diversity and segregation

Nuclei and station

Complexity in organization from low to high level

What is the MAJOR difference between the central and peripheral auditory systems?

The major difference between the central auditory system (CAS) vs the peripheral auditory system (PAS) is the complexity. This helps understand why we call it higher level processing as you move up to the cortex as the task and functions becomes more complex and a lot of the analysis and integration occurs.

What are the 4 main parts of the temporal bone?

Mastoid, squamous section, tympanic bone, petrous section

Which parts of the temporal bones are visible from both the lateral and medial view?

Mastoid, squamous

Which part of the temporal bone is lateral only?

Tympanic

Which part of the temporal bone is medial only?

Petrous

Which part of the temporal bone is the largest section?

Squamous

Which part of the temporal bone is the smallest section?

Tympanic

What is partially responsible for the middle ear resonance?

The air cells that are connected to the middle ear

Why is the mastoid an ideal surface to place the electrode and bone vibrator?

Short distance to inner ear and brainstem

Relatively thin soft tissue, so less attenuation for bone conduction and smaller impedance to electrical signal

Inertia effect for bone conduction

What are two examples of infections that occur at the mastoid? Why is it an easy target?

Mastoiditis and Cholesteatoma. Due to its location and proximity to the middle ear (remember, it has air cells that are connected to the middle ear), it is relatively easy for otitis media to spread to it and infect the air cells.

With reference to the ear canal, how can squamous section be described? (location-wise)

The squamous section forms the roof of the ear canal

With reference to the ear canal, how can the tympanic bone be described (location-wise)?

The tympanic bone forms the floor and walls of the ear canal

What is another name for external auditory meatus?

External ear canal

What is housed in the petrous section?

The sensory organs. That is, the cochlea, the semicircular canals, and the vestibule

What fraction and which location (lateral/medial) is cartilage versus bony composition in the canal?

The lateral 1/3 is cartilage, and the medial 2/3 is bone

What seals the ear canal?

The tympanic membrane

Describe 5 traits of the earlobe

Composed of fibrous cartilage

Covered with cutaneous tissue

No fat under the skin

Rich in blood supply

3 auricular muscles

What was the difference between animal and human pinnae?

Humans can't move their pinnae to help with sound localization like many other mammals can

What are two practical uses of knowing the landmarks?

For reconstruction surgery purposes for ENTS, to make earmolds for hearing aid fittings, to communicate with the HA manufacturers

How is the pinna useful for sound localization (briefly)?

The pinna will reflect the sound differently depending on where it's coming from, thus providing us with cues for sound localization

What are two traits of the general shape of the external canal (external auditory meatus)?

S-shape with one end sealed

What kind of tissue lines the external ear canal?

Cutaneous tissue

What is gain? What is it related to the ear?

In general, gain is the ratio (difference) between the output and input of a system. Gain = output/input

Related to the ear, gain shows the change in signal amplitude between what is entering the ear medially and what is entering the pinna.

What gain ratio shows attenuation and what shows amplification?

Gain of less than 1 is attenuated and greater than 1 it is amplified

What does the pinna do?

The pinna gathers and funnels sound to the ear canal (conduction). It also acts as an acoustic obstacle—its irregular surface alters the frequency and amplitude of sounds coming from different directions.

What effect does the irregular surface of the pinna have?

Filtering and reflection. Altering the frequency amplitude of sound coming from different directions