Auditory and Vestibular Systems

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Auditory and Vestibular Systems
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Sensory systems

• All sensory systems carry information from the
  environment to the brain, where it is processed.
• Sensory receptors in the periphery detect
  physical events
• Stimuli act on these receptors and alter their
  membrane potential
• Sensory transduction is the process of
  transferring stimuli into changes in the membrane
  potential
• Receptor cells lack an axon no action potential
• Receptor potential

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Auditory system

• Concerned with the detection, location and
  recognition of sound
• Sound is the vibration of air molecules
• Alternate compression/rarification of the air
  produces sound waves

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Sound

• Three perceptual dimensions of sound
• The human ear perceives sound with a pitch
  between 30 and 20,000 Hz

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Anatomy of the ear

• Outer ear
• Pinna
• Ear canal
• Tympanic membrane
• Middle ear
• Ossicles bones that conduct magnify
  vibrations
• Malleus
• Incus
• Stapes

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Anatomy of the ear

• Inner ear
• Oval window membrane between the stapes and
  cochlea
• Cochlea fluid-filled structure that conducts
  vibrations contains
• Organ of Corti sensory organ that receives
  vibrations contains
• Hair cells auditory receptor cells
• Basilar membrane membrane where hair cells are
  attached
• Tectorial membrane membrane above the basilar
  membrane
• Round window membrane opposite the oval window

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Transduction of sound

• Sound wave vibrations are conducted and magnified
  by the ossicles of the middle ear
• Stapes vibrates against the oval window
• Causes motion of the fluid in the cochlea
• As membranes move, cilia on the hair cells brush
  against the tectorial membrane
• Bending of the cilia results in a receptor
  potential
• Hair cells synapse with bipolar cells in the
  basilar membrane
• Bipolar cells carry auditory information to the
  brain via the cochlear nerve

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Auditory pathway

• Auditory input synapses at several points before
  reaching the cortex
• Cochlear nucleus (medulla)
• Superior olivary complex (medulla)
• Travel via the lateral lemniscus fiber bundle to
  the
• Inferior colliculus (midbrain)
• Medial geniculate nucleus (thalamus)
• Primary auditory cortex (temporal lobe)

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Auditory pathway

• Each hemisphere receives input from both ears,
  but the majority of input comes from the
  contralateral ear.
• Auditory cortex is organized in a tonotopic
  representation
• Sections along the basilar membrane each respond
  to a certain frequency
• Each section of the membrane corresponds
  spatially with a section of auditory cortex

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Detection of sound

• Pitch is detected in the cochlea by
• Place coding neurons along the basilar membrane
  respond to different frequencies
• Rate coding certain neurons fire in synchrony
  with the motion of the apical end of the basilar
  membrane (lowest frequencies)

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Detection of sound

• Timbre
• Consists of
• Fundamental frequency
• Overtones
• Different sections of the bas. membrane respond
  to each of these tones, producing a unique coded
  pattern of activity in the cochlear nerve
• This pattern of activity is identified in the
  auditory cortex

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Location of sound

• Location detection
• Neurons in the superior olivary complex detect
  differences in the arrival times of sound waves
  hitting each ear
• For continuous sounds, these neurons detect phase
  differences
• Phase differences in high-frequency pitches are
  too short to be detected
• The SOC detects the intensity of the stimulation
  hitting each ear
• Analysis of timbre is used to locate sounds in
  front or behind us
• The external ear enhances and attenuates
  different frequencies, depending on their angle
  of approach

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Cortical auditory processing

• Sound recognition - occurs in the auditory
  association cortex
• Relies on pattern recognition the memory of a
  pattern of auditory activity
• Auditory processing in the cortex is organized
  into two streams
• Dorsal stream perception of sound location
• Ventral stream perception of form (pitch)

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Vestibular system

• Sensory system responsible for balance, head
  position and adjustment of eye movements in
  coordination with the head

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Vestibular anatomy

• Inner ear is comprised of 3 labyrinths
• Cochlea (auditory system)
• Vestibular sacs
• Utricle Saccule
• Semicircular canals

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Vestibular sacs

• Detect the force of gravity to perceive
  orientation of the head
• Contain receptive tissue comprised of hair cells
• Cilia on these hair cells are embedded in a
  gelatinous mass containing calcium carbonate
  crystals
• Cilia detect movement of this mass as the head
  moves

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Semicircular canals

• Correspond to the major planes of the head
  (sagittal, transverse and horizontal)
• Receptors in each canal respond maximally to head
  movements in that plane
• Hair cells in each canal have cilia embedded in a
  gelatinous mass
• Movement of this mass by the fluid in the canal
  stimulates cilia on the hair cell

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Vestibular perception

• Movement of the cilia on the hair cell causes
  changes in the receptor potential
• Receptors synapse with bipolar cells whose axons
  travel into the brain via the vestibular nerve
• Vestibular nerve axons synapse in the vestibular
  nuclei of the medulla
• Some project directly to the cerebellum

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Vestibular perception

• Vestibular nuclei project to
• Cerebellum
• Spinal cord
• Medulla
• Pons
• Temporal cortex responsible for feelings of
  dizziness
• Cranial nerves (3, 4 6) that control eye
  muscles
• Vestibulo-ocular reflex