Hearing: Auditory System

What does an ear have to do?

1. Detect the presence of biologically meaningful sounds

2. Characterize the sound structure for future reference

3. Done through frequency (pitch), loudness, and location (source)

What are the components of sound?

1. Particle motion and pressure waves

1. Tip links on ____________ get stretch-activated ion channels.

1. Hair cell stereocilia

Hair cells are directional (_________): stimulated by movement in a specific direction

Polarized

1. Vm changes more when hair bundle pushed toward _______ stereovilli (0 deg).

Tallest

Hair cell can be slightly hyperpolarized when…?

Bundle pushed backward

_______ waves continuously flow in and out of cochlea.

1. Sound pressure 

   1. Reason why cochlea has 3 chambers

1. What does the outer ear consist of?

1. Pinna

1. What does the middle ear consist of?

1. Malleus, incus, stapes

1. What does the inner ear consist of?

Cochlea

What happens to the basilar membrane as the sound travels up and down the cochlea?

It vibrates

1. What is the sensory apparatus at center of cochlea and atop the basilar membrane?

Organ of Corti

What do deflections of the basilar membrane cause?

1. The hair cell stereocilia to bend when pressed against the tectorial membrane

What initiates auditory transduction?

1. Traveling sound waves

1. How does the ear encode frequency?

1. Frequency to place transformation

   1. Tuning differences based on position in the cochlea

      1. Low frequency sounds cause greatest vibration at apex

      2. High frequency sounds greatest at the base

What is the cochlear amplifier?

1. Outer hair cells contract to increase vibrations in response to quiet sounds

Outer HC contractions increase the magnitude of vibrations, thereby…?

1. Increasing sensitivity to quiet sounds

2. Same contractions can reduce vibrations at very loud sound levels, protecting inner hair cells from overstimulation

What can spontaneous contractions cause?

Tinnitus

1. What is outer hair cell contraction caused by?

A special membrane protein called “Prestin”

1. What is the ascending auditory pathway?

1. Cochlea

2. Cochlear nucleus

   1. Cell bodies of axons projecting to cochlea

3. Superior Olivary Complex

   1. Sound source localization

      1. LSO - IID

      2. MSO - ITD

4. Inferior colliculus

   1. Midbrian center for reconstructing complex sounds

5. MGN of Thalamus

   1. Relay to cortex

6. Primary Auditory Cortex (A1)

   1. Sound sequences and patterns

What is a group of structure in brainstem that calculate where sound came from. Includes the MSO, LSO + MNTB?

1. Superior Olivary Complex

1. What part of the Superior Olivary Complex analyze interaural time differences (ITDs)?

1. MSO

What part of the Superior Olivary Complex analyze interaural intensity differences (IIDs)?

LSO

1. How does the MSO compute the location of sound from interaural time differences?

1. Sound reaches left ear first

2. Action potential begins traveling toward MSO through a delay line

3. Sound reaches right ear a little later

4. Action potential from right ear begins traveling toward MSO

5. Action potentials converge on an MSO neurons that respond most strongly if their arrival is coincident

How does the LSO neurons encode sound location through interaural intensity differences?

1. Stronger stimulus to left ear excited left LSO

2. This stimulus also inhibits right LSO via MNTB interneuron

3. Excitation from left side is greater than inhibition from right side, resulting in net excitation to higher centers

4. Inhibition from left side is greater than excitation from right side, resulting in net inhibition on right and no signal to higher centers

Describe the two tone suppression in auditory system?

1. Sound falling in inhibitory regions suppress responses in excitatory regions

2. Arises from lateral inhibition in ascending auditory pathway (in inferior colliculus) and serves to sharpen receptive fields

What is the frequency of sound?

1. The number of compressed or rarefied patches of air that pass by our ear each second.

What is the intensity of sound?

1. The air pressure difference between peaks and troughs of the sound waves.

2. High intensity waves are louder

What is the pinna?

1. The visible portion of the ear consists primarily of cartilage covered by skin, forming a sort of funnel

What does the 1st stages of the basic auditory pathway look like?

1. Sound wave moves through the tympanic membrane

2. Tympanic membrane moves the ossicles

3. Ossicles move the membrane at the oval window

4. Motion at the oval window moves fluid in the cochlea

5. Movement of fluid in the cochlea causes a response in sensory neurons

Once a neural response to sound is generated in the inner ear, the signal is transferred to and processed by a series of nuclei. Where is the output sent?

1. To a relay in thalamus, the medial geniculate nucleus

2. Projects to primary auditory cortex or A1 in temporal lobe

In what sense are the sensory systems alike?

1. Starts with sensory receptors, which connect to early integration stages (retina for vision and brain stem for audition), then to a thalamic relay, and then to sensory cortex.

1. What are the names of the ossicles?

1. Hammer

   1. Attached to the tympanic membrane 

   2. Forms a rigid connection with the incus

2. Incus

   1. Forms flexible connection with the stapes

3. Staples

1. Describe the movement of the inner ear as air pushed the tympanic membrane.

1. The bottom of the malleus is pushed inward and the lever action of the ossicles makes the footplate of the stapes push inward at the oval window.

2. The pressure pushing at the oval window is greater than that at the tympanic membrane because the surface area of the footplate of the stapes is ambler than the surface area of the tympanic membrane

1. As long as the valve on the Eustachian tube is close (ascending elevation, surrounding air pressure decreases), the air in the middle ear…?

1. Stays at the pressure of the air before you started to climb 

2. Because air in the middle ear is higher than the air pressure outside, the tympanic membrane bulges out, causing discomfort

3. Yawning and swallowing open eustachian tube

The pressure at the oval window will become greater than the pressure at the ear drum if?

1. The force on the oval window membrane is greater than that on the tympanic membrane

2. The surface area of the oval window is smaller than the area of the tympanic membrane

1. What is the attenuation reflex?

1. The onset of loud sound triggers a neural response that causes these muscles to contract

2. Sound attenuation is much greater at low frequencies than at higher frequencies

What are the properties of the auditory pathway?

1. Projections extend broader than those listed.Inferior colliculus sends axons not only to the MGN but also to the superior colliculus (where the integration of auditory and visual information occurs) and to the cerebellum

2. Extensive feedback; brain stem neurons send axons that contact outer hair cells, and auditory cortex send axons to the MGN and inferior colliculus

3. Each cochlear nucleus receives input from just one ear on the ipsilateral side; all other auditory nuclei in the brain stem receive input from both ears,

What is the characteristic frequency?

1. The neuron is most responsive to sound at one frequency