Chapter 11:

  • Sound: physical perception, compression of molecules in the area, a sound wave which gets transduced into the neurobiology that allows us to have the sensation of hearing

Properties of sound waves

  • A waveform: Includes an amplitude (difference between baseline and peak (up or down)) and the frequency (how often it repeats itself every second, Hertz)
  • Ability to perceive sounds 20-20,000 Hz (dog whistle is above this)
  • Frequency gives us a measure of the pitch of the sound wave and amplitude gives us a measure of loudness
    • More often/frequent = higher pitch, higher amplitude = louder
  • Most sounds are not as simple as the graphs shown - actually complex waves, very rare that we hear a pure tone

Complex Tones

  • Fundamental frequency (how often repeat/sec) - directly related to pitch, higher frequency = higher pitch

Neurobiology again

  • Outer ear = pinna captures sound waves to go into the ear canal
  • Ear canal (strongest bones in the body)
    • Has membrane = tympanic membrane, eardrum
    • At the tympanic membrane, any sound is converted into vibration (transduction)
    • Connected to the membrane are three small bones: Hammer, anvil, stapes or stapes, malleus, and incus
  • When the membrane vibrates it causes a mechanical movement of those three small bones against one another
  • The cochlea (shaped like a conch shell) has a basal membrane inside, which has 3 different layers of hair cells (covered in fluid)
  • Sound waves enter the pinna, enter the ear canal, make contact with the tympanic membrane, and are converted into vibrations which cause the mechanical movement of 3 small bones against one another, then the mechanical movement of the small bones causes ripples in the movement of the liquid in the basal membrane of the cochlea which causes the hair cells in the membrane to bend, this bending opens stretch-dependent sodium channels which causes depolarization and if the sound if sufficient magnitude then threshold will be reached and an action potential occurs
  • Figure 11.11
  • In the broad range of frequencies that we have the ability to perceive, how is that modeled in the auditory nerve?
  • Sound wave of a certain frequency causes a certain amount of vibration and so on
  • Neurons have an upper limit on how fast a neuron can fire
  • At very low-frequency sounds, we have phase locking where the number of action potentials just mirror the frequency of the sound wave but we quickly exhaust this

History

  • Layer people in depth so some have time to reload, a volley, and others are able to defend
  • Volley puts fewer bullets out at any moment but produced a constant stream of bullets
  • For medium-frequency sound waves, the auditory nerve works on the volley principle
  • Individual axons
  • Nerve fires and then go into the relative refractory period, next fires and refractory period, and then the next, by this time the first nerve has recovered
  • We are able to perceive sound waves that even the volley principle doesn’t allow us to - different locations (high-frequency sound waves activate different locations on the basilar membrane)
  • Difficulties: begin to lose or damage hair cells on membrane over time due to aging (males suffer more than females as they lose their hearing faster)

Pathway continued

  • Superior olive, bilaterally distributed
  • Comes after the auditory nerve
  • Differential projections to the olive
  • Bifurcating pathway (one part by the closest, ipsilateral, and one crosses the midline, contralateral)
  • Lines should curve, bend, and concave to show EPSP
  • A neuron is either excitatory or inhibitory
  • If it has multiple projections then it is still either one or the other (cannot be both)
  • Interneuron - functions in one location
  • There is an interneuron between the neuron and superior olive on the contralateral side that converts action potentials to fewer actions potentials (inverts the electrophysiological signal that comes in)
  • When neurons stop acting on inhibitory, activity increases
  • Identify locations partially based on loudness, and magnitude represented by the number of action potentials
  • Allows us to locate sounds better by artificially magnifying the difference
  • Olives go to inferior colliculus which orients behaviors to sudden behaviors in your auditory environment
  • From inferior colliculus it goes to the medial geniculate nucleus of the thalamus and then to the auditory cortex

Localize sound

  • Time of arrival of the sound
    • If sound is directly on our midline (facing forward) then it is difficult to location
    • Which ear does the sound enter into first
  • Intensity
    • Which ear is the sound louder at
    • Amplitude changes, related to how loud the sound is
    • Another neuroanatomical structure that contributes to loudness is the auditory pathway (olives and such)

Review

3 different process to perceive or understand the frequency of sound

  • (low) phase locking: the neuron firing action potentisals is locked to the frequency of the sound wave (1-1)
    • (medium) volley principle: (frequency greater than neuron can fires, then neuron replicates parts of it through different neurons firing but when it is summed it shows the full picture) first neuron fires, then absolute refractory period, second fires, absolute regractory period, first goes into relative refraactory period, third fires
  • (high) place theory: super high frequency sounds activate different parts of the basalar membrane
  • Men and older individuals loose high frequency hearing first and occupation plays a role (people who work in high volume locations loose hair cells and high frequency hearing)