Audition Chapter 26

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Audition Chapter 26

• March 9, 2004

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Topics to be discussed

• The Basics Physiology of the Auditory system
• The Mechanics of this sensory system
• Current studies and research pertaining to the
  Auditory system

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Physiology

• The Periphery auditory system is divided into the
  External ear, the Middle ear, and the Inner ear
• They are all interconnected in that, what occurs
  in one section influences what happens in the
  next
• This system functions with the use of
  mechanoreceptor mechanisms

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Physiology

• As with other systems based on mechanoreceptor,
  hair cells allow for the conduction of sensory
  input into the body
• The displacement of these cells begins the
  cascade of events that eventually lead to a
  useful signal being transmitted to the brain
• These receptor cells and other supporting cells
  are housed in the Organ of Corti
• Two types of hair cells are found and named
  either inner or outer hair cells depending on
  where they are found along the cochlear spiral
  and exist in a ratio of about 14 respectively
  (important for later)
• Outer cells are considered to be cochlear
  amplifiers of basilar membrane motion
• (We will discuss the function of inner cells in
  just a moment)
• Motion of the basilar membrane is sensitive in
  its responds to very low sound frequencies and
  sharply tuned to various sound frequencies

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The Basics

• Blood flow to the Inner ear area is reduced in
  order to prevent additional noise associated
  with the functionality of the region
• This reduced need for blood supply is
  accomplished with the use of Na and K gradients
  in and around the hair cells
• These chemical/electrical gradients allow the
  hair cells to be functional without much
  expenditure of energy and thus the reduced need
  for blood flow

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The Basics cont.

• Endolymph is a specialized fluid unique to the
  inner ear, with a low concentration of Na and a
  high concentration of K that has a positive
  electrical potential of about 90mV
• This fluid is contained within three compartment
  in the Inner Ear known as the scalea scala
  tympani, scala media, and scala vestibuli
• Steriocilia are surrounded by endolymph while the
  rest of the hair cells are surrounded by
  perilymph
• Endolymph increases the sensitivity of the hair
  cells it surrounds by increasing the transduction
  current

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• Two types of afferent neurons separately
  innervate the inner and outer hair cells Type I
  and Type II
• Ganglian cells of the spiral ganglion in the
  cochlea provide this innervation- their central
  axons form the Auditory nerve
• They receive synapses from inner and outer hair
  cells respectively
• Fibers from type I are larger and mylinated while
  type II fibers are much thinner and unmylinated
• Type I neurons are associated with the inner hair
  cells which as we know from earlier are
  outnumbered by the outer hair cells 14, but type
  I neurons total about 95 of the afferent
  population
• This translates into Outer hair cells being
  innervated by only a very few afferent neurons
• So we have come to the function of inner hair
  cells
• They are the main channel for sound information
  flowing into the brain

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• There are also two types of neurons responsible
  for efferent innervation lateral OC and medial
  OC neurons
• These are found in the Superior Olivary Complex
  thus the OC of their name
• Lateral OC neurons innervate type I dendrites
  near inner hair cells while medial OC neurons
  innervate outer hair cells
• The lateral neurons are distributed ipsilaterally
  and the media lneurons are distributed
  bilaterally to the innervated cochlea

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Whats really going on in there?

• Sound is determined by the amplitude and
  frequency of its wave.
• The pressure that these waves exude on our
  auditory sensory organs it what we perceive as
  sound
• The range of which the human ear is capable of
  sensing and perceiving is between 0dB and 120dB
• The frequencies that we are sensitive to range
  from about 20 to 20,000Hz with human speech
  falling in the range between about .25 and 3kHz
• The response that occurs once we encounter a
  sound wave is tuned to the frequency of that
  stimuli
• Our Auditory nerve fibers transmit sound waves as
  discrete action potentials to the brain
• Once the brain receives this signal it has to
  decipher the message encoded by the spikes
  (action potentials) presented

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• Each individual auditory nerve fibers response
  increases with the sound level until it is
  saturated and can no longer increase
• In general this increase is between 20 and 30 dB
  with a little increased variation in a few fibers
• This then brings rise to the question How can
  the Auditory nerve signal the range in level of
  audible sound from 0 to 100dB?

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The answer is as follows

• 1. As the level of a given tone increases,
  progressively more fibers that are tuned to other
  characteristic frequencies (CFs) begin to respond
  to the presented frequency because tuning curves
  become broader at higher sound levels
• 2. Auditory fibers vary in sensitivity and as
  sound level increases, the less sensitive fibers
  begin to respond (This is correlated with the
  rate of spontaneous firing which is the rate when
  the stimulus is outside of the tuning curve)
• So in more plain English The collective effort
  of each fiber allows for the expanded range

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From the Inside Out

• Efferent neurons send information from the brain
  to the periphery (External, middle, and Inner
  Ear) with three systems
• 1. Olivocochlear efferents send fibers to the
  organ of Corti
• 2. Middle ear muscle motorneurons send fibers
  to the middle ear muscles
• 3. Inner ear sympathetics send fibers to
  cochlear blood vessels and other targets (little
  is known about the function of this system)

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Olivocochlear Efferents

• Thusly named because they have cell bodies in the
  superior olivary complex of the brain stem and
  project to the cochlea
• There are two groups of OC neurons which we
  discussed earlier
• Activation of these neurons by electrical
  stimulus causes the release of acetylcholine
• Acetylcholine acts as a nicotine receptor at the
  outer hair cells, allowing Ca 2 activated K
  channels to efflux K hyperpolarizing the cell
• This reduces the electromotility of the outer
  hair cell, decreases basilar membrane motion, and
  reduces the responses of inner hair cells and
  auditory nerve fibers
• These decreases collectively shift the responses
  of the auditory nerve fibers to higher sound
  levels preventing or decreasing instances of
  saturation of response
• So the functions of these neurons are to shift
  the gain of the cochlear amplifier, anti-mask,
  and protect the cochlea from damage due to
  intense sound

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Middle ear muscle motorneurons

• Two muscles are attached by tendons to the middle
  ear ossicles
• 1. The Tensor tympani, which is innervated by
  motorneurons from the Vth cranial nerve, is
  connected to the malleus
• 2. The Stapedius is innervated by the
  motorneurons from the VIIth cranial nerve and is
  connected to the stapes
• Both of these muscles are abundantly innervated
  by motorneurons having almost one per muscle
  fiber (need for high degree of control)
• The function of this system is thought to be to
  prevent damage due to intense sounds and to
  attenuate low frequencies in order to prevent
  them from masking higher frequencies (this has
  implications in speech discrimination in a noisy
  environment) Also this may be responsible for
  decreasing response to self generated stimuli ie
  your own voice

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Yet more Work to be Done

• In addition to protecting us from sensory
  overload, the Auditory system also has the task
  of deciphering from which direction an auditory
  input is coming
• And, what signals go with what stimuli
• In order to determine where in space a sound
  source is coming from, the auditory brain stem
  uses binaural cues
• This occurs when cues from both ears are timed
  against one another to determine a locality (two
  types of cues are used)
• 1. Interaural time differences (ITDs)
• 2. Interaural level differences (ILDs)

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Yet more Work to be Done cont.

• ITDs result because sound reaches the ear it is
  closest to sooner than the ear that is farther
  away (this is eventually componsated for in the
  Superior Olivary complex)
• ILDs occur when the head forms a sound shadow,
  which reduces the level of sound that the ear
  away from the source receives
• Which cue is more important in any given instance
  is determined by the frequency of the cue ILDs
  are more important when the frequency of the
  sound is high. ITDs are more important when the
  frequency is low

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Focus of Current research

• Hair cell loss in the mammalian cochlea is
  permanent unlike birds who can regenerate their
  hair cells from nearby supporting cells
• Cochlear implants can restore some hearing in
  individuals with sensorineural hearing loss
• Current research focuses on the improvement of
  the processors and electrodes in order to enable
  more full comprehension of speech
• Also, the plasticity of the central auditory
  response after hearing loss is of much interest
  to researchers. This is because as of now there
  is plenty unknown about the central auditory
  pathways. Perhaps findings related to the
  plasticity of the response can lead to a better
  understanding of how the various systems alter
  the processing of auditory input