Sound

Changes in air pressure that can be heard

One Sound Cycle

The distance between compressed patches of air

Sound Frequency

• Number of sound cycles per second (Hz)
• Primarily determines pitch
• Human Audible Range:20-20,000 Hz

Sound Intensity

• A Measure of the pressure of the compressed air
• Primarily determines loudness
• Measured in decibels
  • 0 dB is inaudible
  • 130 dB is painful

Outer Ear Structure

• Pinna: Part of the ear that we see
• Auditory Canal: ear hole

Middle Ear Structure

• Tympanic Membrane: AKA the ear drum
• Ossicles: Key purpose is to amplify the pressure so that sound can be detected

Ossicles

• Purpose is to amplify pressure (P=F/A)
  • Decreased Area from Tympanic Membrane to Oval Window
  • Increased force do to leaver action of ossicles
• Components:
  • Malleus: "Hammer"
  • Incus: "Anvil"
  • Stapes: "Stirrup"

Middle Ear/Ossicle Muscles

1. Tensor Tympani muscle
2. Stapedius muscle

• Both tense in response to sudden loud sound as a part of the attenuation reflex.
  • This reduces the conduction of sound

Structure of Cochlea

• Divided into three structures (two of which are connected)

1. Scala Vestibuli (Perilymph, connected to #3)
2. Scala Media (Endolymph)
3. Scala Tympani (perilymph, connected to #1)

helicotrema

• connects the scala vestibuli to the scala tympani

Perilymph

• Two of the three fluid sections are filled with perilymph (Scala Tympani and Scala Vestibuli)
• Has a high concentration of Na+
• Has a low concentration of K+

Endolymph

• Fluid that is inside of the Scala Media
• High in K+
• Low in Na+

Apex of Cochlea

• Located at the and of the cochlea
• The Basilar membrane is narrower
• Basilar membrane is not as stiff
• Low frequency waves propagate all the way to the apex (high frequency dissipate earlier)
  • Therefore, low frequency sound is registered in the apex 

Base of Cochlea

• located at the opening of the cochlea
• Basilar membrane is narrower at the base
• Basilar membrane is more ridges at the base
• Base of cochlea detects higher frequency vibrations 

Auditory Hair Cells

• Each hair cell has 100 cilia (kinocilia of mammals degenerate after birth)
• 3 rows of outer hair cells, one row of inner hair cells
• Hair cells are lodged between the basilar membrane and the reticular lamina 

Inner vs. Outer Hair cells

• Inner:
  • Most Auditory Information
  • One cell synapses onto 10 spiral ganglion
• Outer:
  • Multiple cells synapse onto 1 spiral ganglion cell
  • Sound amplifiers-Depolarization shortens cilia

Furosemide

• Eliminates outer hair cell amplification, by inactivating motor proteins which shorten cilia.
• (I don't fully understand this, or the amplification by shortening)

Tip Link

• Connects the stretch activated Ca+ channels at the end of adjacent cilia
• Provides the stretch the opens the channels when the cilia are bent.